Automatic Extended Route Optimization (AERO)

The following sections specify the operation of IP over OMNI links using the AERO service:

AERO Hosts configure an OMNI interface over an underlay interface connected to a Client's ENET and coordinate with both other AERO Hosts and Clients over the ENET. As an implementation matter, the Host either assigns the same (MNP-based) IP address from the underlay interface to the OMNI interface, or configures the "OMNI interface" as a virtual sublayer of the underlay interface itself. AERO Hosts treat the ENET as an ANET, and treat the upstream Client for the ENET as a Proxy/Server. AERO Hosts are seen as OMNI link termination endpoints. AERO Clients can be deployed as fixed infrastructure nodes close to end systems, or as Mobile Nodes (MNs) that can change their network attachment points dynamically. AERO Clients configure OMNI interfaces over underlay interfaces with addresses that may change due to mobility. AERO Clients register their Mobile Network Prefixes (MNPs) with the AERO service, and distribute the MNPs to ENETs (which may connect AERO Hosts and other Clients). AERO Clients provide Proxy/Server-like services for Hosts and other Clients on downstream-attached ENETs. AERO Gateways, Proxy/Servers and Relays are critical infrastructure elements in fixed (i.e., non-mobile) ANET/INET boundary (or standalone INET) deployments and hence have permanent and unchanging INET addresses. Together, they provide access to the AERO service OMNI link virtual overlay for connecting AERO Clients and Hosts. AERO Gateways (together with Proxy/Servers and Relays) provide the secured backbone supporting infrastructure for a Segment Routing Topology (SRT) spanning tree for the OMNI link. AERO Gateways are OMNI link intermediate systems that forward packets both within the same SRT segment and between disjoint SRT segments based on an IPv6 encapsulation mid-layer known as the OMNI Adaptation Layer (OAL). The OMNI interface and OAL provide an adaptation layer forwarding service that the network layer perceives as L2 bridging, since the inner IP TTL/Hop Limit is not decremented. Each Gateway peers with Proxy/Servers, Relays and other Gateways in a dynamic routing protocol instance to provide a Distributed Mobility Management (DMM) service for the list of active MNPs (see ). Gateways assign one or more Mobility Service Prefixes (MSPs) to the OMNI link and configure secured tunnels with Proxy/Servers, Relays and other Gateways; they further maintain forwarding table entries for each MNP or non-MNP prefix in service on the OMNI link. AERO Proxy/Servers distributed across one or more SRT segments provide default forwarding and mobility/multilink services for AERO Client mobile nodes. The Proxy/Server acts as either OMNI link intermediate systems or end systems according to the service model selected by each Client. Proxy/Server also peer with Gateways in an adaptation layer dynamic routing protocol instance to advertise its list of associated MNPs (see ). Hub Proxy/Servers provide prefix delegation/registration services and track the mobility/multilink profiles of each of their associated Clients, where each delegated prefix becomes an MNP taken from an MSP. Proxy/Servers at ANET/INET boundaries provide a primary forwarding service for ANET Client/Host communications with peers in external INETs, while Proxy/Servers in open INETs provide an authentication service for IPv6 ND messages but should be used only a last resort data plane forwarding service when a Client cannot forward directly to an INET peer or Gateway. Source Clients securely coordinate with target Clients by sending control messages via a First-Hop Segment (FHS) Proxy/Server which forwards them over the SRT spanning tree to a Last-Hop Segment (LHS) Proxy/Server which finally forwards them to the target. AERO Relays are Proxy/Servers that provide forwarding services to exchange original IP packets/parcels between the OMNI link and fixed or mobile nodes on other links/networks. Relays run a dynamic routing protocol to discover any non-MNP prefixes in service on other links/networks, and Relays that connect to larger Internetworks (such as the Internet) may originate default routes. The Relay redistributes OMNI link MSP(s) into other links/networks, and redistributes non-MNP prefixes via OMNI link Gateway BGP peerings.

presents the basic OMNI link reference model:

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 Proxy/Servers (S1, S2). 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 non-MNP 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 OMNI Adaptation Layer (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 :

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ]]> 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 treated 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 AERO Multinet service concatenates SRT segments to form a larger network through Gateway-to-Gateway peerings as originally suggested in the "Catenet Model for Internetworking" ; especially follows directly from the illustrations in . The Catenet concept suggested a "network-of-networks" concatenation of independent and diverse Internetwork "segments" to form a much larger network supporting end-to-end services. The Catenet concept first articulated in the 1970's was distorted through the evolution of the Internet in the decades that followed, since the adaptation layer was a critical element missing from the architecture. As a result, while the Internet has been successful beyond measure it has evolved as a monolithic public routing and addressing service interconnecting private domains instead of a true network-of-networks which has impeded flexibility and inhibited end-to-end services. The adaptation layer manifested by AERO and OMNI now provides the means to address these limitations as well as the other "6 Ms of Modern Internetworking" according to the original Catenet network-of-networks vision.

AERO nodes on OMNI links use the Link-Local Address (LLA) prefix fe80::/64 to assign LLAs to the OMNI interface to satisfy the requirements of . AERO Clients configure LLAs constructed from MNPs (i.e., "LLA-MNPs") while AERO infrastructure nodes construct LLAs based on 56-bit random values ("LLA-RNDs") per . Non-MNP routes are also represented the same as for MNPs, but may include a prefix that is not properly covered by an MSP. AERO nodes 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 . The AERO node then uses the prefix {ULA*}::/64 to form "ULA-MNPs" or "ULA-RNDs" as specified in to support OAL addressing. (The prefix {ULA*}::/64 appearing alone and with no suffix represents "default" for that prefix.) AERO Clients also use Temporary Local Addresses (TLAs) and eXtended Local Addresses (XLAs) constructed per , where TLAs are distinguished from ordinary ULAs based on the prefix fd00::/16 and XLAs are distinguished from ULAs/TLAs based on the prefix fd00::/64. Clients use TLA-RNDs only in initial control message exchanges until a stable MNP is assigned, but may sometimes also use them for sustained communications within a local routing region. AERO nodes use XLA-MNPs to provide forwarding information for the global routing table as well as IPv6 ND message addressing information. AERO MSPs, MNPs and non-MNP routes are typically based on Global Unicast Addresses (GUAs), but in some cases may be based on IPv4 private addresses or IPv6 ULA-C's . A GUA block is also reserved for OMNI link anycast purposes. See for a full specification of LLAs, ULAs, TLAs, XLAs, GUAs and anycast addresses used by AERO nodes on OMNI links. Finally, AERO Clients and Proxy/Servers configure node Identification values as specified in .

The AERO routing system comprises a private Border Gateway Protocol (BGP) service coordinated between Gateways and Proxy/Servers (Relays also engage in the routing system as simplified Proxy/Servers). 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 links and 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) 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 only for ULA prefixes for infrastructure elements and XLA-MNPs corresponding to MNP and non-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 non-AERO 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 {ULA}::/48 known as the "SRT prefix". For example, a first segment could assign {ULA}:1000::/56, a second could assign {ULA}:2000::/56, a third could assign {ULA}:3000::/56, etc. Within each segment, each Proxy/Server configures a ULA-RND within the segment's SRT prefix with a 56-bit random value in the interface identifier as specified in . The administrative authorities for each segment must therefore coordinate to assure mutually-exclusive ULA prefix assignments, but internal provisioning of ULAs is an independent local consideration for each administrative authority. For each ULA prefix, the Gateway(s) that connect that segment assign the all-zero's address of the prefix as a Subnet Router Anycast address. For example, the Subnet Router Anycast address for {ULA}:1023::/64 is simply {ULA}:1023::/64. ULA 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 ULA prefixes either via standard BGP routing or static routes. For example, if three Gateways ('A', 'B' and 'C') from different segments serviced {ULA}:1000::/56, {ULA}:2000::/56 and {ULA}:3000::/56 respectively, then the forwarding tables in each Gateway appear as follows: {ULA}:1000::/56->local, {ULA}:2000::/56->B, {ULA}:3000::/56->C {ULA}:1000::/56->A, {ULA}:2000::/56->local, {ULA}:3000::/56->C {ULA}:1000::/56->A, {ULA}:2000::/56->B, {ULA}:3000::/56->local These forwarding table entries rarely change, since they correspond to fixed infrastructure elements in their respective segments. MNP (and non-MNP) routes are instead dynamically advertised in the AERO routing system by Proxy/Servers and Relays that provide service for their corresponding MNPs. The routes are advertised as XLA-MNP prefixes, i.e., as fd00::{MNP} (see: ). For example, if three Proxy/Servers ('D', 'E' and 'F') service the MNPs 2001:db8:1000:2000::/56, 2001:db8:3000:4000::/56 and 2001:db8:5000:6000::/56 then the routing system would include: fd00::2001:db8:1000:2000/120 fd00::2001:db8:3000:4000/120 fd00::2001:db8:5000:6000/120 Note: the MNP length found in an OMNI Neighbor Control sub-option encodes a Preflen between 1 and 64, but the corresponding XLA-MNP is entered into the routing system with length (64 + MNP length). A full discussion of the BGP-based routing system used by AERO is found in .

The distinct {ULA}::/48 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 ULAs, 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 value in the 40-bit ULA Global ID field and assigns an OMNI IPv6 anycast address used for OMNI interface determination in Safety-Based Multilink (SBM) as discussed in . 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 ULA 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 ULA 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 ULAs of multiple OMNI links within a common routing system may be acceptable.

Original IPv6 sources can direct IPv6 packets/parcels to an AERO node by including a standard IPv6 Segment Routing Header (SRH) with the OMNI IPv6 anycast 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 Safety-Based Multilink (SBM).

OMNI interfaces are virtual interfaces configured over one or more underlay interfaces classified as follows: ANET interfaces connect to a protected and secured ANET that is separated from open INETs by Proxy/Servers. The ANET interface may be either on the same L2 link segment as a Proxy/Server, or separated from a Proxy/Server by multiple IP hops. (Note that NATs may appear internally within an ANET and may require NAT traversal on the path to the Proxy/Server the same as for the INET case.) INET interfaces connect to an INET either natively or through one or several IPv4 Network Address Translators (NATs). Native INET interfaces have global IP addresses that are reachable from correspondent on the same INET. NATed INET interfaces typically have private IP addresses and connect to a private network behind one or more NATs with the outermost NAT providing INET access. ENET interfaces connect a Client's downstream-attached networks, where the Client provides forwarding services for ENET Host and Client communications to remote peers. An ENET can be as simple as a small IoT sub-network that travels with a mobile Client to as complex as a large private enterprise network that the Client connects to a larger ANET or INET. VPN interfaces use security encapsulations (e.g. IPsec tunnels) over underlay networks to connect Clients, Proxy/Servers and/or Gateways. VPN interfaces provide security services at lower layers of the architecture (L2/L1) the same as for Direct point-to-point interfaces. Direct point-to-point interfaces securely connect Clients, Proxy/Servers and/or Gateways over physical or virtual media that does not transit any open Internetwork paths. Examples include a line-of-sight link between a remote pilot and an unmanned aircraft, a fiberoptic link between Gateways, etc. OMNI interfaces use OAL encapsulation and fragmentation as discussed in . OMNI interfaces use L2 encapsulation (see: ) to exchange carrier packets with OMNI link neighbors over INET interfaces and IPsec tunnels as well as over ANET interfaces for which the Client and FHS Proxy/Server may be multiple IP hops away. OMNI interfaces use link layer encapsulation only (i.e., and no other L2 encapsulations) over Direct underlay interfaces or ANET interfaces when the Client and FHS Proxy/Server are known to be on the same underlay link. OMNI interfaces maintain an adaptation layer neighbor cache for tracking per-neighbor state. OMNI interfaces use IPv6 ND messages including Router Solicitation (RS), Router Advertisement (RA), Neighbor Solicitation (NS), Neighbor Advertisement (NA), unsolicited Neighbor Advertisement (uNA) and Redirect to manage the neighbor cache. In environments where spoofing may be a threat, OMNI neighbors should invoke OAL Identification window synchronization in their IPv6 ND message exchanges. OMNI interfaces send IPv6 ND messages with an OMNI option formatted as specified in . The OMNI option includes prefix registration information, Interface Attributes and/or AERO Forwarding Parameters (AFPs) containing link information parameters for the OMNI interface's underlay interfaces (as well as any other per-neighbor information). The presence of the OMNI option identifies each IPv6 ND message as an adaptation layer (i.e., and not a network layer) control message. A Host's OMNI interface is configured over an underlay interface connected to an ENET provided by an upstream Client. From the Host's perspective, the ENET appears as an ANET and the upstream Client appears as a Proxy/Server. The Host does not provide OMNI intermediate system services and is therefore a logical termination point for the OMNI link. A Client's OMNI interface may be configured over multiple ANET/INET underlay interfaces. For example, common mobile handheld devices have both wireless local area network ("WLAN") and cellular wireless links. These links are often used "one at a time" with low-cost WLAN preferred and highly-available cellular wireless as a standby, but a simultaneous-use capability could provide benefits. In a more complex example, aircraft frequently have many wireless data link types (e.g. satellite-based, cellular, terrestrial, air-to-air directional, etc.) with diverse performance and cost properties. If a Client's multiple ANET/INET underlay interfaces are used "one at a time" (i.e., all other interfaces are in standby mode while one interface is active), then successive IPv6 ND messages all include OMNI option Interface Attributes, Traffic Selector and/or AFP sub-options with the same underlay interface ifIndex. In that case, the Client would appear to have a single underlay interface but with a dynamically changing link layer address. If the Client has multiple active ANET/INET underlay interfaces, then from the perspective of IPv6 ND it would appear to have multiple link layer addresses. In that case, IPv6 ND message OMNI options MAY include sub-options with different underlay interface ifIndexes. Proxy/Servers on the open Internet include only a single INET underlay interface. INET Clients therefore discover only the L2ADDR information for the Proxy/Server's INET interface. Proxy/Servers on an ANET/INET boundary include both an ANET and INET underlay interface. ANET Clients therefore must discover both the ANET and INET L2ADDR information for their Proxy/Servers. Gateway and Proxy/Server OMNI interfaces are configured over underlay interfaces that provide both secured tunnels for carrying IPv6 ND and BGP protocol control plane messages and open INET access for carrying unsecured data plane messages. The OMNI interface configures a ULA-RND and acts as an OAL source to encapsulate original IP packets/parcels, then fragments the resulting OAL packets, performs L2 encapsulation/fragmentation and sends the resulting carrier packets over the secured or unsecured underlay paths. Note that Gateway and Proxy/Server end-to-end transport protocol sessions used by the BGP run directly over the OMNI interface and use ULA-RND source and destination addresses. The ULA-RND addresses that appear in the original IP packets/parcels of a BGP protocol session may therefore be the same as those that appear in the OAL IPv6 encapsulation header.

AERO Proxy/Servers, Clients and Hosts configure OMNI interfaces as their point of attachment to the OMNI link. AERO nodes assign the MSPs for the link to their OMNI interfaces (i.e., as a "route-to-interface") to ensure that original IP packets/parcels with destination addresses covered by an MNP not explicitly associated with another interface are directed to an OMNI interface. OMNI interface initialization procedures for Proxy/Servers, Clients Hosts and Gateways are discussed in the following sections.

When a Proxy/Server enables an OMNI interface, it assigns a ULA-RND appropriate for the given OMNI link SRT segment. 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 sending carrier packets to OMNI interface neighbors 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 INET/ENET interfaces (see: ). The Relay provisions MNPs to networks on the INET/ENET interfaces (i.e., the same as a Client would do) and advertises the MSP(s) for the OMNI link over the INET/ENET interfaces. The Relay further provides an OMNI link attachment point for non-MNP-based topologies.

When a Client enables an OMNI interface, it assigns either an XLA-MNP or a TLA and sends OMNI-encapsulated RS messages over its ANET/INET underlay interfaces to an FHS Proxy/Server, which coordinates with a Hub Proxy/Server that returns an RA message with corresponding parameters. The RS/RA messages may pass through one or more NATs in the path between the Client and FHS Proxy/Server. (Note: if the Client used a TLA in its initial RS messages, it may discover ULA-MNPs in the corresponding RAs that it receives from FHS Proxy/Servers and begin using these new addresses. If the Client is operating outside the context of AERO infrastructure such as in a Mobile Ad-hoc Network (MANET), however, it may continue using TLAs for Client-to-Client communications either indefinitely or at least until it encounters an infrastructure element that can delegate 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.

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.

AERO Gateways configure an OMNI interface and assign a ULA-RND and corresponding Subnet Router Anycast address for each of 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: ).

Each Client, Proxy/Server and Gateway OMNI interface maintains a network layer conceptual neighbor cache per or the same as for any IP interface. The OMNI interface network layer neighbor cache is maintained through static and/or dynamic neighbor cache entry configurations. Each OMNI interface also maintains a separate internal adaptation layer conceptual neighbor cache that includes a Neighbor Cache Entry (NCE) for each of its active OAL neighbors per . IPv6 ND messages that update the adaptation layer neighbor cache include ULA addresses as well as one or more OMNI options. Throughout this document, the terms "neighbor cache" and "NCE" refer to this adaptation layer neighbor cache unless otherwise specified. Each OMNI interface NCE is indexed by the ULA of a neighbor found in the ND message IPv6 header and determines the context for Identification verification. Clients and Proxy/Servers maintain NCEs through dynamic RS/RA message exchanges, and also maintain NCEs for any active correspondent peers through dynamic NS/NA message exchanges. Hosts maintain NCEs for Clients and other Hosts through the exchange of RS/RA, NS/NA or Redirect messages. Each NCE is indexed by the IP address assigned to the Host ENET interface, which is the same address used for L2 encapsulation (i.e., without the insertion of an OAL header). This encapsulation format identifies the NCE as a Host-based entry where the Host is a leaf end system in the recursively extended OMNI link. Gateways maintain NCEs for Clients within their local segments based on NS/NA route optimization messaging (see: ). When a Gateway creates/updates a NCE for a local segment Client based on NS/NA route optimization, it also maintains AFIB state for messages destined to this local segment Client. Clients establish NCEs for their associated FHS and Hub Proxy/Servers through the exchange of RS/RA messages. When a Client and Proxy/Server establish NCEs, they set a ReachableTime timer to REACHABLE_TIME seconds. Clients determine the service profiles for their FHS and Hub Proxy/Servers by setting the NUD/ARR/RPT flags in RS messages and also by setting/clearing the FMT-Forward and FMT-Mode flags in the Interface Attributes sub-option. When the NUD/ARR/RPT flags are clear, Proxy/Servers forward all NS/NA messages to the Client, while the Client performs mobility update signaling through the transmission of uNA messages to all active neighbors following a mobility event. However, in some environments this may result in excessive NS/NA control message overhead especially for Clients connected to low-end data links. Clients can therefore set the NUD/ARR/RPT flags in RS messages they send to select their Proxy/Server service profiles. If the NUD flag is set, the FHS Proxy/Server that forwards the RS message assumes the role of responding to NS messages and maintains peer NCEs associated with the NCE for this Client. If the ARR flag is set, the Hub Proxy/Server that processes the RS message assumes the role of responding to NS(AR) messages on behalf of this Client NCE. If the RPT flag is set, the Hub Proxy/Server that processes the RS message becomes responsible for maintaining a "Report List" for each Client NCE for the source addresses of NS(AR) messages it forwards on behalf of this Client. When a Client sets the RPT flag, the Hub Proxy/Server maintains Report List entries based on a ReportTime timer initialized to REACHABLE_TIME seconds upon receipt of an NS(AR) and decremented once per second while no additional NS(AR)s arrive. The Hub Proxy/Server then sends uNA messages to each Report List entry when it receives a Client mobility update indication (e.g., through receipt of an RS with updated Interface Attributes and/or Traffic Selectors). When a Report List entry ReportTime timer expires, the Hub Proxy/Server deletes the entry. When a Client NCE timer expires, the Hub Proxy/Server deletes the NCE along with its associated Report List. Clients can also set/clear the FMT-Forward and FMT-Mode flags in the Interface Attributes sub-option of each RS message to express their desired service profile from each FHS Proxy/Server for a specific underlay interface. The FHS Proxy/Server will consider the Client's preferences and either accept or override by setting/clearing the flags in the corresponding RA message reply. Implications for these bit settings are discussed in . Both the Client and its Hub Proxy/Server have full knowledge of the Client's current underlay Interface Attributes and Traffic Selectors, while FHS Proxy/Servers acting in "proxy" mode have knowledge of only the individual Client underlay interfaces they service. Clients determine their FHS and Hub Proxy/Server service models by setting the NUD/ARR/RPT flags in the RS messages they send as discussed above. When an Address Resolution Source (ARS) sends an NS(AR) message toward an Address Resolution Target (ART) Client/Relay, the OMNI link routing system directs the NS(AR) to a Hub Proxy/Server for the ART. The Hub then either acts as an Address Resolution Responder (ARR) on behalf of the ART or forwards the NS(AR) to the ART which acts as an ARR on its own behalf. The ARR returns an NA(AR) response to the ARS, which creates or updates a NCE for the ART while caching L3 and L2 addressing information. The ARS then (re)sets ReachableTime for the NCE to REACHABLE_TIME seconds and performs unicast NS/NA exchanges over specific underlay interface pairs to determine paths for sending carrier packets directly to the ART. The ARS otherwise decrements ReachableTime while no further solicited NA messages arrive. Proxy/Servers add an additional state DEPARTED to the list of NCE states found in Section 7.3.2 of . When a Client terminates its association, the Proxy/Server OMNI interface sets a DepartTime variable for the NCE to DEPART_TIME seconds. DepartTime is decremented unless a new IPv6 ND message causes the state to return to REACHABLE. While a NCE is in the DEPARTED state, the Proxy/Server forwards OAL packets/fragments destined to the target Client to the Client's new FHS/Hub Proxy/Server instead. It is RECOMMENDED that REACHABLE_TIME be set to the default constant value 30 seconds as specified in . It is RECOMMENDED that DEPART_TIME be set to the default constant value 10 seconds to accept any carrier packets that may be in flight. When ReachableTime or DepartTime decrement to 0, the NCE is deleted. AERO nodes also use the value MAX_UNICAST_SOLICIT to limit the number of NS messages sent when a correspondent may have gone unreachable, the value MAX_RTR_SOLICITATIONS to limit the number of RS messages sent without receiving an RA and the value MAX_NEIGHBOR_ADVERTISEMENT to limit the number of uNAs that can be sent based on a single event. It is RECOMMENDED that MAX_UNICAST_SOLICIT, MAX_RTR_SOLICITATIONS and MAX_NEIGHBOR_ADVERTISEMENT be set to 3 the same as specified in . Different values for the above constants MAY be administratively set; however, if different values are chosen, all nodes on the link MUST consistently configure the same values.

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 . OMNI interfaces use ULAs instead of LLAs as adaptation layer IPv6 ND message source and destination addresses. This allows multiple different OMNI links to be joined into a single link at some future time without requiring a global renumbering event. OMNI interfaces normally limit the size of the IPv6 ND messages they send to the IPv6 minimum link MTU, but messages that include a substantial amount of OMNI parameters and/or IP packet/parcel attachments may occasionally exceed that size. The OMNI interface engages IPv6 encapsulation followed by fragmentation to break IPv6 ND messages as large as 65535 octets into fragments no larger than 1280 octets. Whenever possible, OMNI interfaces should send multiple smaller IPv6 ND messages instead of singleton larger messages to minimize fragmentation. For each IPv6 ND message, the OMNI interface includes one or more OMNI options (and any other ND message options) then completely populates all option information. If the OMNI interface includes an authentication 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 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 . 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., ). 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 a ULA-RND while using unicast or anycast OAL/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 ULA-RND of a remote Hub Proxy/Server or the ULA-MNP 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 and optionally to establish sequence number windows. The ARS sends an NS(AR) to the solicited-node multicast 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(AR) messages include a solicited-node multicast destination address to distinguish them from ordinary NS messages. NS/NA(AR) messages must be secured. Ordinary NS/NA messages are used determine target reachability, establish and maintain NAT state, and/or establish multilink forwarding (i.e., AFIB) state. The source sends an NS to the unicast address of the target while optionally including an OMNI AERO Forwarding Parameters (AFP) sub-option naming a specific underlay interface pair, and the target returns a unicast NA that includes a responsive AFP if necessary. 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 window synchronization and/or AFIB state must be secured. Unsolicited NA (uNA) messages are used to signal addressing and/or other 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 non-solicitation IPv6 ND messages (see below). 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 flag in unicast non-solicitation IPv6 ND messages (including RA, NA and Redirect) to request a synchronous (but "unsolicited") uNA response (see: ). The node that processes an SNR/SYN message prepares the response the same as for an ordinary uNA as specified in , including the setting of the R/S/O flags as discussed below. The node sets the uNA Target Address to the unicast destination and uNA destination address to the unicast source of the original message. The node then sets the uNA source address to its own address and includes any necessary OMNI sub-options but MUST NOT itself set the SNR/SYN flags. If the SNR/SYN message included a Nonce and/or Timestamp option, the node includes matching Nonce/Timestamp options in the uNA response. The node finally returns the uNA message to the source of the SNR/SYN message.

As discussed in Section 4.4 of 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 , 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. For further study is whether solicited NAs for anycast targets apply for OMNI links. Since XLA-MNPs must be uniquely assigned to Clients to support correct IPv6 ND protocol operation, however, no role is currently seen for assigning the same XLA-MNP to multiple Clients.

In secured environments (e.g., between secured spanning tree neighbors, between neighbors on the same secured ANET, etc.), OMNI interface neighbors can exchange OAL packets that include randomly-initialized and monotonically-increasing (extended) Identification values (modulo 2**64) without window synchronization. In environments where spoofing is considered a threat, OMNI interface neighbors instead invoke window synchronization by including OMNI Window Synchronization sub-options in RS/RA or NS/NA message exchanges to maintain send/receive window state in their respective neighbor cache and AFIB entries as specified in . In common arrangements, OAL Identification window synchronization is necessary between Clients and Proxy/Servers or other Clients since their message exchanges are typically conducted over unsecured Internetworks. 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. All OAL nodes must verify Identification values of OAL packets addressed to themselves when window synchronization is required; OAL Intermediate systems forward OAL packets/fragments not addressed to themselves without examining their Identification values.

When the network layer forwards an original IP packet/parcel into an OMNI interface, the interface locates or creates a Neighbor Cache Entry (NCE) that matches the destination. The OMNI interface then invokes the OMNI Adaptation Layer (OAL) as discussed in which encapsulates the packet/parcel in an IPv6 header to produce an OAL packet. For example, an original IP packet/parcel with source address 2001:db8:1:2::1 and destination address 2001:db8:1234:5678::1 might cause the OAL encapsulation header to include source address {XLA*}::2001:db8:1:2 (i.e., an XLA-MNP) and destination address {ULA*}::0012:3456:789a:bcde (i.e., a ULA-RND). Following encapsulation, the OAL source then fragments the OAL packet while including an identical Identification value for each fragment that must be within the window for the neighbor. The OAL source includes any necessary OAL IPv6 extension headers including an identical Compressed Routing Header with 32-bit ID fields (CRH-32) with each fragment containing AERO Forwarding Vector Indexes (AFVIs) as discussed in . The OAL source can instead invoke OAL header compression by replacing the OAL IPv6 header, CRH-32 and Extended Fragment Header with an OAL Compressed Header (OCH). For messages that will traverse unsecured paths, the OAL source finally performs L2 encapsulation/fragmentation on each resulting OAL fragment to form a carrier packet, with source address set to its own L2 address (e.g., 192.0.2.100) and destination set to the L2 address of the next hop OAL intermediate system or destination (e.g., 192.0.2.1). The carrier packet encapsulation format in the above example is shown in :

(Note that carrier packets exchanged by Hosts on ENETs do not include the OAL IPv6 or CRH-32 headers, i.e., the OAL encapsulation is NULL and only the L2 encapsulations including any L2 IPv6 extension headers are included.) In this format, the OAL source encapsulates the original IP header and packet/parcel body/fragment in an OAL IPv6 header, the CRH-32 is a Routing Header extension of the OAL header, the Extended Fragment Header identifies each fragment, and the L2 headers are prepared as discussed in . The OAL source sends each such carrier packet into the SRT unsecured spanning tree, where they may be forwarded over multiple OAL intermediate systems until they arrive at the OAL destination. These carrier packets may themselves be subject to L2 fragmentation and reassembly along the path. The OMNI link control plane service distributes Client XLA-MNP prefix information that may change occasionally due to regional node mobility, as well as XLA-MNP prefix information for Relay non-MNPs and per-segment ULA prefix information that rarely changes. OMNI link Gateways and Proxy/Servers use the information to establish and maintain a forwarding plane spanning tree that connects all nodes on the link. The spanning tree supports a virtual bridging service according to link layer (instead of network layer) information, but may often include longer paths than necessary. Each OMNI interface therefore also includes an AERO Forwarding Information Base (AFIB) that caches AERO Forwarding Vectors (AFVs) which can provide both carrier packet Identification context and more direct forwarding "shortcuts" that avoid strict spanning tree paths. As a result, the spanning tree is always available but OMNI interfaces can often use the AFIB entries established through route optimization to greatly improve performance and reduce load on critical infrastructure elements. For OAL packets/fragments undergoing L2 re-encapsulation at an OAL intermediate system, the OMNI interface performs L2 reassembly/decapsulation followed by Identification verification and OAL reassembly only if the OAL packet/fragment is addressed to itself. The OMNI interface then decrements the OAL IPv6 header Hop Limit and discards the packet/fragment if the Hop Limit reaches 0. Otherwise, the OMNI interface updates the OAL addresses if necessary, includes an appropriate Identification, performs OAL fragmentation then for each OAL fragment performs L2 encapsulation/fragmentation to produce carrier packets appropriate for next segment forwarding. When an FHS Gateway forwards an OAL packet/fragment to an LHS Gateway over the unsecured spanning tree, it reconstructs the OAL header based on AFV state, inserts a CRH-32 immediately following the OAL header and adjusts the OAL payload length and destination address field. The FHS Gateway includes a single AFVI in the CRH-32 that the LHS Gateway can use to search its AFIB, then forwards the OAL packet/fragment over the unsecured spanning tree. When the LHS Gateway receives the OAL packet/fragment, it locates the AFV for the next hop based on the CRH-32 AFVI then re-applies header compression (resulting in the removal of the CRH-32) and forwards the OAL packet/fragment to the next hop. OAL packets/fragments that travel over secured spanning tree hops do not include OMNI L2 encapsulations. They are instead admitted into secured links such as IPsec tunnels or direct links where they may be subject to L2 security encapsulations as secured carrier packets. (Note that OMNI protocol L2 encapsulations could be used above the L2 security services, but this could result in excessive encapsulation in some instances.)

When an OAL node receives OAL packets/fragments addressed to another node, it discards the L2 headers and includes new L2 headers appropriate for the next hop in the forwarding path to the OAL destination (after first performing any necessary L2 fragmentation or reassembly). The node then sends these new carrier packets into the next hop underlay interface. When an OAL node receives OAL packets/fragments addressed to itself, it performs L2 reassembly/decapsulation, verifies the Identification, then performs OAL reassembly/decapsulation to obtain the original OAL packet or super-packet (see: ). Next, if the enclosed original IP packet(s)/parcel(s) are destined either to itself or to a destination reached via an interface other than the OMNI interface, the OAL node discards the OAL encapsulation and forwards the original IP packet(s)/parcel(s) to the network layer. If the original IP packet(s)/parcel(s) are destined to another node reached by the OMNI interface, the OAL node instead changes the OAL source to its own address, changes the OAL destination to the ULA of the next-hop node over the OMNI interface, decrements the Hop Limit, then performs L2 encapsulation/fragmentation and forwards these new carrier packets into the next hop underlay interface. Further OMNI link decapsulation details are specified in . Further OMNI link forwarding procedures are specified in .

AERO nodes employ simple data origin authentication procedures. In particular: AERO Gateways and Proxy/Servers accept carrier packets received from the secured spanning tree. AERO Proxy/Servers and Clients accept carrier packets and original IP packets/parcels that originate from within the same secured ANET. AERO Clients and Relays accept original IP packets/parcels from downstream network correspondents based on ingress filtering. AERO Hosts, Clients, Relays, Proxy/Servers and Gateways verify carrier packet L2 encapsulation addresses according to . AERO nodes that invoke window synchronization accept OAL packets/fragments with Identification values within the current window for the OAL source neighbor for a specific underlay interface pair and drop any packets with out-of-window Identification values. AERO nodes silently drop any packets/parcels that do not satisfy the above data origin authentication procedures. Further security considerations are discussed in .

The OMNI interface observes the link nature of tunnels, including the Maximum Transmission Unit (MTU), Effective MTU to Receive (EMTU_R) and the role of fragmentation and reassembly . The OMNI interface employs an OMNI Adaptation Layer (OAL) that accommodates multiple underlay links with diverse MTUs. OMNI interface packet sizing considerations are specified in , where the OMNI interface MTU can essentially be considered "unlimited". When the network layer presents an original IP packet/parcel to the OMNI interface, the OAL source encapsulates and fragments the packet/parcel if necessary. When the network layer presents the OMNI interface with multiple original IP packets/parcels bound to the same OAL destination, the OAL source can concatenate them as a single OAL super-packet as discussed in before applying fragmentation. The OAL source then submits each OAL fragment for L2 encapsulation/fragmentation for transmission as carrier packets over an underlay interface connected to either a physical link (e.g., Ethernet, WiFi, Cellular, etc.) or a virtual link such as an Internet or higher-layer tunnel. Note: Although a CRH-32 may be inserted or removed by a Gateway in the path (see: ), this does not interfere with the destination's ability to reassemble since the CRH-32 is not included in the fragmentable part and its removal/transformation does not invalidate fragment header information.

Original IP packets/parcels enter a node's OMNI interface either from the network layer (i.e., from a local application or the IP forwarding system) while carrier packets enter from the link layer (i.e., from an OMNI interface neighbor). All original IP packets/parcels and carrier packets entering a node's OMNI interface first undergo data origin authentication as discussed in . Those that satisfy data origin authentication are processed further, while all others are dropped silently. Original IP packets/parcels that enter the OMNI interface from the network layer are forwarded to an OMNI interface neighbor using OAL encapsulation and fragmentation to produce carrier packets for transmission over underlay interfaces. (If forwarding state indicates that the original IP packet/parcel should instead be forwarded back to the network layer, the packet/parcel is dropped to avoid looping). Carrier packets that enter the OMNI interface from the link layer are either re-encapsulated and re-admitted into the link layer, or reassembled and forwarded to the network layer where they are subject to either local delivery or IP forwarding. When the network layer forwards an original IP packet/parcel into the OMNI interface, it decrements the TTL/Hop Limit following standard IP router conventions. Once inside the OMNI interface, however, the OAL does not further decrement the original IP packet/parcel TTL/Hop Limit since its adaptation layer forwarding actions occur below the network layer. The original IP packet/parcel's TTL/Hop Limit will therefore be the same when it exits the destination OMNI interface as when it first entered the source OMNI interface. When an OAL intermediate system receives a carrier packet, it performs L2 reassembly/decapsulation to obtain the enclosed OAL packet/fragment. When the intermediate system forwards an OAL packet/fragment not addressed to itself, it decrements the OAL Hop Limit without decrementing the network layer IP TTL/Hop Limit. If decrementing would cause the OAL Hop Limit to become 0, the OAL intermediate system drops the OAL packet/fragment. This ensures that original IP packet(s)/parcel(s) cannot enter an endless loop. OMNI interfaces may have multiple underlay interfaces and/or neighbor cache entries for neighbors with multiple underlay interfaces (see ). The OAL uses Interface Attributes and/or Traffic Selectors to select an outbound underlay interface for each OAL packet and also to select segment routing and/or link layer destination addresses based on the neighbor's target underlay interfaces. AERO implementations SHOULD permit network management to dynamically adjust Traffic Selector values at runtime. If an OAL packet/fragment matches the Interface Attributes and/or Traffic Selectors of multiple outgoing interfaces and/or neighbor interfaces, the OMNI interface replicates the packet and sends a separate copy via each of the (outgoing / neighbor) interface pairs; otherwise, it sends a single copy via an interface with the best matching attributes/selectors. (While not strictly required, the likelihood of successful reassembly may improve when the OMNI interface sends all fragments of the same fragmented OAL packet/fragment consecutively over the same underlay interface pair to avoid complicating factors such as delay variance and reordering.) AERO nodes keep track of which underlay interfaces are currently "reachable" or "unreachable", and only use "reachable" interfaces for forwarding purposes. In addition to standard forwarding based on Interface Attributes and/or Traffic Selectors, nodes may employ a policy engine that would provide further guidance to the forwarding algorithm. For example the policy engine may suggest a load balancing profile over multiple underlay interface pairs, with portions of a traffic flow spread between multiple paths according to Equal Cost MultiPath or Link Aggregation Groups (LAGs) (note that Interface Attributes include an underlay interface group identifier). Other policies may suggest the use of paths with the least cost, best performance, etc. This document therefore specifies mechanisms without mandating any particular policies. The ULA Subnet ID value is used only for subnet coordination within a local OMNI link segment. When a node forwards an OAL packet/fragment addressed to a ULA with a foreign Global and/or Subnet ID value, it forwards the OAL packet/fragment based solely on the OMNI link routing information. For this reason, OMNI link routing and forwarding table entries always include both ULA-RNDs with their associated prefix lengths and XLA-MNPs which encode an MNP while leaving the Global and Subnet ID values set to 0. The following sections discuss the OMNI interface-specific forwarding algorithms for Hosts, Clients, Proxy/Servers and Gateways. In the following discussion, an original IP packet/parcel's destination address is said to "match" if it is the same as a cached address, or if it is covered by a cached prefix (which may be encoded in an {ULA,XLA}-MNP).

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 Section 6.13 of 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.

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 an ANET/INET 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, forwards the resulting OAL packet/fragment to an FHS Proxy/Server, then either invokes address resolution and multilink forwarding procedures per 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 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 to obtain the OAL packet/fragment then examines the OAL destination. If the OAL destination matches one of the Client's ULAs 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 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: ). 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: Clients and their FHS Proxy/Server (and other Client) peers can exchange original IP packets/parcels over ANET underlay interfaces using OMNI L2 encapsulation/fragmentation without invoking the OAL, since the ANET is secured at the link and physical layers. By forwarding original IP packets/parcels without invoking the OAL, the ANET peers use the same L2 encapsulation/fragmentation procedures as specified for Hosts above. Note: The forwarding table entries established in peer Clients of a multihop forwarding region are based on ULA-MNPs and/or TLAs used to seed the multihop routing protocols. When ULA-MNPs are used, the ULA /64 prefix provides topological relevance for the multihop forwarding region, while the 64-bit Interface Identifier encodes the Client MNP. Therefore, Clients can forward atomic fragments with compressed OAL headers that do not include ULA or AFVI information by examining the MNP-based addresses in the original IP packet/parcel header. In other words, each forwarding table entry contains two pieces of forwarding information - the ULA information in the prefix and the MNP information in the interface identifier.

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 ULA 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 is a non-ULA, the 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 . 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, 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 an ANET Client or following reassembly of carrier packets received from an ANET/INET Client if the original IP destination corresponds to the same Client's delegated MNP. Proxy/Servers also drop carrier packets that contain OAL packets/fragments with foreign OAL destinations that do not match their own ULA, the ULA of one of their Clients or a ULA corresponding to one of their GUA routes. These checks are essential to prevent forwarding inconsistencies from accidentally or intentionally establishing endless loops that could congest nodes and/or ANET/INET links. Proxy/Servers process carrier packets that contain OAL packets/fragments with OCH headers or with destinations that match their ULA and also include a CRH-32 header that encodes AFVI information. The Proxy/Server examines the AFVI to locate the corresponding AFV entry in the AFIB. If the carrier packets were not received from the secured spanning tree, the Proxy/Server must then verify that the L2 addresses are "trusted" according to the AFV. If the carrier packets were trusted, 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 ULA but do not include a CRH-32/OCH, 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 ULA 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 . 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 a {ULA,XLA}-MNP 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 ULA 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 Hub 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 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 sends the carrier packets to the Client which must then perform 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 reassembly.) When the Proxy/Server receives a carrier packet that contains an OAL packet/fragment with OAL destination set to a {ULA,XLA}-MNP 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 a GUA route that matches the MNP. 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. Clients and their FHS Proxy/Server peers can exchange original IP packets/parcels over ANET underlay interfaces using L2 encapsulation IPv6 Extended Fragment Headers only and no OAL addressing information, since the ANET is secured at the link and physical layers. (For packets that do not require fragmentation, the peers can even omit the Extended Fragment Header.) FHS Proxy/Servers will then supply an OAL Full (OFH) or Compressed (OCH) header when they forward ANET Client original IP packets/parcels toward final destinations located in other networks. Proxy/Servers forward OAL packets/fragments received in secure control plane carrier packets via the SRT secured spanning tree and forward other OAL packets/fragments via the unsecured spanning tree. 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. When a Proxy/Server receives a carrier packet from the unsecured spanning tree, it applies data origin authentication itself and/or forwards the enclosed unsecured OAL contents toward the destination which must apply data origin authentication on its own behalf. 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 .

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 ULA of a neighboring Gateway or Proxy/Server by examining the OMNI interface's network layer neighbor cache. If so, the Gateway performs OAL encapsulation and 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 that contain OAL packets/fragments with OAL destinations that do not match their ULA or the SRT Subnet Router Anycast address 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 ULA or the SRT Subnet Router Anycast address as the OAL destination. If the OAL packet/fragment contains an OCH or a full OAL header with a CRH-32 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. If the Gateway has a NCE for the target Client with an entry for the target underlay interface and current L2 addresses, the Gateway instead forwards the OAL packet/fragment directly to the target Client while using the final hop AFVI instead of the next hop (see: ). 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 . If the carrier packets arrived over the secured spanning tree and the enclosed OAL packets/fragments are addressed to its ULA, 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 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).

When an AERO node admits an original IP packet/parcel into the OMNI interface, it may receive link and/or network layer error indications. The AERO node may also receive OMNI link error indications in OAL-encapsulated uNA messages that include authentication signatures. A link layer error indication is an ICMP error message generated by a router in an underlay network on the path to the neighbor or by the neighbor itself. The message includes an IP header with the address of the node that generated the error as the source address and with the link layer address of the AERO node as the destination address. The IP header is followed by an ICMP header that includes an error Type, Code and Checksum. Valid type values include "Destination Unreachable", "Time Exceeded", "Parameter Problem" etc. . The ICMP header is followed by the leading portion of the carrier packet that generated the error, also known as the "packet-in-error". For ICMPv6, specifies that the packet-in-error includes: "As much of invoking packet as possible without the ICMPv6 packet exceeding the minimum IPv6 MTU" (i.e., no more than 1280 bytes). For ICMPv4, specifies that the packet-in-error includes: "Internet Header + 64 bits of Original Data Datagram", however Section 4.3.2.3 updates this specification by stating: "the ICMP datagram SHOULD contain as much of the original datagram as possible without the length of the ICMP datagram exceeding 576 bytes". The link layer error message format is shown in :

The AERO node rules for processing these link layer error messages are as follows: When an AERO node receives a link layer Parameter Problem message, it processes the message the same as described as for ordinary ICMP errors in the normative references . When an AERO node receives persistent link layer Time Exceeded messages, the IP ID field may be wrapping before earlier fragments awaiting reassembly have been processed. In that case, the node should begin including integrity checks and/or institute rate limits for subsequent carrier packets. When an AERO node receives persistent link layer Destination Unreachable messages in response to carrier packets that it sends to one of its neighbor correspondents, the node should process the message as an indication that a path may be failing, and optionally initiate NUD over that path. If it receives Destination Unreachable messages over multiple paths, the node should allow future carrier packets destined to the correspondent to flow through a default route and re-initiate route optimization. When an AERO Client receives persistent link layer Destination Unreachable messages in response to carrier packets that it sends to one of its neighbor Proxy/Servers, the Client should mark the path as unusable and use another path. If it receives Destination Unreachable messages on many or all paths, the Client should associate with a new Proxy/Server and release its association with the old Proxy/Server as specified in . When an AERO Proxy/Server receives persistent link layer Destination Unreachable messages in response to carrier packets that it sends to one of its neighbor Clients, the Proxy/Server should mark the underlay path as unusable and use another underlay path. When an AERO Proxy/Server receives link layer Destination Unreachable messages in response to a carrier packet that it sends to one of its permanent neighbors, it treats the messages as an indication that the path to the neighbor may be failing. However, the dynamic routing protocol should soon re-converge and correct the temporary outage. When an AERO Gateway receives a carrier packet for which the network layer destination address is covered by an MSP assigned to a black-hole route, the Gateway drops the carrier packet if there is no more-specific routing information for the destination and returns an OMNI interface Destination Unreachable message subject to rate limiting. When an AERO node receives a carrier packet for which OAL reassembly is currently congested, it returns an OMNI interface Packet Too Big (PTB) message as discussed in (note that the PTB messages could indicate either "hard" or "soft" errors). AERO nodes include ICMPv6 error messages intended for an OAL source as sub-options in the OMNI option of secured uNA messages. When the OAL source receives the uNA message, it can extract the ICMPv6 error message enclosed in the OMNI option and either process it locally or translate it into a network layer error to return to the original source.

AERO nodes observes the Router Discovery and Prefix Registration specifications found in . AERO nodes further coordinate their autoconfiguration actions with the mobility service as discussed in the following sections.

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) , via static configuration, etc. Clients receive the same service regardless of the Proxy/Servers they select. Clients associate each of their ANET/INET 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 Hub Proxy/Server (the Client may instead select a "third-party" Hub 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 Hub Proxy/Server and Client without assuming the Hub role functions themselves. Each Client associates with a single Hub Proxy/Server at a time, while all other Proxy/Servers are candidates for providing the Hub role for other Clients. An FHS Proxy/Server assumes the Hub role when it receives an RS message with its own 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 ULA 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 ULA of a better candidate Proxy/Server to serve as a Hub.) 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 Hub Proxy/Servers include prefix delegation and/or registration parameters in RS/RA messages. The IPv6 ND messages are exchanged between the Client and Hub Proxy/Server (via any FHS Proxy/Servers acting as proxys) according to the prefix management schedule required by the service. If the Client knows its MNP in advance, it can employ prefix registration by including its XLA-MNP as the source address of an RS message and with an OMNI option with valid prefix registration information for the MNP. If the Hub Proxy/Server accepts the Client's MNP assertion, it injects the MNP into the routing system and establishes the necessary neighbor cache state. If the Client does not have a pre-assigned MNP, it can instead employ prefix delegation by including a TLA as the source address of an RS message and with an OMNI option with prefix delegation parameters to request an MNP. The following sections outline Host, Client and Proxy/Server behaviors based on the Router Discovery and Prefix Registration specifications found in Section 15 of . These 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.

AERO Hosts and Clients discover the addresses of candidate Proxy/Servers by resolving the Potential Router List (PRL) in a similar manner as described in . Discovery methods include static configuration (e.g., a flat-file map of Proxy/Server addresses and locations), or through an automated means such as Domain Name System (DNS) name resolution . Alternatively, the Host/Client can discover Proxy/Server addresses through a data link layer login exchange, or through an RA response to a multicast/anycast RS as described below. In the absence of other information, the Host/Client can resolve the DNS Fully-Qualified Domain Name (FQDN) "linkupnetworks.[domainname]" where "linkupnetworks" is a constant text string and "[domainname]" is a DNS suffix for the OMNI link (e.g., "example.com"). The name resolution returns a set of resource records with Proxy/Server address information. The Host/Client then performs RS/RA exchanges over each of its underlay interfaces to associate with (possibly multiple) FHS Proxy/Serves and a single Hub Proxy/Server as specified in Section 15 of . The Host/Client sends each RS (either directly via Direct interfaces, via an IPsec tunnel for VPN interfaces, via an access router for ANET interfaces or via INET encapsulation for INET interfaces) and waits up to RetransTimer milliseconds for an RA message reply (see ) 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 Hub Proxy/Server's RA response, it has assurance that both the Hub and FHS Proxy/Servers have been updated with the new information. If the Host/Client wishes to discontinue use of a Hub Proxy/Server it issues an RS message over any underlay interface with an OMNI Proxy/Server Departure sub-option that encodes the (old) Hub Proxy/Server's ULA. When the Hub Proxy/Server processes the message, it releases the MNP, 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 Hub Proxy/Server withdraws the MNP from the routing system. (Alternatively, when the Host/Client associates with a new FHS/Hub Proxy/Server it can include an OMNI "Proxy/Server Departure" sub-option in RS messages with the ULAs of the Old FHS/Hub Proxy/Servers.)

AERO Proxy/Servers act as both IP routers and IPv6 ND proxys, and support a prefix delegation/registration service for Clients. Proxy/Servers arrange to add their ULAs to the PRL maintained in a static map of Proxy/Server addresses for the link, the DNS resource records for the FQDN "linkupnetworks.[domainname]", etc. before entering service. The PRL should be arranged such that Clients can discover the addresses of Proxy/Servers that are geographically and/or topologically "close" to their underlay network connections. When a FHS/Hub 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 as specified in Section 15 of . When the Hub Proxy/Server receives the RS, it determines the correct MNPs for the Client by processing the XLA-MNP prefix parameters and/or the DHCPv6 OMNI sub-option. When the Hub Proxy/Server returns the MNPs, it also creates an XLA-MNP forwarding table entry for the MNP resulting in a BGP update (see: ). The Hub Proxy/Server then returns an RA to the Client with destination set to the source of the RS (if an FHS Proxy/Server on the return path proxys the RA, it changes the destination to the Client's ULA-MNP). After the initial RS/RA exchange, the Hub 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 Hub Proxy/Server sends an RA response and resets ReachableTime. If the Hub 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 XLA-MNP route 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 Hub Proxy/Server marks the interface as DOWN. If ReachableTime expires before any new RS is received on any individual underlay interface, the Hub Proxy/Server sets the NCE state to STALE and sets a 10 second timer. If the Hub Proxy/Server has not received a new RS or uNA message with a prefix release indication before the 10 second timer expires, it deletes the NCE and withdraws the XLA-MNP from the routing system. The Hub 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 Hub 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 Hub Proxy/Server may also receive carrier packets via the secured spanning tree that contain initial data sent while route optimization is in progress. The Hub 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 Hub Proxy/Server forwards any OAL packets/fragments it receives from the secured spanning tree and destined to the Client to the new Hub Proxy/Server, then deletes the entry after DepartTime expires. Note: Clients SHOULD arrange to notify former Hub Proxy/Servers of their departures, but Hub Proxy/Servers are responsible for expiring neighbor cache entries and withdrawing XLA-MNP routes even if no departure notification is received (e.g., if the Client leaves the network unexpectedly). Hub 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.

AERO Clients register with FHS Proxy/Servers for each underlay interface. Each of the Client's FHS Proxy/Servers must inform a single Hub Proxy/Server of the Client's underlay interface(s) that it services. For Clients on Direct and VPN/IPsec underlay interfaces, the FHS Proxy/Server for each interface is directly connected, for Clients on ANET underlay interfaces the FHS Proxy/Server is located on the ANET/INET boundary, and for Clients on INET underlay interfaces the FHS Proxy/Server is located somewhere in the connected Internetwork. When FHS Proxy/Server "B" processes a Client registration, it must either assume the Hub role or forward a proxyed registration to another Proxy/Server "A" acting as the Hub. Proxy/Servers satisfy these requirements as follows: when FHS Proxy/Server "B" receives a Client RS message, it first verifies that the OAL Identification is within the window for the NCE that matches the {ULA,XLA}-MNP in the RS source address for this Client neighbor and authenticates the message. If no NCE was found, Proxy/Server "B" instead creates one in the STALE state and caches the Client-supplied Interface Attributes, Origin Indication and OMNI Window Synchronization sub-option parameters as well as the Client's observed L2 addresses (noting that they may differ from the Origin addresses if there were NATs on the path). Proxy/Server "B" then examines the RS destination address. If the destination address is the ULA of a different Proxy/Server "A", Proxy/Server "B" prepares a separate proxyed version of the RS message with an OAL header with source set to its own ULA and destination set to Proxy/Server A's ULA. Proxy/Server "B" also writes its own information over the Interface Attributes sub-option supplied by the Client, omits or zeros the Origin Indication sub-option then forwards the message into the OMNI link secured spanning tree. when Hub Proxy/Server "A" receives the RS, it assumes the Hub role, delegates an MNP for the Client if necessary according to the Preflen in a Neighbor Control sub-option included by the Client, and creates/updates a NCE indexed by the Client's XLA-MNP with FHS Proxy/Server "B"'s Interface Attributes as the link layer address information for this FHS ifIndex. Hub Proxy/Server "A" then prepares an RA message with source set to its own ULA, destination set to the source of the RS message, and with a Neighbor Control sub-option with Preflen set to the actual MNP length it will delegate to the Client. Hub Proxy/Server "A" then encapsulates the RA in an OAL header with source set to its own ULA and destination set to the ULA of FHS Proxy/Server "B", then finally performs fragmentation if necessary and sends the resulting carrier packets into the secured spanning tree. when FHS Proxy/Server "B" reassembles the RA, it locates the Client NCE based on the RA destination. If the RA message includes an OMNI "Proxy/Server Departure" sub-option with non zero old FHS/Hub Proxy/Server ULAs that do not match its own ULA, FHS Proxy/Server "B" first sends a uNA to the old FHS/Hub Proxy/Servers named in the sub-option. If the RA message delegates a new XLA-MNP, Proxy/Server "B" then resets the RA destination to the corresponding ULA-MNP for this interface. Proxy/Server "B" then re-encapsulates the message with OAL source set to its own ULA and OAL destination set to ULA that appeared in the Client's RS message OAL source, with an appropriate Identification value, with an authentication signature if necessary, with the Client's Interface Attributes sub-option echoed and with the cached observed L2 addresses written into an Origin Indication sub-option. Proxy/Server "B" sets the P flag in the RA flags field to indicate that the message has passed through a proxy , includes responsive window synchronization parameters, then fragments the RA if necessary and returns the fragments to the Client. The Client repeats this process over each of its additional underlay interfaces while treating each additional FHS Proxy/Server "C", "D", "E", etc. as a proxy to facilitate RS/RA exchanges between Hub "A" and the Client. The Client creates/updates NCEs for each such FHS Proxy/Server as well as the Hub Proxy/Server in the process. After the initial RS/RA exchanges each FHS Proxy/Server forwards any of the Client's carrier packets that contain OAL packets/fragments with destinations for which there is no matching NCE to a Gateway using OAL encapsulation with its own ULA as the source and with destination determined by the Client. The Proxy/Server instead forwards any OAL packets/fragments destined to a neighbor cache target directly to the target according to the OAL or link layer information - the process of establishing neighbor cache entries is specified in . While the Client is still associated with FHS Proxy/Servers "B", "C", "D", "E", etc., each FHS Proxy/Server can send NS, RS and/or uNA messages to update the neighbor cache entries of other AERO nodes on behalf of the Client based on changes in Interface Attributes, Traffic Selectors, etc. This allows for higher-frequency Proxy-initiated RS/RA messaging over well-connected INET infrastructure supplemented by lower-frequency Client-initiated RS/RA messaging over constrained ANET data links. If the Hub Proxy/Server "A" ceases to send solicited RAs, FHS Proxy/Servers "B", "C", "D", "E", etc. can send unsolicited RAs over the Client's underlay interface with destination set to (link-local) All-Nodes multicast and with Router Lifetime set to zero to inform Clients that the Hub Proxy/Server has failed. Although Proxy/Servers "B", "C", "D", "E", etc. can engage in IPv6 ND exchanges on behalf of the Client, the Client can also send IPv6 ND messages on its own behalf, e.g., if it is in a better position to convey state changes. The IPv6 ND messages sent by the Client include the Client's XLA-MNP as the source in order to differentiate them from the IPv6 ND messages sent by a FHS Proxy/Server. If the Client becomes unreachable over all underlay interfaces it serves, the Hub Proxy/Server sets the NCE state to DEPARTED and retains the entry for DepartTime seconds. While the state is DEPARTED, the Hub Proxy/Server forwards any OAL packets/fragments destined to the Client to a Gateway via OAL encapsulation. When DepartTime expires, the Hub Proxy/Server deletes the NCE, withdraws the XLA-MNP route and discards any further carrier packets that contain OAL packets/fragments destined to the former Client. In some ANETs that employ a Proxy/Server, the Client's MNP can be injected into the ANET routing system. In that case, the Client can send original IP packets/parcels without invoking the OAL so that the ANET routing system transports the original IP packets/parcels to the Proxy/Server. This can be beneficial, e.g., if the Client connects to the ANET via low-end data links such as some aviation wireless links. If the ANET first-hop access router is on the same underlay link as the Client and recognizes the AERO/OMNI protocol, the Client can avoid OAL encapsulation for both its control and data messages. When the Client connects to the link, it can send an unencapsulated RS message with source address set to its own XLA-MNP (or to a TLA), and with destination address set to the ULA of the Client's selected Proxy/Server or to link-scoped All-Routers multicast. The Client includes an OMNI option formatted as specified in . The Client then sends the unencapsulated RS message, which will be intercepted by the AERO-aware ANET access router. The ANET access router then performs OAL encapsulation on the RS message and forwards it to a Proxy/Server at the ANET/INET boundary. When the access router and Proxy/Server are one and the same node, the Proxy/Server would share an underlay link with the Client but its message exchanges with outside correspondents would need to pass through a security gateway at the ANET/INET border. The method for deploying access routers and Proxys (i.e. as a single node or multiple nodes) is an ANET-local administrative consideration. Note: When a Proxy/Server alters the IPv6 ND message contents before forwarding (e.g., such as altering the OMNI option contents), the original IPv6 ND message checksum and authentication signature values are invalidated and must be re-calculated. Note: When a Proxy/Server receives a secured Client NS message, it performs the same proxying procedures as for described for RS messages above. The proxying procedures for NS/NA message exchanges is specified in .

In environments where fast recovery from Proxy/Server failure is required, FHS Proxy/Servers SHOULD use proactive Neighbor Unreachability Detection (NUD) to track Hub Proxy/Server reachability in a fashion that parallels Bidirectional Forwarding Detection (BFD) . Each FHS Proxy/Server can then quickly detect and react to failures so that cached information is re-established through alternate paths. The NS/NA control messaging is carried only over well-connected ground domain networks (i.e., and not low-end aeronautical radio links) and can therefore be tuned for rapid response. FHS Proxy/Servers can perform continuous NS/NA exchanges with the Hub Proxy/Server, e.g., one exchange per N seconds. The FHS Proxy/Server sends the NS message via the spanning tree with its own ULA as the source and the ULA of the Hub Proxy/Server as the destination, and the Hub Proxy/Server responds with an NA. When the FHS Proxy/Server also sends RS messages to a Hub Proxy/Server on behalf of Clients, the resulting RA responses can be considered as equivalent hints of forward progress. This means that the FHS Proxy/Server need not also send a periodic NS if it has already sent an RS within the same period. If the Hub Proxy/Server fails (i.e., if the FHS Proxy/Server ceases to receive advertisements), the FHS Proxy/Server can quickly inform Clients by sending unsolicited RA messages The FHS Proxy/Server sends unsolicited RA messages with source address set to the Hub Proxy/Server's address, destination address set to (link-local) All-Nodes multicast, and Router Lifetime set to 0. The FHS Proxy/Server SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated by small delays . Any Clients that had been using the failed Hub Proxy/Server will receive the RA messages and select a different Proxy/Server to assume the Hub role (i.e., by sending an RS with destination set to the ULA of the new Hub).

When a Client is not pre-provisioned with an MNP, it will need for the Hub Proxy/Server to select one or more MNPs on its behalf and set up the correct state in the AERO routing service. (A Client with a pre-provisioned MNP may also request the Hub Proxy/Server to select additional MNPs.) Clients can use the DHCPv6 service to satisfy this requirement. When a Client needs to have the Hub Proxy/Server select MNPs, it sends an RS message with source address set to a TLA and with an OMNI option that includes a DHCPv6 message sub-option with DHCPv6 Prefix Delegation (DHCPv6-PD) parameters. When the Hub Proxy/Server receives the RS message, it extracts the DHCPv6-PD message from the OMNI option. The Hub Proxy/Server then acts as a "Proxy DHCPv6 Client" in a message exchange with the locally-resident DHCPv6 server, which delegates MNPs and returns a DHCPv6-PD Reply message. (If the Hub Proxy/Server wishes to defer creation of MN state until the DHCPv6-PD Reply is received, it can instead act as a Lightweight DHCPv6 Relay Agent per by encapsulating the DHCPv6-PD message in a Relay-forward/reply exchange with Relay Message and Interface ID options.) When the Hub Proxy/Server receives the DHCPv6-PD Reply, it creates an XLA based on the delegated MNP adds an XLA-MNP route to the routing system. The Hub Proxy/Server then sends an RA back to the Client either directly or via an FHS Proxy/Server acting as a proxy. The Proxy/Server that returns the RA directly to the Client sets the (newly-created) ULA-MNP as the destination address and with a DHCPv6-PD Reply message sub-option coded in the OMNI option. When the Client receives the RA, it creates a default route, assigns the Subnet Router Anycast address and sets its {ULA,XLA}-MNP based on the delegated MNP. Note: Further details of the DHCPv6-PD based MNP registration (as well as a minimal MNP delegation alternative that avoids including a DHCPv6 message sub-option in the RS) are found in . Note: when the Hub Proxy/Server forwards an RA to the Client via a different node acting as a FHS Proxy/Server, the Hub sets the RA destination to the same address that appeared in the RS source. The FHS Proxy/Server then subsequently sets the RA destination to the ULA-MNP when it forwards the Proxyed version of the RA to the Client - see for further details.

AERO nodes invoke address resolution, multilink forwarding and route optimization when they need to forward initial original IP packets/parcels to new neighbors over ANET/INET interfaces and for ongoing multilink forwarding coordination with existing neighbors. Address resolution is based on an IPv6 ND NS/NA(AR) messaging exchange between an Address Resolution Source (ARS) and the target neighbor as the Address Resolution Target (ART). Either the ART itself or the ART's current Hub Proxy/Server serves as the Address Resolution Responder (ARR). Address resolution is initiated by the first eligible ARS closest to the original source as follows: For Clients on VPN/IPsec and Direct interfaces, the Client's FHS Proxy/Server is the ARS. For Clients on ANET interfaces, either the FHS Proxy/Server or the Client itself may be the ARS. For Clients on INET interfaces, the Client itself is the ARS. For correspondent nodes on INET/ENET interfaces serviced by a Relay, the Relay is the ARS. For Clients that engage the Hub Proxy/Server in "mobility anchor" mode, the Hub Proxy/Server is the ARS. For peers within the same ANET/ENET, address resolution and route optimization is through receipt of Redirect messages. The AERO routing system directs an address resolution request sent by the ARS to the ARR. The ARR then returns an address resolution reply which must include information that is complete, current, consistent and authentic. Both the ARS and ARR are then jointly responsible for periodically refreshing the address resolution, and for quickly informing each other of any changes. Following address resolution, the ARS and ART perform continuous multilink forwarding and route optimization exchanges to maintain optimal forwarding profiles. During address resolution, multilink forwarding and/or route optimization an NS/NA message source may attach a small number of original IP packets/parcels associated with the message exchange as super-packet extensions per . The authentication signatures and/or lower-layer security features employed at the OAL source and each OAL intermediate system will provide authorization and integrity services for both the NS/NA messages and their IP packet/parcel attachments. When an OAL source or intermediate system forwards a secured NS/NA super-packet, it should perform OAL encapsulation followed by fragmentation using a fragment size no larger than 1280 octets to ensure that the fragments will traverse any possible secured spanning tree paths. The final OAL intermediate system in the path will then securely forward the NS/NA message IP packet/parcel attachments to the OAL target. The address resolution, multilink forwarding and route optimization procedures are specified in the following sections.

When one or more original IP packets/parcels from a source node destined to a target node arrive, the ARS checks for a NCE with an XLA-MNP that matches the target destination. If there is a NCE in the REACHABLE state, the ARS invokes the OAL and sends 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 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: the ULA of the ARS as the source address. the XLA corresponding to the original IP packet/parcel's destination as the Target Address, e.g., for 2001:db8:1:2::10:2000 the Target Address is fd00::2001:db8:1:2. the Solicited-Node multicast address formed from the lower 24 bits of the original IP packet/parcel's destination as the destination address, e.g., for 2001:db8:1:2::10:2000 the NS(AR) destination address is ff02:0:0:0:0:1:ff10:2000. The NS(AR) message also includes an OMNI option with an authentication sub-option (if necessary), with Interface Attributes and/or Traffic Selectors for all of the source Client's underlay interfaces and with a Neighbor Control sub-option with a valid Preflen for its claimed MNP. The ARS then calculates and includes the authentication signature (if necessary) followed by the checksum, then submits the NS(AR) message for OAL encapsulation. The ARS sets the OAL source to its own ULA and sets the OAL destination according to the Client's RS message "RPT" flag (see: ). If the "RPT" flag was set, the ARS sets the OAL destination to the ULA of its Hub Proxy/Server which maintains a Report List; otherwise, the ARS sets the destination to the XLA-MNP corresponding to the ART. The ARS then includes an appropriate Identification value, performs OAL fragmentation and L2 encapsulation/fragmentation, then 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 the ULA of one of its FHS Proxy/Servers as the OAL destination. The ARS Client then performs OAL fragmentation followed by L2 encapsulation/fragmentation then forwards the carrier packets to the FHS Proxy/Server. The FHS Proxy/Server then performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly, verifies the NS(AR) checksum/authentication signature and confirms that the Client's claimed Neighbor Control Preflen is valid for its ULA-MNP source address. The FHS Proxy/Server then changes the OAL source to its own ULA and changes the OAL destination to the ULA of the Hub Proxy/Server or XLA-MNP corresponding to the ART as specified above. The FHS Proxy/Server next includes an appropriate Identification, performs OAL fragmentation, 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 Hub Proxy/Servers include current and accurate information for their multilink Interface Attributes profile. The Hub 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 Hub Proxy/Server.

If the ARS Client's Hub Proxy/Server maintains a Report List, the carrier packets containing the NS(AR) will first arrive at the Hub due to the OAL destination address supplied by the ARS (see above). This source Hub then performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly and records the NS Target Address in the Report List for this source Client. The Hub then leaves the OAL source address unchanged, but changes the OAL destination address to the XLA corresponding to the NS Target Address. The Hub then decrements the OAL header Hop Limit, includes an appropriate Identification, performs OAL fragmentation followed by L2 encapsulation/fragmentation and sends the resulting carrier packets into the secured spanning tree.

When a Gateway receives carrier packets containing the NS(AR), it performs L2 reassembly/decapsulation and determines the next hop by consulting its standard IPv6 forwarding table for the OAL header XLA destination address. The Gateway next decrements the OAL header Hop Limit, performs L2 encapsulation/fragmentation and sends the carrier packet(s) via the secured spanning tree the same as for any IPv6 router where they may traverse multiple OMNI link segments. The final-hop Gateway will deliver the carrier packets via the secured spanning tree to the Hub Proxy/Server (or Relay) that services the ART.

When the Hub Proxy/Server of the ART receives the NS(AR) secured carrier packets with the XLA-MNP of the ART as the OAL destination, it performs L2 reassembly/decapsulation followed by OAL reassembly then either forwards the NS(AR) to the ART or processes it locally if it is acting as a Relay or as the ART's designated ARR. The Hub Proxy/Server processes the message as follows: if the NS(AR) target matches a Client NCE in the DEPARTED state, the (old) Hub Proxy/Server resets the OAL destination address to the ULA of the Client's new Hub Proxy/Server. The old Hub Proxy/Server then decrements the OAL header Hop Limit, performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets over the secured spanning tree. If the NS(AR) target matches a Client NCE in the REACHABLE state, the Hub Proxy/Server notes whether the NS(AR) arrived from the secured spanning tree. If the message arrived via the secured spanning tree the Hub Proxy/Server verifies the NS checksum only; otherwise, it must also verify the message authentication signature. If the Hub Proxy/Server maintains a Report List for the ART, it next records the NS source address in the Report List for this ART. If the Hub Proxy/Server is the ART's designated ARR, it forwards any original IP packet(s)/parcel(s) attached to the NS(AR) super-packet to the ART and prepares to return an NA(AR) as discussed below; otherwise, the Hub Proxy/Server determines the underlay interface for the ART and proceeds as follows: If the Hub Proxy/Server is also the FHS Proxy/Server on the underlay interface used to convey the NS(AR) to the ART, it includes an authentication signature if necessary then recalculates the NS(AR) checksum. The Hub then changes the OAL source to its own ULA and OAL destination to the ULA-MNP of the ART, decrements the OAL Hop Limit, includes an appropriate Identification value, performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets over the underlay interface to the ART. If the Hub Proxy/Server is not the FHS Proxy/Server on the underlay interface used to convey the NS(AR) to the ART, it instead recalculates the NS(AR) checksum, changes the OAL source to its own ULA and changes the OAL destination to the ULA of the FHS Proxy/Server for this ART interface. The Hub Proxy/Server next decrements the OAL Hop Limit, includes an appropriate Identification value, performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets over the secured spanning tree. When the FHS Proxy/Server receives the carrier packets, it performs L2 reassembly/decapsulation, reassembles the NS(AR) and verifies the checksum, then forwards to the ART the same as described above. If the NS(AR) target matches one of its non-MNP routes, the Hub Proxy/Server serves as both a Relay and an ARR, since the Relay forwards original IP packets/parcels toward the (fixed network) target at the network layer. If the ARR is a Relay or the ART itself, it first creates or updates a NCE for the NS(AR) source address while caching all Interface Attributes and Traffic Selector information. Next, the ARR prepares a solicited NA(AR) message to return to the ARS with the source address set to the ART's XLA, the destination address set to the NS(AR) ULA source address and the Target Address set to the same value that appeared in the NS(AR) Target Address. The ARR then includes Interface Attributes and Traffic Selector sub-options for all of the ART's underlay interfaces with current information for each interface and includes a Neighbor Control sub-option with the Preflen to apply to the ART's MNP. The ARR next sets the NA(AR) message R flag to 1 (as a router) and S flag to 1 (as a response to a solicitation) and sets the O flag to 1 (as an authoritative responder). The ARR finally includes an authentication signature if necessary, calculates the NA message checksum, then submits the NA(AR) for OAL encapsulation with source set to its own ULA and destination set to the ULA that appeared in the NS(AR) OAL source while including an appropriate Identification. The ARR then performs OAL fragmentation followed by L2 encapsulation/fragmentation, and forwards the resulting carrier packets. When the ART Proxy/Server receives carrier packets sent by an ART acting as an ARR on its own behalf, it performs L2 reassembly and decapsulation, verifies the Identification, performs OAL reassembly, then verifies the checksum/authentication signature. The Proxy/Server then verifies that the Neighbor Control Preflen is acceptable, changes the OAL source address to its own ULA and changes the OAL destination to the ULA corresponding to the NA(AR) destination. The Proxy/Server next decrements the OAL Hop Limit, includes an appropriate Identification then recalculates the NA checksum. The Proxy/Server finally performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets into the secured spanning tree.

When a Gateway receives NA(AR) carrier packets, it performs L2 reassembly/decapsulation and determines the next hop by consulting its standard IPv6 forwarding table for the OAL header destination address. The Gateway then decrements the OAL header Hop Limit, performs L2 encapsulation/fragmentation and forwards the resulting carrier packets via the SRT secured spanning tree where they may traverse multiple OMNI link segments. The final-hop Gateway will deliver the carrier packets via the secured spanning tree to a Proxy/Server for the ARS.

When the ARS receives NA(AR) carrier packets, it performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly, then searches for a NCE that matches the NA(AR) target address. The ARS then processes the message the same as for standard IPv6 Address Resolution . In the process, it caches all OMNI option information in the NCE for the ART (including Interface Attributes, Traffic Selectors, etc.), and caches the NA(AR) XLA source address as the address of the ART. When the ARS is a Client, the SRT secured spanning tree will first deliver the solicited NA(AR) message to the FHS Proxy/Server, which re-adjusts the OAL header and forwards the message to the Client. If the Client is on a well-managed ANET, physical security and protected spectrum ensures security for the NA(AR) without needing an additional authentication signature; if the Client is on the open INET the Proxy/Server must instead include an authentication signature (while adjusting the OMNI option size, if necessary). The Proxy/Server uses its own ULA as the OAL source and the ULA-MNP of the Client as the OAL destination when it forwards the NA(AR). The Proxy/Server then decrements the OAL Hop Limit, includes an appropriate Identification, performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets over the underlay interface to the Client.

After the ARS transmits the first NS(AR), it should wait up to RETRANS_TIMER seconds to receive a responsive NA(AR). The ARS can then retransmit the NS(AR) up to MAX_UNICAST_SOLICIT times before giving up.

Following address resolution, the ARS and ART (or their respective FHS Proxy/Servers) can assert multilink forwarding paths through underlay interface pairs serviced by the same source/destination ULAs by sending unicast NS/NA messages with OMNI AERO Forwarding Parameter (AFP) sub-options. The unicast NS/NA messages establish multilink forwarding 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 unicast NS messages on behalf of specific underlay interface pairs. (Underlay interface directionality (i.e., in/out) must also be factored into the paths established for multilink forwarding.) When an OAL source asserts a multilink forwarding path through the transmission of a unicast NS message, it includes an IPv6 Minimum Path MTU Hop-by-Hop option for the (adaptation layer) IPv6 header per . Each OAL intermediate node and OAL IPv6 router along the path then updates the minimum MTU per the specification. When the OAL destination responds with a unicast NA message, it returns an IPv6 Minimum Path MTU Hop-by-Hop option based on the one it received in the NS message per . This allows the OAL source to discover any OAL Fragment Size (OFS) limitations for this OAL destination (see: ). For this reason, IPv6 routers that connect SRT segments MUST implement . 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 guide the dispersal of 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. 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 OMNI neighbor underlay interface pairs. The AFIB contains per-interface-pair AERO Forwarding Vectors (AFVs) identified by locally-unique values known as AFV Indexes (AFVIs). The AFVs cache uncompressed OAL header information as well as the previous/next-hop addressing and AFVI information. The AFVs also cache window synchronization state for the specific underlay interface pair. Using the window synchronization state, simple Identification-based data origin authentication is enabled at each OAL source, intermediate system and target node. OMNI interfaces manage the AFIB in conjunction with their internal Neighbor Cache. OMNI interface NCEs link to (possibly) multiple AFVs, with one AVF per underlay interface pair (according to directionality). When OMNI interface peers need to coordinate, they locate a NCE for the peer then use the NCE as a nexus that aggregates potentially many AVFs. In particular, the NCE caches the AFVI to be used to index the local AFV at the head end of the path. OAL source, intermediate system and target nodes create AFVs/AFVIs when they process an NS/NA message with an AFP sub-option with Job code '00' (Initialize; Build B) or a responsive NA message with Job code '01' (Follow B; Build A) (see: ). The OAL source of the NS/NA (which is also the OAL destination of the responsive NA) is considered to reside in the "First Hop Segment (FHS)", while the OAL destination of the NS/NA (which is also the OAL source of the responsive NA) is considered to reside in the "Last Hop Segment (LHS)". The FHS and LHS roles are determined on a per-interface-pair basis. After address resolution, either peer is equally capable of initiating multilink forwarding on behalf of a specific FHS/LHS underlay interface pair. The peer that sends the initiating NS/NA with Job code '00' message for a specific pair becomes the FHS peer while the one that returns the responsive NA becomes the LHS peer for that pair only. It is therefore commonplace that peers may assume the FHS role for some pairs while assuming the LHS role for other pairs, i.e., even though each peer maintains only a single NCE. When an OAL node initiates or forwards an NS/NA with Job code '00', it creates an AFV, records the NS/NA source and destination ULAs then generates and assigns a locally-unique "B" AFVI (while also caching the "B" values for all previous OAL hops on the path from the FHS OAL source). When the OAL node receives future OAL packets/fragments that include "B", it can unambiguously locate the correct AFV and determine directionality without examining addresses. When the AFV is indexed by its "B" AFVI, it returns the ULAs in (dst,src) order the opposite of how they appeared in the OAL header of the original NS to support full header reconstruction for reverse-path forwarding. (If the NS message included a nested OAL encapsulation, the ULAs of both OAL headers are returned.) When an OAL node initiates or forwards a responsive NA with Job code '01', it uses the "B" AFVI to locate the AFV created by the NS then generates and assigns a locally-unique "A" AFVI (while also caching the "A" values for all previous OAL hops on the path from the LHS OAL source). When the OAL node receives future carrier packets that include "A", it can unambiguously locate the correct AFV and determine directionality without examining addresses. When the AFV is indexed by its "A" AFVI, it returns the ULAs in (src,dst) order the same as they appeared in the OAL header of the original NS to support full header reconstruction for forward-path forwarding. (If the NS message included a nested OAL encapsulation, the ULAs of both OAL headers are returned.) OAL nodes generate random non-zero 32-bit values as candidate AFVIs which must first be tested for local uniqueness. If a candidate AFVI is already in use, the OAL node repeats the random generation process until it obtains a unique non-zero value. Since the number of AFVs in service at each OAL node is likely to be much smaller than 2**32, the process will generate a unique value after a small number of tries. Since the uniqueness property is node-local only, an AFVI locally generated by a first OAL node must not be tested for uniqueness by other OAL nodes. OAL nodes cache AFVs for up to ReachableTime seconds following their initial creation. If the node processes another NS or NA 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 and frees its associated AFVIs so they can be reused for future AFVs. The following sections provide the detailed specifications of these NS/NA exchanges for all nodes along the forward and reverse paths.

When an FHS OAL source has an original IP packet/parcel to send toward an LHS OAL target, it first performs multilink address resolution resulting in the creation of a NCE for the XLA of the target then selects a source and target underlay interface pair. The FHS source then uses its cached information for the target interface as LHS information then prepares an NS message with an AFP sub-option with Job code '00', includes window synchronization information, then sets the NS source to the XLA of the FHS Client and the NS target to the XLA of the LHS Client. The FHS source next creates an AFV then generates and assigns a locally-unique "B" AFVI to the AFV while also including it as the first "B" entry in the AFP AFVI List. The FHS source then includes any FHS/LHS addressing information it knows locally in the AFP sub-option, i.e., based on information discovered through address resolution. If the FHS source is the FHS Proxy/Server, it then examines the LHS FMT-Forward code. If FMT-Forward is clear the FHS Proxy/Server sets the NS destination to the ULA of the LHS Proxy/Server; otherwise, it sets the NS destination to the same address as the target. The FHS Proxy/Server then performs OAL encapsulation while setting the OAL source to its own ULA and setting the OAL destination to the FHS Subnet Router Anycast ULA determined by applying the FHS SRT prefix length to its ULA. The FHS Proxy/Server then includes an appropriate Identification value, performs OAL fragmentation followed by L2 encapsulation/fragmentation then forwards the resulting carrier packets into the secured spanning tree which will deliver them to a Gateway interface that assigns the FHS Subnet Router Anycast ULA. If the FHS source is the FHS Client, it instead includes an authentication signature if necessary. If LHS FMT-Forward is clear, the FHS Client sets the NS destination to the ULA of the LHS Proxy/Server; otherwise, it sets the NS destination to the same address as the target. The FHS Client then calculates the NS message checksum, performs OAL encapsulation, sets the OAL source to its own ULA-MNP and sets the OAL destination to the ULA of the FHS Proxy/Server. The FHS Client finally includes an appropriate Identification value for the FHS Proxy/Server, performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets to the FHS Proxy/Server. When the FHS Proxy/Server receives the carrier packets, it performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly if necessary and verifies the NS checksum or authentication signature. The FHS Proxy/Server then creates an AFV (i.e., the same as the FHS Client had done) while caching the AFP "B" entry along with the FHS Client addressing information as previous hop information for this AFV. The FHS Proxy/Server next generates a new locally-unique "B" AFVI, then assigns it as the AFV index and writes it as the next "B" entry in the AFP AFVI List (while also writing any FHS Client and Proxy/Server addressing information). The FHS Proxy/Server then calculates the NS checksum and sets the OAL source address to its own ULA and destination address to the FHS Subnet Router Anycast ULA. The FHS Proxy/Server finally decrements the OAL Hop Limit and includes an Identification appropriate for the secured spanning tree. The FHS Proxy/Server finally performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets into the secured spanning tree.

Gateways in the spanning tree forward OAL packets/fragments not explicitly addressed to themselves, while forwarding those that arrived via the secured spanning tree to the next hop also via the secured spanning tree and forwarding all others via the unsecured spanning tree. When an FHS Gateway receives an OAL packet/fragment over the secured spanning tree addressed to its ULA or the FHS Subnet Router Anycast ULA, it instead performs L2 reassembly/decapsulation, verifies the Identification, then finally performs OAL reassembly to obtain the NS then verifies the NS checksum. The FHS Gateway next creates an AFV (i.e., the same as the FHS Proxy/Server had done) while caching the AFP FHS Client and Proxy/Server addressing information, window synchronization information and corresponding AFVI List "B" values in the AFV to enable future reverse path forwarding to this FHS Client. The FHS Gateway then generates a locally-unique "B" AFVI for the AFV and writes it as the next "B" entry in the NS AFP AFVI List. The FHS Gateway then examines the SRT prefixes corresponding to both FHS and LHS. If the FHS Gateway has a local interface connection to both the FHS and LHS (whether they are the same or different segments), the FHS/LHS Gateway caches the NS AFP LHS information in the AFV, writes its LHS ULA and L2ADDR into the NS AFP LHS fields, then sets its LHS ULA as the OAL source and the ULA of the LHS Proxy/Server as the OAL destination. If the FHS and LHS prefixes are different, the FHS Gateway instead sets its FHS ULA as the OAL source and the LHS Subnet Router Anycast ULA as the OAL destination. The FHS Gateway then decrements the OAL Hop Limit, includes an appropriate Identification, recalculates the NS checksum, performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets into the secured spanning tree. When the FHS and LHS Gateways are different, the LHS Gateway will receive carrier packets over the secured spanning tree from the FHS Gateway, noting there may be many intermediate Gateways in the path between FHS and LHS which will simply forward the enclosed IPv6 OAL packets/fragments without further processing. The LHS Gateway then performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly to obtain the NS, verifies the NS checksum then creates an AFV (i.e., the same as the FHS Gateway had done) while caching the AFP "B" AFVIs and addressing information of previous OAL forwarding hops along with window synchronization information. In particular, the LHS Gateway caches the ULA of the FHS Gateway as the spanning tree address for the previous-hop, caches the LHS information then generates a locally-unique "B" AFVI for the AFV. The LHS Gateway then writes its own LHS ULA and L2ADDR into the AFP sub-option while also writing "B" as the next entry in the AFP AFVI List. The LHS Gateway then sets its own ULA as the OAL source and the ULA of the LHS Proxy/Server as the OAL destination, decrements the OAL Hop Limit, includes an appropriate Identification, recalculates the NS checksum, performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets into the secured spanning tree.

When the LHS Proxy/Server receives the carrier packets from the secured spanning tree, it performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly, verifies the NS checksum then verifies that the LHS information supplied by the FHS source is consistent with its own cached information. If the information is consistent, the LHS Proxy/Server then creates an AFV and caches the AFP "B" AFVIs and addressing information of previous OAL forwarding hops the same as for the prior hop. The LHS Proxy/Server next caches the NS window synchronization parameters in the AFV. If the NS destination is the XLA of the LHS Client, the LHS Proxy/Server also generates a locally-unique "B" AFVI and assigns it both to the AFV and as the next "B" entry in the NS AFVI List. If the NS destination matches its own ULA, the LHS Proxy/Server next prepares to return a responsive NA with Job code '01'. The LHS Proxy/Server next creates or updates an NCE for the NS source address (if necessary) with state set to STALE and with an AFVI pointer to the new AFV state. When the LHS Proxy/Server forwards future carrier packets based on the cached information, it can populate forwarding information in a CRH-32 routing header to enable forwarding based on the cached AFVI List "B" entries. The LHS Proxy/Server then creates an NA with Job code '01' while copying the NS AFP sub-option into the NA and including responsive window synchronization information. The LHS Proxy/Server then generates a locally-unique "A" AFVI and both assigns it to the AFV and includes it as the first "A" entry in the AFP sub-option AFVI List (see: for details on AFVI List A/B processing). The LHS Proxy/Server then encapsulates the NA with OAL source set to its own ULA and OAL destination set to the ULA of the LHS Gateway. The LHS Proxy/Server then includes an appropriate Identification value, calculates the NA checksum, performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets into the secured spanning tree. If the NS destination was the XLA of the LHS Client, the LHS Proxy/Server includes an authentication signature in the NS if necessary, then recalculates the NS checksum, changes the OAL source to its own ULA and changes the OAL destination to the ULA-MNP of the LHS Client. The LHS Proxy/Server then decrements the OAL Hop Limit, includes an appropriate Identification value, performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets to the LHS Client. When the LHS Client receives the carrier packets, it performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly then verifies the NS checksum/authentication signature. The LHS Client then creates a NCE for the NS ULA source address (if necessary) in the STALE state and examines the AFP sub-option. The Client then caches the NS OMNI AFP sub-options in the NCE corresponding to the NS ULA source, then creates an AFV, caches the addressing information and "B" entries of the previous OAL hops then finally generates and assigns a locally-unique "A" AFVI the same as for previous hops. The Client finally caches the new AFVI in the NCE so that future communications can locate the correct AFV. The LHS Client then prepares an NA using exactly the same procedures as for the LHS Proxy/Server above (while including responsive window synchronization information), except that it uses its XLA as the NA source and the NS source as the NA destination. The LHS Client also includes an authentication signature if necessary, calculates the NA message checksum, then encapsulates the NA with OAL source set to its own ULA-MNP and OAL destination set to the ULA of the LHS Proxy/Server. The LHS Client finally includes an appropriate Identification, performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets to the LHS Proxy/Server. When the LHS Proxy/Server receives the carrier packets, it performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly, verifies the NA checksum/authentication signature, then uses the current AFP AFVI List "B" entry to locate the AFV. The LHS Proxy/Server then caches the addressing and "A" information for the LHS Client in the AFV, then generates a locally-unique "A" AFVI and both assigns it to the AFV and writes it as the next AFP AFVI List "A" entry. The LHS Proxy/Server then calculates the NA checksum, sets the OAL source to its own ULA and destination to the ULA of the LHS Gateway, decrements the OAL Hop Limit, includes an appropriate Identification, performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets into the secured spanning tree.

When the LHS Gateway receives the carrier packets containing the NA message, it performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly, verifies the NA checksum then uses the current NA AFP AFVI List "B" entry to locate the AFV. The LHS Gateway then caches the AFP addressing and AFVI List "A" information for the previous hops in the AFV, then generates a locally-unique "A" AFVI and both assigns it to the AFV and writes it as the next AFP AFVI List "A" entry. The LHS Gateway then recalculates the NA checksum. If the LHS Gateway is connected directly to both the FHS and LHS segments (whether the segments are the same or different), the LHS Gateway will have already cached the FHS/LHS information based on the original NS; the LHS Gateway then sets the OAL source to its FHS ULA and OAL destination to the ULA of the FHS Proxy/Server. Otherwise, the LHS Gateway sets the OAL source to its LHS ULA and OAL destination to the ULA of the FHS Gateway. The LHS Gateway then decrements the OAL Hop Limit, includes an appropriate Identification, performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets into the secured spanning tree. When the FHS and LHS Gateways are different, the FHS Gateway will receive carrier packets containing the NA message from the LHS Gateway over the secured spanning tree, where there may have been many intermediate Gateway forwarding hops. The FHS Gateway performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly, verifies the NA checksum and locates the AFV based on the current AFP AFVI List "B" entry. The FHS Gateway then caches the addressing and "A" information for the previous hops in the AFV and generates a locally-unique "A" AFVI. The FHS Gateway then assigns the new "A" value to the AFV, records "A" in the AFP AFVI List then writes its FHS ULA and L2ADDR into the AFP FHS Gateway fields. The FHS Gateway then recalculates the NA checksum, sets its FHS ULA as the OAL source and sets the ULA of the FHS Proxy/Server as the OAL destination. The FHS Gateway then decrements the OAL Hop Limit, includes an appropriate Identification value, performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets into the secured spanning tree.

When the FHS Proxy/Server receives the carrier packets from the secured spanning tree, it performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly, verifies the NA checksum then locates the AFV based on the current AFP AFVI List "B" entry. The FHS Proxy/Server then caches the AFP addressing and "A" information for the previous hops. If the NA destination matches its own ULA, the FHS Proxy/Server locates the NCE for the ULA of the LHS Proxy/Server or XLA of the LHS Client and sets the state to REACHABLE. The FHS Proxy/Server then caches the window synchronization parameters and prepares to return an acknowledgement, if necessary. If the NA destination is the XLA of the FHS Client, the FHS Proxy/Server instead generates a locally-unique "A" AFVI and assigns it both to the AFV and as the next AFP AFVI List "A" entry, then includes an authentication signature/checksum in the NA message. The FHS Proxy/Server then sets the OAL source to its own ULA and sets the OAL destination to the ULA-MNP of the FHS Client. The FHS Proxy/Server then decrements the OAL Hop Limit, includes an appropriate Identification value, performs OAL fragmentation followed by L2 encapsulation/fragmentation and finally forwards the resulting carrier packets to the FHS Client. When the FHS Client receives the carrier packets, it performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly, verifies the NA checksum/authentication signature, then locates the AFV based on the current AFP AFVI List "B" entry. The FHS Client then caches the previous hop addressing and "A" information the same as for prior hops. The FHS Client then locates the NCE for the NS source address and sets the state to REACHABLE, then caches the window synchronization parameters and prepares to return a uNA acknowledgement, if necessary.

If either the FHS Client or FHS Proxy/Server needs to return an acknowledgement to complete window synchronization, it prepares a uNA message with an AFP sub-option with Job code set to '10' (Follow A; Record B). The FHS node sets the uNA source to its own ULA or XLA, then sets the uNA destination to the ULA or XLA of the LHS node. The FHS node next sets the AFP AFVI List to the cached list of "A" entries received in the Job code '01' NA, but need not set any other FHS/LHS information. The FHS node then encapsulates the uNA message in an OAL header with its own ULA as the OAL source. If the FHS node is the Client, it next sets the ULA of the FHS Proxy/Server as the OAL destination, includes an authentication signature/checksum, includes an appropriate Identification value, performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets to the FHS Proxy/Server. The FHS Proxy/Server then verifies the Identification, performs OAL reassembly, verifies the uNA checksum/authentication signature, then uses the current AFVI List "A" entry to locate the AFV. The FHS Proxy/Server then writes its "B" AFVI as the next AFP AFVI List "B" entry, recalculates the uNA checksum then sets its own ULA as the OAL source and the ULA of the FHS Gateway as the OAL destination, The FHS Proxy/Server finally decrements the OAL Hop Limit, includes an appropriate Identification then finally performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets into the secured spanning tree. When the FHS Gateway receives the carrier packets, it performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly, verifies the uNA checksum then uses the current AFVI List "A" entry to locate the AFV. The FHS Gateway then writes its "B" AFVI as the next AFP AFVI List "B" entry, then sets the OAL source to its own ULA. If the FHS Gateway is also the LHS Gateway, it sets the OAL destination to the ULA of the LHS Proxy/Server; otherwise it sets the OAL destination to the ULA of the LHS Gateway. The FHS Gateway recalculates the uNA checksum then decrements the OAL Hop Limit, includes an appropriate Identification, performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets into the secured spanning tree. If an LHS Gateway receives the carrier packets, it processes them exactly the same as the FHS Gateway had done while re-setting the OAL destination to the ULA of the LHS Proxy/Server. When the LHS Proxy/Server receives the carrier packets, it performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly, then verifies the uNA checksum. The LHS Proxy/Server then locates the AFV based on the current AFP AFVI List "A" entry. If the uNA destination matches its own ULA, the LHS Proxy/Server next updates the NCE/AFV for the source ULA based on the uNA window synchronization parameters and MAY compare the AFVI List to the version it had cached in the AFV based on the original NS. If the uNA destination is the XLA of the LHS Client, the LHS Proxy/Server instead writes its "B" AFVI as the next AFP AFVI List "B" entry and includes an authentication signature/checksum. The LHS Proxy/Server then writes its own ULA as the OAL source and the ULA-MNP of the Client as the OAL destination, then decrements the OAL Hop Limit and includes an appropriate Identification. The LHS Proxy/Server finally performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets to the LHS Client. When the LHS Client receives the carrier packets, it performs L2 reassembly/decapsulation, verifies the Identification, performs OAL reassembly, verifies the uNA checksum/authentication signature then processes the message exactly the same as for the LHS Proxy/Server case above. Note: If either the LHS Client or LHS Proxy/Server needs to return an acknowledgement to complete window synchronization, it prepares a uNA message with an AFP sub-option with Job code set to '11' (Follow B; Record A). All other procedures are exactly the opposite as per the FHS case specified above.

Following the initial NS/NA exchange with AFP sub-options, OAL end systems can begin exchanging ordinary carrier packets that include "A/B" AFVIs and with Identification values within their respective send/receive windows without requiring security signatures and/or secured spanning tree traversal. OAL end and intermediate systems can also consult their AFIBs when they receive carrier packets that contain OAL packets/fragments with "A/B" AFVIs to unambiguously locate the correct AFV and can use any discovered "A/B" values of other OAL nodes to forward OAL packets/fragments to nodes that configure the corresponding AFVIs. OAL end systems must then perform continuous NS/NA exchanges to update window state, register new interface pairs for optimized multilink forwarding, confirm reachability and/or refresh AFIB cache state in the path before ReachableTime expires. While the OAL end systems continue to actively exchange OAL packets, they are jointly responsible for updating cache state and per-interface reachability before expiration. Window synchronization state is performed on a per-interface-pair basis and tracked in the AFVs which are also linked to the appropriate NCE. However, the window synchronization exchange only confirms target Client reachability over the specific underlay interface pair. Reachability for other underlay interfaces that share the same window synchronization state must be determined individually using additional NS/NA messages. To update AFIB state in the path, the FHS node that sent the original NS message with AFP Job code '00' can send additional NS messages with AFP sub-options with Job code '10' (Follow "A"; Record "B") and with window synchronization parameters. The message will be processed by all intermediate systems which will refresh AFV timers, cache window synchronization parameters and forward the NS onward toward the LHS node that returned the original NA message. When the LHS node receives the NS, it returns an NA message with AFP Job code '11' (Follow "B"; Record "A"). At the same time, the LHS node that received the original NS message with Job code '00' can send additional NS messages with Job code '11' in order to cause the FHS node to return an NA message with AFP Job code '10'. The process can therefore be coordinated asynchronously with the FHS/LHS nodes initiating an NS/NA exchange independently of one another. The exchanges will succeed as long as the AFIB state in the path remains active. Note that all intermediate system processing of Job code '10' and '11' NS/NA messages is conducted the same as for the initial NS/NA exchange according to the detailed specifications above. OAL sources can also begin including CRH-32s in OAL packets/fragments with AFVI information that OAL intermediate systems can use for shortest-path forwarding based on AFVIs instead of spanning tree addresses. OAL sources and intermediate systems can instead forward OAL packets/fragments with OAL compressed headers termed "OCH" (see: ) that include only a single "A/B" AFVI meaningful to the next hop, since all OAL nodes in the path up to (and sometimes including) the OAL destination have already established AFVs. Note that when an FHS OAL source receives a responsive NA with Job code '01', the AFP sub-option will contain an AFVI List with "A" entries populated in the reverse order needed for populating a CRH-32 routing header. The FHS OAL source must therefore write the AFP AFVI List "A" entries last-to-first when it populates a CRH-32, or must select the correct "A" entry to include in an OCH header based on the intended OAL intermediate system or destination. When a Gateway receives unsecured carrier packets that contain OAL packets/fragments destined to a local SRT segment Client that has asserted direct reachability, the Gateway performs direct forwarding while bypassing the local Proxy/Server based on the Client's advertised AFVIs and discovered NATed L2ADDR information (see: ). If the Client cannot be reached directly (or if NAT traversal has not yet converged), the Gateway instead forwards OAL packets/fragments directly to the local segment Proxy/Server. When a Proxy/Server receives OAL packets/fragments destined to a local SRT segment Client or forwards OAL packets/fragments received from a local segment Client, it first locates the correct AFV. If the OAL packet/fragment includes a secured IPv6 ND message, the Proxy/Server uses the Client's NCE established through RS/RA exchanges to re-encapsulate/re-fragment while sending outbound secured carrier packets via the secured spanning tree and sending inbound secured carrier packets while including an authentication signature/checksum. For ordinary OAL packets/fragments, the Proxy/Server uses the same AFV if directed by AFVI and/or OAL addressing. Otherwise it locates an AFV established through an NS/NA exchange between the Client and the remote SRT segment peer, and forwards the OAL packet/fragments without first reassembling/decapsulating. When a source Client forwards OAL packets/fragments it can employ header compression according to the AFVIs established through an NS/NA exchange with a remote or local peer. When the source Client forwards to a remote peer, it can forward OAL packets/fragments to a local SRT Gateway (following the establishment of L2ADDR information) while bypassing the Proxy/Server following route optimization (see: ). When a target Client receives carrier packets that contain OAL packets/fragments that match a local AFV, the Client first verifies the Identification then decompresses the headers if necessary, reassembles to obtain the OAL packet then decapsulates and delivers the original IP packet/parcel to the network layer. When synchronized peer Clients in the same SRT segment with FMT-Forward and FMT-Mode set discover each other's NATed L2ADDR addresses, they can exchange carrier packets that contain OAL packets/fragments directly with header compression using AFVIs discovered as above (see: ). The FHS Client will have cached the "A" AFVI for the LHS Client, which will have cached the "B" AFVI for the FHS Client. When the FHS Client or FHS Proxy/Server sends an NS for the purpose of establishing multilink forwarding state, it should wait up to RETRANS_TIMER seconds to receive a responsive NA. The FHS node can then retransmit the NS up to MAX_UNICAST_SOLICIT times before giving up. Note that each successive attempt establishes new AFV state in the OAL intermediate systems, but that any abandoned stale AFV state will be quickly reclaimed.

Multilink forwarding can often be invoked in conjunction with Address Resolution in order to reduce control message overhead and round-trip delays. When an ART acting as an ARR receives an NS(AR) with a set of Interface Attributes for the ARS source Client, it can perform "rapid commit" by immediately invoking multilink forwarding as above at the same time as returning the NA(AR). In order to perform rapid commit, the ARR includes an AFP sub-option with Job code '00' and a Window Synchronization sub-option as though it were initiating a multilink coordination NS/NA exchange as specified above. The ARR then includes any Interface Attributes and/or Traffic Selector sub-options as necessary to satisfy the address resolution request. The ARR then returns the NA(AR) to the ARS using the same hop-by-hop OAL addressing disciplines as specified above for an ordinary multilink NS/NA exchange. This will cause the NA(AR) to visit all FHS/LHS OAL intermediate systems on the path towards the ARS. When the NA(AR) traverses the return path to the ARS, OAL intermediate systems in the path process the NS AFP information exactly the same as for an ordinary multilink forwarding exchange as specified above, i.e., without examining the remaining NA(AR) message contents. This results in the ARR node now assuming the FHS role and the ARS assuming the LHS role from the perspective of multilink forwarding coordination. When the NA(AR) arrives, the ARS processes the AFP and window synchronization parameters while also processing all other NA(AR) OMNI option information, thereby eliminating an extraneous message transmission and associated delay. The ARS (now acting as an LHS peer) then completes the exchange by returning a responsive NA with an AFP sub-option with Job code '01'; if no NA response is received within RETRANS_TIMER seconds, the ARR can retransmit the NA(AR) up to MAX_NEIGHBOR_ADVERTISEMENT times before giving up.

Clients with OMNI interfaces configured over underlay interfaces with indeterminant neighborhood properties may be connected to ANETs coordinated as Mobile Ad-hoc NETworks (MANETs). 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 unique IPv6 addresses they configure locally, including TLAs and HHITs. 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 with MANET Designated Router (OSPF-MDR) extensions , OLSR , AODV and others. Other services specific to MANET link-local and/or site-local operations (including SMF , DLEP 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. When two Clients within the same MANET communicate using IP addresses that are advertised in the MANET routing protocol, their OMNI interfaces can avoid OAL encapsulation and treat the IP header supplied by the network layer as if it were an OAL encapsulation header. This includes the application of OAL fragmentation and header compression as discussed in the OMNI specification. 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 header compression state in the MANET is conducted as a MANET-local aspect of the NS/NA multilink forwarding message exchange discussed in . The process can be used to establish either asymmetric or symmetric path header compression state. In the asymmetric case, the forward path from the source Client to the destination Client or a MANET border Proxy/Server may be different than the reverse path. In the symmetric case, both the forward and reverse paths traverse the same set of MANET routers. When the OMNI interface of a MANET source Client sends an NS to establish asymmetric path header compression state, it also includes a CRH-16 extension header and Window Synchronization parameters. The source Client selects a non-zero 16-bit "C" AFVI that is unique for the L2 address of the next MANET forwarding hop for the NS message and writes that value into the first SID field of the CRH-16 while writing the value 0 into the second SID field. The source Client then caches the full OAL header in an AFV for the destination and sends the NS to the next hop. When the next MANET forwarding hop's OMNI interface receives the NS, it creates an AFV and caches the full OAL header as well as the previous hop's "C" AFVI, L2 address and Window Synchronization parameters for the forward path. The OMNI interface then selects its own non-zero/unique "C" AFVI and over-writes that value into the first SID field of the CRH-16. Consecutive MANET forwarding hops then repetitively forward the NS to their respective next hops, which perform the same procedures as above. The process continues until the NS reaches either a final destination within the same MANET or a MANET border Proxy/Server that can forward to destinations in other networks. When the final destination is within the same MANET, the destination OMNI interface returns an NA with a CRH-16 and uses the same non-zero/unique "C" AVFI discipline described above in the reverse path which may travel over a completely different set of MANET routers than those in the forward path. Otherwise, the Proxy/Server that receives the NS forwards it to other networks according to the same multilink forwarding procedures discussed in . When the Proxy/Server eventually receives an NA to return to the original source, the Proxy/Server inserts a CRH-16 (while removing the CRH-32 if present) and performs the same reverse path forwarding that an ordinary MANET destination would perform as described above. When the original source receives the NA, header compression state will have been completely populated in both the forward and reverse paths and the source and destination nodes can begin sending ordinary packets with OCH headers instead of full OAL headers. The same procedures that appear above also apply when an NS originating from a remote network arrives at a MANET border Proxy/Server for a MANET that contains the final destination. The Proxy/Server assumes the source role, inserts a CRH-16 with a non-zero/unique "C" AFVI and forwards it to the next MANET forwarding hop toward the final destination. The forwarding process continues between successive MANET routers until the final destination receives the NS. The final destination then prepares a responsive NA again while inserting a CRH-16 with a non-zero/unique "C" AFVI and returns the NA through the MANET toward the same Proxy/Server that forwarded the NS. Note that it is important that the NA message contains the OAL address of the same Proxy/Server, since that is the only location where state resides to enable the return of the NA message to the original source. In order to establish symmetric MANET paths, the initiating Client can instead send an NS that includes a CRH-16 with a non-zero/unique 2-octet "D" AFVI written into the second SID field and 0 written into the first SID field. The Client then forwards the NS message to the next MANET forwarding hop toward the destination. When the next MANET forwarding hop receives the NS, it creates an AFV and caches the (previous hop) "D" AFVI, then overwrites the second CRH-16 SID field with a newly-generated (next hop) non-zero/unique "D" AFVI value. Consecutive MANET forwarding hops then repetitively forward the NS and create new AFVs in the same fashion until the NS reaches either a final destination within the same MANET or a MANET border Proxy/Server. The destination or Proxy/Server then returns an NA along the reverse path with the (previous hop) "D" AFVI in the second CRH-16 SID field, and with a newly-generated (next hop) non-zero/unique "C" AFVI in the first CRH-16 SID field. When the previous MANET hop processes the NA, it locates the AFV based on the "D" AFVI, caches the "C" AFVI and generates a new non-zero/unique "C" AFVI. The MANET node then overwrites the second CRH-16 SID with its cached previous hop "D" value and overwrites the first CRH-16 SID with the new "C" AFVI value and returns the NA to the previous hop. The process continues until the NA message reaches the original multihop Client that transmitted the NS, at which point header compression state is established in both the forward and reverse directions of the MANET symmetric path. Following the NS/NA exchanges in both the asymmetric and symmetric cases discussed above, each MANET router in the path in both the FHS and LHS MANETs will have established AFVs containing header compression state. The AFVs determine AFVI-based forwarding based on the OCH header contents, and each MANET router only forwards packet with in-window Identification values. MANET routers maintain AFVs for up to ReachableTime seconds unless they are refreshed by either a new NS/NA exchange or the transmission of any data packet with a full OAL header with an in-window Identification value and a CRH-16 extension. 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.

Following multilink route optimization for specific underlay interface pairs, FHS/LHS Clients located on open INETs can invoke Client/Gateway route optimization to improve performance and reduce load and congestion on their respective Proxy/Servers. To initiate Client/Gateway route optimization, the Client prepares an NS message with its own XLA address as the source and the ULA of its Gateway as the destination while creating a NCE for the Gateway if necessary. The NS message must be encapsulated as an atomic fragment and not subject to OAL fragmentation. The Client then includes an Interface Attributes sub-option for its underlay interface as well as an authentication signature but does not include window synchronization parameters. The Client then performs OAL encapsulation with its own ULA-MNP as the source and the ULA of the Gateway as the destination while including a randomly-chosen Identification value, then performs L2 encapsulation/fragmentation on the OAL atomic fragment and forwards the resulting carrier packets directly to the Gateway. When the Gateway receives the carrier packets, it performs L2 decapsulation/reassembly to recover the OAL atomic fragment then verifies the NS checksum/authentication signature and creates a NCE for the Client. The Gateway then caches the L2 encapsulation addresses (which may have been altered by one or more NATs on the path) as well as the Interface Attributes for this Client ifIndex, and marks this Client underlay interface as "trusted". The Gateway then prepares an NA reply with its own ULA as the source and the XLA of the Client as the destination where the NA again must be an atomic fragment. The Gateway then echoes the Client's Interface Attributes, includes an Origin Indication with the Client's observed L2 addresses and includes an authentication signature. The Gateway then performs OAL encapsulation with its own ULA as the source and the ULA-MNP of the Client as the destination while using the same Identification value that appeared in the NS, then performs L2 encapsulation/fragmentation on the OAL atomic fragment and forwards the resulting carrier packets directly to the Client. When the Client receives the NA reply, it caches the carrier packet L2 source address information as the Gateway target address via this underlay interface while marking the interface as "trusted". The Client also caches the Origin Indication L2 address information as its own (external) source address for this underlay interface. After the Client and Gateway have established NCEs as well as "trusted" status for a particular underlay interface pair, each node can begin sending ordinary carrier packets intended for this multilink route optimization directly to one another while omitting the Proxy/Server from the forwarding path while the status is "trusted". The NS/NA messaging will have established the correct state in any NATs in the path so that NAT traversal is naturally supported. The Client and Gateway must maintain timers that watch for activity on the path; if no carrier packets and/or NS/NA messages are sent or received over the path before NAT state is likely to have expired, the underlay interface pair status becomes "untrusted". Thereafter, when the Client sends a carrier packet that contains an OAL packet/fragment toward the Gateway as the next hop, the Client includes the AFVI for the Gateway (discovered during multilink route optimization) instead of the AFVI for its Proxy/Server; the Gateway will accept the OAL packet/fragment from the Client if and only if the AFVI matches the correct AFV and the underlay interface status is trusted. (The same is true in the reverse direction when the Gateway sends carrier packets directly to the Client.) Note: the Client and Gateway each maintain a single NCE, but the NCE may aggregate multiple underlay interface pairs. Each underlay interface pair may use differing source and target L2 addresses according to NAT mappings, and the "trusted/untrusted" status of each pair must be tested independently. When no "trusted" pairs remain, the NCE is deleted. Note: the above method requires Gateways to participate in NS/NA message authentication signature application and verification. In an alternate approach, the Client could instead exchange NS/NA messages with authentication signatures via its Proxy/Server but addressed to the ULA of the Gateway, and the Proxy/Server and Gateway could relay the messages over the secured spanning tree. However, this would still require the Client to send additional messages toward the L2 address of the Gateway to populate NAT state; hence the savings in complexity for Gateways would result in increased message overhead for Clients.

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 the shared segment Gateway) 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 the initial NS/NA exchange 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 for one or both when FMT-Forward and FMT-Mode are both 1). The NS/NA messages then include an Origin Indication (i.e., in addition to an AFP 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 its local chain of NATs for reception of future carrier packets from that L2 address (see: and ). The source Client then prepares an NS message with its own XLA as the source, with the XLA of the target as the destination and with an OMNI option with an Interface Attributes sub-option. The source Client then encapsulates the NS in an OAL header with its own ULA-MNP as the source, with the ULA-MNP of the target Client as the destination and with an in-window Identification for the target. The source Client then performs OAL fragmentation followed by 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, verify the Identification and perform OAL reassembly. Following reassembly, the target Client prepares an NA message with its own XLA as the source, with the XLA 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 in an OAL header with its own ULA-MNP as the source, with the ULA-MNP of the source Client as the destination and with an in-window Identification for the source Client. The target Client then performs OAL fragmentation followed by L2 encapsulation/fragmentation then forwards the resulting carrier packets directly to the source Client. Following the initial NS/NA 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 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 , 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. 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 multilink route optimization exchange. This allows for subsequent NS/NA 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.

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 , and should also include a Neighbor Control sub-option with the Preflen of the MNP found in the Target Address field. 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 ANETs to a peer within the same downstream ANET, the Proxy/Server returns an IPv6 ND Redirect message. All other route optimization functions are conducted per the NS/NA messaging discussed in the previous sections.

AERO nodes perform Neighbor Unreachability Detection (NUD) per either reactively in response to persistent link layer errors (see ) or proactively to confirm reachability. The NUD algorithm is based on periodic control message exchanges and may further be seeded by IPv6 ND hints of forward progress, but care must be taken to avoid inferring reachability based on spoofed information. For example, IPv6 ND message exchanges that include authentication codes and/or in-window Identifications may be considered as acceptable hints of forward progress, while spurious random carrier packets should be ignored. AERO nodes can perform NS/NA exchanges over the OMNI link secured spanning tree (i.e. the same as described above) to test reachability without risk of DoS attacks from nodes pretending to be a neighbor. These NS/NA messages use the unicast XLAs/ULAs of the parties involved in the NUD test. When only reachability information is required without updating any other NCE state, AERO nodes can instead perform NS/NA exchanges directly between neighbors without employing the secured spanning tree as long as they include in-window Identifications and an authentication signature/checksum. After route optimization directs a source FHS peer to a target LHS peer with one or more link layer addresses, either node may invoke multilink forwarding state initialization to establish authentic intermediate system state between specific underlay interface pairs which also tests their reachability. Thereafter, either node acting as the source may perform additional reachability probing through NS messages over the SRT secured or unsecured spanning tree, or through NS messages sent directly to an underlay interface of the target itself. While testing a target underlay interface, the source can optionally continue to forward OAL packets/fragments via alternate interfaces or maintain a small queue of carrier packets until target reachability is confirmed. NS messages are encapsulated, fragmented and transmitted as carrier packets the same as for ordinary original IP data packets/parcels, however the encapsulated destinations are either the ULA or XLA of the source and either the ULA of the LHS Proxy/Server or the XLA of the target itself. The source encapsulates the NS message the same as described in and includes an Interface Attributes sub-option with ifIndex set to identify its underlay interface used for forwarding. The source then includes an in-window Identification, performs OAL fragmentation followed by L2 encapsulation/fragmentation then forwards the resulting carrier packets into the unsecured spanning tree, either directly to the target if it is in the local segment or directly to a Gateway in the local segment. When the target receives the NS carrier packets, it performs L2 reassembly/decapsulation, verifies that it has a NCE for this source and that the Identification is in-window then performs OAL reassembly. The target next verifies the NS checksum/authentication signature, then searches for Interface Attributes in its NCE for the source that match the NS for the NA reply. The target then prepares the NA with the source and destination addresses reversed, encapsulates and sets the OAL source and destination, includes an Interface Attributes sub-option in the NA to identify the ifIndex of the underlay interface the NS arrived on and sets the Target Address to the same value included in the NS. The target next sets the R flag to 1, the S flag to 1 and the O flag to 1, then includes an in-window Identification for the source. The node then performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets into the unsecured spanning tree either directly to the source if it is in the local segment or directly to a Gateway in the local segment. When the source receives the NA, it marks the target underlay interface tested as "trusted". Note that underlay interface states are maintained independently of the overall NCE REACHABLE state, and that a single NCE may have multiple target underlay interfaces in various "trusted/untrusted" states while the NCE state as a whole remains REACHABLE.

AERO is a fully Distributed Mobility Management (DMM) service in which each Proxy/Server is responsible for only a small subset of the Clients on the OMNI link. This is in contrast to a Centralized Mobility Management (CMM) service where there are only one or a few network mobility collective entities for large Client populations. Clients coordinate with their associated FHS and Hub Proxy/Servers via RS/RA exchanges to maintain the DMM profile, and the AERO routing system tracks all current Client/Proxy/Server peering relationships. Hub Proxy/Servers provide a designated router service for their dependent Clients, while FHS Proxy/Servers provide a proxy conduit between the Client and both the Hub and OMNI link in general. Clients are responsible for maintaining neighbor relationships with their Proxy/Servers through periodic RS/RA exchanges, which also serves to confirm neighbor reachability. When a Client's underlay interface attributes change, the Client is responsible for updating the Hub Proxy/Server through new RS/RA exchanges using the FHS Proxy/Server as a first-hop conduit. The FHS Proxy/Server can also act as a proxy to perform some IPv6 ND exchanges on the Client's behalf without consuming bandwidth on the Client underlay interface. Note: when a Client's underlay interface address changes, the Client and/or its (former) FHS Proxy/Server for this interface must invalidate any AFVs based on the (changed) interface. Future data packet forwarding will then trigger a new multilink forwarding NS/NA exchange to re-seed new AFVs in the path. Mobility management considerations are specified in the following sections.

Mobile Clients (and/or their Hub Proxy/Servers) accommodate mobility and/or multilink change events by sending secured uNA messages to each active neighbor. When a node sends a uNA message to each specific neighbor on behalf of a mobile Client, it sets the IPv6 source address to its own ULA or XLA, sets the destination address to the neighbor's ULA or XLA and sets the Target Address to the mobile Client's XLA. The uNA 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 then sets the uNA R flag to 1, S flag to 0 and O flag to 1, then encapsulates the message in an OAL header with source set to its own ULA and destination set to either the specific neighbor's ULA or the FHS Proxy/Server's ULA. Following OAL fragmentation and L2 encapsulation/fragmentation, the carrier packets containing the uNA message will then follow the secured spanning tree and arrive at the specific neighbor. As discussed in Section 7.2.6 of , the transmission and reception of uNA messages is unreliable but provides a useful optimization. In well-connected Internetworks with robust data links uNA messages will be delivered with high probability, but in any case the node can optionally send up to MAX_NEIGHBOR_ADVERTISEMENT uNAs 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 OMNI option header to request a uNA response (see: ). When the FHS/LHS Proxy/Server receives a secured uNA message prepared as above, if the uNA destination was its own 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 message does not provide confirmation that any forward paths to the target Client are working. If the destination was the XLA of the FHS/LHS Client, the Proxy/Server instead changes the OAL source to its own ULA, includes an authentication signature if necessary, and includes an in-window Identification for this Client. Finally, if the uNA message SNR flag was set, the node that processes the uNA also returns a uNA response (see: ).

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 Hub 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 ); otherwise, any stale state in the former FHS Proxy/Server will simply expire after ReachableTime expires with no effect on the Hub 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 messages to all neighbors the same as described in the previous section.

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 Hub 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 messages to all neighbors the same as described above.

When a Client needs to deactivate an existing underlay interface, it sends a uNA message toward the Hub 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 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 messages to all neighbors the same as described above. Note that when a Client deactivates an underlay interface, neighbors that receive the ensuing uNA 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 Client performs the procedures specified in when it first associates with a new Hub Proxy/Server or renews its association with an existing Hub Proxy/Server. When a Client associates with a new Hub Proxy/Server, it sends RS messages to register its underlay interfaces with the new Hub while including the old Hub's ULA in the "Old Hub Proxy/Server ULA" field of a Proxy/Server Departure OMNI sub-option. When the new Hub Proxy/Server returns the RA message via the FHS Proxy/Server (acting as a proxy), the FHS Proxy/Server sends a uNA to the old Hub Proxy/Server (i.e., if the ULA is non-zero and different from its own). The uNA has the XLA of the Client as the source and the ULA of the old hub as the destination and with an OMNI Proxy/Server Departure sub-option as above. The FHS Proxy/Server encapsulates the uNA in an OAL header with the ULA of the new Hub as the source and the ULA of the old Hub as the destination, then performs OAL fragmentation followed by L2 encapsulation/fragmentation and forwards the resulting carrier packets via the secured spanning tree. When the old Hub Proxy/Server receives the carrier packets, it decapsulates and reassembles if necessary to obtain the uNA then changes the Client's NCE state to DEPARTED, resets DepartTime and caches the new Hub Proxy/Server ULA. After a short delay (e.g., 2 seconds) the old Hub Proxy/Server withdraws the Client's MNP from the routing system. While in the DEPARTED state, the old Hub Proxy/Server forwards any carrier packets received via the secured spanning tree destined to the Client's ULA-MNP to the new Hub Proxy/Server's ULA. When DepartTime expires, the old Hub 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 ULA for the "Old FHS Proxy/Server ULA", and the new FHS Proxy/Server will issue a uNA using the same procedures as outlined for the Hub above while using its own ULA 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 Hub 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 Hub Proxy/Server include a Hub 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.

Each Client provides an IGMP (IPv4) or MLD (IPv6) proxy service for its ENETs and/or hosted applications and acts as a Protocol Independent Multicast - Sparse-Mode (PIM-SM, or simply "PIM") Designated Router (DR) on the OMNI link. Proxy/Servers act as OMNI link PIM routers for Clients on ANET, VPN/IPsec or Direct interfaces, and Relays also act as OMNI link PIM routers on behalf of nodes on other links/networks. Clients on VPN/IPsec, Direct or ANET underlay interfaces for which the ANET has deployed native multicast services forward IGMP/MLD messages into the ANET. The IGMP/MLD messages may be further forwarded by a first-hop ANET access router acting as an IGMP/MLD-snooping switch , then ultimately delivered to an ANET (FHS) Proxy/Server. The FHS Proxy/Server then acts as an ARS to send NS(AR) messages to an ARR for the multicast source. Clients on ANET/INET underlay interfaces without native multicast services instead send NS(AR) messages as an ARS to cause their FHS Proxy/Server to forward the message to an ARR. When the ARR prepares an NA(AR) response, it initiates PIM protocol messaging according to the Source-Specific Multicast (SSM) and Any-Source Multicast (ASM) operational modes as discussed in the following sections.

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 . For each S belonging to a prefix reachable via X's OMNI interface, X sends an NS(AR) message (see: ) using its own ULA or XLA as the source address, the solicited node multicast address corresponding to S as the destination and the XLA of S as the target address. X then encapsulates the NS(AR) in an OAL header with source address set to its own ULA and destination address set to the 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 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 XLA 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 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 exchanges to keep the NCE alive for up to t_periodic seconds . 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 message to X the same as specified for unicast mobility in . When X receives the uNA message, it updates its NCE for the XLA for source S and sends new Join messages in NS/NA 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.

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 message with its own ULA or XLA as the source address and the ULA or XLA for R as the destination address, then encapsulates the NS message in an OAL header with its own ULA as the source and the 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 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 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 PIM Register encapsulation. R can then issue a uNA 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 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 . 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.

Bi-Directional PIM (BIDIR-PIM) 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.

An AERO Client can connect to multiple OMNI links the same as for any data link service. In that case, the Client maintains a distinct OMNI interface for each link, e.g., 'omni0' for the first link, 'omni1' for the second, 'omni2' for the third, etc. Each OMNI link would include its own distinct set of Gateways and Proxy/Servers, thereby providing redundancy in case of failures. Each OMNI link could utilize the same or different ANET/INET link layer connections. The links can be distinguished at the link layer via the SRT prefix in a similar fashion as for Virtual Local Area Network (VLAN) tagging (e.g., IEEE 802.1Q) and/or through assignment of distinct sets of MSPs on each link. This gives rise to the opportunity for supporting multiple redundant networked paths (see: ). The Client's network layer can select the outbound OMNI interface appropriate for a given traffic profile while (in the reverse direction) correspondent nodes must have some way of steering their original IP packets/parcels destined to a target via the correct OMNI link. In a first alternative, if each OMNI link services different MSPs the Client can receive a distinct MNP from each of the links. IP routing will therefore assure that the correct OMNI link is used for both outbound and inbound traffic. This can be accomplished using existing technologies and approaches, and without requiring any special supporting code in correspondent nodes or Gateways. In a second alternative, if each OMNI link services the same MSP(s) then each link could assign a distinct "OMNI link Anycast" address that is configured by all Gateways on the link. Correspondent nodes can then perform Segment Routing to select the correct SRT, which will then direct the original IP packet/parcel over multiple hops to the target.

AERO Client MNs and INET correspondent nodes consult the Domain Name System (DNS) the same as for any Internetworking node. When correspondent nodes and Client MNs use different IP protocol versions (e.g., IPv4 correspondents and IPv6 MNs), the INET DNS must maintain A records for IPv4 address mappings to MNs which must then be populated in Relay NAT64 mapping caches. In that way, an IPv4 correspondent node can send original IPv4 packets/parcels to the IPv4 address mapping of the target MN, and the Relay will translate the IPv4 header and destination address into an IPv6 header and IPv6 destination address of the MN. When an AERO Client registers with an AERO Proxy/Server, the Proxy/Server can return the address(es) of DNS servers in RDNSS options . The DNS server provides the IP addresses of other MNs and correspondent nodes in AAAA records for IPv6 or A records for IPv4.

OAL encapsulation ensures that dissimilar INET partitions can be joined into a single unified OMNI link, even though the partitions themselves may have differing protocol versions and/or incompatible addressing plans. However, a commonality can be achieved by incrementally distributing globally routable (i.e., native) IP prefixes to eventually reach all nodes (both mobile and fixed) in all OMNI link segments. This can be accomplished by incrementally deploying AERO Gateways on each INET partition, with each Gateway distributing its MNPs and/or discovering non-MNP IP GUA prefixes on its INET links. This gives rise to the opportunity to eventually distribute native IP addresses to all nodes, and to present a unified OMNI link view even if the INET partitions remain in their current protocol and addressing plans. In that way, the OMNI link can serve the dual purpose of providing a mobility/multilink service and a transition/coexistence service. Alternatively, if an INET partition is transitioned to a native IP protocol version and addressing scheme compatible with the OMNI link MNP-based addressing scheme, the partition and OMNI link can be joined by Gateways. Relays that connect INETs/ENETs with dissimilar IP protocol versions may need to employ a network address and protocol translation function such as NAT64 .

In environments where rapid failure recovery is essential, Proxy/Servers and Gateways SHOULD use Bidirectional Forwarding Detection (BFD) . Nodes that use BFD can quickly detect and react to failures so that cached information is re-established through alternate nodes. BFD control messaging is carried only over well-connected ground domain networks (i.e., and not low-end radio links) and can therefore be tuned for rapid response. Proxy/Servers and Gateways can maintain BFD sessions in parallel with their BGP peerings. If a Proxy/Server or Gateway fails, BGP peers will quickly re-establish routes through alternate paths the same as for common BGP operational practice.

In some use cases, it is desirable, beneficial and efficient for the Client to receive a constant MNP that travels with the Client wherever it moves. For example, this would allow air traffic controllers to easily track aircraft, etc. In other cases, however (e.g., intelligent transportation systems), the MN may be willing to sacrifice a modicum of efficiency in order to have time-varying MNPs that can be changed every so often to defeat adversarial tracking. The DHCPv6 service offers a way for Clients that desire time-varying MNPs to obtain short-lived prefixes (e.g., on the order of a small number of minutes). In that case, the identity of the Client would not be bound to the MNP but rather to a Node Identification value (see: ) that can serve as a Client ID seed for MNP prefix delegation. The Client would then be obligated to renumber its internal networks whenever its MNP (and therefore also its XLA) changes. This should not present problems for Clients with automated network renumbering services, however it can limit the durations of ongoing sessions that would prefer to use a constant address.