Asymmetric Extended Route Optimization (AERO)

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

R1 | |(H1->S1; H2->S2)| | default->R1 | | H1->C1 | +--------+-------+ | H2->C2 | +-------+------+ | +------+-------+ | | | X---+---+-------------------+------------------+---+---X | AERO Link | +-----+--------+ +--------+-----+ |AERO Client C1| |AERO Client C2| | Nbr: S1 | | Nbr: S2 | | default->S1 | | default->S2 | +--------------+ +--------------+ .-. .-. ,-( _)-. ,-( _)-. .-(_ IP )-. .-(_ IP )-. (__ EUN ) (__ EUN ) `-(______)-' `-(______)-' | | +--------+ +--------+ | Host H1| | Host H2| +--------+ +--------+ ]]> presents the AERO link reference model. In this model: Relay R1 acts as a default router for its associated Servers S1 and S2, and connects the AERO link to the rest of the IP Internetwork Servers S1 and S2 associate with Relay R1 and also act as default routers for their associated Clients C1 and C2. Clients C1 and C2 associate with Servers S1 and S2, respectively and also act as default routers for their associated EUNs Hosts H1 and H2 attach to the EUNs served by Clients C1 and C2, respectively Each node maintains a neighbor cache and IP forwarding table. (For example, AERO Relay R1 in the diagram has neighbor cache entries for Servers S1 and S2 and IP forwarding table entries for ACPs H1 and H2.) 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 Servers 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 only, i.e., and not between the AERO link and the native IP Internetwork. AERO Servers maintain an AERO interface neighbor cache entry for each AERO Relay. They also maintain both a neighbor cache entry and an IP forwarding table entry 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 and sub-delegate portions of their ACPs to EUN interfaces. (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 Clients typically configure a TUN/TAP interface as a point-to-point linkage between the IP layer and the AERO interface. The IP layer therefore sees only the TUN/TAP interface, while the AERO interface provides an intermediate conduit between the TUN/TAP interface and the underlying interfaces. AERO Clients that act as hosts assign one or more IP addresses from their ACPs to the TUN/TAP interface, i.e., and not to the AERO interface. 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 is formed as follows: fe80::[ACP] For IPv6, the AERO address begins with the prefix fe80::/64 and includes in its interface identifier 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 formed from the lower 64 bits of an IPv4-mapped IPv6 address that includes the base prefix taken from the Client's IPv4 ACP. For example, if the AERO Client receives the IPv4 ACP: 192.0.2.32/28 it constructs its AERO address as: fe80::FFFF:192.0.2.32 The AERO address remains stable as the Client moves between topological locations, i.e., even if its link-layer addresses change. 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. In that case, the Client MUST accept the address as equivalent to the base address, but then use the base address as the source address of any IPv6 ND message replies. 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 but uses fe80::2001:db8:1000:2000 as the source address of any IPv6 ND replies.

AERO interfaces use encapsulation (see ) to exchange packets with neighbors attached to the AERO link. AERO interfaces maintain a neighbor cache, and AERO Clients and Servers use unicast IPv6 ND messaging. AERO interfaces use unicast 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 redirection message types -- the first known as a Predirect message and the second being the standard Redirect message (see ). AERO links further use link-local-only addressing; hence, AERO nodes ignore any Prefix Information Options (PIOs) they may receive in RA messages over an AERO interface. AERO interface ND messages include one or more Source/Target Link-Layer Address Options (S/TLLAOs) formatted as shown in :

In this format, Link ID is an integer value between 0 and 255 corresponding to an underlying interface of the target node, NDSCPs encodes an integer value between 1 and 64 indicating the number of Differentiated Services Code Point (DSCP) octets that follow. Each DSCP octet is a 6-bit integer DSCP value followed by a 2-bit Preference ("Prf") value. Each DSCP value encodes an integer between 0 and 63 associated with this Link ID, where the value 0 means "default" and other values are interpreted as specified in . The 'Prf' qualifier for each DSCP value is set to the value 0 ("deprecated'), 1 ("low"), 2 ("medium"), or 3 ("high") to indicate a preference level for packet forwarding purposes. UDP Port Number and IP Address are set to the addresses used by the target node when it sends encapsulated packets over the underlying interface. When the encapsulation IP address family is IPv4, IP Address is formed as an IPv4-mapped IPv6 address . 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, 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 Redirect, Predirect and unsolicited NA messages include only a single TLLAO with Link 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 a single link-local address with multiple link-layer addresses. In that case, Redirect, Predirect and unsolicited NA messages MAY include multiple TLLAOs -- each with a different Link ID that corresponds to a specific underlying interface of the Client.

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 ASP prefixes into the native IP Internetwork. Each Relay subsequently maintains an IP forwarding table entry for each Client-Server association, 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-Client 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 but does not update the neighbor cache. The Server further provides a simple conduit between AERO interface neighbors. Therefore, packets enter the Server's AERO interface from the link layer and are forwarded back out the link layer without ever leaving the AERO interface and therefore without ever disturbing the network layer.

When a Client enables an AERO interface, it uses the special address fe80::ffff:ffff:ffff:ffff to obtain an ACP from an AERO Server via DHCPv6 PD. Next, it assigns the corresponding AERO address to the AERO interface and creates a neighbor cache entry for the Server, i.e., the PD exchange bootstraps autoconfiguration of a unique link-local address. 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. Unsolicited NA messages update the cached link-layer addresses for correspondent Clients (e.g., following a link-layer address change due to node mobility) but do not create new neighbor cache entries. NS/NA messages used for NUD update timers in existing neighbor cache entires but do not update link-layer addresses nor create new neighbor cache entries. Finally, the Client need not maintain any IP forwarding table entries for its Servers or correspondent Clients. Instead, it can set a single "route-to-interface" default route in the IP forwarding table, and all forwarding decisions can be made within the AERO interface based on neighbor cache entries. (On systems in which adding a default route would violate security policy, the default route could instead be installed via a "synthesized RA", e.g., as discussed in .)

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.

Relays require full topology knowledge of all ACP/Server associations for the ASP they service, while individual Servers at a minimum only need to know the ACPs for their current set of associated Clients. This is accomplished through the use of an internal instance of the Border Gateway Protocol (BGP) coordinated between Servers and Relays. This internal BGP instance does not interact with the public Internet BGP instance; therefore, the AERO link is presented to the IP Internetwork as a small set of ASPs as opposed to the full set of individual ACPs. In a reference BGP arrangement, 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 ASP it services. By black-holing the ASP, the Relay will maintain active forwarding table entries only for the ACPs that are currently active, and all other ACPs will result in destination unreachable failures due to the black hole route. In this way, the AERO system is incrementally deployable and can support small scale initial deployments followed by deployment of additional Clients, Servers and Relays without disturbing the already-deployed base. Scaling properties of the AERO routing system are therefore 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 Relay. 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 the ASP segment 2001:db8:0100::/40, a third set could service 2001:db8:0200::/40, etc. To support this model, each Relay maintains a route for each other Relay that services a different ASP segment so that inter-Relay forward can be supported. Assuming up to O(10^3) sets of Relays, the system can then accommodate O(10^9) Clients with no additional overhead for Servers and Relays. In this way, each set of Relays services a specific set of ASPs that they advertise to the native routing system outside of the AERO link, and each Server configures ASP-specific routes that list the correct set of Relays as next hops.

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 though DHCPv6 PD exchanges 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 DHCPv6 Reply message response to a Client's DHCPv6 Request, Rebind or Renew message, it creates or updates a static neighbor cache entry based on the AERO address corresponding to the Client's ACP as the network-layer address, 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 as the length to apply to the AERO address. When an AERO Client receives a DHCPv6 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 an IPv6 ND message, and are garbage-collected if not used within a bounded timescale. 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. For dynamic neighbor cache entries, when an AERO Client receives a valid NS message it (re)sets AcceptTime for the neighbor to ACCEPT_TIME. When an AERO Client receives a valid solicited NA message, it (re)sets ForwardTime for the neighbor to FORWARD_TIME and sets MaxRetry to MAX_RETRY. When an AERO Client receives a valid unsolicited NA message, it updates the correspondent's link-layer addresses but DOES NOT reset AcceptTime, ForwardTime or MaxRetry. 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.

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 and sub-IP layer forwarding agents. 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 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 a permanent neighbor cache entry for a Server 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 Relay receives a packet from a Server with a link-local or non-link-local address corresponding to an ASP segment aggregated by another Relay, the first Relay forwards the packet to the second Relay. 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 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 base tunneling specifications (e.g., , , , , , , etc.) except that it inserts a UDP header immediately following the IP encapsulation header. If there are no additional encapsulation headers (and no fragmentation, identification, checksum or signature is needed), the AERO interface next encapsulates the IPv4 or IPv6 packet immediately following the UDP header. In that case, the most significant four bits of the encapsulated packet encode the value '4' for IPv4 or '6' for IPv6. For all other encapsulations, the AERO interface MUST insert an AERO Header between the UDP header and the next encapsulation header as shown in :

a 4-bit "Version" field. MUST be 0 for the purpose of this specification. a 1-bit "Next Header" flag. MUST be 1 for the purpose of this specification to indicate that "Next Header" field is present. "Next Header" encodes the IP protocol number corresponding to the next header in the encapsulation immediately following the AERO header. For example, "Next Header" encodes the value '4' for IPv4, '17' for UDP, '41' for IPv6, '47' for GRE, '50' for ESP, '51' for AH, etc. a 1-bit "Fragment Header" flag. Set to '1' if the "Fragment Offset", "Res", "M", and "Identification" fields are present and collectively referred to as the "AERO Fragment Header"; otherwise, set to '0'. a 1-bit "Checksum" flag. Set to '1' if the "Checksum" field is present; otherwise, set to '0'. When present, the Checksum field contains a checksum of the IP/UDP/AERO encapsulation headers prior to the Checksum field. a 1-bit "Signature" flag. Set to '1' if the "Signature" field is present; otherwise, set to '0'. When present, the Signature field contains a cryptographic signature of the encapsulated packet following the Signature field. The signature is applied prior to any fragmentation; hence' the Signature field only appears in the first fragment of a fragmented packet. (Note: defines an experimental use in which the bits corresponding to (Version, N, F, C, S) are all zero, which can be unambiguously distinguished from the values permitted by this specification.) During encapsulation, the AERO interface copies the "TTL/Hop Limit", "Type of Service/Traffic Class" and "Congestion Experienced" values in the packet's IP header into the corresponding fields in the encapsulation IP header. (When IPv6 is used as the encapsulation protocol, the interface also sets the Flow Label value in the encapsulation header per .) For packets undergoing re-encapsulation, 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. 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 AERO header. For packets sent via 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 also sets the UDP checksum field to zero (see: ) unless an integrity check is required (see: ). The AERO interface next sets the IP protocol number in the encapsulation header to 17 (i.e., the IP protocol number for UDP). When IPv4 is used as the encapsulation protocol, the AERO interface sets the DF bit as discussed in . The AERO interface finally sets the AERO header fields as described in .

AERO interfaces decapsulate packets destined either to the node itself or to a destination reached via an interface other than the AERO interface the packet was received on. When the AERO interface receives a UDP packet, it examines the first octet of the encapsulated packet. If the most significant four bits of the first octet encode the value '4' (i.e., the IP version number value for IPv4) or the value '6' (i.e., the IP version number value for IPv6), the AERO interface discards the encapsulation headers and accepts the encapsulated packet as an ordinary IPv6 or IPv4 data packet, respectively. If the most significant four bits encode the value '0', however, the AERO interface processes the packet according to the appropriate AERO Header fields as specified in .

AERO nodes employ simple data origin authentication procedures for encapsulated packets they receive from other nodes on the AERO link. In particular: AERO Relays and Servers 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 source based on the specific message type. AERO Servers accept encapsulated packets if there is a neighbor cache entry with an AERO address that matches the packet's network-layer source address and with a link-layer address that matches the packet's link-layer source address. AERO Clients accept encapsulated packets if there is a static neighbor cache entry with a link-layer source address that matches the packet's link-layer source address. AERO Clients and Servers 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 point of attachment to the AERO link. AERO links over IP networks have a maximum link MTU of 64KB minus the encapsulation overhead (termed here "ENCAPS"), since the maximum packet size in the base IP specifications is 64KB (while IPv6 jumbograms can be up to 4GB, they are considered optional for IPv6 nodes ). IPv6 specifies a minimum link MTU of 1280 bytes . This is the minimum packet size the AERO interface MUST admit without returning an ICMP Packet Too Big (PTB) message. Although IPv4 specifies a smaller minimum link MTU of 68 bytes , AERO interfaces also observe a 1280 byte minimum for IPv4. Additionally, the vast majority of links in the Internet configure an MTU of at least 1500 bytes. Original source hosts have therefore become conditioned to expect that IP packets up to 1500 bytes in length will either be delivered to the final destination or a suitable PTB message returned. However, PTB messages may be lost in the network resulting in failure of the IP Path MTU Discovery (PMTUD) mechanisms . For these reasons, the source AERO interface (i.e., the tunnel ingress)admit packets into the tunnel subject to their reasonable expectation that PMTUD will convey the correct information to the original source in the event that the packet is too large. In particular, if the original source is within the same well-managed administrative domain as the tunnel ingress, the ingress drops the packet and sends a PTB message back to the original source if the packet is too large to traverse the tunnel in one piece. Similarly, if the tunnel ingress is within the same well-managed administrative domain as the to the destination AERO interface (i.e., the tunnel egress), the ingress can cache MTU values reported in PTB messages received from a router on the path to the egress. In all other cases, AERO interfaces admit all packets up to 1500 bytes in length even if some fragmentation is necessary, and admit larger packets without fragmentation in case they are able to traverse the tunnel in one piece. AERO interfaces are therefore considered to have an indefinite MTU, i.e., instead of clamping the MTU to a finite size. 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 (see: ). 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]. For these reasons, when fragmentation is needed it is performed through insertion of an AERO fragment header (see: ) and application of tunnel fragmentation as described in Section 3.1.7 of . Since the AERO fragment header reduces the room available for packet data, but the original source has no way to control its insertion, the header length MUST be included in the ENCAPS length even for packets in which the header does not appear. The tunnel ingress therefore sends encapsulated packets to the tunnel egress according to the following algorithm: For IP packets that are no larger than (1280-ENCAPS) bytes, the tunnel ingress encapsulates the packet and admits it into the tunnel without fragmentation. For IPv4 AERO links, the tunnel ingress sets the Don't Fragment (DF) bit to 0 so that these packets will be delivered to the tunnel egress even if there is a restricting link in the path, i.e., unless lost due to congestion or routing errors. For IP packets that are larger than (1280-ENCAPS) bytes but no larger than 1500 bytes, the tunnel ingress encapsulates the packet and inserts an AERO fragment header. Next, the tunnel ingress uses the fragmentation algorithm in to break the packet into two non-overlapping fragments where the first fragment (including ENCAPS) is no larger than 1024 bytes and the second is no larger than the first. Each fragment consists of identical UDP/IP encapsulation headers, followed by the AERO header followed by the fragment of the encapsulated packet itself. The tunnel ingress then admits both 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 tunnel egress. When the tunnel egress receives the fragments, it reassembles them into a whole packet per the reassembly algorithm in . The tunnel egress therefore MUST be capable of reassembling packets up to 1500+ENCAPS bytes in length; hence, it is RECOMMENDED that the tunnel egress be capable of reassembling at least 2KB. For IPv4 packets that are larger than 1500 bytes and with the DF bit set to 0, the tunnel ingress uses ordinary IPv4 fragmentation to break the unencapsulated packet into a minimum number of non-overlapping fragments where the first fragment is no larger than 1024-ENCAPS and all other fragments are no larger than the first fragment. The tunnel ingress then encapsulates each fragment (and for IPv4 sets the DF bit to 0) then admits them into the tunnel. These fragments will be delivered to the final destination via the tunnel egress. For all other IP packets, if the packet is too large to enter the underlying interface following encapsulation, the tunnel ingress drops the packet and returns a network-layer (L3) PTB message to the original source with MTU set to the larger of 1500 bytes or the underlying interface MTU minus ENCAPS. Otherwise, the tunnel ingress encapsulates the packet and admits it into the tunnel without fragmentation (and for IPv4 sets the DF bit to 1) and translates any link-layer (L2) PTB messages it may receive from the network into corresponding L3 PTB messages to send to the original source as specified in . Since both L2 and L3 PTB messages may be either lost or contain insufficient information, however, it is RECOMMENDED that original sources that send unfragmentable IP packets larger than 1500 bytes use Packetization Layer Path MTU Discovery (PLPMTUD) . While sending packets according to the above algorithm, the tunnel ingress MAY also send 1500 byte or larger probe packets to determine whether they can reach the tunnel egress without fragmentation. If the probes succeed, the tunnel ingress can discontinue fragmentation and (for IPv4) set DF to 1. Since the path MTU within the tunnel may fluctuate due to routing changes, the tunnel ingress SHOULD continue to send additional probes subject to rate limiting and SHOULD process any L2 PTB messages as an indication that the path MTU may have decreased. If the path MTU within the tunnel becomes insufficient, the source MUST resume fragmentation. To construct a probe, the tunnel ingress prepares an NS message with a Nonce option plus trailing NULL padding octets added to the probe length without including the length of the padding in the IPv6 Payload Length field, but with the length included in the encapsulating IP header. The tunnel ingress then encapsulates the padded NS message in the encapsulation headers (and for IPv4 sets DF to 1) then sends the message to the tunnel egress. If the tunnel egress returns a solicited NA message with a matching Nonce option, the tunnel ingress deems the probe successful. Note that in this process it is essential that probes follow equivalent paths to those used to convey actual data packets. This means that Equal Cost MultiPath (ECMP) and Link Aggregation Gateway (LAG) equipment in the path would need to ensure that probes and data packets follow the same path, which is outside the scope of this specification.

Control messages (i.e., IPv6 ND, DHCPv6, etc.) MUST be accommodated even if some fragmentation is necessary. These packets are therefore accommodated through a modification of the second rule in the above algorithm as follows: For control messages that are larger than (1280-ENCAPS) bytes, the tunnel ingress encapsulates the packet and inserts an AERO fragment header. Next, the tunnel ingress uses the fragmentation algorithm in to break the packet into a minimum number of non-overlapping fragments where the first fragment (including ENCAPS) is no larger than 1024 bytes and the remaining fragments are no larger than the first. The tunnel ingress then encapsulates each fragment (and for IPv4 sets the DF bit to 0) then admits them into the tunnel. Control messages that exceed the 2KB minimum reassembly size rarely occur in the modern era, however the tunnel egress SHOULD be able to reassemble them if they do. This means that the tunnel egress SHOULD include a configuration knob allowing the operator to set a larger reassembly buffer size if large control messages become more common in the future. The tunnel ingress can send large control messages without fragmentation if there is assurance that large packets can traverse the tunnel without fragmentation. The tunnel ingress MAY send 1500 byte or larger probe packets as specified above to determine a size for which fragmentation can be avoided.

When fragmentation is needed, there must be assurance that reassembly can be safely conducted without incurring data corruption. Sources of corruption can include implementation errors, memory errors and misassociations of fragments from a first datagram with fragments of another datagram. The first two conditions (implementation and memory errors) are mitigated by modern systems and implementations that have demonstrated integrity through decades of operational practice. The third condition (reassembly misassociations) must be accounted for by AERO. The AERO fragmentation procedure described in the above algorithms reuses standard IPv6 fragmentation and reassembly code. Since the AERO fragment header includes a 32-bit ID field, there would need to be 2^32 packets alive in the network before a second packet with a duplicate ID enters the system with the (remote) possibility for a reassembly misassociation. For 1280 byte packets, and for a maximum network lifetime value of 60 seconds, this means that the tunnel ingress would need to produce ~(7 *10^12) bits/sec in order for a duplication event to be possible. This exceeds the bandwidth of data link technologies of the modern era, but not necessarily so going forward into the future. Although wireless data links commonly used by AERO Clients support vastly lower data rates, the aggregate data rates between AERO Servers and Relays may be substantial. However, high speed data links in the network core are expected to configure larger MTUs, e.g., 4KB, 8KB or even larger such that unfragmented packets can be used. Hence, no integrity check is included to cover the AERO fragmentation and reassembly procedures. When the tunnel ingress sends an IPv4-encapsulated packet with the DF bit set to 0 in the above algorithms, there is a chance that the packet may be fragmented by an IPv4 router somewhere within the tunnel. Since the largest such packet is only 1280 bytes, however, it is very likely that the packet will traverse the tunnel without incurring a restricting link. Even when a link within the tunnel configures an MTU smaller than 1280 bytes, it is very likely that it does so due to limited performance characteristics . This means that the tunnel would not be able to convey fragmented IPv4-encapsulated packets fast enough to produce reassembly misassociations, as discussed above. However, AERO must also account for the possibility of tunnel paths that include "poorly managed" IPv4 link MTUs due to misconfigurations. Since the IPv4 header includes only a 16-bit ID field, there would only need to be 2^16 packets alive in the network before a second packet with a duplicate ID enters the system. For 1280 byte packets, and for a maximum network lifetime value of 120 seconds, this means that the tunnel ingress would only need to produce ~(5 *10^6) bits/sec in order for a duplication event to be possible - a value that is well within range for many modern wired and wireless data link technologies. Therefore, if there is strong operational assurance that no IPv4 links capable of supporting data rates of 5Mbps or more configure an MTU smaller than 1280 the tunnel ingress MAY omit an integrity check for the IPv4 fragmentation and reassembly procedures; otherwise, the tunnel ingress SHOULD include an integrity check. When an upper layer encapsulation (e.g., IPsec) already includes an integrity check, the tunnel ingress need not include an additional check. Otherwise, the tunnel ingress calculates the UDP checksum over the encapsulated packet and writes the value into the UDP encapsulation header, i.e., instead of writing the value 0. The tunnel egress will then verify the UDP checksum and discard the packet if the checksum is incorrect.

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. For ICMPv6 , the error Types include "Destination Unreachable", "Packet Too Big (PTB)", "Time Exceeded" and "Parameter Problem". For ICMPv4 , the error Types include "Destination Unreachable", "Fragmentation Needed" (a Destination Unreachable Code that is analogous to the ICMPv6 PTB), "Time Exceeded" and "Parameter Problem". 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 IPv4 integrity checks (see: ). 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 routing system is likely in a temporary transitional state that will soon re-converge. When an AERO node receives an L2 PTB message, it translates the message into an L3 PTB message if possible (*) and forwards the message toward the original source as described below. To translate an L2 PTB message to an L3 PTB message, the AERO node first caches the MTU field value of the L2 ICMP header. The node next discards the L2 IP and ICMP headers, and also discards the encapsulation headers of the original L3 packet. Next the node encapsulates the included segment of the original L3 packet in an L3 IP and ICMP header, and sets the ICMP header Type and Code values to appropriate values for the L3 IP protocol. In the process, the node writes the maximum of 1500 bytes and (L2 MTU - ENCAPS) into the MTU field of the L3 ICMP header. The node next 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. Otherwise, the node uses one of its non link-local addresses as the source address of the L3 PTB message. The node finally calculates the ICMP checksum over the L3 PTB message and writes the Checksum in the corresponding field of the L3 ICMP header. The L3 PTB message therefore is formatted as follows:

After the node has prepared the L3 PTB message, it either forwards the message via a link outside of the AERO interface without encapsulation, or encapsulates and forwards the message to the next hop via the AERO interface. 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 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. (*) Note that in some instances the packet-in-error field of an L2 PTB message may not include enough information for translation to an L3 PTB message. In that case, the AERO interface simply discards the L2 PTB message. It can therefore be said that translation of L2 PTB messages to L3 PTB messages can provide a useful optimization when possible, but is not critical for sources that correctly use PLPMTUD.

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 delegation state synchronization is necessary, and Clients can optionally hold separate delegations for the same ACP from multiple Servers. In this way, Clients can associate with multiple Servers, and can receive new delegations from new Servers before deprecating delegations received from existing Servers. AERO Clients and Servers exchange Client link-layer address information using an option format similar to the Client Link Layer Address Option (CLLAO) defined in . Due to practical limitations of CLLAO, however, AERO interfaces instead use Vendor-Specific Information Options as described in the following sections.

AERO Clients discover the link-layer addresses of AERO Servers via static configuration, 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 the connection-specific DNS suffix for the Client's underlying network connection (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 an ACP through a two-message (i.e., Request/Reply) DHCPv6 PD exchange . The Client's Request message includes fe80::ffff:ffff:ffff:ffff as the IPv6 source address, 'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination address and the link-layer address of the Server as the link-layer destination address. The Request message also includes a Client Identifier option with a DHCP Unique Identifier (DUID) and an Identity Association for Prefix Delegation (IA_PD) option. If the Client is pre-provisioned with an ACP associated with the AERO service, it MAY also include the ACP in the IA_PD to indicate its preference to the DHCPv6 server. The Client also SHOULD include an AERO Link-registration Request (ALREQ) option to register one or more links with the Server. The Server will include an AERO Link-registration Reply (ALREP) option in the corresponding DHCPv6 Reply message as specified in . (The Client MAY omit the ALREQ option, in which case the Server will still include an ALREP option in its Reply with "Link ID" set to 0, "DSCP" set to 0, and "Prf" set to 3.) The format for the ALREQ option is shown in :

In the above format, the Client sets 'option-code' to OPTION_VENDOR_OPTS, sets 'option-len (1)' to the length of the option following this field, sets 'enterprise-number' to 45282 (see: "IANA Considerations"), sets opt-code to the value 0 ("OPTION_ALREQ") and sets 'option-len (2)' to the length of the remainder of the option. The Client includes appropriate 'Link ID, 'DSCP' and 'Prf' values for the underlying interface over which the DHCPv6 PD Request will be issued the same as specified for an S/TLLAO . The Client MAY include multiple (DSCP, Prf) values with this Link ID, with the number of values indicated by option-len (2). The Server will register each value with the Link ID in the Client's neighbor cache entry. The Client finally includes any necessary authentication options to identify itself to the DHCPv6 server, and sends the encapsulated DHCPv6 PD Request via the underlying interface corresponding to Link ID. (Note that this implies that the Client must perform additional Renew/Reply DHCPv6 exchanges with the server following the initial Request/Reply using different underlying interfaces and their corresponding Link IDs if it wishes to register additional link-layer addresses and their associated DSCPs.) When the Client receives its ACP via a DHCPv6 Reply from the AERO Server, it 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. The Client then considers the link-layer address of the Server as the primary default encapsulation address for forwarding packets for which no more-specific forwarding information is available. The Client further caches any ASPs included in the ALREP option as ASPs to apply to the AERO link. Next, the Client autoconfigures an AERO address from the delegated ACP, assigns the AERO address to the AERO interface and sub-delegates the ACP to its attached EUNs and/or the Client's own internal virtual interfaces. The Client also assigns a default IP route to the AERO interface as a route-to-interface, i.e., with no explicit next-hop. The Client can then determine the correct next hops for packets submitted to the AERO interface by inspecting the neighbor cache. The Client subsequently renews its ACP delegation through each of its Servers by performing DHCPv6 Renew/Reply exchanges with the link-layer address of a Server as the link-layer destination address and the same options that were used in the initial PD request. Note that if the Client does not issue a DHCPv6 Renew before the delegation expires (e.g., if the Client has been out of touch with the Server for a considerable amount of time) it must re-initiate the DHCPv6 PD procedure. Since the Client's AERO address is obtained from the unique ACP delegation it receives, there is no need for Duplicate Address Detection (DAD) on AERO links. Other nodes maliciously attempting to hijack an authorized Client's AERO address will be denied access to the network by the DHCPv6 server due to an unacceptable link-layer address and/or security parameters (see: Security Considerations).

On some platforms (e.g., popular cell phone operating systems), the act of assigning a default IPv6 route and/or assigning an address to an interface may not be permitted from a user application due to security policy. Typically, those platforms include a TUN/TAP interface that acts as a point-to-point conduit between user applications and the AERO interface. In that case, the Client can instead generate a "synthesized RA" message. The message conforms to and is prepared as follows: the IPv6 source address is the Client's AERO address the IPv6 destination address is all-nodes multicast the Router Lifetime is set to a time that is no longer than the ACP DHCPv6 lifetime the message does not include a Source Link Layer Address Option (SLLAO) the message includes a Prefix Information Option (PIO) with a /64 prefix taken from the ACP as the prefix for autoconfiguration The Client then sends the synthesized RA message via the TUN/TAP interface, where the operating system kernel will interpret it as though it were generated by an actual router. The operating system will then install a default route and use StateLess Address AutoConfiguration (SLAAC) to configure an IPv6 address on the TUN/TAP interface. Methods for similarly installing an IPv4 default route and IPv4 address on the TUN/TAP interface are based on synthesized DHCPv4 messages .

In the initial DHCPv6 PD message exchanges, AERO Clients use the special IPv6 source address 'fe80::ffff:ffff:ffff:ffff' since their AERO addresses are not yet configured. After AERO address autoconfiguration, however, AERO Clients can either continue to use 'fe80::ffff:ffff:ffff:ffff' as the source address for further DHCPv6 messaging or begin using their AERO address as the source address.

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 the DNS resource records for the FQDN "linkupnetworks.[domainname]" before entering service. When an AERO Server receives a prospective Client's DHCPv6 PD Request on its AERO interface, it first authenticates the message. If authentication succeeds, the Server determines the correct ACP to delegate to the Client by searching the Client database. In environments where spoofing is not considered a threat, the Server MAY use the Client's DUID as the identification value. Otherwise, the Server SHOULD use a signed certificate provided by the Client. When the Server delegates the ACP, it also creates an IP forwarding table entry so that the AERO routing system will propagate the ACP to all Relays that aggregate the corresponding ASP (see: ). Next, the Server prepares a DHCPv6 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 Request 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 an IA_PD option with the delegated ACP. Since the Client may experience a fault that prevents it from issuing a DHCPv6 Release before departing from the network, Servers should set a short prefix lifetime (e.g., 40 seconds) so that stale prefix delegation state can be flushed out of the network. The Server also includes an ALREP option that includes the UDP Port Number and IP Address values it observed when it received the ALREQ in the Client's original DHCPv6 message (if present) followed by the ASP(s) for the AERO link. The ALREP option is formatted as shown in :

In the ALREP, the Server sets 'option-code' to OPTION_VENDOR_OPTS, sets 'option-length (1)' to the length of the option, sets 'enterprise-number' to 45282 (see: "IANA Considerations"), sets opt-code to OPTION_ALREP (1), and sets 'option-len (2)' to the length of the remainder of the option. Next, the Server sets 'Link ID' to the same value that appeared in the ALREQ, sets Reserved to 0 and sets 'UDP Port Number' and 'IP address' to the Client's link-layer address. The Server next includes one or more ASP with the IP prefix as it would appear in the interface identifier portion of the corresponding AERO address (see: ), except that the low-order 8 bits of the ASP field encode the prefix length instead of the low-order 8 bits of the prefix. The longest prefix that can therefore appear as an ASP is /56 for IPv6 or /24 for IPv4. (Note that if the Client did not include an ALREQ option in its DHCPv6 message, the Server MUST still include an ALREP option in the corresponding reply with 'Link ID' set to 0.) When the Server admits the DHCPv6 Reply message into the AERO interface, it creates a static neighbor cache entry for the Client's AERO address with lifetime set to no more than the delegation lifetime and the Client's link-layer address as the link-layer address for the Link ID specified in the ALREQ. The Server then uses the Client link-layer address information in the ALREQ option as the link-layer address for encapsulation based on the (DSCP, Prf) information. After the initial DHCPv6 PD exchange, the AERO Server maintains the neighbor cache entry for the Client until the delegation lifetime expires. If the Client issues a Renew/Reply exchange, the Server extends the lifetime. If the Client issues a Release/Reply, or if the Client does not issue a Renew/Reply before the lifetime expires, the Server deletes the neighbor cache entry for the Client and withdraws the IP route 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 acts 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 ALREP option the same as described above then wraps the option in a Relay-Supplied DHCP Option option (RSOO) . The LDRA then incorporates the option 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 ALREP option and authenticates the encapsulated DHCPv6 message. The DHCPv6 server subsequently ignores the ALREQ option itself, since the relay has already included the ALREP option. When the DHCPv6 server prepares a Reply message, it then includes the ALREP option 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.

After an AERO Client registers its Link IDs and their associated (DSCP,Prf) values with the AERO Server, the Client may wish to delete one or more Link registrations, e.g., if an underlying link becomes unavailable. To do so, the Client prepares a DHCPv6 Rebind message that includes an AERO Link-registration Delete (ALDEL) option and sends the Rebind message to the Server over any available underlying link. The ALDEL option is formatted as shown in :

In the ALDEL, the Client sets 'option-code' to OPTION_VENDOR_OPTS, sets 'option-length (1)' to the length of the option, sets 'enterprise-number' to 45282 (see: "IANA Considerations"), sets optcode to OPTION_ALDEL (2), and sets 'option-len (2)' to the length of the remainder of the option. Next, the Server includes each 'Link ID' value that it wishes to delete. If the Client wishes to discontinue use of a Server and thereby delete all of its Link ID associations, it must use a DHCPv6 Release/Reply exchange to delete the entire neighbor cache entry, i.e., instead of using a DHCPv6 Rebind/Reply exchange with one or more ALDEL options.

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 DHCPv6 Request/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. If the Client subsequently disappears without issuing a Release, the Server is responsible for purging stale state by sending synthesized Reply messages to the Forwarding Agent. 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 initiates an intra-domain AERO 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 intradomain route optimization procedure.

depicts the AERO intradomain 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 ('H1') 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 ICMPv6 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-compatible IPv6 address format). the message includes one or more TLLAOs with Link ID and DSCPs 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 Link 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 ICMPv6 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 ICMPv6 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-compatible IPv6 address format). the message includes one or more TLLAOs with Link ID and DSCPs 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 AERO 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 Link 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 ICMPv6 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 Predirect; 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 Predirect to determine the next hop toward Client ('C2') by searching for the AERO address in the neighbor cache. Since Client ('C2') is not a neighbor, Server ('S2') re-encapsulates the Predirect 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 Predirect 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 Predirect 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 Predirect via Server ('S1'). When Server ('S1') receives the Predirect message from Relay ('R1') it determines that Client ('C1') is a neighbor by consulting its neighbor cache. Server ('S1') then re-encapsulates the Predirect 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 ICMPv6 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.

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 refresh 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 proactively 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 a target Client receives an NS message from a source Client, it resets AcceptTime to ACCEPT_TIME if a neighbor cache entry exists; otherwise, it discards the NS message. If ForwardTime is non-zero, the target Client then sends a solicited NA message to the link-layer address of the source Client; otherwise, it sends the solicited NA message to the link-layer address of one of its Servers. When a source Client receives a solicited NA message from a target Client, it resets ForwardTime to FORWARD_TIME if a neighbor cache entry exists; otherwise, it discards the NA message. 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 performs an immediate DHCPv6 Rebind/Reply exchange via each of its Servers using the new link-layer address as the source address and with an ALREQ that includes the correct Link ID and DSCP values. If authentication succeeds, the Server then update its neighbor cache and sends a DHCPv6 Reply. Note that if the Client does not issue a DHCPv6 Rebind before the prefix delegation 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 unsolicited NA messages to each of its correspondent Client neighbors using the same procedures as specified in Section 7.2.6 of , except that it sends the messages as unicast to each neighbor via a Server instead of multicast. In this process, the Client should send no more than MAX_NEIGHBOR_ADVERTISEMENT messages separated by no less than RETRANS_TIMER seconds to each neighbor. With reference to , when Client ('C2') needs to change its link-layer address it sends unicast unsolicited NA messages to Client ('C1') via Server ('S2') 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 136. the Code is set to 0. the Solicited flag is set to 0. the Override flag is set to 1. the Target Address is set to fe80::2001:db8:1:0 (i.e., the AERO address of Client ('C2')). the message includes one or more TLLAOs with Link ID and DSCPs 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. When Server ('S1') receives the NA message, it relays the message in the same way as described for relaying Redirect messages in . In particular, Server ('S1') copies the correct UDP port numbers and IP addresses into the TLLAOs, changes the link-layer source address to its own address, changes the link-layer destination address to the address of Relay ('R1'), then forwards the NA message via the relaying chain the same as for a Redirect. When Client ('C1') receives the NA message, it accepts the message only if it already has a neighbor cache entry for Client ('C2') then updates the link-layer addresses for Client ('C2') based on the addresses in the TLLAOs. Next, Client ('C1') SHOULD initiate the NUD procedures specified in to provide Client ('C2') with an indication that the link-layer source address has been updated, and to refresh ('C2')'s AcceptTime and ('C1')'s ForwardTime timers. If Client ('C2') receives an NS message from Client ('C1') indicating that an unsolicited NA has updated its neighbor cache, Client ('C2') need not send additional unsolicited NAs. If Client ('C2')'s unsolicited NA messages are somehow lost, however, Client ('C1') will soon learn of the mobility event via NUD.

When a Client needs to bring a new underlying interface into service (e.g., when it activates a new data link), it performs an immediate Rebind/Reply exchange via each of its Servers using the new link-layer address as the source address and with an ALREQ that includes the new Link ID and DSCP values. If authentication succeeds, the Server then updates its neighbor cache and sends a DHCPv6 Reply. The Client MAY then send unsolicited NA 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 performs an immediate Rebind/Reply exchange via each of its Servers over any available link with an ALDEL that includes the deprecated Link ID. If authentication succeeds, the Server then updates its neighbor cache and sends a DHCPv6 Reply. The Client SHOULD then send unsolicited NA messages to each of its correspondent Clients to inform them of the deprecated link-layer address as described in .

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 DHCPv6 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 DHCPv6 Release message IP source address and forwards the message to the old Server. When the old Server receives the DHCPv6 Release, it first authenticates the message. The Server then resets the Client's neighbor cache entry lifetime to 5 seconds, rewrites the link-layer address in the neighbor cache entry to the address of the new Server, then returns a DHCPv6 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 MUST retry until it gets Reply indicating that the Release was successful.) 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.

Proxy Mobile IPv6 (PMIPv6) presents a 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 issues a DHCPv6 PD Request to its Server. When the Server delegates an ACP, 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 prefix 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 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 DHCPv6 PD Request message directly to the new Client and queues any arriving data packets addressed to the departed MN. The Request 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 Request 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 DHCPv6 PD Release message to its Server. Finally, the old Client sends unsolicited NA messages to any of the ACP's correspondents with a TLLAO containing the link-layer address of the new Client. This follows the procedure specified in , except that it is the old Client and not the Server that supplies the link-layer address. Upon receiving a reactive handover indication, the new proxy Client creates access link state for the MN's ACP, sends a DHCPv6 PD Request message to its Server, and sends a DHCPv6 PD 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 NA messages to correspondents the same as for the predictive case. When a Server processes a DHCPv6 Request 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 DHCPv6 Release message, it resets the neighbor cache entry lifetime for this ACP to 5 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 NA message, it changes the link-layer address for the ACP's neighbor cache entry to the address of the new proxy Client. The correspondent Client then issues a Predirect/Redirect exchange to establish a new neighbor cache entry in the new 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 AERO 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 a UDP header with source and destination port set to 8060 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 AERO 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 AERO 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 a UDP header with source and destination port set to 8060 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 AERO 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, AERO 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.

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 the Server.

When the underlying network does not support multicast, AERO nodes map IPv6 link-scoped multicast addresses (including 'All_DHCP_Relay_Agents_and_Servers') to the link-layer address of a Server. When the underlying network supports multicast, AERO nodes use the multicast address mapping specification found in for IPv4 underlying networks and use a direct multicast mapping for IPv6 underlying networks. (In the latter case, "direct multicast mapping" means that if the IPv6 multicast destination address of the encapsulated packet is "M", then the IPv6 multicast destination address of the encapsulating header is also "M".)

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 prefix delegation authority through some means outside the scope of this document.

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.

After a tunnel neighbor relationship has been established, neighbors can use a traditional dynamic routing protocol over the tunnel to exchange routing information without having to inject the routes into the AERO routing system.