Asymmetric Extended Route Optimization (AERO)

The following sections specify the operation of IP over Asymmetric Extended Route Optimization (AERO) links:

R1 | |(P1->S1; P2->S2)| | default->R1 | | P1->C1 | | ASP A1 | | P2->C2 | +-------+------+ +--------+-------+ +------+-------+ | | | X---+---+-------------------+------------------+---+---X | AERO Link | +-----+--------+ +--------+-----+ |AERO Client C1| |AERO Client C2| | Nbr: S1 | | Nbr: S2 | | default->S1 | | default->S2 | | ACP P1 | | ACP P2 | +--------------+ +--------------+ .-. .-. ,-( _)-. ,-( _)-. .-(_ IP )-. .-(_ IP )-. (__ EUN ) (__ EUN ) `-(______)-' `-(______)-' | | +--------+ +--------+ | Host H1| | Host H2| +--------+ +--------+ ]]> presents the AERO link reference model. In this model: AERO Relay R1 aggregates AERO Service Prefix (ASP) A1, acts as a default router for its associated Servers S1 and S2, and connects the AERO link to the rest of the IP Internetwork. AERO Servers S1 and S2 associate with Relay R1 and also act as default routers for their associated Clients C1 and C2. AERO Clients C1 and C2 associate with Servers S1 and S2, respectively. They receive AERO Client Prefix (ACP) delegations P1 and P2, and also act as default routers for their associated physical or internal virtual EUNs. (Alternatively, clients can act as multi-addressed hosts without serving any EUNs). Simple hosts H1 and H2 attach to the EUNs served by Clients C1 and C2, respectively. Each AERO node maintains an AERO interface neighbor cache and an IP forwarding table. For example, AERO Relay R1 in the diagram has neighbor cache entries for Servers S1 and S2 as well as IP forwarding table entries for the ACPs delegated to Clients C1 and C2. In common operational practice, there may be many additional Relays, Servers and Clients. (Although not shown in the figure, AERO Forwarding Agents may also be provided for data plane forwarding offload services.)

AERO Relays provide default forwarding services to AERO Servers. Relays forward packets between neighbors connected to the same AERO link and also forward packets between the AERO link and the native IP Internetwork. Relays present the AERO link to the native Internetwork as a set of one or more AERO Service Prefixes (ASPs) and serve as a gateway between the AERO link and the Internetwork. AERO Relays maintain an AERO interface neighbor cache entry for each AERO Server, and maintain an IP forwarding table entry for each AERO Client Prefix (ACP). AERO Relays can also be configured to act as AERO Servers. AERO Servers provide default forwarding services to AERO Clients. Each Server also peers with each Relay in a dynamic routing protocol instance to advertise its list of associated ACPs. Servers configure a DHCPv6 server function to facilitate Prefix Delegation (PD) exchanges with Clients. Each delegated prefix becomes an ACP taken from an ASP. Servers forward packets between AERO interface neighbors, and maintain an AERO interface neighbor cache entry for each AERO Relay. They also maintain both neighbor cache entries and IP forwarding table entries for each of their associated Clients. AERO Servers can also be configured to act as AERO Relays. AERO Clients act as requesting routers to receive ACPs through DHCPv6 PD exchanges with AERO Servers over the AERO link. Each Client MAY associate with a single Server or with multiple Servers, e.g., for fault tolerance, load balancing, etc. Each IPv6 Client receives at least a /64 IPv6 ACP, and may receive even shorter prefixes. Similarly, each IPv4 Client receives at least a /32 IPv4 ACP (i.e., a singleton IPv4 address), and may receive even shorter prefixes. AERO Clients maintain an AERO interface neighbor cache entry for each of their associated Servers as well as for each of their correspondent Clients. AERO Forwarding Agents provide data plane forwarding services as companions to AERO Servers. Note that while Servers are required to perform both control and data plane operations on their own behalf, they may optionally enlist the services of special-purpose Forwarding Agents to offload data plane traffic.

An AERO address is an IPv6 link-local address with an embedded ACP and assigned to a Client's AERO interface. The AERO address remains stable as the Client moves between topological locations, i.e., even if its link-layer addresses change. For IPv6, the AERO address begins with the prefix fe80::/64 and includes in its interface identifier (i.e., the lower 64 bits) the base prefix taken from the Client's IPv6 ACP. The base prefix is determined by masking the ACP with the prefix length. For example, if the AERO Client receives the IPv6 ACP: 2001:db8:1000:2000::/56 it constructs its AERO address as: fe80::2001:db8:1000:2000 For IPv4, the AERO address is based on an IPv4-mapped IPv6 address formed from the ACP and with a Prefix Length of 96 plus the ACP prefix length. For example, for the IPv4 ACP 192.0.2.32/28 the IPv4-mapped IPv6 address is: 0:0:0:0:0:FFFF:192.0.2.32/124 The Client then constructs its AERO address with the prefix fe80::/64 and with the lower 64 bits of the IPv4-mapped IPv6 address in the interface identifier as: fe80::FFFF:192.0.2.32 NOTE: In some cases, prospective neighbors may not have advanced knowledge of the Client's ACP length and may therefore send initial IPv6 ND messages with an AERO destination address that matches the ACP but does not correspond to the base prefix. For example, if the Client receives the IPv6 ACP 2001:db8:1000:2000::/56 then subsequently receives an IPv6 ND message with destination address fe80::2001:db8:1000:2001, it accepts the message as though it were addressed to fe80::2001:db8:1000:2000.

AERO interfaces use encapsulation (see: ) to exchange packets with neighbors attached to the AERO link. AERO interfaces maintain a neighbor cache, and AERO nodes use both DHCPv6 and IPv6 ND control messaging to manage the creation, modification and deletion of neighbor cache entries. AERO Clients send DHCPv6 Solicit, Rebind, Renew and Release messages to AERO Servers, which respond with DHCPv6 Reply messages. AERO nodes use unicast IPv6 ND Neighbor Solicitation (NS), Neighbor Advertisement (NA), Router Solicitation (RS) and Router Advertisement (RA) messages the same as for any IPv6 link. AERO interfaces use two IPv6 ND redirection message types -- the first known as a Predirect message and the second being the standard Redirect message (see ). AERO interface ND messages include one or more Source/Target Link-Layer Address Options (S/TLLAOs) formatted as shown in :

In this format: Type is set to '1' for SLLAO or '2' for TLLAO the same as for IPv6 ND. Length is set to the constant value '5' (i.e., 5 units of 8 octets). Both Reserved fields are set to the value '0' on transmission and ignored on receipt. Interface ID is set to an integer value between 0 and 255 corresponding to an underlying interface of the AERO node. UDP Port Number and IP Address are set to the addresses used by the AERO node when it sends encapsulated packets over the underlying interface. When UDP is not used as part of the encapsulation, UDP Port Number is set to the value '0'. When the encapsulation IP address family is IPv4, IP Address is formed as an IPv4-mapped IPv6 address as specified in . P[i] is a set of 64 Preference values that correspond to the 64 Differentiated Service Code Point (DSCP) values . Each P(i) is set to the value '0' ("disabled"), '1' ("low"), '2' ("medium") or '3' ("high") to indicate a preference level for packet forwarding purposes. AERO interfaces may be configured over multiple underlying interfaces. For example, common mobile handheld devices have both wireless local area network ("WLAN") and cellular wireless links. These links are typically used "one at a time" with low-cost WLAN preferred and highly-available cellular wireless as a standby. 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 IPv6 ND messages include only a single S/TLLAO with Interface ID set to a constant value. If the Client has multiple active underlying interfaces, then from the perspective of IPv6 ND it would appear to have multiple link-layer addresses. In that case, IPv6 ND messages MAY include multiple S/TLLAOs -- each with an Interface ID that corresponds to a specific underlying interface of the AERO node.

When an administrative authority first deploys a set of AERO Relays and Servers that comprise an AERO link, they also assign a unique domain name for the link, e.g., "linkupnetworks.example.com". Next, if administrative policy permits Clients within the domain to serve as correspondent nodes for Internet mobile nodes, the administrative authority adds a Fully Qualified Domain Name (FQDN) for each of the AERO link's ASPs to the Domain Name System (DNS) . The FQDN is based on the suffix "aero.linkupnetworks.net" with a prefix formed from the wildcard-terminated reverse mapping of the ASP , and resolves to a DNS PTR resource record. For example, for the ASP '2001:db8:1::/48' within the domain name "linkupnetworks.example.com", the DNS database contains: '*.1.0.0.0.8.b.d.0.1.0.0.2.aero.linkupnetworks.net. PTR linkupnetworks.example.com' This DNS registration advertises the AERO link's ASPs to prospective correspondent nodes.

When a Relay enables an AERO interface, it first assigns an administratively provisioned link-local address fe80::ID to the interface. Each fe80::ID address MUST be unique among all AERO nodes on the link, and MUST NOT collide with any potential AERO addresses nor the special addresses fe80:: and fe80::ffff:ffff:ffff:ffff. (The fe80::ID addresses are typically taken from the available range fe80::/96, e.g., as fe80::1, fe80::2, fe80::3, etc.) The Relay then engages in a dynamic routing protocol session with all Servers on the link (see: ), and advertises its assigned ASPs into the native IP Internetwork. Each Relay subsequently maintains an IP forwarding table entry for each ACP covered by its ASP(s), and maintains a neighbor cache entry for each Server on the link. Relays exchange NS/NA messages with AERO link neighbors the same as for any AERO node, however they typically do not perform explicit Neighbor Unreachability Detection (NUD) (see: ) since the dynamic routing protocol already provides reachability confirmation.

When a Server enables an AERO interface, it assigns an administratively provisioned link-local address fe80::ID the same as for Relays. The Server further configures a DHCPv6 server function to facilitate DHCPv6 PD exchanges with AERO Clients. The Server maintains a neighbor cache entry for each Relay on the link, and manages per-ACP neighbor cache entries and IP forwarding table entries based on control message exchanges. Each Server also engages in a dynamic routing protocol with each Relay on the link (see: ). When the Server receives an NS/RS message from a Client on the AERO interface it returns an NA/RA message. The Server further provides a simple link-layer conduit between AERO interface neighbors. In particular, when a packet sent by a source Client arrives on the Server's AERO interface and is destined to another of the Server's Clients, the Server forwards the packet at the link layer without ever disturbing the network layer and without ever leaving the AERO interface.

When a Client enables an AERO interface, it uses the special address fe80::ffff:ffff:ffff:ffff to obtain one or more ACPs from an AERO Server via DHCPv6 PD. Next, it assigns the corresponding AERO address(es) to the AERO interface and creates a neighbor cache entry for the Server, i.e., the DHCPv6 PD exchange bootstraps autoconfiguration of unique link-local address(es). The Client maintains a neighbor cache entry for each of its Servers and each of its active correspondent Clients. When the Client receives Redirect/Predirect messages on the AERO interface it updates or creates neighbor cache entries, including link-layer address information.

When a Forwarding Agent enables an AERO interface, it assigns the same link-local address(es) as the companion AERO Server. The Forwarding Agent thereafter provides data plane forwarding services based solely on the forwarding information assigned to it by the companion AERO Server.

The AERO routing system is based on a private instance of the Border Gateway Protocol (BGP) that is coordinated between Relays and Servers and does not interact with either the public Internet BGP routing system or the native IP Internetwork interior routing system. Relays advertise only a small and unchanging set of ASPs to the native routing system instead of the full dynamically changing set of ACPs. In a reference deployment, each AERO Server is configured as an Autonomous System Border Router (ASBR) for a stub Autonomous System (AS) using an AS Number (ASN) that is unique within the BGP instance, and each Server further peers with each Relay but does not peer with other Servers. Similarly, Relays do not peer with each other, since they will reliably receive all updates from all Servers and will therefore have a consistent view of the AERO link ACP delegations. Each Server maintains a working set of associated ACPs, and dynamically announces new ACPs and withdraws departed ACPs in its BGP updates to Relays. Clients are expected to remain associated with their current Servers for extended timeframes, however Servers SHOULD selectively suppress BGP updates for impatient Clients that repeatedly associate and disassociate with them in order to dampen routing churn. Each Relay configures a black-hole route for each of its ASPs. By black-holing the ASPs, the Relay will maintain forwarding table entries only for the ACPs that are currently active, and all other ACPs will correctly result in destination unreachable failures due to the black hole route. Relays do not send BGP updates for ACPs to Servers, but instead originate a default route. In this way, Servers have only partial topology knowledge (i.e., they know only about the ACPs of their directly associated Cliens) and they forward all other packets to Relays which have full topology knowledge. Scaling properties of the AERO routing system are limited by the number of BGP routes that can be carried by Relays. Assuming O(10^6) as a reasonable maximum number of BGP routes, this means that O(10^6) Clients can be serviced by a single set of Relays. A means of increasing scaling would be to assign a different set of Relays for each set of ASPs. In that case, each Server still peers with each Relay, but the Server institutes route filters so that each set of Relays only receives BGP updates for the ASPs they aggregate. For example, if the ASP for the AERO link is 2001:db8::/32, a first set of Relays could service the ASP segment 2001:db8::/40, a second set of Relays could service 2001:db8:0100::/40, a third set could service 2001:db8:0200::/40, etc. Assuming up to O(10^3) sets of Relays, the AERO routing system can then accommodate O(10^9) ACPs with no additional overhead for Servers and Relays (for example, it should be possible to service 4 billion /64 ACPs taken from a /32 ASP and even more for shorter ASPs). In this way, each set of Relays services a specific set of ASPs that they advertise to the native routing system, and each Server configures ASP-specific routes that list the correct set of Relays as next hops. This arrangement also allows for natural incremental deployment, and can support small scale initial deployments followed by dynamic deployment of additional Clients, Servers and Relays without disturbing the already-deployed base. Note that in an alternate routing arrangement each set of Relays could advertise the aggregated ASP for the link into the native routing system even though each Relay services only a segment of the ASP. In that case, a Relay upon receiving a packet with a destination address covered by the ASP segment of another Relay can simply tunnel the packet to the correct Relay. The tradeoff then is the penalty for Relay-to-Relay tunneling compared with reduced routing information in the native routing system. Finally, Realys can express preferences for ACPs learned from multiple Servers by assigning a BGP weight to each Server's peering configuration. In this way Relays can choose the Serevr with the highest weight as the preferred path, and then fail over to a Server with lower weight in case of ACP withdrawl or Server failure.

Each AERO interface maintains a conceptual neighbor cache that includes an entry for each neighbor it communicates with on the AERO link, the same as for any IPv6 interface . AERO interface neighbor cache entires are said to be one of "permanent", "static" or "dynamic". Permanent neighbor cache entries are created through explicit administrative action; they have no timeout values and remain in place until explicitly deleted. AERO Relays maintain a permanent neighbor cache entry for each Server on the link, and AERO Servers maintain a permanent neighbor cache entry for each Relay. Each entry maintains the mapping between the neighbor's fe80::ID network-layer address and corresponding link-layer address. Static neighbor cache entries are created through DHCPv6 PD exchanges as specified in and remain in place for durations bounded by prefix lifetimes. AERO Servers maintain static neighbor cache entries for the ACPs of each of their associated Clients, and AERO Clients maintain a static neighbor cache entry for each of their associated Servers. When an AERO Server sends a Reply message response to a Client's Solicit, Rebind or Renew message, it creates or updates a static neighbor cache entry based on the Client's DHCP Unique Identifier (DUID) as the Client identifier, the AERO address(es) corresponding to the Client's ACP(s) as the network-layer address(es), the prefix lifetime as the neighbor cache entry lifetime, the Client's encapsulation IP address and UDP port number as the link-layer address and the prefix length(s) as the length to apply to the AERO address(es). When an AERO Client receives a Reply message from a Server, it creates or updates a static neighbor cache entry based on the Reply message link-local source address as the network-layer address, the prefix lifetime as the neighbor cache entry lifetime, and the encapsulation IP source address and UDP source port number as the link-layer address. Dynamic neighbor cache entries are created or updated based on receipt of a Predirect/Redirect message as specified in , and are garbage-collected when keepalive timers expire. AERO Clients maintain dynamic neighbor cache entries for each of their active correspondent Client ACPs with lifetimes based on IPv6 ND messaging constants. When an AERO Client receives a valid Predirect message it creates or updates a dynamic neighbor cache entry for the Predirect target network-layer and link-layer addresses plus prefix length. The node then sets an "AcceptTime" variable in the neighbor cache entry to ACCEPT_TIME seconds and uses this value to determine whether packets received from the correspondent can be accepted. When an AERO Client receives a valid Redirect message it creates or updates a dynamic neighbor cache entry for the Redirect target network-layer and link-layer addresses plus prefix length. The Client then sets a "ForwardTime" variable in the neighbor cache entry to FORWARD_TIME seconds and uses this value to determine whether packets can be sent directly to the correspondent. The Client also sets a "MaxRetry" variable to MAX_RETRY to limit the number of keepalives sent when a correspondent may have gone unreachable. It is RECOMMENDED that FORWARD_TIME be set to the default constant value 30 seconds to match the default REACHABLE_TIME value specified for IPv6 ND . It is RECOMMENDED that ACCEPT_TIME be set to the default constant value 40 seconds to allow a 10 second window so that the AERO redirection procedure can converge before AcceptTime decrements below FORWARD_TIME. It is RECOMMENDED that MAX_RETRY be set to 3 the same as described for IPv6 ND address resolution in Section 7.3.3 of . Different values for FORWARD_TIME, ACCEPT_TIME, and MAX_RETRY MAY be administratively set, if necessary, to better match the AERO link's performance characteristics; however, if different values are chosen, all nodes on the link MUST consistently configure the same values. Most importantly, ACCEPT_TIME SHOULD be set to a value that is sufficiently longer than FORWARD_TIME to allow the AERO redirection procedure to converge. When there may be a Network Address Translator (NAT) between the Client and the Server, or if the path from the Client to the Server should be tested for reachability, the Client can send periodic RS messages to the Server to receive RA replies. The RS/RA messaging will keep NAT state alive and test Server reachability without disturbing the DHCPv6 server.

IP packets enter a node's AERO 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 the AERO tunnel virtual link). Packets that enter the AERO interface from the network layer are encapsulated and admitted into the AERO link, i.e., they are tunnelled to an AERO interface neighbor. Packets that enter the AERO interface from the link layer are either re-admitted into the AERO link or delivered to the network layer where they are subject to either local delivery or IP forwarding. Since each AERO node may have only partial information about neighbors on the link, AERO interfaces may forward packets with link-local destination addresses at a layer below the network layer. This means that AERO nodes act as both IP routers/hosts and sub-IP layer forwarding nodes. AERO interface sending considerations for Clients, Servers and Relays are given below. When an IP packet enters a Client's AERO interface from the network layer, if the destination is covered by an ASP the Client searches for a dynamic neighbor cache entry with a non-zero ForwardTime and an AERO address that matches the packet's destination address. (The destination address may be either an address covered by the neighbor's ACP or the (link-local) AERO address itself.) If there is a match, the Client uses a link-layer address in the entry as the link-layer address for encapsulation then admits the packet into the AERO link. If there is no match, the Client instead uses the link-layer address of a neighboring Server as the link-layer address for encapsulation. When an IP packet enters a Server's AERO interface from the link layer, if the destination is covered by an ASP the Server searches for a neighbor cache entry with an AERO address that matches the packet's destination address. (The destination address may be either an address covered by the neighbor's ACP or the AERO address itself.) If there is a match, the Server uses a link-layer address in the entry as the link-layer address for encapsulation and re-admits the packet into the AERO link. If there is no match, the Server instead uses the link-layer address in a permanent neighbor cache entry for a Relay selected through longest-prefix-match as the link-layer address for encapsulation. When an IP packet enters a Relay's AERO interface from the network layer, the Relay searches its IP forwarding table for an entry that is covered by an ASP and also matches the destination. If there is a match, the Relay uses the link-layer address in the corresponding neighbor cache entry as the link-layer address for encapsulation and admits the packet into the AERO link. When an IP packet enters a Relay's AERO interface from the link-layer, if the destination is not a link-local address and does not match an ASP the Relay removes the packet from the AERO interface and uses IP forwarding to forward the packet to the Internetwork. If the destination address is a link-local address or a non-link-local address that matches an ASP, and there is a more-specific ACP entry in the IP forwarding table, the Relay uses the link-layer address in the corresponding neighbor cache entry as the link-layer address for encapsulation and re-admits the packet into the AERO link. When an IP packet enters a Relay's AERO interface from either the network layer or link-layer, and the packet's destination address matches an ASP but there is no more-specific ACP entry, the Relay drops the packet and returns an ICMP Destination Unreachable message (see: ). When an AERO Server receives a packet from a Relay via the AERO interface, the Server MUST NOT forward the packet back to the same or a different Relay. When an AERO Relay receives a packet from a Server via the AERO interface, the Relay MUST NOT forward the packet back to the same Server. When an AERO node re-admits a packet into the AERO link without involving the network layer, the node MUST NOT decrement the network layer TTL/Hop-count. When an AERO node forwards a data packet to the primary link-layer address of a Server, it may receive Redirect messages with an SLLAO that include the link-layer address of an AERO Forwarding Agent. The AERO node SHOULD record the link-layer address in the neighbor cache entry for the neighbor and send subsequent data packets via this address instead of the Server's primary address (see: ). AERO nodes may have multiple underlying interfaces and/or neighbor cache entries for Clients with multiple Interface ID registrations (see ). The AERO node uses the packet's DSCP value to select the outgoing underlying interface based on its own Interface ID preference values and to select the destination link-layer address based on the neighbor's Interface ID with the highest preference value. If multiple Interface IDs have a preference of "high", the AERO node sends one copy of the packet to each of the link-layer addresses (i.e., it replicates the packet); otherwise, the node sends a single copy of the packet.

AERO interfaces encapsulate IP packets according to whether they are entering the AERO interface from the network layer or if they are being re-admitted into the same AERO link they arrived on. This latter form of encapsulation is known as "re-encapsulation". The AERO interface encapsulates packets per the Generic UDP Encapsulation (GUE) encapsulation procedures in , or through an alternate encapsulation format (see: ). For packets entering the AERO link from the IP layer, the AERO interface copies the "TTL/Hop Limit", "Type of Service/Traffic Class" , "Flow Label" .(for IPv6) and "Congestion Experienced" values in the packet's IP header into the corresponding fields in the encapsulation IP header. For packets undergoing re-encapsulation within the AERO link, the AERO interface instead copies the "TTL/Hop Limit", "Type of Service/Traffic Class", "Flow Label" and "Congestion Experienced" values in the original encapsulation IP header into the corresponding fields in the new encapsulation IP header, i.e., the values are transferred between encapsulation headers and *not* copied from the encapsulated packet's network-layer header. When GUE encapsulation is used, the AERO 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 the GUE header (or 0 if GUE direct IP encapsulation is used). For packets sent to a Server, the AERO interface sets the UDP destination port to 8060, i.e., the IANA-registered port number for AERO. For packets sent to a correspondent Client, the AERO interface sets the UDP destination port to the port value stored in the neighbor cache entry for this correspondent. The AERO interface then either includes or omits the UDP checksum according to the GUE specification. For IPv4 encapsulation, the AERO interface sets the DF bit as discussed in .

AERO interfaces decapsulate packets destined either to the AERO node itself or to a destination reached via an interface other than the AERO interface the packet was received on. Decapsulation is per the procedures specified for the appropriate encapsulation format.

AERO nodes employ simple data origin authentication procedures for encapsulated packets they receive from other nodes on the AERO link. In particular: AERO Servers and Relays accept encapsulated packets with a link-layer source address that matches a permanent neighbor cache entry. AERO Servers accept authentic encapsulated DHCPv6 messages from Clients, and create or update a static neighbor cache entry for the Client based on the specific DHCPv6 message type. AERO Clients and Servers accept encapsulated packets if there is a static neighbor cache entry with a link-layer address that matches the packet's link-layer source address. AERO Clients, Servers and Relays accept encapsulated packets if there is a dynamic neighbor cache entry with an AERO address that matches the packet's network-layer source address, with a link-layer address that matches the packet's link-layer source address, and with a non-zero AcceptTime. Note that this simple data origin authentication is effective in environments in which link-layer addresses cannot be spoofed. In other environments, each AERO message must include a signature that the recipient can use to authenticate the message origin.

The AERO interface is the node's attachment to the AERO link. The AERO interface acts as a tunnel ingress when it sends a packet to an AERO link neighbor and as a tunnel egress when it receives a packet from an AERO link neighbor. AERO interfaces observe the packet sizing considerations for tunnels discussed in and as specified below. IPv6 specifies a minimum link Maximum Transmission Unit (MTU) of 1280 bytes . Although IPv4 specifies a smaller minimum link MTU of 68 bytes , AERO interfaces also observe the IPv6 minimum for IPv4 even if the packet may incur fragmentation in the network. IPv6 specifies a minimum Maximum Reassembly Unit (MRU) of 1500 bytes , while the minimum MRU for IPv4 is only 576 bytes (note that IPv6 over IPv4 tunnels assume a larger MRU than the IPv4 minimum). Original sources expect that IP packets will either be delivered to the final destination or a suitable Packet Too Big (PTB) message returned. However, PTB messages may be crafted for malicious purposes such as denial of service, or lost in the network resulting in failure of the IP Path MTU Discovery (PMTUD) mechanisms . For these reasons, AERO links employ operational procedures that avoid all interactions with PMTUD. AERO Servers advertise an MTU that MUST be no smaller than 1280 bytes, MUST be no larger than the minimum MRU among all nodes on the AERO link minus the encapsulation overhead ("ENCAPS"), and SHOULD be no smaller than 1500 bytes. AERO Servers advertise a Maximum Fragment Unit (MFU) as the maximum size for the fragments of an encapsulated packet that require fragmentation. The MFU value MUST NOT be larger than (MTU+ENCAPS) and MUST NOT be larger than 1280 bytes unless there is operational assurance that a larger size can traverse the link along all paths without fragmentation. AERO Clients set the AERO interface MTU/MFU based on the values advertised by their Server, and configure an MRU large enough to reassemble packets up to (MTU+ENCAPS) bytes. All AERO nodes on the link MUST configure the same MTU/MFU values for reasons cited in (in particular, multicast support requires a common MTU value among all nodes on the link). All AERO nodes on the link MUST configure a minimum MRU of (1500+ENCAPS) bytes, and SHOULD be capable of setting a larger MRU accoding to the Server's advertised MTU. In accordnace with these requirements, the ingress accommodates packets of various sizes as follows: First, for each original IPv4 packet that is larger than the AERO interface MTU and with the DF bit set to 0, the ingress uses IPv4 fragmentation to break the packet into a minimum number of non-overlapping fragments where the first fragment is no larger than (MFU-ENCAPS) bytes and the remaining fragments are no larger than the first. Next, for each original IP packet or fragment that is no larger than (MFU-ENCAPS) bytes, the ingress encapsulates the packet and admits it into the tunnel. For IPv4 AERO links, the ingress sets the Don't Fragment (DF) bit to 0 so that these packets will be delivered to the egress even if some fragmentation occurs in the network. For all other original IP packets or fragments, if the packet is larger than the AERO interface MTU, the ingress drops the packet and returns a PTB message to the original source. Otherwise, the ingress encapsulates the packet and fragments the encapsulated packet into a minimum number of non-overlapping fragments where the first fragment is no larger than MFU bytes and the remaining fragments are no larger than the first. The ingress then admits the fragments into the tunnel, and for IPv4 sets the DF bit to 0 in the IP encapsulation header. These fragmented encapsulated packets will be delivered to the egress, which reassembles them into a whole packet. Several factors must be considered when fragmentation of the encapsulated packet is needed. For AERO links over IPv4, the IP ID field is only 16 bits in length, meaning that fragmentation at high data rates could result in data corruption due to reassembly misassociations . For AERO links over both IPv4 and IPv6, studies have also shown that IP fragments are dropped unconditionally over some network paths [I-D.taylor-v6ops-fragdrop]. In environments where IP fragmentation issues could result in operational problems, the ingress SHOULD employ intermediate-layer fragmentation (see: and ) before appending the outer encapsulation headers to each fragment. Since the encapsulation fragment header reduces the room available for packet data, but the original source has no way to control its insertion, the ingress MUST include the fragment header length in the ENCAPS length even for packets in which the header is absent.

When an AERO node admits encapsulated packets into the AERO interface, it may receive link-layer (L2) or network-layer (L3) error indications. An L2 error indication is an ICMP error message generated by a router 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" . (AERO interfaces ignore all L2 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.) 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 L2 error message format is shown in :

The AERO node rules for processing these L2 error messages is as follows: When an AERO node receives an L2 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 L2 IPv4 Time Exceeded messages, the IP ID field may be wrapping before earlier fragments have been processed. In that case, the node SHOULD begin including integrity checks and/or institute rate limits for subseqent packets. When an AERO Client receives persistent L2 Destination Unreachable messages in response to tunneled packets that it sends to one of its dynamic neighbor correspondents, the Client SHOULD test the path to the correspondent using Neighbor Unreachability Detection (NUD) (see ). If NUD fails, the Client SHOULD set ForwardTime for the corresponding dynamic neighbor cache entry to 0 and allow future packets destined to the correspondent to flow through a Server. When an AERO Client receives persistent L2 Destination Unreachable messages in response to tunneled packets that it sends to one of its static neighbor Servers, the Client SHOULD test the path to the Server using NUD. If NUD fails, the Client SHOULD delete the neighbor cache entry and attempt to associate with a new Server. When an AERO Server receives persistent L2 Destination Unreachable messages in response to tunneled packets that it sends to one of its static neighbor Clients, the Server SHOULD test the path to the Client using NUD. If NUD fails, the Server SHOULD cancel the DHCPv6 PD for the Client's ACP, withdraw its route for the ACP from the AERO routing system and delete the neighbor cache entry (see and ). When an AERO Relay or Server receives an L2 Destination Unreachable message in response to a tunneled packet that it sends to one of its permanent neighbors, it discards the message since the AERO routing system is likely in a temporary transitional state that will soon re-converge. In case of a prolonged outage, however, the AERO routing system will compensate for Relays or Servers that have fallen silent. When an AERO Relay receives an L3 packet for which the destination address is covered by an ASP, if there is no more-specific routing information for the destination the Relay drops the packet and returns an L3 Destination Unreachable message. The Relay first writes the IP source address of the original L3 packet as the destination address of the L3 Destination Unreachable message and determines the next hop to the destination. If the next hop is reached via the AERO interface, the Relay uses the IPv6 address "::" or the IPv4 address "0.0.0.0" as the IP source address of the L3 Destination Unreachable message and forwards the message to the next hop within the AERO interface. Otherwise, the Relay uses one of its non link-local addresses as the source address of the L3 Destination Unreachable message and forwards the message via a link outside the AERO interface. When an AERO node receives an encapsulated packet for which the reassembly buffer it too small, it drops the packet and returns an L3 Packet To Big (PTB) message. The node first writes the IP source address of the original L3 packet as the destination address of the L3 PTB message and determines the next hop to the destination. If the next hop is reached via the AERO interface, the node uses the IPv6 address "::" or the IPv4 address "0.0.0.0" as the IP source address of the L3 PTB message and forwards the message to the next hop within the AERO interface. Otherwise, the node uses one of its non link-local addresses as the source address of the L3 PTB message and forwards the message via a link outside the AERO interface. When an AERO node receives any L3 error message via the AERO interface, it examines the destination address in the L3 IP header of the message. If the next hop toward the destination address of the error message is via the AERO interface, the node re-encapsulates and forwards the message to the next hop within the AERO interface. Otherwise, if the source address in the L3 IP header of the message is the IPv6 address "::" or the IPv4 address "0.0.0.0", the node writes one of its non link-local addresses as the source address of the L3 message and recalculates the IP and/or ICMP checksums. The node finally forwards the message via a link outside of the AERO interface.

AERO Router Discovery, Prefix Delegation and Address Configuration are coordinated by the DHCPv6 control messaging protocol as discussed in the following Sections.

Each AERO Server configures a DHCPv6 server function to facilitate PD requests from Clients. Each Server is provisioned with a database of ACP-to-Client ID mappings for all Clients enrolled in the AERO system, as well as any information necessary to authenticate each Client. The Client database is maintained by a central administrative authority for the AERO link and securely distributed to all Servers, e.g., via the Lightweight Directory Access Protocol (LDAP) or a similar distributed database service. Therefore, no Server-to-Server DHCPv6 PD state synchronization is necessary, and Clients can optionally hold separate PDs for the same ACPs from multiple Servers. In this way, Clients can associate with multiple Servers, and can receive new PDs from new Servers before deprecating PDs received from existing Servers. This provides the Client with a natural fault-tolerance and/or load balancing profile. AERO Clients and Servers exchange configuration information using an AERO Vendor-Specific Information Option (AVSIO) formatted as follows:

In this format, "enterprise-number" is set to 45282 (i.e., the IANA-reserved enterprise number for AERO) and "option-length" is set to the total length of the option. A single AVSIO may include one or more AERO-specific (sub)options as defined in the following sections. AERO Clients MUST include an AVSIO in DHCPv6 Solicit and Rebind messages to manage the Server's cached link-layer addresses and preferences. AERO Servers MUST include an AVSIO in DHCPv6 Reply messages that correspond to a Client's DHCPv6 message that also included an AVSIO option. The following sections specify the Client and Server behavior in more detail.

AERO Clients discover the link-layer addresses of AERO Servers via static configuration (e.g., from a flat-file map of Server addresses and locations), or through an automated means such as DNS name resolution. In the absence of other information, the Client resolves the FQDN "linkupnetworks.[domainname]" where "linkupnetworks" is a constant text string and "[domainname]" is a DNS suffix for the Client's underlying network (e.g., "example.com"). After discovering the link-layer addresses, the Client associates with one or more of the corresponding Servers. To associate with a Server, the Client acts as a requesting router to request ACPs through a two-message (i.e., Solicit/Reply) DHCPv6 PD exchange . The Client's includes fe80::ffff:ffff:ffff:ffff as the IPv6 source address of the Solicit message, 'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination address, an underlying interface address of the Client (i.e., the link-layer address) as the link-layer source address and the link-layer address of the Server as the link-layer destination address. The Client also includes a Rapid Commit option, a Client Identifier option with the Client's DUID, and an Identity Association for Prefix Delegation (IA_PD) option. If the Client is pre-provisioned with ACPs associated with the AERO service, it MAY also include the ACPs in the IA_PD to indicate its preferences to the DHCPv6 server. The Client also includes an AVSIO option with one or more AERO Client Link-Layer Address Options (ACLLAOs) to register its link-layer address(es) with the Server. The first ACLLAO MUST be specific to the underlying interface over which the Client will send the Solicit. The Client MAY include additonal ACLLAOs specific to other underlying interfaces, but if so it MUST have assurance that there will be no NATs on the paths to the Server via those interfaces. (Otherwise, the Client MAY issue subsequent Rebind messages after the initial Solicit/Reply exchange to register additional link-layer addresses). The Server will echo the ACLLAOs in the corresponding Reply message as specified in . The format for the ACLLAO is shown in :

In the above format, the Client sets 'opt-code' to 0 ("OPTION_ACLLAO") and sets 'option-len' to 36 (i.e., the length of the option following this field). The Client then includes an "AERO Client Link-Layer Address" in the same format as for S/TLLAOs in beginning with the 'Reserved2' field and extending to the end of the S/TLLAO. The Client then sets 'Reserved2', 'Interface ID', 'UDP Port Number', 'IP address' and 'P(i)' values for the specific underlying interface the same as for S/TLLAO options (see ). The Client finally includes any additional DHCPv6 options (including any necessary authentication options to identify itself to the DHCPv6 server), and sends the encapsulated Solicit message via the underlying interface corresponding to the Interface ID of the first ACLLAO. When the Client attempts to perform a DHCPv6 PD exchange with a Server that is too busy to service the request, the Client may receive an error status code such as "NoPrefixAvail" in the Server's Reply or no Reply at all. In that case, the Client SHOULD discontinue DHCPv6 PD attempts through this Server and try another Server. When the Client receives a Reply from the AERO Server with an AVSIO option and no error status codes, it can compare the UDP Port Number and IP Address values in the first ACLLAO with the values the Client provided in its request. If the values are different, the Client can infer that there is a NAT on the path to the Server via that underlying interface. If the AVSIO option also includes an ALINFO sub-option, the Client also assigns the MTU/MFU values in the ALINFO option to its AERO interface, then caches any ASPs included in the ALINFO option as ASPs to associate with the AERO link (see ). This configuration information applies to the AERO link as a whole, and all Clients will receive the same information. The Client next creates a static neighbor cache entry with the Server's link-local address as the network-layer address and the Server's encapsulation address as the link-layer address. Next, the Client autoconfigures an AERO address for each of the delegated ACPs, assigns the address(es) to the AERO interface and sub-delegates the ACPs to its attached EUNs and/or the Client's own internal virtual interfaces. The Client can then configure as many addresses as it wants from /64 prefixes taken from the ACPs and assign them to either an internal virtual interface ("weak end-system") or to the AERO interface itself ("strong end-system") while black-holing the remaining portions of the /64s. Finally, the Client assigns a default IP route to the AERO interface with the link-local address of the Server as the next hop and with the PD lifetime as the default router lifetime. After the initial Solicit/Reply exchange, the Client SHOULD begin using the AERO address as the source address for further DHCPv6 messaging. The Client subsequently renews its ACP delegations through each of its Servers by sending Renew messages with the link-layer address of a Server as the link-layer destination address. The Client MAY subsequently issue Rebind messages with additional ACLLAOs if it wishes to register additional Interface IDs and/or update the link-layer address information for existing Interface IDs. In that case, the Rebind message MUST be sent over the underlying interface corresponding to the first ACLLAO in the message, i.e., the same as for Solicits. After an AERO Client registers its Interface IDs and their associated 'P(i)' values with the AERO Server, the Client may wish to change one or more Interface ID registrations, e.g., if an underlying interface becomes unavailable, if cost profiles change, etc. To do so, the Client prepares a Rebind message to send over any available underlying interface. The Rebind MUST include the ACLLAO specific to the selected avaialble underlying interface as the first ACLLAO and MAY include any additional ACLLAOs specific to other underlying interfaces. The Client includes fresh 'P(i)' values in each ACLLAO to update the Server's neighbor cache entry. If the Client wishes to disable some or all DSCPs for an underlying interface, it includes an ACLLAO with 'P(i)' values set to 0 ("disabled"). If the Client wishes to discontinue use of a Server it issues a Release to delete the Server's neighbor cache entry.

AERO Servers configure a DHCPv6 server function on their AERO links. AERO Servers arrange to add their encapsulation layer IP addresses (i.e., their link-layer addresses) to a static map of Server addresses for the link and/or the DNS resource records for the FQDN "linkupnetworks.[domainname]" before entering service. When an AERO Server receives a prospective Client's Solicit on its AERO interface, and the Server is too busy to service the message, it SHOULD return a Reply with status code "NoPrefixAvail" per . Otherwise, the Server authenticates the message. If authentication succeeds, the Server determines the correct ACPs to delegate to the Client by searching the Client database. When the Server delegates the ACPs, it also creates IP forwarding table entries so that the AERO BGP-based routing system will propagate the ACPs to all Relays that aggregate the corresponding ASP (see: ). Next, the Server prepares a Reply message to send to the Client while using fe80::ID as the IPv6 source address, the link-local address taken from the Client's Solicit as the IPv6 destination address, the Server's link-layer address as the source link-layer address, and the Client's link-layer address as the destination link-layer address. The Server also includes IA_PD options with the delegated ACPs. For IPv4 ACPs, the prefix included in the IA_PD option is in IPv4-mapped IPv6 address format and with prefix length set as specified in . For AERO links where a Client may experience a fault that prevents it from issuing a Release before departing from the network, Servers should set a short prefix lifetime (e.g., 40 seconds) so that stale PD state can be flushed out of the network. For Replies to Client DHCPv6 messages that include an AVSIO, the Server prepares a new AVSIO to include in the Reply. The Server first copies the ACLLAOs in the body of the Client's AVSIO into the AVSIO that the Server will supply in the Reply message. For the initial ACLLAO, the Server sets 'UDP Port Number' and 'IP address' to the values observed in the outer encapsulating headers of the Client's DHCPv6 message, i.e., even if these values are different than the ones included by the Client. The Server next copies an ALINFO option into the body of the AVSIO (i.e., following the ACLLAO options) formatted as shown in :

In the ALINFO option, the Server sets sets 'opt-code' to 1 ("OPTION_ALINFO") and sets 'option-len' to the length of the remainder of the option. The Server next sets MTU and MFU according to the considerations specified in . The Server finally includes one or more ASPs with 'Prefix Len' set to the ASP prefix length (between 0 and 64), and 'AERO Service Prefix' set to the ASP (between 1 and 8 bytes). When the Server sends the Reply message, it creates or updates a static neighbor cache entry for the Client based on the DUID and AERO addresses with lifetime set to no more than the PD lifetimes and updates the Client's link-layer addresses according to the ACLLAOs. The Server then uses the Client link-layer addresses as the link-layer addresses for encapsulation and uses the 'P(i)' values included in ACLLAOs as preference levels for each DSCP value. After the initial DHCPv6 PD Solicit/Reply exchange, the AERO Server maintains the neighbor cache entry for the Client until the PD lifetimes expire. If the Client issues a Rebind, the Server uses any included ACLLAOs to update the link-layer information in the Client's neighbor cache entry. If the Client issues a Renew, the Server extends the PD lifetimes. If the Client issues a Release, or if the Client does not issue a Renew before the lifetime expires, the Server deletes the neighbor cache entry for the Client and withdraws the IP routes from the AERO routing system.

AERO Clients and Servers are always on the same link (i.e., the AERO link) from the perspective of DHCPv6. However, in some implementations the DHCPv6 server and AERO interface driver may be located in separate modules. In that case, the Server's AERO interface driver module can act as a Lightweight DHCPv6 Relay Agent (LDRA) to relay DHCPv6 messages to and from the DHCPv6 server module. When the LDRA receives a DHCPv6 message from a client, it prepares an AVSIO (including any ACLLAO and ALINFO options as described above) and copies the option into a DHCPv6 Relay-Supplied Option Option (RSOO) . The LDRA then incorporates the RSOO into the Relay-Forward message and forwards the message to the DHCPv6 server. When the DHCPv6 server receives the Relay-Forward message, it caches the AVSIO included in the RSOO and discards the AVSIO included within the Client's message itself. Next, the server authenticates the Client's message and prepares a Reply message if authentication succeeds. When the DHCPv6 server prepares a Reply message, it then includes the relay-supplied AVSIO in the body of the message along with any other options, then wraps the message in a Relay-Reply message. The DHCPv6 server then delivers the Relay-Reply message to the LDRA, which discards the Relay-Reply wrapper and delivers the DHCPv6 message to the Client.

AERO Servers MAY associate with one or more companion AERO Forwarding Agents as platforms for offloading high-speed data plane traffic. When an AERO Server receives a Client's Solicit/Renew/Rebind/Release message, it services the message then forwards the corresponding Reply message to the Forwarding Agent. When the Forwarding Agent receives the Reply message, it creates, updates or deletes a neighbor cache entry with the Client's AERO address and link-layer information included in the Reply message. The Forwarding Agent then forwards the Reply message back to the AERO Server, which forwards the message to the Client. In this way, Forwarding Agent state is managed in conjunction with Server state, with the Client responsible for reliability. When an AERO Server receives a data packet on an AERO interface with a network layer destination address for which it has distributed forwarding information to a Forwarding Agent, the Server returns a Redirect message to the source neighbor (subject to rate limiting) then forwards the data packet as usual. The Redirect message includes a TLLAO with the link-layer address of the Forwarding Engine. When the source neighbor receives the Redirect message, it SHOULD record the link-layer address in the TLLAO as the encapsulation addresses to use for sending subsequent data packets. However, the source MUST continue to use the primary link-layer address of the Server as the encapsulation address for sending control messages.

When a source Client forwards packets to a prospective correspondent Client within the same AERO link domain (i.e., one for which the packet's destination address is covered by an ASP), the source Client MAY initiate an AERO link route optimization procedure. It is important to note that this procedure is initiated by the Client; if the procedure were initiated by the Server, the Server would have no way of knowing whether the Client was actually able to contact the correspondent over the route-optimized path. The procedure is based on an exchange of IPv6 ND messages using a chain of AERO Servers and Relays as a trust basis. This procedure is in contrast to the Return Routability procedure required for route optimization to a correspondent Client located in the Internet as described in . The following sections specify the AERO link route optimization procedure.

depicts the AERO link route optimization reference operational scenario, using IPv6 addressing as the example (while not shown, a corresponding example for IPv4 addressing can be easily constructed). The figure shows an AERO Relay ('R1'), two AERO Servers ('S1', 'S2'), two AERO Clients ('C1', 'C2') and two ordinary IPv6 hosts ('H1', 'H2'):

In , Relay ('R1') assigns the address fe80::1 to its AERO interface with link-layer address L2(R1), Server ('S1') assigns the address fe80::2 with link-layer address L2(S1),and Server ('S2') assigns the address fe80::3 with link-layer address L2(S2). Servers ('S1') and ('S2') next arrange to add their link-layer addresses to a published list of valid Servers for the AERO link. AERO Client ('C1') receives the ACP 2001:db8:0::/48 in a DHCPv6 PD exchange via AERO Server ('S1') then assigns the address fe80::2001:db8:0:0 to its AERO interface with link-layer address L2(C1). Client ('C1') configures a default route and neighbor cache entry via the AERO interface with next-hop address fe80::2 and link-layer address L2(S1), then sub-delegates the ACP to its attached EUNs. IPv6 host ('H1') connects to the EUN, and configures the address 2001:db8:0::1. AERO Client ('C2') receives the ACP 2001:db8:1::/48 in a DHCPv6 PD exchange via AERO Server ('S2') then assigns the address fe80::2001:db8:1:0 to its AERO interface with link-layer address L2(C2). Client ('C2') configures a default route and neighbor cache entry via the AERO interface with next-hop address fe80::3 and link-layer address L2(S2), then sub-delegates the ACP to its attached EUNs. IPv6 host ('H2') connects to the EUN, and configures the address 2001:db8:1::1.

Again, with reference to , when source host ('H1') sends a packet to destination host ('H2'), the packet is first forwarded over the source host's attached EUN to Client ('C1'). Client ('C1') then forwards the packet via its AERO interface to Server ('S1') and also sends a Predirect message toward Client ('C2') via Server ('S1'). Server ('S1') then re-encapsulates and forwards both the packet and the Predirect message out the same AERO interface toward Client ('C2') via Relay ('R1'). When Relay ('R1') receives the packet and Predirect message, it consults its forwarding table to discover Server ('S2') as the next hop toward Client ('C2'). Relay ('R1') then forwards both the packet and the Predirect message to Server ('S2'), which then forwards them to Client ('C2'). After Client ('C2') receives the Predirect message, it process the message and returns a Redirect message toward Client ('C1') via Server ('S2'). During the process, Client ('C2') also creates or updates a dynamic neighbor cache entry for Client ('C1'). When Server ('S2') receives the Redirect message, it re-encapsulates the message and forwards it on to Relay ('R1'), which forwards the message on to Server ('S1') which forwards the message on to Client ('C1'). After Client ('C1') receives the Redirect message, it processes the message and creates or updates a dynamic neighbor cache entry for Client ('C2'). Following the above Predirect/Redirect message exchange, forwarding of packets from Client ('C1') to Client ('C2') without involving any intermediate nodes is enabled. The mechanisms that support this exchange are specified in the following sections.

AERO Redirect/Predirect messages use the same format as for IPv6 ND Redirect messages depicted in Section 4.5 of , but also include a new "Prefix Length" field taken from the low-order 8 bits of the Redirect message Reserved field. For IPv6, valid values for the Prefix Length field are 0 through 64; for IPv4, valid values are 0 through 32. The Redirect/Predirect messages are formatted as shown in :

When a Client forwards a packet with a source address from one of its ACPs toward a destination address covered by an ASP (i.e., toward another AERO Client connected to the same AERO link), the source Client MAY send a Predirect message forward toward the destination Client via the Server. In the reference operational scenario, when Client ('C1') forwards a packet toward Client ('C2'), it MAY also send a Predirect message forward toward Client ('C2'), subject to rate limiting (see Section 8.2 of ). Client ('C1') prepares the Predirect message as follows: the link-layer source address is set to 'L2(C1)' (i.e., the link-layer address of Client ('C1')). the link-layer destination address is set to 'L2(S1)' (i.e., the link-layer address of Server ('S1')). the network-layer source address is set to fe80::2001:db8:0:0 (i.e., the AERO address of Client ('C1')). the network-layer destination address is set to fe80::2001:db8:1:0 (i.e., the AERO address of Client ('C2')). the Type is set to 137. the Code is set to 1 to indicate "Predirect". the Prefix Length is set to the length of the prefix to be assigned to the Target Address. the Target Address is set to fe80::2001:db8:0:0 (i.e., the AERO address of Client ('C1')). the Destination Address is set to the source address of the originating packet that triggered the Predirection event. (If the originating packet is an IPv4 packet, the address is constructed in IPv4-mapped IPv6 address format). the message includes one or more TLLAOs set to appropriate values for Client ('C1')'s underlying interfaces, and with UDP Port Number and IP Address set to 0'. the message SHOULD include a Timestamp option and a Nonce option. the message includes a Redirected Header Option (RHO) that contains the originating packet truncated if necessary to ensure that at least the network-layer header is included but the size of the message does not exceed 1280 bytes. Note that the act of sending Predirect messages is cited as "MAY", since Client ('C1') may have advanced knowledge that the direct path to Client ('C2') would be unusable or otherwise undesirable. If the direct path later becomes unusable after the initial route optimization, Client ('C1') simply allows packets to again flow through Server ('S1').

When Server ('S1') receives a Predirect message from Client ('C1'), it first verifies that the TLLAOs in the Predirect are a proper subset of the Interface IDs in Client ('C1')'s neighbor cache entry. If the Client's TLLAOs are not acceptable, Server ('S1') discards the message. Otherwise, Server ('S1') validates the message according to the Redirect message validation rules in Section 8.1 of , except that the Predirect has Code=1. Server ('S1') also verifies that Client ('C1') is authorized to use the Prefix Length in the Predirect when applied to the AERO address in the network-layer source address by searching for the AERO address in the neighbor cache. If validation fails, Server ('S1') discards the Predirect; otherwise, it copies the correct UDP Port numbers and IP Addresses for Client ('C1')'s links into the (previously empty) TLLAOs. Server ('S1') then examines the network-layer destination address of the Predirect to determine the next hop toward Client ('C2') by searching for the AERO address in the neighbor cache. Since Client ('C2') is not one of its neighbors, Server ('S1') re-encapsulates the Predirect and relays it via Relay ('R1') by changing the link-layer source address of the message to 'L2(S1)' and changing the link-layer destination address to 'L2(R1)'. Server ('S1') finally forwards the re-encapsulated message to Relay ('R1') without decrementing the network-layer TTL/Hop Limit field. When Relay ('R1') receives the Predirect message from Server ('S1') it determines that Server ('S2') is the next hop toward Client ('C2') by consulting its forwarding table. Relay ('R1') then re-encapsulates the Predirect while changing the link-layer source address to 'L2(R1)' and changing the link-layer destination address to 'L2(S2)'. Relay ('R1') then relays the Predirect via Server ('S2'). When Server ('S2') receives the Predirect message from Relay ('R1') it determines that Client ('C2') is a neighbor by consulting its neighbor cache. Server ('S2') then re-encapsulates the Predirect while changing the link-layer source address to 'L2(S2)' and changing the link-layer destination address to 'L2(C2)'. Server ('S2') then forwards the message to Client ('C2').

When Client ('C2') receives the Predirect message, it accepts the Predirect only if the message has a link-layer source address of one of its Servers (e.g., L2(S2)). Client ('C2') further accepts the message only if it is willing to serve as a redirection target. Next, Client ('C2') validates the message according to the Redirect message validation rules in Section 8.1 of , except that it accepts the message even though Code=1 and even though the network-layer source address is not that of it's current first-hop router. In the reference operational scenario, when Client ('C2') receives a valid Predirect message, it either creates or updates a dynamic neighbor cache entry that stores the Target Address of the message as the network-layer address of Client ('C1') , stores the link-layer addresses found in the TLLAOs as the link-layer addresses of Client ('C1') and stores the Prefix Length as the length to be applied to the network-layer address for forwarding purposes. Client ('C2') then sets AcceptTime for the neighbor cache entry to ACCEPT_TIME. After processing the message, Client ('C2') prepares a Redirect message response as follows: the link-layer source address is set to 'L2(C2)' (i.e., the link-layer address of Client ('C2')). the link-layer destination address is set to 'L2(S2)' (i.e., the link-layer address of Server ('S2')). the network-layer source address is set to fe80::2001:db8:1:0 (i.e., the AERO address of Client ('C2')). the network-layer destination address is set to fe80::2001:db8:0:0 (i.e., the AERO address of Client ('C1')). the Type is set to 137. the Code is set to 0 to indicate "Redirect". the Prefix Length is set to the length of the prefix to be applied to the Target Address. the Target Address is set to fe80::2001:db8:1:0 (i.e., the AERO address of Client ('C2')). the Destination Address is set to the destination address of the originating packet that triggered the Redirection event. (If the originating packet is an IPv4 packet, the address is constructed in IPv4-mapped IPv6 address format). the message includes one or more TLLAOs set to appropriate values for Client ('C2')'s underlying interfaces, and with UDP Port Number and IP Address set to '0'. the message SHOULD include a Timestamp option and MUST echo the Nonce option received in the Predirect (i.e., if a Nonce option is included). the message includes as much of the RHO copied from the corresponding Predirect message as possible such that at least the network-layer header is included but the size of the message does not exceed 1280 bytes. After Client ('C2') prepares the Redirect message, it sends the message to Server ('S2').

When Server ('S2') receives a Redirect message from Client ('C2'), it first verifies that the TLLAOs in the Redirect are a proper subset of the Interface IDs in Client ('C2')'s neighbor cache entry. If the Client's TLLAOs are not acceptable, Server ('S2') discards the message. Otherwise, Server ('S2') validates the message according to the Redirect message validation rules in Section 8.1 of . Server ('S2') also verifies that Client ('C2') is authorized to use the Prefix Length in the Redirect when applied to the AERO address in the network-layer source address by searching for the AERO address in the neighbor cache. If validation fails, Server ('S2') discards the Redirect; otherwise, it copies the correct UDP Port numbers and IP Addresses for Client ('C2')'s links into the (previously empty) TLLAOs. Server ('S2') then examines the network-layer destination address of the Redirect to determine the next hop toward Client ('C1') by searching for the AERO address in the neighbor cache. Since Client ('C1') is not a neighbor, Server ('S2') re-encapsulates the Redirect and relays it via Relay ('R1') by changing the link-layer source address of the message to 'L2(S2)' and changing the link-layer destination address to 'L2(R1)'. Server ('S2') finally forwards the re-encapsulated message to Relay ('R1') without decrementing the network-layer TTL/Hop Limit field. When Relay ('R1') receives the Redirect message from Server ('S2') it determines that Server ('S1') is the next hop toward Client ('C1') by consulting its forwarding table. Relay ('R1') then re-encapsulates the Redirect while changing the link-layer source address to 'L2(R1)' and changing the link-layer destination address to 'L2(S1)'. Relay ('R1') then relays the Redirect via Server ('S1'). When Server ('S1') receives the Redirect message from Relay ('R1') it determines that Client ('C1') is a neighbor by consulting its neighbor cache. Server ('S1') then re-encapsulates the Redirect while changing the link-layer source address to 'L2(S1)' and changing the link-layer destination address to 'L2(C1)'. Server ('S1') then forwards the message to Client ('C1').

When Client ('C1') receives the Redirect message, it accepts the message only if it has a link-layer source address of one of its Servers (e.g., ''L2(S1)'). Next, Client ('C1') validates the message according to the Redirect message validation rules in Section 8.1 of , except that it accepts the message even though the network-layer source address is not that of it's current first-hop router. Following validation, Client ('C1') then processes the message as follows. In the reference operational scenario, when Client ('C1') receives the Redirect message, it either creates or updates a dynamic neighbor cache entry that stores the Target Address of the message as the network-layer address of Client ('C2'), stores the link-layer addresses found in the TLLAOs as the link-layer addresses of Client ('C2') and stores the Prefix Length as the length to be applied to the network-layer address for forwarding purposes. Client ('C1') then sets ForwardTime for the neighbor cache entry to FORWARD_TIME. Now, Client ('C1') has a neighbor cache entry with a valid ForwardTime value, while Client ('C2') has a neighbor cache entry with a valid AcceptTime value. Thereafter, Client ('C1') may forward ordinary network-layer data packets directly to Client ('C2') without involving any intermediate nodes, and Client ('C2') can verify that the packets came from an acceptable source. (In order for Client ('C2') to forward packets to Client ('C1'), a corresponding Predirect/Redirect message exchange is required in the reverse direction; hence, the mechanism is asymmetric.)

In some environments, the Server nearest the target Client may need to serve as the redirection target, e.g., if direct Client-to-Client communications are not possible. In that case, the Server prepares the Redirect message the same as if it were the destination Client (see: ), except that it writes its own link-layer address in the TLLAO option. The Server must then maintain a dynamic neighbor cache entry for the redirected source Client.

Although the Client is responsible for initiating route optimization through the transmission of Predirect messages, the Server is the policy enforcement point that determines whether route optimization is permitted. For example, on some AERO links route optimization would allow traffic to circumvent critical network-based traffic interception points. In those cases, the Server can deny route optimization requests by simply discarding any Predirect messages instead of forwarding them.

Clients that receive multiple non-contiguous ACP delegations must perform route optimization for each of the individual ACPs based on demand of traffic with source addresses taken from those prefixes. For example, if Client C1 has already performed route optimization for destination ACP X on behalf of its source ACP Y, it must also perform route optimization for X on behalf of its source ACP Z. As a result, source route optimization state cannot be shared between non-contiguous ACPs and must be managed separately.

AERO nodes perform Neighbor Unreachability Detection (NUD) by sending unicast NS messages to elicit solicited NA messages from neighbors the same as described in . NUD is performed either reactively in response to persistent L2 errors (see ) or proactively to test existing neighbor cache entries. When an AERO node sends an NS/NA message, it MUST use its link-local address as the IPv6 source address and the link-local address of the neighbor as the IPv6 destination address. When an AERO node receives an NS message or a solicited NA message, it accepts the message if it has a neighbor cache entry for the neighbor; otherwise, it ignores the message. When a source Client is redirected to a target Client it SHOULD proactively test the direct path by sending an initial NS message to elicit a solicited NA response. While testing the path, the source Client can optionally continue sending packets via the Server, maintain a small queue of packets until target reachability is confirmed, or (optimistically) allow packets to flow directly to the target. The source Client SHOULD thereafter continue to test the direct path to the target Client (see Section 7.3 of ) periodically in order to keep dynamic neighbor cache entries alive. In particular, while the source Client is actively sending packets to the target Client it SHOULD also send NS messages separated by RETRANS_TIMER milliseconds in order to receive solicited NA messages. If the source Client is unable to elicit a solicited NA response from the target Client after MAX_RETRY attempts, it SHOULD set ForwardTime to 0 and resume sending packets via one of its Servers. Otherwise, the source Client considers the path usable and SHOULD thereafter process any link-layer errors as a hint that the direct path to the target Client has either failed or has become intermittent. When ForwardTime for a dynamic neighbor cache entry expires, the source Client resumes sending any subsequent packets via a Server and may (eventually) attempt to re-initiate the AERO redirection process. When AcceptTime for a dynamic neighbor cache entry expires, the target Client discards any subsequent packets received directly from the source Client. When both ForwardTime and AcceptTime for a dynamic neighbor cache entry expire, the Client deletes the neighbor cache entry.

When a Client needs to change its link-layer address, e.g., due to a mobility event, it issues an immediate Rebind to each of its Servers using the new link-layer address as the source address and with an ACLLAO that includes the updated client link-layer information. If authentication succeeds, the Server then updates its neighbor cache and sends a Reply. Note that if the Client does not issue a Rebind before the PD lifetime expires (e.g., if the Client has been out of touch with the Server for a considerable amount of time), the Server's Reply will report NoBinding and the Client must re-initiate the DHCPv6 PD procedure. Next, the Client sends Predirect messages to each of its correspondent Client neighbors using the same procedures as specified in . The Client sends the Predirect messages via a Server the same as if it was performing the initial route optimization procedure with the correspondent. The Predirect message will update the correspondent's link layer address mapping for the Client.

When a Client needs to bring a new underlying interface into service (e.g., when it activates a new data link), it issues an immediate Rebind to each of its Servers using the new link-layer address as the source address and with an ACLLAO that includes the new client link-layer information. If authentication succeeds, the Server then updates its neighbor cache and sends a Reply. The Client MAY then send Predirect messages to each of its correspondent Clients to inform them of the new link-layer address as described in .

When a Client needs to remove an existing underlying interface from service (e.g., when it de-activates an existing data link), it issues an immediate Rebind to each of its Servers over any available link with an ACLLAO that includes P(i) values set to "disabled". If authentication succeeds, the Server then updates its neighbor cache and sends a Reply. The Client SHOULD then send Predirect messages to each of its correspondent Clients to inform them of the deprecated link-layer address as described in .

AERO Clients and Servers MAY include a configuration knob that allows them to perform implicit mobility management in which no DHCPv6 messaging is used. In that case, the Client only transmits packets over a single interface at a time, and the Server always observes packets arriving from the Client from the same link-layer source address. If the Client's underlying interface address changes (either due to a readdressing of the original interface or switching to a new interface) the Server immediately updates the neighbor cache entry for the Client and begins accepting and sending packets to the Client's new link-layer address. This implicit mobility method applies to use cases such as cellphones with both WiFi and Cellular interfaces where only one of the interfaces is active at a given time, and the Client automatically switches over to the backup interface if the primary interface fails.

When a Client associates with a new Server, it performs the Client procedures specified in . When a Client disassociates with an existing Server, it sends a Release message via a new Server to the unicast link-local network layer address of the old Server. The new Server then writes its own link-layer address in the Release message IP source address and forwards the message to the old Server. When the old Server receives the Release, it first authenticates the message. Next, it resets the Client's neighbor cache entry lifetime to 3 seconds, rewrites the link-layer address in the neighbor cache entry to the address of the new Server, then returns a Reply message to the Client via the old Server. When the lifetime expires, the old Server withdraws the IP route from the AERO routing system and deletes the neighbor cache entry for the Client. The Client can then use the Reply message to verify that the termination signal has been processed, and can delete both the default route and the neighbor cache entry for the old Server. (Note that since Release/Reply messages may be lost in the network the Client SHOULD retry until it gets a Reply indicating that the Release was successful. If the Client does not receive a Reply after MAX_RETRY attempts, the old Server may have failed and the Client should discontinue its Release attempts.) Clients SHOULD NOT move rapidly between Servers in order to avoid causing excessive oscillations in the AERO routing system. Such oscillations could result in intermittent reachability for the Client itself, while causing little harm to the network. 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, etc.

AERO Clients and Servers should maintain a samll queue of packets they have recently sent to an AERO neighbor, e.g., a 1 second window. If the AERO neighbor moves, the AERO node MAY retransmit the queued packets to ensure that they are delviered to the AERO neighbor's new location. Note that this may have performance implications for asymmetric paths. For example, if the AERO neighbor moves from a 50mbps link to a 128kbps link, retransmitting a 1 second window could cause significant congestion. However, any retransmission bursts will subside after the 1 second window.

Proxy Mobile IPv6 (PMIPv6) presents a network-based localized mobility management scheme for use within an access network domain. It is typically used in WiFi and cellular wireless access networks, and allows Mobile Nodes (MNs) to receive and retain an IP address that remains stable within the access network domain without needing to implement any special mobility protocols. In the PMIPv6 architecture, access network devices known as Mobility Access Gateways (MAGs) provide MNs with an access link abstraction and receive prefixes for the MNs from a Local Mobility Anchor (LMA). In a proxy AERO domain, a proxy AERO Client (acting as a MAG) can similarly provide proxy services for MNs that do not participate in AERO messaging. The proxy Client presents an access link abstraction to MNs, and performs DHCPv6 PD exchanges over the AERO interface with an AERO Server (acting as an LMA) to receive ACPs for address provisioning of new MNs that come onto an access link. This scheme assumes that proxy Clients act as fixed (non-mobile) infrastructure elements under the same administrative trust basis as for Relays and Servers. When an MN comes onto an access link within a proxy AERO domain for the first time, the proxy Client authenticates the MN and obtains a unique identifier that it can use as a DHCPv6 DUID then sends a Solicit message to its Server. When the Server delegates an ACP and returns a Reply, the proxy Client creates an AERO address for the MN and assigns the ACP to the MN's access link. The proxy Client then configures itself as a default router for the MN and provides address autoconfiguration services (e.g., SLAAC, DHCPv6, DHCPv4, etc.) for provisioning MN addresses from the ACP over the access link. Since the proxy Client may serve many such MNs simultaneously, it may receive multiple ACP delegations and configure multiple AERO addresses, i.e., one for each MN. When two MNs are associated with the same proxy Client, the Client can forward traffic between the MNs without involving a Server since it configures the AERO addresses of both MNs and therefore also has the necessary routing information. When two MNs are associated with different proxy Clients, the source MN's Client can initiate standard AERO link route optimization to discover a direct path to the target MN's Client through the exchange of Predirect/Redirect messages. When an MN in a proxy AERO domain leaves an access link provided by an old proxy Client, the MN issues an access link-specific "leave" message that informs the old Client of the link-layer address of a new Client on the planned new access link. This is known as a "predictive handover". When an MN comes onto an access link provided by a new proxy Client, the MN issues an access link-specific "join" message that informs the new Client of the link-layer address of the old Client on the actual old access link. This is known as a "reactive handover". Upon receiving a predictive handover indication, the old proxy Client sends a Solicit message directly to the new Client and queues any arriving data packets addressed to the departed MN. The Solicit message includes the MN's ID as the DUID, the ACP in an IA_PD option, the old Client's address as the link-layer source address and the new Client's address as the link-layer destination address. When the new Client receives the Solicit message, it changes the link-layer source address to its own address, changes the link-layer destination address to the address of its Server, and forwards the message to the Server. At the same time, the new Client creates access link state for the ACP in anticipation of the MN's arrival (while queuing any data packets until the MN arrives), creates a neighbor cache entry for the old Client with AcceptTime set to ACCEPT_TIME, then sends a Redirect message back to the old Client. When the old Client receives the Redirect message, it creates a neighbor cache entry for the new Client with ForwardTime set to FORWARD_TIME, then forwards any queued data packets to the new Client. At the same time, the old Client sends a Release message to its Server. Finally, the old Client sends unsolicited Redirect messages to any of the ACP's correspondents with a TLLAO containing the link-layer address of the new Client. Upon receiving a reactive handover indication, the new proxy Client creates access link state for the MN's ACP, sends a Solicit message to its Server, and sends a Release message directly to the old Client. The Release message includes the MN's ID as the DUID, the ACP in an IA_PD option, the new Client's address as the link-layer source address and the old Client's address as the link-layer destination address. When the old Client receives the Release message, it changes the link-layer source address to its own address, changes the link-layer destination address to the address of its Server, and forwards the message to the Server. At the same time, the old Client sends a Predirect message back to the new Client and queues any arriving data packets addressed to the departed MN. When the new Client receives the Predirect, it creates a neighbor cache entry for the old Client with AcceptTime set to ACCEPT_TIME, then sends a Redirect message back to the old Client. When the old Client receives the Redirect message, it creates a neighbor cache entry for the new Client with ForwardTime set to FORWARD_TIME, then forwards any queued data packets to the new Client. Finally, the old Client sends unsolicited Redirect messages to correspondents the same as for the predictive case. When a Server processes a Solicit message, it creates a neighbor cache entry for this ACP if none currently exists. If a neighbor cache entry already exists, however, the Server changes the link-layer address to the address of the new proxy Client (this satisfies the case of both the old Client and new Client using the same Server). When a Server processes a Release message, it resets the neighbor cache entry lifetime for this ACP to 3 seconds if the cached link-layer address matches the old proxy Client's address. Otherwise, the Server ignores the Release message (this satisfies the case of both the old Client and new Client using the same Server). When a correspondent Client receives an unsolicited Redirect message, it changes the link-layer address for the ACP's neighbor cache entry to the address of the new proxy Client. From an architectural perspective, in addition to the use of DHCPv6 PD and IPv6 ND signaling the AERO approach differs from PMIPv6 in its use of the NBMA virtual link model instead of point-to-point tunnels. This provides a more agile interface for Client/Server and Client/Client coordinations, and also facilitates simple route optimization. The AERO routing system is also arranged in such a fashion that Clients get the same service from any Server they happen to associate with. This provides a natural fault tolerance and load balancing capability such as desired for distributed mobility management.

When an enterprise mobile device moves from a campus LAN connection to a public Internet link, it must re-enter the enterprise via a security gateway that has both a physical interface connection to the Internet and a physical interface connection to the enterprise internetwork. This most often entails the establishment of a Virtual Private Network (VPN) link over the public Internet from the mobile device to the security gateway. During this process, the mobile device supplies the security gateway with its public Internet address as the link-layer address for the VPN. The mobile device then acts as an AERO Client to negotiate with the security gateway to obtain its ACP. In order to satisfy this need, the security gateway also operates as an AERO Server with support for AERO Client proxying. In particular, when a mobile device (i.e., the Client) connects via the security gateway (i.e., the Server), the Server provides the Client with an ACP in a DHCPv6 PD exchange the same as if it were attached to an enterprise campus access link. The Server then replaces the Client's link-layer source address with the Server's enterprise-facing link-layer address in all AERO messages the Client sends toward neighbors on the AERO link. The AERO messages are then delivered to other devices on the AERO link as if they were originated by the security gateway instead of by the AERO Client. In the reverse direction, the AERO messages sourced by devices within the enterprise network can be forwarded to the security gateway, which then replaces the link-layer destination address with the Client's link-layer address and replaces the link-layer source address with its own (Internet-facing) link-layer address. After receiving the ACP, the Client can send IP packets that use an address taken from the ACP as the network layer source address, the Client's link-layer address as the link-layer source address, and the Server's Internet-facing link-layer address as the link-layer destination address. The Server will then rewrite the link-layer source address with the Server's own enterprise-facing link-layer address and rewrite the link-layer destination address with the target AERO node's link-layer address, and the packets will enter the enterprise network as though they were sourced from a device located within the enterprise. In the reverse direction, when a packet sourced by a node within the enterprise network uses a destination address from the Client's ACP, the packet will be delivered to the security gateway which then rewrites the link-layer destination address to the Client's link-layer address and rewrites the link-layer source address to the Server's Internet-facing link-layer address. The Server then delivers the packet across the VPN to the AERO Client. In this way, the AERO virtual link is essentially extended *through* the security gateway to the point at which the VPN link and AERO link are effectively grafted together by the link-layer address rewriting performed by the security gateway. All AERO messaging services (including route optimization and mobility signaling) are therefore extended to the Client. In order to support this virtual link grafting, the security gateway (acting as an AERO Server) must keep static neighbor cache entries for all of its associated Clients located on the public Internet. The neighbor cache entry is keyed by the AERO Client's AERO address the same as if the Client were located within the enterprise internetwork. The neighbor cache is then managed in all ways as though the Client were an ordinary AERO Client. This includes the AERO IPv6 ND messaging signaling for Route Optimization and Neighbor Unreachability Detection. Note that the main difference between a security gateway acting as an AERO Server and an enterprise-internal AERO Server is that the security gateway has at least one enterprise-internal physical interface and at least one public Internet physical interface. Conversely, the enterprise-internal AERO Server has only enterprise-internal physical interfaces. For this reason security gateway proxying is needed to ensure that the public Internet link-layer addressing space is kept separate from the enterprise-internal link-layer addressing space. This is afforded through a natural extension of the security association caching already performed for each VPN client by the security gateway.

When an IPv6 host ('H1') with an address from an ACP owned by AERO Client ('C1') sends packets to a correspondent IPv6 host ('H2'), the packets eventually arrive at the IPv6 router that owns ('H2')s prefix. This IPv6 router may or may not be an AERO Client ('C2') either within the same home network as ('C1') or in a different home network. If Client ('C1') is currently located outside the boundaries of its home network, it will connect back into the home network via a security gateway acting as an AERO Server. The packets sent by ('H1') via ('C1') will then be forwarded through the security gateway then through the home network and finally to ('C2') where they will be delivered to ('H2'). This could lead to sub-optimal performance when ('C2') could instead be reached via a more direct route without involving the security gateway. Consider the case when host ('H1') has the IPv6 address 2001:db8:1::1, and Client ('C1') has the ACP 2001:db8:1::/64 with underlying IPv6 Internet address of 2001:db8:1000::1. Also, host ('H2') has the IPv6 address 2001:db8:2::1, and Client ('C2') has the ACP 2001:db8:2::/64 with underlying IPv6 address of 2001:db8:2000::1. Client ('C1') can determine whether 'C2' is indeed also an AERO Client willing to serve as a route optimization correspondent by resolving the AAAA records for the DNS FQDN that matches ('H2')s prefix, i.e.:'0.0.0.0.2.0.0.0.8.b.d.0.1.0.0.2.aero.linkupnetworks.net' If ('C2') is indeed a candidate correspondent, the FQDN lookup will return a PTR resource record that contains the domain name for the AERO link that manages ('C2')s ASP. Client ('C1') can then attempt route optimization using an approach similar to the Return Routability procedure specified for Mobile IPv6 (MIPv6) . In order to support this process, both Clients MUST intercept and decapsulate packets that have a subnet router anycast address corresponding to any of the /64 prefixes covered by their respective ACPs. To initiate the process, Client ('C1') creates a specially-crafted encapsulated Predirect message that will be routed through its home network then through ('C2')s home network and finally to ('C2') itself. Client ('C1') prepares the initial message in the exchange as follows: The encapsulating IPv6 header source address is set to 2001:db8:1:: (i.e., the IPv6 subnet router anycast address for ('C1')s ACP) The encapsulating IPv6 header destination address is set to 2001:db8:2:: (i.e., the IPv6 subnet router anycast address for ('C2')s ACP) The encapsulating IPv6 header is followed by any additional encapsulation headers The encapsulated IPv6 header source address is set to fe80::2001:db8:1:0 (i.e., the AERO address for ('C1')) The encapsulated IPv6 header destination address is set to fe80::2001:db8:2:0 (i.e., the AERO address for ('C2')) The encapsulated Predirect message includes all of the securing information that would occur in a MIPv6 "Home Test Init" message (format TBD) Client ('C1') then further encapsulates the message in the encapsulating headers necessary to convey the packet to the security gateway (e.g., through IPsec encapsulation) so that the message now appears "double-encapsulated". ('C1') then sends the message to the security gateway, which re-encapsulates and forwards it over the home network from where it will eventually reach ('C2'). At the same time, ('C1') creates and sends a second encapsulated Predirect message that will be routed through the IPv6 Internet without involving the security gateway. Client ('C1') prepares the message as follows: The encapsulating IPv6 header source address is set to 2001:db8:1000:1 (i.e., the Internet IPv6 address of ('C1')) The encapsulating IPv6 header destination address is set to 2001:db8:2:: (i.e., the IPv6 subnet router anycast address for ('C2')s ACP) The encapsulating IPv6 header is followed by any additional encapsulation headers The encapsulated IPv6 header source address is set to fe80::2001:db8:1:0 (i.e., the AERO address for ('C1')) The encapsulated IPv6 header destination address is set to fe80::2001:db8:2:0 (i.e., the AERO address for ('C2')) The encapsulated Predirect message includes all of the securing information that would occur in a MIPv6 "Care-of Test Init" message (format TBD) ('C2') will receive both Predirect messages through its home network then return a corresponding Redirect for each of the Predirect messages with the source and destination addresses in the inner and outer headers reversed. The first message includes all of the securing information that would occur in a MIPv6 "Home Test" message, while the second message includes all of the securing information that would occur in a MIPv6 "Care-of Test" message (formats TBD). When ('C1') receives the Redirect messages, it performs the necessary security procedures per the MIPv6 specification. It then prepares an encapsulated NS message that includes the same source and destination addresses as for the "Care-of Test Init" Predirect message, and includes all of the securing information that would occur in a MIPv6 "Binding Update" message (format TBD) and sends the message to ('C2'). When ('C2') receives the NS message, if the securing information is correct it creates or updates a neighbor cache entry for ('C1') with fe80::2001:db8:1:0 as the network-layer address, 2001:db8:1000::1 as the link-layer address and with AcceptTime set to ACCEPT_TIME. ('C2') then sends an encapsulated NA message back to ('C1') that includes the same source and destination addresses as for the "Care-of Test" Redirect message, and includes all of the securing information that would occur in a MIPv6 "Binding Acknowledgement" message (format TBD) and sends the message to ('C1'). When ('C1') receives the NA message, it creates or updates a neighbor cache entry for ('C2') with fe80::2001:db8:2:0 as the network-layer address and 2001:db8:2:: as the link-layer address and with ForwardTime set to FORWARD_TIME, thus completing the route optimization in the forward direction. ('C1') subsequently forwards encapsulated packets with outer source address 2001:db8:1000::1, with outer destination address 2001:db8:2::, with inner source address taken from the 2001:db8:1::, and with inner destination address taken from 2001:db8:2:: due to the fact that it has a securely-established neighbor cache entry with non-zero ForwardTime. ('C2') subsequently accepts any such encapsulated packets due to the fact that it has a securely-established neighbor cache entry with non-zero AcceptTime. In order to keep neighbor cache entries alive, ('C1') periodically sends additional NS messages to ('C2') and receives any NA responses. If ('C1') moves to a different point of attachment after the initial route optimization, it sends a new secured NS message to ('C2') as above to update ('C2')s neighbor cache. If ('C2') has packets to send to ('C1'), it performs a corresponding route optimization in the opposite direction following the same procedures described above. In the process, the already-established unidirectional neighbor cache entries within ('C1') and ('C2') are updated to include the now-bidirectional information. In particular, the AcceptTime and ForwardTime variables for both neighbor cache entries are updated to non-zero values, and the link-layer address for ('C1')s neighbor cache entry for ('C2') is reset to 2001:db8:2000::1. Note that two AERO Clients can use full security protocol messaging instead of Return Routability, e.g., if strong authentication and/or confidentiality are desired. In that case, security protocol key exchanges such as specified for MOBIKE would be used to establish security associations and neighbor cache entries between the AERO clients. Thereafter, NS/NA messaging can be used to maintain neighbor cache entries, test reachability, and to announce mobility events. If reachability testing fails, e.g., if both Clients move at roughly the same time, the Clients can tear down the security association and neighbor cache entries and again allow packets to flow through their home network.

When Servers on the AERO link do not provide DHCPv6 services, operation can still be accommodated through administrative configuration of ACPs on AERO Clients. In that case, administrative configurations of AERO interface neighbor cache entries on both the Server and Client are also necessary. However, this may interfere with the ability for Clients to dynamically change to new Servers, and can expose the AERO link to misconfigurations unless the administrative configurations are carefully coordinated.

In some AERO link scenarios, there may be no Servers on the link and/or no need for Clients to use a Server as an intermediary trust anchor. In that case, each Client acts as a Server unto itself to establish neighbor cache entries by performing direct Client-to-Client IPv6 ND message exchanges, and some other form of trust basis must be applied so that each Client can verify that the prospective neighbor is authorized to use its claimed ACP. When there is no Server on the link, Clients must arrange to receive ACPs and publish them via a secure alternate PD authority through some means outside the scope of this document.

In some environments, the AERO service may be useful for mobile nodes that do not implement the AERO Client function and do not perform encapsulation. For example, if the mobile node has a way of injecting its ACP into the access network routing system an AERO Server connected to the same access network can accept the ACP prefix injection as an indication that a new mobile node has come onto the link. The Server can then inject the ACP into the BGP routing system the same as if an AERO Client/Server DHCPv6 exchange had occurred. If the mobile node subsequently withdraws the ACP from the access network routing system, the Server can then withrdaw the ACP from the BGP routing system. In this arrangement, AERO Servers and Relays are used in exactly the same ways as for environments where DHCPv6 Client/Server exchanges are supported. However, the access network routing systems must be capable of accommodating rapid ACP injections and withrawls from mobile nodes with the understanding that the information must be propagated to all routers in the system. Operational expereince has shown that this kind of routing system "churn" can lead to overall instability and inconsistency in the routing system.

In addition to the dynamic neighbor discovery procedures for AERO link neighbors described above, AERO encapsulation can be applied to manually-configured tunnels. In that case, the tunnel endpoints use an administratively-assigned link-local address and exchange NS/NA messages the same as for dynamically-established tunnels.

In some environments, AERO Servers and Relays may be connected by dedicated point-to-point links, e.g., high speed fiberoptic leased lines. In that case, the Servers and Relays can participate in the AERO link the same as specified above but can avoid encapsulation over the dedicated links. In that case, however, the links would be dedicated for AERO and could not be multiplexed for both AERO and non-AERO communications.

A source Client may connect only to an IPvX underlying network, while the target Client connects only to an IPvY underlying network. In that case, the target and source Clients have no means for reaching each other directly (since they connect to underlying networks of different IP protocol versions) and so must ignore any redirection messages and continue to send packets via their Servers.

When the underlying network does not support multicast, AERO Clients map link-scoped multicast addresses to the link-layer address of a Server, which acts as a multicast forwarding agent. The AERO Client also serves as an IGMP/MLD Proxy for its EUNs and/or hosted applications per while using the link-layer address of the Server as the link-layer address for all multicast packets. When the underlying network supports multicast, AERO nodes use the multicast address mapping specification found in for IPv4 underlying networks and use a TBD site-scoped multicast mapping for IPv6 underlying networks. In that case, border routers must ensure that the encapsulated site-scoped multicast packets do not leak outside of the site spanned by the AERO link.