Automatic Extended Route Optimization (AERO)

4.2.1. AERO/OMNI Reference Model

Figure 1 presents the basic OMNI link reference model:¶

                      +-----------------+
                      | AERO Gateway G1 |
                      | Nbr: S1, S2, P1 |
                      |(X1->S1; X2->S2) |
                      |      MSP M1     |
                      +--------+--------+
    +--------------+           |            +--------------+
    |  AERO P/S S1 |           |            |  AERO P/S S2 |
    |  Nbr: C1, G1 |           |            |  Nbr: C2, G1 |
    |  default->G1 |           |            |  default->G1 |
    |    X1->C1    |           |            |    X2->C2    |
    +-------+------+           |            +------+-------+
            |       OMNI link  |                   |
    X===+===+==================+===================+===+===X
        |                                              |
  +-----+--------+                            +--------+-----+
  |AERO Client C1|                            |AERO Client C2|
  |    Nbr: S1   |                            |   Nbr: S2    |
  | default->S1  |                            | default->S2  |
  |    MNP X1    |                            |    MNP X2    |
  +------+-------+                            +-----+--------+
         |                                          |
        .-.                                        .-.
     ,-(  _)-.     +-------+     +-------+      ,-(  _)-.
  .-(_  IP   )-.   |  AERO |     |  AERO |    .-(_  IP   )-.
(__    ENET     )--|Host H1|     |Host H2|--(__    ENET     )
   `-(______)-'    +-------+     +-------+     `-(______)-'

In this model:¶

• the OMNI link is an overlay network service configured over one or more underlay SRT segments which may be managed by diverse administrative domains using incompatible protocols and/or addressing plans.¶

• AERO Gateway G1 aggregates Mobility Service Prefix (MSP) M1, discovers Mobile Network Prefixes (MNPs) X* and advertises the MSP via BGP peerings over secured tunnels to other Gateways in the SRT (not shown). Together, the set of all Gateways provide the backbone for an SRT spanning tree for the OMNI link.¶

• AERO Proxy/Servers S1 and S2 configure secured tunnels with Gateway G1 and also provide mobility, multilink, multicast and default router services for the MNPs of their associated Clients C1 and C2. (Proxy/Servers that act as Relays can also advertise FNP routes for non-mobile correspondent nodes the same as for MNP Clients.)¶

• AERO Clients C1 and C2 associate with Proxy/Servers S1 and S2, respectively. They receive MNP delegations X1 and X2, and also act as default routers for their associated physical or internal virtual ENETs.¶

• AERO Hosts H1 and H2 attach to the ENETs served by Clients C1 and C2, respectively.¶

An OMNI link configured over a single underlay network appears as a single unified link with a consistent addressing plan; all nodes on the link can exchange carrier packets via simple L2 encapsulation (i.e., following any necessary NAT traversal) since the underlay is connected. In common practice, however, OMNI links are often configured over an SRT spanning tree that bridges multiple distinct underlay network segments managed under different administrative authorities (e.g., as for worldwide aviation service providers such as ARINC, SITA, Inmarsat, etc.). Individual underlay networks may also be partitioned internally, in which case each internal partition appears as a separate segment.¶

The addressing plan of each SRT segment is consistent internally but will often bear no relation to the addressing plans of other segments. Each segment is also likely to be separated from others by network security devices (e.g., firewalls, proxys, packet filtering gateways, etc.), and disjoint segments often have no common physical link connections. Therefore, nodes can only be assured of exchanging carrier packets directly with correspondents in the same segment, and not with those in other segments. The only means for joining the segments therefore is through inter-domain peerings between AERO Gateways.¶

The OMNI link spans multiple SRT segments using the OAL to provide the network layer with a virtual abstraction similar to a bridged campus LAN. The OAL is an OMNI interface sublayer that inserts a mid-layer IPv6 encapsulation header for inter-segment forwarding (i.e., bridging) without decrementing the network layer TTL/Hop Limit of the original IP packet/parcel. An example OMNI link SRT is shown in Figure 2:¶

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
  .                                                                           .
 .                                                                             .
 .     .-(::::::::)                .-(::::::::)               .-(::::::::)     .
 .  .-(::::::::::::)-.   +-+    .-(::::::::::::)-.   +-+   .-(::::::::::::)-.  .
 . (::::    FHS    :::)--|G|--(::: Intermediate ::)--|G|--(::::    LHS    :::) .
 .  `-(::::::::::::)-'   +-+    `-(::Segments::)-'   +-+   `-(::::::::::::)-'  .
 .     `-(::::::)-'                `-(::::::)-'               `-(::::::)-'     .
 .           |                                                      |          .
 .         +---+                                                  +---+        .
 .         |P/S|                                                  |P/S|        .
 .         +---+                                                  +---+        .
 .           |                                                      |          .
 .     .-(::::::::)                                          .-(::::::::)      .
 .  .-(: First Hop :)-.  +-------+             +-------+   .-(: Last Hop :)-.  .
 . (::::  Access  ::::)--| Source|             | Target|--(::::  Access  ::::) .
 .  `-(:: Network ::)-'  | Client|             | Client|     (:: Network ::)-' .
 .     `-(::::::)-'      +-------+             +-------+      `-(::::::)-'     .
 .                                                                             .
 .                                                                             .
 .         <--  Segment Routing Topology (SRT) Spanned by OMNI Link -->        .
   .                                                                          .
     . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

In the Segment Routing Topology, a source Client connects via a first hop access network served by a First Hop Segment (FHS) Proxy/Server. The FHS Proxy/Server then forwards to an FHS Gateway which connects to an arbitrarily complex set of Intermediate Segments. Adjacent intermediate Segments are joined by intermediate Gateways (not shown) that serve as adaptation layer IPv6 routers, with the final segment connected by a Last Hop Segment (LHS) Gateway. The LHS Gateway then forwards to an LHS Proxy/Server which in turn connects to the last hop access network where the target Client resides.¶

Gateway, Proxy/Server and Relay OMNI interfaces are configured over both secured tunnels and open INET underlay interfaces within their respective SRT segments. Within each segment, Gateways configure "hub-and-spokes" BGP peerings with Proxy/Servers and Relays as "spokes". Adjacent SRT segments are joined by Gateway-to-Gateway peerings to collectively form a spanning tree over the entire SRT. The "secured spanning tree" supports authentication and integrity for critical control plane messages (and any trailing data plane message extensions). The "unsecured spanning tree" conveys ordinary carrier packets without security codes and that must be examined by destinations according to data origin authentication procedures. AERO nodes can employ route optimization to cause carrier packets to take more direct paths between OMNI link neighbors without having to follow strict spanning tree paths.¶

The network of networks concept emerged from the earliest days of Internetworking beginning in the 1960's [KAHN]. The concept has carried forward to the present day where the Internet has become successful beyond measure. The AERO Multinet service concatenates SRT segments through Gateway-to-Gateway peerings as suggested in the "Catenet Model for Internetworking (IEN48)" [CERF]. Catenet suggested a concatenation of independent and diverse Internetwork "segments" to form a much larger network supporting end-to-end services.¶

The Catenet vision originally articulated in the 1970's faded from view as the Internet evolved in the decades that followed, and the adaptation layer was omitted from the architecture. As a result, the Internet has evolved to become a monolithic public routing and addressing service interconnecting private domains leading to the rise of the middle and a diminished role for end-to-end [RFC3724]. The adaptation layer manifested by AERO and OMNI now promises to restore the best aspects of end-to-end envisioned by Catenet through incremental deployment in the modern Internet.¶

4.2.2. AERO Addressing

AERO nodes on OMNI links use the Link-Local Address (LLA) prefix fe80::/64 [RFC4291] to assign LLAs to the OMNI interface to satisfy the requirements of [RFC4861]. AERO node LLAs only need to be unique on the local OMNI link segment, however, since there is no way to coordinate duplicate address detection between disjoint OMNI link segments. Therefore, OMNI interface intermediate systems should not forward packets with LLA source and/or destination addresses.¶

AERO Clients assign MANET Local Addresses (MLAs) or Hierarchical Host Identity Tags (HHITs) to their MANET and ALVIF interfaces to support adaptation layer addressing; they also assign the same MLAs or HHITs to the OMNI interface itself. These self generated and unique address types support peer-to-peer communications in multilink local networks when Internetworking infrastructure is either absent or only intermittently available.¶

AERO Clients also use the Unique Local Address (ULA) prefix fd00::/8 followed by a pseudo-random 40-bit Global ID to form the prefix {ULA}::/48, then include a 16-bit Subnet ID '*' to form the prefix {ULA*}::/64 [RFC4291]. AERO Proxy/Servers assign ULAs to Clients as *NET internal addresses in 1x1 correspondence with Globally-Unique Addresses (GUAs) as *NET external addresses according to NPTv6 [I-D.bctb-6man-rfc6296-bis].¶

AERO MSPs, MNPs, FNPs and SNPs are typically based on Global Unicast Addresses (GUAs), but in some cases may be based on IPv4 private addresses [RFC1918] or IPv6 ULA-C's [RFC4193].¶

AERO address selection rules are conducted per [RFC6724] as updated by [I-D.ietf-6man-rfc6724-update].¶

See [I-D.templin-6man-omni3] for a full discussion of the various unicast, anycast and multicast addresses used by AERO nodes on OMNI links.¶

4.2.3. AERO Routing System

The AERO routing system comprises a private Border Gateway Protocol (BGP) [RFC4271] service coordinated between Gateways as interior nodes and Proxy/Servers and Relays as leaf nodes of a spanning tree. The service supports OAL packet/fragment forwarding at a layer below IP and does not interact with the public Internet BGP routing system, but supports redistribution of information for other networks connected by Relays.¶

In a reference deployment, each Proxy/Server is configured as an Autonomous System Border Router (ASBR) for a stub Autonomous System (AS) using a 32-bit AS Number (ASN) [RFC4271] that is unique within the BGP instance, and each Proxy/Server further uses eBGP to peer with one or more Gateways but does not peer with other Proxy/Servers. Each SRT segment in the OMNI link must include one or more Gateways in a "hub" AS, which peer with the Proxy/Servers within that segment as "spoke" ASes. All Gateways within the same segment are members of the same hub AS, and use iBGP to maintain a consistent view of all active routes currently in service. The Gateways of different segments peer with one another using eBGP.¶

Gateways maintain forwarding table entries for the SNP prefixes assigned to Proxy/Serves and the set of all FNP/MNP routes that are currently active; Gateways also maintain black-hole routes for the OMNI link MSPs so that OAL packets/fragments destined to non-existent more-specific routes are flushed from the routing system. In this way, Proxy/Servers and Relays have only partial topology knowledge (i.e., they only maintain routing information for their directly associated Clients and foreign links) and they forward all other OAL packets/fragments to Gateways which have full topology knowledge.¶

Each OMNI link segment assigns a unique sub-prefix of the MSP known as the "SRT prefix". For example, a first segment could assign 2001:db8::/48, a second could assign 2001:db8:1::/48, a third could assign 2001:db8:2::/48, etc. Within each segment, each Proxy/Server and Gateway configures an SNP within the segment's SRT prefix, e.g., the SNPs 2001:db8::/64, 2001:db8:0:1::/64 2001:db8:0:2::/64 all belong to the SRT prefix 2001:db8::/48.¶

The administrative authorities for each segment must therefore coordinate to assure mutually-exclusive SNP assignments, but internal provisioning of SNPs is an independent local consideration for each administrative authority. For each SRT prefix, the Gateway(s) that connect that segment assign the all-zero's address of the prefix as a Subnet Router Anycast (SRA) address. For example, the SRA address for 2001:db8::/48 is simply 2001:db8::. All Proxy/Servers also assign the SRA address taken from their uniquely-assigned more-specific SNP, e.g., the SRA address for the SNP 2001:db8:0:1::/64 is simply 2001:db8:0:1::.¶

SRT prefixes (and their SNP sub-prefixes) are statically represented in Gateway forwarding tables. Gateways join multiple SRT segments into a unified OMNI link over multiple diverse network administrative domains. They support a virtual bridging service by first establishing forwarding table entries for their SRT prefixes either via standard BGP routing or static routes. For example, if three Gateways ('A', 'B' and 'C') from different segments serviced 2001:db8::/48, 2001:db8:1::/48 and 2001:db8:2::/48 respectively, then the forwarding tables in each gateway appear as follows:¶

A:

2001:db8::/48->local, 2001:db8:1::/48->B, 2001:db8:2::/48->C¶

B:

2001:db8::/48->A, 2001:db8:1::/48->local, 2001:db8:2::/48->C¶

C:

2001:db8::/48->A, 2000:db8:1::/48->B, 2001:db8:2::/48->local¶

These forwarding table entries rarely change, since they correspond to fixed infrastructure elements in their respective segments.¶

FNP and MNP routes are instead dynamically advertised in the AERO routing system by Proxy/Servers and Relays that provide anchor points for their corresponding prefixes. For example, if three Proxy/Servers ('D', 'E' and 'F') service the MNPs 2001:db8:1000:1::64/, 2001:db8:1000:2::/64 and 2001:db8:1000:2::/48 then the routing system would include:¶

D:

2001:db8:1000:1::/64¶

E:

2001:db8:1000:2::/64¶

F:

2001:db8:1000:3::/64¶

A full discussion of the BGP-based routing system used by AERO is found in [I-D.ietf-rtgwg-atn-bgp].¶

4.2.4. Segment Routing Topologies (SRTs)

The distinct GUA prefixes in an OMNI link domain identify distinct Segment Routing Topologies (SRTs). Each SRT is a mutually-exclusive OMNI link overlay instance using a distinct set of GUAs, and emulates a bridged campus LAN service for the OMNI link. In some cases (e.g., when redundant topologies are needed for fault tolerance and reliability) it may be beneficial to deploy multiple SRTs that act as independent overlay instances. A communication failure in one instance therefore will not affect communications in other instances.¶

Each SRT is identified by a distinct GUA prefix and assigns an IPv6 SRA address used for OMNI interface determination in Safety-Based Multilink (SBM) as discussed in [I-D.templin-6man-omni3]. Each OMNI interface further applies Performance-Based Multilink (PBM) internally.¶

The Gateways and Proxy/Servers of each independent SRT engage in BGP peerings to form a spanning tree with the Gateways in non-leaf nodes and the Proxy/Servers in leaf nodes. The spanning tree is configured over both secured and unsecured underlay network paths. The secured spanning tree is used to convey secured control messages (and sometimes data message extensions) between Proxy/Servers and Gateways, while the unsecured spanning tree forwards bulk data messages and/or unsecured control messages.¶

Each SRT segment is identified by a unique GUA prefix used by all Proxy/Servers and Gateways in the segment. Each AERO node must therefore discover an SRT prefix that correspondents can use to determine the correct segment, and must publish the SRT prefix in IPv6 ND messages.¶

Note: The distinct GUA prefixes in an OMNI link domain can be carried either in a common BGP routing protocol instance for all OMNI links or in distinct BGP routing protocol instances for different OMNI links. In some SBM environments, such separation may be necessary to ensure that distinct OMNI links do not include any common infrastructure elements as single points of failure. In other environments, carrying the GUAs of multiple OMNI links within a common routing system may be acceptable.¶

4.2.5. Segment Routing For OMNI Link Selection

Original IPv6 sources can direct IPv6 packets/parcels to an AERO node by including a standard IPv6 Segment Routing Header (SRH) [RFC8754] with the IPv6 SRA address for the selected OMNI link as either the IPv6 destination or as an intermediate hop within the SRH. This allows the original source to determine the specific OMNI link SRT an original IPv6 packet/parcel will traverse when there may be multiple alternatives.¶

When an AERO node processes the SRH and forwards the original IPv6 packet/parcel to the correct OMNI interface, the OMNI interface writes the next IPv6 address from the SRH into the IPv6 destination address and decrements Segments Left. If decrementing would cause Segments Left to become 0, the OMNI interface deletes the SRH before forwarding. This form of Segment Routing supports SBM.¶

4.4.1. AERO Gateway Behavior

AERO Gateways configure an OMNI interface and assign an SNP with corresponding SRA GUA for their OMNI link SRT segments. Gateways configure underlay interface secured tunnels with Proxy/Servers in the same SRT segment and other Gateways in the same (or an adjacent) SRT segment. Gateways then engage in a BGP routing protocol session with neighbors over the secured spanning tree (see: Section 4.2.3).¶

4.4.2. AERO Proxy/Server and Relay Behavior

When a Proxy/Server enables an OMNI interface, it assigns an SNP GUA/ULA prefix pair. The Proxy/Server then configures an SRA GUA appropriate for the given OMNI link SRT segment externally and configures an SRA ULA appropriate for the locally attached *NET internally. The Proxy/Server also configures secured underlay interface tunnels and engages in BGP routing protocol sessions over the OMNI interface with one or more neighboring Gateways.¶

The OMNI interface provides a single interface abstraction to the network layer, but internally serves as an NBMA nexus for exchanging carrier packets with other OMNI nodes over underlay interfaces and/or secured tunnels. The Proxy/Server further configures a service to facilitate IPv6 ND exchanges with AERO Clients and manages per-Client neighbor cache entries and IP forwarding table entries based on control message exchanges.¶

Relays are simply Proxy/Servers that run a dynamic routing protocol to redistribute routes between the OMNI interface and foreign networks/links (see: Section 4.2.3). The Relay provisions MNPs and advertises the MSP(s) for the OMNI link over its foreign network interface attachments. The Relay further provides an OMNI link attachment point for FNP-based topologies.¶

4.4.3. AERO Client Behavior

When a Client enables an OMNI interface, it assigns MLAs or HHITs to both its underlying ALVIF or MANET interfaces and the OMNI interface itself. The Client then sends OMNI-encapsulated RS messages with an unspecified source address to FHS Proxy/Servers which then allocate an SNP ULA/GUA address pair and optionally coordinate with a MAP Proxy/Server that delegates one or more MNPs. The MAP/FHS Proxy/Servers then return an RA message to the Client. Note that these RS/RA messages may pass through one or more NATs in the path between the Client and FHS Proxy/Server.¶

If the Client used an unspecified address in its initial RS messages, it will discover ULAs/GUAs in the corresponding RAs that it receives from FHS Proxy/Servers and can then assign the ULAs/GUAs to the OMNI interface. If the Client is operating outside the context of AERO infrastructure, however, it may continue using MLAs or HHITs over its ALVIF, MANET or OMNI interfaces for peer-to-peer communications within the local *NET. The Client can then continue indefinitely or at least until it encounters an infrastructure element that can delegate SNP ULA/GUA pairs and/or MNPs.)¶

A Client can further extend the OMNI link over its (downstream) ENET interfaces where it provides a first-hop router for Hosts and other AERO Clients connected to the ENET. A downstream Client that connects via the ENET serviced by an upstream Client can in turn service further downstream ENETs that connect other Hosts and Clients. This OMNI link extension can be applied recursively over a "chain" of ENET Clients.¶

4.4.4. AERO Host Behavior

When a Host enables an OMNI interface, it assigns an address taken from the ENET underlay interface which may itself be a GUA delegated by the upstream Client. The Host does not assign a link-local address to the OMNI interface, since no autoconfiguration is necessary on that interface. (As an implementation matter, the Host could instead configure the "OMNI interface" as a virtual sublayer of the ENET underlay interface itself.)¶

The Host sends OMNI-encapsulated RS messages over its ENET underlay interface to the upstream Client, which returns encapsulated RAs and provides routing services in the same fashion that Proxy/Servers provides services for Clients. Hosts represent the leaf end systems in recursively-nested chain of concatenated ENETs, i.e., they represent terminating endpoints for the OMNI link.¶

4.5.1. OMNI ND Messages

OMNI interfaces use IPv6 ND messages as the secured control plane messaging service for all adaptation layer neighbor coordination exchanges. OMNI interfaces prepare IPv6 ND messages the same as for standard IPv6 ND, but also include a new option type termed the OMNI option [I-D.templin-6man-omni3]. OMNI interfaces use ULAs/GUAs instead of LLAs as IPv6 ND message source and destination addresses. This allows multiple different OMNI links/segments to be joined into a single link/segment at some future time without requiring a global renumbering event.¶

For each IPv6 ND message, the OMNI interface includes one or more OMNI options (and any other ND message options) then completely populates all sub-option information. If the OMNI interface includes an authentication sub-option, it first writes the value 0 into the authentication signature field then calculates the signature beginning with the first IPv6 ND message octet following the header Checksum field and continuing over the entire length of the message. The OMNI interface next writes the authentication signature value into the appropriate OMNI authentication option field, then 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. The IPv6 ND message checksum therefore provides integrity assurance for the message, while the authentication signature covers the entire packet or super-packet. OMNI interfaces verify integrity and authentication of each message received, and process the message further only following successful verification.¶

OMNI options include per-neighbor information that provides multilink forwarding, link layer address and traffic selector information for the neighbor's underlay interfaces. This information is stored in both the neighbor cache and AERO Forwarding Information Base (AFIB) as basis for the forwarding algorithm specified in Section 4.10. The information is cumulative and reflects the union of the OMNI information from the most recent IPv6 ND messages received from the neighbor.¶

The OMNI option is distinct from any Source/Target Link-Layer Address Options (S/TLLAOs) that may appear in an IPv6 ND message according to the appropriate IPv6 over specific link layer specification (e.g., [RFC2464]). If both OMNI options and S/TLLAOs appear, the former pertains to the adaptation layer to underlay interface address mappings while the latter pertains to the native L2 address format of the underlay media.¶

OMNI interface IPv6 ND messages may also include other IPv6 ND options. In particular, solicitation messages may include a Nonce option if required for verification of advertisement replies. If an OMNI IPv6 ND solicitation message includes a Nonce option, the advertisement reply must echo the same Nonce. If an OMNI IPv6 ND solicitation message includes a Timestamp option, the recipient must also include a Timestamp option in its advertisement reply. All unsolicited advertisement and redirect messages should include a Timestamp option.¶

AERO Clients send RS messages to the link-scoped All-Routers multicast address or the ULA/GUA SRA address of a Proxy/Server while using unicast or anycast OAL and L2 addresses. AERO Proxy/Servers respond by returning unicast RA messages. During the RS/RA exchange, AERO Clients and Proxy/Servers include state synchronization parameters to establish Identification windows and other state.¶

AERO Hosts and Clients on ENET underlay networks send RS messages to the link-scoped All-Routers multicast address, a GUA SRA address of a remote MAP Proxy/Server or the MNP SRA address of an upstream Client while using unicast or anycast OAL/L2 addresses. The upstream AERO Client responds by returning a unicast RA message.¶

AERO nodes use NS/NA messages for the following purposes:¶

• NS/NA(AR) messages are used for address resolution. The ARS sends an NS(AR) to the unicast address of the ART, and an ARR with addressing information for the ART returns a unicast NA(AR) that contains current, consistent and authentic target address resolution information. NS/NA(AR) messages must be secured.¶

• Other NS/NA message exchanges are used to determine target reachability (NS/NA(NUD)), establish/maintain Route Optimization state (NS/NA(RO)) or establish/maintain multilink forwarding state (NS/NA(MF)). The source sends an NS to the unicast address of the target while optionally including an OMNI Multilink Vector sub-option naming a specific underlay interface pair, and the target returns a responsive NA. NS/NA messages that use an in-window sequence number and do not update any other state need not include an authentication signature but must include an IPv6 ND message checksum. NS/NA messages used to establish or update NCE and/or AFIB state must be secured.¶

• Unsolicited NA (uNA) messages are used to signal addressing and/or other mobility management (uNA(MM)) neighbor state changes (e.g., address changes due to mobility, signal degradation, traffic selector updates, etc.). uNA messages can also be also used to acknowledge receipt of other IPv6 ND messages (uNA(ACK)) as well as to securely convey ICMP error information (uNA(ERR)). uNA messages that update state information must be secured.¶

• NS/NA(DAD) messages are not used in AERO, since Duplicate Address Detection is not required.¶

AERO and OMNI together support an added reliability feature not available in ordinary IPv6 ND messaging. In particular, nodes can set the OMNI Neighbor Coordination SNR flag or Window Synchronization SYN/TST flag in unicast non-solicitation IPv6 ND messages (including RA, NA and Redirect) to request a synchronous (but "unsolicited") uNA(ACK) acknowledgement response (see: [I-D.templin-6man-omni3]).¶

The node that processes an SNR/SYN/TST message prepares the response the same as for an ordinary uNA as specified in [RFC4861], including the setting of the R/S/O flags as discussed below. The node sets the uNA(ACK) Target Address to the unicast destination and uNA(ACK) destination address to the unicast source of the original message.¶

The node then sets the uNA(ACK) source address to its own address and includes any necessary OMNI sub-options but MUST NOT itself set the SNR/SYN/TST flags. If the SNR/SYN/TST message included a Nonce and/or Timestamp option, the node includes matching Nonce/Timestamp options in the uNA(ACK) response. The node finally returns the uNA message to the source of the SNR/SYN/TST message.¶

4.5.2. OMNI Neighbor Advertisement Message Flags

As discussed in Section 4.4 of [RFC4861] NA messages include three flag bits R, S and O. OMNI interface NA messages treat the flags as follows:¶

• R: The R ("Router") flag is set to 1 in the NA messages sent by all AERO forwarding nodes on the OMNI link. (AERO Hosts are by definition the only non-forwarding nodes on the OMNI link and therefore set the R flag to 0.)¶

• S: The S ("Solicited") flag is set exactly as specified in Section 4.4. of [RFC4861], i.e., it is set to 1 for Solicited NAs and set to 0 for uNAs (both unicast and multicast).¶

• O: The O ("Override") flag is set to 0 for solicited NAs returned by a Proxy/Server ARR and set to 1 for all other solicited and unsolicited NAs.¶

4.5.3. OMNI Neighbor Window Synchronization

In secured environments (e.g., between secured spanning tree neighbors, between neighbors on the same secured ANET, etc.), OMNI interface neighbors can exchange AERO control messages without including Identification values. In environments where spoofing is considered a threat, OMNI interface neighbors instead invoke Identification window synchronization by including OMNI Multilink Vector sub-options in RS/RA and NS/NA message exchanges to maintain send/receive window state in their respective neighbor caches as well as in AFIB entries of all OAL intermediate nodes in the forward and reverse paths.¶

In common arrangements, OAL Identification window synchronization is necessary for Client to Client, Client to Proxy/Server or Proxy/Server to Proxy/Server message exchanges conducted over unsecured Internetwork paths. Conversely, Proxy/Server to Proxy/Server, Proxy/Server to Gateway and Gateway to Gateway message exchanges carried over the secured spanning tree do not require window synchronization.¶

OAL end system and intermediate nodes verify Identification values of OAL packets that traverse the unsecured spanning tree according to their populated AFIB state. This allows each OAL node to exclude spurious packets injected into the OMNI link from an off-path adversary.¶

4.10.1. Host Forwarding Algorithm

When an original IP packet/parcel enters a Host's OMNI interface from the network layer the Host searches for a NCE that matches the destination. If there is a matching NCE, the Host performs OMNI L2 encapsulation/fragmentation as discussed in [I-D.templin-6man-omni3] then forwards the resulting carrier packets into the ENET addressed to the L2 address of the neighbor. If there is no match, the host instead sends the carrier packets to its upstream Client.¶

After sending carrier packets, the Host may receive an OAL Redirect message from its upstream Client to inform it of another AERO node on the same ENET that would provide a better first hop. The Host authenticates the Redirect message, then updates its neighbor cache accordingly.¶

4.10.2. Client Forwarding Algorithm

When an original IP packet/parcel enters a Client's OMNI interface from the network layer the Client searches for a NCE that matches the destination. If there is a matching NCE for a neighbor reached via a *NET interface (i.e., an upstream interface), the Client selects one or more "reachable" neighbor interfaces in the entry for forwarding purposes. Otherwise, the Client performs OAL encapsulation and fragmentation if necessary, forwards the resulting OAL packet/fragments to an FHS Proxy/Server, then either invokes address resolution and multilink forwarding procedures per Section 4.13 or allows the FHS Proxy/Server to invoke these procedures on its behalf. If there is a matching NCE for a neighbor reached via an ENET interface (i.e., a downstream interface), the Client instead forwards the original IP packet/parcel to the downstream Host or Client using L2 encapsulation and fragmentation if necessary.¶

When a carrier packet enters a Client's OMNI interface from the link layer, the Client performs L2 reassembly/decapsulation if necessary to obtain the OAL packet/fragment then examines the OAL destination (i.e., after locating the correct AFV if the OAL packet header is OCH). If the OAL destination matches one of the Client's addresses the Client (acting as an OAL destination) verifies that the Identification is in-window for the matching AFV, then reassembles/decapsulates as necessary and delivers the original IP packet/parcel to the network layer. If the OAL destination matches a NCE for a dependent peer Client on an ENET interface, the Client instead forwards the OAL packet/fragment to the peer while decrementing the OAL Hop Limit. If the OAL destination matches a NCE for a Host on an ENET interface, the Client instead reassembles then forwards the original IP packet/parcel to the Host while using L2 encapsulation/fragmentation (i.e., without invoking the OAL) if necessary. If the OAL destination does not match, the Client drops the original IP packet/parcel and MAY return a network layer ICMP Destination Unreachable message subject to rate limiting (see: Section 4.11).¶

When a Client forwards an OAL packet/fragment from an ENET Host to a neighbor connected to the same ENET, it also returns a Redirect message to inform the Host that it can reach the neighbor directly as an ENET peer.¶

Note: The forwarding table entries established in peer Clients of a MANET multihop forwarding region are based on MLAs and/or HHITs used to seed the multihop routing protocols. The MLAs or HHITs also appear as OAL source and destination addresses, while the original IP packet source and destination addresses use MLAs, HHITs, ULAs or GUAs. When ULAs are used, the subnet ID in the ULA /64 prefix provides topological relevance for the multihop forwarding region, while the 64-bit Interface Identifier encodes the 1x1 mapping of the MANET-internal ULA to the MANET-external GUA maintained by the Proxy/Server that configures the GUA/ULA SNP.¶

Note: Clients within MANETs support Client-to-Client multihop forwarding when necessary to reach destinations or FHS Proxy/Servers that may be multiple OAL hops away. In this way, forwarding Clients act as OAL intermediate nodes and forward using OCH compression based on AFV state that is indexed by the AFVIs included in each OAL packet/fragment. ULA-based communications are sufficient for Client-to-Client communications within a MANET, while packets that enter or exit the MANET via a FHS Proxy/Server may be subject to NPTv6 [I-D.bctb-6man-rfc6296-bis].¶

4.10.3. Proxy/Server and Relay Forwarding Algorithm

When the network layer admits an original IP packet/parcel into a Proxy/Server's OMNI interface, the OAL drops the packet/parcel to avoid looping if forwarding state indicates that it should be forwarded back to the network layer. Otherwise, the OAL examines the IP destination address to determine if it matches the SNP SRA GUA of a neighboring Gateway found in the OMNI interface's network layer neighbor cache. If so, the Proxy/Server performs OAL encapsulation and fragmentation then performs L2 encapsulation/fragmentation and forwards the resulting carrier packets to the Gateway over a secured link (e.g., an IPsec tunnel, Direct link, etc.) to support control plane functions such as the operation of the BGP routing protocol. If the destination matches an FNP/MNP associated with a (foreign) Proxy/Server or Client, the (local) Proxy/Server instead assumes the Relay role and forwards the original IP packet/parcel in a similar manner as for Clients. Specifically, if there is a matching NCE the Proxy/Server selects one or more "reachable" neighbor interfaces in the entry for forwarding purposes; otherwise, the Proxy/Server performs OAL encapsulation/fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets while invoking address resolution and multilink forwarding procedures per Section 4.13.¶

When the Proxy/Server receives/reassembles carrier packets on underlay interfaces that contain OAL packets/fragments with both a source and destination OAL address that correspond to the same Client's delegated MNP or SNP ULA/GUA, the Proxy/Server drops the carrier packets regardless of their OMNI link point of origin. The Proxy/Server also drops original IP packets/parcels received on underlay interfaces either directly from a (M)ANET Client or following reassembly of carrier packets received from a *NET Client if the original IP destination corresponds to the same Client's delegated MNP or SNP GUA. Proxy/Servers also drop carrier packets that contain OAL packets/fragments with foreign OAL destinations that do not match the SNP/MNP GUA associated with one of their local *NET Clients. These checks are essential to prevent forwarding inconsistencies from accidentally or intentionally establishing endless loops that could congest nodes and/or *NET links.¶

Proxy/Servers process carrier packets that contain OAL packets/fragments with OCH headers or with destinations that match their SNP SRA ULA/GUA and also include a CRH header that encodes AFVI information. The Proxy/Server examines the L2 source address and AFVI to locate the corresponding AFV entry in the AFIB. The Proxy/Server then forwards them according to the AFV state while decrementing the OAL packet/fragment Hop Limit.¶

For OAL packets/fragments with destinations that match their SNP SRA ULA/GUA but do not include a CRH/OCH with a non-zero AFVI, the Proxy/Server instead performs L2 reassembly/decapsulation, verifies the Identification and performs OAL reassembly to obtain the original IP packet/parcel. For data packets/parcels addressed to its own GUA that arrived via the secured spanning tree, the Proxy/Server delivers the original IP packet/parcel to the network layer to support secured BGP routing protocol control messaging. For data packets/parcels originating from one of its dependent Clients, the Proxy/Server instead performs OAL encapsulation/fragmentation followed by L2 encapsulation/fragmentation and sends the resulting carrier packets while invoking address resolution and multilink forwarding procedures per Section 4.13. For IPv6 ND control messages, the Proxy/Server instead authenticates the message and processes it as specified in later sections of this document while updating neighbor cache and/or AFIB state accordingly.¶

When the Proxy/Server receives a carrier packet that contains an OAL packet/fragment with OAL destination set to an SNP ULA or GUA of one of its Client neighbors established through RS/RA exchanges, it accepts the carrier packet only if data origin authentication succeeds. If the NCE state is DEPARTED, the Proxy/Server changes the OAL destination address to the SNP SRA GUA of the new Proxy/Server, decrements the OAL Hop Limit, then performs L2 encapsulation/fragmentation and forwards the resulting carrier packets into the spanning tree which will eventually deliver them to the new Proxy/Server. If the neighbor cache state for the Client is REACHABLE and the Proxy/Server is a MAP responsible for serving as the Client's address resolution responder and/or default router, it verifies the Identification then submits the OAL packet/fragment for reassembly. The Proxy/Server then decapsulates and processes the resulting IPv6 ND message or original IP packet/parcel accordingly. Otherwise, the Proxy/Server decrements the OAL Hop Limit, performs L2 encapsulation/fragmentation and forwards the carrier packets to the Client which then performs data origin verification and reassembly. (In the latter case, the Client may receive fragments of the same original IP packet/parcel from different Proxy/Servers but this will not interfere with correct reassembly.)¶

When the Proxy/Server receives a carrier packet that contains an OAL packet/fragment with OAL destination set to a FNP address that does not match the MSP, it accepts the carrier packet only if data origin authentication succeeds and if there is a network layer forwarding table entry for the FNP. The Proxy/Server then performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly/decapsulation to obtain the original IP packet/parcel, then presents it to the network layer (as a Relay) where it will be delivered according to standard IP forwarding.¶

When a Proxy/Server receives a carrier packet from the secured spanning tree, it considers the message as authentic without having to verify network or higher layer authentication signatures.¶

If the Proxy/Server has multiple original IP packets/parcels to send to the same neighbor, it can concatenate them as a single OAL super-packet [I-D.templin-6man-omni3].¶

4.10.4. Gateway Forwarding Algorithm

When the network layer admits an original IP packet/parcel into the Gateway's OMNI interface, the OAL drops the packet if routing indicates that it should be forwarded back to the network layer to avoid looping. Otherwise, the Gateway examines the IP destination address to determine if it matches the SNP SRA GUA of a neighboring Gateway or Proxy/Server by examining the OMNI interface's network layer neighbor cache. If so, the Gateway performs OAL encapsulation/fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets to the neighboring Gateway or Proxy/Server over a secured link (e.g., an IPsec tunnel, etc.) to support the operation of control plane functions (including the BGP routing protocol) between OAL neighbors.¶

Gateways forward OAL packets/fragments reassembled from spanning tree carrier packets while decrementing the OAL Hop Limit but not the original IP header TTL/Hop Limit. Gateways send carrier packets that contain OAL packets/fragments with critical IPv6 ND control messages or BGP routing protocol control messages via the SRT secured spanning tree, and may send other carrier packets via the secured/unsecured spanning tree or via more direct paths according to AFIB information. When the Gateway receives a carrier packet, it reassembles/decapsulates to obtain the OAL packet/fragment then searches for an AFIB entry that matches the OAL header AFVI or an IP forwarding table entry that matches the OAL destination address.¶

Gateways process carrier packets containing OAL packets/fragments with OAL destinations that do not match their SNP/SRT SRA GUA in the same manner as for traditional IP forwarding within the OAL, i.e., they forward packets not explicitly addressed to themselves. Gateways locally process OAL packets/fragments with OCH headers or full OAL headers with their SNP/SRT SRA GUA as the OAL destination. If the OAL packet/fragment contains an OCH or a full OAL header with a CRH extension, the Gateway examines the AFVI to locate the AFV entry in the AFIB for next hop forwarding. If an AFV is found, the Gateway uses the next hop AFVI to forward the OAL packet/fragment to the next hop while decrementing the OAL Hop Limit but without reassembling. When the Gateway forwards the OAL packet/fragment, it rewrites the OCH/CRH AFVI with the value it will represent to the next OAL hop.¶

If the OAL packet/fragment includes a full OAL header addressed to itself but does not include an AFVI, the Gateway instead reassembles if necessary and processes the OAL packet further. The Gateway first determines whether the OAL packet includes an NS/NA message then processes the message according to the multilink forwarding procedures discussed in Section 4.13. If the carrier packets arrived over the secured spanning tree and the enclosed OAL packets/fragments are addressed to its SNP/SRT SRA GUA, the Gateway instead reassembles then discards the OAL header and forwards the original IP packet/parcel to the network layer to support secured BGP routing protocol control messaging. The Gateway instead drops all other OAL packets.¶

Gateways forward OAL packets/fragments received in carrier packets that arrived from a first segment via the secured spanning tree to the next segment also via the secured spanning tree. Gateways forward OAL packets/fragments received in carrier packets that arrived from a first segment via the unsecured spanning tree to the next segment also via the unsecured spanning tree. Gateways configure a single IPv6 routing table that determines the next hop for a given OAL destination, where the secured/unsecured spanning tree is determined through the selection of the underlay interface to be used for transmission (e.g., an IPsec tunnel or an open INET interface).¶

As for Proxy/Servers, Gateways must verify that the L2 source addresses of carrier packets not received from the secured spanning tree are "trusted" before forwarding according to an AFV (otherwise, the carrier packet must be dropped).¶

4.12.1. AERO Service Model

Each AERO Proxy/Server on the OMNI link is configured to respond to Client prefix delegation/registration requests. Each Proxy/Server is provisioned with a database of MNP-to-Client ID mappings for all Clients enrolled in the AERO service, as well as any information necessary to authenticate each Client. The Client database is maintained by a central administrative authority for the OMNI link and securely distributed to all Proxy/Servers, e.g., via the Lightweight Directory Access Protocol (LDAP) [RFC4511], via static configuration, etc. Clients receive the same service regardless of the Proxy/Servers they select. (Note: an OMNI link can instead delegate non-correlated MNPs to Clients instead of maintaining such a database. In that case, each Client may be delegated a different MNP each time it registers with the OMNI domain and may need to renumber its downstream-attached ENETs.)¶

Clients associate each of their *NET underlay interfaces with FHS Proxy/Servers. Each FHS Proxy/Server locally services one or more of the Client's underlay interfaces, and the Client typically selects one among them to serve as the MAP Proxy/Server (the Client may instead select a "third-party" MAP Proxy/Server that does not directly service any of its underlay interfaces). All of the Client's other FHS Proxy/Servers forward proxyed copies of RS/RA messages between the MAP Proxy/Server and Client without assuming the MAP role functions themselves.¶

Each Client typically associates with a single MAP Proxy/Server, while all other Proxy/Servers are candidates for providing the MAP role for other Clients. An FHS Proxy/Server assumes the MAP role when it receives an RS message with its own SNP SRA GUA/ULA or link-scoped All-Routers multicast as the destination. An FHS Proxy/Server assumes the proxy role when it receives an RS message with the SNP SRA GUA of another Proxy/Server as the destination. (An FHS Proxy/Server can also assume the proxy role when it receives an RS message addressed to link-scoped All-Routers multicast if it can determine the SNP SRA GUA of a better candidate Proxy/Server to serve as a MAP.)¶

Hosts and Clients on ENET interfaces associate with an upstream Client on the ENET the same as a Client would associate with an ANET Proxy/Server. Specifically, the Host/Client sends an RS message via the ENET which directs the message to the upstream Client. The upstream Client then responds to the RS message by returning an RA. In this way, the downstream nodes see the ENET as an ANET and see the upstream Client as a Proxy/Server for that ANET.¶

AERO Hosts, Clients and Proxy/Servers use IPv6 ND messages to maintain adaptation layer NCEs. AERO Proxy/Servers configure their OMNI interfaces as advertising NBMA interfaces, and therefore send unicast RA messages with a short Router Lifetime value (e.g., ReachableTime seconds) in response to a Client's RS message. Thereafter, Clients send additional RS messages to keep Proxy/Server state alive.¶

AERO Clients and FHS/MAP Proxy/Servers include SNP ULA/GUA address delegation (and optionally also MNP prefix delegation) DHCPv6 parameters in RS/RA messages. The IPv6 ND messages are exchanged between the Client and any FHS Proxy/Servers acting as proxys for the MAP Proxy/Server as specified in [I-D.templin-6man-omni3] according to the address/prefix management schedule required by the service. If the Client knows its MNP in advance, it can include the MNP in its DHCPv6 prefix delegation request. If the MAP Proxy/Server accepts the Client's MNP assertion (or if it delegates a new MNP for the Client), it injects the MNP into the routing system and establishes the necessary neighbor cache state.¶

AERO Clients and their FHS Proxy/Servers on MANETs and open INETs must establish and maintain Identification synchronization windows in their RS/RA exchanges. The window synchronization provides a well-managed Identification value that the Client and Proxy/Server can use for validating IPv6 ND messages with authentication signatures.¶

All Host, Client and Proxy/Server behaviors for the exchange of RS/RA messages are conducted according to the Router Discovery and Prefix Delegation specifications found in [I-D.templin-6man-omni3]. The following sections observe all of the OMNI specifications, and include additional specifications of the interactions of Client-Proxy/Server RS/RA exchanges with the AERO mobility service.¶

4.12.2. AERO Host and Client Behavior

AERO Hosts and Clients discover the addresses of candidate FHS Proxy/Servers as specified in the section on "Router Discovery and Prefix Delegation" in [I-D.templin-6man-omni3]. The Host/Client then performs RS/RA exchanges over each of its underlay interfaces to associate with an FHS Proxy/Server for each interface and a single MAP Proxy/Server if necessary. The Host/Client sends each RS (either directly via Direct interfaces, via an IPsec tunnel for VPN interfaces, via an access router for (M)ANET interfaces or via INET encapsulation for INET interfaces) and waits up to RetransTimer milliseconds for an RA message reply (see Section 4.12.3) while retrying up to MAX_RTR_SOLICITATIONS if necessary. If the Host/Client receives no RAs, or if it receives an RA with Router Lifetime set to 0, the Client SHOULD abandon attempts through the first candidate Proxy/Server and try another Proxy/Server.¶

After the Host/Client registers its underlay interfaces, it may wish to change one or more registrations, e.g., if an interface changes address or becomes unavailable, if traffic selectors change, etc. To do so, the Host/Client prepares an RS message to send over any available underlay interface as above. The RS includes an OMNI option with prefix registration/delegation information and with an Interface Attributes sub-option specific to the selected underlay interface. When the Host/Client receives the MAP Proxy/Server's RA response, it has assurance that both the MAP and FHS Proxy/Servers have been updated with the new information.¶

If the Host/Client wishes to discontinue use of a MAP Proxy/Server it issues an RS message over any underlay interface with an OMNI Proxy/Server Departure sub-option that encodes the (old) MAP Proxy/Server's SNP SRA GUA. When the MAP Proxy/Server processes the message, it releases any MNPs, sets the NCE state for the Host/Client to DEPARTED and returns an RA reply with Router Lifetime set to 0. After a short delay (e.g., 2 seconds), the MAP Proxy/Server withdraws the MNP from the routing system. (Alternatively, when the Host/Client associates with a new FHS/MAP Proxy/Server it can include an OMNI "Proxy/Server Departure" sub-option in RS messages with the SNP SRA GUAs of the Old FHS/MAP Proxy/Servers.)¶

4.12.3. AERO Proxy/Server Behavior

AERO Proxy/Servers act as both IP routers and IPv6 ND proxys, to support address and prefix delegation services for Clients. When a FHS/MAP Proxy/Server receives a prospective Client's secured RS message, it SHOULD return an immediate RA reply with Router Lifetime set to 0 if it is currently too busy or otherwise unable to service the Client; otherwise, it processes the RS and performs DHCPv6 address delegation for SNP ULA/GUA pairs while returning the ULA/GUA prefixes per [RFC8028] as specified in [I-D.templin-6man-omni3]. If the RS message also contains DHCPv6 prefix delegation parameters the FHS Proxy/Server processes the prefix delegations locally as a MAP or forwards a proxyed version of the RS to another candidate MAP Proxy/Server.¶

When the MAP Proxy/Server processes the RS, it determines the correct MNPs for the Client by processing OMNI DHCPv6 sub-option(s). When the MAP Proxy/Server returns the MNPs, it also creates a forwarding table entry for each MNP resulting in BGP updates (see: Section 4.2.3). The MAP Proxy/Server then returns an RA to the Client via the FHS Proxy/server as specified in Section 15 of [I-D.templin-6man-omni3].¶

After the initial RS/RA exchange, the MAP Proxy/Server maintains a ReachableTime timer for each of the Client's underlay interfaces individually (and for the Client's NCE collectively) set to expire after ReachableTime seconds. If the Client (or an FHS Proxy/Server) issues additional RS messages, the MAP Proxy/Server sends an RA response and resets ReachableTime. If the MAP Proxy/Server receives an IPv6 ND message with a prefix release indication it sets the Client's NCE to the DEPARTED state and withdraws the MNP routes from the routing system after a short delay (e.g., 2 seconds). If ReachableTime expires before a new RS is received on an individual underlay interface, the MAP Proxy/Server marks the interface as DOWN. If ReachableTime expires before any new RS is received on any individual underlay interface, the MAP Proxy/Server sets the NCE state to STALE and sets a 10 second timer. If the MAP Proxy/Server has not received a new RS or uNA(MM) message with a prefix release indication before the 10 second timer expires, it deletes the NCE and withdraws the MNP routes from the routing system.¶

The MAP Proxy/Server processes any IPv6 ND messages pertaining to the Client while forwarding to the Client or responding on the Client's behalf as necessary. The MAP Proxy/Server may also issue unsolicited RA messages, e.g., with reconfigure parameters to cause the Client to renegotiate its prefix delegation/registrations, with Router Lifetime set to 0 if it can no longer service this Client, etc. The MAP Proxy/Server may also receive carrier packets via the secured spanning tree that contain initial data sent while route optimization is in progress. The MAP Proxy/Server reassembles the enclosed OAL packets/fragments, then re-encapsulates/re-fragments and sends the carrier packets to the target Client via an FHS Proxy/Server if necessary. Finally, If the NCE is in the DEPARTED state, the old MAP Proxy/Server forwards any OAL packets/fragments it receives from the secured spanning tree and destined to the Client to the new MAP Proxy/Server, then deletes the entry after DepartTime expires.¶

Note: Clients SHOULD arrange to notify former MAP Proxy/Servers of their departures, but MAP Proxy/Servers are responsible for expiring neighbor cache entries and withdrawing MNP routes even if no departure notification is received (e.g., if the Client leaves the network unexpectedly). MAP Proxy/Servers SHOULD therefore set Router Lifetime to ReachableTime seconds in solicited RA messages to minimize persistent stale cache information in the absence of Client departure notifications. A short Router Lifetime also ensures that proactive RS/RA messaging between Clients and FHS Proxy/Servers will keep any NAT state alive (see above).¶

Note: All Proxy/Servers on an OMNI link MUST advertise consistent values in the RA Cur Hop Limit, M and O flags, Reachable Time and Retrans Timer fields the same as for any link, since unpredictable behavior could result if different Proxy/Servers on the same link advertised different values.¶

4.13.1. Multilink Address Resolution

When one or more original IP packets/parcels for a flow 3-tuple are forwarded into an OMNI interface, the ARS checks for a NCE with an FNP/MNP SRA address or SNP GUA that matches the target destination. If there is a NCE in the REACHABLE state, the ARS invokes the OAL and forwards the resulting carrier packets according to the cached state then returns from processing.¶

Otherwise, if there is no NCE the ARS creates one in the INCOMPLETE state. The ARS then prepares an NS message for Address Resolution (NS(AR)) to send toward an ART while (optionally) attaching the original IP packet(s)/parcel(s) to the end of the NS(AR) as an OAL super-packet (see above). The resulting NS(AR) message must be sent securely and includes source, destination and target addresses as discussed above. If the source address is an MNP SRA address, the NS(AR) message also includes Route Information Options (RIOs) [RFC4191] for any of the source Client's MNPs.¶

The ARS then includes an OMNI option with an authentication sub-option (if necessary), Interface Attributes and/or Traffic Selectors for all of the source Client's underlay interfaces. The ARS then calculates and includes an authentication signature (if necessary) followed by the checksum, then submits the NS(AR) message for OAL encapsulation.¶

When the ARS is a FHS Proxy/Server, it sets the OAL source to the Client's SNP GUA and sets the OAL destination according to the Client's RS message "RPT" flag (see: [I-D.templin-6man-omni3]). If the "RPT" flag was set, the ARS sets the OAL destination to the SNP SRA GUA of its MAP Proxy/Server which maintains a Report List; otherwise, the ARS sets the OAL destination to the FNP/MNP SRA GUA or SNP GUA corresponding to the ART. The ARS then performs L2 encapsulation/fragmentation and sends the resulting carrier packets into the SRT secured spanning tree without decrementing the network layer TTL/Hop Limit field.¶

When the ARS is a Client, it must instead use its own MLA or HHIT as the OAL source and the MLA or HHIT of the interface-specific FHS Proxy/Server as the OAL destination. If the Client is in a MANET or an open INET, it next calculates and includes an authentication signature and includes an OAL IPv6 Extended Fragment Header with Identification set to an in-window value for this FHS Proxy/Server. The ARS Client then performs L2 encapsulation/fragmentation and forwards the carrier packets to the FHS Proxy/Server.¶

The FHS Proxy/Server then performs L2 reassembly/decapsulation, verifies the Identification, verifies the NS(AR) checksum/authentication signature and confirms that the Client's claimed MNP RIOs and source address are correct. The FHS Proxy/Server then changes the OAL source to the Client's SNP GUA and changes the OAL destination to either the SNP SRA GUA of the MAP Proxy/Server or the GUA corresponding to the ART as specified above. The FHS Proxy/Server next removes the IPv6 Extended Fragment Header, performs L2 encapsulation/fragmentation and sends the resulting carrier packets into the secured spanning tree on behalf of the Client.¶

Note: both the source and target Client/Relay and their MAP Proxy/Servers include current and accurate information for their multilink Interface Attributes profile. The MAP Proxy/Servers can be trusted to provide an authoritative ARR response and/or mobility update message on behalf of the source/target should the need arise. While the source or target itself has no such trust basis, any attempt to mount an attack by providing false Interface Attributes information would only result in black-holing of return traffic, i.e., the "attack" could only result in denial of service to the source/target itself. The source/target's asserted Interface Attributes therefore do not need to be validated by the MAP Proxy/Server.¶

4.13.2. Multilink Forwarding

Following address resolution, the ARS and ART (i.e., the end system Clients or their respective Proxy/Servers) can assert per-flow multilink forwarding paths through underlay interface pairs serviced by the same source/destination ULAs by sending NS/NA messages for Multilink Forwarding (MF) with OMNI Multilink Vector sub-options. The NS/NA(MF) messages establish per-flow multilink forwarding and header compression state in OAL intermediate systems in the path between the ARS and ART. Note that either the ARS or ART can independently initiate multilink forwarding by sending NS(MF) messages on behalf of specific flows over underlay interface pairs.¶

If the original IP packet uses an FNP/MNP GUA as the source, the source Client or Proxy/Server uses the same FNP/MNP GUA as the NS(MF) source. If the original IP packet uses an SNP GUA as the source, the source Client or Proxy/Server uses the same SNP GUA as the NS(MF) source. In both cases, the source Client or Proxy/Server uses the original IP packet destination address as the destination address and uses the SNP GUA associated with the destination as the Target Address of the NS(MF). The source then includes an appropriate Flow Label set according to [RFC6437][RFC6438] as the final element of the flow 3-tuple. When the target Client or Proxy/Server returns an NA(MF), it sets the destination to the same address that appeared in the NS(MF) source, sets the Target Address to the same address that appeared in the NS(MF) target and sets the source address to the SNP GUA of the target Client.¶

When an OAL source asserts a multilink forwarding path through the transmission of an NS(MF) message, it includes an IPv6 Minimum Path MTU Hop-by-Hop Option for the (adaptation layer) IPv6 header per [RFC9268]. Each OAL intermediate node along the path then updates the minimum MTU per the specification. When the OAL destination responds with an NA(MF) message, it returns an IPv6 Minimum Path MTU Option based on the one it received in the NS(MF) message per [RFC9268]. This allows the OAL source to discover any OAL Fragment Size (OFS) limitations for this OAL destination (see: [I-D.templin-6man-omni3]). For this reason, OAL nodes that connect SRT segments MUST implement [RFC9268].¶

The multilink forwarding profile provides support for redundant paths that each OAL node can harness to its best advantage. For example, OAL nodes can use traffic selectors to distribute different traffic types over available multilink paths, while other factors such as metrics, cost, provider, etc. can also provide useful decision points. OAL nodes can also employ multilink forwarding for fault tolerance by sending redundant data over multiple paths simultaneously, or for load balancing where the individual packets of a single traffic flow are spread across multiple independent paths. OAL nodes that engage in multilink forwarding therefore must incorporate a policy engine that selects both inbound and outbound multilink paths for a given traffic profile at a given point in time. This specification therefore provides multilink forwarding mechanisms without mandating any specific multilink policy.¶

All Client, Proxy/Server and Gateway nodes that configure OMNI interfaces and engage in multilink coordination include an additional forwarding table termed the AERO Forwarding Information Base (AFIB) that supports OAL packet/fragment forwarding based on original IP packet flows over specific OMNI neighbor interface pairs. The AFIB contains per-flow AERO Forwarding Vectors (AFVs) identified by the L2 address of the previous OAL hop plus a value known as the AFV Index (AFVI). The AFVs cache uncompressed OAL header information to support forwarding of packets with compressed headers as well as previous/next-hop addressing and AFVI information. The AFVs also cache window synchronization state (i.e., the starting sequence number and window size) for each specific flow. Using the window synchronization state, simple Identification-based data origin authentication is enabled at each OAL source, intermediate system and target node.¶

Client and Proxy/Server OMNI interfaces manage end system AFIB entries in conjunction with their internal Neighbor Cache, where the NCEs link to (possibly) multiple AFVs with one per flow over a specific FHS/LHS interface ifIndex pair. When OMNI interface peers need to coordinate, they locate a NCE for the peer (established through address resolution) then use the NCE as a nexus that aggregates potentially many AVFs which cache AFVIs to be used for multilink forwarding on a per-flow basis. Gateway OMNI interfaces and the OMNI interfaces of Clients or Proxy/Servers acting as OAL intermediate nodes manage transit AFIB entries independently of their internal Neighbor Caches. These transit AFVs are indexed by the L2 address and AFVI supplied by the previous hop.¶

OAL source, intermediate system and target nodes create AFVs/AFVIs when they process an NS/NA(MF) initiation or responsive message with an OMNI Multilink Vector sub-option (see: [I-D.templin-6man-omni3]). The OAL source of the initiating NS/NA(MF) (which is also the OAL destination of the responsive NA(MF)) is considered to reside in the "First Hop Segment (FHS)", while the OAL destination of the NS/NA(MF) (which is also the OAL source of the responsive NA(MF)) is considered to reside in the "Last Hop Segment (LHS)".¶

The FHS and LHS roles are determined on a per-flow and per-interface-pair basis. After address resolution, either peer is equally capable of initiating multilink forwarding on behalf of a specific flow. The peer that sends the initiating NS/NA(MF) message with Multilink Vector for a specific pair becomes the FHS peer while the one that returns the responsive NA(MF) becomes the LHS peer for that (flow, interface pair) only. It is therefore commonplace that peers may assume the FHS role for some flows while assuming the LHS role for others, i.e., even though each peer maintains only a single NCE.¶

When an OAL node sends/forwards an initiating NS/NA(MF) or responsive NA(MF) with a Multilink Vector sub-option, it creates an AFV, caches the Identification window information, caches the NS/NA(MF) and OAL IPv6 headers, records the previous hop L2 address and AFVI, then generates a new next hop AFVI. The next hop AFVI should be selected within the range [1 - (2**16-1)] unless all values within that range are already in active use for this next hop; otherwise, the AFVI must be selected within the range [2**16 - (2**32-1)]. (Note that the AFVI value 0 is used to indicate the unspecified AFV, i.e., one that always matches but includes NULL header compression or forwarding information.) When the OAL node forwards future OAL packets/fragments that include the previous hop L2 address and AFVI, it can unambiguously locate the correct AFV and use the cached forwarding information to forward to the next OAL hop.¶

OAL nodes cache AFVs for up to ReachableTime seconds following their initial creation. If the node processes another NS/NA(MF) message specific to an AFV, it resets ReachableTime to REACHABLE_TIME seconds, i.e., the same as for NCEs. If ReachableTime expires, the node deletes the AFV.¶

The following sections provide the detailed specifications of these NS/NA(MF) exchanges for all nodes along the forward and reverse paths.¶

4.13.3. Mobile Ad-hoc Network (MANET) Forwarding

Clients with OMNI interfaces configured over underlay interfaces with indeterminant neighborhood properties may be connected to a Mobile Ad-hoc NETwork (MANET). Each MANET may be either completely outside of the range of any OMNI link Proxy/Servers or may require multihop traversal between Clients acting as MANET routers to reach Proxy/Servers that connect to the rest of the OMNI link. The former class of MANETs must operate in isolation solely based on the MLA or HHIT unique IPv6 addresses they configure locally. The latter class allows MANET routers to extend infrastructure-based addressing information including MNPs over multiple OMNI link hops as discussed in the OMNI specification.¶

MANET Clients configure their OMNI interfaces over one or more MANET interfaces where multihop forwarding may be necessary. Routing protocols suitable for use over MANET interfaces include OSPFv3 [RFC5340] with MANET Designated Router (OSPF-MDR) extensions [RFC5614], OLSR [RFC7181], AODV [I-D.perkins-manet-aodvv2] and others. Other services specific to MANET link-local and/or site-local operations (including SMF [RFC6621], DLEP [RFC8175] and others) are also considered in-scope. These services strive for optimal use of available radio bandwidth and power consumption in their control message transmissions, but efficient data plane operation is also essential.¶

Clients must therefore reduce overhead through minimal encapsulation and effective header compression whenever possible. For this reason, when the MANET routing protocol discovers a new route the Client configures a lesser-preferred forwarding table entry over the corresponding MANET interface and a more-preferred forwarding table entry over the OMNI interface. This will cause the network layer to direct outbound packets to the OMNI interface, which can apply header compression and underlay MANET interface selection.¶

Proxy/Servers that connect a MANET to the rest of the OMNI link act as regular Proxy/Servers for exchanges with external INETs, but act as Clients over their MANET interfaces. Each such Proxy/Server therefore has at least two underlay interfaces, including an INET interface and a MANET interface. The Proxy/Server therefore services the MANET as if it were an ordinary Client but presents itself as a Proxy/Server to external facing INETs.¶

The process for a multihop Client to establish multilink forwarding and header compression AFV state in the MANET is conducted in the same fashion as described above and using the same NS/NA(MF) message exchanges. Each intermediate MANET node in the path configures AFV state and assigns AFVIs in the same fashion as for intermediate Gateways in the secured spanning tree except that the NS/NA(MF) messages requires an authentication signature (unless neighboring MANET nodes configure IPsec tunnels) and an Identification that is within the window for its serving Proxy/Server if the destination is outside of the local MANET. The NS/NA(MF) messages extend from the initiating FHS MANET Client, then across any intermediate FHS MANET hops to the FHS Proxy/Server, then across the secured SRT spanning tree, then to the LHS Proxy/Server, then finally across any intermediate LHS MANET hops to the responding LHS Client. The LHS responder then returns an NA(MF) along the reverse path. In all other ways, the NS/NA(MF) exchanges are the same as discussed in Section 4.13.2.¶

Following the NS/NA(MF) exchanges, each MANET router in the forward (and optionally also reverse) paths in both the FHS and LHS MANETs will have established AFVs containing multilink forwarding and header compression state for the flow. The AFVs determine AFVI-based forwarding based on the OCH header contents, and each MANET router only forwards packets with in-window Identification values for the flow. MANET routers maintain AFVs for up to ReachableTime seconds unless they are refreshed by an NS/NA(MF) TST/ACK exchange. New window synchronization exchanges must also be performed periodically to avoid window exhaustion and/or spoofing based on predictable Identifications.¶

Note: while the MANET routing protocol runs directly over the node's MANET interfaces to discover routing information, the node configures lesser-preferred forwarding table entries over the MANET interface and corresponding more-preferred forwarding table entries over the OMNI interface. This causes the network layer to forward outbound packets via the OMNI interface which applies encapsulation, fragmentation and/or header compression as necessary before forwarding over the underlying MANET interface. The OMNI protocol designator in the UDP port, IP protocol or Ethernet EtherType field will then cause the packets to visit the OMNI interface of each successive next-hop MANET node.¶

4.13.4. Proxy/Server-to-Proxy/Server Route Optimization

When the FHS and LHS Proxy/Servers are both connected to an IPv6 underlay for the same SRT segment, they can forward NS/NA(MF) exchanges directly over the underlay without engaging SRT spanning tree hops. This is made possible when the FHS and LHS Proxy/Servers inject their SNPs into the underlay network routing system as well as the SRT overlay routing system.¶

If the underlay is not secured, the FHS and LHS Proxy/Servers must include an authentication signature with their NS/NA(MF) messages, which could either be the original authentication signature included by their respective Clients or a new signature included by the Proxy/Server itself. If the Proxy/Server that processes the NS/NA(MF) message determines that the message is authentic, it creates or updates an AFV entry according to the multilink forwarding parameters. This establishes both AFVI and Identification window state to be used for future data traffic forwarding.¶

4.13.5. Gateway-to-Proxy/Server Route Optimization

When the LHS gateway and Proxy/Server are both connected to an IPv6 underlay for the same SRT segment, they can forward NS/NA(MF) exchanges directly over the underlay without engaging additional SRT spanning tree hops. In this arrangement, the LHS Gateway acts the same as the FHS Proxy/Server as discussed in Section 4.13.4 and observes the requirement for including authentication signatures.¶

4.13.6. Client-to-Client Route Optimization

When the FHS/LHS Clients are both located on the same SRT segment, Client-to-Client route optimization is possible following the establishment of any necessary state in NATs in the path. Both Clients will have already established state via their respective shared segment Proxy/Servers (and possibly also any shared segment Gateways) and can begin sending carrier packets directly via NAT traversal while avoiding any Proxy/Server and/or Gateway hops.¶

When the FHS/LHS Clients on the same SRT segment perform initial NS/NA(MF) exchanges to establish AFIB state, they first examine the FMT-Forward and FMT-Mode settings to determine whether direct-path forwarding is even possible for one or both Clients (direct-path forwarding is only possible when FMT-Forward and FMT-Mode are both set). The NS/NA(MF) messages then include an Origin Indication (i.e., in addition to a Multilink Vector sub-option) with the mapped addresses discovered during the RS/RA exchanges with their respective Proxy/Servers. After the AFV paths have been established, both Clients can begin sending carrier packets via strict AFV paths while establishing a direct path for Client-to-Client route optimization.¶

To establish the direct path, either Client (acting as the source) transmits a bubble to the mapped L2 address for the target Client which primes the local chain of NATs for reception of future carrier packets from that L2 address (see: [RFC4380] and [I-D.templin-6man-omni3]). The source Client then prepares an NS(RO) message with its own MNP SRA GUA or SNP GUA as the source, with the MNP SRA GUA or SNP GUA of the target as the destination and with an OMNI option with an Interface Attributes sub-option. The source Client then encapsulates the NS(RO) in an OAL header with its own MLA or HHIT as the source, with the MLA or HHIT of the Proxy/Server as the destination and with an in-window Identification for the target. The source Client then performs L2 encapsulation/fragmentation with L2 headers addressed to its Proxy/Server then sends the resulting carrier packets to the Proxy/Server.¶

When the Proxy/Server receives the carrier packets, it re-encapsulates and sends them as unsecured carrier packets according to AFIB state where they will eventually arrive at the target Client which can perform L2 reassembly/decapsulation. Following reassembly, the target Client prepares an NA(RO) message with its own MNP SRA GUA or SNP GUA as the source, with the MNP SRA GUA or SNP GUA of the source Client as the destination and with an OMNI option with an Interface Attributes sub-option. The target Client then encapsulates the NA(RO) in an OAL header with its own MLA or HHIT as the source, with the MLA or HHIT of the source Client as the destination and with an in-window Identification for the source Client. The target Client then performs L2 encapsulation/fragmentation then forwards the resulting carrier packets directly to the source Client.¶

Following the initial NS/NA(RO) exchange, both Clients mark their respective (source, target) underlay interface pairs as "trusted" for no more than ReachableTime seconds. The Clients can then begin exchanging ordinary data packets as OCH encapsulated carrier packets. While the Clients continue to exchange packets via the direct path avoiding all Proxy/Servers and Gateways, they should perform additional NS/NA(RO) exchanges via their local Proxy/Servers to refresh NCE state as well as send additional bubbles to the peer's Origin address information if necessary to refresh NAT state.¶

Note: these procedures are suitable for a widely-deployed but basic class of NATs. Procedures for advanced NAT classes are outlined in [RFC6081], which provides mechanisms that can be employed equally for AERO using the corresponding sub-options specified by OMNI.¶

Note: each communicating pair of Clients may need to maintain NAT state for peer to peer communications via multiple underlay interface pairs and/or multiple flows. It is therefore important that Origin Indications are maintained with the correct peer interface and that the NCE may cache information for multiple peer interfaces.¶

Note: the source and target Client exchange Origin information during the secured NS/NA(RO) multilink route optimization exchange. This allows for subsequent NS/NA(RO) exchanges to proceed using only the Identification value as a data origin confirmation. However, Client-to-Client peerings that require stronger security may also include authentication signatures for mutual authentication.¶

4.13.7. Intra-(M)ANET/ENET Route Optimization

When a Client forwards an OAL packet (or an original IP packet/parcel) from a Host or another Client connected to one of its downstream ENETs to a peer within the same downstream ENET, the Client returns an IPv6 ND Redirect message to inform the source that that target can be reached directly. The contents of the Redirect message are the same as specified in [RFC4861], and should also include any RIOs with MNP information corresponding to the target.¶

In the same fashion, when a Proxy/Server forwards an OAL packet (or original IP packet/parcel) from a Host or Client connected to one of its downstream *NETs to a peer within the same downstream *NET, the Proxy/Server returns an IPv6 ND Redirect message.¶

All other route optimization functions are conducted per the NS/NA(RO) messaging discussed in the previous sections.¶

4.15.1. Mobility Update Messaging

Mobile Clients (and/or their MAP Proxy/Servers) accommodate mobility and/or multilink change events by sending secured uNA Mobility Management (MM) messages to each active neighbor. When a node sends a uNA(MM) message to each specific neighbor on behalf of a mobile Client, it sets the IPv6 source address to its own MNP SRA GUA or SNP ULA/GUA, sets the destination and target address to the neighbor's SNP ULA/GUA or one of the mobile Client's MNP SRA GUAs. The uNA(MM) also includes an OMNI option with OMNI Interface Attributes and Traffic Selector sub-options for the mobile Client's underlay interfaces and includes an authentication signature if necessary. The node next sets the uNA(MM) R flag to 1, S flag to 0 and O flag to 1, then encapsulates the message in an OAL header. Following OAL and L2 encapsulation/fragmentation, the carrier packets containing the uNA(MM) message will then follow the secured spanning tree and arrive at the specific neighbor.¶

As discussed in Section 7.2.6 of [RFC4861], the transmission and reception of uNA(MM) messages is unreliable but provides a useful optimization. In well-connected Internetworks with robust data links uNA(MM) messages will be delivered with high probability, but in any case the node can optionally send up to MAX_NEIGHBOR_ADVERTISEMENT uNA(MM)s to each neighbor to increase the likelihood that at least one will be received. Alternatively, the node can set the SNR flag in the uNA(MM) OMNI option header to request a uNA(ACK) response (see: Section 4.5.1).¶

When the FHS/LHS Proxy/Server receives a secured uNA(MM) message prepared as above, if the uNA(MM) destination was its own SNP SRA ULA the Proxy/Server uses the included OMNI option information to update its NCE for the target but does not reset ReachableTime since the receipt of a uNA(MM) message does not provide confirmation that any forward paths to the target Client are working. If the destination was the SNP GUA of the FHS/LHS Client, the Proxy/Server instead changes the OAL source to its own SNP SRA GUA/ULA, includes an authentication signature if necessary, and includes an in-window Identification for this Client. Finally, if the uNA(MM) message SNR flag was set, the node that processes the uNA(MM) also returns a uNA(ACK) response (see: Section 4.5.1).¶

4.15.2. Announcing Link-Layer Information Changes

When a Client needs to change its underlay Interface Attributes and/or Traffic Selectors for one or more underlay interfaces (e.g., due to a mobility event), the Client sends RS messages to its MAP Proxy/Server (via first-hop FHS Proxy/Servers if necessary). Each RS includes an OMNI option with Interface Attributes and/or Traffic Selector sub-options for the ifIndex in question.¶

Note that the first FHS Proxy/Server may change due to the underlay interface change. If the Client RS includes an OMNI Proxy/Server Departure sub-option for the former FHS Proxy/Server, the new FHS Proxy/Server can send a departure indication (see Section 4.15.5); otherwise, any stale state in the former FHS Proxy/Server will simply expire after ReachableTime expires with no effect on the MAP Proxy/Server.¶

Up to MAX_RTR_SOLICITATIONS RS messages MAY be sent in parallel with sending carrier packets containing user data in case one or more RAs are lost. If all RAs are lost, the Client SHOULD re-associate with a new Proxy/Server.¶

After performing the RS/RA exchange, the Client sends uNA(MM) messages to all neighbors the same as described in the previous section.¶

4.15.3. Bringing New Links Into Service

When a Client needs to bring new underlay interfaces into service (e.g., when it activates a new data link), it sends an RS message to the MAP Proxy/Server via a FHS Proxy/Server for the underlay interface (if necessary) with an OMNI option that includes an Interface Attributes sub-option with interface parameters and with link layer address information for the new link. The Client then again sends uNA(MM) messages to all neighbors the same as described above.¶

4.15.4. Deactivating Existing Links

When a Client needs to deactivate an existing underlay interface, it sends a uNA(MM) message toward the MAP Proxy/Server via an FHS Proxy/Server with an OMNI option with appropriate Interface Attributes values for the deactivated link.¶

If the Client needs to send uNA(MM) messages over an underlay interface other than the one being deactivated, it MUST include Interface Attributes for any underlay interfaces being deactivated. The Client then again sends uNA(MM) messages to all neighbors the same as described above.¶

Note that when a Client deactivates an underlay interface, neighbors that receive the ensuing uNA(MM) messages need not purge all references for the underlay interface from their neighbor cache entries. The Client may reactivate or reuse the underlay interface and/or its ifIndex at a later point in time, when it will send new RS messages to an FHS Proxy/Server with fresh interface parameters to update any neighbors. The manner in which the Client dynamically manages its local ifIndex to interface mappings is a local decision, but should not be done in a manner that could cause state inconsistencies in the network.¶

4.15.5. Moving Between Proxy/Servers

The Client performs the procedures specified in Section 4.12.2 when it first associates with a new MAP Proxy/Server or renews its association with an existing MAP Proxy/Server.¶

When a Client associates with a new MAP Proxy/Server, it sends RS messages to register its underlay interfaces with the new MAP while including the old MAP's GUA in the "Old MAP Proxy/Server GUA" field of a Proxy/Server Departure OMNI sub-option. When the new MAP Proxy/Server returns the RA message via the FHS Proxy/Server (acting as a proxy), the FHS Proxy/Server sends a uNA(MM) to the old MAP Proxy/Server (i.e., if the GUA is non-zero and different from its own). The uNA(MM) has the MNP SRA GUA of the Client as the source and the SNP SRA GUA of the old MAP as the destination and with an OMNI Proxy/Server Departure sub-option as above. The FHS Proxy/Server encapsulates the uNA(MM) in an OAL header with the SNP SRA GUA of the new MAP as the source and the SNP SRA GUA of the old MAP as the destination, then performs L2 encapsulation/fragmentation and forwards the resulting carrier packets via the secured spanning tree.¶

When the old MAP Proxy/Server receives the carrier packets, it decapsulates and reassembles if necessary to obtain the uNA(MM) then changes the Client's NCE state to DEPARTED, resets DepartTime and caches the new MAP Proxy/Server GUA. After a short delay (e.g., 2 seconds) the old MAP Proxy/Server withdraws the Client's MNP(s) from the routing system. While in the DEPARTED state, the old MAP Proxy/Server forwards any carrier packets received via the secured spanning tree destined to the Client's MNP GUAs or SNP GUA to the new MAP Proxy/Server's SNP GUA. When DepartTime expires, the old MAP Proxy/Server deletes the Client's NCE.¶

Mobility events may also cause a Client to change to a new FHS Proxy/Server over a specific underlay interface at any time such that a Client RS/RA exchange over the underlay interface will engage the new FHS Proxy/Server instead of the old. The Client can arrange to inform the old FHS Proxy/Server of the departure by including a Proxy/Server Departure sub-option with a GUA for the "Old FHS Proxy/Server GUA", and the new FHS Proxy/Server will issue a uNA(MM) using the same procedures as outlined for the MAP above while using its own SNP SRA GUA as the source address. This can often result in successful delivery of carrier packets that would otherwise be lost due to the mobility event.¶

Clients SHOULD NOT move rapidly between MAP Proxy/Servers in order to avoid causing excessive oscillations in the AERO routing system. Examples of when a Client might wish to change to a different MAP Proxy/Server include a MAP Proxy/Server that has become unresponsive, topological movements of significant distance, movement to a new geographic region, movement to a new OMNI link segment, etc.¶

4.15.6. Accommodating Path Changes

After AFV state has been established for a flow, all OAL intermediate systems in the forward path will have AFVs with header compression state and (AFVI, L2ADDR) information for the next hop. However, paths can fluctuate due to factors such as node mobility, routing changes, network membership, etc. If an OAL intermediate system forwarding OAL packets with OCH headers detects that the next hop in the path has changed, it immediately reverts to sending the packets with header compression disabled by including full OFH and IPv6 Extended Fragment Headers (plus full original IP headers) in future packets.¶

If the OAL intermediate system receives an OCH1 packet with the Q bit set and M bit clear during a path change event, it first decompresses the original IP headers of each payload packet in the (packed) OAL packet while retaining the packets as attachments to the (full) OAL header. The OAL intermediate system then processes the OAL packet further.¶

If an OAL packet is larger than the minimum OFS, the OAL intermediate system applies OAL fragmentation to produce (sub-)fragments no larger than the minimum OFS. If the original OAL packet/fragment had a fragment ordinal value N, the OAL intermediate node writes the same value N into each of the (sub-)fragments produced.¶

The OAL intermediate node then encapsulates the OAL packet or fragments as attachments to OAL-encapsulated uNA messages (i.e., the same as for initial packets during a multilink forwarding exchange) but also includes an OMNI Neighbor Control sub-option with the "Path Change (PCH)" bit set (see: [I-D.templin-6man-omni3]. The OAL intermediate node then applies an authentication signature and includes an IPv6 Extended Fragment Header if necessary or admits the uNA-encapsulated packet/fragments into the secured spanning tree. These (sub-)fragments (along with any other OAL fragments) will not be further fragmented by other OAL intermediate nodes on the path and will be reassembled by the OAL destination.¶

When the OAL destination begins to receive OAL packets with full headers and with the PCH bit set, it assumes that the network path has changed and begins sending uNA messages to the OAL source. The OAL destination sends the uNA messages subject to rate limiting, and includes a Multilink Vector OMNI sub-option with both the ACK and RST flags set and with the most recent OAL packet Identification written into the Acknowledgment field.¶

When the OAL source receives the uNA messages with ACK and RST set, it re-initiates multilink forwarding for this flow by issuing a new NS/NA(MF) exchange the same as for a new flow as specified in Section 4.13.2. The AFV state in the former path then simply becomes stale and is soon purged by the former OAL intermediate nodes.¶

4.16.1. Source-Specific Multicast (SSM)

When an ARS "X" (i.e., either a Client or Proxy/Server) acting as PIM router receives a Join/Prune message from a node on its downstream interfaces containing one or more ((S)ource, (G)roup) pairs, it updates its Multicast Routing Information Base (MRIB) accordingly. For each S belonging to a prefix reachable via X's non-OMNI interfaces, X then forwards the (S, G) Join/Prune to any PIM routers on those interfaces per [RFC7761]. The same as for unicast destinations, the 3-tuple of source, destination and Flow Label identifies a flow for multicast group G.¶

For each S belonging to a prefix reachable via X's OMNI interface, X sends an NS(AR) message (see: Section 4.13) using its own MNP SRA GUA or SNP GUA as the source address and the MNP/SNP GUA of S as both the destination and target addresses. X then encapsulates the NS(AR) in an OAL header with source address set to its own MLA/HHIT/GUA/ULA and destination address set to the MLA/HHIT/GUA/ULA for S, then forwards the message into the secured spanning tree which delivers it to ARR "Y" that services S. Y will then return an NA(AR) that includes an OMNI option with Interface Attributes for any underlay interfaces that are currently servicing S.¶

When X processes the NA(AR) it selects one or more underlay interfaces for S and performs an NS/NA(MF) multilink forwarding exchange over the secured spanning tree while including a PIM Join/Prune message for each multicast group of interest in the OMNI option. If S is located behind any Proxys "Z"*, each Z* then updates its MRIB accordingly and maintains the MNP SRA GUA or SNP GUA of X as the next hop in the reverse path. Since Gateways forward messages not addressed to themselves without examining them, this means that the (reverse) multicast tree path is simply from each Z* (and/or S) to X with no other multicast-aware routers in the path.¶

Following the initial combined Join/Prune and NS/NA(MF) messaging, X maintains a NCE for each S the same as if X was sending unicast data traffic to S. In particular, X performs additional NS/NA(MF) exchanges to keep the NCE alive for up to t_periodic seconds [RFC7761]. If no new Joins are received within t_periodic seconds, X allows the NCE to expire. Finally, if X receives any additional Join/Prune messages for (S,G) it forwards the messages over the secured spanning tree.¶

Client C that holds an MNP for source S may later depart from a first Proxy/Server Z1 and/or connect via a new Proxy/Server Z2. In that case, Y sends a uNA(MM) message to X the same as specified for unicast mobility in Section 4.15. When X receives the uNA(MM) message, it updates its NCE for the XLA for source S and sends new Join messages in NS/NA(MF) exchanges addressed to the new target Client underlay interface connection for S. There is no requirement to send any Prune messages to old Proxy/Server Z1 since source S will no longer source any multicast data traffic via Z1. Instead, the multicast state for (S,G) in Proxy/Server Z1 will soon expire since no new Joins will arrive.¶

4.16.2. Any-Source Multicast (ASM)

When an ARS "X" acting as a PIM router receives Join/Prune messages from a node on its downstream interfaces containing one or more (*,G) pairs, it updates its Multicast Routing Information Base (MRIB) accordingly. X first performs an NS/NA(AR) exchange to receive address resolution information for Rendezvous Point (RP) "R" for each G. X then includes a copy of each Join/Prune message in the OMNI option of an NS(MF) message with its own MNP SRA GUA or SNP GUA as the source address and the MNP SRA GUA or SNP GUA for R as the destination and target address, then encapsulates the NS(MF) message in an OAL header with its own MLA/HHIT/GUA/ULA as the source and the MLA/HHIT/GUA/ULA of R's Proxy/Server as the destination then sends the message into the secured spanning tree.¶

For each source "S" that sends multicast traffic to group G via R, Client S* that aggregates S (or its Proxy/Server) encapsulates the original IP packets/parcels in PIM Register messages, includes the PIM Register messages in the OMNI options of uNA(MM) messages, performs OAL encapsulation and fragmentation with Identification values within the receive window for Client R* that aggregates R, then performs L2 encapsulation/fragmentation and forwards the resulting carrier packets. Client R* may then elect to send a PIM Join to S* in the OMNI option of a uNA(MM) over the secured spanning tree. This will result in an (S,G) tree rooted at S* with R as the next hop so that R will begin to receive two copies of the original IP packet/parcel; one native copy from the (S, G) tree and a second copy from the pre-existing (*, G) tree that still uses uNA(MM) PIM Register encapsulation. R can then issue a uNA(MM) PIM Register-stop message over the secured spanning tree to suppress the Register-encapsulated stream. At some later time, if Client S* moves to a new Proxy/Server, it resumes sending original IP packets/parcels via uNA(MM) PIM Register encapsulation via the new Proxy/Server.¶

At the same time, as multicast listeners discover individual S's for a given G, they can initiate an (S,G) Join for each S under the same procedures discussed in Section 4.16.1. Once the (S,G) tree is established, the listeners can send (S, G) Prune messages to R so that multicast original IP packets/parcels for group G sourced by S will only be delivered via the (S, G) tree and not from the (*, G) tree rooted at R. All mobility considerations discussed for SSM apply.¶

4.16.3. Bi-Directional PIM (BIDIR-PIM)

Bi-Directional PIM (BIDIR-PIM) [RFC5015] provides an alternate approach to ASM that treats the Rendezvous Point (RP) as a Designated Forwarder (DF). Further considerations for BIDIR-PIM are out of scope.¶