0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Type (=137)  |  Code (=0/1)  |          Checksum             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Reserved                    | Prefix Length |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                       Target Address                          +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                     Destination Address                       +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Options ...
      +-+-+-+-+-+-+-+-+-+-+-+-

Figure 8: AERO Redirect/Predirect Message Format

3.14.4. Sending Predirects

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 [RFC4861]). 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 Preference 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').

3.14.5. Re-encapsulating and Relaying Predirects

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 [RFC4861], 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').

3.14.6. Processing Predirects and Sending Redirects

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 [RFC4861], 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 Preference 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').

3.14.7. Re-encapsulating and Relaying Redirects

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 [RFC4861]. 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').

3.14.8. Processing Redirects

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 [RFC4861], 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.)

3.14.9. Server-Oriented Redirection

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: Section 3.14.6), except that it writes its own link-layer address in the TLLAO option. The Server must then maintain a neighbor cache entry for the redirected source Client.

3.15. Neighbor Unreachability Detection (NUD)

AERO nodes perform Neighbor Unreachability Detection (NUD) by sending unicast NS messages to elicit solicited NA messages from neighbors the same as described in [RFC4861]. NUD is performed either reactively in response to persistent L2 errors (see Section 3.12) 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 [RFC4861]) 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.

3.16. Mobility Management

3.16.1. Announcing Link-Layer Address Changes

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 and with a CLLAO that includes the correct Link ID and Preference 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 Server has terminated the lease (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 [RFC4861], 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 Figure 7, when Client ('C2') needs to change its link-layer address it sends unicast unsolicited NA messages to Client ('C1') via Server ('S2') as follows:

Section 3.14.7. 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.

• 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 Preference 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

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. However, Client ('C1') MUST NOT update ForwardTime since Client ('C2') will not have updated AcceptTime.

Note that these unsolicited NA messages are unacknowledged; hence, Client ('C2') has no way of knowing whether Client ('C1') has received them. If the messages are somehow lost, however, Client ('C1') will soon learn of the mobility event via the NUD procedures specified in Section 3.15.

3.16.2. Bringing New Links Into Service

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 a CLLAO that includes the new Link ID and Preference 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 Section 3.16.1.

3.16.3. Removing Existing Links from Service

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 a CLLAO that includes the deprecated Link ID and a Preference value of 0. 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 Section 3.16.1.

3.16.4. Moving to a New Server

When a Client associates with a new Server, it performs the Client procedures specified in Section 3.13.2.

When a Client disassociates with an existing Server, it sends a DHCPv6 Release message to the unicast link-local network layer address of the old Server. The Client SHOULD send the message via a new Server (i.e., by setting the link-layer destination address to the address of the new Server) in case the old Server is unreachable at the link layer, e.g., if the old Server is in a different network partition. The new Server will forward the message to a Relay, which will in turn forward the message to the old Server.

When the old Server receives the DHCPv6 Release, it first authenticates the message. If authentication succeeds, the old Server withdraws the IP route from the AERO routing system and deletes the neighbor cache entry for the Client. (The old Server MAY impose a small delay before deleting the neighbor cache entry so that any packets already in the system can still be delivered to the Client.) The old Server then returns a DHCPv6 Reply message via a Relay. 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 the Server's Reply to the Client's Release message may be lost, e.g., if the AERO routing system has not yet converged. Since the Client is responsible for reliability, however, it will retry until it gets an indication 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 due to routing protocol dampening. 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.

3.17. Encapsulation Protocol Version Considerations

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.

3.18. Multicast Considerations

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 [RFC2529] 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".)

3.19. Operation on AERO Links Without DHCPv6 Services

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.

3.20. Operation on Server-less AERO Links

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.

3.21. Proxy AERO

Proxy Mobile IPv6 (PMIPv6) [RFC5213][RFC5844] presents a localized mobility management scheme for use within an access network domain. It is typically used in cellular wireless service provider networks, and allows mobile nodes to receive and retain a stable IP address without needing to implement any special mobility protocols. In the PMIPv6 architecture, access network devices known as Mobility Access Gateways (MAGs) provide mobile nodes with an access link abstraction and receive prefixes for the mobile nodes from a Local Mobility Anchor (LMA).

The AERO Client (acting as a MAG) can similarly provide proxy services for mobile nodes that do not participate in AERO messaging. The proxy Client presents an access link abstraction to mobile nodes, and performs DHCPv6 PD exchanges over the AERO interface with an AERO Server (acting as an LMA) to receive a prefix for address provisioning of the mobile node.

When a mobile node comes onto an access link presented by a proxy Client, the Client authenticates the node and obtains a unique identifier that it can use as the DUID in its DHCPv6 PD messages to the Server. When the Server delegates a prefix, the Client creates a new AERO address for the mobile node and assigns the delegated prefix to the mobile node's access link. The Client then generates address autoconfiguration messages (e.g., IPv6 RA, DHCPv6, DHCPv4, etc.) over the access link and configures itself as a default router for the mobile node. Since the Client may serve many such mobile nodes simultaneously, it may configure multiple AERO addresses, i.e., one for each mobile node.

When two mobile nodes are associated with the same proxy Client, the Client can forward traffic between the mobiles without involving the Server since it configures the AERO addresses of each mobile and therefore also has the necessary routing information. When two mobiles are associated with different Clients, the first mobile node's Client can initiate standard AERO route optimization using the mobile's AERO address as the source for route optimization messaging. This may result in a route optimization where the first mobile node's Client discovers a direct path to the second mobile node's Client.

When a mobile node moves to a new proxy Client, the old proxy Client issues a DHCPv6 Release message and sends unsolicited NA messages to any of the mobile node's correspondents the same as specified for announcing link-layer address changes in Section 3.16.1. However, since the old Client has no way of knowing where the mobile has moved to, it sets the Code field in the NA message to 1. When the correspondent receives such an NA message, it deletes the neighbor cache entry for the departed mobile node and again allows packets to flow through its Server.

In addition to the use of DHCPv6 PD 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-to-Server 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. All other considerations are the same as specified in [RFC5213][RFC5844].

3.22. Extending AERO Links Through Security Gateways

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.

3.23. Extending IPv6 AERO Links to the Internet

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) [RFC6275]. 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 [RFC4555] 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.

4. Implementation Status

An application-layer implementation is in progress.

5. IANA Considerations

IANA is instructed to assign a new 2-octet Hardware Type number "TBD1" for AERO in the "arp-parameters" registry per Section 2 of [RFC5494]. The number is assigned from the 2-octet Unassigned range with Hardware Type "AERO" and with this document as the reference.

IANA is instructed to assign a 4-octet Enterprise Number "TBD2" for AERO in the "enterprise-numbers" registry per [RFC3315].

6. Security Considerations

AERO link security considerations are the same as for standard IPv6 Neighbor Discovery [RFC4861] except that AERO improves on some aspects. In particular, AERO uses a trust basis between Clients and Servers, where the Clients only engage in the AERO mechanism when it is facilitated by a trust anchor. Unless there is some other means of authenticating the Client's identity (e.g., link-layer security), AERO nodes SHOULD also use DHCPv6 securing services (e.g., DHCPv6 authentication, Secure DHCPv6 [I-D.ietf-dhc-sedhcpv6], etc.) for Client authentication and network admission control.

AERO Redirect, Predirect and unsolicited NA messages SHOULD include a Timestamp option (see Section 5.3 of [RFC3971]) that other AERO nodes can use to verify the message time of origin. AERO Predirect, NS and RS messages SHOULD include a Nonce option (see Section 5.3 of [RFC3971]) that recipients echo back in corresponding responses.

AERO links must be protected against link-layer address spoofing attacks in which an attacker on the link pretends to be a trusted neighbor. Links that provide link-layer securing mechanisms (e.g., IEEE 802.1X WLANs) and links that provide physical security (e.g., enterprise network wired LANs) provide a first line of defense that is often sufficient. In other instances, additional securing mechanisms such as Secure Neighbor Discovery (SeND) [RFC3971], IPsec [RFC4301] or TLS [RFC5246] may be necessary.

AERO Clients MUST ensure that their connectivity is not used by unauthorized nodes on their EUNs to gain access to a protected network, i.e., AERO Clients that act as routers MUST NOT provide routing services for unauthorized nodes. (This concern is no different than for ordinary hosts that receive an IP address delegation but then "share" the address with unauthorized nodes via a NAT function.)

On some AERO links, establishment and maintenance of a direct path between neighbors requires secured coordination such as through the Internet Key Exchange (IKEv2) protocol [RFC5996] to establish a security association.

7. Acknowledgements

Discussions both on IETF lists and in private exchanges helped shape some of the concepts in this work. Individuals who contributed insights include Mikael Abrahamsson, Mark Andrews, Fred Baker, Stewart Bryant, Brian Carpenter, Wojciech Dec, Ralph Droms, Sri Gundavelli, Brian Haberman, Joel Halpern, Sascha Hlusiak, Lee Howard, Andre Kostur, Ted Lemon, Joe Touch and Bernie Volz. Members of the IESG also provided valuable input during their review process that greatly improved the document. Special thanks go to Stewart Bryant, Joel Halpern and Brian Haberman for their shepherding guidance.

This work has further been encouraged and supported by Boeing colleagues including Keith Bartley, Dave Bernhardt, Cam Brodie, Balaguruna Chidambaram, Claudiu Danilov, Wen Fang, Anthony Gregory, Jeff Holland, Ed King, Gen MacLean, Kent Shuey, Brian Skeen, Mike Slane, Julie Wulff, Yueli Yang, and other members of the BR&T and BIT mobile networking teams.

Earlier works on NBMA tunneling approaches are found in [RFC2529][RFC5214][RFC5569].

Many of the constructs presented in this second edition of AERO are based on the author's earlier works, including:

• The Internet Routing Overlay Network (IRON) [RFC6179][I-D.templin-ironbis]
• Virtual Enterprise Traversal (VET) [RFC5558][I-D.templin-intarea-vet]
• The Subnetwork Encapsulation and Adaptation Layer (SEAL) [RFC5320][I-D.templin-intarea-seal]
• AERO, First Edition [RFC6706]

8. References

8.1. Normative References

[RFC0768]	Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980.
[RFC0791]	Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
[RFC0792]	Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981.
[RFC2003]	Perkins, C., "IP Encapsulation within IP", RFC 2003, October 1996.
[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460]	Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998.
[RFC2473]	Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 Specification", RFC 2473, December 1998.
[RFC3315]	Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3633]	Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic Host Configuration Protocol (DHCP) version 6", RFC 3633, December 2003.
[RFC3971]	Arkko, J., Kempf, J., Zill, B. and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4213]	Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4861]	Narten, T., Nordmark, E., Simpson, W. and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007.
[RFC4862]	Thomson, S., Narten, T. and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, September 2007.
[RFC6434]	Jankiewicz, E., Loughney, J. and T. Narten, "IPv6 Node Requirements", RFC 6434, December 2011.

8.2. Informative References

[I-D.ietf-dhc-sedhcpv6]	Jiang, S., Shen, S., Zhang, D. and T. Jinmei, "Secure DHCPv6", Internet-Draft draft-ietf-dhc-sedhcpv6-04, September 2014.
[I-D.templin-intarea-seal]	Templin, F., "The Subnetwork Encapsulation and Adaptation Layer (SEAL)", Internet-Draft draft-templin-intarea-seal-68, January 2014.
[I-D.templin-intarea-vet]	Templin, F., "Virtual Enterprise Traversal (VET)", Internet-Draft draft-templin-intarea-vet-40, May 2013.
[I-D.templin-ironbis]	Templin, F., "The Interior Routing Overlay Network (IRON)", Internet-Draft draft-templin-ironbis-16, March 2014.
[RFC0879]	Postel, J., "TCP maximum segment size and related topics", RFC 879, November 1983.
[RFC1035]	Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.
[RFC1191]	Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990.
[RFC1812]	Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, June 1995.
[RFC1930]	Hawkinson, J. and T. Bates, "Guidelines for creation, selection, and registration of an Autonomous System (AS)", BCP 6, RFC 1930, March 1996.
[RFC1981]	McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, August 1996.
[RFC2131]	Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.
[RFC2529]	Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4 Domains without Explicit Tunnels", RFC 2529, March 1999.
[RFC2675]	Borman, D., Deering, S. and R. Hinden, "IPv6 Jumbograms", RFC 2675, August 1999.
[RFC2923]	Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, September 2000.
[RFC3596]	Thomson, S., Huitema, C., Ksinant, V. and M. Souissi, "DNS Extensions to Support IP Version 6", RFC 3596, October 2003.
[RFC3819]	Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J. and L. Wood, "Advice for Internet Subnetwork Designers", BCP 89, RFC 3819, July 2004.
[RFC4271]	Rekhter, Y., Li, T. and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4291]	Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006.
[RFC4301]	Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005.
[RFC4443]	Conta, A., Deering, S. and M. Gupta, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4555]	Eronen, P., "IKEv2 Mobility and Multihoming Protocol (MOBIKE)", RFC 4555, June 2006.
[RFC4592]	Lewis, E., "The Role of Wildcards in the Domain Name System", RFC 4592, July 2006.
[RFC4821]	Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, March 2007.
[RFC4963]	Heffner, J., Mathis, M. and B. Chandler, "IPv4 Reassembly Errors at High Data Rates", RFC 4963, July 2007.
[RFC4994]	Zeng, S., Volz, B., Kinnear, K. and J. Brzozowski, "DHCPv6 Relay Agent Echo Request Option", RFC 4994, September 2007.
[RFC5213]	Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K. and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.
[RFC5214]	Templin, F., Gleeson, T. and D. Thaler, "Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, March 2008.
[RFC5246]	Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5320]	Templin, F., "The Subnetwork Encapsulation and Adaptation Layer (SEAL)", RFC 5320, February 2010.
[RFC5494]	Arkko, J. and C. Pignataro, "IANA Allocation Guidelines for the Address Resolution Protocol (ARP)", RFC 5494, April 2009.
[RFC5522]	Eddy, W., Ivancic, W. and T. Davis, "Network Mobility Route Optimization Requirements for Operational Use in Aeronautics and Space Exploration Mobile Networks", RFC 5522, October 2009.
[RFC5558]	Templin, F., "Virtual Enterprise Traversal (VET)", RFC 5558, February 2010.
[RFC5569]	Despres, R., "IPv6 Rapid Deployment on IPv4 Infrastructures (6rd)", RFC 5569, January 2010.
[RFC5720]	Templin, F., "Routing and Addressing in Networks with Global Enterprise Recursion (RANGER)", RFC 5720, February 2010.
[RFC5844]	Wakikawa, R. and S. Gundavelli, "IPv4 Support for Proxy Mobile IPv6", RFC 5844, May 2010.
[RFC5996]	Kaufman, C., Hoffman, P., Nir, Y. and P. Eronen, "Internet Key Exchange Protocol Version 2 (IKEv2)", RFC 5996, September 2010.
[RFC6146]	Bagnulo, M., Matthews, P. and I. van Beijnum, "Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers", RFC 6146, April 2011.
[RFC6179]	Templin, F., "The Internet Routing Overlay Network (IRON)", RFC 6179, March 2011.
[RFC6204]	Singh, H., Beebee, W., Donley, C., Stark, B. and O. Troan, "Basic Requirements for IPv6 Customer Edge Routers", RFC 6204, April 2011.
[RFC6275]	Perkins, C., Johnson, D. and J. Arkko, "Mobility Support in IPv6", RFC 6275, July 2011.
[RFC6355]	Narten, T. and J. Johnson, "Definition of the UUID-Based DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355, August 2011.
[RFC6438]	Carpenter, B. and S. Amante, "Using the IPv6 Flow Label for Equal Cost Multipath Routing and Link Aggregation in Tunnels", RFC 6438, November 2011.
[RFC6691]	Borman, D., "TCP Options and Maximum Segment Size (MSS)", RFC 6691, July 2012.
[RFC6706]	Templin, F., "Asymmetric Extended Route Optimization (AERO)", RFC 6706, August 2012.
[RFC6864]	Touch, J., "Updated Specification of the IPv4 ID Field", RFC 6864, February 2013.
[RFC6935]	Eubanks, M., Chimento, P. and M. Westerlund, "IPv6 and UDP Checksums for Tunneled Packets", RFC 6935, April 2013.
[RFC6936]	Fairhurst, G. and M. Westerlund, "Applicability Statement for the Use of IPv6 UDP Datagrams with Zero Checksums", RFC 6936, April 2013.
[RFC6939]	Halwasia, G., Bhandari, S. and W. Dec, "Client Link-Layer Address Option in DHCPv6", RFC 6939, May 2013.
[RFC6980]	Gont, F., "Security Implications of IPv6 Fragmentation with IPv6 Neighbor Discovery", RFC 6980, August 2013.
[RFC7078]	Matsumoto, A., Fujisaki, T. and T. Chown, "Distributing Address Selection Policy Using DHCPv6", RFC 7078, January 2014.
[TUNTAP]	Wikipedia, W., "http://en.wikipedia.org/wiki/TUN/TAP", October 2014.

Author's Address