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

Status of This Memo

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Table of Contents

1. Introduction

Air/land/sea/space mobile nodes (e.g., aircraft of various configurations, terrestrial vehicles, seagoing vessels, space systems, enterprise wireless devices, pedestrians with cellphones, etc.) configure mobile routers with multiple interface connections to wireless and/or wired-line data links. These data links often have diverse performance, cost and availability properties that can change dynamically according to mobility patterns, flight phases, proximity to infrastructure, etc. The mobile router acts as a Client of a network-based Mobility Service (MS) by configuring a virtual interface over its underlay interface data link connections.¶

Each Client configures a virtual network interface (termed the "Overlay Multilink Network Interface (OMNI)") as a thin layer over its underlay interfaces (which may themselves connect to virtual or physical links). The OMNI interface is therefore the only interface abstraction exposed to the IP layer and behaves according to the Non-Broadcast, Multiple Access (NBMA) interface principle, while each underlay interface appears as a link layer communication channel in the architecture. The OMNI interface internally employs the "OMNI Adaptation Layer (OAL)" to ensure that original IP packets or parcels [I-D.templin-6man-parcels2], [I-D.templin-intarea-parcels2] are adapted to diverse underlay interfaces with heterogeneous properties.¶

The OMNI interface connects to a virtual overlay known as the "OMNI link". The OMNI link spans one or more Internetworks that may include private-use infrastructures (e.g., enterprise networks, operator networks, etc.) and/or the global public Internet itself. Together, OMNI and the OAL provide the foundational elements required to support the "6 M's of Modern Internetworking", including:¶

1. Multilink - a Client's ability to coordinate multiple diverse underlay interfaces as a single logical unit (i.e., the OMNI interface) to achieve the required communications performance and reliability objectives.¶

2. Multinet - the ability to span the OMNI link over a segment routing topology with multiple diverse administrative domain network segments while maintaining seamless end-to-end communications between mobile Clients and correspondents such as air traffic controllers, fleet administrators, etc.¶

3. Mobility - a Client's ability to change network points of attachment (e.g., moving between wireless base stations) which may result in an underlay interface address change, but without disruptions to ongoing communication sessions with peers over the OMNI link.¶

4. Multicast - the ability to send a single network transmission that reaches multiple Clients belonging to the same interest group, but without disturbing other Clients not subscribed to the interest group.¶

5. Multihop - a mobile Client peer-to-peer relaying capability useful when multiple forwarding hops between peers may be necessary to "reach back" to an infrastructure access point connection to the OMNI link.¶

6. (Performance) Maximization - the ability to exchange large packets/parcels between peers without loss due to a link size restriction, and to adaptively adjust packet/parcel sizes to maintain the best performance profile for each independent traffic flow.¶

Client OMNI interfaces coordinate with the MS and/or OMNI peer nodes through IPv6 Neighbor Discovery (ND) control message exchanges [RFC4861]. The MS consists of a distributed set of service nodes (including Proxy/Servers and other infrastructure elements) that also configure OMNI interfaces. Automatic Extended Route Optimization (AERO) in particular provides a companion MS compatible with the OMNI architecture [I-D.templin-6man-aero3]. AERO discusses details of ND message based multilink forwarding, route optimization, mobility management, and multinet traversal while the fundamental aspects of OMNI link operation are discussed in this document.¶

Each OMNI interface provides a multilink nexus for exchanging inbound and outbound traffic flows via selected underlay interfaces. The IP layer sees the OMNI interface as a point of connection to the OMNI link. Each OMNI link assigns one or more associated Mobility Service Prefixes (MSPs), which are typically IP Global Unicast Address (GUA) prefixes. The MS then delegates Mobile Network Prefixes (MNPs) taken from an MSP to Client end systems as Provider-Independent (PI) address blocks. Clients in local domains also obtain Provider-Aggregated (PA) addresses from internal/external Stable Network Prefixes (SNPs) assigned to Proxy/Servers that connect the local domain to the global topology per [I-D.bctb-6man-rfc6296-bis]. If there are multiple OMNI links, the IP layer will see multiple OMNI interfaces.¶

Clients receive SNP addresses and optionally also MNP prefix delegations through IPv6 ND control message exchanges with Proxy/Servers over MANETs, Access Networks (ANETs) and/or open Internetworks (INETs). Clients sub-delegate MNPs to downstream-attached End-user Networks (ENETs) independently of the underlay interfaces selected for upstream data transport. Each Client acts as a fixed or mobile router on behalf of ENET peers, and uses OMNI interface control messaging to coordinate with Hosts, Proxy/Servers and/or other Clients. The Client iterates its control messaging over each of the OMNI interface's (M)ANET/INET underlay interfaces in order to register each interface with the MS (see Section 13). The Client can also provide multihop forwarding services for a recursively extended chain of other Clients and Hosts connected via downstream-attached ENETs.¶

Clients may connect to multiple distinct OMNI links within the same OMNI domain by configuring multiple OMNI interfaces, e.g., omni0, omni1, omni2, etc. Each OMNI interface is configured over a distinct set of underlay interfaces and provides a nexus for Safety-Based Multilink (SBM) operation. The IP layer applies SBM routing to select a specific OMNI interface, then the selected OMNI interface applies Performance-Based Multilink (PBM) internally to select appropriate underlay interfaces. Applications select SBM topologies based on IP layer Segment Routing [RFC8402], while each OMNI interface orchestrates PBM internally based on OAL Multinet traversal.¶

OMNI provides a link model suitable for a wide range of use cases. For example, the International Civil Aviation Organization (ICAO) Working Group-I Mobility Subgroup is developing a future Aeronautical Telecommunications Network with Internet Protocol Services (ATN/IPS) and has issued a liaison statement requesting IETF adoption [ATN] in support of ICAO Document 9896 [ATN-IPS]. The IETF IP Wireless Access in Vehicular Environments (ipwave) working group has further included problem statement and use case analysis for OMNI in [RFC9365]. Still other communities of interest include AEEC, RTCA Special Committee 228 (SC-228) and NASA programs that examine commercial aviation, Urban Air Mobility (UAM) and Unmanned Air Systems (UAS). Pedestrians with handheld mobile devices, home and small office networks, enterprise networks and many others represent additional large classes of potential OMNI users.¶

This document specifies the transmission of original IP packets/parcels and control messages over OMNI interfaces. The operation of both IP protocol versions (i.e., IPv4 [RFC0791] and IPv6 [RFC8200]) is specified as the network layer data plane, while OMNI interfaces use IPv6 ND messaging in the control plane independently of the data plane protocol(s). OMNI interfaces also provide an adaptation layer based on encapsulation and fragmentation over heterogeneous underlay interfaces as an OAL sublayer between L3 and L2. OMNI and the OAL are specified in detail throughout the remainder of this document.¶

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.¶

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 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] and [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 similar 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) [RFC4193], MANET Local Addresses (MLAs) [I-D.templin-6man-mla] or Hierarchical Host Identity Tags (HHITs) [RFC9374].¶

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 to a local network with indeterminant neighborhood properties over which multihop relaying may be necessary. The MANET interface appears as an Adaptation Layer interface from the viewpoint of the OMNI interface. All MANET interfaces used by AERO/OMNI are IPv6 interfaces and therefore must configure a Maximum Transmission Unit (MTU) no smaller than the IPv6 minimum MTU (1280 octets) even if lower-layer fragmentation is needed.¶

Adaptation-Layer Virtual InterFace (ALVIF)

a virtual interface configured beneath the OMNI interface but over an underlay interface connection to an ANET or INET. The ALVIF provides context for the assignment of adaptation layer IPv6 addresses. MANET interfaces need not configure an ALVIF since they are already adaptation layer interfaces.¶

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 "bridges" 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 *NETs is considered as a leaf OMNI link segment, with each leaf interconnected via links and "bridge" nodes in intermediate OMNI link segments. When the *NETs of multiple leaf segments overlap (e.g., due to network mobility), they can combine to form larger *NETs 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 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-6man-aero3]).¶

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-6man-aero3].¶

Mobility Service Prefix (MSP)

an aggregated IP Global Unicast Address (GUA) prefix (e.g., 2001:db8::/32, 2002:192.0.2.0::/40, etc.) assigned to the OMNI link and from which more-specific Mobile and Stable Network Prefixes (MNPs/SNPs) are delegated, where IPv4 MSPs are represented as "6to4 prefixes" per [RFC3056]. 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 8). 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, 2002:192.0.2.8::/46, 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 *NET 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 *NET groups using IPv6 Unique Local Addresses (ULAs) assigned in 1x1 correspondence to SNP GUAs made visible to external peers through IP network address/prefix translation [RFC6145][RFC6146][RFC6147][I-D.bctb-6man-rfc6296-bis].¶

Foreign Network Prefix (FNP)

a global IP prefix not covered by a MSP and assigned to a link or network outside of the OMNI domain.¶

Subnet Router Anycast (SRA) Address

An IPv6 address taken from an FNP/MNP/SNP in which the remainder of the address beyond 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 2002:192.0.2.0::/40 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 Index 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 10.¶

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 assigned to an IPv6 interface.¶

(OMNI) MANET Local Address (MLA)

an IPv6 address beginning with fee0::/11 followed by a 53-bit random Subnet ID and a 64-bit Interface ID. Each OMNI node assigns an MLA with a /128 prefix length on its adaptation layer interfaces (including MANET and ALVIF interfaces) per [I-D.templin-6man-mla]. The node also assigns the MLA to the OMNI interface itself.¶

(OMNI) Hierarchical Host Identity Tag (HHIT)

an IPv6 address with a /128 prefix length according to [RFC9374]. Each OMNI node assigns an HHIT on its adaptation layer interfaces to domains which provide access to an attestation service. The node also assigns the HHIT to the OMNI interface itself.¶

(OMNI) Unique Local Address (ULA)

an IPv6 address delegated to an OMNI 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]. The OMNI node assigns the ULA to the OMNI interface. (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.)¶

Globally Unique Address (GUA)

a globally unique IPv6 address per the IPv6 addressing architecture [RFC4291] or a globally unique IPv4 address that is not reserved for a special-purpose per [RFC6890].¶

Provider-Aggregated (PA) Address

a GUA delegated to a Client from a SNP assigned to a FHS Proxy/Server is considered Provider-Aggregated (PA) or "Proxy/Server-Aggregated". The Client either assigns the PA address to its own OMNI interface or allows the FHS Proxy/Server to supply the address via Network Prefix Translation for IPv6 (NPTv6) [I-D.bctb-6man-rfc6296-bis].¶

Provider-Independent (PI) Address

a GUA allocated from an MNP delegated to a Client via a MAP Proxy/Server is considered Provider-Independent (PI) or "Proxy/Server-Independent". The Client assigns a PI address to a (downstream) ENET interfaces can sub-delegate the MNP to downstream ENET nodes.¶

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. 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 through intermediate system concatenation of SRT network segments. 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-6man-aero3] 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 SRA address(es) corresponding to the MSP(s) assigned to the domain.¶

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-6man-aero3] 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-6man-aero3] 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 AFVI is always processed as a 4-octet value, but may be transmitted as only the 2 least significant octets when the 2 most significant octets are 0.) 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. The special AFVI value 0 means that no AFVI is assigned.¶

flow

a sequence of packets sent from a particular source to a particular unicast, anycast, or multicast destination that a node desires to label as a flow. The 3-tuple of the Flow Label, Source Address and Destination Address fields enable efficient IPv6 flow classification. The IPv6 Flow Label Specification is observed per [RFC6437] [RFC6438].¶

(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 should be 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.¶

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.¶

3. Requirements

OMNI interfaces limit the size of their IPv6 ND control plane messages (plus any original IP packet/parcel attachments) to the minimum IPv6 link MTU minus overhead for adaptation and link layer encapsulation. If there are sufficient OMNI parameters and/or IP packet/parcel attachments that would exceed this size, the OMNI interface forwards the information as multiple smaller IPv6 ND messages and the recipient accepts the union of all information received. This allows the messages to travel without loss due to a size restriction over secured control plane paths that include IPsec tunnels [RFC4301], secured direct point-to-point links and/or unsecured paths that require an authentication signature.¶

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 IPv6 ND Protocol Constants defined in Section 10 of [RFC4861] are used in their same format and meaning in this document.¶

The L3, adaptation and (virtual) L2 layers each include distinct packet Identification numbering spaces. 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 key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119][RFC8174] when, and only when, they appear in all capitals, as shown here.¶

4. Overlay Multilink Network (OMNI) Interface Model

An OMNI interface is a virtual interface configured over one or more underlay interfaces, which may be physical (e.g., an aeronautical radio link, a cellular wireless link, etc.) or virtual (e.g., an internet-layer or higher-layer "tunnel"). The OMNI interface architectural layering model is the same as in [RFC5558][RFC7847], and augmented as shown in Figure 1. The network layer therefore sees the OMNI interface as a single L3 interface nexus for multiple underlay interfaces that appear as L2 communication channels in the architecture.¶

                                  +----------------------------+
                                  |    Upper Layer Protocol    |
           Session-to-IP    +---->|                            |
           Address Binding  |     +----------------------------+
                            +---->|           IP (L3)          |
           IP Address       +---->|                            |
           Binding          |     +----------------------------+
                            +---->|       OMNI Interface       |
           Logical-to-      +---->|   (OMNI Adaptation Layer)  |
           Physical         |     +----------------------------+
           Interface        +---->|  L2  |  L2  |       |  L2  |
           Binding                |(IF#1)|(IF#2)| ..... |(IF#n)|
                                  +------+------+       +------+
                                  |  L1  |  L1  |       |  L1  |
                                  |      |      |       |      |
                                  +------+------+       +------+

Each underlay interface provides an L2/L1 abstraction according to one of the following models:¶

• (M)ANET interfaces connect to a (M)ANET that is separated from the open INET by Proxy/Servers. The (M)ANET interface may be either on the same link segment as a Proxy/Server, or separated from a Proxy/Server by multiple adaptation layer and/or L2 hops. (Note that NATs may appear internally within a (M)ANET or on the Proxy/Server itself and may require NAT traversal the same as for the INET case.) The OMNI interface configures an ALVIF over each ANET interface, while MANET interfaces already appear as adaptation layer interfaces and need not configure an ALVIF.¶

• INET interfaces connect to an INET either natively or through IP Network Address Translators (NATs). Native INET interfaces have global IP addresses that are reachable from any INET correspondent. NATed INET interfaces typically configure private IP addresses and connect to a private network behind one or more NATs with the outermost NAT providing INET access. The same as for ANET interfaces, the OMNI interface configures an ALVIF over each INET interface.¶

• ENET interfaces connect a Client's downstream-attached networks, where the Client provides forwarding services for ENET Host and Client communications to remote peers. An ENET may be as simple as a small IoT sub-network that travels with a mobile Client to as complex as a large private enterprise network that the Client connects to a larger *NET. Downstream-attached Hosts and Clients see the ENET as a *NET and see the (upstream) Client as a Proxy/Server.¶

• VPN interfaces use security encapsulations (e.g. IPsec tunnels) over underlay networks to connect Client, Proxy/Server or other critical infrastructure nodes. VPN interfaces provide security services at lower layers of the architecture (L2/L1), with securing properties similar to Direct point-to-point interfaces.¶

• Direct point-to-point interfaces securely connect Clients, Proxy/Servers and/or other critical infrastructure nodes over physical or virtual media that does not transit any open Internetwork paths. Examples include a line-of-sight link between a remote pilot and an unmanned aircraft, a fiberoptic link between gateways, etc.¶

The OMNI interface forwards original IP packets/parcels from the network layer using the OMNI Adaptation Layer (OAL) (see: Section 5) as an encapsulation and fragmentation sublayer service. This "OAL source" then further encapsulates the resulting OAL packets/fragments in underlay network headers (e.g., UDP/IP, IP-only, Ethernet-only, etc.) to create L2 encapsulated "carrier packets" for fragmentation and transmission over underlay interfaces. The target OMNI interface then receives the carrier packets from underlay interfaces and performs L2 reassembly/decapsulation.¶

If the resulting OAL packets/fragments are addressed to itself, the OMNI interface performs reassembly/decapsulation as an "OAL destination" and delivers the original IP packet/parcel to the network layer. If the OAL packets/fragments are addressed to another node, the OMNI interface instead re-encapsulates them in new underlay network L2 headers as an "OAL intermediate system" then performs L2 fragmentation and forwards the resulting carrier packets over an underlay interface. The OAL source and OAL destination are seen as "neighbors" on the OMNI link, while OAL intermediate systems provide a virtual bridging service that joins the segments of a (multinet) Segment Routing Topology (SRT).¶

The OMNI interface transports carrier packets over either secured or unsecured underlay interfaces to access the secured/unsecured OMNI link spanning trees as discussed further throughout the document. Carrier packets that carry control plane messages over secured underlay interfaces use secured L2/L1 services such as IPsec, direct encapsulation over secured point-to-point links, etc. Carrier packets that carry data plane messages over unsecured underlay interfaces instead use L2 encapsulations appropriate for public or private Internetworks and are subject for the following sections.¶

The OMNI interface and its OAL can forward original IP packets/parcels over underlay interfaces while including/omitting various lower layer encapsulations including OAL, UDP, IP and (ETH)ernet or other link layer header. The network layer can also engage underlay interfaces directly while bypassing the OMNI interface entirely when necessary. This architectural flexibility may be beneficial for underlay interfaces (e.g., some aviation data links) for which encapsulation overhead is a primary consideration. OMNI interfaces that send original IP packets/parcels directly over underlay interfaces without invoking the OAL can only reach peers located on the same OMNI link segment. Source Clients can instead use the OAL to coordinate with target Clients in the same or different OMNI link segments by sending initial carrier packets to a First-Hop Segment (FHS) Proxy/Server. The FHS Proxy/Sever then sends the carrier packets into the SRT spanning tree, which transports them to a Last-Hop Segment (LHS) Proxy/Server for the target Client.¶

The OMNI interface encapsulation/decapsulation layering possibilities are shown in Figure 2 below. Imaginary vertical lines drawn between the Network Layer at the top of the figure and Underlay Interfaces at the bottom of the figure denote the various encapsulation/decapsulation layering combination possibilities. Common combinations include IP-only (i.e., direct access to underlay interfaces with or without using the OMNI interface), IP/IP, IP/UDP/IP, IP/UDP/IP/ETH, IP/OAL/UDP/IP, IP/OAL/UDP/ETH, etc.¶

 +------------------------------------------------------------+  ^
 |          Network Layer (Original IP packets/parcels)       |  |
 +--+---------------------------------------------------------+ L3
    |         OMNI Interface (virtual sublayer nexus)         |  |
    +--------------------------+------------------------------+  -
                               |      OAL Encaps/Decaps       |  ^
                               +------------------------------+ OAL
                               |        OAL Frag/Reass        |  v
                  +------------+---------------+--------------+  -
                  | UDP Encaps/Decaps/Compress |                 ^
             +----+---+------------+--------+--+  +--------+     |
             | IP E/D |            | IP E/D |     | IP E/D |    L2
        +----+-----+--+----+    +--+----+---+     +---+----+--+  |
        |ETH E/D|  |ETH E/D|    |ETH E/D|             |ETH E/D|  |
 +------+-------+--+-------+----+-------+-------------+-------+  v
 |                    Underlay Interfaces                     |
 +------------------------------------------------------------+

The OMNI/OAL model gives rise to a number of opportunities:¶

• Clients coordinate with the MS and receive both SNP addresses and MNP delegations through IPv6 ND control plane message exchanges with Proxy/Servers. Since GUA and ULA addresses are managed for uniqueness, no Duplicate Address Detection (DAD) or Multicast Listener Discovery (MLD) messaging is necessary over the OMNI interface.¶

• underlay interfaces on the same L2 link segment as a Proxy/Server do not require any L3 addresses (i.e., not even link-local) in environments where communications are coordinated entirely over the OMNI interface.¶

• as underlay interface properties change (e.g., link quality, cost, availability, etc.), any active interface can be used to update the profiles of multiple additional interfaces in a single message. This allows for timely adaptation and service continuity under dynamically changing conditions.¶

• coordinating underlay interfaces in this way allows them to be represented in a unified MS profile with provisions to support the "6 M's of Modern Internetworking".¶

• header compression and path MTU determination is conducted on a per-flow basis, with each flow adapting to the best performance profiles and path selections.¶

• exposing a single virtual interface abstraction to the network layer allows for multilink operation (including QoS based link selection, carrier packet replication, load balancing, etc.) at L2 while still permitting L3 traffic shaping based on, e.g., DSCP, flow label, etc.¶

• the OMNI interface supports multinet traversal over the SRT when communications across different administrative domain network segments are necessary. This mode of operation would not be possible via direct communications over the underlay interfaces themselves.¶

• the OAL supports lossless and adaptive path MTU mitigations not available for communications directly over the underlay interfaces themselves. The OAL supports "packing" of multiple original IP payload packets/parcels within a single OAL "super-packet" and also supports transmission of IP packets/parcels of all sizes up to and including (advanced) jumbograms.¶

• the OAL assigns per-packet Identification values that allow for adaptation/link layer reliability and data origin authentication.¶

• L3 sees the OMNI interface as a point of connection to the OMNI link; if there are multiple OMNI links, L3 will see multiple OMNI interfaces.¶

• Multiple independent OMNI interfaces can be used for increased fault tolerance through Safety-Based Multilink (SBM), with Performance-Based Multilink (PBM) applied within each interface.¶

• Multiple independent OMNI links can be joined together into a single link without requiring renumbering of infrastructure elements, since the GUAs/ULAs assigned by Proxy/Servers of the different links will be mutually exclusive.¶

• the OMNI/OAL model supports transmission of new forms of IP packets known as "IP parcels and Advanced Jumbos (AJs)" that improve performance and efficiency for both transport layer protocols and networked paths.¶

• OMNI provides robust support for both Provider-Aggregated (PA) and Provider-Independent (PI) addressing resulting in a versatile service for all Client use cases.¶

Figure 3 depicts the architectural model for a source Client with an attached ENET connecting to the OMNI link via multiple independent *NETs. The Client's OMNI interface forwards adaptation layer IPv6 ND solicitation messages over available *NET underlay interfaces using any necessary L2 encapsulations. The IPv6 ND messages traverse the *NETs until they reach an FHS Proxy/Server (FHS#1, FHS#2, ..., FHS#n), which returns an IPv6 ND advertisement message and/or forwards a proxyed version of the message over the SRT to an LHS Proxy/Server near the target Client (LHS#1, LHS#2, ..., LHS#m). The Hop Limit in IPv6 ND messages is not decremented due to encapsulation; hence, the source and target Client OMNI interfaces appear to be attached to a common link.¶

                        +--------------+
                        |Source Client |
                        +--------------+        (:::)-.
                        |OMNI interface|<-->.-(::ENET::)
                        +----+----+----+      `-(::::)-'
               +--------|IF#1|IF#2|IF#n|------ +
              /         +----+----+----+        \
             /                 |                 \
            /                  |                  \
           v                   v                   v
        (:::)-.              (:::)-.              (:::)-.
   .-(::*NET:::)        .-(::*NET:::)        .-(::*NET:::)
     `-(::::)-'           `-(::::)-'           `-(::::)-'
      +-----+              +-----+              +-----+
 ...  |FHS#1|  .........   |FHS#2|   .........  |FHS#n|  ...
.     +--|--+              +--|--+              +--|--+     .
.        |                    |                    |
.        \                    v                    /        .
.         \                                       /         .
.           v                 (:::)-.           v            .
.                        .-(::::::::)                       .
.                    .-(::: Segment :::)-.                  .
.                  (:::::   Routing   ::::)                 .
.                     `-(:: Topology ::)-'                  .
.                         `-(:::::::-'                      .
.                  /          |          \                  .
.                 /           |           \                 .
.                v            v            v
.     +-----+              +-----+              +-----+     .
 ...  |LHS#1|  .........   |LHS#2|   .........  |LHS#m|  ...
      +--|--+              +--|--+              +--|--+
          \                   |                    /
           v                  v                   v
                    <-- Target Clients -->

After the initial IPv6 ND message exchange, the source Client (as well as any nodes on its attached ENETs) can send carrier packets to the target Client via the OMNI interface. OMNI interface multilink services will send the carrier packets via FHS Proxy/Servers for the correct underlay *NETs. The FHS Proxy/Server then re-encapsulates the carrier packets and forwards them over the SRT which delivers them to an LHS Proxy/Server, and the LHS Proxy/Server in turn re-encapsulates and forwards them to the target Client. (Note that when the source and target Client are on the same SRT segment, the FHS and LHS Proxy/Servers may be one and the same.)¶

Mobile Clients select a MAP Proxy/Server (not shown in the figure), which will often be one of their FHS Proxy/Servers but could also be any Proxy/Server on the OMNI link. Clients then register all of their *NET underlay interfaces with the MAP Proxy/Server via per interface FHS Proxy/Servers in a pure proxy role. The MAP Proxy/Server then provides a designated router that advertises the Client's MNPs into the OMNI link routing system, and the Client can quickly migrate to a new MAP Proxy/Server if the former becomes unresponsive.¶

Clients therefore use Proxy/Servers as gateways into the SRT to reach OMNI link correspondents via a spanning tree established in a manner outside the scope of this document. Proxy/Servers forward critical MS control messages via the secured spanning tree and forward other messages via the unsecured spanning tree (see Security Considerations). When AERO route optimization is applied, Clients can instead forward directly to correspondents in the same SRT segment to reduce Proxy/Server and/or Gateway load.¶

Note: while not shown in the figure, a Client's ENET may connect many additional Hosts and even other Clients in a recursive extension of the OMNI link. This OMNI virtual link extension will be discussed more fully throughout the document.¶

Note: Original IP packets/parcels sent into an OMNI interface will receive consistent consideration according to their size as discussed in the following sections, while those sent directly over underlay interfaces that exceed the underlay network path MTU are dropped with an ordinary ICMP Packet Too Big (PTB) message returned. These PTB messages are subject to loss the same as for any non-OMNI IP interface [RFC2923].¶

5. OMNI Interface Maximum Transmission Unit (MTU)

The OMNI interface observes the link nature of tunnels, including the Maximum Transmission Unit (MTU), Effective MTU to Send (EMTU_S), Effective MTU to Receive (EMTU_R) and the role of fragmentation and reassembly [I-D.ietf-intarea-tunnels]. The OMNI interface is configured over one or more underlay interfaces as discussed in Section 4, where underlay links and network paths may have diverse MTUs. OMNI interface considerations for accommodating original IP packets/parcels of various sizes are discussed in the following sections.¶

IPv6 underlay interfaces are REQUIRED to configure a minimum MTU of 1280 octets and a minimum EMTU_R of 1500 octets [RFC8200]. Therefore, the minimum IPv6 path MTU is 1280 octets since routers on the path are not permitted to perform network fragmentation even though the destination is required to reassemble more. The network therefore MUST forward original IP packets/parcels as large as 1280 octets without generating an IPv6 Path MTU Discovery (PMTUD) Packet Too Big (PTB) message [RFC8201]. Since each OAL intermediate system must configure an EMTU_R of at least 65535 octets (see: Section 6.3), the source can apply "source fragmentation" for carrier packets as large as that size but this does not affect the minimum IPv6 path MTU.)¶

IPv4 underlay interfaces are REQUIRED to configure a minimum MTU of 68 octets [RFC0791] and a minimum EMTU_R of 576 octets [RFC0791][RFC1122]. Therefore, when the Don't Fragment (DF) bit in the IPv4 header is set to 0 the minimum IPv4 path MTU is 576 octets since routers on the path support network fragmentation and the destination is required to reassemble at least that much. The OMNI interface therefore SHOULD set DF to 0 in the IPv4 encapsulation headers of carrier packets no larger than 576 octets, and SHOULD set DF to 1 in larger carrier packets unless it has a way to determine the EMTU_R of the next OAL hop as discussed in Section 6.15. This limitation is therefore relaxed by the requirement that each OAL intermediate system must configure a minimum EMTU_R of 65535 octets (see: Section 6.3) allowing for IPv4 fragmentation and reassembly for larger carrier packets.¶

The OMNI interface itself sets an "unlimited" MTU of (2**32 - 1) octets. The network layer therefore unconditionally admits all original IP packets/parcels into the OMNI interface, where the adaptation layer accommodates them if possible according to their size. For each parcel that it accommodates, the OAL source within the OMNI interface first performs "parcellation" if necessary to break large parcels into smaller sub-parcels that can transit the OAL path (see: Section 5.1). The OAL source then invokes adaptation layer encapsulation/fragmentation services to transform all original IP packets and (sub-)parcels no larger that 65535 octets into OAL packets/fragments. The OAL source then applies L2 encapsulation and fragmentation if necessary to form carrier packets and finally forwards the carrier packets via underlay interfaces.¶

When the OAL source performs IPv6 encapsulation and fragmentation (see: Section 6), the Payload Length field limits the maximum-sized original IP packet/parcel that the OAL can accommodate while applying IPv6 fragmentation to (2**16 - 1) = 65535 octets (i.e., not including the OAL encapsulation header lengths). The OAL source is also permitted to forward packets/parcels larger than this size as a best-effort delivery service if the L2 path can accommodate them through "jumbo-in-jumbo" encapsulation (see: Section 5.2); otherwise, the OAL source discards the packet and arranges to return a PTB "hard error" to the original source (see: Section 6.9).¶

Each OMNI interface therefore sets a minimum EMTU_R of 65535 octets (plus the length of the OAL encapsulation headers), and each OAL destination must consistently either accept or reject still larger whole packets that arrive over any of its underlay interfaces according to their size. If an underlay interface presents a whole packet larger than the OAL destination is prepared to accept (e.g., due to a buffer size restriction), the OAL destination discards the packet and arranges to return a PTB "hard error" to the OAL source which in turn forwards the PTB to the original source (see: Section 6.9).¶

6. The OMNI Adaptation Layer (OAL)

The OMNI interface forwards original IP packets/parcels from the network layer for transmission over one or more underlay interfaces. The OMNI Adaptation Layer (OAL) acting as the OAL source then 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. Each successive OAL intermediate system performs L2 reassembly (if necessary) then re-encapsulates by removing the L2 headers of the first underlay network and appending L2 headers appropriate for the next underlay network 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 performs L2 reassembly (if necessary) then discards the L2 headers and reassembles the resulting OAL fragments 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-6man-aero3]) 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.¶

   +-+-+-+-+-+-+-+-+
   | Parcel ID |P|S|
   +-+-+-+-+-+-+-+-+

     +----------+-------------------------+---------------+
     |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

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~               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.  ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type  | Traffic Class | OAL Hop Limit | Parcel ID |P|S|Q|F|A|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                OAL Identification (4 octets)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AFVI (2 or 4 octets)      /  Payload Len (0 or 2 octets)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | L3 Next Header|  L3 Hop Limit |Header Checksum (0 or 2 octets)|
   +~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type  | Traffic Class | OAL Hop Limit |   Index   |Resvd|F|A|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Identification (4 octets)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AFVI (2 or 4 octets)      /  Payload Len (0 or 2 octets)  |
   +~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type  | Traffic Class | OAL Hop Limit | Parcel ID |P|S|Res|A|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  OAL Identification (4 octets)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AFVI (2 or 4 octets)      /  Payload Len (0 or 2 octets)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | L3 Next Header| L3 Hop Limit  |      Fragment Offset    |Res|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       IPv6 Identification                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type  |Type of Service| OAL Hop Limit | Parcel ID |P|S|Res|A|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  OAL Identification (4 octets)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     AFVI (2 or 4 octets)      /  Payload Len (0 or 2 octets)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Version|  IHL  |      IPv4 Identification      |Flags|Offset(1)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Offset(2)   | Time to Live  |    Protocol   |  Checksum (1) |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Checksum (2) |            Options            |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    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

                <------- 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 --->

7. Ethernet-Compatible Link Layer Frame Format

When the OMNI interface forwards original IP packets/parcels from the network layer it first invokes OAL encapsulation and fragmentation, then wraps each resulting OAL packet/fragment in any necessary L2 headers to produce carrier packets according to the native frame format of the underlay interface. For example, for Ethernet-compatible interfaces the frame format is specified in [RFC2464], for aeronautical radio interfaces the frame format is specified in standards such as ICAO Doc 9776 (VDL Mode 2 Technical Manual), for various forms of tunnels the frame format is found in the appropriate tunneling specification, etc.¶

When the OMNI interface encapsulates an OAL packet/fragment directly over an Ethernet-compatible link layer, the over-the-wire transmission format is shown in Figure 15:¶

   +--- ~~~ ---+-------~~~------+---------~~~---------+--- ~~~ ---+
   |  eth-hdr  | OMNI Ext. Hdrs | OAL Packet/Fragment | eth-trail |
   +--  ~~~ ---+-------~~~------+---------~~~---------+--- ~~~ ---+
               |<-------   Ethernet Payload   -------->|

The format includes a standard Ethernet Header ("eth-hdr") with EtherType TBD2 (see: Section 21.2) followed by an Ethernet Payload that includes zero or more OMNI Extension Headers followed by an OAL (or native IPv6/IPv4) Packet/Fragment. The Ethernet Payload is then followed by a standard Ethernet Trailer ("eth-trail").¶

The first OMNI extension header and the OAL Packet/Fragment both begin with a 4-bit "Type/Version" as discussed in Section 6.2. When "Type/Version" encodes an OMNI extension header type, the length of the extension headers is limited by [I-D.ietf-6man-eh-limits] and the length of the OAL Packet/Fragment is determined by the IP header fields that follow the extension headers.¶

When "Type/Version" encodes OMNI-OFH, OMNI-OCH1/2, OMNI-IP4 or OMNI-IP6 the length of the OAL Packet/Fragment is determined by the {Total, Payload} Length field found in the full/compressed header according to the specific protocol rules.¶

See Figure 2 for a map of the various L2 layering combinations possible. For any layering combination, the final layer (e.g., UDP, IP, Ethernet, etc.) must have an assigned number and frame format representation that is compatible with the selected underlay interface.¶

8. OMNI Addressing

OMNI addressing follows the IPv6 addressing architecture [RFC4291] which states that: "IPv6 addresses of all types are assigned to interfaces, not nodes. An IPv6 unicast address refers to a single interface. Since each interface belongs to a single node, any of that node's interfaces' unicast addresses may be used as an identifier for the node." OMNI addressing further follows the IPv6 address preference policies specified in [RFC6724] as updated by [I-D.ietf-6man-rfc6724-update].¶

Each OMNI interface is configured over a set of MANET and/or ALVIF interfaces, and each ALVIF is in turn configured over a single *NET underlay interface. OMNI nodes assign IP addresses to their *NET interfaces according to the native underlay network autoconfiguration service(s) or through manual configuration. OMNI nodes assign IPv6 addresses to their OMNI, MANET and ALVIF interfaces as specified in this section.¶

[RFC4861] requires that hosts and routers assign Link-Local Addresses (LLAs) to all interfaces (including the OMNI interface), and that routers use their LLAs as the source address for RA and Redirect messages. Since the OMNI "link" comprises the concatenation of potentially many OMNI link segments, however, LLA uniqueness assurance is possible only on a per-segment basis and not across the entire OMNI link. For example, a Proxy/Server and all of the Client's that connect through it via a local *NET all share a common OMNI link segment over which LLA uniqueness applies. However, the LLAs used within a local *NET may overlap with those in other *NETs which represent different segments. Finally, MANET and ALVIF interfaces need not ensure uniqueness for the LLAs they assign since they will not be used as adaptation layer source or destination addresses nor as an identifier for the node.¶

[I-D.templin-6man-mla] specifies the MANET Local Address (MLA), and [RFC9374] specifies a Hierarchical Host Identity Tag (HHIT). OMNI nodes assign a unique MLA or HHIT to each MANET and ALVIF interface as well as to the OMNI interface itself. According to the IPv6 scoped addressing architecture [RFC4007], the node may assign the same MLA/HHIT to multiple ALVIF or MANET interfaces that connect to the same *NET. MLAs and HHITs are considered as adaptation layer addresses in the architecture, but nodes may also use them as the source and destination addresses of original IP packets exchanged between peers in isolated MANETs with no connection to the global Internet. When the node uses the OMNI interface, the original IP packet with MLA or HHIT addresses is subject to OAL encapsulation with an IPv6 header that also uses MLA or HHIT addresses. When the node uses a MANET or ALVIF interface, the original IP packet with MLA or HHIT addresses is presented for L2 encapsulation without including an OAL IPv6 header.¶

OMNI interfaces assign IPv6 Unique-Local Addresses (ULAs) and use them as the source and destination addresses in IPv6 packets forwarded over the OMNI interface within the local *NET. ULAs are routable only within the scope of each individual *NET, and are derived from the IPv6 prefix fd00::/8 (i.e., the ULA prefix fc00::/7 followed by the L bit set to 1). The 56 bits following fd00::/8 encode a 40-bit Global ID followed by a 16-bit Subnet ID followed by a 64-bit Interface Identifier as specified in Section 3 of [RFC4193].¶

When a Proxy/Server configures a ULA prefix for OMNI, it selects a 40-bit Global ID for the OMNI link segment initialized to a candidate pseudo-random value as specified in Section 3 of [RFC4193]. All nodes on the same OMNI link segment use the same Global ID, and statistical uniqueness of the pseudo-random Global ID provides a unique OMNI link segment identifier. This property allows different link segments to join together in the future without requiring renumbering even if the segments come in contact with one another and overlap, e.g., as a result of a mobility event.¶

Proxy/Servers for each OMNI link segment use the DHCPv6 service to delegate 1x1 mapped ULA/GUA SNP addresses for each Client that requests an address delegation. Clients in turn assign the ULA/GUA delegations to their OMNI interfaces which ensures that the addresses are available for use and that no duplicates will be assigned within each subnet. Considerations for 1x1 ULA/GUA address mapping are discussed in [I-D.ietf-v6ops-ula-usage-considerations] and [I-D.bctb-6man-rfc6296-bis].¶

The ULA presents an IPv6 address format that is routable within the local OMNI link segment and can be used to convey link-scoped (i.e., single-hop) IPv6 ND messages across multiple hops through OAL IPv6 encapsulation. The OMNI link extends across one or more underlying Internetworks to include all Proxy/Servers and other service nodes. All Clients are also considered to be connected to the OMNI link, however unnecessary encapsulations are omitted whenever possible to conserve bandwidth (see: Section 12).¶

OMNI domains manage MSPs delegated from the IP GUA prefix space [RFC4291] from which the MS delegates MNPs to support Client PI addressing. OMNI Proxy Servers also configure SNPs paired with a ULA configured as above to delegate PA internal (ULA) and external (GUA) addresses to Clients within their local *NETs.¶

For IPv6, MSPs are assigned to the OMNI link by IANA and/or an associated Regional Internet Registry [IPV6-GUA] such that the link can be interconnected to the global IPv6 Internet without causing inconsistencies in the routing system. Instead of GUAs, an OMNI link could use ULAs with the 'L' bit set to 0 (i.e., from the "ULA-C" prefix fc00::/8) [RFC4193], however this would require IPv6 NAT if the domain were ever connected to the global IPv6 Internet.¶

For IPv4, MSPs are assigned to the OMNI link by IANA and/or an associated RIR [IPV4-GUA] such that the link can be interconnected to the global IPv4 Internet without causing routing inconsistencies. An OMNI *NET could instead use private IPv4 prefixes (e.g., 10.0.0.0/8, etc.) [RFC3330], however this would require IPv4 NAT at the *NET boundary. OMNI interfaces advertise IPv4 MSPs into IPv6 routing systems as "6to4 prefixes" [RFC3056] (e.g., the IPv6 prefix for the IPv4 MSP "V4ADDR/24" is 2002:V4ADDR::/40).¶

IPv4 routers that configure OMNI interfaces advertise the prefix TBD3/N (see: IANA Considerations) into the routing systems of their connected *NETs and assign the IPv4 OMNI anycast address TBD3.1 to their *NET interfaces. IPv6 routers that configure OMNI interfaces advertise the prefix 2002:TBD3::/(N+16) into the routing systems of their connected *NETs and assign the IPv6 OMNI anycast address 2002:TBD3:: to their *NET interfaces.¶

Proxy/Server OMNI interfaces configure ULA/GUA IPv6 SNP SRA addresses per [RFC4291] and accept packets addressed to the SRA the same as for any IPv6 router. Proxy/Servers also configure the global IPv6 SRA address for each MSP managed by this OMNI link and accept packets addressed to the SRA address on their internal interfaces to support Client OMNI link discovery. Client OMNI interfaces configure the IPv6 SRA address corresponding to their MNP delegations.¶

OMNI interfaces use their OMNI IPv6 and IPv4 anycast addresses to support control plane Service Discovery in the spirit of [RFC7094], i.e., the addresses are not intended for use in supporting longer term data plane flows. Specific applications for OMNI IPv6 and IPv4 anycast addresses are discussed throughout the document as well as in [I-D.templin-6man-aero3].¶

9. Node Identification

OMNI Clients and Proxy/Servers that connect over open Internetworks include a unique node identification value for themselves in the OMNI options of their IPv6 ND messages (see: Section 10.2.3). An example identification value alternative is the (H)HIT per [RFC7401] and [RFC9374]. (Another example is the Universally Unique IDentifier (UUID) [RFC9562] which can be self-generated by a node without supporting infrastructure with very low probability of collision.)¶

When a Client is truly outside the context of any infrastructure, it may have no addressing information at all. In that case, the Client can use an MLA or HHIT as an IPv6 source/destination address for sustained communications in Vehicle-to-Vehicle (V2V) and (multihop) Vehicle-to-Infrastructure (V2I) scenarios. The Client can also propagate the MLA or HHIT into the multihop routing tables of (collective) Mobile/Vehicular Ad-hoc Networks (MANETs/VANETs) using only the vehicles themselves as communications relays.¶

MLAs and HHITs provide an especially useful construct since they appear as properly-formed IPv6 addresses and can therefore be assigned to interfaces. Clients may assign an MLA or HHIT to their MANET or ALVIF interfaces to support peer-to-peer communications with other nodes within the same OMNI link segment without the need for OMNI encapsulation. Clients also assign the same MLA or HHIT to the OMNI interface itself so that the MLA or HHIT can appear as the source address of an original IP packet that becomes subject to OMNI encapsulation. Clients may inject their MLA or HHIT into the local routing system of each OMNI link segment, but Proxy/Servers must not inject MLAs or HHITs into the OMNI link global routing system.¶

10. Address Mapping - Unicast

OMNI interfaces maintain a network layer conceptual neighbor cache per [RFC1256] or [RFC4861] the same as for any IP interface. 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 each of its active OAL neighbors. 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.¶

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

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

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

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

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

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

                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     | S-Type=3|    Sub-length=N     |      Type     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~    Hashed Message Authentication Code (HMAC) (N-1 Octets)     ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     | 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              ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ...

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

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

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

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

                                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     |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                         ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

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

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

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

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

11. Address Mapping - Multicast

The multicast address mapping of the native underlay interface applies. The Client mobile router also serves as an IGMP/MLD Proxy for its ENETs and/or hosted applications per [RFC4605].¶

The Client uses Multicast Listener Discovery (MLDv2) [RFC3810] to coordinate with Proxy/Servers, and underlay network elements use MLD snooping [RFC4541]. The Client can also employ multicast routing protocols to coordinate with network-based multicast sources as specified in [I-D.templin-6man-aero3].¶

Since the OMNI link model is NBMA, OMNI links support link-scoped multicast through iterative unicast transmissions to individual multicast group members (i.e., unicast/multicast emulation).¶

12. Multilink Conceptual Sending Algorithm

The Client's network layer selects the outbound OMNI interface according to SBM considerations when forwarding original IP packets/parcels from local or ENET applications to external correspondents. Each OMNI interface maintains an internal OAL neighbor cache maintained the same as discussed in [RFC4861], but also includes additional state for multilink coordination. Each Client OMNI interface maintains default routes via Proxy/Servers discovered as discussed in Section 13, and may configure more-specific routes discovered through means outside the scope of this specification.¶

For each original IP packet/parcel it forwards, the OMNI interface selects one or more source underlay interfaces based on PBM factors (e.g., traffic attributes, cost, performance, message size, etc.) and one or more target underlay interfaces for the neighbor based on Interface Attributes received in IPv6 ND messages (see: Section 10.2.5). Multilink forwarding may also direct carrier packet replication across multiple underlay interface pairs for increased reliability at the expense of duplication. The set of all Interface Attributes and Traffic Selectors received in IPv6 ND messages determines the multilink forwarding profile for selecting target underlay interfaces.¶

When the OMNI interface forwards an original IP packet/parcel over a selected source underlay interface, it first employs OAL encapsulation and fragmentation as discussed in Section 5, then performs L2 encapsulation as directed by the appropriate AFV. The OMNI interface also performs L2 encapsulation (following OAL encapsulation) when the nearest Proxy/Server is located multiple hops away as discussed in Section 13.2.¶

OMNI interface multilink service designers MUST observe the BCP guidance in Section 15 [RFC3819] in terms of implications for reordering when original IP packets/parcels from the same flow may be spread across multiple underlay interfaces having diverse properties.¶

13. Router Discovery and Prefix Delegation

Clients engage their FHS Proxy/Servers and the MS by sending OAL encapsulated 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 may in turn forward a proxyed copy to a MAP Proxy/Server located in a local or remote SRT segment if the Client requires MNP service. The MAP Proxy/Server then returns an OAL encapsulated 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 10.2.5. 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 (normally 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 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 OMNI link segment and advertises its L2ADDR(s) 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 an SNP ULA for the local OMNI link segment if it has one (otherwise with source set to the unspecified address ("::/128") per [RFC4861]) and with destination set to either the SRA GUA of a specific (MAP) Proxy/Server or link-scoped All-Routers multicast (ff02::2) [RFC4291]. The Client includes an OMNI option per Section 10 with a Neighbor Control sub-option with the RS NUD, ARR and RPT flags set or cleared as necessary.¶

Clients in MANETs and open INET deployments also include an OMNI Multilink Vector sub-option with FHS ifIndex set to the ifIndex of its own underlay interface and with LHS ifIndex set to 0 (i.e., the default ifIndex configured by all Proxy/Servers). The Client also sets AFVI to 0, sets Sequence Number to a randomly-chosen 8-octet value and sets the Flow Label in the IPv6 header to 0. The resulting exchange will establish symmetric Identification windows for the Client and Proxy/Server for use in authenticating control messages.¶

The Client next includes an Interface Attributes sub-option for the underlay interface, a DHCPv6 Solicit sub-option with IA_NA and (optionally) IA_PD DHCPv6 options, 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 its desired FHS Proxy/Server service model as described in Section 10.2.5.¶

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 if necessary followed by the RS message checksum. The OMNI interface next sets the OAL source address to the MLA or HHIT for the outgoing MANET or ALVIF interface and sets the OAL destination to site-scoped All-Routers multicast (ff05::2) [RFC4291], a known Proxy/Server SNP SRA ULA or an anycast address. 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 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 performs L2 reassembly if necessary, sets aside the L2 and OAL headers, 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 (in which case 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 searches for DHCPv6 IA_NA options in the OMNI DHCPv6 sub-option. If 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 if present (see: Section 10.1) and examines the RS destination address.¶

If the destination matches one of its own addresses and the OMNI DHCPv6 sub-option includes DHCPv6 IA_PD options, the FHS Proxy/Server assumes the MAP role 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 10.1).¶

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 responsive Multilink Vector sub-option with AFVI set to 0 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 10.2.5 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 10.2.17.1), 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 2 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 SNP 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 MLA or HHIT and destination to the OAL source that appeared in the RS, 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 (ff02::2), 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 caches the Multilink Vector sub-option and removes it from the RS message, 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 RS message checksum, sets the OAL source to the Client's SNP GUA and destination to the MAP Proxy/Server SNP SRA GUA, 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 and OAL 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 RA message checksum then encapsulates the RA as an OAL packet with source set to the RS message destination (i.e., its own SNP SRA GUA) and destination set to the RS message source (i.e., the Client's SNP GUA). The MAP Proxy/Server finally performs 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 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 ULA 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 2 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 from 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 calculates the authentication signature and RA message checksum, 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 decapsulation to obtain the RA message. The Client next verifies the authentication signature/checksum, then matches the RA 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 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 for use in future IPv6 ND message exchanges via this FHS Proxy/Server. 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-6man-aero3]).¶

• 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 SNP SRA GUA 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 can either declare this underlay interface as DOWN or continue to use the interface to support any peer-to-peer local communications with peers located in the same *NET. 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-6man-aero3].¶

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: 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 SRA GUA 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 forwards the resulting carrier packets without disturbing the MAP.¶

14. Secure Redirection

If the *NET link model is multiple access, the FHS Proxy/Server is responsible for assuring that address duplication cannot corrupt the neighbor caches of other nodes on the link through the use of the DHCPv6 address delegation service. When the Client sends an RS message on a multiple access *NET, the Proxy/Server verifies that the Client is authorized to use the address and responds with an RA (or forwards the RS to the MAP) only if the Client is authorized.¶

After verifying Client authorization and returning an RA, the Proxy/Server MAY return IPv6 ND Redirect messages in response to subsequent data plane packet transmissions to direct Clients located on the same *NET to exchange OAL packets directly without transiting the Proxy/Server. In that case, the Clients can exchange OAL packets according to their unicast L2 addresses discovered from the Redirect message instead of using the dogleg path through the Proxy/Server. In some *NETs, however, such direct communications may be undesirable and continued use of the dogleg path through the Proxy/Server may provide better performance. In that case, the Proxy/Server can refrain from sending Redirects, and/or Clients can ignore them.¶

15. Proxy/Server Resilience

*NETs SHOULD deploy Proxy/Servers in Virtual Router Redundancy Protocol (VRRP) [RFC5798] configurations so that service continuity is maintained even if one or more Proxy/Servers fail. Using VRRP, the Client is unaware which of the (redundant) FHS Proxy/Servers is currently providing service, and any service discontinuity will be limited to the failover time supported by VRRP. Widely deployed public domain implementations of VRRP are available.¶

Proxy/Servers SHOULD use high availability clustering services so that multiple redundant systems can provide coordinated response to failures. As with VRRP, widely deployed public domain implementations of high availability clustering services are available. Note that special-purpose and expensive dedicated hardware is not necessary, and public domain implementations can be used even between lightweight virtual machines in cloud deployments.¶

16. Detecting and Responding to Proxy/Server Failures

In environments where fast recovery from Proxy/Server failure is essential, FHS Proxy/Servers SHOULD use proactive Neighbor Unreachability Detection (NUD) in a manner that parallels Bidirectional Forwarding Detection (BFD) [RFC5880] to track MAP Proxy/Server reachability. FHS Proxy/Servers can then quickly detect and react to failures so that cached information is re-established through alternate paths. Proactive NUD control messaging is carried only over well-connected ground domain networks (i.e., and not low-end links such as aeronautical radios) and can therefore be tuned for rapid response.¶

FHS Proxy/Servers perform proactive NUD for MAP Proxy/Servers for which there are currently active Clients. If a MAP Proxy/Server fails, the FHS Proxy/Server can quickly inform Clients of the outage by sending multicast RA messages. The FHS Proxy/Server sends RA messages to Clients with source set to the ULA of the MAP, with destination address set to link-scoped All-Nodes multicast (ff02::1) [RFC4291] and with Router Lifetime set to 0.¶

The FHS Proxy/Server SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated by small delays [RFC4861]. Any Clients that have been using the (now defunct) MAP Proxy/Server will receive the RA messages.¶

17. Transition Considerations

When a Client connects to a *NET link for the first time, it sends an RS message with an OMNI option. If the first hop router recognizes the option, it responds according to the appropriate FHS/MAP Proxy/Server role resulting in an RA message with an OMNI option returned to the Client. The Client then engages this FHS Proxy/Sever according to the OMNI link model specified above. If the first hop router is a legacy IPv6 router, however, it instead returns an RA message with no OMNI option and with an ordinary unicast source LLA as specified in [RFC4861]. In that case, the Client engages the *NET according to the legacy IPv6 link model and without the OMNI extensions specified in this document.¶

If the *NET link model is multiple access, there must be assurance that address duplication cannot corrupt the neighbor caches of other nodes on the link. When the Client sends an RS message on a multiple access *NET link with an OMNI option, first hop routers that recognize the option ensure that the Client is authorized to use the address and return an RA with a non-zero Router Lifetime only if the Client is authorized. First hop routers that do not recognize the OMNI option instead return an RA that makes no statement about the Client's authorization to use the source address. In that case, the Client should perform Duplicate Address Detection to ensure that it does not interfere with other nodes on the link.¶

An alternative approach for multiple access *NET links to ensure isolation for Client-Proxy/Server communications is through link layer address mappings as discussed in Appendix E. This arrangement imparts a (virtual) point-to-point link model over the (physical) multiple access link.¶

18. OMNI Interfaces on Open Internetworks

Client OMNI interfaces configured over IPv6-enabled underlay interfaces on an open Internetwork without an OMNI-aware first-hop router receive IPv6 RA messages with no OMNI options, while OMNI interfaces configured over IPv4-only underlay interfaces receive no IPv6 RA messages at all (but may receive IPv4 RA messages per [RFC1256]). Client OMNI interfaces that receive RA messages with OMNI options configure addresses, on-link prefixes, etc. on the underlay interface that received the RA according to standard IPv6 ND and address resolution conventions [RFC4861] [RFC4862]. Client OMNI interfaces configured over IPv4-only underlay interfaces configure IPv4 address information on the underlay interfaces using mechanisms such as DHCPv4 [RFC2131].¶

Client OMNI interfaces configured over underlay interfaces connected to open Internetworks can apply lower layer security services such as VPNs (e.g., IPsec tunnels) to connect to a Proxy/Server, or can establish a secured direct point-to-point link to the Proxy/Server through some other means (see Section 4). In environments where lower layer security may be impractical or undesirable, Client OMNI interfaces can instead send IPv6 ND messages with OMNI options that include authentication signatures.¶

OMNI interfaces use UDP/IP as L2 encapsulation headers for transmission over open Internetworks with UDP service port number 8060 for both IPv4 and IPv6 underlay interfaces. The OMNI interface submits original IP packets/parcels for OAL encapsulation, then encapsulates the resulting OAL fragments in UDP/IP L2 headers to form carrier packets. (The first 4 bits following the UDP header determine whether the OAL headers are uncompressed/compressed as discussed in Section 6.5.) The OMNI interface sets the UDP length to the encapsulated OAL fragment length and sets the IP length to an appropriate value at least as large as the UDP datagram.¶

When necessary, sources include an OMNI option with an authentication sub-option in IPv6 ND messages. The source can employ a simple Hashed Message Authentication Code (HMAC) as specified in [RFC2104][RFC6234], EdDSA [RFC8032], or a message-based authentication service such as HIP [RFC7401], QUIC-TLS [RFC9000][RFC9001], etc., by using the IPv6 ND message OMNI option as a "shipping container". Before calculating the authentication signature, the source fully populates any necessary OMNI sub-options as well as any ordinary IPv6 ND options as necessary.¶

The source then sets both the IPv6 ND message Checksum and authentication signature fields to 0 and calculates the authentication signature over the full length of the IPv6 ND message beginning after the IPv6 ND message checksum field and extending over the length of the message. (If the IPv6 ND message is part of an OAL super-packet, the source instead continues to calculate the authentication signature over the entire length of the super-packet.) The source next writes the authentication signature into the appropriate sub-option field, calculates and writes the message checksum, then forwards the message.¶

After establishing a secured underlay link or preparing for UDP/IP encapsulation, OMNI interfaces send RS/RA messages for Client-Proxy/Server coordination (see: Section 13) and NS/NA messages for multilink forwarding, route optimization, and mobility management (see: [I-D.templin-6man-aero3]). These control plane messages must be authenticated while other control and data plane messages are delivered the same as for ordinary best effort traffic with source address and/or Identification window-based data origin verification. Transport and higher layer protocol sessions over OMNI interfaces that connect over open Internetworks without an explicit underlay link security services should therefore employ security at their layers to ensure authentication, integrity and/or confidentiality.¶

Clients should avoid using INET Proxy/Servers as general-purpose routers for steady streams of carrier packets that do not require authentication. Clients should therefore perform route optimization to coordinate with other INET nodes that can provide forwarding services (or preferably coordinate with peer Clients directly) instead of burdening the Proxy/Server. Procedures for coordinating with peer Clients and discovering INET nodes that can provide better forwarding services are discussed in [I-D.templin-6man-aero3].¶

Clients that attempt to contact peers over INET underlay interfaces often encounter NATs in the path. OMNI interfaces accommodate NAT traversal using UDP/IP encapsulation and the mechanisms discussed in [I-D.templin-6man-aero3]. FHS Proxy/Servers include Origin Indications in RA messages to allow Clients to detect the presence of NATs.¶

Note: Following the initial IPv6 ND message exchange, OMNI interfaces configured over INET underlay interfaces maintain neighbor relationships by transmitting periodic IPv6 ND messages with OMNI options that include authentication signatures. Other authentication services that use their own IPv6 ND option types such as [RFC3971] and [RFC8928] can also be used in addition to any OMNI authentication services.¶

Note: OMNI interfaces configured over INET underlay interfaces should employ the Identification window synchronization mechanisms specified in Section 6.7 in order to exclude spurious carrier packets that might otherwise clutter the reassembly cache. This is especially important in environments where carrier packet spoofing and/or corruption is a threat.¶

Note: NATs may be present on the path from a Client to its FHS Proxy/Server, but never on the path from the FHS Proxy/Server to the MAP where only INET and/or spanning tree hops occur. Therefore, the FHS Proxy/Server does not communicate Client origin information to the MAP where it would serve no purpose.¶

19. Time-Varying MNPs

In some use cases, it is desirable, beneficial and efficient for the Client to receive a constant MNP that travels with the Client wherever it moves. For example, this would allow air traffic controllers to easily track aircraft, etc. In other cases, however (e.g., intelligent transportation systems), the Client may be willing to sacrifice a modicum of efficiency in order to have time-varying MNPs that can be changed occasionally to defeat adversarial tracking.¶

The prefix delegation services discussed in Section 13.3 allows Clients that desire time-varying MNPs to obtain short-lived prefixes to send RS messages with an OMNI option with DHCPv6 IA_PD sub-options. The Client would then be obligated to renumber its internal networks whenever its MNPs change. This should not present a challenge for Clients with automated network renumbering services, but may disrupt persistent sessions that would prefer to use a constant address.¶

20. Error Messages

An OAL destination or intermediate system may need to return ICMPv6-like error messages (e.g., Destination Unreachable, Packet Too Big, Time Exceeded, etc.) [RFC4443] to an OAL source. Since ICMPv6 error messages do not themselves include authentication codes, OAL nodes can instead return error messages as an OMNI ICMPv6 Error sub-option in a secured IPv6 ND uNA message.¶

21. IANA Considerations

The following IANA actions are requested in accordance with [RFC8126] and [RFC8726]:¶

   Addresses      Usage                                         Reference
   ---------      -----                                         ---------
   00-52-14       Overlay Multilink Network (OMNI) Interface    [RFCXXXX]

   Code            Name                         Reference
   ---             ----                         ---------
   0               PTB Hard Error               [RFC4443]
   1 (suggested)   PTB Soft Error (no loss)     [RFCXXXX]
   2 (suggested)   PTB Soft Error (loss)        [RFCXXXX]

   Code            Name                         Reference
   ---             ----                         ---------
   0               Reserved                     [RFCXXXX]
   1 (suggested)   PTB Soft Error (no loss)     [RFCXXXX]
   2 (suggested)   PTB Soft Error (loss)        [RFCXXXX]

   Value    Sub-Type name                  Reference
   -----    -------------                  ----------
   0        Pad1                           [RFCXXXX]
   1        PadN                           [RFCXXXX]
   2        Node Identification            [RFCXXXX]
   3        Authentication                 [RFCXXXX]
   4        Neighbor Control               [RFCXXXX]
   5        Interface Attributes           [RFCXXXX]
   6        Traffic Selector               [RFCXXXX]
   7        Multilink Vector               [RFCXXXX]
   8        Geo Coordinates                [RFCXXXX]
   9        DHCPv6 Message                 [RFCXXXX]
   10       PIM-SM Message                 [RFCXXXX]
   11       HIP Message                    [RFCXXXX]
   12       QUIC-TLS Message               [RFCXXXX]
   13       Fragmentation Report           [RFCXXXX]
   14       ICMPv6 Error                   [RFCXXXX]
   15       Proxy/Server Departure         [RFCXXXX]
   16-29    Unassigned
   30       Sub-Type Extension             [RFCXXXX]
   31       Reserved by IANA               [RFCXXXX]

   Value    Sub-Type name                  Reference
   -----    -------------                  ----------
   0        UUID                           [RFCXXXX]
   1        HIT                            [RFCXXXX]
   2        HHIT                           [RFCXXXX]
   3        Network Access Identifier      [RFCXXXX]
   4        FQDN                           [RFCXXXX]
   5        IPv6 Address                   [RFCXXXX]
   6-252    Unassigned                     [RFCXXXX]
   253-254  Reserved for Experimentation   [RFCXXXX]
   255      Reserved by IANA               [RFCXXXX]

   Value    Sub-Type name                  Reference
   -----    -------------                  ----------
   0        NULL                           [RFCXXXX]
   1-252    Unassigned                     [RFCXXXX]
   253-254  Reserved for Experimentation   [RFCXXXX]
   255      Reserved by IANA               [RFCXXXX]

   Value    Sub-Type name                  Reference
   -----    -------------                  ----------
   0        RFC4380 UDP/IP Header Option   [RFCXXXX]
   1        RFC6081 UDP/IP Trailer Option  [RFCXXXX]
   2-252    Unassigned
   253-254  Reserved for Experimentation   [RFCXXXX]
   255      Reserved by IANA               [RFCXXXX]

   Value    Sub-Type name                  Reference
   -----    -------------                  ----------
   0        Origin Indication (IPv4)       [RFC4380]
   1        Authentication Encapsulation   [RFC4380]
   2        Origin Indication (IPv6)       [RFCXXXX]
   3-252    Unassigned
   253-254  Reserved for Experimentation   [RFCXXXX]
   255      Reserved by IANA               [RFCXXXX]

   Value    Sub-Type name                  Reference
   -----    -------------                  ----------
   0        Unassigned
   1        Nonce                          [RFC6081]
   2        Unassigned
   3        Alternate Address (IPv4)       [RFC6081]
   4        Neighbor Discovery Option      [RFC6081]
   5        Random Port                    [RFC6081]
   6        Alternate Address (IPv6)       [RFCXXXX]
   7-252    Unassigned
   253-254  Reserved for Experimentation   [RFCXXXX]
   255      Reserved by IANA               [RFCXXXX]

22. Security Considerations

Security considerations for IPv4 [RFC0791], IPv6 [RFC8200] and IPv6 Neighbor Discovery [RFC4861] apply. OMNI interface IPv6 ND messages SHOULD include Nonce and Timestamp options [RFC3971] when transaction confirmation and/or time synchronization is needed.¶

OMNI interfaces configured over secured ANET/ENET interfaces inherit the physical and/or link layer security properties (i.e., "protected spectrum") of the connected networks. OMNI interfaces configured over open *NET interfaces can use symmetric securing services such as IPsec tunnels [RFC4301] or can by some other means establish a direct point-to-point link secured at lower layers. When lower layer security may be impractical or undesirable, however, control message integrity and authorization services such as those specified in [RFC7401], [RFC4380], [RFC6234], [RFC8032], [RFC9000], etc. must be employed.¶

OMNI link mobility services MUST support strong network layer authentication for control plane messages and forwarding path integrity for data plane messages. In particular, the AERO service [I-D.templin-6man-aero3] constructs a secured spanning tree with Proxy/Servers as leaf nodes and secures the spanning tree links with network layer security services based on IPsec [RFC4301] with IKEv2 [RFC7296]. (Note that direct point-to-point links secured at lower layers can also be used instead of or in addition to network layer security.) These network (and/or lower-layer) services together provide connectionless integrity and data origin authentication with optional protection against replays.¶

Control plane messages that affect the routing system or neighbor state are constrained to travel only over secured spanning tree paths and are therefore protected by network (and/or lower-layer) security. Other control and data plane messages can travel over unsecured route optimized paths that do not strictly follow the spanning tree, therefore end-to-end sessions should employ transport or higher layer security services (e.g., TLS/SSL [RFC8446], DTLS [RFC6347], etc.). Additionally, the OAL Identification value can provide a first level of data origin authentication to mitigate off-path spoofing.¶

Identity-based key verification infrastructure services such as iPSK may be necessary for verifying the identities claimed by Clients. This requirement should be harmonized with the manner in which identifiers such as (H)HITs are attested in a given operational environment.¶

Security considerations for specific access network interface types are covered under the corresponding IP-over-(foo) specification (e.g., [RFC2464], [RFC2492], etc.).¶

Security considerations for IPv6 fragmentation and reassembly are discussed in Section 6.15. In environments where spoofing is considered a threat, OMNI nodes SHOULD employ Identification window synchronization and OAL destinations SHOULD configure an (end-system-based) firewall.¶

23. Implementation Status

AERO/OMNI Release-3.2 was tagged on March 30, 2021, and was subject to internal testing. The implementation is not planned for public release.¶

A new implementation architecture based on a clean-slate has been developed and will incorporate updated aspects of the AERO/OMNI specs, with the goal of producing a reference implementation for future release.¶

24. Document Updates

This document suggests that the following could be updated through future IETF initiatives:¶

• [RFC1191]¶

• [RFC2675]¶

• [RFC4443]¶

• [RFC8200]¶

• [RFC8201]¶

Updates can be through, e.g., standards action, the errata process, etc. as appropriate.¶

25. Acknowledgements

The first version of this document was prepared per the consensus decision at the 7th Conference of the International Civil Aviation Organization (ICAO) Working Group-I Mobility Subgroup on March 22, 2019. Consensus to take the document forward to the IETF was reached at the 9th Conference of the Mobility Subgroup on November 22, 2019. Attendees and contributors included: Guray Acar, Danny Bharj, Francois D´Humieres, Pavel Drasil, Nikos Fistas, Giovanni Garofolo, Bernhard Haindl, Vaughn Maiolla, Tom McParland, Victor Moreno, Madhu Niraula, Brent Phillips, Liviu Popescu, Jacky Pouzet, Aloke Roy, Greg Saccone, Robert Segers, Michal Skorepa, Michel Solery, Stephane Tamalet, Fred Templin, Jean-Marc Vacher, Bela Varkonyi, Tony Whyman, Fryderyk Wrobel and Dongsong Zeng.¶

The following individuals are acknowledged for their useful comments: Amanda Baber, Scott Burleigh, Stuart Card, Donald Eastlake, Adrian Farrel, Michael Matyas, Robert Moskowitz, Madhu Niraula, Greg Saccone, Stephane Tamalet, Eliot Lear, Eduard Vasilenko, Eric Vyncke. Pavel Drasil, Zdenek Jaron and Michal Skorepa are especially recognized for their many helpful ideas and suggestions. Akash Agarwal, Madhuri Madhava Badgandi, Sean Dickson, Don Dillenburg, Joe Dudkowski, Vijayasarathy Rajagopalan, Ron Sackman, Bhargava Raman Sai Prakash and Katherine Tran are acknowledged for their hard work on the implementation and technical insights that led to improvements for the spec.¶

Discussions on the IETF 6man and atn mailing lists during the fall of 2020 suggested additional points to consider. The authors gratefully acknowledge the list members who contributed valuable insights through those discussions. Eric Vyncke and Erik Kline were the intarea ADs, while Bob Hinden and Ole Troan were the 6man WG chairs at the time the document was developed; they are all gratefully acknowledged for their many helpful insights. Many of the ideas in this document have further built on IETF experiences beginning in the 1990s, with insights from colleagues including Ron Bonica, Brian Carpenter, Ralph Droms, Tom Herbert, Bob Hinden, Christian Huitema, Thomas Narten, Dave Thaler, Joe Touch, Pascal Thubert, and many others who deserve recognition.¶

Early observations on IP fragmentation performance implications were noted in the 1986 Digital Equipment Corporation (DEC) "qe reset" investigation, where fragment bursts from NFS UDP traffic triggered hardware resets resulting in communication failures. Jeff Chase, Fred Glover and Chet Juzsczak of the Ultrix Engineering Group led the investigation, and determined that setting a smaller NFS mount block size reduced the amount of fragmentation and suppressed the resets. Early observations on L2 media MTU issues were noted in the 1988 DEC FDDI investigation, where Raj Jain, KK Ramakrishnan and Kathy Wilde represented architectural considerations for FDDI networking in general including FDDI/Ethernet bridging. Jeff Mogul (who led the IETF Path MTU Discovery working group) and other DEC colleagues who supported these early investigations are also acknowledged.¶

Throughout the 1990's and into the 2000's, many colleagues supported and encouraged continuation of the work. Beginning with the DEC Project Sequoia effort at the University of California, Berkeley, then moving to the DEC research lab offices in Palo Alto CA, then to Sterling Software at the NASA Ames Research Center, then to SRI in Menlo Park, CA, then to Nokia in Mountain View, CA and finally to the Boeing Company in 2005 the work saw continuous advancement through the encouragement of many. Those who offered their support and encouragement are gratefully acknowledged.¶

This work is aligned with the NASA Safe Autonomous Systems Operation (SASO) program under NASA contract number NNA16BD84C.¶

This work is aligned with the FAA as per the SE2025 contract number DTFAWA-15-D-00030.¶

This work is aligned with the Boeing Information Technology (BIT) Mobility Vision Lab (MVL) program.¶

Honoring life, liberty and the pursuit of happiness.¶

Appendix A. IPv4 Reassembly Checksum Algorithm

The IPv4 reassembly checksum algorithm adopts the 8-bit Fletcher algorithm specified in Appendix I of [RFC1146] as also analyzed in [CKSUM]. [RFC6247] declared [RFC1146] historic for the reason that the algorithms had never seen widespread use with TCP, however this document adopts the 8-bit Fletcher algorithm for a different purpose. Quoting from Appendix I of [RFC1146], the IPv4 Fragmentation Checksum Algorithm proceeds as follows:¶

• "The 8-bit Fletcher Checksum Algorithm is calculated over a sequence of data octets (call them D[1] through D[N]) by maintaining 2 unsigned 1's-complement 8-bit accumulators A and B whose contents are initially zero, and performing the following loop where i ranges from 1 to N:¶

  • A := A + D[i]¶

  • B := B + A¶

  It can be shown that at the end of the loop A will contain the 8-bit 1's complement sum of all octets in the datagram, and that B will contain (N)D[1] + (N-1)D[2] + ... + D[N]."¶

To calculate the IPv4 reassembly checksum, the above algorithm is applied over the N-octets of the L2-encapsulated OAL packet/fragment body beginning immediately after the L2 encapsulation header(s).¶

Appendix B. IPv6 Compatible Addresses

Section 2.5.5.1 of [RFC4291] defines an "IPv4-Compatible IPv6 Address" with the following structure:¶

   |                80 bits               | 16 |      32 bits        |
   +--------------------------------------+----+---------------------+
   |0000..............................0000|0000|    IPv4 address     |
   +--------------------------------------+----+---------------------+

Although [RFC4291] deprecates the address format from its former use in IPv6 transition mechanisms, this document now assigns new uses and therefore updates [RFC4291].¶

When an IPv4-Compatible IPv6 address appears in a packet sent over the wire, the most significant 96 bits are 0 and the least significant 32 bits include an IPv4 address as shown above.¶

When the address format is used for temporary local address conversions to IPv6, however, it can also be used to represent EUI-48 and EUI-64 addresses as shown below:¶

   |                80 bits               |          48 bits         |
   +--------------------------------------+--------------------------+
   |0000..............................0000|      EUI-48 address      |
   +--------------------------------------+--------------------------+

   |             64 bits            |             64 bits            |
   +--------------------------------+--------------------------------+
   |0000........................0000|         EUI-64 address         |
   +--------------------------------+--------------------------------+

The above EUI-48 and EUI-64 compatible IPv6 forms MAY be used for temporary local address conversions, such as when converting EUI addresses to IPv6 to support IPv6 fragmentation/reassembly. The address forms MUST NOT appear in the IPv6 headers of packets sent over the wire, however they MAY appear in the body of a packet if also accompanied by a Type designator.¶

Appendix C. IPv6 ND Message Authentication and Integrity

OMNI interface IPv6 ND messages are subject to authentication and integrity checks at multiple levels. When an OMNI interface sends an IPv6 ND message over an INET interface, it includes an authentication sub-option with a valid signature if necessary and always includes an IPv6 ND message checksum. The OMNI interface that receives the message verifies the IPv6 ND message checksum followed by the authentication signature (if present) to ensure IPv6 ND message integrity and authenticity.¶

When an OMNI interface sends an IPv6 ND message over an underlay interface connected to a secured network, it omits authentication (sub-)options but always calculates/includes an IPv6 ND message checksum beginning with a pseudo-header of the IPv6 header and extending to the end of the IPv6 ND message only with the Checksum field itself set to 0. When an OMNI interface sends an IPv6 ND message over an underlay interface connected to an unsecured network, it first includes an authentication (sub-)option and calculates the signature beginning with the first octet following the IPv6 ND message header Checksum field and extending to the end of the entire packet or super-packet with the authentication signature field set to 0. The OMNI interface next writes the signature into the signature field, then calculates the IPv6 ND message checksum as above.¶

The OMNI interface that receives the message applies any link layer authentication and integrity checks, then verifies the IPv6 ND message checksum. If the checks are correct, the OMNI interface next verifies the authentication signature. The OMNI interface then processes the packet further only if all checksums and authentication signatures were correct.¶

OAL destinations also discard carrier packets with unacceptable Identifications and submit the encapsulated fragments in all others for reassembly. The reassembly algorithm rejects any fragments with unacceptable sizes, offsets, etc. and reassembles all others. During reassembly, the extended Identification value provides an integrity assurance vector that compliments any integrity checks already applied by lower layers as well as a first-pass filter for any checks that will be applied later by upper layers.¶

Appendix D. VDL Mode 2 Considerations

ICAO Doc 9776 is the "Technical Manual for VHF Data Link Mode 2" (VDLM2) that specifies an essential radio frequency data link service for aircraft and ground stations in worldwide civil aviation air traffic management. The VDLM2 link type is "multicast capable" [RFC4861], but with considerable differences from common multicast links such as Ethernet and IEEE 802.11.¶

First, the VDLM2 link data rate is only 31.5Kbps - multiple orders of magnitude less than most modern wireless networking gear. Second, due to the low available link bandwidth only VDLM2 ground stations (i.e., and not aircraft) are permitted to send broadcasts, and even so only as compact link layer "beacons". Third, aircraft employ the services of ground stations by performing unicast RS/RA exchanges upon receipt of beacons instead of listening for multicast RA messages and/or sending multicast RS messages.¶

This beacon-oriented unicast RS/RA approach is necessary to conserve the already-scarce available link bandwidth. Moreover, since the numbers of beaconing ground stations operating within a given spatial range must be kept as sparse as possible, it would not be feasible to have different classes of ground stations within the same region observing different protocols. It is therefore highly desirable that all ground stations observe a common language of RS/RA as specified in this document.¶

Note that links of this nature may benefit from compression techniques that reduce the bandwidth necessary for conveying the same amount of data. The IETF lpwan working group is considering possible alternatives: [https://datatracker.ietf.org/wg/lpwan/documents].¶

Appendix E. Client-Proxy/Server Isolation Through Link-Layer Address Mapping

Per [RFC4861], IPv6 ND messages may be sent to either a multicast or unicast link-scoped IPv6 destination address. However, IPv6 ND messaging should be coordinated between the Client and Proxy/Server only without invoking other nodes on the underlay network. This implies that Client-Proxy/Server control messaging should be isolated and not overheard by other nodes on the link.¶

To support Client-Proxy/Server isolation on some links, Proxy/Servers can maintain an OMNI-specific unicast link layer address ("MSADDR"). For Ethernet-compatible links, this specification reserves one Ethernet unicast address TBD5 (see: IANA Considerations). For non-Ethernet statically-addressed links MSADDR is reserved per the assigned numbers authority for the link layer addressing space. For still other links, MSADDR may be dynamically discovered through other means, e.g., link layer beacons.¶

Clients map the L3 addresses of all IPv6 ND messages they send (i.e., both multicast and unicast) to MSADDR instead of to an ordinary unicast or multicast link layer address. In this way, all of the Client's IPv6 ND messages will be received by Proxy/Servers that are configured to accept carrier packets destined to MSADDR. Note that multiple Proxy/Servers on the link could be configured to accept carrier packets destined to MSADDR, e.g., as a basis for supporting redundancy.¶

Therefore, Proxy/Servers must accept and process carrier packets destined to MSADDR, while all other devices must not process carrier packets destined to MSADDR. This model has well-established operational experience in Proxy Mobile IPv6 (PMIP) [RFC5213][RFC6543].¶

Appendix F. Change Log

<< RFC Editor - remove prior to publication >>¶

Differences from earlier versions:¶

• Submit for review.¶

Author's Address