Transmission of IPv6 Packets over AERO Links

The following sections specify the operation of IPv6 over Asymmetric Extended Route Optimization (AERO) links:

AERO Relays relay packets between nodes connected to the same AERO link and also forward packets between the AERO link and the native IPv6 network. The relaying process entails re-encapsulation of IPv6 packets that were received from a first AERO node and are to be forwarded without modification to a second AERO node. AERO Servers configure their AERO interfaces as router interfaces, and provide default routing services to AERO Clients. AERO Servers configure a DHCPv6 relay or server function and facilitate DHCPv6 Prefix Delegation (PD) exchanges. An AERO Server may also act as an AERO Relay. AERO Clients act as requesting routers to receive IPv6 prefixes through a DHCPv6 PD exchange via AERO Servers over the AERO link. (Clients typically associate with a single Server at a time; Clients MAY associate with multiple Servers, but associating with many Servers may result in excessive control message overhead.) Each AERO Client receives at least a /64 prefix delegation, and may receive even shorter prefixes. AERO Clients that act as routers configure their AERO interfaces as router interfaces and sub-delegate portions of their received prefix delegations to links on EUNs. End system applications on AERO Clients that act as routers bind to EUN interfaces (i.e., and not the AERO interface). AERO Clients that act as ordinary hosts configure their AERO interfaces as host interfaces and assign one or more IPv6 addresses taken from their received prefix delegations to the AERO interface but DO NOT assign the delegated prefix itself to the AERO interface. Instead, the host assigns the delegated prefix to a "black hole" route so that unused portions of the prefix are nullified. End system applications on AERO Clients that act as hosts bind directly to the AERO interface.

An AERO address is an IPv6 link-local address assigned to an AERO interface and with an IPv6 prefix embedded within the interface identifier. The AERO address is formatted as: fe80::[IPv6 prefix] Each AERO Server configures the AERO address 'fe80::'; this corresponds to the IPv6 prefix '::/0' (i.e., "default") and provides a handle for Clients to insert into a neighbor cache entry. Each AERO Client configures an AERO address based on the prefix it has received from the AERO link prefix delegation authority (e.g., the DHCPv6 server). The address begins with the prefix fe80::/64 and includes in its interface identifier the base /64 prefix taken from the Client's delegated IPv6 prefix. The base prefix is determined by masking the delegated prefix with the prefix length. For example, if an AERO Client has received the prefix delegation: 2001:db8:1000:2000::/56 it would construct its AERO address as: fe80::2001:db8:1000:2000 The AERO address remains stable as the Client moves between topological locations, i.e., even if its underlying address changes.

AERO interfaces use IPv6-in-IPv6 encapsulation to exchange tunneled packets with AERO neighbors attached to an underlying IPv6 network, and use IPv6-in-IPv4 encapsulation to exchange tunneled packets with AERO neighbors attached to an underlying IPv4 network. AERO interfaces can also operate over secured tunnel types such as IPsec or TLS in environments where strong authentication and confidentiality are required. When Network Address Translator (NAT) traversal and/or filtering middlebox traversal may be necessary, a UDP header is further inserted immediately above the IP encapsulation header. Servers assign the AERO address fe80:: to their AERO interfaces. Servers and Relays also use (non-AERO) administratively-assigned link-local addresses to support the operation of the inter-Server/Relay routing system (see: ). Clients initially use a temporary IPv6 link-local address in the DHCPv6 PD exchanges used to receive an IPv6 prefix and derive an AERO address. If the Client is provisioned with an IPv6 prefix associated with the AERO service, it SHOULD use the AERO address derived from the prefix as the temporary address. Otherwise, the Client uses any randomly-selected link-local address as the temporary address. After the Client receives a prefix delegation, it assigns the corresponding AERO address to the AERO interface. DHCPv6 is therefore used to bootstrap the assignment of unique link-local addresses on the AERO interface for subsequent use in IPv6 ND messaging. AERO interfaces maintain a neighbor cache and use an adaptation of standard 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 being the standard Redirect message and the second known as a Predirect message (see Section 3.9). AERO links further use link-local-only addressing; hence, Clients ignore any Prefix Information Options (PIOs) they may receive in RA messages. AERO interface Redirect/Predirect messages use Target Link Layer Address Options (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 source/target node, and Preference is an integer value between 0 and 255 indicating the node's preference for this underlying interface, with 0 being highest preference and 255 being lowest. 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 no UDP encapsulation is used, UDP Port Number is set to 0. When the encapsulation IP address family is IPv4, IP Address is formed as an IPv4-compatible IPv6 address . AERO interface Redirect/Predirect messages can both update and create neighbor cache entries. Redirect/Predirect messages SHOULD include a Timestamp option (see Section 5.3 of ) that other AERO nodes can use to verify the message time of origin. AERO interface NS/NA/RS/RA do not include Source/Target Link Layer Address Options; they may only update existing neighbor cache entires and do not create new neighbor cache entries. NS/RS messages SHOULD include a Nonce option (see Section 5.3 of ). If an NS/RS message contains a Nonce option, the recipient MUST echo the option back in the corresponding NA/RA response. Unsolicited NA/RA messages are not used on AERO interfaces, and SHOULD be ignored on receipt.

AERO interfaces may be configured over multiple underlying interfaces. From the perspective of IPv6 Neighbor Discovery, the AERO interface therefore appears as a single logical interface with multiple link-layer addresses the same as described for "Inbound Load Balancing" in Section 3 of . The load balancing paradigm applies to AERO Servers that are connected to stable backhaul networks, but may not necessarily be appropriate for AERO Clients that connect via multiple diverse media types. For example, common handheld devices of the modern era have both wireless local area network (aka "WiFi") and cellular wireless links. These links are typically used "one at a time" with low-cost WiFi preferred and highly-available cellular wireless as a cold standby. In a more complex example, aircraft frequently have many wireless data link types (e.g. satellite-based, terrestrial, directional point-to-point, 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 disabled when one interface is active), then Predirect/Redirect messages MUST include only a single TLLAO with the Link ID and Preference values set to 0. If the Client enables multiple underlying interfaces, Predirect/Redirect messages MAY include multiple TLLAOs that each use a different Link ID value. Coordination of multiple active underlying interfaces is outside the scope of this specification and MAY be defined in future specifications.

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 (see ). Neighbor cache entries are created and maintained as follows: When an AERO Server relays a DHCPv6 Reply message to an AERO Client, it creates or updates a neighbor cache entry for the Client based on the AERO address corresponding to the prefix in the IA_PD option as the Client's network layer address and with the Client's encapsulation IP address and UDP port number as the link-layer address. When an AERO Client receives a DHCPv6 Reply message from an AERO Server, it creates or updates a neighbor cache entry for the Server based on fe80:: as the network layer address and the Server's encapsulation IP address and UDP port number as the link-layer address. When an AERO Client receives a valid Predirect message it creates or updates a neighbor cache entry for the Predirect target network-layer and link-layer addresses, and also creates an IPv6 forwarding table entry for the predirected (source) prefix. The node then sets an "ACCEPT" timer for the neighbor and uses this timer to determine whether messages received from the predirected neighbor can be accepted. When an AERO Client receives a valid Redirect message it creates or updates a neighbor cache entry for the redirected target network-layer and link-layer addresses, and also creates an IPv6 forwarding table entry for the redirected (destination) prefix. The node then sets a "FORWARD" timer for the neighbor and uses this timer to determine whether packets can be sent directly to the redirected neighbor. The node also maintains a constant value MAX_RETRY to limit the number of keepalives sent when a neighbor may have gone unreachable. When an AERO Client receives a valid NS message it (re)sets the ACCEPT timer for the neighbor to ACCEPT_TIME. When an AERO Client receives a valid NA message, it (re)sets the FORWARD timer for the neighbor to FORWARD_TIME. 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 neighbor discovery . 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 the ACCEPT timer decrements below FORWARD_TIME. It is RECOMMENDED that MAX_RETRY be set to 3 the same as described for IPv6 neighbor discovery 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. In particular, ACCEPT_TIME SHOULD be set to a value that is sufficiently longer than FORWARD_TIME to allow the AERO redirection procedure to converge.

AERO nodes use a simple data origin authentication for encapsulated packets they receive from other nodes. In particular, AERO nodes accept encapsulated packets with a link-layer source address belonging to one of their current AERO Servers and accept encapsulated packets with a link-layer source address that is correct for the network-layer source address. The AERO node considers the link-layer source address correct for the network-layer source address if there is an IPv6 forwarding table entry that matches the network-layer source address as well as a neighbor cache entry corresponding to the next hop that includes the link-layer address and the ACCEPT timer is non-zero.

The AERO link Maximum Transmission Unit (MTU) is 64KB minus the encapsulation overhead for IPv4 and 4GB minus the encapsulation overhead for IPv6 . This is the most that IPv4 and IPv6 (respectively) can convey within the constraints of protocol constants, but actual sizes available for tunneling will frequently be much smaller. The base tunneling specifications for IPv4 and IPv6 typically set a static MTU on the tunnel interface to 1500 bytes minus the encapsulation overhead or smaller still if the tunnel is likely to incur additional encapsulations on the path. This can result in path MTU related black holes when packets that are too large to be accommodated over the AERO link are dropped, but the resulting ICMP Packet Too Big (PTB) messages are lost on the return path. As a result, AERO nodes use the following MTU mitigations to accommodate larger packets. AERO nodes set their AERO interface MTU to the larger of the underlying interface MTU minus the encapsulation overhead, and 1500 bytes. (If there are multiple underlying interfaces, the node sets the AERO interface MTU according to the largest underlying interface MTU, or 64KB /4G minus the encapsulation overhead if the largest MTU cannot be determined.) AERO nodes optionally cache other per-neighbor MTU values in the underlying IP path MTU discovery cache initialized to the underlying interface MTU. AERO nodes admit packets that are no larger than 1280 bytes minus the encapsulation overhead (*) as well as packets that are larger than 1500 bytes into the tunnel without fragmentation, i.e., as long as they are no larger than the AERO interface MTU before encapsulation and also no larger than the cached per-neighbor MTU following encapsulation. For IPv4, the node sets the "Don't Fragment" (DF) bit to 0 for packets no larger than 1280 bytes minus the encapsulation overhead (*) and sets the DF bit to 1 for packets larger than 1500 bytes. If a large packet is lost in the path, the node may optionally cache the MTU reported in the resulting PTB message or may ignore the message, e.g., if there is a possibility that the message is spurious. For packets destined to an AERO node that are larger than 1280 bytes minus the encapsulation overhead (*) but no larger than 1500 bytes, the node uses IP fragmentation to fragment the encapsulated packet into two pieces (where the first fragment contains 1024 bytes of the original IPv6 packet) then admits the fragments into the tunnel. If the encapsulation protocol is IPv4, the node admits each fragment into the tunnel with DF set to 0 and subject to rate limiting to avoid reassembly errors . For both IPv4 and IPv6, the node also sends a 1500 byte probe message (**) to the neighbor, subject to rate limiting. To construct a probe, the node prepares an NS message with a Nonce option plus trailing padding octets added to a length of 1500 bytes without including the length of the padding in the IPv6 Payload Length field. The node then encapsulates the NS in the encapsulation headers (while including the length of the padding in the encapsulation header length fields), sets DF to 1 (for IPv4) and sends the padded NS message to the neighbor. If the neighbor returns an NA message with a correct Nonce value, the node may then send whole packets within this size range and (for IPv4) relax the rate limiting requirement. (Note that the trailing padding SHOULD NOT be included within the Nonce option itself but rather as padding beyond the last option in the NS message; otherwise, the (large) Nonce option would be echoed back in the solicited NA message and may be lost at a link with a small MTU along the reverse path.) AERO nodes MUST be capable of reassembling packets up to 1500 bytes plus the encapsulation overhead length. It is therefore RECOMMENDED that AERO nodes be capable of reassembling at least 2KB. (*) Note that if it is known without probing that the minimum Path MTU to an AERO node is MINMTU bytes (where 1280 < MINMTU < 1500) then MINMTU can be used instead of 1280 in the fragmentation threshold considerations listed above. (**) It is RECOMMENDED that no probes smaller than 1500 bytes be used for MTU probing purposes, since smaller probes may be fragmented if there is a nested tunnel somewhere on the path to the neighbor. Probe sizes larger than 1500 bytes MAY be used, but may be unnecessary since original sources are expected to implement when sending large packets.

AERO interfaces encapsulate IPv6 packets according to whether they are entering the AERO interface for the first time or if they are being forwarded out the same AERO interface that they arrived on. This latter form of encapsulation is known as "re-encapsulation". AERO interfaces encapsulate packets per the specifications in except that the interface copies the "Hop Limit", "Traffic Class" and "Congestion Experienced" values in the packet's IPv6 header into the corresponding fields in the encapsulation header. For packets undergoing re-encapsulation, the AERO interface instead copies the "TTL/Hop Limit", "Type of Service/Traffic Class" and "Congestion Experienced" values in the original encapsulation header into the corresponding fields in the new encapsulation header (i.e., the values are transferred between encapsulation headers and *not* copied from the encapsulated packet's network-layer header). When AERO UDP encapsulation is used, the AERO interface encapsulates the packet per the specifications in except that it inserts a UDP header between the encapsulation header and IPv6 packet header. The AERO interface sets the UDP source port to a constant value that it will use in each successive packet it sends, sets the UDP checksum field to zero (see: ) and sets the UDP length field to the length of the IPv6 packet plus 8 bytes for the UDP header itself. 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) when AERO-only encapsulation is used. For packets sent to a neighboring Client, the AERO interface sets the UDP destination port to the port value stored in the neighbor cache entry for this neighbor. The AERO interface next sets the IP protocol number in the encapsulation header to the appropriate value for the first protocol layer within the encapsulation (e.g., IPv6, UDP, IPsec, etc.). When IPv6 is used as the encapsulation protocol, the interface then sets the flow label value in the encapsulation header the same as described in . When IPv4 is used as the encapsulation protocol, the AERO interface sets the DF bit as discussed in Section 3.6. AERO interfaces decapsulate packets destined either to the node itself or to a destination reached via an interface other than the receiving AERO interface. When AERO UDP encapsulation is used (i.e., when a UDP header with destination port 8060 is present) the interface examines the first octet of the encapsulated packet. If the most significant four bits of the first octet encode the value '0110' (i.e., the version number value for IPv6), the packet is accepted and the encapsulating UDP header is discarded; otherwise, the packet is discarded. Further decapsulation then proceeds according to the appropriate tunnel type .

AERO Clients observe the IPv6 node requirements defined in . AERO Clients first 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 Fully-Qualified Domain Name (FQDN) "linkupnetworks.domainname", where "domainname" is the DNS domain appropriate for the Client's attached underlying network. The Client then creates a neighbor cache entry with fe80:: as the link-local address and the discovered addresses of one or more Servers as the link-layer addresses. Next, the Client acts as a requesting router to request an IPv6 prefix through DHCPv6 PD using a temporary link-local address (see Section 3.3) as the IPv6 source address and fe80:: as the IPv6 destination address. The Client includes a DHCPv6 Unique Identifier (DUID) in the Client Identifier option of its DHCPv6 messages , where the DUID uniquely identifies the Client to the Server. The Client also includes any additional authenticating information necessary to authenticate itself to the DHCPv6 server. If the Client is pre-provisioned with an IPv6 prefix associated with the AERO service, it MAY also include the prefix in an IA_PD option in its DHCPv6 Request to indicate its preferred prefix to the DHCPv6 server. The Client then sends the encapsulated DHCPv6 request via an underlying interface. After the Client receives its prefix delegation, it assigns the link-local AERO address taken from the prefix to the AERO interface and sub-delegates the prefix to nodes and links within its attached EUNs (the AERO link-local address thereafter remains stable as the Client moves). The Client also sets both the ACCEPT and FORWARD timers for each Server to infinity, since the Client will remain with this Server unless it explicitly terminates the association. The Client further renews its prefix delegation by performing DHCPv6 Renew/Reply exchanges with its AERO address as the IPv6 source address, fe80:: as the IPv6 destination address and the same DUID value in the Client Identifier option. If the Client wishes to associate with multiple Servers, it can perform DHCPv6 Renew/Reply exchanges via each of the Servers, which will result in the creation of neighbor cache entries. The Client then sends an RS message to each of its associated Servers to receive an RA message with a default router lifetime and any other link-specific parameters. When the Client receives an RA message, it configures a default route according to the default router lifetime but ignores any Prefix Information Options (PIOs) included in the RA message since the AERO link is link-local-only. The Client further ignores any RS messages it might receive, since only Servers may process RS messages. The Client then sends periodic RS messages to each Server (subject to rate limiting) to obtain new RA messages for Neighbor Unreachability Detection (NUD), to refresh any network state, and to update the default router lifetime and any other link-specific parameters. The Client can also forward IPv6 packets destined to networks beyond its local EUNs via a Server as an IPv6 default router. The Server may in turn return a redirection message informing the Client of a neighbor on the AERO link that is topologically closer to the final destination as specified in . Note that, since the Client's AERO address is configured from the unique DHCPv6 prefix 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 due to an unacceptable link-layer address and/or security parameters (see: Security Considerations).

AERO Servers observe the IPv6 router requirements defined in and further configure a DHCPv6 relay function on their AERO links. When the AERO Server relays a Client's DHCPv6 PD messages to the DHCPv6 server, it wraps each message in a "Relay-forward" message per and includes a DHCPv6 Interface Identifier option that encodes a value that identifies the AERO link to the DHCPv6 server. The Server then includes the Client's link-layer address in a DHCPv6 Client Link Layer Address Option (CLLAO) with the link-layer address format shown in (i.e., Link ID followed by Preference followed by UDP Port Number followed by IP Address). The Server sets the CLLAO 'option-length' field to 22 (2 plus the length of the link-layer address) and sets the 'link-layer type' field to TBD (see: IANA Considerations). The Server finally includes a DHCPv6 Echo Request Option (ERO) that encodes the option code for the CLLAO in a 'requested-option-code-n' field. The CLLAO information will therefore subsequently be echoed back in the DHCPv6 Server's "Relay-reply" message. When the DHCPv6 server issues the IPv6 prefix delegation in a "Relay-reply" message via the AERO Server (acting as a DHCPv6 relay), the AERO Server obtains the Client's link-layer address from the echoed CLLAO option and obtains the Client's delegated prefix from the included IA_PD option. The Server then creates a neighbor cache entry for the Client's AERO address with the Client's link-layer address as the link-layer address for the neighbor cache entry. The neighbor cache entry is created with both ACCEPT and FORWARD timers set to infinity, since the Client will remain with this Server unless it explicitly terminates the association. The Server also configures an IPv6 forwarding table entry that lists the Client's AERO address as the next hop toward the delegated IPv6 prefix with a lifetime derived from the DHCPv6 lease lifetime. The Server finally injects the Client's prefix as an IPv6 route into the inter-Server/Relay routing system (see: ) then relays the DHCPv6 message to the Client while using fe80:: as the IPv6 source address, the link-local address found in the "peer address" field of the Relay-reply message as the IPv6 destination address, and the Client's link-layer address as the destination link-layer address. Servers respond to RS/NS messages from Clients on their AERO interfaces by returning an RA/NA message. The Server SHOULD NOT include PIOs in the RA messages it sends to Clients, since the Client will ignore any such options. Servers ignore any RA messages they may receive from a Client. Servers MAY examine RA messages received from other Servers for consistency verification purposes. When the Server forwards a packet via the same AERO interface on which it arrived, it initiates an AERO route optimization procedure as specified in .

depicts the AERO redirection reference operational scenario. The figure shows an AERO Server('A'), two AERO Clients ('B', 'D') and three ordinary IPv6 hosts ('C', 'E', 'F'):

B; E->D) | +--------------+ fe80:: L2(A) | X-----+-----------+-----------+--------X | AERO Link | L2(B) L2(D) fe80::2001:db8:0:0 fe80::2001:db8:1:0 .-. +--------------+ +--------------+ ,-( _)-. | AERO Client B| | AERO Client D| .-(_ IPv6 )-. | (default->A) | | (default->A) |--(__ EUN ) +--------------+ +--------------+ `-(______)-' 2001:DB8:0::/48 2001:DB8:1::/48 | | 2001:db8:1::1 .-. +-------------+ ,-( _)-. 2001:db8:0::1 | Host E | .-(_ IPv6 )-. +-------------+ +-------------+ (__ EUN )--| Host C | `-(______)-' +-------------+ ]]> In , AERO Server ('A') connects to the AERO link and connects to the IPv6 Internet, either directly or via an AERO Relay (not shown). Server ('A') assigns the address fe80:: to its AERO interface with link-layer address L2(A). Server ('A') next arranges to add L2(A) to a published list of valid Servers for the AERO link. AERO Client ('B') receives the IPv6 prefix 2001:db8:0::/48 in a DHCPv6 PD exchange via AERO Server ('A') then assigns the address fe80::2001:db8:0:0 to its AERO interface with link-layer address L2(B). Client ('B') configures a default route and neighbor cache entry via the AERO interface with next-hop address fe80:: and link-layer address L2(A), then sub-delegates the prefix 2001:db8:0::/48 to its attached EUNs. IPv6 host ('C') connects to the EUN, and configures the address 2001:db8:0::1. AERO Client ('D') receives the IPv6 prefix 2001:db8:1::/48 in a DHCPv6 PD exchange via AERO Server ('A') then assigns the address fe80::2001:db8:1:0 to its AERO interface with link-layer address L2(D). Client ('D') configures a default route and neighbor cache entry via the AERO interface with next-hop address fe80:: and link-layer address L2(A), then sub-delegates the prefix 2001:db8:1::/48 to its attached EUNs. IPv6 host ('E') connects to the EUN, and configures the address 2001:db8:1::1. Finally, IPv6 host ('F') connects to an IPv6 network outside of the AERO link domain. Host ('F') configures its IPv6 interface in a manner specific to its attached IPv6 link, and assigns the address 2001:db8:2::1 to its IPv6 link interface.

With reference to , when the IPv6 source host ('C') sends a packet to an IPv6 destination host ('E'), the packet is first forwarded via the EUN to AERO Client ('B'). Client ('B') then forwards the packet over its AERO interface to AERO Server ('A'), which then re-encapsulates and forwards the packet to AERO Client ('D'), where the packet is finally forwarded to the IPv6 destination host ('E'). When Server ('A') re-encapsulates and forwards the packet back out on its advertising AERO interface, it must arrange to redirect Client ('B') toward Client ('D') as a better next-hop node on the AERO link that is closer to the final destination. However, this redirection process applied to AERO interfaces must be more carefully orchestrated than on ordinary links since the parties may be separated by potentially many underlying network routing hops. Consider a first alternative in which Server ('A') informs Client ('B') only and does not inform Client ('D') (i.e., "classical redirection"). In that case, Client ('D') has no way of knowing that Client ('B') is authorized to forward packets from the claimed source address, and it may simply elect to drop the packets. Also, Client ('B') has no way of knowing whether Client ('D') is performing some form of source address filtering that would reject packets arriving from a node other than a trusted default router, nor whether Client ('D') is even reachable via a direct path that does not involve Server ('A'). Consider a second alternative in which Server ('A') informs both Client ('B') and Client ('D') separately, via independent redirection control messages (i.e., "augmented redirection"). In that case, if Client ('B') receives the redirection control message but Client ('D') does not, subsequent packets sent by Client ('B') could be dropped due to filtering since Client ('D') would not have a route to verify the claimed source address. Also, if Client ('D') receives the redirection control message but Client ('B') does not, subsequent packets sent in the reverse direction by Client ('D') would be lost. Since both of these alternatives have shortcomings, a new redirection technique (i.e., "AERO redirection") is needed.

Again, with reference to , when source host ('C') sends a packet to destination host ('E'), the packet is first forwarded over the source host's attached EUN to Client ('B'), which then forwards the packet via its AERO interface to Server ('A'). Server ('A') then re-encapsulates and forwards the packet out the same AERO interface toward Client ('D') and also sends an AERO "Predirect" message forward to Client ('D') as specified in . The Predirect message includes Client ('B')'s network- and link-layer addresses as well as information that Client ('D') can use to determine the IPv6 prefix used by Client ('B') . After Client ('D') receives the Predirect message, it process the message and returns an AERO Redirect message destined for Client ('B') via Server ('A') as specified in . During the process, Client ('D') also creates or updates a neighbor cache entry for Client ('B') and creates an IPv6 forwarding table entry for Client ('B')'s IPv6 prefix. When Server ('A') receives the Redirect message, it re-encapsulates the message and forwards it on to Client ('B') as specified in . The message includes Client ('D')'s network- and link-layer addresses as well as information that Client ('B') can use to determine the IPv6 prefix used by Client ('D'). After Client ('B') receives the Redirect message, it processes the message as specified in . During the process, Client ('B') also creates or updates a neighbor cache entry for Client ('D') and creates an IPv6 forwarding table entry for Client ('D')'s IPv6 prefix. Following the above Predirect/Redirect message exchange, forwarding of packets from Client ('B') to Client ('D') without involving Server ('A) as an intermediary 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 (valid values for the Prefix Length field are 0 through 64). The Redirect/Predirect messages are formatted as shown in :

When a Server forwards a packet out the same AERO interface that it arrived on, the Server sends a Predirect message forward toward the AERO Client nearest the destination instead of sending a Redirect message back to the Client nearest the source. In the reference operational scenario, when Server ('A') forwards a packet sent by Client ('B') toward Client ('D'), it also sends a Predirect message forward toward Client ('D'), subject to rate limiting (see Section 8.2 of ). Server ('A') prepares the Predirect message as follows: the link-layer source address is set to 'L2(A)' (i.e., the underlying address of Server ('A')). the link-layer destination address is set to 'L2(D)' (i.e., the underlying address of Client ('D')). the network-layer source address is set to fe80::2001:db8:0:0 (i.e., the AERO address of Client ('B')). the network-layer destination address is set to fe80::2001:db8:1:0 (i.e., the AERO address of Client ('D')). 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 applied to Target address. the Target Address is set to fe80::2001:db8:0::0 (i.e., the AERO address of Client ('B')). the Destination Address is set to the IPv6 source address of the packet that triggered the Predirection event. the message includes a TLLAO with UDP Port Number and IP Address set to 'L2(B)'. the message includes a Timestamp option. the message includes a Redirected Header Option (RHO) that contains the originating packet truncated to ensure that at least the network-layer header is included but the size of the message does not exceed 1280 bytes. Server ('A') then sends the message forward to Client ('D').

When Client ('D') receives a Predirect message, it accepts the message only if the message has a link-layer source address of the Server, i.e. 'L2(A)'. Client ('D') further accepts the message only if it is willing to serve as a redirection target. Next, Client ('D') validates the message according to the ICMPv6 Redirect message validation rules in Section 8.1 of . In the reference operational scenario, when Client ('D') receives a valid Predirect message, it either creates or updates a neighbor cache entry that stores the Target Address of the message as the network-layer address of Client ('B') and stores the link-layer address found in the TLLAO as the link-layer address(es) of Client ('B'). Client ('D') then sets the neighbor cache entry ACCEPT timer with timeout value ACCEPT_TIME. Next, Client ('D') applies the Prefix Length to the Interface Identifier portion of the Target Address and records the resulting IPv6 prefix in its IPv6 forwarding table. After processing the message, Client ('D') prepares a Redirect message response as follows: the link-layer source address is set to 'L2(D)' (i.e., the link-layer address of Client ('D')). the link-layer destination address is set to 'L2(A)' (i.e., the link-layer address of Server ('A')). the network-layer source address is set to fe80::2001:db8:1:0 (i.e., the AERO address of Client ('D')). the network-layer destination address is set to fe80::2001:db8:0:0 (i.e., the AERO address of Client ('B')). 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 and Destination address. the Target Address is set to fe80::2001:db8:1:0 (i.e., the AERO address of Client ('D')). the Destination Address is set to the IPv6 destination address of the packet that triggered the Redirection event. the message includes a TLLAO with UDP port number and IP address set to '0'. the message includes a Timestamp option. 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 ('D') prepares the Redirect message, it sends the message to Server ('A').

When Server ('A') receives a Redirect message from Client ('D'), it accepts the message only if it has a neighbor cache entry that associates the message's link-layer source address with the network-layer source address. Next, Server ('A') validates the message according to the ICMPv6 Redirect message validation rules in Section 8.1 of and also verifies that Client ('D') is authorized to use the Prefix Length in the Redirect message when applied to the AERO address in the network-layer source of the Redirect message. If validation fails, Server ('A') discards the message; otherwise, it copies the correct UDP port numbers and IP addresses into the TLLAO supplied by Client ('D'). Server ('A') then re-encapsulates the Redirect and relays it on to Client ('B') by changing the link-layer source address of the message to 'L2(A)' and changing the link-layer destination address to 'L2(B)' . Server ('A') finally forwards the re-encapsulated message to the ingress node ('B') without decrementing the network-layer IPv6 header Hop Limit field. While not shown in , AERO Relays relay Redirect and Predirect messages in exactly this same fashion described above. See in Appendix A for an extension of the reference operational scenario that includes Relays.

When Client ('B') receives the Redirect message, it accepts the message only if it has a link-layer source address of the Server, i.e. 'L2(A)'. Next, Client ('B') validates the message according to the ICMPv6 Redirect message validation rules in Section 8.1 of . Following validation, Client ('B') then processes the message as follows. In the reference operational scenario, when Client ('B') receives the Redirect message, it either creates or updates a neighbor cache entry that stores the Target Address of the message as the network-layer address of Client ('D') and stores the link-layer address found in the TLLAO as the link-layer address of Client ('D'). Client ('D') then sets the neighbor cache entry FORWARD timer with timeout value FORWARD_TIME. Next, Client ('B') applies the Prefix Length to the Interface Identifier portion of the Target Address and records the resulting IPv6 prefix in its IPv6 forwarding table. Now, Client ('B') has an IPv6 forwarding table entry for Client('D')'s prefix and a neighbor cache entry with a valid FORWARD time, while Client ('D') has an IPv6 forwarding table entry for Client ('B')'s prefix with a valid ACCEPT time. Thereafter, Client ('B') may forward ordinary network-layer data packets directly to Client ("D") without involving Server ('A') and Client ('D') can verify that the packets came from an acceptable source. (In order for Client ('D') to forward packets to Client ('B') a corresponding Predirect/Redirect message exchange is required in the reverse direction.)

AERO nodes send unicast NS messages to elicit NA messages from neighbors the same as described for Neighbor Unreachability Detection (NUD) in . When an AERO node sends an NS/NA message, it MUST use its AERO address as the IPv6 source address and the AERO address of the neighbor as the IPv6 destination address. When an AERO node receives an NS/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 test the direct path to the target by sending an initial NS message to elicit a solicited NA response. While testing the path, the source Client SHOULD continue sending packets via the Server until target Client reachability has been confirmed. The source Client SHOULD thereafter continue to test the direct path to the target Client (see Section 7.3 of ) in order to keep neighbor cache entries alive. In particular, the source Client sends NS messages to the target Client subject to rate limiting in order to receive solicited NA messages. If at any time the direct path appears to be failing, the source Client can resume sending packets via the Server which may or may not result in a new redirection event. When a target Client receives an NS message from a source Client, it resets the ACCEPT timer to ACCEPT_TIME if a neighbor cache entry exists; otherwise, it discards the NS message. When a source Client receives a solicited NA message from a target Client, it resets the FORWARD timer to FORWARD_TIME if a neighbor cache entry exists; otherwise, it discards the NA message. When the FORWARD timer on a neighbor cache entry expires, the source Client resumes sending any subsequent packets via the Server and may (eventually) receive a new Redirect message. When the ACCEPT timer on a neighbor cache entry expires, the target Client discards any subsequent packets received directly from the source Client. When both the FORWARD and ACCEPT timers on a neighbor cache entry expire, the Client deletes both the neighbor cache entry and the corresponding IPv6 forwarding table entry. If the source Client is unable to elicit an NA response from the target Client after MAX_RETRY attempts, it SHOULD consider the direct path unusable for forwarding purposes. 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 Client needs to change its link-layer address (e.g., due to a mobility event), it performs an immediate DHCPv6 Renew/Reply via each of its Servers using the new link-layer address as the source. The DHCPv6 Renew/Reply exchange will update each Server's neighbor cache. Next, the Client sends a Predirect message to each of its active neighbors via a Server using the new link-layer address as the encapsulation source address. The Predirect message includes a TLLAO with UDP Port Number and IP Address set to 0. The Server then copies the correct UDP port number and IP address into the TLLAO supplied by the Client and forwards the Predirect message towards the target as specified in Section 3.9. When the target receives the Predirect message, it returns a Redirect message which the Client processes as an indication that the target has received the update and is ready to accept encapsulated packets with the new link-layer address. When a Client needs to associate with a new Server, it issues a new DHCPv6 Renew message via the new Server as the DHCPv6 relay. The new Server then relays the message to the DHCPv6 server and processes the resulting exchange. After the Client receives the resulting DHCPv6 Reply message, it sends an RS message to the new Server to receive a new RA message. When a Client disassociates with an existing Server, it sends a "terminating RS" message to the old Server. The terminating RS message is prepared exactly the same as for an ordinary RS message, except that the Code field contains the value '1'. When the old Server receives the terminating RS message, it withdraws the IPv6 route from the routing system and deletes the neighbor cache entry and IPv6 forwarding table entry for the Client. The old Server then returns an RA message with default router lifetime set to 0 which the Client can use to verify that the termination signal has been processed. The client then deletes both the default route and the neighbor cache entry for the old Server. (Note that the Client and the old Server MAY impose a small delay before deleting the neighbor cache and IPv6 forwarding table entries so that any packets already in the system can still be delivered to the Client.)

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 underlying IP 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 the AERO link does not provide DHCPv6 services, operation can still be accommodated through administrative configuration of prefixes on AERO Clients. In that case, administrative configurations of IPv6 routes and AERO interface neighbor cache entries on both the Server and Client are also necessary. However, this may preclude 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 can then act as its own Server to establish neighbor cache entries and IPv6 forwarding table entries by performing direct Client-to-Client Predirect/Redirect 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 prefix. When there is no Server on the link, Clients must arrange to receive prefix delegations and publish the delegations via a secure alternate prefix delegation authority through some means outside the scope of this document.

IPv6 hosts serviced by an AERO Client can reach IPv4-only services via a NAT64 gateway within the IPv6 network. AERO nodes can use the Default Address Selection Policy with DHCPv6 option the same as on any IPv6 link. All other (non-multicast) functions that operate over ordinary IPv6 links operate in the same fashion over AERO links.