Automatic Extended Route Optimization (AERO)

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

AERO (Proxy/)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 that obtain MNPs register them with the AERO service, and distribute the MNPs to EUNs (which may connect other Clients). AERO Clients can also provide Proxy services for other Clients on downstream-attached networks. AERO Gateways, Proxy/Servers and Relays are infrastructure elements in *NET boundary (or standalone INET) deployments and hence have INET addresses that rarely (if ever) change. Together, they provide access to the AERO service OMNI link virtual overlay for connecting AERO Clients. AERO Gateways (together with Proxy/Servers and Relays) provide the secured backbone supporting infrastructure for an OMNI link Segment Routing Topology (SRT) spanning tree. AERO Gateways are Proxy/Servers deployed as 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 as 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 IPsec tunnels with Proxy/Servers, Relays and other Gateways; they further maintain forwarding table entries for each FNP/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. Each Proxy/Server acts as either an OMNI link intermediate system or end system according to the service model selected by the Client. Each Proxy/Server also peers with Gateways in an adaptation layer dynamic routing protocol instance to advertise its list of associated MNPs (see: ). MAP Proxy/Servers provide prefix delegation 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 *NET boundaries provide a primary forwarding service for (M)ANET Client communications with peers in external INETs. Proxy/Servers in open INETs provide an authentication service for control messages but should be considered as a less preferred data plane forwarding service when a Client cannot forward directly to an INET peer. 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 between the OMNI link and fixed or mobile nodes on other links/networks. Relays run a dynamic routing protocol to discover any FNP prefixes in service on foreign 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 FNPs 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 )-. |IP end | |IP end | .-(_ IP )-. (__ EUN )--|system | |system |--(__ EUN ) `-(______)-' +-------+ +-------+ `-(______)-' ]]> In this model: the OMNI link is an overlay network service configured over one or more underlay SRT segments which may be managed by diverse administrative domains using incompatible protocols and/or addressing plans. AERO Gateway G1 aggregates Mobility Service Prefix (MSP) M1, discovers Mobile Network Prefixes (MNPs) X* and advertises the MSP via BGP peerings over secured tunnels to other Gateways in the SRT (not shown). Together, the set of all Gateways provide the backbone for an SRT spanning tree for the OMNI link. AERO Proxy/Servers S1 and S2 configure secured tunnels with Gateway G1 and also provide mobility, multilink, multicast and default router services for the MNPs of their associated Clients C1 and C2. (Proxy/Servers that act as Relays can also advertise FNP routes for non-mobile correspondent nodes the same as for MNP Clients.) AERO Clients C1 and C2 associate with Proxy/Servers S1 and S2, respectively. They receive MNP delegations X1 and X2, and also act as default routers for their associated physical or internal virtual EUNs. (While not shown, AERO Clients can also be recursively nested in an arbitrarily-deep chain of (Proxy/)Clients between a Proxy/Server and the ultimate IP end systems.) IP end systems attach to the EUNs served by Clients C1 and C2, respectively. (Although not depicted here, there may be multiple Proxy/Client intermediate systems between Clients C1 and C2 and the ultimate IP end systems.) 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 underlay encapsulation (i.e., following any necessary NAT traversal) since the underlay is connected. In common practice, however, OMNI links are often configured over an SRT spanning tree that bridges multiple distinct underlay network segments managed under different administrative authorities (e.g., as for worldwide aviation service providers such as ARINC, SITA, Inmarsat, etc.). Individual underlay networks may also be partitioned internally, in which case each internal partition appears as a separate segment. The addressing plan of each SRT segment is consistent internally but will often bear no relation to the addressing plans of other segments. Each segment is also likely to be separated from others by network security devices (e.g., firewalls, proxys, packet filtering gateways, etc.), and disjoint segments often have no common physical link connections. Therefore, nodes can only be assured of exchanging carrier packets directly with correspondents in the same segment, and not with those in other segments. The only means for joining the segments therefore is through inter-domain peerings between AERO Gateways. The OMNI link spans multiple SRT segments using the OAL to provide the network layer with a virtual abstraction similar to a bridged campus LAN. The OAL is an OMNI interface sublayer that inserts a mid-layer IPv6 encapsulation header for inter-segment forwarding (i.e., bridging) without decrementing the network layer TTL/Hop Limit of the original IP packet. 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 examined by destinations according to data origin authentication procedures. AERO nodes can employ route optimization to cause carrier packets to take more direct paths between OMNI link neighbors without having to follow strict spanning tree paths. The network of networks concept emerged from the earliest days of Internetworking . The concept has carried forward to the present day where the Internet has become successful beyond measure. The AERO Multinet service concatenates SRT segments through Gateway-to-Gateway peerings as first suggested in "Interconnection of Packet switching Networks" and later formalized in the "Catenet Model for Internetworking (IEN48)" . The catenet model in particular suggests an interconnection of independent and diverse packet switching network "segments" to form a much larger Internetwork supporting end-to-end services. The catenet vision faded into obscurity as the Internet evolved in the decades that followed, and the adaptation layer was omitted from the architecture. As a result, the Internet has flourished as a monolithic public routing and addressing service interconnecting private domains leading to the rise of the middle (including NATs) and a diminished role for end-to-end . The adaptation layer manifested by AERO and OMNI now promises to restore the best aspects of end-to-end through incremental deployment of catenet constructs in the modern Internet.

AERO nodes on OMNI links assign an external Link-Local Address (LLA) and link-layer address on the OMNI interface as required by . AERO nodes also assign different internal LLA and link-layer addresses to support the operation of an adaptation layer internal virtual router entity. AERO nodes assign a Multilink Local Address (MLA) to the OMNI interface per . The node assigns an MLA to an OMNI interface the same as suggested for "sites" in the IPv6 scoped addressing architecture , i.e., as a single adaptation layer address assigned to a virtual interface configured over potentially multiple underlying interfaces. MLAs are considered as adaptation layer addresses in the architecture and can appear as OAL encapsulation addresses, but nodes may also use them as the Source and Destination Addresses of original IP packets exchanged between peers within the same OMNI link segment. Each original IP packet with MLA addresses is subject to OAL encapsulation with an IPv6 header that also uses MLA addresses. AERO Clients receive Mobile Network Prefix (MNP) delegations during Proxy/Server RS/RA exchanges and assign the MNPs to EUN interfaces. AERO MSPs, MNPs and Foreign Network Prefixes (FNPs) are typically based on GUAs, but in some cases may be based on IPv4 private addresses or IPv6 ULA-C's . AERO address selection rules are conducted per as updated by . See for a full discussion of the various unicast, anycast and multicast addresses used by AERO nodes on OMNI links.

The AERO routing system comprises a private Border Gateway Protocol (BGP) service coordinated between Gateways as interior nodes and Proxy/Servers and Relays as leaf nodes of a spanning tree. The service supports OAL packet/fragment forwarding at a layer below IP and does not interact with the public Internet BGP routing system, but supports redistribution of information for other networks connected by Relays. In a reference deployment, each Proxy/Server is configured as an Autonomous System Border Router (ASBR) for a stub Autonomous System (AS) using a 32-bit AS Number (ASN) that is unique within the BGP instance, and each Proxy/Server further uses eBGP to peer with one or more Gateways but does not peer with other Proxy/Servers. Each SRT segment in the OMNI link must include one or more Gateways in a "hub" AS, which peer with the Proxy/Servers within that segment as "spoke" ASes. All Gateways within the same segment are members of the same hub AS, and use iBGP to maintain a consistent view of all active routes currently in service. The Gateways of different segments peer with one another using eBGP. Gateways maintain forwarding table entries for the set of all MLAs and FNP/MNP routes that are currently active for the OMNI link; Gateways also maintain black-hole routes for the OMNI link MSPs so that OAL packets/fragments destined to non-existent more-specific routes are flushed from the routing system. In this way, Proxy/Servers and Relays have only partial topology knowledge (i.e., they only maintain routing information for their directly associated Clients and foreign links) and they forward all other OAL packets/fragments to Gateways which have full topology knowledge. MLAs and FNP/MNP routes are dynamically advertised in the AERO routing system by Proxy/Servers and Relays that provide anchor points for their corresponding prefixes. For example, if three Proxy/Servers ('D', 'E' and 'F') service the MNPs 2001:db8:1000:1::64/, 2001:db8:1000:2::/64 and 2001:db8:1000:2::/48 then the routing system would include: 2001:db8:1000:1::/64 2001:db8:1000:2::/64 2001:db8:1000:3::/64 A full discussion of the BGP-based routing system used by AERO is found in .

The distinct GUA prefixes in an OMNI link domain identify distinct Segment Routing Topologies (SRTs). Each SRT is a mutually-exclusive OMNI link overlay instance using a distinct set of GUAs, and emulates a bridged campus LAN service for the OMNI link. In some cases (e.g., when redundant topologies are needed for fault tolerance and reliability) it may be beneficial to deploy multiple SRTs that act as independent overlay instances. A communication failure in one instance therefore will not affect communications in other instances. Each SRT is identified by a distinct MSP prefix and assigns an IPv6 SRA 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 MSP 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 control messages. Note: The distinct MSP 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 MSPs of multiple OMNI links within a common routing system may be acceptable.

OMNI interfaces are virtual interfaces configured over one or more underlay interfaces classified as follows: (M)ANET interfaces connect to a protected and secured ANET or an open MANET that connects to an INET via Proxy/Servers. The (M)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 a (M)ANET and may require NAT traversal on the path to the Proxy/Server the same as for the INET case.) MANETs are special cases of ANETs in which adaptation layer multihop forwarding may be necessary, and protected secured underlay links cannot always be assumed. 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. EUN interfaces connect a Client's downstream-attached networks, where the Client provides forwarding services for EUN end system communications to remote peers. An EUN 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 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 underlay 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 neighbor may be multiple IP hops away. OMNI interfaces use link layer encapsulation only (i.e., and no other underlay encapsulations) over Direct underlay interfaces or (M)ANET interfaces when the Client and neighbor are known to be on the same underlay link. OMNI interfaces maintain an adaptation layer view of the neighbor cache for tracking per-neighbor state. IP nodes that configure OMNI interfaces use ND messages including Router Solicitation (RS), Router Advertisement (RA), Neighbor Solicitation (NS), Neighbor Advertisement (NA), unsolicited Neighbor Advertisement (uNA) and Redirect to manage both the network and adaptation layer views of the neighbor cache. The adaptation layer further uses a multilink forwarding message set termed Multilink Initiate (MI), Multilink Respond (MR) and Multilink Control (MC) which use the same ICMPv6 Type value as the standard NS message but with different Code values. OMNI neighbors invoke per-flow OAL Identification window synchronization in their ND message exchanges to enable Source Address verification, header compression and robust fragmentation/reassembly. OMNI interfaces include OMNI options formatted as specified in in the control messages they forward. The OMNI option includes parameters for coordinating the OMNI interface's underlay interfaces. A Client's OMNI interface may be configured over multiple *NET 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 *NET underlay interfaces are used "one at a time" (i.e., all other interfaces are in standby mode while one interface is active), then successive control messages all include OMNI option Interface Attributes, Traffic Selector and/or Neighbor Synchronization 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 underlay address. If the Client has multiple active *NET underlay interfaces, then from the perspective of IPv6 ND it would appear to have multiple underlay addresses. In that case, control message OMNI options MAY include multiple Interface Attribute sub-options; each with a different underlay interface ifIndex. Proxy/Servers on the open Internet include only a single INET underlay interface. INET Clients therefore discover only the UNX information for the Proxy/Server's INET interface. Proxy/Servers on a (M)ANET/INET boundary include both (M)ANET and INET underlay interfaces. (M)ANET Clients therefore must discover both the (M)ANET and INET UNX information for their Proxy/Servers. Gateway and Proxy/Server OMNI interface connections to the SRT are configured over both secured IPsec tunnels for carrying IPv6 ND and BGP protocol control plane messages and open INET paths for carrying unsecured data plane messages. The OMNI interface configures an MLA and acts as an OAL source to encapsulate original IP packets, then fragments the resulting OAL packets, performs underlay encapsulation 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 MLA IPv6 Source and Destination Addresses.

AERO Proxy/Servers and Clients 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 with Destination Addresses covered by an MNP not explicitly associated with another interface are directed to an OMNI interface where address resolution is applied. OMNI interface initialization procedures for Gateways, Proxy/Servers and Clients are discussed in the following sections.

AERO Gateways configure an OMNI interface and assign an MLA. 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 an adaptation layer BGP routing protocol session with neighbors over the secured spanning tree (see: ).

When a Proxy/Server enables an OMNI interface, it assigns both an LLA and MLA. The Proxy/Server also configures secured underlay interface tunnels and engages in adaptation layer BGP routing protocol sessions over the OMNI interface with one or more neighboring Gateways. The OMNI interface provides a single interface abstraction to the network layer, but internally serves as an NBMA nexus for exchanging carrier packets with other OMNI nodes over underlay interfaces and/or secured tunnels. The Proxy/Server further configures a service to facilitate control message exchanges with AERO Clients and manages per-Client Neighbor Cache Entries (NCEs) and IP forwarding table entries based on control message exchanges. Relays are simply Proxy/Servers that run a dynamic routing protocol to redistribute routes between the OMNI interface and foreign networks/links (see: ). The Relay provisions MNPs and advertises the MSP(s) for the OMNI link over its foreign network interface attachments. The Relay further provides an OMNI link attachment point for FNP-based topologies.

When a Client enables an OMNI interface, it assigns different administratively assigned internal and external LLAs and link-layer addresses as well as a globally unique MLA to the OMNI interface. The adaptation layer within the interface then issues an internally-generated RA message to the network layer to establish itself as a (virtual) default router for the OMNI link. The network layer then issues a DHCPv6 Prefix Delegation (PD) request per . When it receives the PD request (or after a brief timeout), the adaptation layer then sends OMNI-encapsulated RS messages to FHS Proxy/Servers which optionally coordinate with a MAP Proxy/Server that delegates one or more MNPs. The MAP/FHS Proxy/Servers then return an RA message to the Client which may pass through one or more NATs in the path. When the Client sends initial RS messages, it will discover MNPs in the corresponding RAs that it receives from FHS Proxy/Servers and can then associate them with the OMNI interface. If the Client is operating outside the context of AERO infrastructure, however, it may continue using MLAs over its underlay or OMNI interfaces for peer-to-peer communications within the local *NET.

Each Client and Proxy/Server OMNI interface maintains a network layer conceptual Neighbor and Destination Cache per the same as for any IP interface. The OMNI interface network layer neighbor cache (NLNC) is maintained through static and/or dynamic neighbor cache entry (NLNCE) configurations. The IP layer initiates and terminates IP ND messaging exchanges to manage the network layer view of the neighbor cache. Each OMNI interface also maintains an internal adaptation layer view of the neighbor cache (ALNC) that includes an entry (ALNCE) for each of its active OAL neighbors. ALNCE state is managed according to neighbor cache entry states per Section 7.3.2 of the same as the NLNC. The adaptation layer indexes the ALNC by the neighbor's MLA and includes routing information for any of the neighbor's FNPs/MNPs. Control messages that update the ALNC include an OMNI option with zero or more sub-options. Each OMNI interface ALNCE is indexed by the IPv6 MLA of a neighbor found in an ND message 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 IPv6 NS/NA message exchanges. Clients establish NCEs and establish adaptation layer service profiles for their associated FHS and MAP Proxy/Servers through the exchange of RS/RA messages as specified in . When a Client and Proxy/Server establish NCEs, they set a ReachableTime timer to REACHABLE_TIME seconds. Both the Client and its MAP 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. 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 MAP Proxy/Server for the ART. The MAP 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 NCE state for the ART while caching network layer and underlay addressing information. The ARS then (re)sets ReachableTime for the NCE to REACHABLE_TIME seconds and performs multilink forwarding ND message 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 ND 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 control 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/MAP 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(NUD) 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 solicited IPv6 ND advertisements 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. IPv6 ND messages sent over OMNI interfaces include or omit the Source/Target Link Layer Address Option (S/TLLAO) as discussed in . OMNI interfaces forward IPv6 ND messages to and from the IP layer the same as for standard IPv6 ND, but also append/remove a trailing OMNI pseudo-option during encapsulation/decapsulation . For each IPv6 ND message, the OMNI interface includes a trailing OMNI option following any other ND message options then completely populates all sub-option information. If the OMNI interface includes an authentication sub-option, it calculates and includes a digital signature per the OMNI specification. OMNI interfaces verify integrity and authenticity of each message received, and process the message further only following successful verification. OMNI options include per-neighbor information that provides multilink forwarding, address resolution and traffic selector information for the neighbor's underlay interfaces. This information is stored in both the neighbor cache and AERO Flow 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. AERO/OMNI Clients send RS messages to cause Proxy/Servers to respond with RA messages that include autoconfiguration and addressing parameters as specified in . AERO nodes use NS/NA messages as follows: NS/NA(AR) messages are used for address resolution. When an ARS prepares an NS(AR) it sets the Source Address to the IPv6 MLA assigned to the OMNI interface. The ARS also sets the Target Address to the IP Destination Address of the invoking packet and sets the Destination Address to the solicited-node multicast address corresponding to the (unicast) Target Address. After the ARS sends the NS(AR), an ARR with addressing information for the ART returns a unicast NA(AR) that contains current, consistent and authentic Target Address resolution information. The ARR sets the NA(AR) Source Address to an IPv6 MLA assigned to the ART's OMNI interface, sets the Destination Address to the Source Address of the NS(AR) and sets the Target Address to the Target Address of the NS(AR). NS/NA(AR) messages must be secured. Other NS/NA message exchanges are used to determine target reachability (NS/NA(NUD)). These messages include the same addresses as for NS/NA(AR) with the exception that the source sets the NS(NUD) Destination and Target Address to the same unicast address. The target then returns a responsive NA(NUD). NS/NA(NUD) messages used to establish or update NCE state must be secured. Unsolicited NA messages (uNAs) are used to update a neighbor's cache when an underlay interface address changes due to a mobility event. Nodes also use uNAs during Route Optimization. NS/NA(DAD) messages are not used in AERO, since Duplicate Address Detection is not required on OMNI links. When the network layer issues an NS(DAD) message over an OMNI interface, the interface simply discards the message. AERO also introduces three IPv6 ND "pseudo-messages" for Multilink Forwarding. These messages use the same Type value and message formatting specifications as the standard NS messages but include different Code values. The messages are: Multilink Initiate (MI) (Type 135; Code TBD1) - sent as an adaptation layer control message used to initiate state needed to support multilink forwarding. Recipients of MI messages respond by returning a Multilink Respond (MR). Multilink Respond (MR) (Type 135; Code TBD2) - an adaptation layer control message multilink forwarding response to an MI message. Recipients of MR messages optionally return a Multilink Control (MC). Multilink Control (MC) (Type 135; Code TBD3) - a muti-purpose secured standalone adaptation layer control message used to confirm a previous MI/MR exchange, establish multilink forwarding state, forward error/informational messages or transport passenger packets. Unlike standard IPv6 ND messages, the pseudo message set (MI/MR/MC) is used internally within the adaptation layer only and the messages are never exposed to the network layer. (Any MI/MR/MC messages accidentally exposed to the network layer would be silently discarded due to NA message validation rules per since they include non-zero Code values.) IPv6 ND pseudo messages MI and MC set the Source Address to the FNP/MNP/MLA Source Address of the invoking packet, set the Destination Address to the MLA of the LHS Proxy/Server and set the Target Address to the FNP/MNP/MLA Destination Address of the invoking packet. IPv6 ND pseudo message MR sets the Source Address to the Destination Address of the invoking packet, sets the Destination Address to the MLA of the FHS Proxy/Server and sets the Target Address to the Source Address of the invoking packet. IPv6 ND (pseudo) messages sent on OMNI links that must be examined by transit OAL intermediate systems on the path require a special codepoint for control message recognition. The OAL source therefore sets the DSCP field in the IPv6 OAL encapsulation header of such messages to the special value '111111' (see: ). The control planes of transit OAL intermediate systems can then intercept and process these messages before forwarding them to the next OAL hop. IPv6 ND (pseudo) messages also include an SRH extension to the OAL IPv6 header as discussed in . The pseudo messages MI/MR/MC are distinguished from ordinary NS messages by the message Code field value alone, and must not elicit an NA response.

In secured environments (e.g., between secured spanning tree neighbors, between neighbors on the same secured ANET, etc.), OMNI interface neighbors can exchange AERO control messages without including Identification values. In environments where spoofing is considered a threat, OMNI interface neighbors instead invoke Identification window synchronization by including OMNI Neighbor Synchronization sub-options in control message exchanges to maintain send/receive window state in their respective neighbor caches as well as in AFIB entries of all OAL intermediate nodes in the forward and reverse paths. In common arrangements, OAL Identification window synchronization is necessary for Client to Client, Client to Proxy/Server or Proxy/Server to Proxy/Server message exchanges conducted over unsecured Internetwork paths. Conversely, Proxy/Server to Proxy/Server, Proxy/Server to Gateway and Gateway to Gateway message exchanges carried over the secured spanning tree do not require window synchronization. OAL end system and intermediate nodes verify Identification values of OAL packets that traverse the unsecured spanning tree according to their populated AFIB state. This allows each OAL node to exclude spurious packets injected into the OMNI link from an off-path adversary.

When the network layer forwards an original IP packet into an OMNI interface, the interface locates a NLNCE corresponding to the destination (which may only match "default"). The OMNI interface then invokes the OAL as discussed in which removes the virtual Ethernet header and encapsulates the packet in an IPv6 header to form an OAL packet according to ALNCE information. 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 flow over the interface pair selected for the neighbor. The OAL source includes any necessary OAL IPv6 extension headers including an identical SRH with each fragment. The OAL source can instead invoke OAL header compression by replacing the full OAL IPv6 header, SRH and Extended Fragment Header with an OAL Compressed Header (OCH) that includes an AERO Flow Vector Index (AFVI) (see: ). For messages that will traverse unsecured paths, the OAL source finally performs underlay encapsulation on each resulting OAL fragment to form a carrier packet, with Source Address set to its own address (e.g., 192.0.2.100) and Destination Address set to the 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 :

In this format, the OAL source encapsulates the original IP header and packet body/fragment in an OAL IPv6 header. The OAL source then adds an SRH plus Extended Fragment Header as OAL IPv6 header extensions for each fragment and prepends underlay headers 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 fragmentation and reassembly along the concatenated underlay network path segments. The OMNI link control plane service distributes Client MNP prefix information that may change occasionally due to regional node mobility, as well as more static information for Relay FNPs that rarely change. 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 Flow Information Base (AFIB) that caches AERO Flow 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 underlay re-encapsulation at an OAL intermediate system, the OMNI interface performs underlay 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 underlay encapsulation to produce a carrier packet appropriate for next segment forwarding.

When an OAL node receives OAL packets/fragments addressed to another node, it discards the previous hop underlay headers and includes new underlay headers appropriate for the next hop in the forwarding path to the OAL destination. 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 underlay decapsulation, verifies the Identification, then performs OAL reassembly/decapsulation to obtain the original OAL packet or composite packet (see: ). Next, if the enclosed original IP packet(s) are addressed either to itself or to a destination reached via an interface other than the OMNI interface, the OAL node replaces the OAL encapsulation IPv6 header with a virtual Ethernet header and forwards the original IP packet(s) to the network layer. If the original IP packet(s) are destined to another node reached by the OMNI interface, the OAL node instead decrements the Hop Limit, then performs underlay encapsulation and forwards these new carrier packets into an underlay interface for the next segment. 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 that originate from within the same secured ANET. AERO Clients and Relays accept original IP packets from downstream network correspondents based on ingress filtering. AERO Clients, Relays, Proxy/Servers and Gateways verify carrier packet underlay encapsulation addresses according to . OAL end systems and intermediate systems forward/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 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 the OAL to accommodate 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 to the OMNI interface, the OAL source encapsulates and fragments the packet if necessary. When the network layer presents the OMNI interface with multiple original IP packets addressed to the same IPv6 flow, the OAL source can concatenate them as a single OAL composite packet as discussed in before applying fragmentation. The OAL source then submits each OAL fragment for underlay encapsulation and transmission as a carrier packet via 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.

Original IP packets 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 underlay (i.e., from an OMNI interface neighbor). All original IP packets 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 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 should instead be forwarded back to the network layer, the packet is dropped to avoid looping). Carrier packets that enter the OMNI interface from the underlay are either re-encapsulated and re-admitted into the underlay, or reassembled and forwarded to the network layer where they are subject to either local delivery or IP forwarding. When the network layer of a router forwards an original IP packet 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 decrement the original IP packet TTL/Hop Limit further since its adaptation layer forwarding actions occur below the network layer. The original IP packet'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 underlay decapsulation to obtain the enclosed OAL packet/fragment. When the intermediate system forwards an OAL packet/fragment not addressed to itself (or one addressed to itself but that also includes an SRH with Segments Left greater than 0), 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) cannot enter an endless loop. OMNI interfaces may have multiple underlay interfaces and/or NCEs 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 underlay network 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 is often greatest 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 use only "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. All Clients, Proxy/Servers and Gateways serve as OAL intermediate nodes for the purpose of forwarding OAL packets/fragments that include an SRH or OCH with non-zero AFVI over the unsecured spanning tree based on AFIB entries. When an OAL intermediate node forwards an OAL packet/fragment with an underlay Source Address and AFVI that matches an AFV, the node first verifies that the Identification is in sequence. The OAL intermediate node then rewrites the packet's AFVI with a value that will be recognized by the next OAL hop and forwards the packet. (For OAL packets/fragments with uncompressed headers and with AFVI set to 0, the OAL intermediate node instead forwards based on matching the OAL IPv6 Destination Address with a standard IPv6 forwarding table entry after applying SRH processing.) The chain of OAL source, intermediate and destination nodes may therefore traverse many (Proxy/)Clients, Proxy/Servers and Gateways on the path. The following sections discuss the OMNI interface-specific forwarding algorithms for Clients, Proxy/Servers and Gateways. In the following discussion, an original IP packet's Destination Address is said to "match" if it is the same as a cached address, or if it is covered by a cached FNP/MNP.

When the network layer presents an original IP packet to a Client's OMNI interface (i.e., following next-hop determination), the Client forwards the packet to the OMNI interface virtual router function if the Destination is off-link. The virtual router then either forwards the packet to a Proxy/Server or initiates adaptation layer address resolution and forwards the packet according to ALNCE information. If the Destination is on-link, the Client instead invokes network layer address resolution if there is no NLNCE. The Client holds the packet in a queue until address resolution completes, then admits the packet into the OMNI interface. The OMNI interface then searches for an ALNCE that matches the Destination. If there is a matching ALNCE for a neighbor reached via a *NET interface (i.e., an upstream interface), the Client selects one or more "reachable" neighbor interfaces in the entry for forwarding purposes. When a carrier packet enters a Client's OMNI interface from the underlay, the Client performs underlay decapsulation if necessary to obtain the OAL packet/fragment then examines the OAL Destination Address (i.e., after locating the correct AFV if the OAL packet header is OCH). If the OAL Destination Address matches one of the Client's addresses and the packet includes an SRH with Segments Left greater than 0, the Client rewrites the OAL Destination Address and forwards the packet to the peer Client or Proxy/Server indicated by the next hop SRH address. Otherwise, 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 to the network layer. If the OAL Destination Address does not match, the Client drops the original IP packet and MAY return a network layer ICMP Destination Unreachable message subject to rate limiting (see: ). Note: The forwarding table entries established in peer Clients are based on MLAs which also appear as OAL Source or Destination Addresses within (M)ANETs. The original IP packet Source and Destination Addresses instead use MNP or MLA addresses. Note: Clients within MANETs support Client-to-Client multihop forwarding when necessary to reach destinations or FHS Proxy/Servers that may be multiple OAL hops away. In this way, forwarding Clients act as OAL intermediate nodes and forward using OCH compression based on AFV state that is indexed by the AFVIs included in each OAL packet/fragment.

When the network layer admits an original IP packet into a Proxy/Server's OMNI interface, the OAL drops the packet 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 MLA 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 underlay encapsulation and forwards the resulting carrier packet 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 IP Destination Address matches an FNP/MNP associated with a (foreign) Proxy/Server or Client, the (local) Proxy/Server instead assumes the Relay role and forwards the original IP packet in the same manner as for Client forwarding while including an SRH. 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 underlay encapsulation and forwards the resulting carrier packets while invoking address resolution and multilink forwarding procedures per . When the Proxy/Server receives 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 MLA, the Proxy/Server drops the carrier packets regardless of their OMNI link point of origin. The Proxy/Server also drops original IP packets received on underlay interfaces either directly from a *NET Client if the original IP Destination Address corresponds to the same Client's delegated MNP. Proxy/Servers also drop carrier packets that contain OAL packets/fragments with foreign OAL Destination Addresses (MLAs) that do not match one of their local *NET Clients. These checks are essential to prevent forwarding inconsistencies from accidentally or intentionally establishing endless loops that could congest nodes and/or *NET links. Proxy/Servers process carrier packets that contain OAL packets/fragments with OCH headers or with Destination Addresses that match their MLA and also include an SRH. In the first case, the Proxy/Server examines the underlay Source Address and AFVI to locate the corresponding AFV entry in the AFIB. In the second case, the Proxy/Server applies standard SRH processing procedures. The Proxy/Server then forwards them according to the AFV or IPv6 routing state while decrementing the OAL packet/fragment Hop Limit. For OAL packets/fragments with Destination Addresses that match their MLA and also include an SRH, the Proxy/Server performs any necessary local processing then rewrites the OAL Destination Address according to the next hop SRH address. For those that do not include an OCH or SRH with additional next hop addresses, the Proxy/Server instead performs underlay decapsulation, verifies the Identification and performs OAL reassembly to obtain the original IP packet. For data packets addressed to its own MLA that arrived via the secured spanning tree, the Proxy/Server delivers the original IP packet to the network layer to support secured BGP routing protocol control messaging. For data packets originating from one of its dependent Clients, the Proxy/Server instead performs OAL encapsulation/fragmentation followed by underlay encapsulation and sends the resulting carrier packets while invoking address resolution and multilink forwarding procedures per . For 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 carrier packets that contain OAL packets with OAL Destination Address set to the MLA of one of its Client neighbors established through RS/RA exchanges, it accepts the carrier packets only if data origin authentication succeeds. If the NCE state is DEPARTED, the Proxy/Server changes the OAL Destination Address to the MLA of the Client's new Proxy/Server, decrements the OAL Hop Limit, then performs underlay encapsulation and forwards the resulting carrier packets into the spanning tree which will eventually deliver them to the new Proxy/Server. If the neighbor cache state for the Client is REACHABLE and the Proxy/Server is a MAP responsible for serving as the Client's address resolution responder and/or default router, it verifies the Identification then submits the OAL packet/fragment for reassembly. The Proxy/Server then decapsulates and processes the resulting control message or original IP packet accordingly. Otherwise, the Proxy/Server changes the OAL Destination Address to the local Client's MLA, includes an SRH with intermediate hop MLAs, decrements the OAL Hop Limit, performs underlay encapsulation and forwards the carrier packet to the Client which then performs data origin verification and reassembly. (In the mobility case, the Client may receive fragments of the same original IP packet from different Proxy/Servers but this will not interfere with correct reassembly.) When the Proxy/Server receives carrier packets that contain OAL packets with OAL Source Address set to the MLA of one of its Client neighbors established through RS/RA exchanges, it accepts the carrier packets only if data origin authentication succeeds. The local Proxy/Server then forwards the packet according to the IP destination or AFVI state previously established through multilink control messaging. When the Proxy/Server receives carrier packets that contain OAL packets with OAL Destination Address set to a FNP address that does not match the MSP, it accepts the carrier packets only if data origin authentication succeeds and if there is a network layer forwarding table entry for the FNP. The Proxy/Server then performs underlay decapsulation, verifies the Identification, performs OAL reassembly/decapsulation to obtain the original IP packet, then presents it to the network layer (as a Relay) where it will be delivered according to standard IP forwarding. When a Proxy/Server receives a carrier packet from the secured spanning tree, it considers the message as authentic without having to verify network or higher layer authentication signatures. If the Proxy/Server has multiple original IP packets to send to the same neighbor, it can concatenate them as a single OAL composite packet .

When the network layer admits an original IP packet 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 MLA of a neighboring Gateway or Proxy/Server by examining the OMNI interface's network layer neighbor cache. If so, the Gateway performs OAL encapsulation/fragmentation followed by underlay encapsulation 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 BGP routing protocol or other 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 decapsulates to obtain the OAL packet/fragment then searches for an AFIB entry that matches the OAL AFVI or an IPv6 forwarding table entry that matches the OAL Destination Address. Gateways process carrier packets containing OAL packets/fragments with OAL Destination Addresses that do not match their MLAs 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 MLA as the OAL Destination Address. If the OAL packet/fragment contains an OCH or a full OAL header with an SRH extension, the Gateway forwards the OAL packet/fragment to the next hop while decrementing the OAL Hop Limit but without reassembling. When the Gateway forwards the OAL packet/fragment, it either rewrites the OCH AFVI with the value it will represent to the next OAL hop or follows standard SRH processing procedures. If the OAL packet/fragment includes a full OAL header but does not include an AFVI, the Gateway instead examines the OAL packet. The Gateway first determines whether the OAL packet includes an MI/MR/MC 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 MLA, the Gateway instead reassembles then discards the OAL header and forwards the original IP packet 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 an IPv6 routing table that determines the next hop for a given OAL Destination Address, 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 underlay Source Addresses of carrier packets not received from the secured spanning tree are "trusted" before forwarding according to an AFV (otherwise, the carrier packet must be dropped).

When an AERO node admits an original IP packet 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 MC 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 next OAL hop or by the next OAL hop itself. The message includes an IP header with the address of the node that generated the error as the Source Address and with the underlay 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", "Packet Too Big", "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 Packet Too Big messages, there may be a restricting link on the path or the next OAL hop may be experiencing reassembly cache congestion. In both cases, the node should adaptively decrease the size of the OAL fragments it sends to this OAL next hop (note that the PTB messages could indicate either "hard" or "soft" errors). 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 adaptively decrease the size of the OAL fragments it sends to this OAL next hop. 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. AERO nodes include ICMPv6 error messages intended for an OAL source as sub-options in the OMNI option of secured MC messages. When the OAL source receives the MC 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. An AERO/OMNI intermediate system may discover that a transit packet has no matching AFIB state to support forwarding to the next adaptation layer hop. In that case, the intermediate system should return a Destination Unreachable error sub-option in a secured MC message. The OAL source should process the message as an indication that AFIB multilink forwarding state for a particular flow must be refreshed.

AERO nodes observes the Router Discovery and Prefix Registration specifications and coordinate their autoconfiguration actions with the mobility service through RS/RA message exchanges as specified in .

Each AERO Proxy/Server on the OMNI link is configured to respond to Client MNP prefix delegation/registration requests based on the DHCPv6 service. Each Proxy/Server is provisioned with a database of MNP-to-Client ID mappings for all Clients enrolled in the AERO service. 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 MNP service regardless of the Proxy/Servers they select and provision their MNPs for downstream-attached node addressing on EUN interfaces. (Note: an OMNI link can instead delegate non-correlated MNPs to Clients instead of maintaining a common synchronized database. In that case, each Client may receive a different MNP delegation each time it registers with the OMNI domain and may need to renumber its downstream-attached EUNs.) Clients associate each of their *NET underlay interfaces with FHS Proxy/Servers. Each FHS Proxy/Server locally services one or more of the Client's underlay interfaces, and the Client typically selects one among them to serve as the MAP Proxy/Server (the Client may instead select a "third-party" MAP Proxy/Server that does not directly service any of its underlay interfaces). All of the Client's other FHS Proxy/Servers forward proxyed copies of RS/RA messages between the MAP Proxy/Server and Client without assuming the MAP role functions themselves. Each Client typically associates with a single MAP Proxy/Server, while all other Proxy/Servers are candidates for providing the MAP role for other Clients. A Client can select both an FHS and MAP Proxy/Server in a single message by including an SRH in the RS message OAL header when it already knows the MAP's address. An FHS Proxy/Server assumes the MAP role when it receives an RS message with a Destination Address that matches its own MLA or link-scoped All-Routers multicast. An FHS Proxy/Server assumes the proxy role when it receives an RS message with an SRH or with a Destination Address that matches another Proxy/Server. (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 MLA of another Proxy/Server to serve as a MAP.) AERO Clients and Proxy/Servers use control messages to maintain NLNCEs and ALNCEs. 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., 2 * ReachableTime seconds) in response to a Client's RS message. Thereafter, Clients send additional RS messages to keep Proxy/Server state alive. AERO Clients and FHS/MAP Proxy/Servers include MNP prefix delegation parameters in RS/RA messages. The control messages are exchanged between the Client and any FHS Proxy/Servers acting as proxys for the MAP Proxy/Server as specified in according to the address/prefix management schedule required by the service. If the Client knows its MNP in advance, it can include the MNP in its prefix delegation request. If the MAP Proxy/Server accepts the Client's MNP assertion (or if it delegates a new MNP for the Client), it injects the MNP into the routing system and establishes the necessary neighbor cache state. AERO Clients and their FHS Proxy/Servers on MANETs and open INETs must establish and maintain Identification synchronization windows in their RS/RA exchanges. The window synchronization provides a well-managed Identification value that the Client and Proxy/Server can use for validating control messages with authentication signatures. All Client and Proxy/Server behaviors for the exchange of RS/RA messages are conducted according to the Router Discovery and Prefix Delegation specifications found in .

AERO nodes invoke address resolution, multilink forwarding and route optimization when they need to forward the initial original IP packets of flows to new neighbors over (M)ANET/INET interfaces as well as to maintain continuous multilink forwarding coordination with existing neighbors. As specified in , "when a node has a unicast packet to send to a neighbor, but does not know the neighbor's link-layer address, it performs address resolution". Possible Source and Destination Addresses for original IP packets that traverse a local (M)ANET/INET and/or the rest of the OMNI link include addresses taken from an FNP, MNP or MLA assigned to a Client. The flow is then identified by the 3-tuple consisting of the IPv6 Source Address, Destination Address and Flow Label. The Address Resolution Source (ARS) considers candidate original IP packet destinations as either on-link or off-link on the OMNI interface. For destinations that match an on-link prefix, the ARS invokes address resolution as a network layer function if there is no NLNCE matching the destination. For destinations that match an off-link prefix, the ARS forwards the packet to a virtual router function within the OMNI interface that invokes address resolution as an adaptation layer function if there is no ALNCE matching the destination. The first such address resolution may return prefix information sufficient to satisfy future resolutions for addresses covered by the same prefix locally and without the need for additional control messaging over the network. Address resolution is based on an IPv6 ND NS/NA(AR) messaging exchange between the ARS and the target neighbor as the Address Resolution Target (ART). The ARS engages address resolution by sending NS(AR) messages over a selected underlay interface to determine adaptation and underlay address mappings for the ART network layer address. (The ARS may select any available underlay interface to carry the NS(AR) with a likely emphasis on the best performing, least congested, etc.) The ARS then discovers address resolution information from any OMNI Interface Attributes sub-options included in NA(AR) messages returned by the ART. Both the ARS and ART can update their neighbor caches based on the address resolution information and cache any information received in Route Information Options (RIOs) included in the NS/NA(AR) exchange. Note that the NS/NA(AR) RIOs are included as adaptation layer information in the OMNI option per and are not delivered to the network layer. The original source or its current FHS/MAP Proxy/Server serves as the ARS. Either the ART itself or the current LHS/MAP Proxy/Server (or Relay) for the ART serves as the Address Resolution Responder (ARR), i.e., the NA(AR) source. 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 (M)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 FNP correspondent nodes on foreign links/networks serviced by a Relay, the Relay is the ARS. For Clients that engage the MAP Proxy/Server in "mobility anchor" mode, the MAP Proxy/Server is the ARS. For peer Clients within the same (M)ANET/EUN, 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 subsequent multilink forwarding and route optimization exchanges to maintain optimal forwarding profiles for each distinct flow. During address resolution, multilink forwarding and/or route optimization a control message source may attach a small number of original IP packets associated with the message exchange as composite 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 IPv6 ND messages and their IP packet attachments. The final OAL intermediate system in the path will then securely forward the control message IP packet attachments to the target. The source can attach original IP packets to the subject control message, but this may cause the message size to exceed the IPv6 minimum MTU and/or result in sub-optimal forwarding for the IP packet attachments. In that case, the source can instead create small MC "pilot" messages used to transport the original IP packets as attachments over shortest paths determined by routing. The OAL source can attach as many IP packets as will fit without causing the OAL packet to exceed the adaptation layer path MTU using the composite packet construct discussed in . When the target Proxy/Server or Client receives a pilot MC, it removes all passenger attachment original IP packets then delivers them to the destination. This service supports assured (but sub-optimal) short-term delivery of protocol data while neighbor coordination is in progress without creating network state. The address resolution, multilink forwarding and route optimization procedures are specified in the following sections.

Address resolution over OMNI interfaces is conceptually the same as specified in Section 7 of including the sending and receiving of NS/NA(AR) messages as well as their implications for neighbor cache entry creation and state management. The NS/NA(AR) messages include addresses and S/TLLAOs in the same manner as for any interface and as discussed in . The OMNI interface therefore exhibits an IP layer behavior that is indistinguishable from an ordinary interface while managing adaptation layer state at a layer below IP. For destinations that match an off-link prefix, Address Resolution over OMNI interfaces is driven by NS/NA(AR) messaging as an adaptation layer function without disturbing the NLNCE. The network layer forwards packets that match an off-link prefix to a virtual router function within the OMNI interface. The virtual router function then either forwards the packets to an FHS Proxy/Server to act as the ARS or initiates an NS/NA(AR) exchange at the adaptation layer (without including S/TLLAOs) while acting as an ARS on its own behalf. In that case, the OMNI interface caches any updated adaptation layer addressing information received in NS/NA(AR) messages in the ALNC. For destinations that match an on-link prefix, Address Resolution over OMNI interfaces is driven by network layer NS/NA(AR) messaging the same as for any IP interface. The OMNI interface then removes the S/TLLAO upon transmission of all NS/NA(AR) messages and includes an S/TLLAO with the OMNI interface internal link-layer address when delivering an NS/NA(AR) message to the network layer. When one or more original IP packets matching an on-link prefix are forwarded over an OMNI interface, the ARS checks the Destination Cache to determine whether there is a NCE that matches the Destination Address. If there is a NCE in the REACHABLE state, the ARS invokes the OAL and forwards the resulting carrier packets according to the cached state then returns from processing. If there is no NLNCE but the ARS is able to determine that adaptation layer mapping state for the IP destination is available, e.g., by snooping the ALNC, it can also create a NLNCE in the REACHABLE state and admit the original IP packets into the interface without requiring an NS/NA(AR) exchange. Otherwise, if there is no NLNCE the ARS creates one in the INCOMPLETE state. The ARS then prepares an Address Resolution NS(AR) message to send toward an ART. The resulting NS(AR) message must be sent securely and includes Source, Destination and Target Addresses as discussed in . When the ARS admits the NS(AR) message into the OMNI interface, the adaptation layer returns an immediate NA(AR) if the ALNC already contains fresh address resolution information for the FNP/MNP prefix or MLA that covers the IP Destination Address. Otherwise, the adaptation layer prepares to forward the NS(AR) while removing the SLLAO (if present) since the locally-unique link-layer address has no useful information for prospective neighbors. For both the on-link and off-link cases of forwarding an NS(AR) at the adaptation layer, the ARS then includes an OMNI option with an authentication sub-option (if necessary). If the ARS can commit to managing its own dynamic mobility updates, it then includes Interface Attributes and/or Traffic Selectors for all of the source Client's underlay interfaces plus RIOs for any of the Client's MNPs. The ARS then then submits the NS(AR) message for OAL encapsulation and transmission.

When the ARS is a FHS Proxy/Server, it sets the OAL Source Address to its own MLA and sets the OAL Destination Address to the NS(AR) Target Address. The ARS then performs underlay encapsulation and sends the resulting carrier packet into the SRT secured spanning tree without decrementing the network layer TTL/Hop Limit field. When the ARS is a Client, it instead uses its own MLA as the OAL Source Address and the MLA of the next OAL hop as the OAL Destination Address while including an SRH with the MLAs of OAL intermediate systems ending with the MLA of the interface-specific FHS Proxy/Server. If the Client is in a MANET or an open INET, it next calculates and includes an authentication signature. In all *NET cases, the Client then includes an OAL IPv6 Extended Fragment Header with Identification set to an in-window value for this FHS Proxy/Server. The ARS Client then performs underlay encapsulation and forwards the carrier packet to the FHS Proxy/Server. The FHS Proxy/Server then performs underlay decapsulation, verifies the Identification, verifies the NS(AR) OAL checksum/authentication signature and confirms that the Client's claimed FNP/MNP RIO(s) and Source Address are correct. The FHS Proxy/Server then changes the OAL Source Address to its own MLA and changes the OAL Destination Address to the NS(AR) Target Address. The FHS Proxy/Server next removes the IPv6 Extended Fragment Header, performs underlay encapsulation and sends the resulting carrier packet into the secured spanning tree on behalf of the Client. Note: both the source and target Client/Relay and their MAP Proxy/Servers include current and accurate information for their multilink Interface Attributes profile. The MAP Proxy/Servers can be trusted to provide an authoritative ARR response and/or mobility update message on behalf of the source/target if necessary.

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

When the LHS/MAP Proxy/Server (or Relay) of the ART receives the NS(AR) secured carrier packets with the target address of the ART as the OAL Destination Address, it performs underlay decapsulation then either forwards the NS(AR) to the ART or processes it locally if it is acting as the ART's designated ARR. The LHS/MAP Proxy/Server (or Relay) processes the message as follows: if the NS(AR) target matches a Client NCE in the DEPARTED state, the (old) MAP Proxy/Server resets the OAL Destination Address to the MLA of its new MAP Proxy/Server. The old MAP Proxy/Server then decrements the OAL header Hop Limit, performs underlay encapsulation and forwards the resulting carrier packet over the secured spanning tree. If the NS(AR) target matches a Client NCE in the REACHABLE state, the LHS/MAP Proxy/Server (or Relay) notes whether the NS(AR) arrived from the secured spanning tree. If the message arrived via the secured spanning tree the LHS/MAP Proxy/Server (or Relay) verifies the NS(AR) OAL checksum only; otherwise, it must also verify the authentication signature. If the LHS/MAP Proxy/Server maintains a Report List for the ART (see: ), it next records the NS(AR) Source Address in the Report List for this ART. If the MAP Proxy/Server is the ART's designated ARR, it forwards any original IP packet(s) attached to the NS(AR) composite packet to the ART and prepares to return an NA(AR) as discussed below; otherwise, the LHS/MAP Proxy/Server determines the underlay interface for the ART and proceeds as follows: If the LHS/MAP Proxy/Server is also the LHS Proxy/Server on the underlay interface used to convey the NS(AR) to the ART, it includes an OAL IPv6 Extended Fragment Header with an in-window Identification for the ART Client plus an SRH and authentication signature if necessary then recalculates the OAL checksum. The Proxy/Server then changes the OAL Source Address to its own MLA, changes the OAL Destination Address to the next hop MLA on the path to the ART, decrements the OAL Hop Limit, performs underlay encapsulation and forwards the resulting carrier packet over the underlay interface to the ART. If the MAP Proxy/Server is not the LHS Proxy/Server on the underlay interface used to convey the NS(AR) to the ART, it instead changes the OAL Destination Address to the MLA of the LHS Proxy/Server for the selected ART interface. The MAP Proxy/Server next decrements the OAL Hop Limit, performs underlay encapsulation and forwards the resulting carrier packet over the secured spanning tree. When the LHS Proxy/Server receives the carrier packets, it performs underlay decapsulation, verifies the NS(AR) OAL checksum, then forwards to the ART while changing the OAL addresses as above. The LHS Proxy/Server also includes an IPv6 Extended Fragment Header plus an SRH and authentication signature if necessary while recalculating the checksum the same as described above. If the NS(AR) target matches one of its FNP routes, the MAP/LHS Proxy/Server serves as both a Relay and an ARR, since the Relay forwards original IP packets toward FNP target nodes at the network layer. If the ARR is a Relay or the ART itself, it first creates or updates an ALNCE for the NS(AR) Source MLA while caching all Interface Attributes and Traffic Selector information in the ALNCE and caching any IPv6 addresses for the original source found in the Interface Attributes in the Destination Cache. The ARR then installs any RIO MNP prefixes in the ALNCE. (If the NS(AR) does not include address resolution information, the ART will simply need to initiate another unidirectional address resolution request if it has return traffic to send back to the ARS.) Next, if the NS(AR) target is on-link on the ARR/ART's OMNI interface the ART delivers the NS(AR) to the network layer while including an SLLAO with the OMNI interface internal link-layer address. The network layer will return an NA(AR) with a TLLAO which the ARR removes before forwarding the NA(AR) back to the ARS. For off-link targets, the ARR instead prepares a solicited NA(AR) message to return to the ARS as an adaptation layer function without exposing the NS(AR) to the network layer. In both the on- and off-link cases, the ARR includes RIOs for all of the ART's FNPs/MNPs, where the RIO for a destination that matches only "default" includes a /64 FNP that covers the 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. 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, an IPv6 Extended Fragment Header and an OAL SRH with MLA addressing information for the LHS hops on the path to the LHS Proxy/Server and ending with the MNP of the ARS MAP/FHS Proxy/Server that appeared in the NS(AR) SRH. The ARR next calculates the NA(AR) OAL checksum then submits the NA(AR) for encapsulation with OAL Source Address set to its own MLA and Destination Address set to either the MLA that appeared in the NS(AR) OAL source for (M)ANET traversal or the NS(AR) source itself for INET traversal. The ARR then performs underlay encapsulation and forwards the resulting carrier packet. When the ART's LHS Proxy/Server receives carrier packets sent by an ART acting as an ARR on its own behalf, it performs underlay decapsulation then verifies the NA(AR) message OAL Identification and checksum/authentication signature. The Proxy/Server then verifies that any RIO information is acceptable, changes the OAL Source Address to its own MLA and changes the OAL Destination Address to the MLA of the ARS MAP/FHS Proxy/Server. The Proxy/Server next decrements the OAL Hop Limit, removes the OAL Extended Fragment Header, performs underlay encapsulation and finally forwards the resulting carrier packet into the secured spanning tree.

When a Gateway receives NA(AR) carrier packets, it performs underlay 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 underlay encapsulation and forwards the resulting carrier packet via the SRT secured spanning tree where it may traverse multiple intermediate OMNI link segments interconnected by other Gateways. The final-hop Gateway will deliver the carrier packets via the secured spanning tree to a FHS Proxy/Server for the ARS.

When the ARS receives NA(AR) carrier packets, it performs underlay decapsulation then searches for an ALNCE that matches the MLA corresponding to the NA(AR) Source. The ARS then processes the message the same as for standard IPv6 Address Resolution . In the process, it caches all OMNI option Interface Attributes and Traffic Selectors in the ALNCE for the NA(AR) MLA Source Address and caches any IPv6 addresses for the ART found in the Interface Attributes in the Destination Cache. The ARS then caches any RIO FNP/MNP prefixes in the ALNCE indexed by the neighbor's MLA. All included Interface Attributes sub-options plus RIOs together provide the address mapping information necessary to satisfy address resolution. For targets that match an on-link prefix, the adaptation layer of the ARS then includes a TLLAO with the OMNI interface internal link-layer address then delivers the NA(AR) to the network layer. The network layer will then set the NLNCE for this neighbor to REACHABLE while caching the link-layer address. Future original IP packet transmissions over the OMNI interface will use this IP to link-layer address mapping the same as for any IPv6 interface. When the ARS is a Client, the SRT secured spanning tree will first deliver the solicited NA(AR) message to the Client's FHS Proxy/Server, which includes an OAL Extended Fragment Header with an in-window Identification for this Client, 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 or Identification; if the Client is in a MANET or in the open INET the Proxy/Server must instead include an Identification and authentication signature. The Proxy/Server then includes an SRH, changes the OAL Source Address to its own MLA and changes the OAL Destination Address to the MLA of the next hop on the path to the Client when it forwards the NA(AR). The Proxy/Server then decrements the OAL Hop Limit, performs underlay encapsulation and forwards the resulting carrier packet over the underlay interface to the Client. When the Client receives the NA(AR), it caches the adaptation layer information as above then removes the OMNI option and forwards the NS(AR) to the network layer if the target is on-link. The Client is then responsible for informing the ART if any of its adaptation layer addressing changes (e.g., due to mobility) before the ALCNE expires.

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 (i.e., the end system Clients or their respective Proxy/Servers) can assert per-flow multilink forwarding paths through underlay interface pairs serviced by the same Source/Destination Addresses and Flow Label by sending MI/MR/MC messages with OMNI Neighbor Synchronization sub-options and with an OAL SRH with an AVFI option with (I)nitialize set to 1. The MI/MR/MC messages establish per-flow multilink forwarding and header compression state in OAL intermediate systems in the path between the ARS and ART. Note that either the ARS or ART can independently initiate multilink forwarding by sending MI messages on behalf of specific flows over underlay interface pairs. The source Client or FHS Proxy/Server sets the MI address fields as discussed in then includes the Flow Label of the original IP packet set according to . The source finally performs OAL encapsulation with Source and Destination addresses set the same as for address resolution while including an SRH extension. The flow 3-tuple is then identified by the MI Source Address, Destination Address and Flow Label. When the target Client or LHS Proxy/Server returns an MR, it sets the address fields as discussed in . The target Client or LHS Proxy/Server then sets the Flow Label to value that appeared in the MI Flow Label and finally performs OAL encapsulation while including an SRH extension with the MLA of the FHS Proxy/Server that appeared in the MI SRH as the ultimate hop. The multilink forwarding profile provides support for redundant paths that each OAL node can harness to its best advantage. For example, OAL nodes can use traffic selectors to distribute different traffic types over available multilink paths, while other factors such as metrics, cost, provider, etc. can also provide useful decision points. OAL nodes can also employ multilink forwarding for fault tolerance by sending redundant data over multiple paths simultaneously, or for load balancing where the individual packets of a single traffic flow are spread across multiple independent paths. OAL nodes that engage in multilink forwarding therefore must incorporate a policy engine that selects both inbound and outbound multilink paths for a given traffic profile at a given point in time. This specification therefore provides multilink forwarding mechanisms without mandating any specific multilink policy. All Client, Proxy/Server and Gateway nodes that configure OMNI interfaces and engage in multilink coordination include an additional forwarding table termed the AERO Flow Information Base (AFIB) that supports OAL packet/fragment forwarding based on original IP packet flows over specific OMNI neighbor interface pairs. The AFIB contains per-flow AERO Flow Vectors (AFVs) identified by the underlay address of the previous OAL hop plus a value known as the AFV Index (AFVI). The AFVs cache uncompressed OAL header information to support forwarding of packets with compressed headers as well as previous/next-hop addressing and AFVI information. The AFVs also cache window synchronization state (i.e., the starting sequence number and window size) for each specific flow. Using the window synchronization state, simple Identification-based data origin authentication is enabled at each OAL source, intermediate system and target node. Client and Proxy/Server OMNI interfaces manage end system AFIB entries in conjunction with their internal ALNC, where the ALNCEs link to (possibly) multiple AFVs with one per flow over a specific FHS/LHS interface ifIndex pair. When OMNI interface peers need to coordinate, they locate a NLNCE for the peer (established through address resolution) then use the ALNCE as a nexus that aggregates potentially many AVFs which cache AFVIs to support multilink forwarding on a per-flow basis. Gateway OMNI interfaces and the OMNI interfaces of Clients or Proxy/Servers acting as OAL intermediate nodes manage transit AFIB entries independently of their internal neighbor caches. These transit AFVs are indexed by the underlay address and AFVI supplied by the previous hop. OAL source, intermediate system and target nodes create or update AFVs/AFVIs when they process an MI/MR/MC initiation or response message with an OMNI Neighbor Synchronization sub-option with the SYN flag set (see: ). The source of the initiating MI is considered to reside in the "First Hop Segment (FHS)", while the destination is considered to reside in the "Last Hop Segment (LHS)". The FHS and LHS roles are determined on a per-flow and per-interface-pair basis. After address resolution, either peer is equally capable of initiating multilink forwarding on behalf of a specific flow. The peer that sends the initiating MI message with Neighbor Synchronization for a specific pair becomes the FHS peer while the one that returns the responsive MR becomes the LHS peer for that (flow, interface pair) only. It is therefore commonplace that peers may assume the FHS role for some flows while assuming the LHS role for others, i.e., even though each peer maintains only a single NCE. When an OAL node sends/forwards an initiating MI or responsive MR with a Neighbor Synchronization sub-option with the SYN flag set, it creates or updates an AFV, caches the Identification window information, caches the MI/MR OAL IPv6 header and caches the flow 3-tuple for compression/decompression of the original IP packet header. The OAL node also records the previous hop underlay address and AFVI, then generates a new next hop AFVI or updates the lifetime of an already-established AFVI. The next hop AFVI should be selected within the range [1 - (2**16-1)] unless all values within that range are already in active use. Otherwise, the AFVI must be selected within the range [2**16 - (2**32-1)] while the value 0 indicates "AFVI unspecified". When the OAL node forwards future OAL packets/fragments that include the previous hop underlay address and AFVI, it can unambiguously locate the correct AFV and use the cached information to forward to the next OAL hop. OAL nodes cache AFVs for up to ReachableTime seconds following their initial creation. If the node processes another MI/MR message specific to an AFV, it updates ReachableTime to REACHABLE_TIME seconds, i.e., the same as for NCEs. If ReachableTime expires, the node deletes the AFV. The following sections provide the detailed specifications of these MI/MR/MC exchanges for all nodes along the forward and reverse paths.

When an FHS OAL source has an original IP packet to send toward an LHS OAL target, it first performs address resolution resulting in the creation of an ALNCE for the MLA 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 MI message with a Neighbor Synchronization sub-option while setting the MI Source, Target and Destination Addresses as specified above. The FHS source next creates an AFV then generates and assigns an AFVI for the flow over this interface pair; the AFVI (coupled with its UNX address) must be unique for its communications to this next OAL hop. The FHS source then includes an SRH with segment addressing information. The FHS source finally includes an OMNI Neighbor Synchronization sub-option with window synchronization parameters and prepares the MI message for transmission while also caching the window synchronization parameters in the local AFV. If the FHS source is the FHS Proxy/Server, it performs OAL encapsulation while setting the OAL Source Address to its own MLA and setting the OAL Destination Address to the MLA found in the target Interface Attributes. The FHS Proxy/Server then performs underlay encapsulation and forwards the resulting carrier packet into the secured spanning tree which will deliver it to an FHS Gateway. If the FHS source is the FHS Client, it instead includes an authentication signature and OAL Extended Fragment Header with an in-window Identification for its FHS Proxy/Server if necessary. If FMT-Forward and FMT-Mode are both set, the Client sets the Neighbor Synchronization LHS ifIndex to the ifIndex of the target; otherwise, it sets the ifIndex to 0 to allow the FHS Proxy/Server to select the target ifIndex. The FHS Client then performs OAL encapsulation while including an SRH per , sets the OAL Source Address to its own MLA and sets the OAL Destination Address to the MLA of the first hop toward the FHS Proxy/Server. The FHS Client finally performs underlay encapsulation and forwards the resulting carrier packet to the FHS Proxy/Server. If there are multiple OAL hops between the Client and FHS Proxy/Server, the first OAL intermediate node receives the carrier packets containing the MI then also verifies the OAL checksum and authentication signature. The OAL intermediate node then caches the FHS/LHS Client addressing, AFVI and window synchronization information as previous hop information in a new or existing AFV. The OAL intermediate hop then creates a new unique AFVI to forward to the next OAL hop, then both caches the AFVI and writes it into the control message AVI field, i.e., while over-writing the value supplied by the previous hop. The OAL intermediate node then forwards to the next OAL hop toward the FHS Proxy/Server which then performs the same functions as the previous OAL hop. When the FHS Proxy/Server receives the carrier packets, it performs underlay decapsulation, verifies the Identification, and verifies the MI OAL checksum and authentication signature. The FHS Proxy/Server then creates an AFV (i.e., the same as the FHS Client had done) while caching the FHS/LHS Client addressing, AFVI and window synchronization information as previous hop information for this AFV. The FHS Proxy/Server next generates a new unique AFVI to forward to the next OAL hop, then both caches the AFVI in the AFV and writes it into the MI AFVI field. The FHS Proxy/Server next calculates the MI OAL checksum then decrements the OAL Hop Limit, removes the OAL Extended Fragment Header, performs underlay encapsulation and forwards the resulting carrier packet 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 MI packet over the secured spanning tree, it performs underlay decapsulation then verifies the MI OAL checksum. The FHS Gateway next creates an AFV based on the previous hop Neighbor Synchronization information, i.e., the same as the FHS Proxy/Server had done. The FHS Gateway then generates a locally-unique AFVI for the next hop and both caches the value in the AFV and copies it into the MI AFVI. The FHS Gateway then examines the SRT prefixes corresponding to both the 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 MI Neighbor Synchronization information in the AFV, and writes a new locally-unique AFVI for the next hop into the AFV and MI AFVI. The FHS Gateway then decrements the OAL Hop Limit, performs underlay encapsulation and forwards the resulting carrier packet 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 update their transit AFVs in the same fashion while selecting new locally-unique AFVIs for the next hop based on Neighbor Synchronization and SRH information. The LHS Gateway then performs underlay decapsulation, verifies the Identification, verifies the MI OAL checksum then creates an AFV (i.e., the same as all previous hop Gateways had done) while caching the Neighbor Synchronization information from the previous hop and creating a new AFVI for the next hop. The LHS Gateway then decrements the OAL Hop Limit, performs underlay encapsulation and forwards the resulting carrier packet into the secured spanning tree.

When the LHS Proxy/Server receives the carrier packets from the secured spanning tree, it performs underlay decapsulation, verifies the MI OAL checksum then creates an AFV and caches the previous hop Neighbor Synchronization and addressing information. If the MI Destination Address matches the target and the LHS Proxy/Server is configured to respond on the target's behalf, (i.e., if FMT-Forward is set) it next prepares to return a responsive MR. The LHS Proxy/Server next creates or updates an NCE for the MI Source Address (if necessary) with state set to STALE. The LHS Proxy/Server then creates an MR while copying the Neighbor Synchronization sub-option from the MI and including responsive window synchronization information. The LHS Proxy/Server sets the MR addresses as discussed in then encapsulates the MR with an SRH, with OAL Source Address set to the MI OAL Destination Address and with OAL Destination Address set to the MI OAL Source Address. If the LHS Proxy/Server will set the MR message SYN flag, it also writes a non-zero value in the MR AFVI field; otherwise, it writes the value 0. The LHS Proxy/Server then calculates the MR OAL checksum, performs underlay encapsulation and forwards the resulting carrier packet into the secured spanning tree. The LHS Proxy/Server then creates a locally-unique AFVI for the Client and both caches it in the newly-created AFV and writes it into the MR AFVI field. If FMT-Forward is clear and FMT-Mode is set, the LHS Proxy/Server next resets the Neighbor Synchronization FHS ifIndex to 0. The LHS Proxy/Server next includes an authentication signature in the MI if necessary, changes the OAL Source Address to its own MLA, changes the Destination Address to the MLA of the next hop toward the LHS Client and includes an SRH with the MLAs of intermediate systems. The LHS Proxy/Server then decrements the OAL Hop Limit, includes an OAL Extended Fragment Header with an appropriate Identification value if necessary, performs underlay encapsulation and forwards the resulting carrier packet to the LHS Client. If there are multiple OAL hops between the LHS Proxy/Server and LHS Client, the first OAL intermediate node receives the carrier packet containing the MI then also verifies the OAL checksum and authentication signature. The OAL intermediate node then caches the FHS/LHS Client addressing, AFVI and window synchronization information as previous hop information in a new or existing AFV. The OAL intermediate hop then creates a new unique AFVI to forward to the next OAL hop, then both caches the AFVI and writes it into the MI, i.e., while over-writing the value supplied by the previous hop. The OAL intermediate node then forwards to the next OAL hop toward the LHS Client which then performs the same functions as the previous OAL hop. When the LHS Client receives the carrier packet, it performs underlay decapsulation, verifies the Identification, then verifies the MI OAL checksum/authentication signature. The LHS Client then creates an ALNCE for the MI Source Address (if necessary) in the STALE state and caches the MI Neighbor Synchronization information in a new AFV associated with the NCE corresponding to the MI Source Address. If the LHS Client will request reverse path state establishment, it finally generates and assigns a locally-unique AFVI for a flow to be forwarded to the previous hop, which it caches in the new AFV. Otherwise, the LHS Client sets the reverse path AFVI to 0. The LHS Client then prepares an MR using the same procedures as for the LHS Proxy/Server above while including responsive window synchronization information, the new AFVI in the MR and with SRH addressing information for OAL forwarding in the reverse path. The LHS Client includes an authentication signature if necessary, calculates the MR OAL checksum, then encapsulates the MR with OAL Source Address set to its own MLA and OAL Destination Address set to the MLA of the next hop toward the LHS Proxy/Server. The LHS Client finally includes an OAL SRH and Extended Fragment Header with an appropriate Identification if necessary, performs underlay encapsulation and forwards the resulting carrier packet to the LHS Proxy/Server. If there are multiple OAL hops between the LHS Client and LHS Proxy/Server, the first OAL intermediate node receives the carrier packet containing the MR then also verifies the OAL checksum and authentication signature. If the MR Neighbor Synchronization sub-option SYN flag is set, the OAL intermediate node then caches the LHS/FHS Client addressing, AFVI and window synchronization information as previous hop information in a new or existing AFV. The OAL intermediate hop then creates a new unique AFVI to forward to the next OAL hop, then both caches the AFVI and writes it into the MR, i.e., while over-writing the value supplied by the previous hop. The OAL intermediate node then forwards to the next OAL hop toward the LHS Proxy/Server which then performs the same functions as the previous OAL hop.

When the LHS Proxy/Server receives the carrier packet, it performs underlay decapsulation, verifies the Identification and verifies the MR OAL checksum/authentication signature. The LHS Proxy/Server removes the OAL Extended Fragment Header and forwards the MR over the reverse path toward the initiating FHS node, where it may traverse many intermediate Gateways. Each Gateway along the reverse path processes the Neighbor Synchronization information in the MR message in the same way that the Gateways in the forward path had processed the MI. If the FHS Proxy/Server forwards the message, it changes the OAL Source Address to its own MLA, changes the Destination Address to the next hop MLA toward the FHS Client and includes an SRH. The MR will eventually arrive at the initiating FHS node as confirmation that AFV state is established in all end and intermediate systems in the forward path. Flow state is normally unidirectional from the source to the destination, however the LHS node can elect bidirectional state by setting the SYN flag in the MR Neighbor Synchronization sub-option. In that case, the forward and reverse paths between the FHS (initiator) Proxy/Server and LHS (responder) Proxy/Server may traverse different sets of intermediate nodes but the source and destination are still able to coordinate the value used in the Flow Label. The initiator should then complete the three-way handshake by returning an MC or a data packet with an in-window Identification value to confirm that the SYN was received. Under nominal conditions when the source and destination do not coordinate their flow 3-tuples, if AFV state is needed in the reverse path a separate MI/MR exchange between the LHS initiator and FHS responder based on asynchronous packet arrivals may be needed. In that case, the forward and reverse path flows bear no relation to one another and will often include different flow 3-tuple information.

Following the initial MI/MR/MC exchange OAL end systems can begin exchanging ordinary carrier packets for synchronized flows that include AFVIs and with Identification values within their respective send 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 AFVIs to unambiguously locate the correct AFV and can use the AFV state to forward OAL packets/fragments to the next hop. OAL end systems must then perform continuous MI/MR/MC exchanges to update window state, register new flows 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-flow 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 NCE must be determined individually using additional MI/MR/MC messages that include Neighbor Synchronization information. OAL sources can then begin including OCHs in OAL packets/fragments with an AFVI that OAL intermediate systems can use for shortest-path forwarding based on AFVIs instead of spanning tree OAL IPv6 addresses. Forwarding based on the limited OCH information is supported since all OAL nodes in the path up to (and sometimes including) the OAL destination have already established AFVs. 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 control message, the Proxy/Server uses the Client's ALNCE established through RS/RA exchanges to re-encapsulate while sending outbound secured carrier packets via the secured spanning tree and sending inbound secured carrier packets while including an OAL 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 MI/MR/MC 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 AFVs established through an MI/MR/MC exchange with a remote or local peer. 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 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 UNX addresses, they can exchange carrier packets that contain OAL packets/fragments directly with header compression using AFVIs discovered as above (see: ). When the FHS Client or FHS Proxy/Server sends an MI for the purpose of establishing multilink forwarding state, it should wait up to RETRANS_TIMER seconds to receive a responsive MR. The FHS node can then retransmit the MI up to MAX_UNICAST_SOLICIT times before giving up.

Following address resolution, either the ARS or ART acting as an OAL source may immediately begin forwarding original IP packets for a flow as composite packet attachments to "pilot" MC messages without waiting for an initial MI/MR round-trip exchange. The messages will include an authentication signature if necessary that covers the entire composite packet including the MC pilot and all original IP packet attachments. Each MC message OMNI option contains a Neighbor Synchronization sub-option with the SYN flag set to cause OAL intermediate systems in the forward path to create or update AFIB state. The MC sets its Source/Destination/Target Addresses and Neighbor Synchronization flags the same as described for MI messages above. All OAL intermediate systems in the forward path will process the MC message the same as specified for MI messages above. The OAL source should continue to forward additional original IP packets for the flow that arrive during a brief convergence interval as secured MC composite packets, e.g., up to N composite packets within a 1 second window. The OAL source can then begin forwarding new IP packet arrivals for the flow via the unsecured spanning tree while applying OAL encapsulation, fragmentation and header compression. The OAL source can continue forwarding under the optimistic expectation that AFIB state is securely established in the forward path and/or infer forward progress through receipt of return traffic. As AFIB state becomes stale, the OAL source can resume forwarding secured MC composite packets during a new convergence interval to refresh/renew state as above before resuming via the unsecured spanning tree. The OAL source can also forward OAL-fragmented packets in this manner by including the fragment body preceded by an OMNI Fragment Header (OMNI-FH) immediately following the OAL-encapsulated MC message. Each fragment MUST be no larger than the minimum OAL Fragment Size (OFS) of 1024 octets to ensure they will transit the secured spanning tree without encountering a size restriction. The fragment size is determined by the MC message payload length minus the initial payload length up to the beginning of the OMNI-FH, i.e., the fragment must appear as the final trailing component of the composite packet. Further information on OMNI fragmentation and reassembly can be found in .

Clients with OMNI interfaces configured over underlay interfaces with indeterminant neighborhood properties may be connected to a Mobile Ad-hoc NETwork (MANET). Each MANET may be either completely outside of the range of any OMNI link Proxy/Servers or may require multihop traversal between Clients acting as MANET routers to reach Proxy/Servers that connect to the rest of the OMNI link. The former class of MANETs must operate in isolation solely based on the unique IPv6 MLAs they configure locally. The latter class allows MANET routers to extend infrastructure-based addressing information including MNPs over multiple OMNI link hops as discussed in the OMNI specification. MANET Clients configure their OMNI interfaces over one or more MANET interfaces where multihop forwarding may be necessary. Routing protocols suitable for use over MANET interfaces include OSPFv3 with MANET Designated Router (OSPF-MDR) extensions , OLSRv2 , Babel , AODVv2 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 MLA-based 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 as specified in . This will cause the network layer to direct outbound packets to the OMNI interface, which can apply header compression and underlay MANET interface selection. Multilink Clients that connect a MANET to the rest of the OMNI link act as regular Clients for exchanges with external INETs, but act as Proxy/Servers over their MANET interfaces. Each such Client therefore has at least two underlay interfaces, including both INET and MANET interfaces. The Client therefore services the MANET as if it were a Proxy/Server but presents itself as a Client to external facing INETs. This class of Clients are also known as "Proxy/Clients". The process for a multihop Client to establish multilink forwarding and header compression AFV state in the MANET is conducted in the same fashion as described above and using the same MI/MR/MC or unidirectional MC message exchanges. Each MANET forwarding node in the path creates or updates AFV state in the same fashion as for intermediate Gateways in the secured spanning tree except that the MI/MR/MC messages require authentication signatures (unless neighboring MANET nodes configure IPsec tunnels) and an Identification that is within the window for its serving Proxy/Server if the destination is outside of the local MANET cluster. The MI/MR/MC messages extend from the initiating FHS MANET Client, then across any MANET hops over intermediate FHS Proxy/Clients, then to the FHS Proxy/Server, then across the secured SRT spanning tree to the LHS Proxy/Server, then finally across any intermediate LHS MANET hops to the responding LHS Client. (When the source and target Client are both within the same local MANET cluster, however, the process is conducted directly between the two Clients without engaging the FHS Proxy/Server.) In all other ways, the MI/MR/MC or unidirectional MC exchanges are the same as discussed in . Following the MI/MR/MC exchanges, each MANET router in the forward (and optionally also reverse) path in both the FHS and LHS MANETs will have established AFVs containing multilink forwarding and header compression state for the flow. The AFVs determine AFVI-based forwarding based on the OCH header contents, and each MANET router only forwards packets with in-window Identification values for the flow. MANET routers maintain AFVs for up to ReachableTime seconds unless they are refreshed by a new MI/MR/MC message. 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.

When the FHS and LHS Proxy/Servers are both connected to an IPv6 underlay for the same SRT segment, they can forward MI/MR/MC exchanges directly over the underlay without engaging SRT spanning tree hops. If the underlay is not secured, the FHS and LHS Proxy/Servers must include an authentication signature with their MI/MR/MC messages, which could either be the original authentication signature included by their respective Clients or a new signature included by the Proxy/Server itself. If the Proxy/Server that processes the MI/MR/MC message determines that the message is authentic, it creates or updates an AFV entry according to the multilink forwarding parameters. This establishes both AFVI and Identification window state to be used for future data traffic forwarding.

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

When the FHS/LHS Clients are both located on the same SRT segment, Client-to-Client route optimization is possible following the establishment of any necessary state in NATs in the path. Both Clients will have already established state via their respective shared segment Proxy/Servers (and possibly also any shared segment Gateways) and can begin sending carrier packets directly via NAT traversal while avoiding any Proxy/Server and/or Gateway hops. When the FHS/LHS Clients on the same SRT segment perform initial MI/MR/MC exchanges to establish AFIB state, they first examine the FMT-Forward and FMT-Mode settings to determine whether direct-path forwarding is even possible for one or both Clients (direct-path forwarding is only possible when FMT-Forward and FMT-Mode are both set). The MI/MR/MC messages then include an Interface Attributes sub-option (i.e., in addition to a Neighbor Synchronization sub-option) with the mapped UNX information 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 UNX for the target Client which primes the local chain of NATs for reception of future carrier packets from that UNX (see: and ). The source Client then prepares an MI message with the FNP/MNP address of a subject packet as the Source Address, with the FNP/MNP address of the target as the Target and Destination Address and with an OMNI option with an Interface Attributes sub-option. The source Client then encapsulates the MI in an OAL header and SRH extension with its own MLA as the Source Address, with the MLA of the next OAL hop toward the Proxy/Server as the Destination Address and with an in-window Identification for the target. The source Client then performs underlay encapsulation and 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. The target Client then prepares an MR message with the MI Destination Address as the Source Address, with the MI Source Address as the Target and Destination Address and with an OMNI option with an Interface Attributes sub-option. The target Client then encapsulates the MR in an OAL header and SRH extension with its own MLA as the Source Address, with the MLA of the source Client as the Destination Address and with an in-window Identification for the source Client. The target Client then performs underlay encapsulation then forwards the resulting carrier packet directly to the source Client. Following the initial MI/MR 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 MI/MR exchanges via their local Proxy/Servers to refresh NCE state as well as send additional bubbles to the peer's UNX if necessary to refresh NAT state. Note: these procedures apply 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 and/or multiple flows. It is therefore important that UNX information is maintained with the correct peer interface and that the NCE may cache information for multiple peer interfaces. Note: the source and target Client exchange UNX information during the secured MI/MR/MC multilink route optimization exchange. This allows for subsequent MI/MR/MC 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) from another Client connected to one of its downstream EUNs to a peer within the same downstream EUN, the Client returns an IPv6 ND Redirect message to inform the source that the target can be reached directly. The contents of the Redirect message are the same as specified in except that the message includes MLAs instead of LLAs, and should also include any OMNI option RIOs with MNP information corresponding to the target. In the same fashion, when a Proxy/Server forwards an OAL packet (or original IP packet) from a Client connected to one of its downstream *NETs to a peer within the same downstream *NET, the Proxy/Server returns an IPv6 ND Redirect message. All other route optimization functions are conducted per the MI/MR/MC messaging discussed in the previous sections.

The address resolution procedures discussed in previous sections are based on the exchange of NS/NA(AR) messages the same as for any link and apply for bounded-scale MLA address and/or FNP/MNP prefix populations. When the number of deployed addresses/prefixes exceeds the scaling capacity of the AERO BGP routing system, an alternate address resolution service may be needed for some or all target addresses. The FHS Proxy/Server engages such alternate services based on the Target Address found in a Client's NS(AR). For targets that are covered by BGP routing system prefixes, the FHS Proxy/Server forwards the NS(AR) into the routing system as above and expects to receive a responsive NA(AR). For targets that match prefixes beyond the BGP routing system's scaling capacity, the FHS Proxy/Server instead contacts a TBD mapping system that can maintain real-time address resolution mapping information for extremely large address/prefix populations. The BGP routing system at a minimum must carry the MLAs of all Proxy/Servers and Gateways on the OMNI link, and should also carry selected (sub)sets of FNP/MNP prefixes while all other information is maintained in the TBD mapping system. When the FHS Proxy/Server process an NS(AR) received from one of its Clients and with Target Address not covered by a BGP routing system prefix, it makes a side-call to the TBD mapping system instead of forwarding the NS(AR) into the routing system. The mapping system then returns an NA(AR) that includes the same OMNI information that would have resulted from a MAP/Client response to the NS(AR). The mapping system must remember each such transaction so that it can return future mobility update messages if mapping information changes the same as a MAP Proxy/Server would do. In all other ways, address resolution is satisfied for arbitrarily large population sizes. Since the numbers of Clients may far exceed the scaling capacity of the BGP routing system, Client MLAs are an example of information that may be better maintained in the TBD mapping system. This applies especially when the shortest possible Client MLA prefix length is /128.

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 on-link destinations, NUD messaging is driven by the network layer under the same conditions as for any interface; for off-link destinations, the process is driven by the adaptation layer. The NS/NA(NUD) messages follow the same forwarding and processing rules as for address resolution and confirm that the target neighbor is still reachable over a selected underlay interface path. The messages may also update NLNCE/ALNCE state if they include address resolution information and an authentication signature. In order to test additional underlay interface paths, the adaptation layer can independently send an MI message with an SRH with an AFVI with the (I)nitialize flag set to 0 in order to receive an MR response. The MI messages may be sent while ordinary data packets are flowing either as independent messages or as pilot messages of composite packets. The MI messages follow already-established paths without updating state and therefore need not include an authentication signature but should include an OMNI Nonce sub-option. When the source receives the MR, 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 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 MAP Proxy/Servers via RS/RA exchanges to maintain the DMM profile, and the AERO routing system tracks all current Client/Proxy/Server peering relationships. MAP Proxy/Servers provide an address resolution aggregation point for their dependent Clients, while FHS Proxy/Servers provide a proxy conduit between the Client and both the MAP and OMNI link in general. Clients are responsible for maintaining neighbor relationships with their Proxy/Servers through periodic RS/RA exchanges, which also serve to confirm neighbor reachability. When a Client's underlay interface attributes change, the Client is responsible for updating the MAP 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 MI/MR/MC exchange to re-populate new AFVs in the path. Mobility management considerations are specified in the following sections.

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), it sends RS messages to its MAP Proxy/Server via new 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 FHS Proxy/Server may change due to an underlay interface connectivity change or an intentional switch to a new Proxy/Server. 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 MAP Proxy/Server. Up to MAX_RTR_SOLICITATIONS RS messages MAY be sent in parallel with sending carrier packets containing user data in case one or more RAs are lost. If all RAs are lost, the Client SHOULD re-associate with a new Proxy/Server. When a Client needs to bring new underlay interfaces into service (e.g., when it activates a new data link), it sends an RS message to the MAP Proxy/Server via a FHS Proxy/Server for the underlay interface (if necessary) with an OMNI option that includes an Interface Attributes sub-option with interface parameters and with address resolution information for the new link.

When a Client needs to deactivate an existing underlay interface, it sends an RS message toward the MAP Proxy/Server via an FHS Proxy/Server with an OMNI option with appropriate Interface Attributes values for the deactivated link. If the Client needs to send RS messages over an underlay interface other than the one being deactivated, it MUST include current Interface Attributes for the link used as the first sub-option as well as additional Interface Attributes for any underlay interfaces being deactivated as additional sub-options with ifMetric set to 'ffffffff'. The Client (or its MAP Proxy/Server) 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 NCEs. The Client may reactivate or reuse the underlay interface and/or its ifIndex at a later point in time, when it will send new RS messages to an FHS Proxy/Server with fresh interface parameters to update any neighbors. The manner in which the Client dynamically manages its local ifIndex to interface mappings is a local decision, but should not be done in a manner that could cause state inconsistencies in the network.

When a Client associates with a new MAP Proxy/Server, it sends RS messages to register its underlay interfaces with the new MAP while including the departed MAP's MLA in a Proxy/Server Control OMNI sub-option. When the new MAP Proxy/Server returns an RA message via the FHS Proxy/Server (acting as a proxy), the FHS Proxy/Server sends a uNA to the departed MAP Proxy/Server if the departed MLA is other than ::/128. Note that the FHS Proxy/Server defers the uNA transmission until after the new MAP has responded for reliability purposes; if even greater reliability is needed, the FHS Proxy/Server can instead send an NS(NUD) message to receive an NA(NUD) response. The FHS Proxy/Server sets the uNA Source Address to the MLA of the new MAP, sets the Target Address to the Client's MLA and sets the Destination Address to the MLA of the departed MAP. The FHS Proxy/Server then includes a Proxy/Server Control OMNI sub-option with the M flag set to 1 and all other flags set to 0 and with no departed addresses included. The FHS Proxy/Server then encapsulates the uNA in an OAL header with its own MLA as the Source Address and the MLA of the departed MAP as the Destination Address, then performs underlay encapsulation and forwards the resulting carrier packet via the secured spanning tree. When the departed MAP Proxy/Server receives the carrier packet, it decapsulates to obtain the uNA and examines the Proxy/Server Control sub-option M flag. If the M flag is 1 the departed MAP Proxy/Server then changes the Client's NCE state to DEPARTED, resets DepartTime and caches the new MAP Proxy/Server MLA. After a short delay (e.g., 2 seconds) the departed MAP Proxy/Server withdraws the Client's MNP(s) from the routing system. While in the DEPARTED state, the departed MAP Proxy/Server forwards any carrier packets received via the secured spanning tree destined to the Client's MNP addresses to the new MAP Proxy/Server's MLA. When DepartTime expires, the departed MAP Proxy/Server deletes the Client's NCE. Mobility events may also cause a Client to change to a new FHS Proxy/Server over a specific underlay interface at any time such that a Client RS/RA exchange over the underlay interface will engage the new FHS Proxy/Server instead of the old. The Client can arrange to inform the old FHS Proxy/Server of the departure by including a Proxy/Server Control sub-option with the MLA of the departed FHS Proxy/Server. The new FHS Proxy/Server will issue a uNA (or NS/NA) using the same procedures as outlined for the MAP above. The new FHS Proxy/Server sets the uNA Source Address its own MLA, sets the Target Address to the Client's MLA and sets the Destination Address to the MLA of the departed FHS Proxy/Server. The new FHS Proxy/Server then includes a Proxy/Server Control OMNI sub-option with the P flag set to 1 and all other flags set to 0 and with no departed addresses included. The new FHS Proxy/Server then encapsulates the uNA in an OAL header with its own MLA as the Source Address and the MLA of the old FHS Proxy/Server as the Destination Address, then performs underlay encapsulation and forwards the resulting carrier packet via the secured spanning tree. When the departed FHS Proxy/Server receives the uNA, it updates the Client's MLA to the address of the new FHS Proxy/Server and sets the NCE state to DEPARTED. The departed FHS Proxy/Server can then forward any packets it receives for the Client to its new FHS Proxy/Server. The departed FHS Proxy/Server's NCE for the Client will then naturally expire if no further RS message arrive. This can often result in successful delivery of carrier packets that would otherwise be lost temporarily due to the mobility event. Clients SHOULD NOT move rapidly between MAP Proxy/Servers in order to avoid causing excessive oscillations in the AERO routing system. Examples of when a Client might wish to change to a different MAP Proxy/Server include a MAP Proxy/Server that has become unresponsive, topological movements of significant distance, movement to a new geographic region, movement to a new OMNI link segment, etc. Note that a Client simultaneous change to both a new FHS and MAP Proxy/Server is signaled by the new FHS when it sends a uNA to both the departed FHS and MAP Proxy/Server. If both the departed FHS/MAP are one and the same, and the new FHS/MAP are one and the same, the new FHS sends a single uNA (or NS/NA) with both the M and P flags set to 1, i.e., it need not send multiple messages.

Mobile Clients (and/or their MAP Proxy/Servers) accommodate mobility and/or multilink change events by sending secured uNA messages to each neighbor that previously received address resolution information. When a node sends a uNA message to each specific neighbor on behalf of a mobile Client, it sets the IPv6 Source and Target Address to the Client's MLA then sets the Destination Address to the neighbor's MLA. The node then encapsulates the uNA in an OAL header with Source set to its own MLA and Destination set to the MLA of the neighbor. The node also includes an OMNI option with Interface Attributes and Traffic Selector sub-options for any of the mobile Client's underlay interfaces that may have changed values and includes an authentication signature if necessary. The node next sets the uNA R flag to 1, S flag to 0 and O flag to 1, then encapsulates the message in an OAL header. Following OAL and underlay encapsulation, the carrier packet 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 reliability, 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 send an MI(MM) message to solicit an MR(MM) response as discussed in . When the neighbor receives a uNA with address resolution changes, it marks any AFVs in its ALNCE for the uNA source that were established based on now-obsolete information as STALE. When the neighbor forwards the next packet for an affected flow, it initiates a new multilink forwarding exchange as specified in to refresh AFVI state for the path.

After AFV state has been established for a flow, all OAL intermediate systems in the forward path will have AFVs with header compression state and (AFVI, UNX) information for the next hop. However, paths can fluctuate due to factors such as node mobility, routing changes, network membership, etc. If an OAL intermediate system forwarding OAL packets with OCH headers detects that the next hop in the path has changed, it immediately reverts to sending the packets with header compression disabled by including full OAL IPv6 and Extended Fragment Headers (plus full original IP headers) in future packets. When the OAL destination begins to receive OAL packets with full headers (i.e., when it had previously received OCH compressed headers), it assumes that the network path for this flow has changed and begins sending MC messages to the OAL source. The OAL destination sends the MC messages subject to rate limiting, and includes a Neighbor Synchronization OMNI sub-option with both the ACK and RST flags set and with the most recent OAL packet Identification recorded in the Acknowledgment field. When the OAL source receives the MC messages with ACK and RST set, it re-initiates multilink forwarding for this flow by issuing a new MI/MR/MC or unidirectional MC exchange the same as for a new flow as specified in . The AFV state in the former path then simply expires and is soon purged by the former OAL intermediate nodes.

Each Client provides an IGMP (IPv4) or MLD (IPv6) proxy service for its EUNs 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 (M)ANET underlay interfaces for which the *NET has deployed native multicast services forward IGMP/MLD messages into the *NET. The IGMP/MLD messages may be further forwarded by a first-hop *NET access router acting as an IGMP/MLD-snooping switch , then ultimately delivered to a *NET (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 *NET 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 . The same as for unicast destinations, the 3-tuple of Source Address, Destination Address and Flow Label identifies a flow for multicast group G. For each S belonging to a prefix reachable via X's OMNI interface, X sends an NS(AR) message (see: ) 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, Traffic Selectors and RIOs for S. When X processes the NA(AR) it selects one or more underlay interfaces for S and performs an MI/MR/MC multilink forwarding exchange over the secured spanning tree while including a PIM Join/Prune message OMNI sub-option for each multicast group of interest. If S is located behind any Proxys "Z"*, each Z* then updates its MRIB accordingly and maintains the FNP/MNP Source Address 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 MI/MR/MC 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 MI/MR/MC 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 an MC message to X the same as specified for unicast mobility in . When X receives the MC message, it updates its NCE for the MLA for source S and sends new Join messages in MI/MR/MC 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 MI/MR/MC message, then encapsulates the MI/MR/MC message in an OAL header and 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 in PIM Register messages, includes the PIM Register messages in the OMNI options of MC messages, performs OAL encapsulation and fragmentation with Identification values within the receive window for Client R* that aggregates R, then performs underlay encapsulation and forwards the resulting carrier packets. Client R* may then elect to send a PIM Join to S* in the OMNI option of a MC 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; one native copy from the (S, G) tree and a second copy from the pre-existing (*, G) tree that still uses MC PIM Register encapsulation. R can then issue an MC with a 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 via MC 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 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 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 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 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 FNPs 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/EUNs 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 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.