Asymmetric Extended Route Optimization (AERO)

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

AERO Clients are Mobile Nodes (MNs) that configure OMNI interfaces over underlying interfaces with addresses that may change when the Client moves to a new network connection point. AERO Clients register their Mobile Network Prefixes (MNPs) with the AERO service, and distribute the MNPs to nodes on EUNs. AERO Bridges, Servers, Proxys and Relays are critical infrastructure elements in fixed (i.e., non-mobile) INET deployments and hence have permanent and unchanging INET addresses. Together, they constitute the AERO service which provides an OMNI link virtual overlay for connecting AERO Clients. AERO Bridges provide hybrid routing/bridging services (as well as a security trust anchor) for nodes on an OMNI link. Bridges use standard IPv6 routing to forward packets both within the same INET partition and between disjoint INET partitions based on an IPv6 encapsulation mid-layer known as the OMNI Adaptation Layer (OAL) . During forwarding, the inner IP layer experiences a virtual bridging service since the inner IP TTL/Hop Limit is not decremented. Each Bridge also peers with Servers and other Bridges in a dynamic routing protocol instance to provide a Distributed Mobility Management (DMM) service for the list of active MNPs (see ). Bridges present the OMNI link as a set of one or more Mobility Service Prefixes (MSPs) and configure secured tunnels with Servers, Relays, Proxys and other Bridges; they further maintain IP forwarding table entries for each MNP and any other reachable non-MNP prefixes. AERO Servers in distributed INET locations provide default forwarding and mobility/multilink services for AERO Client Mobile Nodes (MNs). Each Server also peers with Bridges in a dynamic routing protocol instance to advertise its list of associated MNPs (see ). Servers facilitate prefix delegation/registration exchanges with Clients, where each delegated prefix becomes an MNP taken from an MSP. Servers forward packets between OMNI interface neighbors and track each Client's mobility profiles. Servers may further act as Servers for some sets of Clients and as Proxys for others. AERO Proxys provide a conduit for ANET Clients to associate with Servers in external INETs. Client and Servers exchange control plane messages via the Proxy acting as a bridge between the ANET/INET boundary. The Proxy forwards data packets between Clients and the OMNI link according to forwarding information in the neighbor cache. The Proxy function is specified in . Proxys may further act as Proxys for some sets of Clients and as Servers for others. AERO Relays are Servers that provide forwarding services between the OMNI interface and INET/EUN interfaces. Relays are provisioned with MNPs the same as for an AERO Client, and also run a dynamic routing protocol to discover any non-MNP IP routes. The Relay advertises the MSP(s) to its connected networks, and distributes all of its associated MNPs and non-MNP IP GUA routes via BGP peerings with Bridges

presents the basic OMNI link reference model:

S1; X2->S2)| | MSP M1 | +-+---------+--+-+ +--------------+ | Secured | | +--------------+ |AERO Server S1| | tunnels | | |AERO Server S2| | Nbr: C1, B1 +-----+ | +-----+ Nbr: C2, B1 | | default->B1 | | | default->B1 | | X1->C1 | | | X2->C2 | +-------+------+ | +------+-------+ | OMNI link | | X===+===+===================+==)===============+===+===X | | | | +-----+--------+ +--------+--+-----+ +--------+-----+ |AERO Client C1| | AERO Proxy P1 | |AERO Client C2| | Nbr: S1 | |(Proxy Nbr Cache)| | Nbr: S2 | | default->S1 | +--------+--------+ | default->S2 | | MNP X1 | | | MNP X2 | +------+-------+ .--------+------. +-----+--------+ | (- Proxyed Clients -) | .-. `---------------' .-. ,-( _)-. ,-( _)-. .-(_ IP )-. +-------+ +-------+ .-(_ IP )-. (__ EUN )--|Host H1| |Host H2|--(__ EUN ) `-(______)-' +-------+ +-------+ `-(______)-' ]]> In this model: the OMNI link is an overlay network service configured over one or more underlying INET partitions which may be managed by different administrative authorities and have incompatible protocols and/or addressing plans. AERO Bridge B1 aggregates Mobility Service Prefix (MSP) M1, discovers Mobile Network Prefixes (MNPs) X* and advertises the MSP via BGP peerings over secured tunnels to Servers (S1, S2). Bridges connect the disjoint segments of a partitioned OMNI link. AERO Servers/Relays S1 and S2 configure secured tunnels with Bridge B1 and also provide mobility, multilink and default router services for their associated Clients C1 and C2. AERO Clients C1 and C2 associate with Servers S1 and S2, respectively. They receive Mobile Network Prefix (MNP) delegations X1 and X2, and also act as default routers for their associated physical or internal virtual EUNs. Simple hosts H1 and H2 attach to the EUNs served by Clients C1 and C2, respectively. AERO Proxy P1 configures a secured tunnel with Bridge B1 and provides proxy services for AERO Clients in secured enclaves that cannot associate directly with other OMNI link neighbors. An OMNI link configured over a single INET appears as a single unified link with a consistent underlying network addressing plan. In that case, all nodes on the link can exchange packets via simple INET encapsulation, since the underlying INET is connected. In common practice, however, an OMNI link may be partitioned into multiple "segments", where each segment is a distinct INET potentially managed under a different administrative authority (e.g., as for worldwide aviation service providers such as ARINC, SITA, Inmarsat, etc.). Individual INETs may also themselves be partitioned internally, in which case each internal partition is seen as a separate segment. The addressing plan of each 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 in many cases disjoint segments may not even have any common physical link connections. Therefore, nodes can only be assured of exchanging 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 Bridges. The same as for traditional campus LANs, multiple OMNI link segments can be joined into a single unified link via a virtual bridging service using the OMNI Adaptation Layer (OAL) which inserts a mid-layer IPv6 encapsulation header that supports inter-segment forwarding (i.e., bridging) without decrementing the network-layer TTL/Hop Limit. This bridging of OMNI link segments is shown in :

. . . . . . . . . . . . . . .. . . . . . . . . ]]> Bridges, Servers, Relays and Proxys connect via secured INET tunnels over their respective segments in a spanning tree topology rooted at the Bridges. The secured spanning tree supports strong authentication for IPv6 ND control messages and may also be used to convey the initial data packets in a flow. Route optimization can then be employed to cause data packets to take more direct paths between OMNI link neighbors without having to strictly follow the spanning tree.

AERO nodes on OMNI links use the Link-Local Address (LLA) prefix fe80::/64 to assign LLAs used for network-layer addresses in link-scoped IPv6 ND and data messages. AERO Clients use LLAs constructed from MNPs (i.e., "MNP-LLAs") while other AERO nodes use LLAs constructed from administrative identification values ("ADM-LLAs") as specified in . AERO nodes also use the Unique Local Address (ULA) prefix fd00::/8 followed by a pseudo-random 40-bit OMNI domain identifier to form the prefix [ULA]::/48, then include a 16-bit OMNI link identifier '*' to form the prefix [ULA*]::/64 . The AERO node then uses the prefix [ULA*]::/64 to form "MNP-ULAs" or "ADM-ULA"s as specified in to support OAL addressing. AERO Clients also use Temporary ULAs constructed per , where the addresses are typically used only in initial control message exchanges until a stable MNP-LLA/ULA is assigned. AERO MSPs and MNPs are typically based on Global Unicast Addresses (GUAs), but in some cases may be based on private-use addresses. See for a full specification of LLAs, ULAs and GUAs used by AERO nodes on OMNI links. Finally, AERO Clients and Servers configure node identification values as specified in .

The AERO routing system comprises a private instance of the Border Gateway Protocol (BGP) that is coordinated between Bridges and Servers and does not interact with either the public Internet BGP routing system or any underlying INET routing systems. In a reference deployment, each 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 Server further uses eBGP to peer with one or more Bridges but does not peer with other Servers. Each INET of a multi-segment OMNI link must include one or more Bridges, which peer with the Servers and Proxys within that INET. All Bridges within the same INET are members of the same hub AS, and use iBGP to maintain a consistent view of all active MNPs currently in service. The Bridges of different INETs peer with one another using eBGP. Bridges advertise the OMNI link's MSPs and any non-MNP routes to each of their Servers. This means that any aggregated non-MNPs (including "default") are advertised to all Servers. Each Bridge configures a black-hole route for each of its MSPs. By black-holing the MSPs, the Bridge will maintain forwarding table entries only for the MNPs that are currently active, and packets destined to all other MNPs will correctly incur Destination Unreachable messages due to the black-hole route. In this way, Servers have only partial topology knowledge (i.e., they know only about the MNPs of their directly associated Clients) and they forward all other packets to Bridges which have full topology knowledge. Each OMNI link segment assigns a unique ADM-ULA sub-prefix of [ULA*]::/96. For example, a first segment could assign [ULA*]::1000/116, a second could assign [ULA*]::2000/116, a third could assign [ULA*]::3000/116, etc. Within each segment, each Proxy/Server configures an ADM-ULA within the segment's prefix, e.g., the Proxy/Servers within [ULA*]::2000/116 could assign the ADM-ULAs [ULA*]::2011/116, [ULA*]::2026/116, [ULA*]::2003/116, etc. The administrative authorities for each segment must therefore coordinate to assure mutually-exclusive ADM-ULA prefix assignments, but internal provisioning of ADM-ULAs an independent local consideration for each administrative authority. For each ADM-ULA prefix, the Bridge(s) that connect that segment assign the all-zero's address of the prefix as a Subnet Router Anycast address. For example, the Subnet Router Anycast address for [ULA*]::1023/116 is simply [ULA*]::1000. ADM-ULA prefixes are statically represented in Bridge forwarding tables. Bridges join multiple segments into a unified OMNI link over multiple diverse administrative domains. They support a bridging function by first establishing forwarding table entries for their ADM-ULA prefixes either via standard BGP routing or static routes. For example, if three Bridges ('A', 'B' and 'C') from different segments serviced [ULA*]::1000/116, [ULA*]::2000/116 and [ULA*]::3000/116 respectively, then the forwarding tables in each Bridge are as follows: [ULA*]::1000/116->local, [ULA*]::2000/116->B, [ULA*]::3000/116->C [ULA*]::1000/116->A, [ULA*]::2000/116->local, [ULA*]::3000/116->C [ULA*]::1000/116->A, [ULA*]::2000/116->B, [ULA*]::3000/116->local These forwarding table entries are permanent and never change, since they correspond to fixed infrastructure elements in their respective segments. MNP ULAs are instead dynamically advertised in the AERO routing system by Servers and Relays that provide service for their corresponding MNPs. For example, if three Servers ('D', 'E' and 'F') service the MNPs 2001:db8:1000:2000::/56, 2001:db8:3000:4000::/56 and 2001:db8:5000:6000::/56 then the routing system would include: [ULA*]:2001:db8:1000:2000/120 [ULA*]:2001:db8:3000:4000/120 [ULA*]:2001:db8:5000:6000/120 A full discussion of the BGP-based routing system used by AERO is found in .

With the Client and partition prefixes in place in Bridge forwarding tables, the OMNI interface sends control and data packets toward AERO destination nodes located in the same segment using simple INET encapsulation without using the spanning tree. For destinations located in different OMNI link segments, the OMNI interface instead sends packets over the spanning tree via mid-layer encapsulation using the IPv6 OMNI Adaptation Layer (OAL) header . When necessary, the OMNI interface also includes an OMNI Routing Header (ORH) as an extension to the OAL header if final segment forwarding information is available, e.g., in the neighbor cache. The ORH is formatted as shown in :

In this format: Next Header identifies the type of header immediately following the ORH. Hdr Ext Len is the length of the Routing header in 8-octet units (not including the first 8 octets), with trailing padding added if necessary to produce an integral number of 8-octet units. Routing Type is set to TBD (see IANA Considerations). Segments Left is omitted, and replaced by a 5-bit SRT and 3-bit FMT field. SRT - a 5-bit Segment Routing Topology prefix length value that (when added to 96) determines the prefix length to apply to the ADM-ULA formed from concatenating [ULA*]::/96 with the 32 bit LHS value that follows. For example, the value 16 corresponds to the prefix length 112. FMT - a 3-bit "Framework/Mode/Type" code corresponding to the included Link Layer Address as follows: When the most significant bit (i.e., "Framework") is set to 1, L2ADDR is the INET encapsulation address for the target Client itself; otherwise L2ADDR is the address of the Server/Proxy named in the LHS. When the next most significant bit (i.e., "Mode") is set to 1, the Framework node is (likely) located behind an INET Network Address Translator (NAT); otherwise, it is on the open INET. When the least significant bit (i.e., "Type") is set to 0, L2ADDR includes a UDP Port Number followed by an IPv4 address; otherwise, it includes a UDP Port Number followed by an IPv6 address. LHS - the 32 bit ID of a node in the Last Hop Segment that services the target. When SRT and LHS are both set to 0, the LHS is considered unspecified. When SRT is set to 0 and LHS is non-zero, the prefix length is set to 128. SRT and LHS provide guidance to the OMNI interface forwarding algorithm. Specifically, if SRT/LHS is located in the local OMNI link segment then the OMNI interface can omit OAL/ORH encapsulation and (following any necessary NAT traversal messaging) send directly to the target using INET encapsulation according to FMT/L2ADDR. Otherwise, it must perform INET/OAL/ORH encapsulation and forward according to the OMNI link spanning tree. Link Layer Address (L2ADDR) - Formatted according to FMT, and identifies the link-layer address (i.e., the encapsulation address) of the target. The UDP Port Number appears in the first two octets and the IP address appears in the next 4 octets for IPv4 or 16 octets for IPv6. The Port Number and IP address are recorded in network byte order, and in ones-compliment "obfuscated" form per . The OMNI interface forwarding algorithm uses FMT/L2ADDR to determine the INET encapsulation address for local forwarding when SRT/LHS is located in the same OMNI link segment. Note that if the target is behind a NAT, L2ADDR will contain the mapped INET address stored in the NAT; otherwise, L2ADDR will contain the native INET information of the target itself. Destination Suffix is a 64-bit field included only for OAL-encapsulated packets that require OAL fragmentation. Present only when Hdr Ext Len indicates that at least 8 bytes follow L2ADDR. When present, encodes the 64-bit MNP-ULA suffix for the target Client. For example, if the MNP-ULA is [ULA*]:2001:db8:1:2 then the Destination suffix encodes the value 2001:db8:1:2. Null Padding contains zero-valued octets as necessary to pad the ORH to an integral number of 8-octet units. AERO nodes use OAL encapsulation when the target destination is located in a different OMNI link segment, or when OAL fragmentation is necessary (otherwise, OAL encapsulation is omitted and simple INET encapsulation is used). When an AERO node uses OAL encapsulation for a packet with source address 2001:db8:1:2::1 and destination address 2001:db8:1234:5678::1, it sets the OAL header source address to its own ULA (e.g., [ULA*]::1000:2000) and sets the destination address to the MNP-ULA corresponding to the IP destination address (e.g., [ULA*]::2001:db8:1234:5678). If the neighbor cache information indicates that the target is in a different segment, the node next inserts an ORH immediately following the OAL header while including the correct SRT, FMT, LHS, L2ADDR and Destination Suffix if fragmentation is needed (in this case, the Destination Suffix is 2001:db8:1234:5678). Next, the node overwrites the OAL header destination address with the LHS Subnet Router Anycast address (for example, for LHS 3000:4567 with SRT 16, the Subnet Router Anycast address is [ULA*]::3000:0000). (Note: if ADM-ULA of the last-hop Proxy/Server is known but the SRT, FMT, LHS and L2ADDR are not (yet) known, the node instead overwrites the OAL header destination address with the ADM-ULA.) The node then fragments the OAL/ORH packet if necessary, with each resulting fragment including the OAL/ORH headers while only the first fragment includes the original IPv6 header. The node finally encapsulates each resulting OAL/ORH packet/fragment in an INET header with source address set to its own INET address (e.g., 192.0.2.100) and destination set to the INET address of a Bridge (e.g., 192.0.2.1). The encapsulation format in the above example is shown in :

In this format, the inner IP header and packet body are the original IP packet, the OAL header is an IPv6 header prepared according to , the ORH is a Routing Header extension of the OAL header, and the INET header is prepared as discussed in . This gives rise to a routing system that contains both Client prefix routes that may change dynamically due to regional node mobility and partition prefix routes that rarely if ever change. The Bridges can therefore provide link-layer bridging by sending packets over the spanning tree instead of network-layer routing according to MNP routes. As a result, opportunities for packet loss due to node mobility between different segments are mitigated. In normal operations, IPv6 ND messages are conveyed over secured paths between OMNI link neighbors so that specific Proxys, Servers or Relays can be addressed without being subject to mobility events. Conversely, only the first few packets destined to Clients need to traverse secured paths until route optimization can determine a more direct path. Note: When the source OAL has multiple inner IP packets to send to the same neighbor, it can perform "packing" to generate a "super-packet" as discussed in Section 5.2 of . In that case, the OAL/ORH super-packet may include up to N inner packets as long as the total length of the super-packet does not exceed the OMNI interface MTU. Note: Use of an IPv6 "minimal encapsulation" format (i.e., an IPv6 variant of ) based on extensions to the ORH was considered and abandoned. In the approach, the ORH would be inserted as an extension header to the original IPv6 packet header. The IPv6 destination address would then be written into the ORH, and the ULA corresponding to the destination would be overwritten in the IPv6 destination address. This would seemingly convey enough forwarding information so that OAL encapsulation could be avoided. However, this "minimal encapsulation" IPv6 packet would then have a non-ULA source address and ULA destination address, an incorrect value in upper layer protocol checksums, and a Hop Limit that is decremented within the spanning tree when it should not be. The insertion and removal of the ORH would also entail rewriting the Payload Length and Next Header fields - again, invalidating upper layer checksums. These irregularities would result in implementation challenges and the potential for operational issues, e.g., since actionable ICMPv6 error reports could not be delivered to the original source. In order to address the issues, still more information such as the original IPv6 source address could be written into the ORH. However, with the additional information the benefit of the "minimal encapsulation" savings quickly diminishes, and becomes overshadowed by the implementation and operational irregularities.

The 64-bit sub-prefixes of [ULA]::/48 identify up to 2^16 distinct Segment Routing Topologies (SRTs). Each SRT is a mutually-exclusive OMNI link overlay instance using a distinct set of ULAs, and emulates a Virtual LAN (VLAN) service for the OMNI link. In some cases (e.g., when redundant topologies are needed for fault tolerance and reliability) it may be beneficial to deploy multiple SRTs that act as independent overlay instances. A communication failure in one instance therefore will not affect communications in other instances. Each SRT is identified by a distinct value in bits 48-63 of [ULA]::/48, i.e., as [ULA0]::/64, [ULA1]::/64, [ULA2]::/64, etc. Each OMNI interface is identified by a unique interface name (e.g., omni0, omni1, omni2, etc.) and assigns an anycast ADM-ULA corresponding to its SRT prefix length. The anycast ADM-ULA is used for OMNI interface determination in Safety-Based Multilink (SBM) as discussed in . Each OMNI interface further applies Performance-Based Multilink (PBM) internally.

An original IPv6 source can direct an IPv6 packet to an AERO node by including a standard IPv6 Segment Routing Header (SRH) with the anycast ADM-ULA for the selected SRT as either the IPv6 destination or as an intermediate hop within the SRH. This allows the original source to determine the specific OMNI link topology a packet will traverse when there may be multiple alternatives. When the AERO node processes the SRH and forwards the packet to the correct OMNI interface, the OMNI interface writes the next IPv6 address from the SRH into the IPv6 destination address and decrements Segments Left. If decrementing would cause Segments Left to become 0, the OMNI interface deletes the SRH before forwarding. This form of Segment Routing supports Safety-Based Multilink (SBM).

AERO node OMNI interfaces can insert OAL/ORH headers for Segment Routing within the OMNI link to influence the paths of packets destined to targets in remote segments without requiring all packets to traverse strict spanning tree paths. When an AERO node's OMNI interface has a packet to send to a target discovered through route optimization located in the same OMNI link segment, it encapsulates the packet in OAL/ORH headers if necessary as discussed above. The node then uses the target's Link Layer Address (L2ADDR) information for INET encapsulation. When an AERO node's OMNI interface has a packet to send to a route optimization target located in a remote OMNI link segment, it encapsulates the packet in OAL/ORH headers as discussed above while forwarding the packet to a Bridge with destination set to the Subnet Router Anycast address for the final OMNI link segment. When a Bridge receives a packet destined to its Subnet Router Anycast address with an OAL/ORH with SRT/LHS values corresponding to the local segment, it examines the L2ADDR according to FMT and removes the ORH from the packet; if the packet is not a fragment, the Bridge also removes the OAL header. If the packet was a fragment, the Bridge instead writes the MNP-ULA corresponding to the ORH Destination Suffix into the OAL destination address. The Bridge then encapsulates the packet/fragment in an INET header according to L2ADDR and forwards the packet within the INET either to the LHS Server/Proxy or directly to the target Client itself. In this way, the Bridge participates in route optimization to reduce traffic load and suboptimal routing through strict spanning tree paths.

OMNI interfaces are virtual interfaces configured over one or more underlying interfaces classified as follows: INET interfaces connect to an INET either natively or through one or several NATs. Native INET interfaces have global IP addresses that are reachable from any INET correspondent. All Server, Relay and Bridge interfaces are native interfaces, as are INET-facing interfaces of Proxys. NATed INET interfaces connect to a private network behind one or more NATs that provide INET access. Clients that are behind a NAT are required to send periodic keepalive messages to keep NAT state alive when there are no data packets flowing. ANET interfaces connect to an ANET that is separated from the open INET by a Proxy. Proxys can actively issue control messages over the INET on behalf of the Client to reduce ANET congestion. VPNed interfaces use security encapsulation over the INET to a Virtual Private Network (VPN) server that also acts as a Server or Proxy. Other than the link-layer encapsulation format, VPNed interfaces behave the same as Direct interfaces. Direct (i.e., single-hop point-to-point) interfaces connect a Client directly to a Server or Proxy without crossing any ANET/INET paths. An example is a line-of-sight link between a remote pilot and an unmanned aircraft. The same Client considerations apply as for VPNed interfaces. OMNI interfaces use OAL/ORH encapsulation as necessary as discussed in . OMNI interfaces use link-layer encapsulation (see: ) to exchange packets with OMNI link neighbors over INET or VPNed interfaces as well as over ANET interfaces for which the Client and Proxy may be multiple IP hops away. OMNI interfaces do not use link-layer encapsulation over Direct underlying interfaces or ANET interfaces when the Client and Proxy are known to be on the same underlying link. OMNI interfaces maintain a neighbor cache for tracking per-neighbor state the same as for any interface. OMNI interfaces use ND messages including Router Solicitation (RS), Router Advertisement (RA), Neighbor Solicitation (NS) and Neighbor Advertisement (NA) for neighbor cache management. OMNI interfaces send ND messages with an OMNI option formatted as specified in . The OMNI option includes prefix registration information and Interface Attributes containing link information parameters for the OMNI interface's underlying interfaces. Each OMNI option may include multiple Interface Attributes sub-options, each identified by an ifIndex value. A Client's OMNI interface may be configured over multiple underlying interface connections. 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 underlying interfaces are used "one at a time" (i.e., all other interfaces are in standby mode while one interface is active), then ND message OMNI options include only a single Interface Attributes sub-option set to constant values. In that case, the Client would appear to have a single interface but with a dynamically changing link-layer address. If the Client has multiple active underlying interfaces, then from the perspective of ND it would appear to have multiple link-layer addresses. In that case, ND message OMNI options MAY include multiple Interface Attributes sub-options - each with values that correspond to a specific interface. Every ND message need not include Interface Attributes for all underlying interfaces; for any attributes not included, the neighbor considers the status as unchanged. Bridge, Server and Proxy OMNI interfaces may be configured over one or more secured tunnel interfaces. The OMNI interface configures both an ADM-LLA and its corresponding ADM-ULA, while the underlying secured tunnel interfaces are either unnumbered or configure the same ULA. The OMNI interface encapsulates each IP packet in OAL/ORH headers and presents the packet to the underlying secured tunnel interface. Routing protocols such as BGP that run over the OMNI interface do not employ OAL/ORH encapsulation, but rather present the routing protocol messages directly to the underlying secured tunnels while using the ULA as the source address. This distinction must be honored consistently according to each node's configuration so that the IP forwarding table will associate discovered IP routes with the correct interface.

AERO Servers, Proxys 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 packets with destination addresses covered by an MNP not explicitly assigned to a non-OMNI interface are directed to the OMNI interface. OMNI interface initialization procedures for Servers, Proxys, Clients and Bridges are discussed in the following sections.

When a Server enables an OMNI interface, it assigns an ADM-{LLA,ULA} appropriate for the given OMNI link segment. The Server also configures secured tunnels with one or more neighboring Bridges and engages in a BGP routing protocol session with each Bridge. The OMNI interface provides a single interface abstraction to the IP layer, but internally comprises multiple secured tunnels as well as an NBMA nexus for sending encapsulated data packets to OMNI interface neighbors. The Server further configures a service to facilitate ND exchanges with AERO Clients and manages per-Client neighbor cache entries and IP forwarding table entries based on control message exchanges. Relays are simply Servers that run a dynamic routing protocol to redistribute routes between the OMNI interface and INET/EUN interfaces (see: ). The Relay provisions MNPs to networks on the INET/EUN interfaces (i.e., the same as a Client would do) and advertises the MSP(s) for the OMNI link over the INET/EUN interfaces. The Relay further provides an attachment point of the OMNI link to a non-MNP-based global topology.

When a Proxy enables an OMNI interface, it assigns an ADM-{LLA, ULA} and configures permanent neighbor cache entries the same as for Servers. The Proxy also configures secured tunnels with one or more neighboring Bridges and maintains per-Client neighbor cache entries based on control message exchanges. Importantly Proxys are often configured to act as Servers, and vice-versa.

When a Client enables an OMNI interface, it assigns either an MNP-{LLA, ULA} or a Temporary ULA and sends RS messages with ND parameters over its underlying interfaces to a Server, which returns an RA message with corresponding parameters. The RS/RA messages may pass through a Proxy in the case of a Client's ANET interface, or through one or more NATs in the case of a Client's INET interface. (Note: if the Client used a Temporary ULA in its initial RS message, it will discover an MNP-{LLA, ULA} in the corresponding RA that it receives from the Server and begin using these new addresses while deprecating the Temporary addresses. If the Client is operating outside the context of AERO infrastructure, however, it may continue using Temporary ULAs for Client-to-Client communications until it encounters an infrastructure element that can provide an MNP.)

AERO Bridges configure an OMNI interface and assign the ADM-ULA Subnet Router Anycast address for each OMNI link segment they connect to. Bridges configure secured tunnels with Servers, Proxys and other Bridges, and engage in a BGP routing protocol session with neighbors on the spanning tree (see: ).

Each OMNI interface maintains a conceptual neighbor cache that includes an entry for each neighbor it communicates with on the OMNI link per . OMNI interface neighbor cache entries are said to be one of "permanent", "symmetric", "asymmetric" or "proxy". Permanent neighbor cache entries are created through explicit administrative action; they have no timeout values and remain in place until explicitly deleted. AERO Bridges maintain permanent neighbor cache entries for their associated Proxys/Servers (and vice-versa). Each entry maintains the mapping between the neighbor's network-layer LLA and corresponding INET address. Symmetric neighbor cache entries are created and maintained through RS/RA exchanges as specified in , and remain in place for durations bounded by prefix lifetimes. AERO Servers maintain symmetric neighbor cache entries for each of their associated Clients, and AERO Clients maintain symmetric neighbor cache entries for each of their associated Servers. Asymmetric neighbor cache entries are created or updated based on route optimization messaging as specified in , and are garbage-collected when keepalive timers expire. AERO ROSs maintain asymmetric neighbor cache entries for active targets with lifetimes based on ND messaging constants. Asymmetric neighbor cache entries are unidirectional since only the ROS (and not the ROR) creates an entry. Proxy neighbor cache entries are created and maintained by AERO Proxys when they process Client/Server ND exchanges, and remain in place for durations bounded by ND and prefix lifetimes. AERO Proxys maintain proxy neighbor cache entries for each of their associated Clients. Proxy neighbor cache entries track the Client state and the address of the Client's associated Server(s). To the list of neighbor cache entry states in Section 7.3.2 of , Proxy and Server OMNI interfaces add an additional state DEPARTED that applies to symmetric and proxy neighbor cache entries for Clients that have recently departed. The interface sets a "DepartTime" variable for the neighbor cache entry to "DEPART_TIME" seconds. DepartTime is decremented unless a new ND message causes the state to return to REACHABLE. While a neighbor cache entry is in the DEPARTED state, packets destined to the target Client are forwarded to the Client's new location instead of being dropped. When DepartTime decrements to 0, the neighbor cache entry is deleted. It is RECOMMENDED that DEPART_TIME be set to the default constant value REACHABLE_TIME plus 10 seconds (40 seconds by default) to allow a window for packets in flight to be delivered while stale route optimization state may be present. When an ROR receives an authentic NS message used for route optimization, it searches for a symmetric neighbor cache entry for the target Client. The ROR then returns a solicited NA message without creating a neighbor cache entry for the ROS, but creates or updates a target Client "Report List" entry for the ROS and sets a "ReportTime" variable for the entry to REPORT_TIME seconds. The ROR resets ReportTime when it receives a new authentic NS message, and otherwise decrements ReportTime while no authentic NS messages have been received. It is RECOMMENDED that REPORT_TIME be set to the default constant value REACHABLE_TIME plus 10 seconds (40 seconds by default) to allow a window for route optimization to converge before ReportTime decrements below REACHABLE_TIME. When the ROS receives a solicited NA message response to its NS message used for route optimization, it creates or updates an asymmetric neighbor cache entry for the target network-layer and link-layer addresses. The ROS then (re)sets ReachableTime for the neighbor cache entry to REACHABLE_TIME seconds and uses this value to determine whether packets can be forwarded directly to the target, i.e., instead of via a default route. The ROS otherwise decrements ReachableTime while no further solicited NA messages arrive. It is RECOMMENDED that REACHABLE_TIME be set to the default constant value 30 seconds as specified in . AERO nodes also use the value MAX_UNICAST_SOLICIT to limit the number of NS keepalives 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 unsolicited NAs 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 DEPART_TIME, REPORT_TIME, REACHABLE_TIME, MAX_UNICAST_SOLICIT, MAX_RTR_SOLCITATIONS and MAX_NEIGHBOR_ADVERTISEMENT MAY be administratively set; however, if different values are chosen, all nodes on the link MUST consistently configure the same values. Most importantly, DEPART_TIME and REPORT_TIME SHOULD be set to a value that is sufficiently longer than REACHABLE_TIME to avoid packet loss due to stale route optimization state.

OMNI interface IPv6 ND messages include OMNI options with Interface Attributes that provide Link-Layer Address and QoS Preference information for the neighbor's underlying interfaces. This information is stored in the neighbor cache and provides the basis for the forwarding algorithm specified in . The information is cumulative and reflects the union of the OMNI information from the most recent ND messages received from the neighbor; it is therefore not required that each ND message contain all neighbor information. The OMNI option Interface Attributes for each underlying interface includes a two-part "Link-Layer Address" consisting of a simple IP encapsulation address determined by the FMT and L2ADDR fields and an ADM-ULA determined by the SRT and LHS fields. If the neighbor is located in the local OMNI link segment (and, if any necessary NAT state has been established) forwarding via simple IP encapsulation can be used; otherwise, OAL encapsulation must be used. Underlying interfaces are further selected based on their associated preference values "high", "medium", "low" or "disabled". Note: the OMNI option is distinct from any Source/Target Link-Layer Address Options (S/TLLAOs) that may appear in an ND message according to the appropriate IPv6 over specific link layer specification (e.g., ). If both an OMNI option and S/TLLAO appear, the former pertains to encapsulation addresses while the latter pertains to the native L2 address format of the underlying media.

As discussed in Section 4.4 of NA messages include three flag bits R, S and O. OMNI interface NA messages treat the flags as follows: R: The R ("Router") flag is set to 1 in the NA messages sent by all AERO/OMNI node types. Simple hosts that would set R to 0 do not occur on the OMNI link itself, but may occur on the downstream links of Clients and Relays. S: The S ("Solicited") flag is set exactly as specified in Section 4.4. of , i.e., it is set to 1 for Solicited NAs and set to 0 for Unsolicited NAs (both unicast and multicast). O: The O ("Override") flag is set to 0 for solicited proxy NAs and set to 1 for all other solicited and unsolicited NAs. For further study is whether solicited NAs for anycast targets apply for OMNI links. Since MNP-LLAs must be uniquely assigned to Clients to support correct ND protocol operation, however, no role is currently seen for assigning the same MNP-LLA to multiple Clients.

The OMNI Adaptation Layer (OAL) inserts mid-layer IPv6 headers known as the OAL/ORH headers when necessary as discussed in the following sections. After either inserting or omitting the OAL/ORH headers, the OMNI interface also inserts or omits an outer ANET/INET encapsulation header as discussed below. OMNI interfaces avoid outer encapsulation over Direct underlying interfaces and ANET underlying interfaces for which the Client and Proxy are connected to the same underlying link. Otherwise, OMNI interfaces encapsulate packets according to whether they are entering the OMNI interface from the network layer or if they are being re-admitted into the same OMNI link they arrived on. This latter form of encapsulation is known as "re-encapsulation". For packets entering the OMNI interface from the network layer, the OMNI interface copies the "TTL/Hop Limit", "Type of Service/Traffic Class" , "Flow Label" (for IPv6) and "Congestion Experienced" values in the inner packet's IP header into the corresponding fields in the OAL and outer encapsulation header(s). For packets undergoing re-encapsulation, the OMNI interface instead copies these values from the original encapsulation header into the new encapsulation header, i.e., the values are transferred between encapsulation headers and *not* copied from the encapsulated packet's network-layer header. (Note especially that by copying the TTL/Hop Limit between encapsulation headers the value will eventually decrement to 0 if there is a (temporary) routing loop.) OMNI interfaces configured over ANET underlying interfaces which employ a different IP protocol version (and/or when the Client and Proxy may be separated by multiple ANET IP hops) use IP-in-IP encapsulation so that the inner packet can traverse the ANET without decrementing the TTL/Hop-Limit. IPv6 underlying ANET interfaces use encapsulation, while IPv4 interfaces use the appropriate encapsulation per one of . OMNI interfaces configured over INET underlying interfaces encapsulate packets in INET headers according to the next hop determined in the forwarding algorithm in . If the next hop is reached via a secured tunnel, the OMNI interface uses an encapsulation format specific to the secured tunnel type (see: ). If the next hop is reached via an unsecured INET interface, the OMNI interface instead uses UDP/IP encapsulation per . When UDP/IP encapsulation is used, the OMNI interface next sets the UDP source port to a constant value that it will use in each successive packet it sends, and sets the UDP length field to the length of the encapsulated packet plus 8 bytes for the UDP header itself plus the length of any included extension headers or trailers. The encapsulated packet may be either IPv6 or IPv4, as distinguished by the version number found in the first four bits following the UDP header. For UDP/IP-encapsulated packets sent to a Server, Relay or Bridge, the OMNI interface sets the UDP destination port to 8060, i.e., the IANA-registered port number for AERO. For packets sent to a Client, the OMNI interface sets the UDP destination port to the port value stored in the neighbor cache entry for this Client. The OMNI interface finally includes/omits the UDP checksum according to . When a Proxy, Relay or Server re-encapsulates a packet received from a Client that includes an OAL but no ORH, it inserts an ORH immediately following the OAL header and adjusts the OAL payload length and destination address field. The inserted ORH will be removed by the final-hop Bridge, but its insertion and removal will not interfere with reassembly at the final destination. For this reason, Clients must reserve 40 bytes for a maximum-length ORH when they perform OAL encapsulation (see: ).

OMNI interfaces decapsulate packets destined either to the AERO node itself or to a destination reached via an interface other than the OMNI interface the packet was received on. When the encapsulated packet arrives in multiple OAL fragments, the OMNI interface reassembles as discussed in . Further decapsulation steps are performed according to the appropriate encapsulation format specification.

AERO nodes employ simple data origin authentication procedures. In particular: AERO Bridges, Servers and Proxys accept encapsulated data packets and control messages received from the (secured) spanning tree. AERO Proxys and Clients accept packets that originate from within the same secured ANET. AERO Clients and Relays accept packets from downstream network correspondents based on ingress filtering. AERO Clients, Relays and Servers verify the outer UDP/IP encapsulation addresses according to . 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), Maximum Reassembly Unit (MRU) and the role of fragmentation and reassembly . The OMNI interface employs an OMNI Adaptation Layer (OAL) for accommodating multiple underlying links with diverse MTUs. The functions of the OAL and the OMNI interface MTU/MRU are specified in Section 5 of , with MTU/MRU both set to the constant value 9180 bytes. When the network layer presents an IP packet to the OMNI interface, the OAL encapsulates the packet in OAL and ORH headers if necessary. When the network layer presents the OMNI interface with multiple IP packets bound to the same neighbor, the OAL can concatenate multiple IP packets together into a single OAL super-packet as discussed in Section 5.2 of . The OAL then fragments the encapsulated packet if necessary such that the OAL/ORH headers appear in each fragment while the original IP packet header appears only in the first fragment. The OAL then transmits each OAL/ORH packet/fragment over an underlying interface connected to either a physical link such as Ethernet, WiFi and the like or a virtual link such as an Internet or higher-layer tunnel (see the definition of link in ). Note: A Client that does not (yet) have neighbor cache state for a target may omit the ORH in packets with the understanding that a Proxy/Server may insert an ORH on its behalf. For this reason, Clients reserve 40 bytes for the largest possible ORH in their OAL fragment size calculations. Note: Although the ORH may be removed by a Bridge on the path (see: ), this does not interfere with the destination's ability to reassemble in the event that the packet was fragmented. This is due to the fact that the ORH is not included in the fragmentable part; therefore, its removal does not invalidate the offset values in any fragment headers.

IP packets enter a node's OMNI interface either from the network layer (i.e., from a local application or the IP forwarding system) or from the link layer (i.e., from an OMNI interface neighbor). All packets entering a node's OMNI interface first undergo data origin authentication as discussed in . Packets that satisfy data origin authentication are processed further, while all others are dropped silently. The OMNI interface OAL wraps accepted packets in OAL/ORH headers if necessary as discussed above. Packets that enter the OMNI interface from the network layer are forwarded to an OMNI interface neighbor. Packets that enter the OMNI interface from the link layer are either re-admitted into the OMNI link or forwarded to the network layer where they are subject to either local delivery or IP forwarding. In all cases, the OMNI interface itself MUST NOT decrement the network layer TTL/Hop-count since its forwarding actions occur below the network layer. OMNI interfaces may have multiple underlying interfaces and/or neighbor cache entries for neighbors with multiple underlying interfaces (see ). The OMNI interface uses interface attributes and/or traffic classifiers (e.g., DSCP value, port number, flow specification, etc.) to select an outgoing underlying interface for each packet based on the node's own QoS preferences, and also to select a destination link-layer address based on the neighbor's underlying interface with the highest preference. AERO implementations SHOULD allow for QoS preference values to be modified at runtime through network management. If multiple outgoing interfaces and/or neighbor interfaces have a preference of "high", the AERO node replicates the packet and sends one copy via each of the (outgoing / neighbor) interface pairs; otherwise, the node sends a single copy of the packet via an interface with the highest preference. AERO nodes keep track of which underlying interfaces are currently "reachable" or "unreachable", and only use "reachable" interfaces for forwarding purposes. The following sections discuss the OMNI interface forwarding algorithms for Clients, Proxys, Servers and Bridges. In the following discussion, a 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 prefix (which may be encoded in an MNP-LLA).

When an IP packet enters a Client's OMNI interface from the network layer the Client searches for an asymmetric neighbor cache entry that matches the destination. If there is a match, the Client uses one or more "reachable" neighbor interfaces in the entry for packet forwarding. If there is no asymmetric neighbor cache entry, the Client instead forwards the packet toward a Server (the packet is intercepted by a Proxy if there is a Proxy on the path). The Client encapsulates the packet in OAL/ORH headers if necessary and fragments according to MTU requirements (see: ). The Client sets the OAL destination address to the MNP-ULA of the neighbor if it has an asymmetric neighbor cache entry; otherwise, it sets the OAL destination to the ADM-ULA of the Proxy/Server. If the Client has multiple IP packets to send to the same neighbor, it can concatenate them in a single super-packet as discussed in Section 5.2 of . If there is an asymmetric neighbor cache entry and the neighbor interface selected for forwarding is located on the same OMNI link segment and not behind a NAT, the Client forwards the packet/fragments directly according to the L2ADDR information for the neighbor. If the neighbor interface is behind a NAT on the same OMNI link segment, the Client instead forwards the initial packet/fragments to its Server and initiates NAT traversal procedures. If the Client's intended source underlying interface is also behind a NAT and located on the same OMNI link segment, it sends a "direct bubble" over the interface per to the L2ADDR found in the neighbor cache in order to establish state in its own NAT by generating traffic toward the neighbor (note that no response to the bubble is expected). The Client next sends an NS(NUD) message toward the MNP-ULA of the neighbor via its Server as discussed in . If the Client receives an NA(NUD) from the neighbor over the underlying interface, it marks the neighbor interface as "trusted" and sends future packets/fragments directly to the L2ADDR information for the neighbor instead of indirectly via the Server. The Client must honor the neighbor cache maintenance procedure by sending additional direct bubbles and/or NS/NA(NUD) messages as discussed in in order to keep NAT state alive as long as packets are still flowing. When an IP packet enters a Client's OMNI interface from the link-layer, if the destination matches one of the Client's MNPs or link-local addresses the Client reassembles and decapsulates as necessary and delivers the inner packet to the network layer. Otherwise, the Client drops the packet and MAY return a network-layer ICMP Destination Unreachable message subject to rate limiting (see: ).

For control messages originating from or destined to a Client, the Proxy intercepts the message and updates its proxy neighbor cache entry for the Client. The Proxy then forwards a (proxyed) copy of the control message. (For example, the Proxy forwards a proxyed version of a Client's NS/RS message to the target neighbor, and forwards a proxyed version of the neighbor's NA/RA reply to the Client.) When the Proxy receives a data packet from a Client within the ANET, the Proxy reassembles and re-fragments if necessary then searches for an asymmetric neighbor cache entry that matches the destination and forwards as follows: if the destination matches an asymmetric neighbor cache entry, the Proxy uses one or more "reachable" neighbor interfaces in the entry for packet forwarding using OAL/ORH encapsulation if necessary according to the cached link-layer address information. If the neighbor interface is in a different OMNI link segment, the Proxy forwards the packet to a Bridge; otherwise, it forwards the packet directly to the neighbor. If the neighbor is behind a NAT, the Proxy instead forwards initial packets via a Bridge while sending an NS(NUD) to the neighbor. When the Proxy receives the NA(NUD), it can begin forwarding packets directly to the neighbor the same as discussed in while sending additional NUD messages as necessary to maintain NAT state. Note that no direct bubbles are necessary since the Proxy is by definition not located behind a NAT. else, the Proxy uses OAL/ORH encapsulation and forwards the packet to a Bridge while using the MNP-ULA corresponding to the packet's destination as the destination address. When the Proxy receives an encapsulated data packet from an INET neighbor or from a secured tunnel from a Bridge, it accepts the packet only if data origin authentication succeeds and if there is a proxy neighbor cache entry that matches the inner destination. Next, the Proxy reassembles the packet (if necessary) and continues processing. If the reassembly is complete and the neighbor cache state is REACHABLE, the Proxy then returns a PTB if necessary (see: ) then either drops or forwards the packet to the Client while performing OAL/ORH re-encapsulation and re-fragmentation if necessary. If the neighbor cache entry state is DEPARTED, the Proxy instead changes the destination address to the address of the new Server and forwards it to a Bridge while performing OAL/ORH re-encapsulation if necessary. Note: When the Proxy receives fragmented OAL packets with destination address set to the MNP-ULA of the Client, it can either reassemble first and then re-encapsulate/re-fragment before forwarding to the Client or forward the raw fragments on to the Client which then must reassemble. In the former case, the Proxy can re-fragment to a size that better matches the link MTU for the Client, which may be important for low-end links with large MTUs. In the latter case, the Client may receive fragments that are smaller than its link MTU but can still be reassembled; this case may provide an important performance benefit to Proxys by permitting them to avoid excessive reassembly and re-fragmentation overhead.) Note: If the Proxy has multiple IP packets to send to the same neighbor, it can concatenate them in a single super-packet as discussed in Section 5.2 of .

For control messages destined to a target Client's MNP-LLA that are received from a secured tunnel, the Server intercepts the message and returns a Proxyed response on behalf of the Client. (For example, the Server sends a Proxyed NA message reply in response to an NS message directed to one of its associated Clients.) If the Client's neighbor cache entry is in the DEPARTED state, however, the Server instead forwards the packet to the Client's new Server using OAL/ORH re-encapsulation the same as discussed for Proxys in . When the Server receives an encapsulated data packet from an INET neighbor or from a secured tunnel, it accepts the packet only if data origin authentication succeeds. If the packet was an OAL fragment with destination set to the Server's ADM-ULA, the Server submits the packet for reassembly (if the destination was the MNP-ULA of one of its Clients, the Server instead either submits the packet for reassembly or forwards the fragment directly to the target Client). If reassembly is complete (or if no reassembly was required), the Server then (re)fragments if necessary and forwards as follows: if the network layer destination matches a symmetric neighbor cache entry in the REACHABLE state the Server prepares the packet for forwarding to the destination Client. The Server then forwards the packet as specified in . else, if the destination matches a symmetric neighbor cache entry in the DEPARETED state the Server re-encapsulates the packet and forwards it using the ADM-ULA of the Client's new Server as the destination. else, if the destination matches an asymmetric neighbor cache entry, the Server proceeds the same as specified for a Proxy in . Again, the Server need not send direct bubbles since it is not located behind a NAT. else, if the destination matches a non-MNP route in the IP forwarding table or an ADM-LLA assigned to the Server's OMNI interface, the Server decapsulates the packet and releases it to the network layer for local delivery or IP forwarding. else, the Server drops the packet. When the Server's OMNI interface receives a data packet from the network layer or from a VPNed or Direct Client, it performs OAL/ORH encapsulation and fragmentation if necessary, then processes the packet according to the network-layer destination address as follows: if the destination matches a symmetric or asymmetric neighbor cache entry the Server processes the packet as above. else, the Server encapsulates the packet in OLA/ORH headers and forwards it to a Bridge using its own ADM-ULA as the source and the MNP-ULA corresponding to the destination as the destination. Note: The same as for Proxys, when the Server receives fragmented OAL packets with destination address set to the MNP-ULA of the target, it can either reassemble first and then re-encapsulate/re-fragment before sending or forward the raw fragments on to the target which then must reassemble. In the former case, the Server can re-fragment to a size that better matches the link MTU for the Client, which may be important for low-end links with large MTUs. In the latter case, the target may receive fragments that are smaller than its link MTU but can still be reassembled; this case may provide an important performance benefit to Servers by permitting them to avoid excessive reassembly and re-fragmentation overhead.) Note: If the Server has multiple IP packets to send to the same neighbor, it can concatenate them in a single super-packet as discussed in Section 5.2 of .

Bridges forward OAL/ORH-encapsulated packets over secured tunnels the same as any IP router. When the Bridge receives an OAL/ORH-encapsulated packet via a secured tunnel, it removes the outer INET header and searches for a forwarding table entry that matches the OAL destination address. The Bridge then processes the packet as follows: if the destination matches its ADM-ULA Subnet Router Anycast address the Bridge processes the packet locally before forwarding. The Bridge drops the packet if the next header is not an ORH; otherwise, for NA(NUD) messages the Bridge replaces the OMNI option Interface Attributes sub-option with information for its own interface while retaining the ifIndex value supplied by the NA(NUD) message source. For all packet types, the Bridge next examines the ORH FMT code. If the code indicates the destination is a Client on the open INET (or, a Client behind a NAT for which NAT traversal procedures have already converged) the Bridge removes the ORH then writes the MNP-ULA formed from the ORH Destination Suffix into the OAL destination if the packet is a fragment (otherwise, the Bridge also removes the OAL header). The Bridge then forwards the packet/fragment via INET encapsulation to the ORH L2ADDR. For all other destination cases, the Bridge instead writes the ADM-ULA formed from the ORH SRT/LHS into the OAL destination address and forwards the OAL/ORH encapsulated packet to the ADM-ULA Server while invoking NAT traversal procedures the same as for Proxys and Servers if necessary, noting that no direct bubbles are necessary since only the Client and not the Bridge is behind a NAT. else, if the destination matches one of the Bridge's own addresses, the Bridge submits the packet for local delivery. else, if the destination matches a forwarding table entry the Bridge forwards the packet via a secured tunnel to the next hop. If the destination matches an MSP without matching an MNP, however, the Bridge instead drops the packet and returns an ICMP Destination Unreachable message subject to rate limiting (see: ). else, the Bridge drops the packet and returns an ICMP Destination Unreachable as above. As for any IP router, the Bridge decrements the TTL/Hop Limit when it forwards the packet. Therefore, when an OAL header is present only the Hop Limit in the OAL header is decremented and not the TTL/Hop Limit in the inner packet header. Bridges do not insert OAL/ORH headers themselves; instead, they act as IPv6 routers and forward packets based on the destination address found in the headers of packets they receive.

When an AERO node admits a packet into the OMNI interface, it may receive link-layer or network-layer error indications. A link-layer error indication is an ICMP error message generated by a router in the INET on the path to the neighbor or by the neighbor itself. The message includes an IP header with the address of the node that generated the error as the source address and with the link-layer address of the AERO node as the destination address. The IP header is followed by an ICMP header that includes an error Type, Code and Checksum. Valid type values include "Destination Unreachable", "Time Exceeded" and "Parameter Problem" . (OMNI interfaces ignore all link-layer IPv4 "Fragmentation Needed" and IPv6 "Packet Too Big" messages since they only emit packets that are guaranteed to be no larger than the IP minimum link MTU as discussed in .) The ICMP header is followed by the leading portion of the 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 (where, "L2" and "L3" refer to link-layer and network-layer, respectively):

The AERO node rules for processing these link-layer error messages are as follows: When an AERO node receives a link-layer Parameter Problem message, it processes the message the same as described as for ordinary ICMP errors in the normative references . When an AERO node receives persistent link-layer Time Exceeded messages, the IP ID field may be wrapping before earlier fragments awaiting reassembly have been processed. In that case, the node should begin including integrity checks and/or institute rate limits for subsequent packets. When an AERO node receives persistent link-layer Destination Unreachable messages in response to encapsulated packets that it sends to one of its asymmetric 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 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 encapsulated packets that it sends to one of its symmetric neighbor 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 Server and release its association with the old Server as specified in . When an AERO Server receives persistent link-layer Destination Unreachable messages in response to encapsulated packets that it sends to one of its symmetric neighbor Clients, the Server should mark the underlying path as unusable and use another underlying path. When an AERO Server or Proxy receives link-layer Destination Unreachable messages in response to an encapsulated 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 reconverge and correct the temporary outage. When an AERO Bridge receives a packet for which the network-layer destination address is covered by an MSP, the Bridge drops the packet if there is no more-specific routing information for the destination and returns a network-layer Destination Unreachable message subject to rate limiting. The Bridge writes the network-layer source address of the original packet as the destination address and uses one of its non link-local addresses as the source address of the message. When an AERO node receives an encapsulated packet for which the reassembly buffer it too small, it drops the packet and returns a network-layer Packet Too Big (PTB) message. The node first writes the MRU value into the PTB message MTU field, writes the network-layer source address of the original packet as the destination address and writes one of its non link-local addresses as the source address.

AERO Router Discovery, Prefix Delegation and Autoconfiguration are coordinated as discussed in the following Sections.

Each AERO Server on the OMNI link is configured to facilitate Client prefix delegation/registration requests. Each Server is provisioned with a database of MNP-to-Client ID mappings for all Clients enrolled in the AERO service, as well as any information necessary to authenticate each Client. The Client database is maintained by a central administrative authority for the OMNI link and securely distributed to all Servers, e.g., via the Lightweight Directory Access Protocol (LDAP) , via static configuration, etc. Clients receive the same service regardless of the Servers they select. AERO Clients and Servers use ND messages to maintain neighbor cache entries. AERO Servers configure their OMNI interfaces as advertising NBMA interfaces, and therefore send unicast RA messages with a short Router Lifetime value (e.g., ReachableTime seconds) in response to a Client's RS message. Thereafter, Clients send additional RS messages to keep Server state alive. AERO Clients and Servers include prefix delegation and/or registration parameters in RS/RA messages (see ). The ND messages are exchanged between Client and Server according to the prefix management schedule required by the service. If the Client knows its MNP in advance, it can employ prefix registration by including its MNP-LLA as the source address of an RS message and with an OMNI option with valid prefix registration information for the MNP. If the Server (and Proxy) accept the Client's MNP assertion, they inject the prefix into the routing system and establish the necessary neighbor cache state. If the Client does not have a pre-assigned MNP, it can instead employ prefix delegation by including the unspecified address (::) as the source address of an RS message and with an OMNI option with a DHCPv6 message option to request an MNP. The following sections specify the Client and Server behavior.

AERO Clients discover the addresses of Servers in a similar manner as described in . Discovery methods include static configuration (e.g., from a flat-file map of Server addresses and locations), or through an automated means such as Domain Name System (DNS) name resolution . Alternatively, the Client can discover Server addresses through a layer 2 data link login exchange, or through a unicast RA response to a multicast/anycast RS as described below. In the absence of other information, the Client can resolve the DNS Fully-Qualified Domain Name (FQDN) "linkupnetworks.[domainname]" where "linkupnetworks" is a constant text string and "[domainname]" is a DNS suffix for the OMNI link (e.g., "example.com"). To associate with a Server, the Client acts as a requesting router to request MNPs. The Client prepares an RS message with prefix management parameters and includes a Nonce and Timestamp option if the Client needs to correlate RA replies. If the Client already knows the Server's ADM-LLA, it includes the LLA as the network-layer destination address; otherwise, the Client includes the (link-local) All-Routers multicast as the network-layer destination. If the Client already knows its own MNP-LLA, it can use the MNP-LLA as the network-layer source address and include an OMNI option with prefix registration information. Otherwise, the Client uses the unspecified address (::) as the network-layer source address and includes a DHCPv6 sub-option in the OMNI option (see: ). The Client includes Interface Attributes corresponding to the underlying interface over which it will send the RS message, and MAY include additional Interface Attributes specific to other underlying interfaces. The Client then sends the RS message (either directly via Direct interfaces, via a VPN for VPNed interfaces, via a Proxy for ANET interfaces or via INET encapsulation for INET interfaces) while using OAL encapsulation/fragmentation if necessary, then waits for an RA message reply (see ). The Client retries up to MAX_RTR_SOLICITATIONS times until an RA is received. If the Client receives no RAs, or if it receives an RA with Router Lifetime set to 0, the Client SHOULD abandon this Server and try another Server. Otherwise, the Client processes the prefix information found in the RA message. When the Client processes an RA, it first performs OAL reassembly if necessary then creates a symmetric neighbor cache entry with the Server's ADM-LLA as the network-layer address and the Server's encapsulation and/or link-layer addresses as the link-layer address. The Client next records the RA Router Lifetime field value in the neighbor cache entry as the time for which the Server has committed to maintaining the MNP in the routing system via this underlying interface, and caches the other RA configuration information including Cur Hop Limit, M and O flags, Reachable Time and Retrans Timer. The Client then autoconfigures MNP-LLAs for any delegated MNPs and assigns them to the OMNI interface. The Client also caches any MSPs included in Route Information Options (RIOs) as MSPs to associate with the OMNI link, and assigns the MTU value in the MTU option to the underlying interface. The Client then registers additional underlying interfaces with the Server by sending RS messages via each additional interface as described above. The RS messages include the same parameters as for the initial RS/RA exchange, but with destination address set to the Server's ADM-LLA. The Client finally sub-delegates the MNPs to its attached EUNs and/or the Client's own internal virtual interfaces as described in to support the Client's downstream attached "Internet of Things (IoT)". The Client then sends additional RS messages over each underlying interface before the Router Lifetime received for that interface expires. After the Client registers its underlying interfaces, it may wish to change one or more registrations, e.g., if an interface changes address or becomes unavailable, if QoS preferences change, etc. To do so, the Client prepares an RS message to send over any available underlying interface as above. The RS includes an OMNI option with prefix registration/delegation information, with Interface Attributes specific to the selected underlying interface, and with any additional Interface Attributes specific to other underlying interfaces. When the Client receives the Server's RA response, it has assurance that the Server has been updated with the new information. If the Client wishes to discontinue use of a Server it issues an RS message over any underlying interface with an OMNI option with a prefix release indication. When the Server processes the message, it releases the MNP, sets the symmetric neighbor cache entry state for the Client to DEPARTED and returns an RA reply with Router Lifetime set to 0. After a short delay (e.g., 2 seconds), the Server withdraws the MNP from the routing system.

AERO Servers act as IP routers and support a prefix delegation/registration service for Clients. Servers arrange to add their ADM-LLAs to a static map of Server addresses for the link and/or the DNS resource records for the FQDN "linkupnetworks.[domainname]" before entering service. Server addresses should be geographically and/or topologically referenced, and made available for discovery by Clients on the OMNI link. When a Server receives a prospective Client's RS message on its OMNI interface, it SHOULD return an immediate RA reply with Router Lifetime set to 0 if it is currently too busy or otherwise unable to service the Client. Otherwise, the Server performs OAL reassembly on the RS message if necessary, then authenticates the message and processes the prefix delegation/registration parameters. The Server first determines the correct MNPs to provide to the Client by processing the MNP-LLA prefix parameters and/or the DHCPv6 OMNI sub-option. When the Server returns the MNPs, it also creates a forwarding table entry for the MNP-ULA corresponding to each MNP so that the MNPs are propagated into the routing system (see: ). For IPv6, the Server creates an IPv6 forwarding table entry for each MNP. For IPv4, the Server creates an IPv6 forwarding table entry with the IPv4-compatibility MNP-ULA prefix corresponding to the IPv4 address. The Server next creates a symmetric neighbor cache entry for the Client using the base MNP-LLA as the network-layer address and with lifetime set to no more than the smallest prefix lifetime. Next, the Server updates the neighbor cache entry by recording the information in each Interface Attributes sub-option in the RS OMNI option. The Server also records the actual OAL/INET addresses in the neighbor cache entry. Next, the Server prepares an RA message using its ADM-LLA as the network-layer source address and the network-layer source address of the RS message as the network-layer destination address. The Server sets the Router Lifetime to the time for which it will maintain both this underlying interface individually and the symmetric neighbor cache entry as a whole. The Server also sets Cur Hop Limit, M and O flags, Reachable Time and Retrans Timer to values appropriate for the OMNI link. The Server includes the MNPs, any other prefix management parameters and an OMNI option with no Interface Attributes. The Server then includes one or more RIOs that encode the MSPs for the OMNI link, plus an MTU option (see ). The Server finally forwards the message to the Client using OAL/INET, INET, or NULL encapsulation and OAL fragmentation as necessary. After the initial RS/RA exchange, the Server maintains a ReachableTime timer for each of the Client's underlying interfaces individually (and for the Client's symmetric neighbor cache entry collectively) set to expire after ReachableTime seconds. If the Client (or Proxy) issues additional RS messages, the Server sends an RA response and resets ReachableTime. If the Server receives an ND message with a prefix release indication it sets the Client's symmetric neighbor cache entry to the DEPARTED state and withdraws the MNP from the routing system after a short delay (e.g., 2 seconds). If ReachableTime expires before a new RS is received on an individual underlying interface, the Server marks the interface as DOWN. If ReachableTime expires before any new RS is received on any individual underlying interface, the Server sets the symmetric neighbor cache entry state to STALE and sets a 10 second timer. If the Server has not received a new RS or ND message with a prefix release indication before the 10 second timer expires, it deletes the neighbor cache entry and withdraws the MNP from the routing system. The Server processes any ND messages pertaining to the Client and returns an NA/RA reply in response to solicitations. The Server may also issue unsolicited RA messages, e.g., with reconfigure parameters to cause the Client to renegotiate its prefix delegation/registrations, with Router Lifetime set to 0 if it can no longer service this Client, etc. Finally, If the symmetric neighbor cache entry is in the DEPARTED state, the Server deletes the entry after DepartTime expires. Note: Clients SHOULD notify former Servers of their departures, but Servers are responsible for expiring neighbor cache entries and withdrawing routes even if no departure notification is received (e.g., if the Client leaves the network unexpectedly). Servers SHOULD therefore set Router Lifetime to ReachableTime seconds in solicited RA messages to minimize persistent stale cache information in the absence of Client departure notifications. A short Router Lifetime also ensures that proactive Client/Server RS/RA messaging will keep any NAT state alive (see above). Note: All Servers on an OMNI link MUST advertise consistent values in the RA Cur Hop Limit, M and O flags, Reachable Time and Retrans Timer fields the same as for any link, since unpredictable behavior could result if different Servers on the same link advertised different values.

When a Client is not pre-provisioned with an MNP-LLA, it will need for the Server to select one or more MNPs on its behalf and set up the correct state in the AERO routing service. (A Client with a pre-provisioned MNP may also request the Server to select additional MNPs.) The DHCPv6 service is used to support this requirement. When a Client needs to have the Server select MNPs, it sends an RS message with source address set to the unspecified address (::) and with an OMNI option that includes a DHCPv6 message sub-option with DHCPv6 Prefix Delegation (DHCPv6-PD) parameters. When the Server receives the RS message, it extracts the DHCPv6-PD message from the OMNI option. The Server then acts as a "Proxy DHCPv6 Client" in a message exchange with the locally-resident DHCPv6 server, which delegates MNPs and returns a DHCPv6-PD Reply message. (If the Server wishes to defer creation of MN state until the DHCPv6-PD Reply is received, it can instead act as a Lightweight DHCPv6 Relay Agent per by encapsulating the DHCPv6-PD message in a Relay-forward/reply exchange with Relay Message and Interface ID options.) When the Server receives the DHCPv6-PD Reply, it adds a route to the routing system and creates an MNP-LLA based on the delegated MNP. The Server then sends an RA back to the Client with the (newly-created) MNP-LLA as the destination address and with the DHCPv6-PD Reply message coded in the OMNI option. When the Client receives the RA, it creates a default route, assigns the Subnet Router Anycast address and sets its MNP-LLA based on the delegated MNP.

Clients may connect to protected-spectrum ANETs that employ physical and/or link-layer security services to facilitate communications to Servers in outside INETs. In that case, the ANET can employ an AERO Proxy. The Proxy is located at the ANET/INET border and listens for RS messages originating from or RA messages destined to ANET Clients. The Proxy acts on these control messages as follows: when the Proxy receives an RS message from a new ANET Client, it first authenticates the message then examines the network-layer destination address. If the destination address is a Server's ADM-LLA, the Proxy proceeds to the next step. Otherwise, if the destination is (link-local) All-Routers multicast, the Proxy selects a "nearby" Server that is likely to be a good candidate to serve the Client and replaces the destination address with the Server's ADM-LLA. Next, the Proxy creates a proxy neighbor cache entry and caches the Client and Server link-layer addresses along with the OMNI option information and any other identifying information including Transaction IDs, Client Identifiers, Nonce values, etc. The Proxy finally encapsulates the (proxyed) RS message in an OAL header with source set to the Proxy's ADM-ULA and destination set to the Server's ADM-ULA. The Proxy also includes an OMNI header with an Interface Attributes option that includes its own INET address plus a unique Port Number for this Client, then forwards the message into the OMNI link spanning tree. (Note: including a unique Port Number allows the Server to distinguish different Clients located behind the same Proxy at the link-layer, whereas the link-layer addresses would otherwise be indistinguishable) when the Server receives the RS, it authenticates the message then creates or updates a symmetric neighbor cache entry for the Client with the Proxy's ADM-ULA, INET address and Port Number as the link-layer address information. The Server then sends an RA message back to the Proxy via the spanning tree. when the Proxy receives the RA, it authenticates the message and matches it with the proxy neighbor cache entry created by the RS. The Proxy then caches the prefix information as a mapping from the Client's MNPs to the Client's link-layer address, caches the Server's advertised Router Lifetime and sets the neighbor cache entry state to REACHABLE. The Proxy then optionally rewrites the Router Lifetime and forwards the (proxyed) message to the Client. The Proxy finally includes an MTU option (if necessary) with an MTU to use for the underlying ANET interface. After the initial RS/RA exchange, the Proxy forwards any Client data packets for which there is no matching asymmetric neighbor cache entry to a Bridge using OAL encapsulation with its own ADM-ULA as the source and the MNP-ULA corresponding to the Client as the destination. The Proxy instead forwards any Client data destined to an asymmetric neighbor cache target directly to the target according to the OAL/link-layer information - the process of establishing asymmetric neighbor cache entries is specified in . While the Client is still attached to the ANET, the Proxy sends NS, RS and/or unsolicited NA messages to update the Server's symmetric neighbor cache entries on behalf of the Client and/or to convey QoS updates. This allows for higher-frequency Proxy-initiated RS/RA messaging over well-connected INET infrastructure supplemented by lower-frequency Client-initiated RS/RA messaging over constrained ANET data links. If the Server ceases to send solicited advertisements, the Proxy sends unsolicited RAs on the ANET interface with destination set to (link-local) All-Nodes multicast and with Router Lifetime set to zero to inform Clients that the Server has failed. Although the Proxy engages in ND exchanges on behalf of the Client, the Client can also send ND messages on its own behalf, e.g., if it is in a better position than the Proxy to convey QoS changes, etc. For this reason, the Proxy marks any Client-originated solicitation messages (e.g. by inserting a Nonce option) so that it can return the solicited advertisement to the Client instead of processing it locally. If the Client becomes unreachable, the Proxy sets the neighbor cache entry state to DEPARTED and retains the entry for DepartTime seconds. While the state is DEPARTED, the Proxy forwards any packets destined to the Client to a Bridge via OAL/ORH encapsulation. When DepartTime expires, the Proxy deletes the neighbor cache entry and discards any further packets destined to this (now forgotten) Client. In some ANETs that employ a Proxy, the Client's MNP can be injected into the ANET routing system. In that case, the Client can send data messages without encapsulation so that the ANET routing system transports the unencapsulated packets to the Proxy. This can be very beneficial, e.g., if the Client connects to the ANET via low-end data links such as some aviation wireless links. If the first-hop ANET access router is on the same underlying link and recognizes the AERO/OMNI protocol, the Client can avoid encapsulation for both its control and data messages. When the Client connects to the link, it can send an unencapsulated RS message with source address set to its own MNP-LLA (or to a Temporary LLA), and with destination address set to the ADM-LLA of the Client's selected Server or to (link-local) All-Routers multicast. The Client includes an OMNI option formatted as specified in . The Client then sends the unencapsulated RS message, which will be intercepted by the AERO-Aware access router. The access router then encapsulates the RS message in an ANET header with its own address as the source address and the address of a Proxy as the destination address. The access router further remembers the address of the Proxy so that it can encapsulate future data packets from the Client via the same Proxy. If the access router needs to change to a new Proxy, it simply sends another RS message toward the Server via the new Proxy on behalf of the Client. In some cases, the access router and Proxy may be one and the same node. In that case, the node would be located on the same physical link as the Client, but its message exchanges with the Server would need to pass through a security gateway at the ANET/INET border. The method for deploying access routers and Proxys (i.e. as a single node or multiple nodes) is an ANET-local administrative consideration. Note: The Proxy can apply packing as discussed in Section 5.2 of if an opportunity arises to concatenate multiple payload packets that will be destined to the same neighbor.

Clients may need to connect directly to Servers via INET, Direct and VPNed interfaces (i.e., non-ANET interfaces). If the Client's underlying interfaces all connect via the same INET partition, then it can connect to a single controlling Server via all interfaces. If some Client interfaces connect via different INET partitions, however, the Client still selects a set of controlling Servers and sends RS messages via their directly-connected Servers while using the ADM-LLA of the controlling Server as the destination. When a Server receives an RS with destination set to the ADM-LLA of a controlling Server, it acts as a Proxy to forward the message to the controlling Server while forwarding the corresponding RA reply to the Client.

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

In environments where Client messaging over ANETs is bandwidth-limited and/or expensive, Clients can enlist the services of the Proxy to coordinate with multiple Servers in a single RS/RA message exchange. The Client can send a single RS message to (link-local) All-Routers multicast that includes the ID's of multiple Servers in MS-Register sub-options of the OMNI option. When the Proxy receives the RS and processes the OMNI option, it sends a separate RS to each MS-Register Server ID. When the Proxy receives an RA, it can optionally return an immediate "singleton" RA to the Client or record the Server's ID for inclusion in a pending "aggregate" RA message. The Proxy can then return aggregate RA messages to the Client including multiple Server IDs in order to conserve bandwidth. Each RA includes a proper subset of the Server IDs from the original RS message, and the Proxy must ensure that the message contents of each RA are consistent with the information received from the (aggregated) Servers. Clients can thereafter employ efficient point-to-multipoint Server coordination under the assistance of the Proxy to reduce the number of messages sent over the ANET while enlisting the support of multiple Servers for fault tolerance. Clients can further include MS-Release sub-options in IPv6 ND messages to request the Proxy to release from former Servers via the procedures discussed in . The OMNI interface specification provides further discussion of the Client/Proxy RS/RA messaging involved in point-to-multipoint coordination.

While data packets are flowing between a source and target node, route optimization SHOULD be used. Route optimization is initiated by the first eligible Route Optimization Source (ROS) closest to the source as follows: For Clients on VPNed and Direct interfaces, the Server is the ROS. For Clients on ANET interfaces, the Proxy is the ROS. For Clients on INET interfaces, the Client itself is the ROS. For correspondent nodes on INET/EUN interfaces serviced by a Relay, the Relay is the ROS. The route optimization procedure is conducted between the ROS and the target Server/Relay acting as a Route Optimization Responder (ROR) in the same manner as for IPv6 ND Address Resolution and using the same NS/NA messaging. The target may either be a MNP Client serviced by a Server, or a non-MNP correspondent reachable via a Relay. The procedures are specified in the following sections.

While data packets are flowing from the source node toward a target node, the ROS performs address resolution by sending an NS message for Address Resolution (NS(AR)) to receive a solicited NA(AR) message from the ROR. When the ROS sends an NS(AR), it includes: the LLA of the ROS as the source address. the data packet's destination as the Target Address. the Solicited-Node multicast address formed from the lower 24 bits of the data packet's destination as the destination address, e.g., for 2001:db8:1:2::10:2000 the NS destination address is ff02:0:0:0:0:1:ff10:2000. The NS(AR) message includes an OMNI option with no Interface Attributes, such that the target will not create a neighbor cache entry. The Prefix Length in the OMNI option is set to the Prefix Length associated with the ROS's LLA. The ROS then encapsulates the NS(AR) message in an OAL header with source set to its own ULA and destination set to the MNP-ULA corresponding to the target, then sends the message into the spanning tree without decrementing the network-layer TTL/Hop Limit field. (When the ROS is a Client, it instead securely sends the NS(AR) to one of its current Servers as specified in . The Server then forwards an adapted version of the NS(AR) into the spanning tree on behalf of the Client.) Note: The ROS can apply packing as discussed in Section 5.2 of to concatenate the NS(AR) onto the same OAL packet used to carry one or more data packets that triggered route optimization.

When the Bridge receives the NS(AR) message from the ROS, it discards the INET header and determines that the ROR is the next hop by consulting its standard IPv6 forwarding table for the OAL header destination address. The Bridge then forwards the message toward the ROR via the spanning tree the same as for any IPv6 router. The final-hop Bridge in the spanning tree will deliver the message via a secured tunnel to the ROR.

When the ROR receives the NS(AR) message, it examines the Target Address to determine whether it has a neighbor cache entry and/or route that matches the target. If there is no match, the ROR drops the message. Otherwise, the ROR continues processing as follows: if the target belongs to an MNP Client neighbor in the DEPARTED state the ROR changes the NS(AR) message OAL destination address to the ADM-ULA of the Client's new Server, forwards the message into the spanning tree and returns from processing. If the target belongs to an MNP Client neighbor in the REACHABLE state, the ROR instead adds the AERO source address to the target Client's Report List with time set to ReportTime. If the target belongs to a non-MNP route, the ROR continues processing without adding an entry to the Report List. The ROR then prepares a (solicited) NA(AR) message to send back to the ROS but does not create a neighbor cache entry. The ROR sets the NA(AR) source address to the MNP-LLA corresponding to the target, sets the Target Address to the target of the solicitation, and sets the destination address to the source of the solicitation. The ROR then includes an OMNI option with Prefix Length set to the length associated with the MNP-LLA. If the target is an MNP Client, the ROR next includes Interface Attributes in the OMNI option for each of the target Client's underlying interfaces with current information for each interface and with the S/T-ifIndex field in the OMNI header set to 0 to indicate that the message originated from the ROR and not the Client. For each Interface Attributes sub-option, the ROR sets the L2ADDR according to its own INET address for VPNed or Direct interfaces, to the INET address of the Proxy or to the Client's INET address for INET interfaces. The ROR then includes the lower 32 bits of its own ADM-ULA (or the ADM-ULA of the Proxy) as the LHS, encodes the ADM-ULA prefix length code in the SRT field and sets the FMT code accordingly as specified in . The ROR then sets the NA(AR) message R flag to 1 (as a router), S flag to 1 (as a response to a solicitation), and O flag to 0 (as a proxy). The ROR finally encapsulates the NA(AR) message in an OAL header with source set to its own ULA and destination set to the source ULA of the NS(AR) message, then forwards the message into the spanning tree without decrementing the network-layer TTL/Hop Limit field.

When the Bridge receives the NA(AR) message from the ROR, it discards the INET header and determines that the ROS is the next hop by consulting its standard IPv6 forwarding table for the OAL header destination address. The Bridge then forwards the OAL-encapsulated NA(AR) message toward the ROS the same as for any IPv6 router. The final-hop Bridge in the spanning tree will deliver the message via a secured tunnel to the ROS.

When the ROS receives the NA(AR) message, it processes the message the same as for standard IPv6 Address Resolution . In the process, it caches the source MNP-ULA then creates an asymmetric neighbor cache entry for the target and caches all information found in the OMNI option. The ROS finally sets the asymmetric neighbor cache entry lifetime to ReachableTime seconds. (When the ROS is a Client, the solicited NA(AR) message will first be delivered via the spanning tree to one of its current Servers, which then securely forwards the message to the Client as discussed in .)

Following route optimization, the ROS forwards future data packets destined to the target via the addresses found in the cached link-layer information. The route optimization is shared by all sources that send packets to the target via the ROS, i.e., and not just the source on behalf of which the route optimization was initiated. While new data packets destined to the target are flowing through the ROS, it sends additional NS(AR) messages to the ROR before ReachableTime expires to receive a fresh NA(AR) message the same as described in the previous sections (route optimization refreshment strategies are an implementation matter, with a non-normative example given in ). The ROS uses the cached ADM-ULA of the ROR as the NS(AR) OAL destination address (i.e., instead of using the ULA corresponding to the target as was the case for the initial NS(AR)), and sends up to MAX_MULTICAST_SOLICIT NS(AR) messages separated by 1 second until an NA(AR) is received. If no NA(AR) is received, the ROS assumes that the current ROR has become unreachable and deletes the target neighbor cache entry. Subsequent data packets will trigger a new route optimization with an NS with OAL destination address set to the MNP-ULA corresponding to the target per to discover a new ROR while initial data packets travel over a suboptimal route. If an NA(AR) is received, the ROS then updates the asymmetric neighbor cache entry to refresh ReachableTime, while (for MNP destinations) the ROR adds or updates the ROS address to the target's Report List and with time set to ReportTime. While no data packets are flowing, the ROS instead allows ReachableTime for the asymmetric neighbor cache entry to expire. When ReachableTime expires, the ROS deletes the asymmetric neighbor cache entry. Any future data packets flowing through the ROS will again trigger a new route optimization. The ROS may also receive unsolicited NA messages from the ROR at any time (see: ). If there is an asymmetric neighbor cache entry for the target, the ROS updates the link-layer information but does not update ReachableTime since the receipt of an unsolicited NA does not confirm that any forward paths are working. If there is no asymmetric neighbor cache entry, the ROS simply discards the unsolicited NA. In this arrangement, the ROS holds an asymmetric neighbor cache entry for the target via the ROR, but the ROR does not hold an asymmetric neighbor cache entry for the ROS. The route optimization neighbor relationship is therefore asymmetric and unidirectional. If the target node also has packets to send back to the source node, then a separate route optimization procedure is performed in the reverse direction. But, there is no requirement that the forward and reverse paths be symmetric.

AERO nodes perform Neighbor Unreachability Detection (NUD) per either reactively in response to persistent link-layer errors (see ) or proactively to confirm reachability and/or establish NAT state. The NUD algorithm is based on periodic control message exchanges. The algorithm may further be seeded by ND hints of forward progress, but care must be taken to avoid inferring reachability based on spoofed information. For example, authentic IPv6 ND message exchanges may be considered as acceptable hints of forward progress, while spurious data packets should not be. AERO nodes can use (OAL/ORH-encapsulated) standard NS/NA exchanges sent over the OMNI link spanning tree to securely test reachability without risk of DoS attacks from nodes pretending to be a neighbor (these NS/NA(NUD) messages use the unicast LLAs and ULAs of the two parties involved in the NUD test the same as for standard IPv6 ND, and both messages flow over the spanning tree). Proxys can further perform NUD to securely verify Server reachability on behalf of their proxyed Clients. However, a means for an ROS to test the unsecured target route optimized paths is also necessary. When an ROR directs an ROS to a target neighbor with one or more link-layer addresses, the ROS can proactively test each such unsecured route optimized path through secured NS(NUD) messages over the spanning tree that invoke an unsecured NA(NUD) reply that travels over the route optimized path.. (The NS(NUD) messages must therefore include Nonce and Timestamp options that will be echoed in the unsecured NA(NUD) replies.) While testing the paths, the ROS can optionally continue to send packets via the spanning tree, maintain a small queue of packets until target reachability is confirmed, or (optimistically) allow packets to flow via the route optimized paths. When the ROS sends an NS(NUD) message, it sets the IPv6 source to its own address, sets the destination to the MNP-LLA of the target, and sets the target's MNP Subnet-Router anycast address as the Target Address. The ROS also includes an OMNI option with a single Interface Attributes sub-option with the SRT, FMT, LHS and L2ADDR information for its own underlying interface it wishes to test, but sets the S/T-ifIndex field to the index for target's underlying interface to be tested. The ROS includes a Nonce and Timestamp option, then encapsulates the message in OAL/INET headers with its own ULA as the source and the ULA of the target as the destination. The ROS then forwards the NS(NUD) message toward the target via a Server or Bridge. When the target receives the NS(NUD) message, it creates an NA(NUD) by reversing the OAL and IPv6 addresses and including an Interface Attributes sub-option with attributes for its own interface identified by the NS(NUD) S/T-ifIndex. The target sets the NA(NUD) S/T-ifIndex to the index of the ROS, sets the Target Address to the same value that was in the NS(NUD), and returns the message using its own underlying interface identified by S/T-ifIndex and destined to the ROS's interface identified by the original Interface Attributes sub-option. When the ROS receives the NA(NUD) message, it can determine from the Nonce, Timestamp and Target Address that the message matched its NS(NUD) and that it transited the direct path from the ROR using the selected underlying interface pair. The ROS marks route optimization target paths that pass these NUD tests as "reachable", and those that do not as "unreachable". These markings inform the OMNI interface forwarding algorithm specified in . Note: If the target determines that the OMNI option Interface Attributes in the NS(NUD) is located in a different OMNI link segment than its own interface named in the S/T-ifIndex, it instead returns the NA(NUD) via the spanning tree while including an ORH and setting the OAL destination address to the Subnet Router Anycast address used by Bridges on the ROS segment. When a Bridge on the ROS segment receives the NA(NUD), it replaces the Interface Attributes with information for its own interface while using the ifIndex value specific to the target.

AERO is a Distributed Mobility Management (DMM) service. Each Server is responsible for only a subset of the Clients on the OMNI link, as opposed to a Centralized Mobility Management (CMM) service where there is a single network mobility collective entity for all Clients. Clients coordinate with their associated Servers via RS/RA exchanges to maintain the DMM profile, and the AERO routing system tracks all current Client/Server peering relationships. Servers provide default routing and mobility/multilink services for their dependent Clients. Clients are responsible for maintaining neighbor relationships with their Servers through periodic RS/RA exchanges, which also serves to confirm neighbor reachability. When a Client's underlying interface address and/or QoS information changes, the Client is responsible for updating the Server with this new information. Note that when there is a Proxy in the path, the Proxy can also perform some RS/RA exchanges on the Client's behalf. Mobility management messaging is based on the transmission and reception of unsolicited Neighbor Advertisement (uNA) messages. Each uNA message sets the IPv6 destination address to (link-local) All-Nodes multicast to convey a general update of Interface Attributes to (possibly) multiple recipients, or to a specific unicast LLA to announce a departure event to a specific recipient. Implementations must therefore examine the destination address to determine the nature of the mobility event (i.e., update vs departure). Mobility management considerations are specified in the following sections.

Servers accommodate Client mobility, multilink and/or QoS change events by sending unsolicited NA (uNA) messages to each ROS in the target Client's Report List. When a Server sends a uNA message, it sets the IPv6 source address to the Client's MNP-LLA, sets the destination address to (link-local) All-Nodes multicast and sets the Target Address to the Client's Subnet-Router anycast address. The Server also includes an OMNI option with Prefix Length set to the length associated with the Client's MNP-LLA, with Interface Attributes for the target Client's underlying interfaces and with the OMNI header S/T-ifIndex set to 0. The Server then sets the NA R flag to 1, the S flag to 0 and the O flag to 1, then encapsulates the message in an OAL header with source set to its own ADM-ULA and destination set to the ULA of the ROS and sends the message into the spanning tree. As discussed in Section 7.2.6 of , the transmission and reception of uNA messages is unreliable but provides a useful optimization. In well-connected Internetworks with robust data links uNA messages will be delivered with high probability, but in any case the Server can optionally send up to MAX_NEIGHBOR_ADVERTISEMENT uNAs to each ROS to increase the likelihood that at least one will be received. When the ROS receives a uNA message prepared as above, it ignores the message if there is no existing neighbor cache entry for the Client. Otherwise, it uses the included OMNI option information to update the neighbor cache entry, but does not reset ReachableTime since the receipt of an unsolicited NA message from the target Server does not provide confirmation that any forward paths to the target Client are working. If uNA messages are lost, the ROS may be left with stale address and/or QoS information for the Client for up to ReachableTime seconds. During this time, the ROS can continue sending packets according to its stale neighbor cache information. When ReachableTime is close to expiring, the ROS will re-initiate route optimization and receive fresh link-layer address information. In addition to sending uNA messages to the current set of ROSs for the Client, the Server also sends uNAs to the MNP-ULA associated with the link-layer address for any underlying interface for which the link-layer address has changed. These uNA messages update an old Proxy/Server that cannot easily detect (e.g., without active probing) when a formerly-active Client has departed. When the Server sends the uNA, it sets the IPv6 source address to the Client's MNP-LLA, sets the destination address to the old Proxy/Server's ADM-LLA, and sets the Target Address to the Client's Subnet-Router anycast address. The Server also includes an OMNI option with Prefix Length set to the length associated with the Client's MNP-LLA, with Interface Attributes for the changed underlying interface, and with the OMNI header S/T-ifIndex set to 0. The Server then sets the NA R flag to 1, the S flag to 0 and the O flag to 1, then encapsulates the message in an OAL header with source set to its own ADM-ULA and destination set to the ADM-ULA of the old Proxy/Server and sends the message into the spanning tree.

When a Client needs to change its underlying interface addresses and/or QoS preferences (e.g., due to a mobility event), either the Client or its Proxys send RS messages to the Server via the spanning tree with an OMNI option that includes Interface attributes with the new link quality and address information. Up to MAX_RTR_SOLICITATIONS RS messages MAY be sent in parallel with sending actual data packets in case one or more RAs are lost. If all RAs are lost, the Client SHOULD re-associate with a new Server. When the Server receives the Client's changes, it sends uNA messages to all nodes in the Report List the same as described in the previous section.

When a Client needs to bring new underlying interfaces into service (e.g., when it activates a new data link), it sends an RS message to the Server via the underlying interface with an OMNI option that includes Interface Attributes with appropriate link quality values and with link-layer address information for the new link.

When a Client needs to deactivate an existing underlying interface, it sends an RS or uNA message to its Server with an OMNI option with appropriate Interface Attribute values - in particular, the link quality value 0 assures that neighbors will cease to use the link. If the Client needs to send RS/uNA messages over an underlying interface other than the one being deactivated, it MUST include Interface Attributes with appropriate link quality values for any underlying interfaces being deactivated. Note that when a Client deactivates an underlying interface, neighbors that have received the RS/uNA messages need not purge all references for the underlying interface from their neighbor cache entries. The Client may reactivate or reuse the underlying interface and/or its ifIndex at a later point in time, when it will send RS/uNA messages with fresh Interface Attributes to update any neighbors.

The Client performs the procedures specified in when it first associates with a new Server or renews its association with an existing Server. The Client also includes MS-Release identifiers in the RS message OMNI option per if it wants the new Server to notify any old Servers from which the Client is departing. When the new Server receives the Client's RS message, it returns an RA as specified in and sends up to MAX_NEIGHBOR_ADVERTIISEMENT uNA messages to any old Servers listed in OMNI option MS-Release identifiers. When the new Server sends a uNA message, it sets the IPv6 source address to the Client's MNP-LLA, sets the destination address to the old Server's ADM-LLA, and sets the Target Address to the Client's Subnet-Router anycast address. The new Server also includes an OMNI option with Prefix Length set to the length associated with the Client's MNP-LLA, with Interface Attributes for its own underlying interface, and with the OMNI header S/T-ifIndex set to 0. The new Server then sets the NA R flag to 1, the S flag to 0 and the O flag to 1, then encapsulates the message in an OAL header with source set to its own ADM-ULA and destination set to the ADM-ULA of the old Server and sends the message into the spanning tree. When an old Server receives the uNA, it changes the Client's neighbor cache entry state to DEPARTED, sets the link-layer address of the Client to the new Server's ADM-ULA, and resets DepartTime. After a short delay (e.g., 2 seconds) the old Server withdraws the Client's MNP from the routing system. After DepartTime expires, the old Server deletes the Client's neighbor cache entry. The old Server also iteratively forwards a copy of the uNA message to each ROS in the Client's Report List by changing the OAL destination address to the ULA of the ROS while leaving all other fields of the message unmodified. When the ROS receives the uNA, it examines the Target address to determine the correct asymmetric neighbor cache entry and verifies that the IPv6 destination address matches the old Server. The ROS then caches the IPv6 source address as the new Server for the existing asymmetric neighbor cache entry and marks the entry as STALE. While in the STALE state, the ROS allows new data packets to flow according to any existing cached link-layer information and sends new NS(AR) messages using its own ULA as the OAL source and the ADM-ULA of the new Server as the OAL destination address to elicit NA messages that reset the asymmetric neighbor cache entry state to REACHABLE. If no new NA message is received for 10 seconds while in the STALE state, the ROS deletes the neighbor cache entry. Clients SHOULD NOT move rapidly between 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 Server include a Server that has gone unreachable, topological movements of significant distance, movement to a new geographic region, movement to a new OMNI link segment, etc. When a Client moves to a new Server, some of the fragments of a multiple fragment packet may have already arrived at the old Server while others are en route to the new Server, however no special attention in the reassembly algorithm is necessary when re-routed fragments are simply treated as loss.

The AERO Client provides an IGMP (IPv4) or MLD (IPv6) proxy service for its EUNs and/or hosted applications . The Client forwards IGMP/MLD messages over any of its underlying interfaces for which group membership is required. The IGMP/MLD messages may be further forwarded by a first-hop ANET access router acting as an IGMP/MLD-snooping switch , then ultimately delivered to an AERO Proxy/Server acting as a Protocol Independent Multicast - Sparse-Mode (PIM-SM, or simply "PIM") Designated Router (DR) . AERO Relays also act as PIM routers (i.e., the same as AERO Proxys/Servers) on behalf of nodes on INET/EUN networks. The behaviors identified in the following sections correspond to Source-Specific Multicast (SSM) and Any-Source Multicast (ASM) operational modes.

When an ROS (i.e., an AERO Proxy/Server/Relay) "X" acting as PIM router receives a Join/Prune message from a node on its downstream interfaces containing one or more ((S)ource, (G)roup) pairs, it updates its Multicast Routing Information Base (MRIB) accordingly. For each S belonging to a prefix reachable via X's non-OMNI interfaces, X then forwards the (S, G) Join/Prune to any PIM routers on those interfaces per . For each S belonging to a prefix reachable via X's OMNI interface, X originates a separate copy of the Join/Prune for each (S,G) in the message using its own LLA as the source address and ALL-PIM-ROUTERS as the destination address. X then encapsulates each message in an OAL header with source address set to the ULA of X and destination address set to S then forwards the message into the spanning tree, which delivers it to AERO Server/Relay "Y" that services S. At the same time, if the message was a Join, X sends a route-optimization NS message toward each S the same as discussed in . The resulting NAs will return the LLA for the prefix that matches S as the network-layer source address and with an OMNI option with the ULA corresponding to any underlying interfaces that are currently servicing S. When Y processes the Join/Prune message, if S located behind any INET, Direct, or VPNed interfaces Y acts as a PIM router and updates its MRIB to list X as the next hop in the reverse path. If S is located behind any Proxys "Z"*, Y also forwards the message to each Z* over the spanning tree while continuing to use the LLA of X as the source address. Each Z* then updates its MRIB accordingly and maintains the LLA of X as the next hop in the reverse path. Since the Bridges do not examine network layer control messages, this means that the (reverse) multicast tree path is simply from each Z* (and/or Y) to X with no other multicast-aware routers in the path. If any Z* (and/or Y) is located on the same OMNI link segment as X, the multicast data traffic sent to X directly using OAL/INET encapsulation instead of via a Bridge. Following the initial Join/Prune and NS/NA messaging, X maintains an asymmetric neighbor cache entry for each S the same as if X was sending unicast data traffic to S. In particular, X performs additional NS/NA exchanges to keep the neighbor cache entry alive for up to t_periodic seconds . If no new Joins are received within t_periodic seconds, X allows the neighbor cache entry to expire. Finally, if X receives any additional Join/Prune messages for (S,G) it forwards the messages to each Y and Z* in the neighbor cache entry over the spanning tree. At some later time, Client C that holds an MNP for source S may depart from a first Proxy Z1 and/or connect via a new Proxy Z2. In that case, Y sends an unsolicited NA message to X the same as specified for unicast mobility in . When X receives the unsolicited NA message, it updates its asymmetric neighbor cache entry for the LLA for source S and sends new Join messages to any new Proxys Z2. There is no requirement to send any Prune messages to old Proxys Z1 since source S will no longer source any multicast data traffic via Z1. Instead, the multicast state for (S,G) in Proxy Z1 will soon time out since no new Joins will arrive. After some later time, C may move to a new Server Y2 and depart from old Sever Y1. In that case, Y1 sends Join messages for any of C's active (S,G) groups to Y2 while including its own LLA as the source address. This causes Y2 to include Y1 in the multicast forwarding tree during the interim time that Y1's symmetric neighbor cache entry for C is in the DEPARTED state. At the same time, Y1 sends an unsolicited NA message to X with an OMNI option with S/T-ifIndex in the header set to 0 and a release indication to cause X to release its asymmetric neighbor cache entry. X then sends a new Join message to S via the spanning tree and re-initiates route optimization the same as if it were receiving a fresh Join message from a node on a downstream link.

When an ROS X acting as a PIM router receives a Join/Prune from a node on its downstream interfaces containing one or more (*,G) pairs, it updates its Multicast Routing Information Base (MRIB) accordingly. X then forwards a copy of the message to the Rendezvous Point (RP) R for each G over the spanning tree. X uses its own LLA as the source address and ALL-PIM-ROUTERS as the destination address, then encapsulates each message in an OAL header with source address set to the ULA of X and destination address set to R, then sends the message into the spanning tree. At the same time, if the message was a Join X initiates NS/NA route optimization the same as for the SSM case discussed in . For each source S that sends multicast traffic to group G via R, the Proxy/Server Z* for the Client that aggregates S encapsulates the packets in PIM Register messages and forwards them to R via the spanning tree, which may then elect to send a PIM Join to Z*. This will result in an (S,G) tree rooted at Z* with R as the next hop so that R will begin to receive two copies of the packet; one native copy from the (S, G) tree and a second copy from the pre-existing (*, G) tree that still uses PIM Register encapsulation. R can then issue a PIM Register-stop message to suppress the Register-encapsulated stream. At some later time, if C moves to a new Proxy/Server Z*, it resumes sending packets via PIM Register encapsulation via the new Z*. 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 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 Bridges, Servers and Proxys, thereby providing redundancy in case of failures. Each OMNI link could utilize the same or different ANET 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, with each VLAN distinguished by a different SRT "color" (see: ). The Client's IP layer can select the outgoing OMNI interface appropriate for a given traffic profile while (in the reverse direction) correspondent nodes must have some way of steering their packets destined to a target via the correct OMNI link. In a first alternative, if each OMNI link services different MSPs, then 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 Bridges. 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 Bridges on the link. Correspondent nodes can then perform Segment Routing to select the correct SRT, which will then direct the 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 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 Server, the 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 Relays on each INET partition, with each Relay distributing its MNPs and/or discovering non-MNP IP GUA prefixes on its INET links. This gives rise to the opportunity to eventually distribute native IP addresses to all nodes, and to present a unified OMNI link view even if the INET partitions remain in their current protocol and addressing plans. In that way, the OMNI link can serve the dual purpose of providing a mobility/multilink service and a transition service. Or, if an INET partition is transitioned to a native IP protocol version and addressing scheme that is compatible with the OMNI link MNP-based addressing scheme, the partition and OMNI link can be joined by Relays. 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 required, Servers and Bridges 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. Servers and Bridges maintain BFD sessions in parallel with their BGP peerings. If a Server or Bridge fails, BGP peers will quickly re-establish routes through alternate paths the same as for common BGP deployments. Similarly, Proxys maintain BFD sessions with their associated Bridges even though they do not establish BGP peerings with them. Proxys SHOULD use proactive NUD for Servers for which there are currently active ANET Clients in a manner that parallels BFD, i.e., by sending unicast NS messages in rapid succession to receive solicited NA messages. When the Proxy is also sending RS messages on behalf of ANET Clients, the RS/RA messaging can be considered as equivalent hints of forward progress. This means that the Proxy need not also send a periodic NS if it has already sent an RS within the same period. If a Server fails, the Proxy will cease to receive advertisements and can quickly inform Clients of the outage by sending multicast RA messages on the ANET interface. The Proxy sends multicast RA messages with source address set to the Server's address, destination address set to (link-local) All-Nodes multicast, and Router Lifetime set to 0. The Proxy SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated by small delays . Any Clients on the ANET interface that have been using the (now defunct) Server will receive the RA messages and associate with a new Server.

AERO Clients that connect to the open Internet via INET interfaces can establish a VPN or direct link to securely connect to a Server in a "tethered" arrangement with all of the Client's traffic transiting the Server. Alternatively, the Client can associate with an INET Server using UDP/IP encapsulation and control message securing services as discussed in the following sections. When a Client's OMNI interface enables an INET underlying interface, it first determines whether the interface is likely to be behind a NAT. For IPv4, the Client assumes it is on the open Internet if the INET address is not a special-use IPv4 address per . Similarly for IPv6, the Client assumes it is on the open Internet if the INET address is a Global Unicast Address (GUA) . Otherwise, the Client assumes it may be behind one or several NATs. The Client then prepares an RS message with IPv6 source address set to its MNP-LLA, with IPv6 destination set to (link-local) All-Routers multicast and with an OMNI option with underlying interface attributes. If the Client believes that it is on the open Internet, it SHOULD include an L2ADDR in the Interface Attributes sub-option corresponding to the underlying interface; otherwise, it MAY omit L2ADDR. If the underlying address is IPv4, the Client includes the Port Number and IPv4 address written in obfuscated form as discussed in . If the underlying interface address is IPv6, the Client instead includes the Port Number and IPv6 address in obfuscated form. The Client finally includes a HIP "Initiator" message sub-option in the OMNI option to provide message authentication and performs OAL encapsulation and fragmentation if necessary. The Client then encapsulates each fragment in UDP/IP headers, sets the UDP/IP source to its INET address and UDP port, sets the UDP/IP destination to the Server's INET address and the AERO service port number (8060), then sends the (whole or fragmented) message to the Server. When the Server receives the RS, it performs OAL reassembly if necessary then authenticates the message and registers the Client's MNP and INET interface information according to the OMNI option parameters. If the RS message OMNI option includes Interface Attributes with an L2ADDR, the Server compares the encapsulation IP address and UDP port number with the (unobfuscated) values. If the values are the same, the Server caches the Client's information as "INET" addresses meaning that the Client is likely to accept direct messages without requiring NAT traversal exchanges. If the values are different (or, if the OMNI option did not include an L2ADDR) the Server instead caches the Client's information as "mapped" addresses meaning that NAT traversal exchanges may be necessary. The Server then prepares an RA message with IPv6 source and destination set corresponding to the addresses in the RS, and with an OMNI option with an Origin Indication sub-option per with the mapped and obfuscated Port Number and IP address observed in the encapsulation headers. The Server also includes a HIP "Responder" message sub-option per that contains an acknowledgement of the update sent by the Client. The Server then performs OAL encapsulation and fragmentation if necessary, and encapsulates each fragment in UDP/IP headers with adresses set per the L2ADDR information in the neighbor cache entry for the Client. When the Client receives the RA message, it performs OAL reassembly if necessary, authenticates the HIP "Responder" message, then compares the mapped Port Number and IP address from the Origin Indication sub-option with its own address. If the addresses are the same, the Client assumes the open Internet / Cone NAT principle; if the addresses are different, the Client instead assumes that further qualification procedures are necessary to detect the type of NAT and proceeds according to standard procedures . After the Client has registered its INET interfaces in such RS/RA exchanges it sends periodic RS messages to receive fresh RA messages before the Router Lifetime received on each INET interface expires. The Client also maintains default routes via its Servers, i.e., the same as described in earlier sections. When the Client sends messages to target IP addresses, it also invokes route optimization per using IPv6 ND address resolution messaging. The Client sends the NS(AR) message to the Server with an OMNI option with a HIP "Update/Sequence" message sub-option. The Client sets the NS source address to the Client's MNP-LLA and destination address to the target solicited node multicast address. The Client wraps the NS message in an OAL header with source address set to its own MNP-ULA and destination address set to the Server's ADM-ULA. The Client then performs OAL fragmentation if necessary, wraps each fragment in a UDP/IP header and sends it to the Server. When the Server receives the OAL-encapsulated NS, it performs OAL reassembly if necessary. The Server then authenticates the message by processing the HIP message sub-option and sends a corresponding NS(AR) message over the spanning tree the same as if it were the ROS, but with the OAL source address set to the Server's ADM-ULA, with destination set to the MNP-ULA of the target, and with an OMNI option that includes no sub-options. When the ROR receives the NS(AR), it adds the Server's ADM-ULA and Client's MNP-LLA to the target's Report List, and returns an NA(AR) with OMNI option information for the target including all of the target's Interface Attributes. The ROR sets the NA(AR) source address to the target's MNP-LLA and sets the destination address to the Client's MNP-LLA, then sets the OAL source address to the ADM-ULA of the ROR and the destination to the ADM-ULA of the Server. When the Server receives the NA(AR) message, it rewrites the OAL source address to its own ADM-ULA and the destination address to the MNP-ULA of the Client, then includes a HIP "Update/Acknowledge" message sub-option in the OMNI option, performs OAL fragmentation if necessary, wraps each fragment in UDP/IP headers, and sends it to the Client. Following route optimization for targets in the same OMNI link segment, if the target's L2ADDR is on the open INET, the Client forwards data packets directly to the target INET address. If the target is behind a NAT, the Client first establishes NAT state for the L2ADDR using the "direct bubble" and NUD mechanisms discussed in . The Client continues to send data packets via its Server until NAT state is populated, then begins forwarding packets via the direct path through the NAT to the target. For targets in different OMNI link segments, the Client uses OAL/ORH encapsulation and forwards data packets to the Bridge that returned the NA message. The ROR may return uNAs via the Server if the target moves, and the Server will send corresponding uNAs to the Client with a HIP "Notify" authentication message. The Client can also send NUD messages to test forward path reachability even though there is no security association between the Client and the target. The Client sends UDP/IP encapsulated IPv6 packets to route-optimized neighbors in the same OMNI link segment no larger than 576 bytes in one piece and without OAL encapsulation. Otherwise, the Client inserts an OAL header with source set to its own MNP-ULA and destination set to the MNP-ULA of the target and uses OAL fragmentation if necessary according to . The Client then encapsulates each fragment in a UDP/IP header and sends the fragments to the next hop. Note: The NAT traversal procedures specified in this document are applicable for Cone, Address-Restricted and Port-Restricted NATs only. While future updates to this document may specify procedures for other NAT variations (e.g., hairpinning and various forms of Symmetric NATs), it should be noted that continuous communications are always possible through forwarding via a Server even if NAT traversal is not employed. Note: Following the initial HIP Initiator/Responder exchange, AERO Clients with OMNI interfaces configured over the open Internet maintain HIP associations through the transmission of IPv6 ND messages that include OMNI options with HIP "Update" and "Notify" messages. OMNI interfaces use the HIP "Update" message when an acknowledgement is required, and use the "Notify" message in unacknowledged isolated IPv6 ND messages (e.g., unsolicited NAs).

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 the Client's identity would be bound to the DHCPv6 Device Unique Identifier (DUID) and used as the seed for Prefix Delegation. The Client would then be obligated to renumber its internal networks whenever its MNP (and therefore also its MNP-LLA) changes. This should not present a challenge for Clients with automated network renumbering services, however presents limits for the durations of ongoing sessions that would prefer to use a constant address.