Transmission of IP Packets over Overlay Multilink Network (OMNI) Interfaces

Internet-Draft	IPv6 over OMNI Interfaces	April 2024
Templin	Expires 4 October 2024	[Page]

Abstract

Air/land/sea/space mobile nodes (e.g., aircraft of various configurations, terrestrial vehicles, seagoing vessels, space systems, enterprise wireless devices, pedestrians with cell phones, etc.) communicate with networked correspondents over wireless and/or wired-line data links and configure mobile routers to connect end user networks. This document presents a multilink virtual interface specification that enables mobile nodes to coordinate with a network-based mobility service, fixed node correspondents and/or other mobile node peers. The virtual interface provides an adaptation layer service suited for both mobile and more static environments such as enterprise and home networks. Both Provider-Aggregated (PA) and Provider-Independent (PI) addressing services are supported. This document specifies the transmission of IP packets over Overlay Multilink Network (OMNI) Interfaces.¶

2. Terminology

The terminology in the normative references applies; especially, the terms "link" and "interface" are the same as defined in the IPv6 [RFC8200] and IPv6 Neighbor Discovery (ND) [RFC4861] specifications. This document assumes the following IPv6 ND control plane message types: Router Solicitation (RS), Router Advertisement (RA), Neighbor Solicitation (NS), Neighbor Advertisement (NA), unsolicited NA (uNA) and Redirect.¶

OMNI interfaces normally limit the size of their IPv6 ND control plane messages to the minimum IPv6 link MTU, but some messages may exceed this size if there are sufficient OMNI parameters and/or IP packet/parcel attachments. These larger messages can still travel over secured underlying network control plane paths that include IPsec tunnels [RFC4301] and/or secured direct point-to-point links without loss due to a size restriction by engaging OMNI IPv6 encapsulation/fragmentation as necessary up to a maximum size of 65535 octets.¶

Host, Client and Proxy/Server OMNI interfaces that employ IPv6 ND control plane messaging maintain per-neighbor state in Neighbor Cache Entries (NCEs). Each NCE is indexed by the neighbor's network layer address(es) while the neighbor's OAL encapsulation address provides context for Identification verification.¶

The Protocol Constants defined in Section 10 of [RFC4861] are used in their same format and meaning in this document. The terms "All-Routers multicast", "All-Nodes multicast" and "Subnet-Router anycast" are the same as defined in [RFC4291]. Also, IPv6 ND state names, variables and constants including REACHABLE, ReachableTime and REACHABLE_TIME are the same as defined in [RFC4861].¶

The term "IP" is used to refer collectively to either Internet Protocol version (i.e., IPv4 [RFC0791] or IPv6 [RFC8200]) when a specification at the layer in question applies equally to either version.¶

The terms Host, Client and Proxy/Server are intentionally capitalized to denote an instance of that particular node type that also configures an OMNI interface and engages the OMNI Adaptation Layer.¶

The terms "application layer (L5 and higher)", "transport layer (L4)", "network layer (L3)", "(data) link layer (L2)" and "physical layer (L1)" are used consistently with common Internetworking terminology, with the understanding that reliable delivery protocol users of UDP are considered as transport layer elements. The OMNI specification further defines an "adaptation layer" positioned below the network layer but above the link layer, which may include physical links and Internet- or higher-layer tunnels. A (network) interface is a node's attachment to a link (via L2), and an OMNI interface is therefore a node's attachment to an OMNI link (via the adaptation layer).¶

The L3, adaptation and (virtual) L2 layers each include distinct packet Identification numbering spaces although L3 and L2 packet headers often omit the Identification for unfragmented packets. The adaptation layer employs an 8-octet Identification numbering space that is distinct from L3/L2 spaces, with an Identification value appearing in an IPv6 Extended Fragment Header [I-D.templin-6man-ipid-ext2] or an OMNI Compressed Header (OCH) (see: Section 6.5) in each adaptation layer encapsulation.¶

The terms "IP jumbogram", "advanced jumbo (AJ)" and "IP parcel" refer to special packet formats that enable a new link model for the Internet as discussed in [I-D.templin-6man-parcels2] [I-D.templin-intarea-parcels2].¶

The following terms are defined within the scope of this document:¶

L3: The Network layer in the OSI network model. Also known as "layer 3", "IP layer", etc.¶
L2: The Data Link layer in the OSI network model. Also known as "layer 2", "link layer", "sub-IP layer", etc.¶
Adaptation layer: An encapsulation mid-layer that adapts L3 to a diverse collection of L2 underlay interfaces and their encapsulations. (No layer number is assigned, since numbering was an artifact of the legacy reference model that need not carry forward in the modern architecture.) The adaptation layer sees the network layer as "L3" and sees all link layer encapsulations as "L2 encapsulations", which may include UDP, IP and true link layer (e.g., Ethernet, etc.) headers.¶
Access Network (ANET): a connected network region (e.g., an aviation radio access network, corporate enterprise network, satellite service provider network, cellular operator network, residential WiFi network, etc.) that connects Clients to the rest of the OMNI link. Physical and/or data link level security is assumed (sometimes referred to as "protected spectrum" for wireless domains). ANETs such as private enterprise networks and ground domain aviation service networks often provide multiple secured IP hops between the Client's physical point of connection and the nearest Proxy/Server.¶
Mobile Ad-hoc NETwork (MANET): a connected network region that shares the same properties as an ANET except that links often have undetermined connectivity properties, lower layer security services cannot always be assumed and multihop forwarding between Clients acting as MANET routers may be necessary.¶
Internetwork (INET): a connected network region with a coherent IP addressing plan that provides transit forwarding services between ANETs and/or OMNI nodes that coordinate with the Mobility Service over unprotected media. Since physical and/or data link level security cannot always be assumed, security must be applied by the network and/or higher layers if necessary. The global public Internet itself is an example.¶
End-user Network (ENET): a simple or complex "downstream" network tethered to a Client as a single logical unit that travels together. The ENET could be as simple as a single link connecting a single Host, or as complex as a large network with many links, routers, bridges and end user devices. The ENET provides an "upstream" link for arbitrarily many low-, medium- or high-end devices dependent on the Client for their upstream connectivity, i.e., as Internet of Things (IoT) entities. The ENET can also support a recursively-descending chain of additional Clients such that the ENET of an upstream Client is seen as the ANET of a downstream Client.¶
*NET: a "wildcard" term used when a given specification applies equally to all MANET/ANET/INET cases. From the Client's perspective, *NET interfaces are "upstream" interfaces that connect the Client to the Mobility Service, while ENET interfaces are "downstream" interfaces that the Client uses to connect downstream ENETs, Hosts and/or other Clients. Local communications between correspondents within the same *NET can often be conducted based on IPv6 Unique Local Addresses (ULAs) at a layer below IP but without requiring adaptation layer encapsulation.¶
underlay interface: a *NET or ENET interface over which an OMNI interface is configured. The OMNI interface is seen as an L3 interface by the network layer, and each underlay interface is seen as an L2 interface by the OMNI interface. The underlay interface either connects directly to the physical communications media or coordinates with another node where the physical media is hosted.¶
MANET Interface: a node's underlay interface connection to a local network with indeterminant neighborhood properties over which multihop relaying may be necessary. All MANET interfaces used by AERO/OMNI are IPv6 interfaces and therefore must configure a Maximum Transmission Unit (MTU) at least as large as the IPv6 minimum MTU (1280 octets) even if lower-layer fragmentation is needed.¶
OMNI link: a Non-Broadcast, Multiple Access (NBMA) virtual overlay configured over one or more INETs and their connected (M)ANETs/ENETs. An OMNI link may comprise multiple distinct "segments" joined by L2 forwarding devices the same as for any link; the addressing plans in each segment may be mutually exclusive and managed by different administrative entities. Proxy/Servers and other infrastructure elements extend the link to support communications between Clients as single-hop neighbors.¶
OMNI link segment: a Proxy/Server and all of its constituent Clients within any attached local networks is considered as a leaf OMNI link segment, with each leaf interconnected via links and "bridge" nodes in intermediate OMNI link segments. When the local networks of multiple leaf segments overlap (e.g., due to network mobility), they can combine to form larger local networks but with no changes to Client-to-Proxy/Server relationships. The OMNI link consists of the concatenation of all OMNI link leaf and intermediate segments as a loop-free spanning tree.¶
OMNI interface: a node's attachment to an OMNI link, and configured over one or more underlay interfaces. If there are multiple OMNI links in an OMNI domain, a separate OMNI interface is configured for each link. The OMNI interface configures a Maximum Transmission Unit (MTU) and an Effective MTU to Receive (EMTU_R) the same as any interface.¶
OMNI Adaptation Layer (OAL): an OMNI interface sublayer service that encapsulates original IP packets/parcels admitted into the interface in an IPv6 header and/or subjects them to fragmentation and reassembly. The OAL is also responsible for generating MTU-related control messages as necessary, and for providing addressing context for OMNI link SRT traversal. The OAL presents a new layer in the Internet architecture known simply as the "adaptation layer". The OMNI link is an example of a limited domain [RFC8799] at the adaptation layer although its segments may be joined over open Internetworks at L2.¶
(OMNI) Host: an end user device that extends the OMNI link over an ENET interface serviced by a Client. (As an implementation matter, the Host either assigns the same IP address from the ENET (underlay) interface to an (overlay) OMNI interface, or configures an OMNI-like function as a virtual sublayer of the ENET interface itself.) The IP addresses assigned to each Host ENET interface remain stable even if the Client's upstream *NET interface connections change.¶
(OMNI) Client: a network platform/device mobile router that configures one or more OMNI interfaces over distinct sets of underlay interfaces grouped as logical OMNI link units. The Client coordinates with the Mobility Service via upstream networks over *NET interfaces, and provides Proxy/Server services for Hosts and other Clients on ENET interface downstream networks. The Client's *NET interface addresses and performance characteristics may change over time (e.g., due to node mobility, link quality, etc.) while downstream-attached Hosts and other Clients see the ENET as a stable ANET.¶
(OMNI) Proxy/Server: a segment routing topology edge node that configures an OMNI interface and connects Clients to the Mobility Service. As a server, the Proxy/Server responds directly to some Client IPv6 ND messages. As a proxy, the Proxy/Server forwards other Client IPv6 ND messages to other Proxy/Servers and Clients. As a router, the Proxy/Server provides a forwarding service for ordinary data messages that may be essential in some environments and a last resort in others. Proxy/Servers at (M)ANET boundaries configure both an (M)ANET downstream interface and *NET upstream interface, while INET-based Proxy/Servers configure only an INET interface. All Proxy/Servers configure a Stable Network Prefix (SNP) and manage 1x1 mappings of internal Unique Local Addresses (ULAs) and external Globally Unique Addresses (GUAs) according to [I-D.bctb-6man-rfc6296-bis].¶
First-Hop Segment (FHS) Proxy/Server: a Proxy/Server connected to the source Client's *NET that forwards OAL packets sent by the source into the segment routing topology. FHS Proxy/Servers allocate Provider-Aggregated (Proxy/Server-Aggregated) addresses to Clients within their local networks. FHS Proxy/Servers also act as intermediate forwarding systems to facilitate RS/RA-based Provider-Independent Prefix Delegation exchanges between Clients and Mobility Anchor Point (MAP) Proxy/Servers.¶
Last-Hop Segment (LHS) Proxy/Server: a Proxy/Server connected to the target Client's *NET that forwards OAL packets received from the segment routing topology to the target.¶
Mobility Anchor Point (MAP) Proxy/Server: a single Proxy/Server selected by the Client that provides a designated router service for any *NET underlay networks that register the Client's Mobile Network Prefix (MNP). Since all Proxy/Servers provide equivalent services, Clients normally select the first FHS Proxy/Server they coordinate with to serve as the MAP. However, the MAP can instead be any available Proxy/Server for the OMNI link, i.e., and not necessarily one of the Client's FHS Proxy/Servers. This flexible arrangement supports a fully distributed mobility management service.¶
Segment Routing Topology (SRT): a multinet forwarding region configured over one or more INETs between the FHS Proxy/Server and LHS Proxy/Server. The SRT spans the OMNI link on behalf of communicating peer nodes using segment routing in a manner outside the scope of this document (see: [I-D.templin-intarea-aero2]).¶
Mobility Service (MS): a mobile routing service that tracks Client movements and ensures that Clients remain continuously reachable even across mobility events. The MS consists of the set of all Proxy/Servers plus all other OMNI link supporting infrastructure nodes. Specific MS details are out of scope for this document, with an example found in [I-D.templin-intarea-aero2].¶
Mobility Service Prefix (MSP): an aggregated IP Global Unicast Address (GUA) prefix (e.g., 2001:db8::/32, 192.0.2.0/24, etc.) assigned to the OMNI link and from which more-specific Mobile Network Prefixes (MNPs) are delegated. OMNI link administrators typically obtain MSPs from an Internet address registry, however private-use prefixes can also be used subject to certain limitations (see: Section 10). OMNI links that connect to the global Internet advertise their MSPs to their interdomain routing peers.¶
Mobile Network Prefix (MNP): a longer IP prefix delegated from an MSP (e.g., 2001:db8:1000:2000::/56, 192.0.2.8/30, etc.) and assigned to a Client. Clients receive MNPs from MAP Proxy/Servers and sub-delegate them to routers, Hosts and other Clients located in ENETs.¶
Stable Network Prefix (SNP): a global IP and unique-local IP prefix pair assigned to one or more Proxy/Servers that connect local (M)ANETs or INET Client groups to the rest of the OMNI link. Clients request address delegations from the SNP that can be used to support global and local-scoped communications. Clients communicate internally within (M)ANETs and INET groups using IPv6 Unique Local Addresses (ULAs) assigned in 1x1 correspondence to SNP Globally Unique Addresses (GUAs) made visible to external peers through IP network address/prefix translation [RFC6145][RFC6146][RFC6147][I-D.bctb-6man-rfc6296-bis].¶
Subnet Router Anycast (SRA) Address: An IPv6 address taken from an MNP or SNP in which the remainder of the address beyond the final bit of the prefix is set to the value "all-zeros". For example, the SRA for 2001:db8:1::/48 is simply 2001:db8:1:: (i.e., with the 80 least significant bits set to 0). For IPv4, the IPv6 SRA corresponding to the IPv4 prefix 192.0.2.0/24 is 2002:V4ADDR::/40, where "V4ADDR" is the hexadecimal value corresponding to 192.0.2.0 per [RFC3056].¶
original IP packet/parcel: a whole IP packet/parcel or fragment admitted into the OMNI interface by the network layer prior to OAL encapsulation/fragmentation, or an IP packet/parcel delivered to the network layer by the OMNI interface following OAL reassembly/decapsulation.¶
OAL packet: an original IP packet/parcel encapsulated in an OAL IPv6 header with an IPv6 Extended Fragment Header extension that includes an 8-octet (64-bit) OAL Identification value. Each OAL packet is then subject to OAL fragmentation and reassembly.¶
OAL fragment: a portion of an OAL packet following fragmentation but prior to L2 encapsulation/fragmentation, or following L2 reassembly/decapsulation but prior to OAL reassembly.¶
(OAL) atomic fragment: an OAL packet that can be forwarded without fragmentation, but still includes an IPv6 Extended Fragment Header with an 8-octet (64-bit) OAL Identification value and with Fragment Offset and More Fragments both set to 0.¶
(L2) carrier packet: an encapsulated OAL fragment following L2 encapsulation or prior to L2 decapsulation. OAL sources and destinations exchange carrier packets over underlay interfaces, and may be separated by one or more OAL intermediate systems. OAL intermediate systems may perform re-encapsulation on carrier packets by removing the L2 headers of the first hop network and replacing them with new L2 headers for the next hop network. Carrier packets may themselves be subject to fragmentation and reassembly in L2 underlay networks at a layer below the OAL. Carrier packets sent over unsecured paths use OMNI protocol L2 encapsulations, while those sent over secured paths use L2 security encapsulations such as IPsec [RFC4301], etc. (The term "carrier" honors agents of the service postulated by [RFC1149] and [RFC6214].)¶
OAL source: an OMNI interface acts as an OAL source when it encapsulates original IP packets/parcels to form OAL packets, then performs OAL fragmentation and encapsulation to create carrier packets which may themselves be subject to fragmentation at their layer. Every OAL source is also an OMNI link ingress.¶
OAL destination: an OMNI interface acts as an OAL destination when it decapsulates carrier packets (while reassembling first, if necessary), then performs OAL reassembly/decapsulation to derive the original IP packet/parcel. Every OAL destination is also an OMNI link egress.¶
OAL intermediate system: an OMNI interface acts as an OAL intermediate system when it reassembles/decapsulates carrier packets received from a first segment to obtain the original OAL packet/fragment, then re-encapsulates in new L2 headers appropriate for the next segment and sends these new carrier packets into the next segment (while re-fragmenting first, if necessary). OAL intermediate systems decrement the Hop Limit in OAL packets/fragments during forwarding, and discard the OAL packet/fragment if the Hop Limit reaches 0. OAL intermediate systems do not decrement the TTL/Hop Limit of the original IP packet/parcel, which can only be updated by the network and higher layers.¶
OMNI Option: an IPv6 Neighbor Discovery Option providing multilink parameters for the OMNI interface as specified in Section 12.¶
Interface Identifier (IID): the least significant 64 bits of an IPv6 address, as specified in the IPv6 addressing architecture [RFC4291].¶
(OMNI) Link Local Address (LLA): an IPv6 address beginning with fe80::/64 per the IPv6 addressing architecture [RFC4291] and with either a 64-bit MNP (LLA-MNP) or a 56-bit random value (LLA-RND) encoded in the IID as specified in Section 8.¶
(OMNI) Unique Local Address (ULA): an IPv6 address delegated to a *NET node beginning with fd00::/8 followed by a 40-bit Global ID, a 16-bit Subnet ID and a 64-bit Interface ID per [RFC4193]. (Note that [RFC4193] specifies a second form of ULAs based on the prefix fc00::/8, which are referred to as "ULA-C" throughout this document to distinguish them from the ULAs defined here.)¶
(OMNI) Hierarchical Host Identity Tag (HHIT): a 128-bit IPv6 address according to [RFC9374]. Each Clients assigns a unique HHIT used to bootstrap autoconfiguration in the presence of OMNI link infrastructure or for sustained communications in the absence of infrastructure. When a Client receives a PA SNP GUA/ULA delegation from a Proxy/Server or a PI prefix delegation from a MAP, it can begin using PA/PI addresses instead of its HHIT for Internetworking communications.¶
Provider-Independent (PI) Address: a global unicast IP address allocated from an MNP delegated to a Client via a MAP Proxy/Server is considered Provider-Independent (Proxy/Server-Independent) or "PI".¶
Provider-Aggregated (PA) Address: a global unicast IP address delegated to a Client from a SNP assigned to a FHS Proxy/Server is considered Provider-Aggregated (Proxy/Server-Aggregated) or "PA".¶
Multilink: a Client OMNI interface's manner of managing multiple diverse *NET underlay interfaces as a single logical unit. The OMNI interface provides a single unified interface to the network layer, while underlay interface selections are performed on a per-flow basis considering traffic selectors such as DSCP, flow label, application policy, signal quality, cost, etc. Multilink selections are coordinated in both the outbound and inbound directions based on source/target underlay interface pairs.¶
Multinet: an intermediate system's manner of spanning multiple diverse IP Internetwork and/or private enterprise network "segments" through OAL encapsulation. Through intermediate system concatenation of SRT network segments, multiple diverse Internetworks (such as the global public IPv4 and IPv6 Internets) can serve as transit segments in an end-to-end OAL forwarding path. This OAL concatenation capability provides benefits such as supporting IPv4/IPv6 transition and coexistence, joining multiple diverse operator networks into a cooperative single service network, etc. See: [I-D.templin-intarea-aero2] for further information.¶
Multihop: an iterative relaying of carrier packets between Client's over an OMNI underlay interface technology (such as omnidirectional wireless) without support of fixed infrastructure. Multihop services entail Client-to-Client relaying within a Mobile/Vehicular Ad-hoc Network (MANET/VANET) for Vehicle-to-Vehicle (V2V) communications and/or for Vehicle-to-Infrastructure (V2I) "range extension" where Clients within range of communications infrastructure elements provide forwarding services for other Clients.¶
Mobility: any action that results in a change to a Client underlay interface address. The change could be due to, e.g., a handover to a new wireless base station, loss of link due to signal fading, an actual physical node movement, etc.¶
Safety-Based Multilink (SBM): A means for ensuring fault tolerance through redundancy by connecting multiple OMNI interfaces within the same domain to independent routing topologies (i.e., multiple independent OMNI links).¶
Performance Based Multilink (PBM): A means for selecting one or more underlay interface(s) for carrier packet transmission and reception within a single OMNI interface.¶
OMNI Domain: The set of all SBM/PBM OMNI links that collectively provides services for a common set of MSPs. All OMNI links within the same domain configure, advertise and respond to the same OMNI IPv6 Anycast address(es).¶
AERO Forwarding Information Base (AFIB): A multilink forwarding table on each OAL source, destination and intermediate system that includes AERO Forwarding Vectors (AFV) with both next hop forwarding instructions and context for reconstructing compressed headers for specific underlay interface pairs used to communicate with peers. See: [I-D.templin-intarea-aero2] for further discussion.¶
AERO Forwarding Vector (AFV): An AFIB entry that includes soft state for each underlay interface pairwise communication session between peer neighbors. AFVs are identified by an AFV Index (AFVI) paired with the previous hop L2 address, with the pair established based on an IPv6 ND solicitation and solicited IPv6 ND advertisement response. The AFV also caches underlay interface pairwise Identification sequence number parameters to support carrier packet filtering. See: [I-D.templin-intarea-aero2] for further discussion.¶
AERO Forwarding Vector Index (AFVI): A 2-octet or 4-octet integer value supplied by a first hop OAL node when it requests a next hop OAL node to create an AFV. The next hop OAL node caches the AFVI and L2 address supplied by the previous hop as header compression/decompression state for future OAL packets with compressed headers. The first hop OAL node must ensure that the AFVI values it assigns to the next hop via a specific underlay interface are distinct and reused only after their useful lifetimes expire. See: [I-D.templin-intarea-aero2] for further discussion.¶
(OMNI) L2 encapsulation: the OMNI protocol encapsulation of OAL packets/fragments in an outer header or headers to form carrier packets that can be routed within the scope of the local *NET underlay network partition. The OAL node that performs encapsulation is known as the "L2 source" while the OAL node that performs decapsulation is known as the "L2 destination"; both OAL end and intermediate systems can also act as an L2 source or destination. Common L2 encapsulation combinations include UDP, IP and/or Ethernet using a port/protocol/type number for OMNI.¶
L2 address (L2ADDR): an address that appears in the OMNI protocol L2 encapsulation for an underlay interface and also in IPv6 ND message OMNI options. L2ADDR can be either an IP address for IP encapsulations or an IEEE EUI address [EUI] for direct data link encapsulation. (When UDP/IP encapsulation is used, the UDP port number is considered an ancillary extension of the IP L2ADDR.)¶
OAL Fragment Size (OFS): the current size for OAL source fragmentation which must be no smaller than 1024 octets and no larger than 65279 octets (allowing for up to 256 octets of L2 encapsulations for each OAL fragment). Each OAL source maintains an OFS in AERO Forwarding Vectors (AFVs) for each OAL destination. The source discovers the "maximum OFS" through IPv6 Minimum Path MTU Options [RFC9268] and maintains an equal or smaller value "effective OFS" according to dynamic network control message feedback. The OAL source should adaptively seek to use the largest possible effective OFS under current network conditions to provide better performance for upper layers. OAL fragments prepared by the source must not be further fragmented by OAL intermediate systems on the path to the OAL destination.¶
Carrier Fragment Size (CFS): the current size for L2 carrier packet fragments including the headers, trailers and OAL fragment body. The OAL L2 source applies source fragmentation if necessary to each L2-encapsulated OAL fragment under the default CFS of 1280 octets (i.e., the IPv6 minimum MTU) until it can either engage IPv4 network fragmentation or determine whether a larger CFS is possible through Packetization Layer Path MTU Discovery for Datagram Transports [RFC8899]. The L2 source should adaptively seek to maximize CFS to provide better performance for upper layers.¶

6. The OMNI Adaptation Layer (OAL)

When an OMNI interface forwards an original IP packet/parcel from the network layer for transmission over one or more underlay interfaces, the OMNI Adaptation Layer (OAL) acting as the OAL source applies IPv6 encapsulation to form OAL packets subject to OAL fragmentation producing fragments suitable for L2 encapsulation and transmission as carrier packets. These carrier packets may in turn be subject to IP fragmentation over underlay interface paths as described in Section 6.1. The carrier packets/fragments then travel over one or more underlay networks spanned by OAL intermediate systems in the SRT, which first reassemble (if necessary) then re-encapsulate by removing the L2 headers of the first underlay network and appending L2 headers appropriate for the next underlay network in succession while re-fragmenting if necessary. (This process supports the multinet concatenation capability needed for joining multiple diverse networks.) Following any forwarding by OAL intermediate systems, the carrier packets arrive at the OAL destination.¶

When the OAL destination receives the carrier packets, it reassembles (if necessary) then discards the L2 headers and reassembles the resulting OAL fragments (if necessary) into an OAL packet as described in Section 6.3. The OAL destination next decapsulates the OAL packet to obtain the original IP packet/parcel which it then delivers to the network layer. The OAL source may be either the source Client or its FHS Proxy/Server, while the OAL destination may be either the LHS Proxy/Server or the target Client. Proxy/Servers (and SRT Gateways as discussed in [I-D.templin-intarea-aero2]) may also serve as OAL intermediate systems.¶

The OAL presents an OMNI sublayer abstraction similar to ATM Adaptation Layer 5 (AAL5). Unlike AAL5 which performs segmentation and reassembly with fixed-length 53-octet cells over ATM networks, however, the OAL uses IPv6 encapsulation, fragmentation and reassembly with larger variable-length cells over heterogeneous networks. Detailed operations of the OAL are specified in the following sections.¶

6.1. OAL Source Encapsulation and Fragmentation

When the network layer forwards an original IP packet/parcel into the OMNI interface, it sets the TTL/Hop Limit for locally-generated packets or decrements it according to standard IP forwarding rules for forwarded packets. The OAL source next creates an "OAL packet" by prepending an IPv6 OAL encapsulation header in the spirit of [RFC2473] but with Next Header set to TBD1 (see: IANA Considerations) and with the IPv6 OAL header followed by an IPv4 or IPv6 original packet. The OAL source copies the "Type of Service/Traffic Class" [RFC2983] and "Explicit Congestion Notification (ECN)" [RFC3168] values in the original packet/parcel's IP header into the corresponding fields in the OAL header, then sets the OAL header "Flow Label" as specified in [RFC6438]. The OAL source next sets the OAL header IPv6 Payload Length to the length of the original IP packet/parcel and sets Hop Limit to a value that MUST NOT be larger than 63 yet is still sufficiently large to support loop-free forwarding over multiple concatenated OAL intermediate hops.¶

The OAL source next selects OAL packet source and destination addresses. Client OMNI interfaces set the OAL source address to a Unique Local Address (ULA) or Globally Unique Address (GUA) based on SNP addresses received from a Proxy/Server. When a Client OMNI interface does not (yet) have a ULA/GUA and/or MNP, it can instead use a (Hierarchical) Host Identity Tag ((H)HIT [RFC9374] as an OAL address.¶

The OAL source next inserts any necessary extension headers following the OAL IPv6 header but before the payload packet as specified in Section 6.4. The source first inserts any per-fragment extension headers (e.g., Hop-by-Hop, Routing, etc.) then inserts an IPv6 Extended Fragment Header (see: [I-D.templin-6man-ipid-ext2]) with an 8-octet (64-bit) OAL packet Identification. Note that the extension header insertions could cause the IPv6 Payload Length to exceed 65535 octets by a small amount when the original IPv6 packet is (nearly) the maximum length. The OAL source then fragments the OAL packet if necessary according to an OAL Fragment Size (OFS) maintained in AERO Forwarding Vectors (AVFs) for each OAL destination. (OAL packets that are no larger than the OFS and original IP packets/parcels larger than 65535 octets are instead processed as "atomic fragments".) OAL fragments prepared by the source must not be fragmented further by OAL intermediate systems on the path to the OAL destination.¶

OAL packets that contain original IP parcels no larger than (64*65535) octets may be first subject to OMNI interface parcellation, after which the (sub-)parcels (as well as OAL packets that contain original IP packets no larger than 65535 octets) are subject to OAL fragmentation-based assured delivery. Advanced Jumbos (AJs) larger than 65535 octets (see: [I-D.templin-6man-parcels2] [I-D.templin-intarea-parcels2]) are not eligible for OAL fragmentation but instead engage a best effort jumbo-in-jumbo encapsulation service as discussed in Section 5.2. (Note: the original source can optionally elect this best-effort jumbo-in-jumbo delivery service for any parcel/AJ regardless of its size.)¶

OAL fragmentation is conducted the same as for standard IPv6 fragmentation [RFC8200] with the exception that an Extended Fragment Header is used and the IPv6 Payload Length may exceed 65535 by at most the length of the extension headers. The OAL source MUST set a "maximum OFS" to a size no smaller than 1024 octets and no larger than 65279 octets and thereafter reduce or increase the "effective OFS" according to dynamic network control message feedback. (Note that these sizes allow for up to 256 octets of L2 encapsulation relative to the IPv6 minimum MTU of 1280 octets and the maximum-sized reassembled packet of 65535 octets.) Specifically, if an OAL intermediate system or the OAL destination advertises a reduced size, the OAL source SHOULD reduce the effective OFS accordingly (to a size no smaller than 1024 octets) and can later increase the effective OFS (to a size no larger than the maximum OFS) as network conditions improve. When the OAL source performs fragmentation, it SHOULD produce the minimum number of fragments under the effective OFS constraints, where the fragments MUST be non-overlapping and the portion of each non-final fragment following the IPv6 Extended Fragment Header MUST be equal in length while that of the final fragment may be a different length.¶

The OAL source discovers the maximum OFS by including an IPv6 Minimum Path MTU Hop-by-Hop Option [RFC9268] in the OAL encapsulation header of its Neighbor Solicitation (NS) / Neighbor Advertisement (NA) exchanges over the secured spanning tree used to establish multilink forwarding state (see: [I-D.templin-intarea-aero2]). Each OAL intermediate system on the path sets the minimum path MTU in the NS message OAL extension header to the maximum OFS capable of traversing the next segment. (Note that segments traversed by L2 encapsulations such as IPsec tunnels can normally regard the MTU for their unsecured overlay network segments as 65535 octets while those traversed by direct point-to-point links and multihop MANET links must regard the link MTU as a restricting size; therefore, each OAL intermediate system MUST correctly recognize and honor the IPv6 Minimum Path MTU Hop-by-Hop Option. Note also that OAL intermediate systems forward the NS/NA messages in the control plane, but the returned MTU reflects the maximum OFS for the data plane.) When the OAL destination returns an NA message with an OAL header containing an IPv6 Minimum Path MTU Hop-by-Hop Option, the OAL source can then set the maximum OFS for this AFV by deducting 256 from the returned MTU. The OAL source can later adaptively increase or decrease the effective OFS if it receives dynamic path MTU feedback from an OAL intermediate node or destination with the understanding that larger OFS sizes may provide better performance but also increase the retransmission unit in case of loss.¶

During fragmentation, the OAL source replaces the IPv6 Extended Fragment Header 1-octet "Reserved" field with OMNI-specific encodings as shown in Figure 4:¶

   +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+
   | Parcel ID |P|S|     |  Ordinal  |Res|
   +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+
   a) First fragment     a) Non-first fragment

Figure 4: IPv6 Extended Fragment Header Reserved Field Coding

For the first fragment (i.e., the one with Fragment Offset set to 0), the OAL source sets "Parcel ID", "(P)arcel" and "More (S)egments" as specified in Section 6.13. For each non-first fragment, the OAL source instead writes a monotonically-increasing "Ordinal" value between 1 and 63. Specifically, the OAL source writes the Ordinal value '1' for the first non-first fragment, '2' for the second, '3' for the third, etc. up to the final fragment. The first fragment is logically considered Ordinal number '0' while the final fragment may assign an Ordinal as large as '63'; therefore at most 64 fragments are possible. For this reason, OAL fragments produced by OAL source fragmentation must not be subjected to further adaptation layer fragmentation by an OAL intermediate system or IPv6 router on the path.¶

The OAL source finally encapsulates the fragments in L2 headers to form carrier packets for transmission over underlay interfaces, while retaining the fragments and their ordinal numbers (i.e., #0, #1, #2, etc.) for a brief period to support adaptation layer retransmissions (see: Section 6.8). OAL fragment and carrier packet formats are shown in Figure 5 (note that IPv4 carrier packets with DF=0 may include trailing checksums ("Csum") as discussed in Section 6.2).¶

     +----------+-------------------------+---------------+
     |OAL Header| Original Packet Headers |    Frag #0    |
     +----------+-------------------------+---------------+
     +----------+----------------+
     |OAL Header|     Frag #1    |
     +----------+----------------+
     +----------+----------------+
     |OAL Header|     Frag #2    |
     +----------+----------------+
                 ....
     +----------+----------------+
     |OAL Header|   Frag #(N-1)  |
     +----------+----------------+
     a) OAL fragmentation


     +----------+-----------------------------+
     |OAL Header|  Original IP packet/parcel  |
     +----------+-----------------------------+
     b) An OAL atomic fragment


     +--------+----------+----------------+------+
     |L2 Hdrs |OAL Header|     Frag #i    | Csum |
     +--------+----------+----------------+------+
     c) OAL carrier packet after L2 encapsulation

Figure 5: OAL Fragments and Carrier Packets

6.2. OAL L2 Encapsulation and Re-Encapsulation

The OAL source or intermediate system next encapsulates each OAL fragment (with either full or compressed headers) in L2 encapsulation headers to create a carrier packet. The OAL source or intermediate system (i.e., the L2 source) includes a UDP header as the innermost sublayer if NATs and/or filtering middleboxes might occur on the path. Otherwise, the L2 source includes a full/compressed IP header and/or an actual link layer header (e.g., such as for Ethernet-compatible links) as the innermost sublayer. The L2 source also appends any additional encapsulation sublayer headers necessary (e.g., IPsec AH/ESP, jumbo-in-jumbo encapsulation, etc.).¶

The L2 source encapsulates the OAL information immediately following the innermost L2 sublayer header. The L2 source next interprets the first four bits following the L2 headers as a Type field that determines the type of OAL header that follows. The L2 source sets Type to "OMNI-OFH" for an uncompressed IPv6 OMNI Full Header (OFH) or "OMNI-OCH1/2" for an OMNI Compressed Header, Type 1 (OCH1) or 2 (OCH2) as specified in Section 6.5. For IP packets/parcels that do not include an OAL header, the L2 source instead interprets the first four bits as a Version field that encodes '4' (OMNI-IP4) for an ordinary IPv4 packet/parcel or '6' (OMNI-IP6) for an ordinary IPv6 packet/parcel. Other Type values (including a Type for a Hop-by-Hop Options header that includes a Parcel Payload Option) may also appear as specified in Section 6.5.¶

The OAL node prepares the L2 encapsulation headers for OAL packets/fragments as follows:¶

For UDP/IP encapsulation, the L2 source sets the UDP source port to 8060 (i.e., the port number reserved for AERO/OMNI). When the L2 destination is a Proxy/Server or Gateway, the L2 source sets the UDP destination port to 8060; otherwise, the L2 source sets the UDP destination port to its cached port number value for the peer. The L2 source next sets the UDP Length the same as specified in [I-D.ietf-tsvwg-udp-options]. (If the OAL packet is submitted for jumbo-in-jumbo encapsulation, the L2 source instead includes a Hop-by-Hop Options header with a Parcel Payload Option with Advanced Jumbo Type 0 following the L2 UDP/IP header with the length of the L2 UDP header included in the Jumbo Payload Length.) The L2 source then sets the IP {Protocol, Next Header} to '17' (the UDP protocol number) and sets the {Total, Payload} Length the same as specified in the base IP protocol specifications for IP parcels and Advanced Jumbos [I-D.templin-6man-parcels2][I-D.templin-intarea-parcels2] or for ordinary IP packets [RFC0791][RFC8200][I-D.ietf-tsvwg-udp-options]. The L2 source then continues to set the remaining IP header fields as discussed below.¶
For raw IP encapsulation, the L2 source sets the IP {Protocol, Next Header} to TBD1 (see: IANA Considerations) and sets the {Total, Payload} Length the same as specified in [RFC0791] or [RFC8200]. (If the OAL header includes a Parcel Payload Option with an Advanced Jumbo Type, the L2 source includes an Parcel Payload Option with AJ Type 0 in the L2 IP header.) The L2 source then continues to set the remaining IP header fields as discussed below.¶
For IPsec AH/ESP encapsulation, the L2 source sets the appropriate IP or UDP header to indicate AH/ESP then sets the AH/ESP Next Header field to TBD1 the same as for raw IP encapsulation.¶
For direct encapsulations over Ethernet-compatible links, the L2 source prepares an Ethernet Header with EtherType set to TBD2 (see: Section 24.2) (see: Section 7).¶
For OAL packet/fragment encapsulations over secured underlay interface connections to the secured spanning tree, the L2 source applies any L2 security encapsulations according to the protocol (e.g., IPsec). These secured carrier packets are then subject to lower layer security services including fragmentation and reassembly.¶

When an L2 source includes a UDP header, it SHOULD calculate and include a UDP checksum in carrier packets with full OAL headers to prevent mis-delivery and/or detect IPv4 reassembly corruption; the L2 source MAY set UDP checksum to 0 (disabled) in carrier packets with compressed OAL headers (see: Section 6.5) or when reassembly corruption is not a concern. If the L2 source discovers that a path is dropping carrier packets with UDP checksums disabled, it should supply UDP checksums in future carrier packets sent to the same L2 destination. If the L2 source discovers that a path is dropping carrier packets that do not include a UDP header, it should include a UDP header in future carrier packets.¶

When an L2 source sends carrier packets with compressed OAL headers and with UDP checksums disabled, mis-delivery due to corruption of the AERO Forwarding Vector Index (AFVI) is possible but unlikely since the corrupted index would somehow have to match valid state in the (sparsely-populated) AERO Forwarding Information Base (AFIB). In the unlikely event that a match occurs, an OAL destination may receive carrier packets that contain a mis-delivered OAL fragment but can immediately reject any with incorrect Identifications. If the Identification value is somehow accepted, the OAL destination may submit the mis-delivered OAL fragment to the reassembly cache where it will most likely be rejected due to incorrect reassembly parameters. If a reassembly that includes the mis-delivered OAL fragment somehow succeeds (or, for atomic fragments) the OAL destination will verify any included checksums to detect corruption. Finally, any spurious data that somehow eludes all prior checks will be detected and rejected by end-to-end upper layer integrity checks. See: [RFC6935] [RFC6936] for further discussion.¶

For UDP/IP or IP-only L2 encapsulations, when the L2 source is also the OAL source it next copies the "Type of Service/Traffic Class" [RFC2983] and "Explicit Congestion Notification (ECN)" [RFC3168] values in the OAL header into the corresponding fields in the L2 IP header, then (for IPv6) set the L2 IPv6 header "Flow Label" as specified in [RFC6438]. The L2 source then sets the L2 IP TTL/Hop Limit the same as for any host (i.e., it does not copy the Hop Limit value from the OAL header) and finally sets the source and destination IP addresses to direct the carrier packet to the next OAL hop. For carrier packets subject to re-encapsulation, the OAL intermediate system as the L2 source reassembles if necessary then removes the L2 header(s). The L2 source then decrements the OAL header Hop Limit and discards the OAL packet/fragment if the value reaches 0. The L2 source then copies the Type of Service/Traffic Class and ECN values from the previous segment L2 encapsulation header into the next segment L2 encapsulation header while setting the next segment L2 source and destination IP addresses the same as above. (The L2 source also writes the ECN value into the OAL full/compressed header.)¶

The L2 source then applies source fragmentation if necessary by inserting an IPv6 Fragment Header between the L2 headers and the (compressed) OAL header then applying IP fragmentation per [RFC8200] or [I-D.herbert-ipv4-eh] to produce carrier packet fragments no larger than the current Carrier Fragment Size (CFS). (Note that the OMNI protocol L2 headers appear in each fragment and the Fragment Header Next Header field is adjusted as described in Section 6.4 following fragmentation.) The L2 source should prepare carrier packet fragments no larger than 1280 octets (i.e., the IPv6 minimum MTU) until it can determine whether a larger CFS is possible, e.g., through dynamic path probing to the L2 destination. For IPv4, until a probed CFS is determined the L2 source must set DF to 0 and include ancillary integrity checks; these IPv4 carrier packet fragments may be (further) fragmented by intermediate systems in the L2 network.¶

For UDP/IPv4 carrier packets/fragments that set DF to 0, the L2 source calculates the UDP checksum and also includes a trailing 2-octet IPv4 reassembly checksum as specified in Appendix A. The L2 source calculates the checksums simultaneously in a single pass over the UDP pseudo-header plus the remainder of the packet following the header, then writes the UDP result in the UDP header and the IPv4 fragmentation result as the final 2 octets of the packet while incrementing the IPv4 length by 2. For raw IPv4 carrier packet (re-)encapsulation with DF set to 0, the source instead includes a trailing 2-octet IPv4 payload checksum followed by a 2-octet IPv4 reassembly checksum (calculated as above) while incrementing the IPv4 length by 4. The source calculates the IPv4 payload checksum the same as specified for UDP checksums [RFC0768], except that instead of the UDP length the pseudo header includes the length of the IPv4 payload only without including the IPv4 header or trailing checksum lengths. The source calculates the IPv4 payload and reassembly checksums simultaneously in a single pass over the pseudo header plus IPv4 payload the same as for the UDP case without extending to cover the trailing checksum fields themselves. (In both the UDP/IPv4 and raw IPv4 cases, the trailing checksum lengths will not cause the carrier packet to exceed 65535 octets since each OAL fragment reserves space for up to 256 L2 encapsulation octets.)¶

The L2 source then sends the resulting carrier packet fragments over one or more underlay interfaces. Underlay interfaces often connect directly to physical media on the local platform (e.g., an aircraft with a radio frequency link, a laptop computer with WiFi, etc.), but in some configurations the physical media may be hosted on a separate Local Area Network (LAN) node. In that case, the OMNI interface can establish a Layer-2 VLAN or a point-to-point tunnel (at a layer below the underlay interface) to the node hosting the physical media. The OMNI interface may also apply encapsulation at the underlay interface layer (e.g., as for a tunnel virtual interface) such that carrier packets would appear "double-encapsulated" on the LAN; the node hosting the physical media in turn removes the LAN encapsulation prior to transmission or inserts it following reception. Finally, the underlay interface must monitor the node hosting the physical media (e.g., through periodic keepalives) so that it can convey up-to-date Interface Attribute information to the OMNI interface.¶

Note: UDP/IPv4 and IPv4 L2 encapsulations that use IPsec AH/ESP do not include payload or reassembly integrity checks since the security encapsulations already include strong integrity checks.¶

Note: the L2 source must include a suitable Identification value in the IPv6 Fragment Header when it performs source fragmentation and must also include a suitable Identification value in the IPv4 header when it sets DF=0.¶

6.2.1. Carrier Fragment Size (CFS) Determination

For paths that cannot rely on network fragmentation to deliver carrier packets that exceed the path MTU, the L2 source should actively probe the path to determine the largest possible Carrier Fragment Size (CFS) for the L2 destination under current path conditions. The L2 source conducts probing in the spirit of "Packetization Layer Path MTU Discovery for Datagram Transports" [RFC8899] using a probe packet such as an NS message that includes Nonce and Timestamp options [RFC3971] plus a discard trailing packet attachment as specified in Section 6.10. The L2 source then encapsulates the message in L2 headers as a whole carrier packet and sends the message over the unsecured underlay interface (for IPv4, the L2 source also sets the probe packet DF flag to 1.)¶

Prior to any probing, the L2 source assumes a nominal CFS of 1280 octets (the IPv6 minimum MTU) for both IPv6 and IPv4. Since this size is greater than the IPv4 minimum MTU, the L2 source must set the DF bit to 0 in each carrier packet to increase the likelihood that it will reach the L2 destination. When the L2 source sets DF to 0, it must include IPv4 payload/reassembly checksum(s) as discussed above.¶

When the L2 source engages probing, it will receive NA responses from the L2 destination to confirm delivery of its OAL and L2 encapsulated padded NS messages. When the L2 source receives an NA with a matching Nonce, it can then advance CFS to the size of the NS probe. The L2 source must then continuously probe to confirm the current CFS or advance to even larger CFS values using the probing strategies specified in [RFC8899].¶

After the L2 source confirms a CFS through probing, it can send carrier packet fragments up to CFS octets in length and with DF set to 1 for IPv4. If the path changes, the L2 source may receive a PTB message from a router on the path and should then reduce and/or re-probe the CFS accordingly.¶

6.3. Reassembly and Decapsulation

All OAL intermediate systems and destinations MUST configure an L2 EMTU_R of 65535 octets on all unsecured underlay interfaces to enable successful reassembly of fragmented carrier packets no larger than that size (conversely, secured underlay interfaces use an EMTU_R specific to the L2 security service such as IPsec). OAL nodes are permitted to accept still larger unfragmented parcels/AJs as a best-effort service. OAL nodes must further recognize and honor the extended Identifications included in the IPv6 Extended Fragment Header [I-D.templin-6man-ipid-ext2].¶

When an OAL node reassembles an IPv4 or IPv6 carrier packet with an extended Identification, it accepts the reassembled packet following UDP checksum verification if necessary. When an OAL node reassembles an IPv4 carrier packet with DF set to 0, it must verify both the UDP or IPv4 payload checksum and the IPv4 reassembly checksum. The OAL node then accepts the reassembled packet only if the included checksums are correct, then trims the trailing payload/reassembly checksum(s) by decrementing the IPv4 length before processing the packet further. When an OAL node detects a checksum error or failed reassembly for either IPv4 or IPv6 carrier packets, and the IP first fragment includes enough of the OAL packet header, the OAL node returns a uNA message with an OMNI Fragmentation Report (FRAGREP) option to the OAL source as specified in Section 6.8. The FRAGREP provides immediate feedback allowing the OAL source to quickly retransmit the corrupted OAL fragment(s).¶

If the carrier packet encodes OMNI L2 extension headers per Section 6.4, the OAL node instead removes the UDP header if necessary and submits the packet for IPv6 extension header processing per [RFC8200] (while converting IPv4/Ethernet headers to IPv6 and converting IPv4/EUI addresses to IPv6 compatible addresses if necessary as specified above). The OAL node first sets the IPv6 Next Header field to the 8 bit protocol value for the first extension. When an (Extended) Fragment Header is included, reassembly then restores the (fragmented) L2 packet included by the previous hop.¶

When an OMNI interface processes a (reassembled) carrier packet from an underlay interface, it copies the ECN value from the L2 encapsulation headers into the OAL header if the carrier packet contains an OAL first-fragment. The OMNI interface next discards the L2 encapsulation headers and examines the OAL header of the enclosed OAL fragment according to the value in the Type field as discussed in Section 6.2. If the OAL fragment is addressed to a different node, the OMNI interface (acting as an OAL intermediate system) performs L2 encapsulation and fragmentation if necessary then forwards while decrementing the OAL Hop Limit as discussed in Section 6.2. If the OAL fragment is addressed to itself, the OMNI interface (acting as an OAL destination) accepts or drops the fragment based on the (Source, Destination, Identification)-tuple.¶

The OAL destination next drops all ordinal OAL non-first fragments that would overlap or leave "holes" with respect to other ordinal fragments already received. The OAL destination updates a checklist of accepted ordinal fragments of the same OAL packet but admits all accepted fragments into the reassembly cache.¶

During reassembly at the OAL destination, the reassembled OAL packet may exceed 65535 by a small amount equal to the size of the OAL encapsulation extension headers. The OAL destination does not write this (too-large) value into the OAL header Payload Length field, but rather remembers the value during reassembly. When reassembly is complete, the OAL destination finally removes the OAL headers and delivers the original IP packet/parcel to the network layer. The original IP packet/parcel may therefore be as large as 65535 octets, or larger still for large parcels/AJs delivered through jumbo-in-jumbo encapsulation without invoking fragmentation.¶

When an OAL path traverses an IPv6 network with routers that perform adaptation layer forwarding based on full IPv6 headers with OAL addresses, the OAL intermediate system at the head of the IPv6 path forwards the OAL packet/fragment the same as an ordinary IPv6 packet without decapsulating and delivering to the network layer. Once within the IPv6 network, these OAL packets/fragments may traverse arbitrarily-many IPv6 hops before arriving at an OAL intermediate system which may again encapsulate the OAL packets/fragments as carrier packets for transmission over underlay interfaces.¶

Note: carrier packets often traverse paths with underlying links that use integrity checks such as CRC-32 which provide adequate hop-by-hop integrity assurance for payloads up to ~9K octets [CRC]. However, other paths may traverse links (such as fragmenting tunnels over IPv4 - see: [RFC4963]) that do not include adequate checks. The end-to-end integrity checks in IP parcels and AJs therefore allow the final destination to detect any link errors that may have accumulated along the path even if the links themselves do not provide adequate error checking.¶

6.4. OMNI-Encoded IPv6 Extension Headers

The IPv6 specification [RFC8200] defines extension headers that follow the base IPv6 header, while Upper Layer Protocols (ULPs) are specified in other documents. Each extension header present is identified by a "Next Header" octet in the previous (extension) header and encodes a "Next Header" field in the first octet that identifies the next extension header or ULP instance. The OMNI specification supports encoding of IPv6 extension header chains immediately following the OMNI L2 UDP, IP or Ethernet header even if the L2 IP protocol version is IPv4. In all cases, the length of the IPv6 extension header chain is limited by [I-D.ietf-6man-eh-limits].¶

The OAL source prepares an OMNI extension header chain by setting the first 4 bits of the first IPv6 extension header in the chain to a Type value for the extension header itself immediately following the OMNI L2 protocol header. The source then sets the next 4 bits to a Next value that identifies either a terminating ULP or the next extension header in the chain. The source then sets the first 8 bits of each subsequent IPv6 extension header in the chain to the standard Next Header encoding as shown in Figure 6:¶

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               OMNI L2 UDP, IP or Ethernet Header              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type |  Next |           Extension Header #1                 ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |           Extension Header #2                 ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |           Extension Header #3                 ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          ...                         ...                          ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |           Extension Header #N                 ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~  OMNI Full/Compressed, IPv6/IPv4, TCP/UDP, ICMPv6, ESP, etc.  ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 6: OMNI Extension Header Chains

The following Type/Next values are currently defined:¶

0 (OMNI-RES) - Reserved for experimentation.¶
1 (OMNI-OCH1) - OMNI Compressed Header, Type 1 per Section 6.5.¶
2 (OMNI-OCH2) - OMNI Compressed Header, Type 2 per Section 6.5.¶
3 (OMNI-OFH) - OMNI Full Header, per Section 6.5.¶
4 (OMNI-IP4) - IPv4 header per [RFC0791].¶
5 (OMNI-HBH) - Hop-by-Hop Options per Section 4.3 of [RFC8200].¶
6 (OMNI-IP6) - IPv6 header per [RFC8200].¶
7 (OMNI-RH) - Routing Header per Section 4.4 of [RFC8200].¶
8 (OMNI-FH) - Fragment Header per Section 4.5 of [RFC8200].¶
9 (OMNI-DO) - Destination Options per Section 4.6 of [RFC8200].¶
10 (OMNI-AH) - Authentication Header per [RFC4302].¶
11 (OMNI-ESP) - Encapsulating Security Payload per [RFC4303].¶
12 (OMNI-NNH) - No Next Header per Section 4.7 of [RFC8200].¶
13 (OMNI-TCP) - TCP Header per [RFC9293].¶
14 (OMNI-UDP) - UDP Header per [RFC0768].¶
15 (OMNI-ULP) - Upper Layer Protocol shim (see below).¶

Entries OMNI-OCH1 through OMNI-AH in the above list follow the convention that the OMNI Type/Version appears in the first 4 bits of the extension header (or IP header) itself. Conversely, entries OMNI-ESP through OMNI-UDP represent commonly-used ULPs which do not encode a Type/Version in the first 4 bits.¶

Entries OMNI-HBH, OMNI-RH, OMNI-FH, OMNI-DO and OMNI-AH represent true IPv6 extension headers encoded for OMNI, which may be chained. Source and destination processing of OMNI extension headers follows exactly per their definitions in the normative references, with the exception of the special (Type, Next) coding in the first 8 bits of the first extension header.¶

When a ULP not found in the above table immediately follows the OMNI L2 UDP, IP or Ethernet header, the source includes a 2-octet "Type 1 ULP Shim" before the ULP where both the first 4 bit (Type) and next 4 bit (Next) fields encode the special value 15 (OMNI-ULP). The source then includes a Next Header field that encodes the IP protocol number of the ULP. The source then includes the ULP data immediately after the shim as shown in Figure 7.¶

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Type=15|Next=15|  Next Header  |   Upper Layer Protocol        ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 7: OMNI Upper Layer Protocol (ULP) Shim (Type 1)

When a ULP "OMNI-(N)" found in the above table immediately follows the OMNI L2 UDP, IP or Ethernet header, the source includes a 1-octet "Type 2 ULP Shim" before the ULP where the first 4 bits encode the special Type value 15 (OMNI-ULP) and the next 4 bits encode the Next ULP type "N" taken from the table above. The source then includes the ULP data immediately after the shim as shown in Figure 8.¶

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Type=15| Next=N|          Upper Layer Protocol                 ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 8: OMNI Upper Layer Protocol (ULP) Shim (Type 2)

When a ULP not found in the above table follows a first OMNI extension header, the source sets the extension header Next field to OMNI-ULP (15) and includes a 1-octet "Type 3 ULP Shim" that encodes the IP protocol number for the Next Header of the ULP data that follows as shown in Figure 9.¶

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |           Upper Layer Protocol                ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 9: OMNI Upper Layer Protocol (ULP) Shim (Type 3)

When a ULP "OMNI-(N)" found in the above table follows a first OMNI extension header, the source sets the extension header Next field to the ULP Type "N" and does not include a shim. The ULP then begins immediately after the first OMNI extension header.¶

When a ULP of any kind follows a non-first OMNI extension header, the source sets the extension header Next Header field to the IP protocol number for the ULP and does not include a shim. The ULP then begins immediately after the non-first OMNI extension header.¶

Note: The L2 UDP header (when present) is logically considered as the first L2 extension header in the chain. If an Advanced Jumbo extension header is also present, its Jumbo Payload length includes the length of the L2 UDP header.¶

Note: After a node parses the extension header chain, it changes the "Type/Next" field in the first extension header back to the correct "Next Header" value before processing the first extension header.¶

6.5. OMNI Full and Compressed Headers (OFH/OCH)

OAL sources that send OAL packets with OMNI Full Headers (OFH) include a full IPv6 header with Compressed Routing Header (CRH) [I-D.ietf-6man-comp-rtg-hdr] and IPv6 Extended Fragment Header extensions for segment-by-segment forwarding based on an AERO Forwarding Information Base (AFIB) in each OAL intermediate system. OAL sources, intermediate systems and destinations can also establish header compression state through IPv6 ND NS/NA message exchanges. After an initial NS/NA exchange, OAL nodes can apply OMNI Header Compression to significantly reduce OAL encapsulation overhead. OAL nodes can also apply these same OMNI header compression methods for native IPv6 packets that use OMNI L2 encapsulation but do not apply OAL encapsulation.¶

Each OAL node establishes AFIB soft state entries known as AERO Forwarding Vectors (AFVs) which support both OAL packet/fragment forwarding and OAL/IPv6 header compression/decompression. For FHS OAL sources, each AFV is referenced by a single AERO Forwarding Vector Index (AFVI) which in conjunction with the previous hop L2ADDR provides compression/decompression and next hop forwarding context.¶

When an OAL node sends carrier packets that contain OAL packets/fragments to a next hop, it includes an OFH with a CRH containing AFVI forwarding information followed by an Extended Fragment Header. When an OAL node sends ordinary IPv6 packets, it includes an ordinary IPv6 header with a CRH followed by an (Extended) Fragment Header only when an Identification is necessary. In the first case, the first four bits following the L2 headers must encode the Type OMNI-OFH to signify that an uncompressed OFH (plus extensions) is present. In the second case, the first four bits must encode the value OMNI-IP6 as a Type/Version value for IPv6.¶

When an OAL intermediate system forwards an OAL packet with Type OMNI-OFH to an IPv6 router for the SRT, it discards the L2 encapsulation headers, includes non-OMNI L2 headers for the next hop link, and resets the Type/Version field value to OMNI-IP6. When an OAL intermediate system forwards an OAL packet received from an SRT IPv6 router, it resets the Type/Version field value to OMNI-OFH and includes new OMNI L2 encapsulation headers. In this case (and also for multihop forwarding in mobile ad-hoc networks) the OMNI-OFH coding is necessary to distinguish native IPv6 from OMNI encapsulation.¶

Whenever possible, the OAL source should omit significant portions of the OAL header (plus extensions) by applying OMNI header compression when sufficient AFV state is available. For OAL first fragments (including atomic fragments), the OAL node uses OMNI Compressed Header, Type 1 (OCH1) Format (a) as shown in Figure 10:¶

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type  |A|R|ECN|  Next Header  | Hop Limit | Parcel ID |P|S|F|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Identification (4 octets)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AFVI (2 or 4 octets)      /  Payload Len (0 or 2 octets)  |
   +~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+

Figure 10: OMNI Compressed Header (OCH1) Format (a)

The format begins with a 4-bit Type followed by (A)FVI extension; (R)eserved flag bits and a 2-bit Explicit Congestion Notification (ECN) field. The format then includes an 8-bit Next Header, a 6-bit Hop Limit, a 6-bit Parcel ID and four flag bits. The header next includes the 4 least significant octets of the Identification followed by a 2/4-octet AFVI according to whether the A flag is set to 0/1, respectively. The format finally includes a 2-octet Payload Length only if the L2 header does not include a length field.¶

The OAL node sets Type to OMNI-OCH1, sets Hop Limit to the minimum of the uncompressed IPv6 header Hop Limit and 63 and sets the ECN bits and Next Header the same as for an uncompressed IPv6 header. The OAL node next sets (F)irst to 1 as a first fragment and finally sets (M)ore Fragments, Parcel ID, ((P)arcel, and More (S)egments the same as for an uncompressed (Extended) Fragment Header.¶

The body of the OAL first fragment is then included immediately following the OCH1 header, and the L2 header length field (if present) is reduced by the difference in length between the compressed and full-length headers. If the L2 header includes a length field, the OAL destination can determine the Payload Length by examining the L2 header; otherwise, the OCH1 header itself includes a 2-octet Payload Length field that encodes the length of the rest of the packet (including any extension headers) that follow the OCH1. Note that first fragments (and atomic packets) are logically considered Ordinal fragment 0 even though no Ordinal value is transmitted.¶

For OAL non-first fragments (i.e., those with non-zero Fragment Offsets), the OAL uses OMNI Compressed Header, Type 1 (OCH1) Format (b) as shown in Figure 11:¶

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type  | Hop Limit |  Ordinal  |    Fragment Offset      |A|F|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Identification (4 octets)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AFVI (2 or 4 octets)      /  Payload Len (0 or 2 octets)  |
   +~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+

Figure 11: OMNI Compressed Header (OCH1) Format (b)

The format begins with a 4-bit Type, a 6-bit Hop Limit, a 6-bit Ordinal, a 13-bit Fragment Offset, an (A)FVI extension flag, a (F)irst) flag and a (M)ore Fragments flag. The format next includes the least-significant 4 octets of the (extended) Identification followed by a 2/4-octet AFVI according to the A flag followed by a 0/2-octet Payload Length field the same as for an OCH1 first fragment. The OAL node sets Type to OMNI-OCH1, sets Hop Limit to the minimum of the uncompressed IPv6 header Hop Limit and 63, and sets (Ordinal, Fragment Offset, (F)irst, (M)ore Fragments, Identification) to their appropriate values as a non-first fragment. In particular, the OAL Node sets Ordinal to a monotonically increasing value beginning with 1 for the first non-first fragment, 2 for the second non-first fragment, 3 for the third non-first fragment, etc., up to at most 63 for the final fragment.¶

The OAL non-first fragment body is then included immediately following the OCH1 header, and the L2 header length field (if present) is reduced by the difference in length between the compressed headers and full-length OFH/IPv6 header plus extensions. The OAL destination will then be able to determine the Payload Length by examining the L2 header length field if present; otherwise by examining the 2-octet OCH1 Payload Length the same as for first fragments.¶

The header compression state in the AFV of each OAL node in the path records the Traffic Class and Flow Label of the most recent packet forwarded via the AFV. When a new packet traverses the AFV using the same values for Traffic Class / Flow Label as those that are stored in the header compression state, the OAL node can use the fully-compressed OCH1 formats specified above. When a new packet with different Traffic Class / Flow Label values traverses the AFV, the OAL node instead records the new values in the header compression state and includes one of the "Type 2" OMNI Compressed Header (OCH2) formats shown in Figure 12 and Figure 13:¶

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type  | Traffic Class |           Flow Label                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Reservd|A|Resvd|  Next Header  | Hop Limit | Parcel ID |P|S|F|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Identification (4 octets)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AFVI (2 or 4 octets)      /  Payload Len (0 or 2 octets)  |
   +~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+

Figure 12: OMNI Compressed Header, Type 2 (OCH2) Format (a)

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type  | Traffic Class |           Flow Label                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Reservd| Hop Limit |  Ordinal  |    Fragment Offset      |A|F|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Identification (4 octets)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AFVI (2 or 4 octets)      /  Payload Len (0 or 2 octets)  |
   +~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+

Figure 13: OMNI Compressed Header, Type 2 (OCH2) Format (b)

In the above formats, the first 4 octets of the OCH2 include the same information that would appear in the first 4 octets of a corresponding OFH or uncompressed IPv6 header and the next 4 octets include the same information that would appear in the first 4 octets of a corresponding OCH1 header. (Note however that the ECN bits are recorded in the normal location in the Traffic Class field instead of the special encoding used for OCH1 first fragments). The remainder of the OCH2 encodes exactly the same information that would appear in a corresponding OCH1.¶

When an OAL destination or intermediate system receives a carrier packet, it determines the length of the encapsulated OAL information and verifies that the innermost next header field indicates OMNI (see: Section 6.2), then processes any included OMNI L2 extension headers as specified in Section 6.4. The OAL destination then examines the Next Header field of the final L2 extension header. If the Next Header field contains the value TBD1, and the 4-bit Type that follows encodes a value OMNI-IP6, OMNI-OFH, OMNI-OCH1 or OMNI-OCH2 the OAL node processes the remainder of the OAL header as a full or compressed header as specified above.¶

The OAL node then uses the AFVI to locate the cached AFV which determines the next hop. During forwarding for compressed headers, the OAL node changes the OCH AFVI to the cached value for the AFV next hop. If the OAL node is the destination, it instead reconstructs the IPv6/OFH header then for non-atomic fragments adds the resulting OAL fragment to the reassembly cache if the Identification is acceptable.¶

For all OCH1/2 types, the source node sets all Reserved fields and bits to 0 on transmission and the destination node ignores the values on reception. For OCH1, ECN information is only compiled for first fragments, and not for non-first fragments.¶

Finally, if an IPv6 Hop-by-Hop (HBH) extension header is required to appear along with each OAL packet/fragment that uses OCH1/2 compression the OAL node inserts an OMNI-HBH header as the first extension following the L2 header then inserts the OMNI-OCH1/2 as the next header. In this arrangement, the special (Type, Next) coding in the first 8 bits of the HBH extension header would include the values ("OMNI-HBH", "OMNI-OCH1/2").¶

6.6. OAL and L2 Encapsulation Avoidance

When the OAL source and OAL destination are on the same OMNI link segment as determined by neighbor discovery, the OMNI interface forwards packets directly to the specific underlay interface without applying OAL encapsulation. In that case, the OAL source treats the IPv6 header of the original packet the same as if it had applied an OAL encapsulation header. The Next Header field will therefore encode a value specific to the transport layer protocol (e.g., '6' for TCP, '17' for UDP, etc.) since the OAL does not insert an IPv6 encapsulation header. The OAL source then applies fragmentation, header compression and L2 encapsulation the same as described above even though a single IPv6 header (and not an additional OAL encapsulation header) is present.¶

The OAL source can also apply these same encapsulation avoidance procedures for IPv4 by first translating the IPv4 header of the original packet into an IPv6 header and translating the IPv4 addresses into IPv4-compatible IPv6 addresses as discussed in Appendix B. These translated headers can then be manipulated the same as for IPv6 headers as described above, including fragmentation, header compression, etc.¶

When an OAL node and its next OAL hop are known to be connected to the same underlay link, or when the node's underlay interface connects to a Mobile Ad-Hoc Network (MANET) where MANET-local IPv6 routing protocols are applied, the node does not include full UDP/IP headers as part of the carrier packet L2 encapsulation and instead uses link layer encapsulation using EtherType TBD2 for Ethernet-compatible data links. The MANET-local IPv6 routing protocols will then direct the packets to the correct destination which may be one or more MANET routing hops away from the source.¶

When the OAL node is unable to determine whether the next OAL hop is connected to the same underlay link, it should perform carrier packet L2 encapsulation for initial packets sent via the next hop over a specific underlay interface by including full UDP/IP headers and with the UDP port numbers set as discussed in Section 6.2. The node can thereafter attempt to send an NS to the next OAL hop in carrier packet(s) that omit the UDP header and set the IP protocol number to TBD1. If the OAL node receives an NA reply, it can omit the UDP header in subsequent packets. The node can further attempt to send an NS in carrier packet(s) that omit both the UDP and IP headers and set EtherType to TBD2. If the source receives an NA reply, it can begin omitting both the UDP and IP headers in subsequent packets.¶

Note: in the above, "next OAL hop" refers to the first OAL node encountered on the optimized path to the destination over a specific underlay interface as determined through route optimization (e.g., see: [I-D.templin-intarea-aero2]). The next OAL hop could be a Proxy/Server, Gateway or the OAL destination itself.¶

6.7. OAL Identification Window Maintenance

The OAL encapsulates each original IP packet/parcel as an OAL packet then performs fragmentation to produce one or more carrier packets with the same 8-octet Identification value. In environments where spoofing is not considered a threat, OMNI interfaces send OAL packets with Identifications beginning with an unpredictable Initial Send Sequence (ISS) value [RFC7739] monotonically incremented (modulo 2**64) for each successive OAL packet sent to either a specific neighbor or to any neighbor. (The OMNI interface may later change to a new unpredictable ISS value as long as the Identifications are assured unique within a timeframe that would prevent the fragments of a first OAL packet from becoming associated with the reassembly of a second OAL packet.) In other environments, OMNI interfaces should maintain explicit per-interface-pair send and receive windows to detect and exclude spurious carrier packets that might clutter the reassembly cache as discussed below.¶

OMNI interface neighbors use a window synchronization service similar to TCP [RFC9293] to maintain unpredictable ISS values incremented (modulo 2**64) for each successive OAL packet and re-negotiate windows often enough to maintain an unpredictable profile. OMNI interface neighbors exchange IPv6 ND messages that include OMNI Multilink Vector sub-options (see: Section 12.2.8) that include TCP-like information fields and flags to manage streams of OAL packets instead of streams of octets. As a link layer service, the OAL provides low-persistence best-effort retransmission with no mitigations for duplication, reordering or deterministic delivery. Since the service model is best-effort and only control message sequence numbers are acknowledged, OAL nodes can select unpredictable new initial sequence numbers outside of the current window without delaying for the Maximum Segment Lifetime (MSL).¶

OMNI interface end neighbors and intermediate systems maintain current and previous per-interface-pair window state in IPv6 ND NCEs and/or AFVs to support dynamic rollover to a new window while still sending OAL packets and accepting carrier packets from the previous windows. OMNI interface neighbors synchronize windows through asymmetric and/or symmetric IPv6 ND message exchanges. When OMNI end and intermediate systems receive an IPv6 ND message with new interface pair-based window information, it resets the previous window state based on the current window then resets the current window based on new and/or pending information.¶

The IPv6 ND message OMNI option Multilink Vector sub-option includes TCP-like information fields including Sequence Number, Acknowledgement Number, Window and flags (see: Section 12). OMNI interface neighbors and intermediate systems maintain the following TCP-like state variables on a per-interface-pair basis (i.e., through a combination of NCE and/or AFV state):¶

    Send Sequence Variables (current, previous and pending)

      SND.NXT - send next
      SND.WND - send window
      ISS     - initial send sequence number

    Receive Sequence Variables (current and previous)

      RCV.NXT - receive next
      RCV.WND - receive window
      IRS     - initial receive sequence number

OMNI interface neighbors "OAL A" and "OAL B" exchange IPv6 ND messages per [RFC4861] with OMNI options that include TCP-like information fields as well as interface pair parameters such as Interface Attributes. When OAL A synchronizes with OAL B, it maintains both a current and previous SND.WND beginning with a new unpredictable ISS and monotonically increments SND.NXT for each successive OAL packet transmission. OAL A initiates synchronization by including the new ISS in the Sequence Number of an authentic IPv6 ND message with the SYN flag set and with Window set to M (up to 2**24) as a tentative receive window size while creating a NCE in the INCOMPLETE state if necessary. OAL A caches the new ISS as pending, uses the new ISS as the Identification for OAL encapsulation, then sends the resulting OAL packet to OAL B and waits up to RetransTimer milliseconds to receive an IPv6 ND message response with the ACK flag set (retransmitting up to MAX_UNICAST_SOLICIT times if necessary).¶

When OAL B receives the SYN, it creates a NCE in the STALE state and also an AFV if necessary, resets its RCV variables, caches the tentative (send) window size M, and selects a (receive) window size N (up to 2**24) to indicate the number of OAL packets it is willing to accept under the current RCV.WND. (The RCV.WND should be large enough to minimize control message overhead yet small enough to provide an effective filter for spurious carrier packets.) OAL B then prepares an IPv6 ND message with the ACK flag set, with the Acknowledgement Number set to OAL A's next sequence number, and with Window set to N. Since OAL B does not assert an ISS of its own, it uses the IRS it has cached for OAL A as the Identification for OAL encapsulation then sends the ACK to OAL A.¶

When OAL A receives the ACK, it notes that the Identification in the OAL header matches its pending ISS. OAL A then sets the NCE state to REACHABLE and resets its SND variables based on the Window size and Acknowledgement Number (which must include the sequence number following the pending ISS). OAL A can then begin sending OAL packets to OAL B with Identification values within the (new) current SND.WND for this interface pair for up to ReachableTime milliseconds or until the NCE is updated by a new IPv6 ND message exchange. This implies that OAL A must send a new SYN before sending more than N OAL packets within the current SND.WND, i.e., even if ReachableTime is not nearing expiration. After OAL B returns the ACK, it accepts carrier packets received from OAL A via this interface pair within either the current or previous RCV.WND as well as any new authentic NS/RS SYN messages received from OAL A even if outside the windows.¶

OMNI interface neighbors can employ asymmetric window synchronization as described above using two independent (SYN -> ACK) exchanges (i.e., a four-message exchange), or they can employ symmetric window synchronization using a modified version of the TCP three-way handshake as follows:¶

OAL A prepares a SYN with an unpredictable ISS not within the current SND.WND and with Window set to M as a tentative receive window size. OAL A caches the new ISS and Window size as pending information, uses the pending ISS as the Identification for OAL encapsulation, then sends the resulting OAL packet to OAL B and waits up to RetransTimer milliseconds to receive an ACK response (retransmitting up to MAX_UNICAST_SOLICIT times if necessary).¶
OAL B receives the SYN, then resets its RCV variables based on the Sequence Number while caching OAL A's tentative receive Window size M and a new unpredictable ISS outside of its current window as pending information. OAL B then prepares a response with Sequence Number set to the pending ISS and Acknowledgement Number set to OAL A's next sequence number. OAL B then sets both the SYN and ACK flags, sets Window to N and sets the OPT flag according to whether an explicit concluding ACK is optional or mandatory. OAL B then uses the pending ISS as the Identification for OAL encapsulation, sends the resulting OAL packet to OAL A and waits up to RetransTimer milliseconds to receive an acknowledgement (retransmitting up to MAX_UNICAST_SOLICIT times if necessary).¶
OAL A receives the SYN/ACK, then resets its SND variables based on the Acknowledgement Number (which must include the sequence number following the pending ISS) and OAL B's advertised Window N. OAL A then resets its RCV variables based on the Sequence Number and marks the NCE as REACHABLE. If the OPT flag is clear, OAL A next prepares an immediate unsolicited NA message with the ACK flag set, the Acknowledgement Number set to OAL B's next sequence number, with Window set a value that may be the same as or different than M, and with the OAL encapsulation Identification to SND.NXT, then sends the resulting OAL packet to OAL B. If the OPT flag is set and OAL A has OAL packets queued to send to OAL B, it can optionally begin sending their carrier packets under the (new) current SND.WND as implicit acknowledgements instead of returning an explicit ACK. In that case, the tentative Window size M becomes the current receive window size.¶
OAL B receives the implicit/explicit acknowledgement(s) then resets its SND state based on the pending/advertised values and marks the NCE as REACHABLE. If OAL B receives an explicit acknowledgement, it uses the advertised Window size and abandons the tentative size. (Note that OAL B sets the OPT flag in the SYN/ACK to assert that it will interpret timely receipt of carrier packets within the (new) current window as an implicit acknowledgement. Potential benefits include reduced delays and control message overhead, but use case analysis is outside the scope of this specification.)¶

Following synchronization, OAL A and OAL B hold updated NCEs and AFVs, and can exchange OAL packets with Identifications set to SND.NXT for each interface pair while the state remains REACHABLE and there is available window capacity. (Intermediate systems that establish AFVs for the per-interface-pair window synchronization exchanges can also use the Identification window for source validation.) Either neighbor may at any time send a new SYN to assert a new ISS. For example, if OAL A's current SND.WND for OAL B is nearing exhaustion and/or ReachableTime is nearing expiration, OAL A can continue sending OAL packets under the current SND.WND while also sending a SYN with a new unpredictable ISS. When OAL B receives the SYN, it resets its RCV variables and may optionally return either an asymmetric ACK or a symmetric SYN/ACK to also assert a new ISS. While sending SYNs, both neighbors continue to send OAL packets with Identifications set to the current SND.NXT for each interface pair then reset the SND variables after an acknowledgement is received.¶

While the optimal symmetric exchange is efficient, anomalous conditions such as receipt of old duplicate SYNs can cause confusion for the algorithm as discussed in Section 3.5 of [RFC9293]. For this reason, the OMNI Multilink Vector sub-option includes an RST flag which OAL nodes set in solicited NA responses to ACKs received with incorrect acknowledgement numbers. The RST procedures (and subsequent synchronization recovery) are conducted exactly as specified in [RFC9293].¶

OMNI interfaces that employ the window synchronization procedures described above observe the following requirements:¶

OMNI interfaces MUST select new unpredictable ISS values that are at least a full window outside of the current SND.WND.¶
OMNI interfaces MUST set the initial SYN message Window field to a tentative value to be used only if no concluding NA ACK is sent.¶
OMNI interfaces MUST send IPv6 ND messages used for window synchronization securely while using unpredictable initial Identification values until synchronization is complete.¶

It is essential to understand that the above window synchronization operations between nodes OAL(A) and OAL(B) are conducted in IPv6 ND message exchanges over multihop paths with potentially many OAL(i) intermediate hops in the forward and reverse paths (which may be disjoint). Each such forward path OAL(i) caches the sequence number and window size advertised from OAL(A) to OAL(B) in its AFV entry indexed by the previous hop L2ADDR and AFVI, while each such revers path OAL(i) caches the sequence number, window size and AFVI advertised from OAL(B) to OAL(A). (The forward/reverse path OAL(i) nodes then select new unique next-hop AFVIs before forwarding.) Since the forward and reverse paths may be disjoint, this means that both OAL(A) and OAL(B) must consistently advertise the same window size values for both sending and receiving so that OAL(i) intermediate nodes can cache consistent values. Since OAL(A) initiates the window synchronization, this means that its initial window value N determines the symmetric window value that both nodes must use consistently until the next synchronization event.¶

Note: Although OMNI interfaces employ TCP-like window synchronization and support uNA ACK responses to SYNs, all other aspects of the IPv6 ND protocol (e.g., control message exchanges, NCE state management, timers, retransmission limits, etc.) are honored exactly per [RFC4861]. OMNI interfaces further manage per-interface-pair window synchronization parameters in one or more AFVs for each neighbor pair.¶

Note: Recipients of OAL-encapsulated IPv6 ND messages index the NCE based on the message source address, which also determines the carrier packet Identification window. However, IPv6 ND messages may contain a message source address that does not match the OMNI encapsulation source address when the recipient acts as a proxy.¶

Note: OMNI interface neighbors apply separate send and receive windows for all of their (multilink) underlay interface pairs that exchange carrier packets. Each interface pair represents a distinct underlay network path, and the set of paths traversed may be highly diverse when multiple interface pairs are used. OMNI intermediate systems therefore become aware of each distinct set of interface pair window synchronization parameters based on periodic IPv6 ND message updates to their respective AFVs.¶

6.8. OAL Fragmentation Reports and Retransmissions

The OAL source should maintain a short-term cache of the OAL fragments it sends to OAL destinations in case timely best-effort selective retransmission is requested. The OAL destination in turn maintains a checklist for (Source, Destination, Identification)-tuples of recently received OAL fragments and notes the ordinal numbers of OAL fragments already received (i.e., as ordinals #0, #1, #2, #3, etc.). The timeframe for maintaining the OAL source and destination caches determines the link persistence (see: [RFC3366]).¶

If the OAL destination notices some fragments missing after most other fragments within the same link persistence timeframe have already arrived, it may issue an Automatic Repeat Request (ARQ) with Selective Repeat (SR) by sending a uNA message to the OAL source. The OAL destination creates a uNA message with an OMNI option with one or more Fragmentation Report (FRAGREP) sub-options that include (Identification, Bitmap)-tuples for fragments received and missing from this OAL source (see: Section 12). The OAL destination includes an authentication signature if necessary, performs OAL encapsulation (with the its own address as the OAL source and the source address of the message that prompted the uNA as the OAL destination) and sends the message to the OAL source.¶

If an OAL intermediate system or OAL destination processes an OAL fragment for which corruption is detected, it may similarly issue an immediate ARQ/SR the same as described above. The FRAGREP provides an immediate (rather than time-bounded) indication to the OAL source that a retransmission is required.¶

When the OAL source receives the uNA message, it authenticates the message then examines any enclosed FRAGREPs. For each (Source, Destination, Identification)-tuple, the OAL source determines whether it still holds the corresponding OAL fragments in its cache and retransmits any for which the Bitmap indicates a loss event. For example, if the Bitmap indicates that ordinal fragments #3, #7, #10 and #13 from the OAL packet with Identification 0x0123456789abcdef are missing the OAL source only retransmits those fragments. When the OAL destination receives the retransmitted OAL fragments, it admits them into the reassembly cache and updates its checklist. If some fragments are still missing, the OAL destination may send a small number of additional uNA ARQ/SRs within the link persistence timeframe.¶

The OAL therefore provides a link layer low-to-medium persistence ARQ/SR service consistent with [RFC3366] and Section 8.1 of [RFC3819]. The service provides the benefit of timely best-effort link layer retransmissions which may reduce OAL fragment loss and avoid some unnecessary end-to-end delays. This best-effort network-based service therefore compliments transport and higher layer end-to-end protocols responsible for true reliability.¶

6.9. OMNI Interface MTU Feedback Messaging

When the OMNI interface forwards original IP packets/parcels from the network layer, it invokes the OAL and returns internally-generated Path MTU Discovery (PMTUD) ICMPv4 "Fragmentation Needed and Don't Fragment Set" [RFC1191] or ICMPv6 "Packet Too Big (PTB)" [RFC8201] messages as necessary. This document refers to both message types as "PTBs" and introduces a distinction between PTB "hard" and "soft" errors as discussed below.¶

Ordinary PTB messages are hard errors that always indicate loss due to a real MTU restriction has occurred. However, the OMNI interface can also forward original IP packets/packets via OAL encapsulation and fragmentation while at the same time returning PTB soft error messages (subject to rate limiting) to the original source to suggest smaller sizes due to factors such as link performance characteristics, number of fragments needed, reassembly congestion, etc.¶

This ensures that the path MTU is adaptive and reflects the current path used for a given data flow. The OMNI interface can therefore continuously forward original IP packets/parcels without loss while returning PTB soft error messages recommending a smaller size if necessary. Original sources that receive the soft errors in turn reduce the size of the original IP packets/parcels they send, i.e., the same as for hard errors but not necessarily due to a loss event. The original source can then resume sending larger packets/parcels without delay if the soft errors subside.¶

OAL destinations and intermediate systems may experience reassembly cache congestion, and can return uNA messages to the OAL source that include OMNI encapsulated PTB messages with a PTB soft error Code to OAL sources that originate the fragments (subject to rate limiting). The OAL node creates a uNA message with an authentication signature and an OMNI option containing an ICMPv6 Error sub-option. The OAL node encodes a PTB message in the sub-option with MTU set to a reduced value and with the leading portion an OAL first fragment containing the header of an original IP packet/parcel for which the source must be notified (see: Section 12).¶

The OAL node that sends the uNA encapsulates the leading portion of the OAL first fragment (beginning with the OAL header) in the PTB "packet in error" field, signs the message if an authentication signature is included, performs OAL encapsulation (with the its own address as the OAL source and the source address of the message that prompted the uNA as the OAL destination) and sends the message to the OAL source.¶

When the OAL source receives a uNA message from an OAL intermediate system, it can reduce its OFS estimate and begin sending smaller OAL fragments and/or reduce its CFS estimate and begin sending smaller carrier packet fragments. When the OAL source receives a uNA message from the OAL destination, it sends a corresponding network layer PTB soft error to the original source to recommend a smaller size.¶

The OAL source prepares the PTB soft error by first setting the Type field to 2 for IPv6 [RFC4443] or TBD6 for IPv4 (see: IANA considerations). The OAL source then sets the Code field to "PTB Soft Error (no loss)" if the OAL destination forwarded the original IP packet/parcel successfully or "PTB Soft Error (loss)" if it was dropped (see: IANA considerations). The OAL source next sets the PTB destination address to the original IP packet/parcel source, and sets the source address to one of its OMNI interface addresses that is reachable from the perspective of the original source.¶

The OAL source then sets the MTU field to a value smaller than the original IP packet/parcel size but no smaller than 1280, writes as much of the original IP packet/parcel first fragment as possible into the "packet in error" field such that the entire PTB including the IP header is no larger than 1280 octets for IPv6 or 576 octets for IPv4. The OAL source then calculates and sets the ICMP Checksum and returns the PTB to the original source.¶

An original sources that receives these PTB soft errors first verifies that the ICMP Checksum is correct and the packet-in-error contains the leading portion of one of its recent packet/parcel transmissions. The original source can then adaptively tune the size of the original IP packets/parcels it sends to produce the best possible throughput and latency, with the understanding that these parameters may fluctuate over time due to factors such as congestion, mobility, network path changes, etc. Original sources should therefore consider receipt or absence of soft errors as hints of when decreasing or increasing packet/parcel sizes may provide better performance.¶

The OMNI interface supports continuous transmission and reception of packets/parcels of various sizes in the face of dynamically changing network conditions. Moreover, since PTB soft errors do not indicate a hard limit, original sources that receive soft errors can resume sending larger packets/parcels without waiting for the recommended 10 minutes specified for PTB hard errors [RFC1191][RFC8201]. The OMNI interface therefore provides an adaptive service that accommodates MTU diversity especially well-suited for dynamic multilink networks.¶

The OMNI interface may also return PTB messages with Parcel Report and/or Jumbo Report Codes in response to parcels and/or AJs delivered by the network layer and forwarded through jumbo-in-jumbo encapsulation. These Parcel/Jumbo Report messages are prepared the same as for PTB soft errors discussed above. IP parcels and AJs are discussed in [I-D.templin-6man-parcels2][I-D.templin-intarea-parcels2].¶

6.10. OAL Super-Packets

The OAL source ordinarily includes a 40-octet IPv6 encapsulation header for each original IP packet/parcel during OAL encapsulation. The OAL source then performs fragmentation such that a copy of the 40-octet IPv6 header plus a 16-octet IPv6 Extended Fragment Header is included in each OAL fragment (when a Routing Header is added, the OAL encapsulation headers become larger still). However, these encapsulations may represent excessive overhead in some environments.¶

OAL header compression can dramatically reduce the amount of encapsulation overhead, however a complimentary technique known as "packing" (see: [I-D.ietf-intarea-tunnels]) supports encapsulation of multiple original IP packets/parcels and/or control messages within a single OAL "super-packet". (The super-packet normally includes a control message as the first message of the packet, with one or more original IP packets/parcels included as trailing attachments.)¶

When the OAL source has multiple original IP packets/parcels to send to the same OAL destination with total length no larger than the OAL destination EMTU_R, it can concatenate them into a super-packet encapsulated in a single OAL header. Within the OAL super-packet, the IP header of the first original IP packet/parcel (iHa) followed by its data (iDa) is concatenated immediately following the OAL header, then the IP header of the next original packet/parcel (iHb) followed by its data (iDb) is concatenated immediately following the first, etc. The OAL super-packet format is transposed from [I-D.ietf-intarea-tunnels] and shown in Figure 14:¶

                <------- Original IP packets ------->
                +-----+-----+
                | iHa | iDa |
                +-----+-----+
                      |
                      |     +-----+-----+
                      |     | iHb | iDb |
                      |     +-----+-----+
                      |           |
                      |           |     +-----+-----+
                      |           |     | iHc | iDc |
                      |           |     +-----+-----+
                      |           |           |
                      v           v           v
     +----------+-----+-----+-----+-----+-----+-----+
     |  OAL Hdr | iHa | iDa | iHb | iDb | iHc | iDc |
     +----------+-----+-----+-----+-----+-----+-----+
     <--- OAL "Super-Packet" with single OAL Hdr --->

Figure 14: OAL Super-Packet Format

When the OAL source prepares a super-packet, it applies OAL fragmentation then applies L2 encapsulation/fragmentation and sends the resulting carrier packets to the OAL destination. When the OAL destination receives the super-packet it first reassembles if necessary. The OAL destination then selectively extracts each original IP packet/parcel (e.g., by setting pointers into the super-packet buffer and maintaining a reference count, by copying each packet into a separate buffer, etc.) and forwards each one to the network layer. During extraction, the OAL determines the IP protocol version of each successive original IP packet/parcel 'j' by examining the four most-significant bits of iH(j), and determines the length of each one by examining the rest of iH(j) according to the IP protocol version.¶

When an OAL source prepares a super-packet that includes an IPv6 ND message with an authentication signature as the first original IP packet/parcel (i.e., iHa/iDa), it calculates the authentication signature over the remainder of super-packet. Security and integrity for forwarding initial data messages in conjunction with IPv6 ND messages used to establish NCE state are therefore supported. (A second common use case entails a path MTU probe beginning with an unsigned IPv6 ND message followed by a suitably large NULL packet (e.g., an IP packet with padding octets added beyond the IP header and with {Protocol, Next Header} set to 59 ("No Next Header"), a UDP/IP packet with port number set to '9' ("discard") [RFC0863], etc.)¶

The OAL header of a super packet may also include a Parcel Payload Option with AJ Type 0 if the total length of all payload packets/parcels exceeds 65535 octets. In that case, the super-packet must be forwarded as an atomic fragment over OAL paths that support such large sizes.¶

6.11. OAL Bubbles

OAL sources may send NULL OAL packets known as "bubbles" for the purpose of establishing Network Address Translator (NAT) state on the path to the OAL destination. The OAL source prepares a bubble by crafting an OAL header with appropriate IPv6 source and destination ULAs, with the IPv6 Next Header field set to the value 59 ("No Next Header" - see [RFC8200]) and with 0 or more octets of NULL protocol data immediately following the IPv6 header.¶

The OAL source includes a random Identification value then encapsulates the OAL packet in L2 headers destined to either the mapped address of the OAL destination's first-hop ingress NAT or the L2 address of the OAL destination itself. When the OAL source sends the resulting carrier packet, any egress NATs in the path toward the L2 destination will establish state based on the activity but the bubble will be harmlessly discarded by either an ingress NAT on the path to the OAL destination or by the OAL destination itself.¶

The bubble concept for establishing NAT state originated in [RFC4380] and was later updated by [RFC6081]. OAL bubbles may be employed by mobility services such as AERO.¶

6.12. OMNI Hosts

OMNI Hosts are end systems that connect to the OMNI link over ENET underlay interfaces (i.e., either via an OMNI interface or as a sublayer of the ENET interface itself). Each ENET connects to the rest of the OMNI link via a Client that receives an MNP delegation. Clients delegate MNP addresses and/or sub-prefixes to ENET nodes (i.e., Hosts, other Clients, routers and non-OMNI hosts) using standard mechanisms such as DHCP [RFC8415][RFC2131] and IPv6 Stateless Address AutoConfiguration (SLAAC) [RFC4862]. Clients forward original IP packets/parcels between their ENET Hosts and peers on external networks acting as routers and/or OAL intermediate systems.¶

OMNI Hosts coordinate with Clients and/or other Hosts connected to the same ENET using OMNI L2 encapsulation of OMNI IPv6 ND messages. The L2 encapsulation headers and ND messages both use the MNP-based addresses assigned to ENET underlay interfaces as source and destination addresses (i.e., instead of ULAs). For IPv4 MNPs, the ND messages use IPv4-Compatible IPv6 addresses [RFC4291] in place of the IPv4 addresses.¶

Hosts discover Clients by sending encapsulated RS messages using an OMNI link IP anycast address (or the unicast address of the Client) as the RS L2 encapsulation destination as specified in Section 15. The Client configures the IPv4 and/or IPv6 anycast addresses for the OMNI link on its ENET interface and advertises the address(es) into the ENET routing system. The Client then responds to the encapsulated RS messages by sending an encapsulated RA message that uses its ENET unicast address as the source. (To differentiate itself from an INET border Proxy/Server, the Client sets the RA message OMNI Interface Attributes sub-option LHS field to 0 for the Host's interface index. When the RS message includes an L2 anycast destination address, the Client also includes an Interface Attributes sub-option for interface index 0 to inform the Host of its L2 unicast address - see: Section 15 for full details on the RS and RA message contents.)¶

Hosts coordinate with peer Hosts on the same ENET by sending encapsulated NS messages to receive an NA reply. (Hosts determine whether a peer is on the same ENET by matching the peer's IP address with the MNP (sub)-prefix for the ENET advertised in the Client's RA message [RFC8028].) Each ENET peer then creates a NCE and synchronizes Identification windows the same as for OMNI link neighbors, and the Host can then engage in OMNI link transactions with the Client and/or other ENET Hosts. The Host therefore regards the Client as if it were an ANET Proxy/Server, and the Client provides the same services that a Proxy/Server would provide. By coordinating with other Hosts, the peers can exchange large IP packets/parcels over the ENET using encapsulation and fragmentation if necessary.¶

When a Host prepares an original IP packet/parcel, it uses the IP address of its OMNI interface (which is the same as the IP address of the underlying native ENET interface) as the source and the IP address of the (remote) peer as the destination. The Host next performs parcellation if necessary (see: Section 6.13) then encapsulates the packet(s)/(sub-)parcel(s) in OMNI L2 headers while setting the L2 source to the L3 source address and L2 destination to either the L3 destination address if the peer is on the local ENET, or to the IP address of the Client otherwise. The Host can then proceed to exchange packets/parcels with the destination, either directly or via the Client as an intermediate system.¶

The encapsulation procedures are coordinated per Section 6.1, except that the OMNI L2 encapsulation header is followed by an IPv6 (Extended) Fragment Header. When the L2 encapsulation is based on an EUI or IPv4 address, the Host next translates the encapsulation header into an IPv6 header with IPv6 compatible addresses per Appendix B. Next, for IPv4 ENETs the Host sets the {IPv6 Traffic Class, Payload Length, Next Header, Hop Limit} fields according to the IPv4 {Type of Service, Total Length, Protocol, TTL} fields, respectively and also sets Flow Label as specified in [RFC6438]. The Host then applies IPv6 fragmentation to produce IPv6 fragments no smaller than the effective OFS described in Section 6.1. The Host next translates the IPv6 encapsulation headers back to OMNI L2 headers for the native ENET address format and with Type set to indicate the presence of the L2 IPv6 (Extended) Fragment Header. The Host finally sends the resultant carrier packets to the ENET peer.¶

When the ENET peer receives the carrier packets, it first translates the OMNI L2 headers back to IPv6 headers with compatible addresses. The peer then reassembles then removes the encapsulation headers and applies parcel reunification if necessary. The peer then either delivers the original IP packet/parcel to the transport layers if it is also the final destination or forwards the packet/parcel via the next hop if it is a Client acting as an intermediate system.¶

Hosts and Clients that initiate OMNI-based original IP packet/parcel transactions should first test the path toward the final destination using the parcel path qualification procedure specified in [I-D.templin-6man-parcels2][I-D.templin-intarea-parcels2]. An OMNI Host that sends and receives parcels need not implement the full OMNI interface abstraction but MUST implement enough of the OAL to be capable of fragmenting and reassembling maximum-length encapsulated IP packets/parcels and sub-parcels as discussed above and in the following section.¶

Note: Hosts and their peer Clients/Hosts on the same ANET/ENET can improve efficiency by forwarding original IP packets/parcels that do not require fragmentation as direct encapsulations within the OMNI L2 header and without including a L2 IPv6 (Extended) Fragment Header. In that case, the first four bits immediately following the OMNI L2 encapsulation header encode the value '4' for IPv4 or '6' for IPv6. Note that this savings comes at the expense of omitting a well-behaved Identification, but this may be an acceptable tradeoff in many secured ANET/ENET instances.¶

6.13. IP Parcels

IP parcels are formed by an OMNI Host or Client transport layer protocol entity identified by the "5-tuple" (source address, destination address, source port, destination port, protocol number) when it produces a {TCP,UDP} protocol data unit containing the concatenation of multiple transport layer protocol segments. The transport layer protocol entity then presents the buffer and non-final segment size to the network layer which appends a single {TCP,UDP}/IP header (plus any extension headers) before presenting the parcel to the OMNI Interface. Transport and network protocol formatting and processing rules as well as parcellation and reunification procedures for IP parcels are specified in [I-D.templin-6man-parcels2][I-D.templin-intarea-parcels2], while detailed OAL encapsulation and fragmentation procedures are specified here.¶

When the network layer forwards a parcel, the OMNI interface invokes the OAL which forwards it to either an intermediate system or the final destination itself. The OAL source first invokes parcellation by subdividing the parcel into sub-parcels if necessary with each sub-parcel no larger than 65535 (minus headers). The OAL source also maintains a Parcel ID for each sub-parcel of the same original parcel that along with the Identification value for this OAL packet supports reassembly; the OAL source increments Parcel ID (modulo 64) for each successive parcel.¶

The OAL source next performs encapsulation on each sub-parcel with destination set to the next hop address. If the next hop is reached via a (M)ANET/INET interface, the OAL source inserts an OAL header the same as discussed in Section 6.1 and sets the destination to the ULA of the target Client. If the next hop is reached via an ENET interface, the OAL source instead inserts an IP header of the appropriate protocol version for the underlay ENET (i.e., even if the encapsulation header is IPv4) and sets the destination to the ENET IP address of the next hop. The OAL source inserts the encapsulation header even if no actual fragmentation is needed and/or even if the Parcel Payload Option is present.¶

The OAL source next assigns an appropriate Identification number that is monotonically-incremented for each consecutive sub-parcel, then performs IPv6 fragmentation over the sub-parcel if necessary to create fragments small enough to traverse the path to the next hop. (If the encapsulation header is IPv4, the OAL source first translates the encapsulation header into an IPv6 header with IPv4-Compatible IPv6 addresses during fragmentation/reassembly while inserting the IPv6 Extended Fragment Header.) The OAL source then writes the "Parcel ID" and sets/clears the "(P)arcel" and "More (S)egments" bits in the Reserved field of the IPv6 Extended Fragment Header of the first fragment (see: Figure 4). (The OAL source sets P to 1 for a parcel or to 0 for a non-parcel. When P is 1, the OAL next sets S to 1 for non-final sub-parcels or to 0 if the sub-parcel contains the final segment.) The OAL source then sends each resulting carrier packet to the next hop, i.e., after first translating the IPv6 encapsulation header back to IPv4 if necessary.¶

When the OAL destination receives the carrier packets, it reassembles if necessary (i.e., after first translating the IPv4 encapsulation header to IPv6 if necessary). If the P flag in the first fragment is 0, the OAL destination then processes the reassembled entity as an ordinary IP packet; otherwise it continues processing as a sub-parcel. If the OAL destination is not the final destination, it can optionally retain the sub-parcels along with their Parcel ID and Identification values for a brief time for opportunistic reunification with peer sub-parcels of the same original parcel identified by the 4-tuple consisting of the adaptation layer (OAL source, OAL destination, Parcel ID, Identification). (Note that the OAL destination must not consult the parcel's network layer "5-tuple" at the adaptation layer, since it is possible that multiple sub-parcels of the same parcel may be forwarded over different network paths).¶

The OAL destination performs adaptation layer reunification by concatenating the segments included in sub-parcels with the same Parcel ID and Identification values within 64 of one another to create a larger sub-parcel possibly even as large as the entire original (sub)parcel. Order of concatenation is determined by increasing Identification values, noting that a sub-parcel that sets any TCP control flags must occur as a first concatenation, and the final sub-parcel (i.e., the one with S set to 0) must occur as a final concatenation and not as an intermediate. The OAL destination then appends common {TCP,UDP}/IP headers plus extensions to each reunified sub-parcel as specified in [I-D.templin-6man-parcels2][I-D.templin-intarea-parcels2].¶

When the OAL destination is not the final destination, it next forwards the reunified (sub-)parcel(s) to the next hop toward the final destination while ensuring that the S flag remains set to 0 in the sub-parcel that contains the final segment. When the parcel or sub-parcels arrive at the final destination, it performs network layer reunification to form the largest possible (sub)-parcels (while honoring the S flag) then delivers them to the transport layer entity which acts on the enclosed 5-tuple information supplied by the original source.¶

Note: IP parcels may also originate from a non-OMNI original source and travel over multiple parcel-capable IP links before reaching an OMNI link ingress node (i.e., either a Client or Proxy/Server acting as a "relay"). The ingress node then forwards the parcel into the OMNI link according to the rules established above for locally-generated parcels, with the exception that the parcel IP TTL/Hop Limit is decremented. Similarly, when the IP parcel arrives at the OMNI link egress node (i.e., either a Client or Proxy/Server acting as a "relay"), the parcel may travel over multiple parcel-capable IP links before reaching the final destination.¶

Note: The OAL destination process of reunifying parcels at the adaptation layer is optional, and should be avoided in cases where performance could be negatively impacted. It is always acceptable (albeit sometimes sub-optimal) for the OAL destination to forward sub-parcels on toward the final destination without performing adaptation layer reunification, since each sub-parcel will contain a well-formed header and an integral number of transport layer protocol segments and with the Parcel ID field and P, S flag set appropriately. The final destination can then optionally perform network layer reunification independently of any adaptation layer reunification that may have been applied by the OAL.¶

Note: The "Parcel ID" that appears in the OAL Extended Fragment Header and OCH1/2 headers is an adaptation layer value that encodes the same value for all sub-parcels of the original parcel at the adaptation layer. This is different than the "(Parcel) Index" that appears in the Parcel Payload Option header as well as L2/L3 IPv6 Extended Fragment Headers, which is a network layer value that encodes a transport layer segment index.¶

Note: Parcel Path Qualification procedures require two additional ICMP PTB message Code values to identify a Parcel Report and Jumbo Report. These Code values are specified in [I-D.templin-6man-parcels2] for IPv6 and [I-D.templin-intarea-parcels2] for IPv4.¶

6.14. OAL Requirements

In light of the above, OAL sources, destinations and intermediate systems observe the following normative requirements:¶

OAL sources MUST forward original IP packets/parcels either larger than the OMNI interface minimum EMTU_R or smaller than the minimum OFS as atomic fragments (i.e., and not as multiple fragments).¶
OAL sources MUST produce non-final fragments with payloads no smaller than the minimum OFS during fragmentation.¶
OAL intermediate systems SHOULD and OAL destinations MUST unconditionally drop any non-final OAL fragments with payloads smaller than the minimum OFS.¶
OAL destinations MUST drop any new OAL fragments with offset and length that would overlap with other fragments and/or leave holes smaller than the minimum OFS between fragments that have already been received.¶

Note: Under the minimum OFS, an ordinary 1500-octet original IP packet/parcel would require at most 2 OAL fragments, with the first fragment containing 1024 payload octets and the final fragment containing the remainder. For all packet/parcel sizes, the likelihood of successful reassembly may improve when the OMNI interface sends all fragments of the same fragmented OAL packet consecutively over the same underlay interface pair instead of spread across multiple underlay interface pairs. Finally, an assured minimum OFS allows continuous operation over all paths including those that traverse bridged L2 media with dissimilar MTUs.¶

Note: Certain legacy network hardware of the past millennium was unable to accept IP fragment "bursts" resulting from a fragmentation event - even to the point that the hardware would reset itself when presented with a burst. This does not seem to be a common problem in the modern era, where fragmentation and reassembly can be readily demonstrated at line rate (e.g., using tools such as 'iperf3') even over fast links on ordinary hardware platforms. Even so, while the OAL destination is reporting reassembly congestion (see: Section 6.9) the OAL source could impose "pacing" by inserting an inter-fragment delay and increasing or decreasing the delay according to congestion indications.¶

6.15. OAL Fragmentation Security Implications

As discussed in Section 3.7 of [RFC8900], there are four basic threats concerning IPv6 fragmentation; each of which is addressed by effective mitigations as follows:¶

Overlapping fragment attacks - reassembly of overlapping fragments is forbidden by [RFC8200]; therefore, this threat does not apply to the OAL.¶
Resource exhaustion attacks - this threat is mitigated by providing a sufficiently large OAL reassembly cache and instituting "fast discard" of incomplete reassemblies that may be part of a buffer exhaustion attack. The reassembly cache should be sufficiently large so that a sustained attack does not cause excessive loss of good reassemblies but not so large that (timer-based) data structure management becomes computationally expensive. The cache should also be indexed based on the arrival underlay interface such that congestion experienced over a first underlay interface does not cause discard of incomplete reassemblies for uncongested underlay interfaces.¶
Attacks based on predictable fragment Identification values - in environments where spoofing is possible, this threat is mitigated through the use of Identification windows beginning with unpredictable values per Section 6.7. By maintaining windows of acceptable Identifications, OAL neighbors can quickly discard spurious carrier packets that might otherwise clutter the reassembly cache. The OAL additionally provides an integrity check to detect corruption that may be caused by spurious fragments received with in-window Identification values.¶
Evasion of Network Intrusion Detection Systems (NIDS) - since the OAL source employs a robust OFS, network-based firewalls can inspect and drop OAL fragments containing malicious data thereby disabling reassembly by the OAL destination. However, since OAL fragments may take different paths through the network (some of which may not employ a firewall) each OAL destination must also employ a firewall.¶

IPv4 includes a 2-octet (16-bit) Identification (IP ID) field with only 65535 unique values such that even at moderate data rates the field could wrap and apply to new carrier packets while the fragments of old carrier packets using the same IP ID are still alive in the network [RFC4963]. Carrier packets sent via an IPv4 path with DF set to 0 and with trailing payload/reassembly checksum(s) therefore ensure sufficient integrity to detect and discard reassembly errors. Since IPv6 provides a 4-octet (32-bit) Identification value, IP ID wraparound for IPv6 fragmentation may only be a concern at extreme data rates (e.g., 1Tbps or more). Note that these limitations are fully addressed through the Extended Identification format supported by [I-D.templin-6man-ipid-ext2].¶

Fragmentation security concerns for large IPv6 ND messages are documented in [RFC6980]. These concerns are addressed when the OMNI interface employs the OAL instead of directly fragmenting the IPv6 ND message itself. For this reason, OMNI interfaces MUST employ OAL encapsulation and fragmentation for IPv6 ND messages larger than the effective OFS for this OAL destination.¶

Unless the path is secured at the network layer or below (i.e., in environments where spoofing is possible), OMNI interfaces MUST NOT send OAL packets/fragments with Identification values outside the current window and MUST secure IPv6 ND messages used for address resolution or window state synchronization. OAL destinations SHOULD therefore discard without reassembling any out-of-window OAL fragments received over an unsecured path.¶

12. Address Mapping - Unicast

OMNI interfaces maintain a network layer conceptual neighbor cache per [RFC1256] or [RFC4861] the same as for any IP interface, and (for IPv6) use the link-local address format specified in Section 8. The network layer maintains state through static and/or dynamic Neighbor Cache Entry (NCE) configurations.¶

Each OMNI interface also maintains a separate internal adaptation layer conceptual neighbor cache that includes a NCE for the unique-local address of each of its active OAL neighbors (see: Section 8). For each peer NCE, OAL neighbors also maintain AERO Forwarding Vectors (AFVs) which map per-interface-pair parameters. Throughout this document, the terms "neighbor cache", "NCE" and "AFV" refer to this OAL neighbor information unless otherwise specified.¶

IPv6 Neighbor Discovery (ND) [RFC4861] messages sent over OMNI interfaces without OAL encapsulation observe the native underlay interface Source/Target Link-Layer Address Option (S/TLLAO) format (e.g., for Ethernet the S/TLLAO is specified in [RFC2464]). IPv6 ND messages sent from within the OMNI interface using OAL encapsulation do not include S/TLLAOs, but instead include a new option type that encodes OMNI link-specific information. Hence, this document does not define a new S/TLLAO format but instead defines a new option type termed the "OMNI option" designed for these purposes. (Note that OMNI interface IPv6 ND messages sent without encapsulation may include both OMNI options and S/TLLAOs, but the information conveyed in each is mutually exclusive.)¶

For each IPv6 ND message, the OMNI interface includes one or more OMNI options (and any other ND message options) then completely populates all option information. OMNI options should be padded when necessary to ensure that they end on their natural 64-bit boundaries the same as for any IPv6 ND message option.¶

If the OMNI interface includes an OMNI option with an authentication signature, it first sets the signature field to 0 then calculates the authentication signature beginning after the IPv6 ND message header checksum field. The OMNI interface extends the calculation over the entire length of the ND message (as well as any concatenated extensions in the case of a super-packet) then writes the authentication signature value into the appropriate OMNI authentication sub-option field.¶

The OMNI interface then applies any non-OMNI authentication signatures, calculates the IPv6 ND message checksum per [RFC4443] beginning with a pseudo-header of the IPv6 header and writes the value into the Checksum field. OMNI interfaces verify first integrity then authenticity of each IPv6 ND message or super-packet received, and process the message further only following successful verification.¶

OMNI interface Clients such as aircraft typically have multiple wireless data link types (e.g. satellite-based, cellular, terrestrial, air-to-air directional, etc.) with diverse performance, cost and availability properties. The OMNI interface would therefore appear to have multiple L2 connections, and may include information for multiple underlay interfaces in a single IPv6 ND message exchange. OMNI interfaces manage their dynamically-changing multilink profiles by including OMNI options in IPv6 ND messages as discussed in the following subsections.¶

12.1. The OMNI Option

OMNI options appear in IPv6 ND messages formatted as shown in Figure 16:¶

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Type     |     Length    |         Sub-Options           ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 16: OMNI Option Format

In this format:¶

Type is set to TBD4 (see: IANA Considerations).¶
Length is set to the number of 8-octet blocks in the option. The value 0 is invalid, while the values 1 through 255 (i.e., 8 through 2040 octets, respectively) indicate the total length of the OMNI option. If multiple OMNI option instances appear in the same IPv6 ND message, the union of the contents of all OMNI options is accepted unless otherwise qualified for specific sub-options below.¶
Sub-Options is a Variable-length field padded with Pad1/N sub-options if necessary (see below) such that the complete OMNI option is an integer multiple of 8 octets long. The Sub-Options field contains zero or more sub-options as specified in Section 12.2.¶

The OMNI option is included in OMNI interface IPv6 ND messages; the option is processed by receiving interfaces that recognize it and otherwise ignored. The OMNI interface processes all OMNI option instances received in the same IPv6 ND message in the consecutive order in which they appear. The OMNI option(s) included in each IPv6 ND message may include full or partial information for the neighbor. The OMNI interface therefore retains the union of the information in the most recently received OMNI options in the corresponding NCE.¶

12.2. OMNI Sub-Options

Each OMNI option includes a Sub-Options block containing zero or more individual sub-options. Each consecutive sub-option is concatenated immediately following its predecessor. All sub-options except Pad1 (see below) are in an OMNI-specific type-length-value (TLV) format encoded as follows:¶

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
     | Sub-Type|      Sub-Length     | Sub-Option Data ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Figure 17: Sub-Option Format

Sub-Type is a 5-bit field that encodes the sub-option type. Sub-option types defined in this document are:¶

     Sub-Option Name             Sub-Type
     Pad1                           0
     PadN                           1
     Node Identification            2
     Authentication                 3
     Neighbor Control               4
     Interface Attributes           5
     Traffic Selector               6
     Multilink Vector               7
     Geo Coordinates                8
     DHCPv6 Message                 9
     PIM-SM Message                10
     HIP Message                   11
     QUIC-TLS Message              12
     Fragmentation Report          13
     ICMPv6 Error                  14
     Proxy/Server Departure        15
     Sub-Type Extension            30

Figure 18

Sub-Types 16-29 are available for future assignment for major protocol functions, while Sub-Type 30 supports scalable extension to include other functions. Sub-Type 31 is reserved by IANA.¶

Sub-Length is an 11-bit field that encodes the length of the Sub-Option Data in octets.¶
Sub-Option Data is a block of data with format determined by Sub-Type and length determined by Sub-Length. Note that each sub-option is concatenated consecutively with the previous and may therefore begin and/or end on an arbitrary octet boundary.¶

The OMNI interface codes each sub-option with a 2-octet header that includes Sub-Type in the most significant 5 bits followed by Sub-Length in the next most significant 11 bits. Each sub-option encodes a maximum Sub-Length value of 2038 octets minus the lengths of the OMNI option header and any preceding sub-options. This allows ample Sub-Option Data space for coding large objects (e.g., ASCII strings, domain names, protocol messages, security codes, etc.), while a single OMNI option is limited to 2040 octets the same as for any IPv6 ND option.¶

The OMNI interface codes initial sub-options in a first OMNI option instance and any additional sub-options in additional instances in the same IPv6 ND message in the intended order of processing. If the size of all OMNI options with their sub-options would cause the IPv6 ND message to exceed the OMNI interface MTU, the OMNI interface can code any remaining sub-options in additional IPv6 ND messages.¶

The OMNI interface processes all OMNI options received in an IPv6 ND message while skipping over and ignoring any unrecognized sub-options. The OMNI interface processes the sub-options of all OMNI option instances in the consecutive order in which they appear in the IPv6 ND message, beginning with the first instance and continuing through any additional instances to the end of the message. If an individual sub-option length would cause processing to exceed the OMNI option instance and/or IPv6 ND message lengths, the OMNI interface accepts any sub-options already processed and ignores the remainder of that instance. The interface then processes any remaining OMNI option instances in the same fashion to the end of the IPv6 ND message.¶

IPv6 ND messages that require OMNI authentication services MUST include a Node Identification sub-option as the first sub-option of the first OMNI option, and MUST include some form of authentication (e.g., HMAC, HIP, QUIC, etc.) as the immediately next sub-option whether in the same or different OMNI option. A single IPv6 ND messages may include only one OMNI authentication service sub-option; if multiple are included, the first sub-option is processed and all others are ignored. The IPv6 ND message may also include non-OMNI authentication options such as those specified in [RFC3971] or [RFC8928] either instead of or in addition to an OMNI authentication option. Nodes that receive IPv6 ND messages over unsecured underlying networks first verify the IPv6 ND message checksum then authenticate the message by processing any authentication options/sub-options.¶

Note: large objects that exceed the maximum Sub-Option Data length are not supported under the current specification; if this proves to be limiting in practice, future specifications may define support for fragmenting large sub-options across multiple OMNI options within the same IPv6 ND message (or even across multiple IPv6 ND messages, if necessary).¶

The following sub-option types and formats are defined in this document:¶

12.2.1. Pad1

     +-+-+-+-+-+-+-+-+
     | S-Type=0|x|x|x|
     +-+-+-+-+-+-+-+-+

Figure 19: Pad1

Sub-Type is set to 0. If multiple instances appear in OMNI options of the same message all are processed.¶
Sub-Type is followed by 3 'x' bits, set to any value on transmission (typically all-zeros) and ignored on reception. Pad1 therefore consists of a single octet with the most significant 5 bits set to 0, and with no Sub-Length or Sub-Option Data fields following.¶

If more than a single octet of padding is required, the PadN option, described next, should be used, rather than multiple Pad1 options.¶

12.2.2. PadN

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
     | S-Type=1|    Sub-length=N     | N padding octets ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Figure 20: PadN

Sub-Type is set to 1. If multiple instances appear in OMNI options of the same message all are processed.¶
Sub-Length is set to N that encodes the number of padding octets that follow.¶
Sub-Option Data consists of N octets, set to any value on transmission (typically all-zeros) and ignored on receipt.¶

When a proxy forwards an IPv6 ND message with OMNI options, it can employ PadN to void any non-Pad1 sub-options that should not be processed by the next hop by simply writing the value '1' over the Sub-Type. When the proxy alters the IPv6 ND message contents in this way, any included authentication and integrity checks are invalidated. See: Appendix C for a discussion of IPv6 ND message authentication and integrity.¶

12.2.3. Node Identification

The Node Identification sub-option includes a form of identification for the node, and (when present) must appear as the first sub-option of the first OMNI option in each IPv6 ND message.¶

At least one instance of the sub-option must be present in messages that also include an OMNI authentication service sub-option. If multiple instances appear in OMNI options of the same IPv6 ND message the first instance of a specific ID-Type is processed and all other instances of the same ID-Type are ignored. (It is therefore possible for a single IPv6 ND message to convey multiple distinct Node Identifications - each with a different ID-Type.)¶

The format and contents of the sub-option are shown in Figure 21:¶

                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     | S-Type=2|    Sub-length=N     |    ID-Type    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~            Node Identification Value (N-1 octets)             ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 21: Node Identification

Sub-Type is set to 2. Multiple instances are processed as discussed above.¶
Sub-Length is set to N that encodes the number of Sub-Option Data octets that follow. The ID-Type field is always present; hence, the maximum Node Identification Value length is limited by the remaining available space in this OMNI option.¶
ID-Type is a 1-octet field that encodes the type of the Node Identification Value. The following ID-Type values are currently defined:¶
- 0 - Universally Unique IDentifier (UUID) [RFC4122]. Indicates that Node Identification Value contains a 16-octet UUID.¶
- 1 - Host Identity Tag (HIT) [RFC7401]. Indicates that Node Identification Value contains a 16-octet HIT.¶
- 2 - Hierarchical HIT (HHIT) [RFC9374]. Indicates that Node Identification Value contains a 16-octet HHIT.¶
- 3 - Network Access Identifier (NAI) [RFC7542]. Indicates that Node Identification Value contains an (N-1)-octet NAI.¶
- 4 - Fully-Qualified Domain Name (FQDN) [RFC1035]. Indicates that Node Identification Value contains an (N-1)-octet FQDN.¶
- 5 - IPv6 Address. Indicates that Node Identification contains a 16-octet IPv6 address that is not a (H)HIT. The IPv6 address type is determined according to the IPv6 addressing architecture [RFC4291].¶
- 6 - 252 - Unassigned.¶
- 253 - 254 - reserved for experimentation, as recommended in [RFC3692].¶
- 255 - reserved by IANA.¶
Node Identification Value is an (N-1)-octet field encoded according to the appropriate the "ID-Type" reference above.¶

OMNI interfaces code Node Identification Values used for DHCPv6 messaging purposes as a DHCP Unique IDentifier (DUID) using the "DUID-EN for OMNI" format with enterprise number 45282 (see: Section 24) as shown in Figure 22:¶

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |         DUID-Type (2)         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Enterprise Number (45282)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    ID-Type    |                                               |
     +-+-+-+-+-+-+-+-+                                               ~
     ~                   Node Identification Value                   ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 22: DUID-EN for OMNI Format

In this format, the OMNI interface codes the ID-Type and Node Identification Value fields from the OMNI sub-option following a 6-octet DUID-EN header, then includes the entire "DUID-EN for OMNI" in a DHCPv6 message per [RFC8415].¶

12.2.4. Authentication

The Authentication sub-option includes a Hashed Message Authentication Code (HMAC) computed according to [RFC2104] and [RFC6234].¶

The Authentication sub-option is formatted as shown in Figure 23:¶

                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     | S-Type=3|    Sub-length=N     |      Type     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~          Hashed Message Authentication Code (HMAC)            ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 23: Authentication

Sub-Type is set to 3. The Authentication sub-option must appear at most once in any IPv6 ND message; if multiple instances appear in OMNI options of the same message the first is processed and all others are ignored.¶
Sub-Length is set to N, i.e., the length of the option in octets beginning immediately following the Sub-Length field and extending to the end of the HMAC. The length of the HMAC is therefore limited by the remaining available space for this sub-option.¶
Type encodes the authentication algorithm type found in the IANA "ICMPv6 Parameters - Trust Anchor Option (Type 15) Name Field" registry, and determines the length of the HMAC. For example, when Type is 3 the authentication algorithm is SHA-1 and the HMAC is 160 bits (20 octets) in length, when Type is 5 the algorithm is SHA-256 and the HMAC is 256 bits (32 octets) in length, etc. A full list of available Types is found in the registry, which cites [RFC6495] for several well-known Types. The Type value TBD7 is reserved for the Edwards-Curve Digital Signature Algorithm (EdDSA) (see IANA Considerations) with the HMAC (i.e., digital signature) including 64 octets for Ed25519 or 114 octets for Ed448 per [RFC8032].¶
HMAC includes the Hashed Message Authentication Code or digital signature for this IPv6 ND message with field length corresponding to Type.¶

12.2.5. Neighbor Control

IPv6 ND messages used to manage neighbor relationships between Clients and their Proxy/Servers (and also between Clients and their peer Clients) include a Neighbor Control OMNI sub-option. Each IPv6 ND message includes at most one Neighbor Control sub-option which must be specific to the underlying interface over which the ND message is sent.¶

The Neighbor Control sub-option is formatted as follows:¶

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     | S-Type=4|    Sub-length=4     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |N|A|R|S|                                                       |
     |U|R|P|N|                     Reserved                          |
     |D|R|T|R|                                                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 24: Neighbor Control

Sub-Type is set to 4. If multiple instances appear in OMNI options of the same message, the first is processed and all others are ignored.¶
Sub-Length is set to 4.¶
Sub-Option Data includes a 4-octet neighbor control flags field. Clients set the Neighbor Unreachability Detection (NUD), Address Resolution Responder (ARR) and Report (RPT) flags in RS messages to control the operation of their Proxy/Server neighbors as discussed in Section 15. Nodes set the Synchronous (u)NA Required (SNR) flag in non-solicitation IPv6 ND messages (i.e., solicited/unsolicited NA/RA and Redirects) for which they require a synchronous (but technically "unsolicited") NA reply (see: [I-D.templin-intarea-aero2]). The remaining flags are Reserved and must be set to 0; future specifications may define new flags.¶

12.2.6. Interface Attributes

The Interface Attributes sub-option provides neighbors with forwarding information for the multilink conceptual sending algorithm discussed in Section 14. Neighbors use the forwarding information to select among potentially multiple candidate underlay interfaces that can be used to forward carrier packets to the neighbor based on factors such as traffic selectors and link metrics. Interface Attributes further include link layer address information to be used for either direct INET encapsulation for targets in the local SRT segment or spanning tree forwarding for targets in remote SRT segments.¶

OMNI nodes include Interface Attributes for some/all of a source or target Client's underlay interfaces in NS/NA and uNA messages used to publish Client information (see: [I-D.templin-intarea-aero2]). At most one Interface Attributes sub-option for each distinct ifIndex may be included; if an IPv6 ND message includes multiple Interface Attributes sub-options for the same ifIndex, the first is processed and all others are ignored. OMNI nodes that receive NS/NA messages can use all of the included Interface Attributes and/or Traffic Selectors to formulate a map of the prospective source or target node as well as to seed the information to be populated in future neighbor exchanges.¶

OMNI Clients and Proxy/Servers also include Interface Attributes sub-options in RS/RA messages used to initialize, discover and populate routing and addressing information. Each RS message MUST contain exactly one Interface Attributes sub-option with an ifIndex corresponding to the Client's underlay interface used to transmit the message, and each RA message MUST echo the same Interface Attributes sub-option with any (proxyed) information populated by the FHS Proxy/Server to provide operational context.¶

When an FHS Proxy/Server receives an RS message destined to an anycast L2 address, it MUST include an additional Interface Attributes sub-option with ifIndex '0' that encodes its own unicast L2 address relative to the Client's underlay interface in the solicited RA response. Any additional Interface Attributes sub-options that appear in RS/RA messages (i.e., besides those for the Client's own ifIndex and ifIndex '0') are ignored.¶

The Interface Attributes sub-option is formatted as shown below:¶

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     | S-Type=5|    Sub-length=N     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            ifIndex                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            ifType                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            ifProvider                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            ifMetric                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            ifGroup                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      SRT      |      FMT      |                               ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               ~
     ~                         LHS GUA/L2ADDR                        ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                    Traffic Selector Blocks                    ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ...

Figure 25: Interface Attributes

Sub-Type is set to 5. Multiple instances are processed as discussed above.¶
Sub-Length is set to N that encodes the number of Sub-Option Data octets that follow.¶
Sub-Option Data contains an "Interface Attributes" option encoded as follows:¶
- ifIndex is a 4-octet index value corresponding to a specific underlay interface. Client OMNI interfaces MUST number each distinct underlay interface with a non-zero ifIndex value assigned by network management per [RFC2863] and include the value in this field. The ifIndex value '0' denotes "unspecified".¶
- ifType is a 4-octet type value corresponding to this underlay interface. The value is coded per the 'IANAifType-MIB' registry [http://www.iana.org].¶
- ifProvider is a 4-octet provider identifier corresponding to this underlay interface. This document defines the single provider identifier value '0' (undefined). Future documents may define other values.¶
- ifMetric encodes a 4-octet interface metric. Lower values indicate higher priorities, and the highest value indicates an interface that should not be selected. The ifMetric setting provides an instantaneous indication of the interface bandwidth, link quality, signal strength, cost, etc.; hence, its value may change in successive IPv6 ND messages.¶
- ifGroup is a 4-octet identifier for a Link Aggregation Group (LAG) [IEEE802.1AX] corresponding to the underlay interface identified by ifIndex. Interface attributes for ifIndex members of the same group will encode the same value in ifGroup. This document defines the single ifGroup value '0' meaning "no group assigned". Future documents will specify the setting of other values.¶
- SRT is a 1-octet Segment Routing Topology prefix length value between 0 and 128 that determines the prefix length associated with the LHS ULA.¶
- FMT - a 1-octet "Forward/Mode/Type" code interpreted as follows:¶
  - The most significant two bits (i.e., "FMT-Forward" and "FMT-Mode") are interpreted in conjunction with one another. When FMT-Forward is clear, the LHS Proxy/Server performs OAL reassembly and decapsulation to obtain the original IP packet/parcel before forwarding. If the FMT-Mode bit is clear, the LHS Proxy/Server then forwards the original IP packet/parcel at L3; otherwise, it invokes the OAL to re-encapsulate, re-fragment and sends the resulting carrier packets to the Client via the selected underlay interface. When FMT-Forward is set, the LHS Proxy/Server forwards unsecured OAL fragments to the Client without reassembling, while reassembling secured OAL fragments before re-fragmenting and forwarding to the Client. If FMT-Mode is clear, all carrier packets destined to the Client must always be sent via the LHS Proxy/Server; otherwise the Client is eligible for direct forwarding over the open INET where it may be located behind one or more NATs.¶
  - The value encoded in the least significant 6 bits (i.e., "FMT-Type") determines the type and length of the L2ADDR field. The following values are currently defined:¶
    - 0 - L2ADDR is 0 octets in length and unused.¶
    - 1 - L2ADDR is 4 octets in length and encodes an IPv4 address.¶
    - 2 - L2ADDR is 6 octets in length and encodes an EUI-48 address [EUI].¶
    - 3 - L2ADDR is 8 octets in length and encodes an EUI-64 address [EUI].¶
    - 4 - L2ADDR is 16 octets in length and encodes an IPv6 address.¶
- LHS GUA/L2ADDR - encodes the 16 octet SNP IPv6 GUA of the node relative to the LHS Proxy/Server followed by the L2ADDR field formatted as above. When SRT and are both set to 0, the LHS Proxy/Server is considered unspecified in this IPv6 ND message. FMT, SRT and LHS together provide guidance for the OMNI interface forwarding algorithm. Specifically, if LHS::/SRT is located in the local OMNI link segment, then the source can address the target Client either through its dependent Proxy/Server or through direct encapsulation following NAT traversal according to FMT. Otherwise, the target Client is located on a different SRT segment and the path from the source must employ a combination of route optimization and spanning tree hop traversals. L2ADDR identifies the LHS Proxy/Server's INET-facing interface not located behind NATs, therefore no UDP port number is included since port number 8060 is used when the L2 encapsulation includes a UDP header. Instead, L2ADDR includes only an L2 address with type and length determined by FMT-Type as described above. When L2ADDR includes an IPv4 or IPv6 address, it is recorded in network byte order in ones-compliment "obfuscated" form per [RFC4380].¶
- Traffic Selector Blocks(s) - zero or more Traffic Selector blocks follow, with their total length determined by the number of octets remaining in the Interface Attributes sub-option beyond the end of the LHS Proxy/Server information. Each Traffic Selector block is formatted the same as specified in Section 12.2.7 and processed consecutively, with its length subtracted from the remaining length of the Interface Attributes sub-option.¶

12.2.7. Traffic Selector

The Traffic Selector sub-option provides forwarding information for the multilink conceptual sending algorithm discussed in Section 14. The sub-option includes traffic selector information per [RFC6088] as ancillary information for an Interface Attributes sub-option with the same ifIndex value, or as discrete information for the included ifIndex when no Interface Attributes sub-option is present.¶

IPv6 ND messages may include multiple Traffic Selectors for some or all of the source/target Client's underlay interfaces (see: [I-D.templin-intarea-aero2] for further discussion). Traffic Selectors must be honored by all implementations in the format shown below:¶

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     | S-Type=6|    Sub-length=N     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            ifIndex                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   TS Length   |   TS Format   |A|B|C|D|E|F|G|H|I|J|K|L|M|N|RES|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 (A)Start Source Address                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 (B)End Source Address                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 (C)Start Destination Address                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 (D)End Destination Address                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     (E)Start IPsec SPI                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      (F)End IPsec SPI                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   (G)Start Source port        |   (H)End Source port          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   (I)Start Destination port   |   (J)End Destination port     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  (K)Start DS  |  (L)End DS    |(M)Start Prot. | (N) End Prot. |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~               Additional Traffic Selector Blocks              ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ...

Figure 26: Traffic Selector

Sub-Type is set to 6. Multiple instances with the same or different ifIndex values may appear in the same IPv6 ND message. When multiple instances appear, all are processed and the cumulative information from all is accepted.¶
Sub-Length is set to N that encodes the number of Sub-Option Data octets that follow.¶
Sub-Option data begins with a 4-octet ifIndex value corresponding to a specific underlay interface.¶
The remainder of Sub-Option Data contains one or more "Traffic Selector" blocks for this ifIndex that each begin with 1-octet "TS Length" and "TS Format" fields. TS length encodes the combined lengths of the TS* fields plus the Traffic Selector body that follows (i.e. a value between 2-255 octets). When TS Format encodes the value 1 or 2, the Traffic Selector body encodes an IPv4 or IPv6 traffic selector per [RFC6088] beginning with 16 flag bits ("A-N" plus 2 "Reserved"); when TS Format encodes any other value the Traffic Selector block is skipped and processing resumes beginning with the next Traffic Selector block (if any). The Traffic Selector block elements then appear immediately after the flags (with no 16-bit Reserved field included) and encode the information corresponding to any set flag bit(s) in order the same as specified in [RFC6088]. Each included Traffic Selector block is processed consecutively, with its length subtracted from the remaining sub-option length until all blocks are processed. If the length of any Traffic Selector block would exceed the remaining length for the entire sub-option, the remainder of the sub-option is ignored.¶

12.2.8. Multilink Vector

Clients and their correspondents exchange NS/NA messages to populate AERO Forwarding Vector (AFV) state in OAL intermediate and end systems in the path between their respective underlay interfaces. The Multilink Vector sub-option provides the necessary information allowing OAL intermediate and end systems in the path to establish (multilink) vectors to support future packet forwarding.¶

The NS/NA message used to initiate AFV state is termed the "initiator" and the responsive NA message is termed the "responder", i.e., in some cases the "initiator" may be an NA response to an NS. Each IPv6 NS/NA message may contain at most one Multilink Vector sub-option; if multiple are present, the first is recognized and all others are ignored.¶

The Multilink Vector sub-option is formatted as follows:¶

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     | S-Type=7|    Sub-length=32    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Initiator ifIndex                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Responder ifIndex                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              AERO Forwarding Vector Index (AFVI)              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                        Sequence Number                        ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                     Acknowledgment Number                     ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |R|R|O|A|R|R|S|R|                                               |
     |E|E|P|C|E|S|Y|E|                   Window                      |
     |S|S|T|K|S|T|N|S|                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 27: Multilink Vector

Sub-Type is set to 7. If multiple instances appear in OMNI options of the same message, the first is processed and all others are ignored.¶
Sub-Length is set to 32.¶
the first 4 octets of Sub-Option Data include the 4-octet ifIndex of the initiator, i.e., the one that includes window control flags with SYN set and ACK not set or with ACK set and SYN not set - see below.¶
the next 4 octets include the 4-octet ifIndex of the responder, i.e., the one that includes window control flags with both SYN and ACK set - see below.¶
the next 4 octets includes the AERO Forwarding Vector Index (AFVI) that the initiator/responder provides to the next OAL hop. When the SYN flag is set, the next hop records this AFVI in an AERO Forwarding Information Base (AFIB) AERO Forwarding Vector (AFV) indexed by both the previous hop L2 address and the AFVI. The next hop then rewrites the NS/NA AFVI field to one of its own chosen values and forwards the message to the following OAL hop. When the SYN flag is not set, the next hop instead uses the NS/NA AFVI value to verify that an AFV already exists without creating a new one, then resets the NS/NA AFVI to its cached value for the following OAL hop. The manner for populating AFV information is specified in further detail in [I-D.templin-intarea-aero2].¶
the final 20 octets of Sub-Option Data is modeled from the Transmission Control Protocol (TCP) header specified in Section 3.1 of [RFC9293]. The field is formatted as an 8-octet Sequence Number, followed by an 8-octet Acknowledgement Number, followed by a 1-octet flags field followed by a 3-octet Window size. The TCP (ACK, RST, SYN) flags are used for TCP-like window synchronization, while the TCP (CWR, ECE, URG, PSH, FIN) flags are unused. The OPT flag (discussed in Section 6.7) is an OMNI-specific replacement for the TCP URG flag, and the four remaining unused flags appear as reserved (RES). Together, these fields support the OAL window synchronization services specified in Section 6.7.¶

12.2.9. Geo Coordinates

                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     | S-Type=8|     Sub-length=N    |    Geo Type   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                        Geo Coordinates                        ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 28: Geo Coordinates

Sub-Type is set to 8. If multiple instances appear in OMNI options of the same message all are processed.¶
Sub-Length is set to N that encodes the number of Sub-Option Data octets that follow.¶
Geo Type is a 1-octet field that encodes a type designator that determines the format and contents of the Geo Coordinates field that follows. The following types are currently defined:¶
- 0 - NULL, i.e., the Geo Coordinates field is zero-length.¶
Geo Coordinates is a type-specific format field of length up to the remaining available space for this OMNI option. New formats to be specified in future documents and may include attributes such as latitude/longitude, altitude, heading, speed, etc.¶

12.2.10. Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Message

The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) sub-option may be included in the OMNI options of Client RS messages and Proxy/Server RA messages.¶

Note that OMNI DHCPv6 messages do not include a Checksum field since integrity is protected by the IPv6 ND message checksum, authentication signature and/or link or physical layer authentication and integrity checks.¶

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     | S-Type=9|    Sub-length=N     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    msg-type   |               transaction-id                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                        DHCPv6 options                         ~
     ~                 (variable number and length)                  ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 29: DHCPv6 Message

Sub-Type is set to 9. If multiple instances appear in OMNI options of the same message the first is processed and all others are ignored.¶
Sub-Length is set to N that encodes the number of Sub-Option Data octets that follow. The 'msg-type' and 'transaction-id' fields are always present; hence, the length of the DHCPv6 options is limited by the remaining available space for this OMNI option.¶
'msg-type' and 'transaction-id' are coded according to Section 8 of [RFC8415].¶
A set of DHCPv6 options coded according to Section 21 of [RFC8415] follows.¶

12.2.11. PIM-SM Message

The Protocol Independent Multicast - Sparse Mode (PIM-SM) Message sub-option may be included in the OMNI options of IPv6 ND messages. PIM-SM messages are formatted as specified in Section 4.9 of [RFC7761], with the exception that the Checksum field is omitted since the IPv6 ND message is already protected by the IPv6 ND message checksum, authentication signature and/or link or physical layer authentication and integrity checks.¶

The PIM-SM message sub-option format is shown in Figure 30:¶

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |S-Type=10|    Sub-length=N     |PIM Ver| Type  |   Reserved    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                         PIM-SM Message                        ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 30: PIM-SM Message Option Format

Sub-Type is set to 10. If multiple instances appear in OMNI options of the same message all are processed.¶
Sub-Length is set to N, i.e., the length of the option in octets beginning immediately following the Sub-Length field and extending to the end of the PIM-SM message. The length of the entire PIM-SM message is therefore limited by the remaining available space for this OMNI option.¶
The PIM-SM message is coded exactly as specified in Section 4.9 of [RFC7761], except that the Checksum field is omitted, and the Reserved field is set to 0 on transmission and ignored on reception. The "PIM Ver" field encodes the value 2, and the "Type" field encodes the PIM message type. (See Section 4.9 of [RFC7761] for a list of PIM-SM message types and formats.)¶

12.2.12. Host Identity Protocol (HIP) Message

The Host Identity Protocol (HIP) Message sub-option (when present) provides an authentication service alternative for IPv6 ND messages exchanged between Clients and FHS Proxy/Servers (or between Clients and their peers) over an open Internetwork. When the HIP service is used, FHS Proxy/Servers verify the HIP authentication signatures in source Client IPv6 ND messages then remove the HIP message sub-option and securely forward the ND messages to other OMNI nodes. LHS Proxy/Servers that receive secured IPv6 ND messages from other OMNI nodes that do not already include a security sub-option can insert HIP authentication signatures before forwarding them to the target Client.¶

OMNI interfaces that use the HIP service include the HIP message sub-option when they forward IPv6 ND messages that require security over INET underlay interfaces, i.e., where authentication and integrity is not already assured by link/physical layers or other OMNI layer services. The OMNI interface calculates the authentication signature over the entire length of the OAL packet (or super-packet) beginning after the IPv6 ND message header and extending over the remainder of the OAL packet or super-packet. OMNI interfaces that process OAL packets containing secured IPv6 ND messages verify the signature then either process the rest of the message locally or forward a proxyed copy to the next hop.¶

When an FHS Client inserts a HIP message sub-option in an IPv6 ND message destined to a target in a remote spanning tree segment, it must ensure that the insertion does not cause the message to exceed the OMNI interface MTU. If the LHS Proxy/Server cannot create sufficient space through any means without causing the OMNI option to exceed 2040 octets or causing the IPv6 ND message to exceed the OMNI interface MTU, it returns a suitable error (see: Section 12.2.15) and drops the message.¶

The HIP message sub-option is formatted as shown below:¶

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |S-Type=11|    Sub-length=N     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0| Packet Type |Version| RES.|1|           Controls            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                Sender's Host Identity Tag (HIT)               ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~               Receiver's Host Identity Tag (HIT)              ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                        HIP Parameters                         ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 31: HIP Message

Sub-Type is set to 11. If multiple instances appear in OMNI options of the same message the first is processed and all others are ignored.¶
Sub-Length is set to N, i.e., the length of the option in octets beginning immediately following the Sub-Length field and extending to the end of the HIP parameters. The length of the entire HIP message is therefore limited by the remaining available space for this OMNI option.¶
The HIP message is coded per Section 5 of [RFC7401], except that the OMNI "Sub-Type" and "Sub-Length" fields replace the first 2 octets of the HIP message header (i.e., the Next Header and Header Length fields). Also, since the IPv6 ND message is already protected by its own checksum, the 2-octet HIP message Checksum field is omitted.¶

Note: In some environments, maintenance of a Host Identity Tag (HIT) namespace may be unnecessary for securely associating an OMNI node with an IPv6 address-based identity. In that case, IPv6 ULAs can be used instead of HITs in the authentication signature as long as the address can be uniquely associated with the Sender/Receiver.¶

12.2.13. QUIC-TLS Message

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |S-Type=12|     Sub-length=N    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                         QUIC-TLS Message                      ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 32: QUIC-TLS Message

Sub-Type is set to 12. If multiple instances appear in OMNI options of the same IPv6 ND message, the first is processed and all others are ignored.¶
Sub-Length is set to N that encodes the number of Sub-Option Data octets that follow.¶
The QUIC-TLS message [RFC9000][RFC9001][RFC9002] encodes the QUIC and TLS message parameters necessary to support QUIC connection establishment.¶

IPv6 ND messages serve as couriers to transport the QUIC and TLS parameters necessary to establish a secured QUIC connection.¶

12.2.14. Fragmentation Report (FRAGREP)

Fragmentation Report (FRAGREP) sub-options may be included in the OMNI options of uNA messages sent from an OAL destination to an OAL source. The message consists of (N/16)-many (Identification, Bitmap)-tuples which include the Identification values of OAL fragments received plus a Bitmap marking the ordinal positions of individual non-first fragments received and missing.¶

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |S-Type=13|    Sub-Length=N     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-+-+-+-          Identification (0) (64 bits)           -+-+-+-+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-+-+-+-              Bitmap (0) (64 bits)               -+-+-+-+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-+-+-+-          Identification (1) (64 bits)           -+-+-+-+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-+-+-+-              Bitmap (1) (64 bits)               -+-+-+-+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           ...                                 |

Figure 33: Fragmentation Report (FRAGREP)

Sub-Type is set to 13. If multiple instances appear in OMNI options of the same message all are processed.¶
Sub-Length is set to N which must be a multiple of 16, i.e., the combined lengths of each (Identification, Bitmap) pair beginning immediately following the Sub-Length field and extending to the end of the sub-option.¶
Identification(i) includes the 8-octet Identification value found in a received OAL fragment.¶
Bitmap(i) includes a 64-bit checklist of up to 64 ordinal fragments for this Identification, with each bit set to 1 for a fragment received or 0 for a fragment corrupted, lost or still in transit. For example, for a 20-fragment OAL packet with ordinal fragments #3, #10, #13 and #17 missing or corrupted and all other fragments received or still in transit, Bitmap(i) encodes the following:¶
```
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
     |1|1|1|0|1|1|1|1|1|1|0|1|1|0|1|1|1|0|1|1|0|0|0|...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
```
Figure 34

12.2.15. ICMPv6 Error

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |S-Type=14|     Sub-length=N    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Type      |     Code      |           Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                    ICMPv6 Error Message Body                  ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 35: ICMPv6 Error

Sub-Type is set to 14. If multiple instances appear in OMNI options of the same IPv6 ND message all are processed.¶
Sub-Length is set to N that encodes the number of Sub-Option Data octets that follow.¶
Sub-Option Data includes an N-octet ICMPv6 Error Message body encoded exactly as per Section 2.1 of [RFC4443], i.e., with the IPv6 header omitted. OMNI interfaces include as much of the "packet in error" in the ICMPv6 error message body as possible without causing the IPv6 ND message that includes the OMNI option to exceed the IPv6 minimum MTU. While all ICMPv6 error message types are supported, OAL destinations in particular often include ICMPv6 PTB messages in uNA messages to provide MTU feedback information via the OAL source (see: Section 6.9). Note: ICMPv6 informational messages must not be included and must be ignored if received.¶

12.2.16. Proxy/Server Departure

OMNI Clients include a Proxy/Server Departure sub-option in RS messages when they associate with a new FHS and/or MAP Proxy/Server and need to send a departure indication to an old FHS and/or MAP Proxy/Server. The Proxy/Server Departure sub-option is formatted as shown below:¶

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |S-Type=15|   Sub-length=32     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                Old FHS Proxy/Server GUA (16 octets)           ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                Old MAP Proxy/Server GUA (16 octets)           ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 36: Proxy/Server Departure

Sub-Type is set to 15. If multiple instances appear in OMNI options of the same message, the first is processed and all others are ignored.¶
Sub-Length is set to 32.¶
Sub-Option Data contains the 16-octet GUA for the "Old FHS Proxy/Server" followed by a 16-octet GUA for an "Old MAP Proxy/Server. (If the Old FHS/MAP is a different node, the corresponding GUA includes the address of the (foreign) Proxy/Server. If the Old FHS/MAP is the local node, the corresponding GUA includes the node's own address. If the FHS/MAP is unspecified, the corresponding GUA instead includes the value 0.)¶

12.2.17. Sub-Type Extension

Since the Sub-Type field is only 5 bits in length, future specifications of major protocol functions may exhaust the remaining Sub-Type values available for assignment. This document therefore defines Sub-Type 30 as an "extension", meaning that the actual sub-option type is determined by examining a 1-octet "Extension-Type" field immediately following the Sub-Length field. The Sub-Type Extension is formatted as shown in Figure 37:¶

                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |S-Type=30|     Sub-length=N    | Extension-Type|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                       Extension-Type Body                     ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 37: Sub-Type Extension

Sub-Type is set to 30. If multiple instances appear in OMNI options of the same message all are processed, where each individual extension defines its own policy for processing multiple of that type.¶
Sub-Length is set to N that encodes the number of Sub-Option Data octets that follow. The Extension-Type field is always present, and the maximum Extension-Type Body length is limited by the remaining available space in this OMNI option.¶
Extension-Type contains a 1-octet Sub-Type Extension value between 0 and 255.¶
Extension-Type Body contains an (N-1)-octet block with format defined by the given extension specification.¶

Extension-Type values 0 and 1 are defined in the following subsections, while Extension-Type values 2 through 252 are available for assignment by future specifications which must also define the format of the Extension-Type Body and its processing rules. Extension-Type values 253 and 254 are reserved for experimentation, as recommended in [RFC3692], and value 255 is reserved by IANA.¶

12.2.17.1. RFC4380 Header Extension Option

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |S-Type=30|      Sub-length=N   |   Ext-Type=0  |   Header Type |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                      Header Option Value                      ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 38: RFC4380 Header Extension Option (Extension-Type 0)

Sub-Type is set to 30.¶
Sub-Length is set to N that encodes the number of Sub-Option Data octets that follow. The Extension-Type and Header Type fields are always present, and the Header Option Value is limited by the remaining available space in this OMNI option.¶
Extension-Type is set to 0. Each instance encodes exactly one header option per Section 5.1.1 of [RFC4380], with Ext-Type and Header Type representing the first 2 octets of the option. If multiple instances of the same Header Type appear in OMNI options of the same message the first instance is processed and all others are ignored.¶
Header Type and Header Option Value are coded exactly as specified in Section 5.1.1 of [RFC4380]; the following types are currently defined:¶
- 0 - Origin Indication (IPv4) - value coded as a UDP port number followed by a 4-octet IPv4 address both in "obfuscated" form per Section 5.1.1 of [RFC4380].¶
- 1 - Authentication Encapsulation - value coded per Section 5.1.1 of [RFC4380].¶
- 2 - Origin Indication (IPv6) - value coded as a UDP port number followed by an IP address both in "obfuscated" form per Section 5.1.1 of [RFC4380], except that the IP address is a 16-octet IPv6 address instead of a 4-octet IPv4 address.¶
Header Type values 3 through 252 are available for assignment by future specifications, which must also define the format of the Header Option Value and its processing rules. Header Type values 253 and 254 are reserved for experimentation, as recommended in [RFC3692], and value 255 is reserved by IANA.¶

12.2.17.2. RFC6081 Trailer Extension Option

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |S-Type=30|      Sub-length=N   |   Ext-Type=1  |  Trailer Type |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                     Trailer Option Value                      ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 39: RFC6081 Trailer Extension Option (Extension-Type 1)

Sub-Type is set to 30.¶
Sub-Length is set to N that encodes the number of Sub-Option Data octets that follow. The Extension-Type and Trailer Type fields are always present, and the maximum-length Trailer Option Value is limited by the remaining available space in this OMNI option.¶
Extension-Type is set to 1. Each instance encodes exactly one trailer option per Section 4 of [RFC6081]. If multiple instances of the same Trailer Type appear in OMNI options of the same message the first instance is processed and all others ignored.¶
Trailer Type and Trailer Option Value are coded exactly as specified in Section 4 of [RFC6081]; the following Trailer Types are currently defined:¶
- 0 - Unassigned¶
- 1 - Nonce Trailer - value coded per Section 4.2 of [RFC6081].¶
- 2 - Unassigned¶
- 3 - Alternate Address Trailer (IPv4) - value coded per Section 4.3 of [RFC6081].¶
- 4 - Neighbor Discovery Option Trailer - value coded per Section 4.4 of [RFC6081].¶
- 5 - Random Port Trailer - value coded per Section 4.5 of [RFC6081].¶
- 6 - Alternate Address Trailer (IPv6) - value coded per Section 4.3 of [RFC6081], except that each address is a 16-octet IPv6 address instead of a 4-octet IPv4 address.¶
Trailer Type values 7 through 252 are available for assignment by future specifications, which must also define the format of the Trailer Option Value and its processing rules. Trailer Type values 253 and 254 are reserved for experimentation, as recommended in [RFC3692], and value 255 is reserved by IANA.¶

15. Router Discovery and Prefix Delegation

Clients engage their FHS Proxy/Servers and the MS by sending RS messages with OMNI options under the assumption that one or more Proxy/Server will process the message and respond. The RS message is received by a FHS Proxy/Server, which in turn forwards a proxyed copy to a MAP Proxy/Server located on the same or different SRT segment if the Client requires MNP service. The MAP Proxy/Server then returns an RA message either directly to the Client or via the original FHS Proxy/Server acting as a proxy.¶

To support Client to service coordination, OMNI defines flag bits in the OMNI Neighbor Control sub-option discussed in Section 12.2.5. Clients set or clear the NUD, ARR and/or RPT flags in RS messages as directives to the Mobility Service FHS/MAP Proxy/Servers. Proxy/Servers interpret the flags as follows:¶

When an FHS Proxy/Server forwards or processes an RS with the NUD flag set, it responds directly to future NS Neighbor Unreachability Detection (NUD) messages with the Client as the target by returning NA(NUD) replies; otherwise, it forwards NS(NUD) messages to the Client.¶
When the MAP Proxy/Server receives an RS with the ARR flag set, it responds directly to future NS Address Resolution (AR) messages with the Client as the target by returning NA(AR) replies; otherwise, it forwards NS(AR) messages to the Client.¶
When the MAP Proxy/Server receives an RS with the RPT flag set, it maintains a Report List of recent NS(AR) message sources for the source or target Client and sends uNA messages to all list members if any aspects of the Client's underlay interfaces change.¶

Mobility Service Proxy/Servers function according to the NUD, ARR and RPT flag settings received in the most recent RS message to support dynamic Client updates.¶

Clients and FHS Proxy/Servers include an authentication signature as an OMNI sub-option in their RS/RA exchanges when necessary but always include a valid IPv6 ND message checksum as the final step. FHS and MAP Proxy/Server RS/RA message exchanges over the SRT secured spanning tree instead always include the checksum and omit the authentication signature. Clients and Proxy/Servers use the information included in RS/RA messages to establish NCE state and OMNI link autoconfiguration information as discussed in this section.¶

For each underlay interface, the Client sends RS messages with OMNI options to coordinate with a (potentially) different FHS Proxy/Server for each interface but typically with a limited set of MAP Proxy/Servers (often only one). All Proxy/Servers are identified by their ULA/GUA SRA addresses and accept carrier packets addressed to their anycast/unicast L2ADDRs; the MAP Proxy/Server may be chosen among any of the Client's FHS Proxy/Servers or may be any other Proxy/Server for the OMNI link. Example ULA/L2ADDR discovery methods appear in [RFC5214] and include data link login parameters, name service lookups, static configuration, a DHCP option, a static "hosts" file, etc. In the absence of other information, the Client can resolve the DNS Fully-Qualified Domain Name (FQDN) "linkupnetworks.[domainname]" where "linkupnetworks" is a constant text string and "[domainname]" is a DNS suffix for the OMNI link (e.g., "example.com"). The name resolution will return a set of DNS resource records with the addresses of Proxy/Servers for the local OMNI link segment. When the underlay *NET does not support standard unicast server-based name resolution [RFC1035] the Client can engage a multicast service such as mDNS [RFC6762] within the local OMNI link segment.¶

Each FHS Proxy/Server configures a SNP SRA ULA/GUA address pair for the local link segment and advertises the GUA/L2ADDR combination for discovery as above. The Client can then manage its own SNP ULA/GUA addresses through DHCPv6 address autoconfiguration exchanges with FHS Proxy/Servers. The FHS Proxy/Servers discovered over multiple of the Client's underlay interfaces may configure the same or different SNP SRA ULAs/GUAs, and the Client's ULA for each underlay interface will fall within the ULA OMNI link segment relative to each FHS Proxy/Server.¶

Clients configure OMNI interfaces that observe the properties discussed in previous sections. The OMNI interface and its underlay interfaces are said to be in either the "UP" or "DOWN" state according to administrative actions in conjunction with the interface connectivity status. An OMNI interface transitions to UP/DOWN through administrative action and/or through underlay interface state transitions. When a first underlay interface transitions to UP, the OMNI interface also transitions to UP. When all underlay interfaces transition to DOWN, the OMNI interface also transitions to DOWN.¶

When a Client OMNI interface transitions to UP, it sends RS messages to register an initial set of underlay interfaces that are also UP and to optionally register/request an MNP. The Client sends additional RS messages to refresh lifetimes and to register/deregister underlay interfaces as they transition to UP or DOWN. The Client's OMNI interface sends initial RS messages over an UP underlay interface with source set to a ULA delegation for the local OMNI link segment if it has one (otherwise with source set to the unspecified address "::" per [RFC4861]) and with destination set to either the SRA GUA of a specific (MAP) Proxy/Server or link-scoped All-Routers multicast. The Client includes an OMNI option per Section 12 with a Neighbor Control sub-option with the RS NUD, ARR and RPT flags set or cleared as necessary and a Multilink Vector sub-option with "initiator ifIndex" set to the ifIndex of its own underlay interface and with "responder ifIndex" set to 0 (i.e., the default ifIndex configured by all Proxy/Serves). The Client also includes a DHCPv6 Solicit sub-option with IA_NA and (optionally) IA_PD DHCPv6 options, an Interface Attributes sub-option for the underlay interface, and with any other necessary OMNI sub-options such as authentication, Proxy/Server Departure, etc. The OMNI interface finally sets or clears the Interface Attributes FMT-Forward and FMT-Mode bits according to the behavior it would like to receive from the FHS Proxy/Server as described in Section 12.2.5.¶

The Client next prepares to forward the RS over the underlay interface using OAL encapsulation. The OMNI interface first includes a Nonce and/or Timestamp if necessary, then calculates and sets the authentication signature followed by the RS message checksum. The OMNI interface next selects an Identification value (see: Section 6.7), sets the OAL source address to its SNP ULA previously delegated by a Proxy/Server (otherwise to its HHIT), sets the OAL destination to fd00::/8 (i.e., the generic ULA SRA address) or to a known Proxy/Server SRA ULA, then performs OAL fragmentation if necessary. When L2 encapsulation is used, the Client next includes the discovered FHS Proxy/Server L2ADDR or an anycast address as the L2 destination then fragments if necessary and forwards the resulting carrier packet(s) into the underlay network. Note that the Client does not yet create a NCE, but instead caches the Identification, Nonce and/or Timestamp values included in its RS message transmissions to match against any received RA messages.¶

When an FHS Proxy/Server receives the carrier packets containing an RS it reassembles if necessary, sets aside the L2 headers, verifies the Identifications, performs OAL reassembly if necessary, sets aside the OAL header, then verifies the RS checksum/authentication signature. The FHS Proxy/Server then creates/updates a NCE indexed by the RS ULA source address unless unspecified; otherwise, indexed by the OAL source address. The FHS Proxy/Server then caches the OMNI Interface Attributes and any Traffic Selector sub-options while also caching the L2 (UDP/IP) and OAL source and destination address information. The FHS Proxy/Server then examines the OMNI DHCPv6 sub-option and looks for DHCPv6 IA_NA options. If any IA_NA options are present, the FHS Proxy/Server coordinates with the local DHCPv6 server to either allocate new SNP GUA/ULA pairs or extend the lease lifetime for existing SNP GUA/ULA pairs for the Client. The FHS Proxy/Server next caches the SNP GUA/ULA in the (newly-created) NCE, then caches the RS Neighbor Control NUD flag and Multilink Vector parameters (see: Section 12.1) and examines the RS destination address.¶

If the destination matches one of its own unicast/anycast addresses and the OMNI DHCPv6 sub-option includes one or more DHCPv6 IA_PD options, the FHS Proxy/Server assumes the MAP role and acts as a default router entry point for injecting the Client's MNP(s) into the OMNI link routing system (i.e., after performing any necessary prefix delegation operations). The FHS/MAP Proxy/Server then caches the RS ARR and RPT flags to determine its role in processing NS(AR) messages and generating uNA messages (see: Section 12.1).¶

The FHS/MAP Proxy/Server then prepares to return an RA message directly to the Client by first populating the Cur Hop Limit, Flags, Router Lifetime, Reachable Time and Retrans Timer fields with values appropriate for the OMNI link. The FHS/MAP Proxy/Server next includes as the first RA message option an OMNI option with a Neighbor Control sub-option and a Multilink Vector sub-option with its own chosen AFVI and with responsive window synchronization information. The FHS/MAP Proxy/Server also includes an authentication sub-option if necessary and a (proxyed) copy of the Client's original Interface Attributes sub-option with its INET-facing interface information written in the FMT, SRT and LHS Proxy/Server GUA/L2ADDR fields. The Proxy/Server also includes a DHCPv6 Reply sub-option with any IA_NA/IA_PD options that have been processed/populated by the DHCPv6 exchange(s).¶

The FHS/MAP Proxy/Server next sets or clears the FMT-Forward and FMT-Mode flags if necessary to convey its capabilities to the Client, noting that it should honor the Client's stated preferences for those parameters if possible or override otherwise. The FMT-Forward/Mode flags thereafter remain fixed unless and until a new RS/RA exchange establishes different values (see: Section 12.2.5 for further discussion). If the FHS/MAP Proxy/Server's Client-facing interface is different than its INET-facing interface, the Proxy/Server next includes a second Interface Attributes sub-option with ifIndex set to '0', with a unicast L2 address for its Client-facing interface in the L2ADDR field and with its SRA ULA in the GUA field.¶

The FHS/MAP Proxy/Server next includes an Origin Indication sub-option that includes the RS L2 source L2ADDR information (see: Section 12.2.17.1), then includes any other necessary OMNI sub-options (either within the same OMNI option or in additional OMNI options). Following the OMNI option(s), the FHS/MAP Proxy/Server next includes any other necessary RA options including two PIOs with (A=0; L=0) that include the ULA/GUA SNP prefixes for the segment per [RFC8028], RIOs with more-specific routes per [RFC4191], Nonce and Timestamp options, etc. The FHS/MAP Proxy/Server then sets the RA source address to its own SRA GUA and destination address to the (new) SNP ULA for the Client, then calculates the authentication signature/checksum. The FHS/MAP Proxy/Server finally performs OAL encapsulation while setting the source to its own ULA and destination to the OAL source that appeared in the RS, then includes an appropriate Identification, performs OAL fragmentation if necessary, performs L2 encapsulation/fragmentation with L2 source and destination address information reversed from the RS L2 information and returns the resulting carrier packets to the Client over the same underlay interface the RS arrived on.¶

When an FHS Proxy/Server receives an RS with a valid checksum and authentication signature with destination set to link-scoped All-Routers multicast, it can either assume the MAP role itself the same as above or act as a proxy and select the SNP SRA GUA of another Proxy/Server to serve as the MAP. When an FHS Proxy/Server assumes the proxy role or receives an RS with destination set to the SNP SRA GUA of another Proxy/Server, it forwards the message as a proxy. The FHS Proxy/Server creates or updates a NCE for the Client (i.e., based on the RS source address) and caches the OAL source, Neighbor Control, Multilink Vector and Interface Attributes addressing information as above. The FHS Proxy/Server then locally processes any DHCPv6 IA_NA options found in the RS OMNI option and assigns the SNP ULA/GUA address pairs to the Client NCE. The FHS Proxy/Server then writes its own INET-facing FMT, SRT and LHS Proxy/Server GUA/L2ADDR information into the appropriate Interface Attributes sub-option fields (while also setting/clearing FMT-Forward and FMT-Type as above) where the GUA is the Client's SNP GUA address. Next, the FHS Proxy/Server removes the Multilink Vector sub-option, sets the RS source address to the Client's SNP GUA and sets the RS destination to the SNP SRA address of the MAP Proxy/Server. The FHS Proxy/Server then calculates and includes the checksum, performs OAL encapsulation while setting the source to the Client's SNP GUA and destination SNP SRA GUA of the MAP Proxy/Server, includes an appropriate Identification, performs OAL fragmentation, performs L2 encapsulation/fragmentation and sends the resulting carrier packets into the SRT secured spanning tree.¶

When the MAP Proxy/Server receives the carrier packets, it performs L2 reassembly/decapsulation, performs OAL reassembly/decapsulation to obtain the proxyed RS, verifies the checksum, then performs DHCPv6 Prefix Delegation (PD) to obtain or update any MNPs for the Client. The MAP Proxy/Server then creates/updates a NCE for the Client's MNP(s) and caches any state (including the ARR and RPT flags, IA_NA addresses, OAL addresses, Interface Attributes information and Traffic Selectors), then finally performs routing protocol injection. The MAP Proxy/Server then returns an RA that echoes the Client's (proxyed) Interface Attributes sub-option and with any RA parameters the same as specified for the FHS/MAP Proxy/Server case above. The MAP Proxy/Server sets the RA source address to its own SNP SRA GUA and destination address to the RS source address (i.e., the Client SNP GUA). The MAP Proxy/Server next calculates the checksum then encapsulates the RA as an OAL packet with source set to its own SNP SRA GUA and destination set to the SNP SRA GUA of the FHS Proxy/Server. The MAP Proxy/Server finally includes an appropriate Identification, performs OAL fragmentation followed by L2 encapsulation/fragmentation and sends the resulting carrier packets into the secured spanning tree.¶

When the FHS Proxy/Server receives the carrier packets it performs L2 reassembly/decapsulation followed by OAL reassembly/decapsulation to obtain the RA message, verifies checksums then updates the OMNI interface NCE for the Client and creates/updates a NCE for the MAP. The FHS Proxy/Server then sets the P flag in the RA flags field [RFC4389] and proxys the RA by changing the OAL source to its SNP SRA GUA and changing the OAL destination to the source address from the Client's original RS message while also recording any DHCPv6 IA_NA SNP GUA/ULA address pairs as alternate indexes into the Client NCE. The FHS Proxy/Server then includes two PIOs with (A=0; L=0) with the SNP ULA/GUA prefixes for the segment per [RFC8028]. The FHS Proxy/Server next includes Neighbor Control parameters responsive to those in the Client's RS and a Multilink Vector sub-option with its responses to its cached initiations for the Client. The FHS Proxy/Server also includes an Interface Attributes sub-option with ifIndex '0' and with its Client-facing interface unicast L2 address if necessary (see above), an Origin Indication sub-option with the Client's cached L2ADDR and an authentication sub-option if necessary. The FHS Proxy/Server finally includes an Identification value per Section 6.7, calculates the authentication signature/checksum, performs OAL fragmentation, performs L2 encapsulation/fragmentation with addresses taken from the Client's NCE and sends the resulting carrier packets via the same underlay interface over which the RS was received.¶

When the Client receives the carrier packets, it performs L2 reassembly/decapsulation followed by OAL reassembly/decapsulation to obtain the RA message. The Client next verifies the authentication signature/checksum, then matches the RA message with its previously-sent RS by comparing the RS Sequence Number with the RA Acknowledgement Number and also comparing the Nonce and/or Timestamp values if present. If the values match, the Client then creates/updates OMNI interface NCEs for both the MAP and FHS Proxy/Server and caches the information in the RA message. The Client also caches the RA source address as the MAP Proxy/Server SNP SRA GUA and uses the OAL source address to configure the SNP SRA ULA of this FHS Proxy/Server. The Client next discovers its own SNP ULA by examining the RA destination address, discovers its own SNP GUA by examining the IA_NA DHCPv6 delegated addresses, and discovers the SNP ULA/GUA PIO prefixes for the OMNI link segment per [RFC8028]. If the Client has multiple underlay interfaces, it creates additional FHS Proxy/Server NCEs as necessary when it receives RAs over those interfaces (noting that multiple of the Client's underlay interfaces may be serviced by the same or different FHS Proxy/Servers). The Client finally adds the MAP Proxy/Server SRA GUA to the default router list if necessary.¶

For each underlay interface, the Client next caches the (filled-out) Interface Attributes for its own ifIndex and Origin Indication information that it received in an RA message over that interface so that it can include them in future NS/NA messages to provide neighbors with accurate FMT/SRT/LHS information. (If the message includes an Interface Attributes sub-option with ifIndex '0', the Client also caches the L2ADDR as the underlay network-local unicast address of the FHS Proxy/Server via that underlay interface.) The Client then compares the Origin Indication L2ADDR information with its own underlay interface addresses to determine whether there may be NATs on the path to the FHS Proxy/Server; if the L2ADDR information differs, the Client is behind one or more NATs and must supply the Origin information in IPv6 ND message exchanges with prospective neighbors on the same SRT segment. The Client then caches the Multilink Vector responsive window synchronization parameters and AFVI for use in future carrier packet exchanges that use the FHS Proxy/Server for this underlay interface as a router. The Client finally configures default routes and assigns the IPv6 SRA address corresponding to the MNP (e.g., 2001:db8:1:2::) to the OMNI interface.¶

Following the initial exchange, the FHS Proxy/Server MAY later send additional periodic and/or event-driven unsolicited RA messages per [RFC4861]. (The unsolicited RAs may be initiated either by the FHS Proxy/Server itself or by the MAP via the FHS as a proxy.) The Client then continuously manages its underlay interfaces according to their states as follows:¶

When an underlay interface transitions to UP, the Client sends an RS over the underlay interface with an OMNI option with sub-options as specified above.¶
When an underlay interface transitions to DOWN, the Client sends unsolicited NA messages over any UP underlay interface with an OMNI option containing Interface Attributes sub-options for the DOWN underlay interface with ifMetric set to 'ffffffff'. The Client sends isolated unsolicited NAs when reliability is not thought to be a concern (e.g., if redundant transmissions are sent on multiple underlay interfaces), or may instead set the SNR flag in an OMNI Neighbor Control sub-option to trigger an unsolicited NA reply (see: [I-D.templin-intarea-aero2]).¶
When the Router Lifetime for the MAP Proxy/Server nears expiration, the Client sends an RS over any underlay interface to receive a fresh RA from the MAP. If no RA messages are received over a first underlay interface (i.e., after retrying), the Client marks the underlay interface as DOWN and should attempt to contact the MAP Proxy/Server via a different underlay interface. If the MAP Proxy/Server is unresponsive over additional underlay interfaces, the Client sends an RS message with destination set to the ULA of another Proxy/Server which will then assume the MAP role.¶
When all of a Client's underlay interfaces have transitioned to DOWN (or if a prefix delegation lifetime expires), the MAP Proxy/Server withdraws the MNP the same as if it had received a message with a release indication.¶

The Client is responsible for retrying each RS exchange up to MAX_RTR_SOLICITATIONS times separated by RTR_SOLICITATION_INTERVAL seconds until an RA is received. If no RA is received over an UP underlay interface (i.e., even after attempting to contact alternate Proxy/Servers), the Client declares this underlay interface as DOWN. When changing to a new FHS/MAP Proxy/Server, the Client also includes a Proxy/Server Departure OMNI sub-option in new RS messages; the (new) FHS Proxy/Server will in turn send uNA messages to the old FHS and/or MAP Proxy/Server to announce the Client's departure as discussed in [I-D.templin-intarea-aero2].¶

The network layer sees the OMNI interface as an ordinary IPv6 interface. Therefore, when the network layer sends an RS message the OMNI interface returns an internally-generated RA message as though the message originated from an IPv6 router. The internally-generated RA message contains configuration information consistent with the information received from the RAs generated by the MAP Proxy/Server. Whether the OMNI interface IPv6 ND messaging process is initiated from the receipt of an RS message from the network layer or independently of the network layer is an implementation matter. Some implementations may elect to defer the OMNI interface internal RS/RA messaging process until an RS is received from the network layer, while others may elect to initiate the process independently. Still other deployments may elect to administratively disable network layer RS/RA messaging over the OMNI interface, since the messages are not required to drive the OMNI interface internal RS/RA process. (Note that this same logic applies to IPv4 implementations that employ "ICMP Router Discovery" [RFC1256].)¶

Note: The Router Lifetime value in RA messages indicates the time before which the Client must send another RS message over this underlay interface (e.g., 600 seconds), however that timescale may be significantly longer than the lifetime the MS has committed to retain the prefix registration (e.g., REACHABLE_TIME seconds). Proxy/Servers are therefore responsible for keeping MS state alive on a shorter timescale than the Client may be required to do on its own behalf.¶

Note: On certain multicast-capable underlay interfaces, Clients should send periodic unsolicited multicast NA messages and Proxy/Servers should send periodic unsolicited multicast RA messages as "beacons" that can be heard by other nodes on the link. If a node fails to receive a beacon after a timeout value specific to the link, it can initiate Neighbor Unreachability Detection (NUD) exchanges to test reachability.¶

Note: If a single FHS Proxy/Server services multiple of a Client's underlay interfaces, window synchronization will initially be repeated for the RS/RA exchange over each underlay interface, i.e., until the Client discovers the many-to-one relationship. This will naturally result in a single window synchronization that applies over the Client's multiple underlay interfaces for the same FHS Proxy/Server.¶

Note: Although the Client's FHS Proxy/Server is a first-hop segment node from its own perspective, the Client stores the Proxy/Server's FMT/SRT/GUA/L2ADDR as last-hop segment (LHS) information to supply to neighbors. This allows both the Client and MAP Proxy/Server to supply the information to neighbors that will perceive it as LHS information on the return path to the Client.¶

Note: The MAP Proxy/Server injects Client MNPs into the OMNI link routing system by simply creating a route-to-interface forwarding table entry for MNP::/N via the OMNI interface. The dynamic routing protocol will notice the new entry and propagate the route to its peers. If the MAP receives additional RS messages, it need not re-create the forwarding table entry (nor disturb the dynamic routing protocol) if an entry is already present. If the MAP ceases to receive RS messages from any of the Client's interfaces, it removes the Client MNP(s) from the forwarding table (i.e., after a short delay) which also results in their removal from the routing system.¶

Note: If the Client's initial RS message includes an anycast L2 destination address, the FHS Proxy/Server returns the solicited RA using the same anycast address as the L2 source while including an Interface Attributes sub-option with ifIndex '0' and its true unicast address in the L2ADDR. When the Client sends additional RS messages, it includes this FHS Proxy/Server unicast address as the L2 destination and the FHS Proxy/Server returns the solicited RA using the same unicast address as the L2 source. This will ensure that RS/RA exchanges are not impeded by any NATs on the path while avoiding long-term exposure of messages that use an anycast address as the source.¶

Note: The Origin Indication sub-option is included only by the FHS Proxy/Server and not by the MAP (unless the MAP is also serving as an FHS).¶

Note: Clients should set the NUD, ARR and RPT flags consistently in successive RS messages and only change those settings when an FHS/MAP Proxy/Server service profile update is necessary.¶

Note: Although the Client adds the MAP Proxy/Server SNP GUA SRA address to the default router list, it also caches the ULAs of the FHS Proxy/Servers on the path to the MAP over each underlying interface. When the Client needs to send an original IP packet/parcel to a default router, it engages OAL encapsulation/fragmentation while using a destination ULA corresponding to the selected interface which directs the packet to an FHS Proxy/Server for that interface. The FHS Proxy/Server then performs L2 encapsulation/fragmentation and sends the resulting carrier packets without disturbing the MAP.¶

15.1. Window Synchronization

The RS/RA exchanges discussed above observe the principles specified in Section 6.7. Window synchronization is conducted between the Client and each FHS Proxy/Server used to contact the MAP Proxy/Server, i.e., and not between the Client and the MAP. This is due to the fact that the MAP Proxy/Server is responsible only for forwarding control and data messages via the secured spanning tree to FHS Proxy/Servers, and is not responsible for forwarding messages directly to the Client under a synchronized window over the unsecured spanning tree. Also, in the reverse direction the FHS Proxy/Servers handle all default forwarding actions without forwarding Client-initiated data to the MAP.¶

When a Client sends an RS to perform window synchronization via a new FHS Proxy/Server, it includes an OMNI Multilink Vector sub-option with window synchronization parameters with the SYN flag set and ACK flag clear, and with an initial Sequence Number for Window Synchronization. The Client then sets the RS source address to its own MNP SRA GUA or SNP ULA and destination address to the SNP SRA ULA of the MAP Proxy/Server (or to All-Routers multicast in an initial RS). The Client then performs OAL encapsulation/fragmentation using its own ULA/HHIT as the source and the ULA of the FHS Proxy/Server (or ULA anycast address) as the destination. The Client finally includes an Interface Attributes sub-option then performs L2 encapsulation/fragmentation and sends the resulting carrier packets to the FHS Proxy/Server. When the FHS Proxy/Server receives the carrier packets, it performs L2 and OAL reassembly/decapsulation to extract the RS message and caches the Multilink Vector parameters then re-encapsulates/fragments with its own SNP SRA GUA as the OAL source and the SNP SRA GUA of the MAP Proxy/Server as the OAL destination. In the process, the FHS Proxy/Server removes the Multilink Vector sub-option itself, since the path to the MAP Proxy/Server is not included in window synchronization.¶

The FHS Proxy/Server then performs L2 encapsulation/fragmentation and sends the resulting carrier packets via the secured spanning tree to the MAP Proxy/Server, which updates the Client's Interface Attributes and returns a unicast RA message with source set to its own SNP SRA GUA and destination set to the RS source address and with the Client's Interface Attributes echoed. The MAP Proxy/Server then performs OAL encapsulation/fragmentation using its own SNP SRA GUA as the source and the SNP SRA GUA of the FHS Proxy/Server as the destination, then performs L2 encapsulation/fragmentation and sends the carrier packets via the secured spanning tree to the FHS Proxy/Server. The FHS Proxy/Server then proxys the message as discussed in the previous section and includes responsive window synchronization information. The FHS Proxy/Server then forwards the message to the Client via OAL encapsulation with SNP ULAs which updates its window synchronization information for the FHS Proxy/Server as necessary.¶

Following the initial RS/RA-driven window synchronization, the Client can re-assert new windows with specific FHS Proxy/Servers by performing RS/RA exchanges between its own ULAs and the ULAs of the FHS Proxy/Servers at any time without having to disturb the MAP (when the Client needs to refresh MAP state, it can set the RS destination address to the MAP SNP SRA address).¶

15.2. Router Discovery in IP Multihop and IPv4-Only Networks

On some *NETs, a Client may be located multiple intermediate OAL hops away from the nearest OMNI link Proxy/Server. Clients in multihop networks perform route discovery through the application of a routing protocol (e.g., a MANET/VANET routing protocol over omnidirectional wireless interfaces, an inter-domain routing protocol in an enterprise network, etc.) then apply corresponding forwarding entries to the OMNI interface. Example routing protocols optimized for MANET/VANET operations include OSPFv3 [RFC5340] with MANET Designated Router (OSPF-MDR) extensions [RFC5614], OLSRv2 [RFC7181], AODVv2 [I-D.perkins-manet-aodvv2] and others. Clients employ the routing protocol according to the link model found in [RFC5889] and subnet model articulated in [RFC5942]. For unique identification, Clients use an HHIT as a Router ID or set an administrative ULA value that is managed for uniqueness within the MANET/VANET.¶

A Client located potentially multiple OAL hops away from the nearest Proxy/Server prepares an RS message, sets the source address to its ULA or unspecified, and sets the destination to link-scoped All-Routers multicast or the SNP SRA GUA of a Proxy/Server the same as discussed above. The OMNI interface then employs OAL encapsulation, sets the OAL source address to its HHIT/ULA and sets the OAL destination to an SNP SRA ULA or a 6to4-encapsulated IPv4 address (see: Section 10).¶

For IPv6-enabled *NETs where the underlay interface observes the MANET properties discussed above, the Client injects the HHIT/ULA into the IPv6 multihop routing system and forwards the message without further encapsulation. Otherwise, the Client encapsulates the message in UDP/IPv6 L2 headers, sets the source to the underlay interface IPv6 address and sets the destination to the same SNP SRA ULA address. The Client then forwards the message into the IPv6 multihop routing system which conveys it to the nearest Proxy/Server that advertises a matching SNP SRA ULA address. If the nearest Proxy/Server is too busy, it should forward (without Proxying) the OAL-encapsulated RS to another nearby Proxy/Server connected to the same IPv6 (multihop) network that also advertises the matching OMNI IPv6 anycast prefix.¶

For IPv4-only *NETs, the Client encapsulates the RS message in UDP/IPv4 L2 headers, sets the source to the underlay interface IPv4 address and sets the destination to the OMNI IPv4 anycast address TBD3. The Client then forwards the message into the IPv4 multihop routing system which conveys it to the nearest Proxy/Server that advertises the corresponding IPv4 prefix. If the nearest Proxy/Server is too busy and/or does not configure the specified OMNI IPv6 SRA address, it should forward (without Proxying) the OAL-encapsulated RS to another nearby Proxy/Server connected to the same IPv4 (multihop) network that configures the OMNI IPv6 anycast address. (In environments where reciprocal RS forwarding cannot be supported, the first Proxy/Server should instead return an RA based on its own MSP(s).)¶

When an OAL intermediate node that participates in the routing protocol receives the encapsulated RS, it caches any window synchronization and AFVI information, rewrites the RS AFVI with a new value for the next OAL hop, and forwards the message according to its routing tables (note that an OAL intermediate system could be a fixed infrastructure element such as a roadside unit or another MANET/VANET Client). This process repeats iteratively until the RS message is received by a penultimate OAL hop within single-hop communications range of a Proxy/Server, which forwards the message to the Proxy/Server final hop.¶

When a Proxy/Server that configures the OMNI IPv6 anycast OAL destination receives the message, it decapsulates the RS and assumes either the MAP or FHS role (in which case, it may forward the RS to a candidate MAP). The MAP/FHS Proxy/Server then prepares an RA message using the same addressing disciplines as discussed in Section 15 and forwards the RA either to the FHS Proxy/Server or directly to the Client.¶

When the MAP or FHS Proxy/Server forwards the RA to the Client, it encapsulates the message in L2 encapsulation headers (if necessary) with (src, dst) set to the (dst, src) of the RS L2 encapsulation headers. The Proxy/Server then forwards the message to an OAL node within communications range, which caches the window synchronization and AFVI information and forwards the message according to the next OAL hop according to its routing tables while rewriting the AFVI to provide next hop forwarding state. The multihop forwarding process within the *NET continues repetitively until the message arrives at the original Client, which decapsulates the message and performs autoconfiguration the same as if it had received the RA directly from a Proxy/Server on the same physical link. The Client then injects the delegated ULA/MNP(s) into the IPv6 multihop routing system.¶

Note: When the RS message includes anycast OAL and/or L2 encapsulation destinations, the FHS Proxy/Server must use the same anycast addresses as the OAL and/or L2 encapsulation sources to support forwarding of the RA message plus any initial data messages. The FHS Proxy/Server then sends the resulting carrier packets over any NATs on the path. When the Client receives the RA, it will discover the FHS Proxy/Server unicast ULAs and/or L2 encapsulation addresses and can send future carrier packets using the unicast (instead of anycast) addresses to populate NAT state in the forward path. (If the Client does not have immediate data to send to the FHS Proxy/Server, it can instead send an OAL "bubble" - see Section 6.11.) After the Client begins using unicast OAL/L2 encapsulation addresses in this way, the FHS Proxy/Server should also begin using the same unicast addresses in the reverse direction.¶

Note: When an OMNI interface configures a HHIT, any nodes that forward an encapsulated RS message with the HHIT as the OAL source must not consider the message as being specific to a particular OMNI link segment. HHITs can therefore also serve as the source and destination addresses of unencapsulated IPv6 data communications within the local routing region, and if the HHITs are injected into the local network routing protocol their prefix length must be set to 128.¶

Note: Each node normally conducts the multi-hop relaying between intermediate forwarding systems using the same underlay interface in both the inbound and outbound directions, i.e. as opposed to different underlay interfaces. The final forwarding node within range of a Proxy/Server could use the same or a different underlay interface to exchange carrier packets with the Proxy/Server, but may not be well positioned to perform multilink selections over multiple underlay interfaces on behalf of multihop dependent peers.¶

15.3. DHCPv6-based Prefix Registration

When a Client requires SNP ULA/GUA delegations via a specific Proxy/Server (or, when the Client requires MNP delegations for the OMNI link), it invokes the DHCPv6 service [RFC8415] in conjunction with its OMNI RS/RA message exchanges.¶

When a Client requires the MS to delegate PA ULA/GUA pairs or PI MNPs, it sends an RS message to a FHS Proxy/Server. If the Client requires one or more address or MNP delegations, it includes a DHCPv6 Message sub-option containing a Client Identifier, one or more IA_NA/IA_PD options and a Rapid Commit option then sets the 'msg-type' field to "Solicit" and includes a 3-octet 'transaction-id'. The Client then sets the RS destination to link-scoped All-Routers multicast and sends the message using OAL encapsulation and fragmentation if necessary as discussed above.¶

When the FHS/MAP Proxy/Server receives the RS message, it performs OAL reassembly if necessary. Next, if the OMNI option includes a DHCPv6 message sub-option, the FHS/MAP Proxy/Server acts as a "Proxy DHCPv6 Client" in a message exchange with the locally-resident DHCPv6 server. The FHS/MAP Proxy/Server then sends the DHCPv6 message to the DHCPv6 Server, which delegates SNP ULA/GUA pairs or MNPs and returns a DHCPv6 Reply message with autoconfiguration parameters.¶

When the FHS Proxy/Server receives a DHCPv6 Reply with delegated addresses, it records the delegated SNP ULA/GUA pairs in the NCE for the Client, then forwards the RS message to the MAP Proxy/Server for prefix delegation if necessary; otherwise, it returns an immediate RA message to the Client.¶

When the MAP Proxy/Server receives a DHCPv6 Reply with delegated prefixes, it creates OMNI interface MNP forwarding table entries (i.e., to prompt the dynamic routing protocol). The MAP Proxy/Server then sends an RA back to the FHS Proxy/Server with the DHCPv6 Reply message included in an OMNI DHCPv6 message sub-option, and the FHS Proxy/Server returns the RA to the Client.¶

15.4. OMNI Link Extension

Clients can provide an OMNI link ingress point for other nodes on their (downstream) ENETs that also act as Clients. When Client A has already coordinated with an (upstream) (M)ANET/INET Proxy/Server, Client B on an ENET serviced by Client A can send OAL-encapsulated RS messages with addresses set the same as specified in Section 15.2. When Client A receives the RS message, it infers from the OAL encapsulation that Client B is seeking to establish itself as a Client instead of just a simple ENET Host.¶

Client A then returns an RA message the same as a Proxy/Server would do as specified in Section 15.2 except that it instead uses its own MNP SRA GUA as the RA and OAL source addresses and performs (recursive) DHCPv6 Prefix Delegation. The MNP delegation in the RA message must be a sub-MNP from the MNP delegated to Client A. For example, if Client A receives the MNP 2001:db8:1000::/48 it can provide a sub-delegation such as 2001:db8:1000:2000::/56 to Client B. Client B can in turn sub-delegate 2001:db8:1000:2000::/56 to its own ENET(s), where there may be a further prospective Client C that would in turn request OMNI link services via Client B.¶

To support this Client-to-Client chaining, Clients send IPv6 ND messages addressed to the OMNI link anycast address via their *NET (i.e., upstream) interfaces, but advertise the OMNI link anycast address into their ENET (i.e., downstream) networks where there may be further prospective Clients wishing to join the chain. The ENET of the upstream Client is therefore seen as an ANET by downstream Clients, and the upstream Client is seen as a Proxy/Server by downstream Clients.¶

29. References

29.1. Normative References

[I-D.templin-6man-ipid-ext2]: Templin, F., "IPv6 Extended Fragment Header", Work in Progress, Internet-Draft, draft-templin-6man-ipid-ext2-01, 19 February 2024, <https://datatracker.ietf.org/doc/html/draft-templin-6man-ipid-ext2-01>.
[I-D.templin-6man-parcels2]: Templin, F., "IPv6 Parcels and Advanced Jumbos (AJs)", Work in Progress, Internet-Draft, draft-templin-6man-parcels2-02, 19 February 2024, <https://datatracker.ietf.org/doc/html/draft-templin-6man-parcels2-02>.
[I-D.templin-intarea-parcels2]: Templin, F., "IPv4 Parcels and Advanced Jumbos (AJs)", Work in Progress, Internet-Draft, draft-templin-intarea-parcels2-02, 19 February 2024, <https://datatracker.ietf.org/doc/html/draft-templin-intarea-parcels2-02>.
[RFC0768]: Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, August 1980, <https://www.rfc-editor.org/info/rfc768>.
[RFC0791]: Postel, J., "Internet Protocol", STD 5, RFC 791, DOI 10.17487/RFC0791, September 1981, <https://www.rfc-editor.org/info/rfc791>.
[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC2473]: Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473, December 1998, <https://www.rfc-editor.org/info/rfc2473>.
[RFC3971]: Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, DOI 10.17487/RFC3971, March 2005, <https://www.rfc-editor.org/info/rfc3971>.
[RFC4191]: Draves, R. and D. Thaler, "Default Router Preferences and More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191, November 2005, <https://www.rfc-editor.org/info/rfc4191>.
[RFC4193]: Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005, <https://www.rfc-editor.org/info/rfc4193>.
[RFC4291]: Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC4443]: Conta, A., Deering, S., and M. Gupta, Ed., "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", STD 89, RFC 4443, DOI 10.17487/RFC4443, March 2006, <https://www.rfc-editor.org/info/rfc4443>.
[RFC4861]: Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007, <https://www.rfc-editor.org/info/rfc4861>.
[RFC4862]: Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, DOI 10.17487/RFC4862, September 2007, <https://www.rfc-editor.org/info/rfc4862>.
[RFC6088]: Tsirtsis, G., Giarreta, G., Soliman, H., and N. Montavont, "Traffic Selectors for Flow Bindings", RFC 6088, DOI 10.17487/RFC6088, January 2011, <https://www.rfc-editor.org/info/rfc6088>.
[RFC8028]: Baker, F. and B. Carpenter, "First-Hop Router Selection by Hosts in a Multi-Prefix Network", RFC 8028, DOI 10.17487/RFC8028, November 2016, <https://www.rfc-editor.org/info/rfc8028>.
[RFC8174]: Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8200]: Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017, <https://www.rfc-editor.org/info/rfc8200>.
[RFC8201]: McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., "Path MTU Discovery for IP version 6", STD 87, RFC 8201, DOI 10.17487/RFC8201, July 2017, <https://www.rfc-editor.org/info/rfc8201>.
[RFC8415]: Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., Richardson, M., Jiang, S., Lemon, T., and T. Winters, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 8415, DOI 10.17487/RFC8415, November 2018, <https://www.rfc-editor.org/info/rfc8415>.
[RFC9268]: Hinden, R. and G. Fairhurst, "IPv6 Minimum Path MTU Hop-by-Hop Option", RFC 9268, DOI 10.17487/RFC9268, August 2022, <https://www.rfc-editor.org/info/rfc9268>.
[RFC9293]: Eddy, W., Ed., "Transmission Control Protocol (TCP)", STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022, <https://www.rfc-editor.org/info/rfc9293>.

29.2. Informative References

[ATN]: Maiolla, V., "The OMNI Interface - An IPv6 Air/Ground Interface for Civil Aviation, IETF Liaison Statement #1676, https://datatracker.ietf.org/liaison/1676/", 3 March 2020.
[ATN-IPS]: "ICAO Document 9896 (Manual on the Aeronautical Telecommunication Network (ATN) using Internet Protocol Suite (IPS) Standards and Protocol), Draft Edition 3 (work-in-progress)", 10 December 2020.
[CKSUM]: Stone, J., Greenwald, M., Partridge, C., and J. Hughes, "Performance of Checksums and CRC's Over Real Data, IEEE/ACM Transactions on Networking, Vol. 6, No. 5", October 1998.
[CRC]: Jain, R., "Error Characteristics of Fiber Distributed Data Interface (FDDI), IEEE Transactions on Communications", August 1990.
[EUI]: "IEEE Guidelines for Use of Extended Unique Identifier (EUI), Organizationally Unique Identifier (OUI), and Company ID, https://standards.ieee.org/wp-content/uploads/import/documents/tutorials/eui.pdf", 3 August 2017.
[I-D.bctb-6man-rfc6296-bis]: Cullen, M., Baker, F., Trøan, O., and N. Buraglio, "RFC 6296bis IPv6-to-IPv6 Network Prefix Translation", Work in Progress, Internet-Draft, draft-bctb-6man-rfc6296-bis-02, 26 January 2024, <https://datatracker.ietf.org/doc/html/draft-bctb-6man-rfc6296-bis-02>.
[I-D.herbert-ipv4-eh]: Herbert, T., "IPv4 Extension Headers and Flow Label", Work in Progress, Internet-Draft, draft-herbert-ipv4-eh-03, 22 February 2024, <https://datatracker.ietf.org/doc/html/draft-herbert-ipv4-eh-03>.
[I-D.ietf-6man-comp-rtg-hdr]: Bonica, R., Kamite, Y., Alston, A., Henriques, D., and L. Jalil, "The IPv6 Compact Routing Header (CRH)", Work in Progress, Internet-Draft, draft-ietf-6man-comp-rtg-hdr-04, 18 March 2024, <https://datatracker.ietf.org/doc/html/draft-ietf-6man-comp-rtg-hdr-04>.
[I-D.ietf-6man-eh-limits]: Herbert, T., "Limits on Sending and Processing IPv6 Extension Headers", Work in Progress, Internet-Draft, draft-ietf-6man-eh-limits-12, 18 December 2023, <https://datatracker.ietf.org/doc/html/draft-ietf-6man-eh-limits-12>.
[I-D.ietf-intarea-tunnels]: Touch, J. D. and M. Townsley, "IP Tunnels in the Internet Architecture", Work in Progress, Internet-Draft, draft-ietf-intarea-tunnels-13, 26 March 2023, <https://datatracker.ietf.org/doc/html/draft-ietf-intarea-tunnels-13>.
[I-D.ietf-tsvwg-udp-options]: Touch, J. D., "Transport Options for UDP", Work in Progress, Internet-Draft, draft-ietf-tsvwg-udp-options-32, 21 March 2024, <https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-udp-options-32>.
[I-D.perkins-manet-aodvv2]: Perkins, C. E., Dowdell, J., Steenbrink, L., and V. Pritchard, "Ad Hoc On-demand Distance Vector Version 2 (AODVv2) Routing", Work in Progress, Internet-Draft, draft-perkins-manet-aodvv2-04, 3 March 2024, <https://datatracker.ietf.org/doc/html/draft-perkins-manet-aodvv2-04>.
[I-D.templin-intarea-aero2]: Templin, F., "Automatic Extended Route Optimization (AERO)", Work in Progress, Internet-Draft, draft-templin-intarea-aero2-05, 27 March 2024, <https://datatracker.ietf.org/doc/html/draft-templin-intarea-aero2-05>.
[IEEE802.1AX]: "Institute of Electrical and Electronics Engineers, Link Aggregation, IEEE Standard 802.1AX-2008, https://standards.ieee.org/ieee/802.1AX/6768/", 29 May 2020.
[IPV4-GUA]: Postel, J., "IPv4 Address Space Registry, https://www.iana.org/assignments/ipv4-address-space/ipv4-address-space.xhtml", 14 December 2020.
[IPV6-GUA]: Postel, J., "IPv6 Global Unicast Address Assignments, https://www.iana.org/assignments/ipv6-unicast-address-assignments/ipv6-unicast-address-assignments.xhtml", 14 December 2020.
[RFC0863]: Postel, J., "Discard Protocol", STD 21, RFC 863, DOI 10.17487/RFC0863, May 1983, <https://www.rfc-editor.org/info/rfc863>.
[RFC1035]: Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1122]: Braden, R., Ed., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989, <https://www.rfc-editor.org/info/rfc1122>.
[RFC1146]: Zweig, J. and C. Partridge, "TCP alternate checksum options", RFC 1146, DOI 10.17487/RFC1146, March 1990, <https://www.rfc-editor.org/info/rfc1146>.
[RFC1149]: Waitzman, D., "Standard for the transmission of IP datagrams on avian carriers", RFC 1149, DOI 10.17487/RFC1149, April 1990, <https://www.rfc-editor.org/info/rfc1149>.
[RFC1191]: Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, November 1990, <https://www.rfc-editor.org/info/rfc1191>.
[RFC1256]: Deering, S., Ed., "ICMP Router Discovery Messages", RFC 1256, DOI 10.17487/RFC1256, September 1991, <https://www.rfc-editor.org/info/rfc1256>.
[RFC2104]: Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997, <https://www.rfc-editor.org/info/rfc2104>.
[RFC2131]: Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, DOI 10.17487/RFC2131, March 1997, <https://www.rfc-editor.org/info/rfc2131>.
[RFC2464]: Crawford, M., "Transmission of IPv6 Packets over Ethernet Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998, <https://www.rfc-editor.org/info/rfc2464>.
[RFC2492]: Armitage, G., Schulter, P., and M. Jork, "IPv6 over ATM Networks", RFC 2492, DOI 10.17487/RFC2492, January 1999, <https://www.rfc-editor.org/info/rfc2492>.
[RFC2675]: Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms", RFC 2675, DOI 10.17487/RFC2675, August 1999, <https://www.rfc-editor.org/info/rfc2675>.
[RFC2863]: McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB", RFC 2863, DOI 10.17487/RFC2863, June 2000, <https://www.rfc-editor.org/info/rfc2863>.
[RFC2923]: Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, DOI 10.17487/RFC2923, September 2000, <https://www.rfc-editor.org/info/rfc2923>.
[RFC2983]: Black, D., "Differentiated Services and Tunnels", RFC 2983, DOI 10.17487/RFC2983, October 2000, <https://www.rfc-editor.org/info/rfc2983>.
[RFC3056]: Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4 Clouds", RFC 3056, DOI 10.17487/RFC3056, February 2001, <https://www.rfc-editor.org/info/rfc3056>.
[RFC3068]: Huitema, C., "An Anycast Prefix for 6to4 Relay Routers", RFC 3068, DOI 10.17487/RFC3068, June 2001, <https://www.rfc-editor.org/info/rfc3068>.
[RFC3168]: Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, September 2001, <https://www.rfc-editor.org/info/rfc3168>.
[RFC3330]: IANA, "Special-Use IPv4 Addresses", RFC 3330, DOI 10.17487/RFC3330, September 2002, <https://www.rfc-editor.org/info/rfc3330>.
[RFC3366]: Fairhurst, G. and L. Wood, "Advice to link designers on link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366, DOI 10.17487/RFC3366, August 2002, <https://www.rfc-editor.org/info/rfc3366>.
[RFC3692]: Narten, T., "Assigning Experimental and Testing Numbers Considered Useful", BCP 82, RFC 3692, DOI 10.17487/RFC3692, January 2004, <https://www.rfc-editor.org/info/rfc3692>.
[RFC3810]: Vida, R., Ed. and L. Costa, Ed., "Multicast Listener Discovery Version 2 (MLDv2) for IPv6", RFC 3810, DOI 10.17487/RFC3810, June 2004, <https://www.rfc-editor.org/info/rfc3810>.
[RFC3819]: Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. Wood, "Advice for Internet Subnetwork Designers", BCP 89, RFC 3819, DOI 10.17487/RFC3819, July 2004, <https://www.rfc-editor.org/info/rfc3819>.
[RFC4122]: Leach, P., Mealling, M., and R. Salz, "A Universally Unique IDentifier (UUID) URN Namespace", RFC 4122, DOI 10.17487/RFC4122, July 2005, <https://www.rfc-editor.org/info/rfc4122>.
[RFC4301]: Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, December 2005, <https://www.rfc-editor.org/info/rfc4301>.
[RFC4302]: Kent, S., "IP Authentication Header", RFC 4302, DOI 10.17487/RFC4302, December 2005, <https://www.rfc-editor.org/info/rfc4302>.
[RFC4303]: Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, December 2005, <https://www.rfc-editor.org/info/rfc4303>.
[RFC4380]: Huitema, C., "Teredo: Tunneling IPv6 over UDP through Network Address Translations (NATs)", RFC 4380, DOI 10.17487/RFC4380, February 2006, <https://www.rfc-editor.org/info/rfc4380>.
[RFC4389]: Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April 2006, <https://www.rfc-editor.org/info/rfc4389>.
[RFC4429]: Moore, N., "Optimistic Duplicate Address Detection (DAD) for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006, <https://www.rfc-editor.org/info/rfc4429>.
[RFC4541]: Christensen, M., Kimball, K., and F. Solensky, "Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006, <https://www.rfc-editor.org/info/rfc4541>.
[RFC4605]: Fenner, B., He, H., Haberman, B., and H. Sandick, "Internet Group Management Protocol (IGMP) / Multicast Listener Discovery (MLD)-Based Multicast Forwarding ("IGMP/MLD Proxying")", RFC 4605, DOI 10.17487/RFC4605, August 2006, <https://www.rfc-editor.org/info/rfc4605>.
[RFC4821]: Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, <https://www.rfc-editor.org/info/rfc4821>.
[RFC4963]: Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly Errors at High Data Rates", RFC 4963, DOI 10.17487/RFC4963, July 2007, <https://www.rfc-editor.org/info/rfc4963>.
[RFC5213]: Gundavelli, S., Ed., Leung, K., Devarapalli, V., Chowdhury, K., and B. Patil, "Proxy Mobile IPv6", RFC 5213, DOI 10.17487/RFC5213, August 2008, <https://www.rfc-editor.org/info/rfc5213>.
[RFC5214]: Templin, F., Gleeson, T., and D. Thaler, "Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, DOI 10.17487/RFC5214, March 2008, <https://www.rfc-editor.org/info/rfc5214>.
[RFC5237]: Arkko, J. and S. Bradner, "IANA Allocation Guidelines for the Protocol Field", BCP 37, RFC 5237, DOI 10.17487/RFC5237, February 2008, <https://www.rfc-editor.org/info/rfc5237>.
[RFC5340]: Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, <https://www.rfc-editor.org/info/rfc5340>.
[RFC5558]: Templin, F., Ed., "Virtual Enterprise Traversal (VET)", RFC 5558, DOI 10.17487/RFC5558, February 2010, <https://www.rfc-editor.org/info/rfc5558>.
[RFC5614]: Ogier, R. and P. Spagnolo, "Mobile Ad Hoc Network (MANET) Extension of OSPF Using Connected Dominating Set (CDS) Flooding", RFC 5614, DOI 10.17487/RFC5614, August 2009, <https://www.rfc-editor.org/info/rfc5614>.
[RFC5798]: Nadas, S., Ed., "Virtual Router Redundancy Protocol (VRRP) Version 3 for IPv4 and IPv6", RFC 5798, DOI 10.17487/RFC5798, March 2010, <https://www.rfc-editor.org/info/rfc5798>.
[RFC5880]: Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, <https://www.rfc-editor.org/info/rfc5880>.
[RFC5889]: Baccelli, E., Ed. and M. Townsley, Ed., "IP Addressing Model in Ad Hoc Networks", RFC 5889, DOI 10.17487/RFC5889, September 2010, <https://www.rfc-editor.org/info/rfc5889>.
[RFC5942]: Singh, H., Beebee, W., and E. Nordmark, "IPv6 Subnet Model: The Relationship between Links and Subnet Prefixes", RFC 5942, DOI 10.17487/RFC5942, July 2010, <https://www.rfc-editor.org/info/rfc5942>.
[RFC6081]: Thaler, D., "Teredo Extensions", RFC 6081, DOI 10.17487/RFC6081, January 2011, <https://www.rfc-editor.org/info/rfc6081>.
[RFC6145]: Li, X., Bao, C., and F. Baker, "IP/ICMP Translation Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011, <https://www.rfc-editor.org/info/rfc6145>.
[RFC6146]: Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146, April 2011, <https://www.rfc-editor.org/info/rfc6146>.
[RFC6147]: Bagnulo, M., Sullivan, A., Matthews, P., and I. van Beijnum, "DNS64: DNS Extensions for Network Address Translation from IPv6 Clients to IPv4 Servers", RFC 6147, DOI 10.17487/RFC6147, April 2011, <https://www.rfc-editor.org/info/rfc6147>.
[RFC6214]: Carpenter, B. and R. Hinden, "Adaptation of RFC 1149 for IPv6", RFC 6214, DOI 10.17487/RFC6214, April 2011, <https://www.rfc-editor.org/info/rfc6214>.
[RFC6234]: Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 10.17487/RFC6234, May 2011, <https://www.rfc-editor.org/info/rfc6234>.
[RFC6247]: Eggert, L., "Moving the Undeployed TCP Extensions RFC 1072, RFC 1106, RFC 1110, RFC 1145, RFC 1146, RFC 1379, RFC 1644, and RFC 1693 to Historic Status", RFC 6247, DOI 10.17487/RFC6247, May 2011, <https://www.rfc-editor.org/info/rfc6247>.
[RFC6347]: Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC6438]: Carpenter, B. and S. Amante, "Using the IPv6 Flow Label for Equal Cost Multipath Routing and Link Aggregation in Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011, <https://www.rfc-editor.org/info/rfc6438>.
[RFC6495]: Gagliano, R., Krishnan, S., and A. Kukec, "Subject Key Identifier (SKI) SEcure Neighbor Discovery (SEND) Name Type Fields", RFC 6495, DOI 10.17487/RFC6495, February 2012, <https://www.rfc-editor.org/info/rfc6495>.
[RFC6543]: Gundavelli, S., "Reserved IPv6 Interface Identifier for Proxy Mobile IPv6", RFC 6543, DOI 10.17487/RFC6543, May 2012, <https://www.rfc-editor.org/info/rfc6543>.
[RFC6706]: Templin, F., Ed., "Asymmetric Extended Route Optimization (AERO)", RFC 6706, DOI 10.17487/RFC6706, August 2012, <https://www.rfc-editor.org/info/rfc6706>.
[RFC6762]: Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, DOI 10.17487/RFC6762, February 2013, <https://www.rfc-editor.org/info/rfc6762>.
[RFC6935]: Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and UDP Checksums for Tunneled Packets", RFC 6935, DOI 10.17487/RFC6935, April 2013, <https://www.rfc-editor.org/info/rfc6935>.
[RFC6936]: Fairhurst, G. and M. Westerlund, "Applicability Statement for the Use of IPv6 UDP Datagrams with Zero Checksums", RFC 6936, DOI 10.17487/RFC6936, April 2013, <https://www.rfc-editor.org/info/rfc6936>.
[RFC6980]: Gont, F., "Security Implications of IPv6 Fragmentation with IPv6 Neighbor Discovery", RFC 6980, DOI 10.17487/RFC6980, August 2013, <https://www.rfc-editor.org/info/rfc6980>.
[RFC7042]: Eastlake 3rd, D. and J. Abley, "IANA Considerations and IETF Protocol and Documentation Usage for IEEE 802 Parameters", BCP 141, RFC 7042, DOI 10.17487/RFC7042, October 2013, <https://www.rfc-editor.org/info/rfc7042>.
[RFC7094]: McPherson, D., Oran, D., Thaler, D., and E. Osterweil, "Architectural Considerations of IP Anycast", RFC 7094, DOI 10.17487/RFC7094, January 2014, <https://www.rfc-editor.org/info/rfc7094>.
[RFC7181]: Clausen, T., Dearlove, C., Jacquet, P., and U. Herberg, "The Optimized Link State Routing Protocol Version 2", RFC 7181, DOI 10.17487/RFC7181, April 2014, <https://www.rfc-editor.org/info/rfc7181>.
[RFC7217]: Gont, F., "A Method for Generating Semantically Opaque Interface Identifiers with IPv6 Stateless Address Autoconfiguration (SLAAC)", RFC 7217, DOI 10.17487/RFC7217, April 2014, <https://www.rfc-editor.org/info/rfc7217>.
[RFC7296]: Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. Kivinen, "Internet Key Exchange Protocol Version 2 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 2014, <https://www.rfc-editor.org/info/rfc7296>.
[RFC7401]: Moskowitz, R., Ed., Heer, T., Jokela, P., and T. Henderson, "Host Identity Protocol Version 2 (HIPv2)", RFC 7401, DOI 10.17487/RFC7401, April 2015, <https://www.rfc-editor.org/info/rfc7401>.
[RFC7421]: Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S., Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit Boundary in IPv6 Addressing", RFC 7421, DOI 10.17487/RFC7421, January 2015, <https://www.rfc-editor.org/info/rfc7421>.
[RFC7542]: DeKok, A., "The Network Access Identifier", RFC 7542, DOI 10.17487/RFC7542, May 2015, <https://www.rfc-editor.org/info/rfc7542>.
[RFC7739]: Gont, F., "Security Implications of Predictable Fragment Identification Values", RFC 7739, DOI 10.17487/RFC7739, February 2016, <https://www.rfc-editor.org/info/rfc7739>.
[RFC7761]: Fenner, B., Handley, M., Holbrook, H., Kouvelas, I., Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March 2016, <https://www.rfc-editor.org/info/rfc7761>.
[RFC7847]: Melia, T., Ed. and S. Gundavelli, Ed., "Logical-Interface Support for IP Hosts with Multi-Access Support", RFC 7847, DOI 10.17487/RFC7847, May 2016, <https://www.rfc-editor.org/info/rfc7847>.
[RFC8032]: Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, January 2017, <https://www.rfc-editor.org/info/rfc8032>.
[RFC8126]: Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/info/rfc8126>.
[RFC8402]: Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8446]: Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/info/rfc8446>.
[RFC8726]: Farrel, A., "How Requests for IANA Action Will Be Handled on the Independent Stream", RFC 8726, DOI 10.17487/RFC8726, November 2020, <https://www.rfc-editor.org/info/rfc8726>.
[RFC8799]: Carpenter, B. and B. Liu, "Limited Domains and Internet Protocols", RFC 8799, DOI 10.17487/RFC8799, July 2020, <https://www.rfc-editor.org/info/rfc8799>.
[RFC8892]: Thaler, D. and D. Romascanu, "Guidelines and Registration Procedures for Interface Types and Tunnel Types", RFC 8892, DOI 10.17487/RFC8892, August 2020, <https://www.rfc-editor.org/info/rfc8892>.
[RFC8899]: Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T. Völker, "Packetization Layer Path MTU Discovery for Datagram Transports", RFC 8899, DOI 10.17487/RFC8899, September 2020, <https://www.rfc-editor.org/info/rfc8899>.
[RFC8900]: Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O., and F. Gont, "IP Fragmentation Considered Fragile", BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020, <https://www.rfc-editor.org/info/rfc8900>.
[RFC8928]: Thubert, P., Ed., Sarikaya, B., Sethi, M., and R. Struik, "Address-Protected Neighbor Discovery for Low-Power and Lossy Networks", RFC 8928, DOI 10.17487/RFC8928, November 2020, <https://www.rfc-editor.org/info/rfc8928>.
[RFC8981]: Gont, F., Krishnan, S., Narten, T., and R. Draves, "Temporary Address Extensions for Stateless Address Autoconfiguration in IPv6", RFC 8981, DOI 10.17487/RFC8981, February 2021, <https://www.rfc-editor.org/info/rfc8981>.
[RFC9000]: Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, May 2021, <https://www.rfc-editor.org/info/rfc9000>.
[RFC9001]: Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021, <https://www.rfc-editor.org/info/rfc9001>.
[RFC9002]: Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection and Congestion Control", RFC 9002, DOI 10.17487/RFC9002, May 2021, <https://www.rfc-editor.org/info/rfc9002>.
[RFC9365]: Jeong, J., Ed., "IPv6 Wireless Access in Vehicular Environments (IPWAVE): Problem Statement and Use Cases", RFC 9365, DOI 10.17487/RFC9365, March 2023, <https://www.rfc-editor.org/info/rfc9365>.
[RFC9374]: Moskowitz, R., Card, S., Wiethuechter, A., and A. Gurtov, "DRIP Entity Tag (DET) for Unmanned Aircraft System Remote ID (UAS RID)", RFC 9374, DOI 10.17487/RFC9374, March 2023, <https://www.rfc-editor.org/info/rfc9374>.