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 = 2   |  Length = 1/3 |           Reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Link ID    |   Preference  |     UDP Port Number (or 0)    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +--                                                           --+
     |                                                               |
     +--       IP Address (Redirect/Predirect messages only)       --+
     |                                                               |
     +--                                                           --+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: AERO S/TLLAO Format

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). For Redirect/Predirect messages, Length is set to 3 and UDP Port Number/IP Address are set to the addresses used by the target node when it sends encapsulated packets over the underlying interface. For other ND messages, Length is set to 1, UDP Port Number is set to 0 and IP Address is omitted.

Each AERO Redirect/Predirect message includes one or more TLLAOs, i.e., one for each underlying interface the target node wishes to publish. 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 [RFC4291].

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 [RFC3971]) that other AERO nodes can use to verify the message time of origin.

Each AERO NS/NA/RS/RA message includes a single S/TLLAO with UDP Port Number set to 0 and IP Address omitted (since the node has no way of knowing whether it is behind a NAT and hence may be unable to convey the correct values). Instead, AERO nodes determine the link-layer addresses of neighbors by examining the encapsulation header IP address and UDP port number of any NS/NA/RS/RA messages they receive.

AERO interface NS/NA/RS/RA messages 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 [RFC3971]). 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.

3.3.1. Coordination of Multiple Underlying Interfaces

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 [RFC4861]. 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 to 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 the S/TLLAOs in ND messages SHOULD use the same Link ID and Preference values regardless of the underlying interface and Predirect/Redirect messages SHOULD include only a single TLLAO. If the Client enables multiple underlying interfaces, it instead SHOULD use a different Link ID and Preference value for the S/TLLAO corresponding to each interface and Predirect/Redirect messages MAY include multiple TLLAOs. In that case, the Client would need to send separate NS/RS messages to each of its neighbors for each active underlying interface.

3.4. AERO Interface Neighbor Cache Maintenace

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 [RFC4861]). 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 Redirect 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 [RFC4861].

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 [RFC4861].

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.

3.5. AERO Interface Data Origin Authentication

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. An exception is that neighbor discovery messages may include a different link-layer address than the one currently in the neighbor cache for this Link ID, and the new link-layer address updates the neighbor cache entry.

3.6. AERO Interface MTU Considerations

The AERO link Maximum Transmission Unit (MTU) is 64KB minus the encapsulation overhead for IPv4 [RFC0791] and 4GB minus the encapsulation overhead for IPv6 [RFC2675]. 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 [RFC4963][RFC6864]. 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 SLLAO, 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 [RFC4821] when sending large packets. Also, if the neighbor can be reached by multiple underlying interfaces the paths via each interface must be probed independently.

3.7. AERO Interface Encapsulation, Re-encapsulation and Decapsulation

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 [RFC2473][RFC4213][RFC4301][RFC5246] 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 [RFC2473][RFC4213] 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: [RFC6935][RFC6936]) 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 [RFC6438]. 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 [RFC2473][RFC4213][RFC4301][RFC5246].

3.8. AERO Router Discovery, Prefix Delegation and Address Configuration

3.8.1. AERO Client Behavior

AERO Clients observe the IPv6 node requirements defined in [RFC6434]. 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 [RFC3633] via each AERO Server it wishes to associate with 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 [RFC3315][RFC6355] and 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 one of its active underlying interfaces (i.e., the "primary" underlying interface for DHCPv6 transactions).

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 via standard DHCPv6 procedures by sending DHCPv6 Renew messages 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.

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. (If the Client has multiple active underlying interfaces, it sends periodic RS messages over each underlying interface.) 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 Section 3.9.

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).

3.8.2. AERO Server Behavior

AERO Servers observe the IPv6 router requirements defined in [RFC6434] 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 [RFC3315] 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) [RFC6939] with the link-layer address format shown in Figure 1. 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) [RFC4994] 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: [IRON]) 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. When the Server receives an RS/NS message, it updates the neighbor cache entry using the network-layer source address as the neighbor's network-layer address and using the link-layer source address of the RS/NS message as the neighbor's link-layer address. 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 Section 3.9.

3.9. AERO Redirection

3.9.1. Reference Operational Scenario

Figure 2 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'):

                 .-(::::::::)
              .-(::: IPv6 :::)-.   +-------------+
             (:::: Internet ::::)--|    Host F   |
              `-(::::::::::::)-'   +-------------+
                 `-(::::::)-'       2001:db8:2::1
                      |
               +--------------+
               | AERO Server A|
               | (C->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   |
    `-(______)-'    +-------------+

       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 ...
      +-+-+-+-+-+-+-+-+-+-+-+-

                          .-(::::::::)
                       .-(::: IPv6 :::)-.
                      (:: Internetwork ::)
                       `-(::::::::::::)-'
                          `-(::::::)-'
                               |
    +--------------+    +------+-------+    +--------------+
    |AERO Server C |    | AERO Relay D |    |AERO Server E |
    | (default->D) |    | (A->C; G->E) |    | (default->D) |
    |    (A->B)    |    +-------+------+    |    (G->F)    |
    +-------+------+            |           +------+-------+
            |                   |                  |
    X---+---+-------------------+------------------+---+---X
        |                  AERO Link                   |
  +-----+--------+                            +--------+-----+
  |AERO Client B |                            |AERO Client F |
  | (default->C) |                            | (default->E) |
  +--------------+                            +--------------+
        .-.                                         .-.
     ,-(  _)-.                                   ,-(  _)-.
  .-(_ IPv6  )-.                              .-(_ IPv6  )-.
 (__    EUN      )                           (__    EUN      )
    `-(______)-'                                `-(______)-'
         |                                           |
     +--------+                                  +--------+
     | Host A |                                  | Host G |
     +--------+                                  +--------+