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 = 3  |           Reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Link ID    |   Preference  |     UDP Port Number (or 0)    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +--                                                           --+
     |                                                               |
     +--                        IP Address                         --+
     |                                                               |
     +--                                                           --+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: AERO Target Link-Layer Address Option (TLLAO) Format

In this format, Link ID is an integer value between 0 and 255 corresponding to an underlying interface of the 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 [RFC4291], i.e., 96 bits of leading 0's followed by a 32-bit IPv4 address

AERO interface Redirect/Predirect messages can both update and create neighbor cache entries, including link-layer address information. 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.

AERO interface NS/NA/RS/RA messages used for neighbor reachability verification update timers in existing neighbor cache entires but do not update link-layer addresses nor create new neighbor cache entries. AERO interface unsolicited NA messages are used to update a neighbor's cached link-layer address for the sender, e.g., following a link-layer address change due to node mobility. AERO interface NS/RS messages SHOULD include a Nonce option (see Section 5.3 of [RFC3971]) that the recipient echoes back in the corresponding NA/RA response.

3.3.1. Coordination of Multiple Underlying Interfaces

AERO interfaces may be configured over multiple underlying interfaces. For example, common handheld devices have both wireless local area network ("WLAN") and cellular wireless links. These links are typically used "one at a time" with low-cost WLAN preferred and highly-available cellular wireless as a standby. In a more complex example, aircraft frequently have many wireless data link types (e.g. satellite-based, terrestrial, air-to-air directional, etc.) with diverse performance and cost properties.

If a Client's multiple underlying interfaces are used "one at a time" (i.e., all other interfaces are in standby mode while one interface is active), then Predirect/Redirect messages include only a single TLLAO with Link ID set to 0.

If the Client has multiple active underlying interfaces, then from the perspective of IPv6 ND it would appear to have a single link-local address with multiple link-layer addresses. In that case, Predirect/Redirect messages MAY include multiple TLLAOs -- each with a different Link ID that corresponds to an underlying interface of the Client. Further details on coordination of multiple active underlying interfaces are outside the scope of this specification.

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 [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 Client's prefix delegation as the 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 the Reply message link-local source address as the network-layer address, and the encapsulation IP source address and UDP source 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 IP forwarding table entry for the predirected (source) prefix. The node then sets an "AcceptTime" variable for the neighbor and uses this value 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 IP forwarding table entry for the redirected (destination) prefix. The node then sets a "ForwardTime" variable for the neighbor and uses this value 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 AcceptTime for the neighbor to ACCEPT_TIME.

When an AERO Client receives a valid solicited NA message, it (re)sets ForwardTime for the neighbor to FORWARD_TIME. (When an AERO Client receives a valid unsolicited NA message, it updates the neighbor's link-layer address but DOES NOT reset ForwardTime.)

It is RECOMMENDED that FORWARD_TIME be set to the default constant value 30 seconds to match the default REACHABLE_TIME value specified for IPv6 ND [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 AcceptTime decrements below FORWARD_TIME.

It is RECOMMENDED that MAX_RETRY be set to 3 the same as described for IPv6 ND address resolution in Section 7.3.3 of [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 IP 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 AcceptTime is non-zero.

Note that this simple data origin authentication only applies to environments in which link-layer addresses cannot be spoofed. Additional security mitigations may be necessary in other environments.

3.6. AERO Interface MTU Considerations

The AERO link Maximum Transmission Unit (MTU) is 64KB minus the encapsulation overhead for IPv4 as the link-layer [RFC0791] and 4GB minus the encapsulation overhead for IPv6 as the link layer [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 IP packet) then admits the fragments into the tunnel. If the link-layer 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 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.

3.7. AERO Interface Encapsulation, Re-encapsulation and Decapsulation

AERO interfaces encapsulate IP 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 [RFC2003][RFC2473][RFC4213][RFC4301][RFC5246] except that the interface copies the "TTL/Hop Limit", "Type of Service/Traffic Class" and "Congestion Experienced" values in the packet's IP header into the corresponding fields in the encapsulation 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 [RFC2003][RFC2473][RFC4213] except that it inserts a UDP header between the encapsulation header and the packet's IP 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 IP 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., IPv4, 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) or the value '0100' (i.e., the version number value for IPv4), 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 [RFC2003][RFC2473][RFC4213][RFC4301][RFC5246].

3.8. AERO Router Discovery, Prefix Delegation and Address Configuration

3.8.1. AERO Client Behavior

AERO Clients discover the link-layer addresses of AERO Servers via static configuration, or through an automated means such as DNS name resolution. In the absence of other information, the Client resolves the Fully-Qualified Domain Name (FQDN) "linkupnetworks.domainname", where "domainname" is the DNS domain appropriate for the Client's attached underlying network. After discovering the link-layer addresses, the Client associates with one or more of the corresponding Servers.

To associate with a Server, the Client acts as a requesting router to request an IP prefix through DHCPv6 PD [RFC3315][RFC3633][RFC6355] using fe80::ffff:ffff:ffff:ffff as the IPv6 source address (see Section 3.3), 'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination address and the link-layer address of the Server as the link-layer destination address. The Client includes a DHCPv6 Unique Identifier (DUID) in the Client Identifier option of its DHCPv6 messages (as well as a DHCPv6 authentication option if necessary) to identify itself to the DHCPv6 server. If the Client is pre-provisioned with an IP prefix associated with the AERO service, it MAY also include the prefix in its DHCPv6 PD Request to indicate its preferred prefix to the DHCPv6 server. The Client then sends the encapsulated DHCPv6 request via an underlying interface.

When the Client receives its prefix delegation via a Reply from the DHCPv6 server, it creates a neighbor cache entry with the Server's link-local address (i.e., fe80::ID) as the network-layer address and the Server's encapsulation address as the link-layer addresses. Next, the Client assigns the AERO address derived from the delegated prefix to the AERO interface and sub-delegates the prefix to nodes and links within its attached EUNs (the AERO address thereafter remains stable as the Client moves). The Client also sets both AcceptTime and ForwardTime for each Server to the constant value REACHABLE_TIME. The Client further renews its prefix delegation by performing DHCPv6 Renew/Reply exchanges with its AERO address as the IPv6 source address, 'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination address, the link-layer address of a Server as the link-layer destination address and the same DUID and authentication information. If the Client wishes to associate with multiple Servers, it can perform DHCPv6 Renew/Reply exchanges via each of the Servers.

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 or updates 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 before AcceptTime and ForwardTime expire 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. When the Client receives a new RA message, it resets AcceptTime and ForwardTime to REACHABLE_TIME. The Client can also forward IP packets destined to networks beyond its local EUNs via a Server as a 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 (see 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 access to the network by the DHCPv6 server due to an unacceptable link-layer address and/or security parameters (see: Security Considerations).

3.8.2. AERO Server Behavior

AERO Servers configure a DHCPv6 relay function on their AERO links. AERO Servers arrange to add their encapsulation layer IP addresses (i.e., their link-layer addresses) to the DNS resource records for the FQDN "linkupnetworks.domainname" before entering service.

When an AERO Server relays a prospective 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. Without creating internal state, 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 (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) [RFC4994] that encodes the option code for the CLLAO in a 'requested-option-code-n' field, then relays the message to the DHCPv6 server. The CLLAO information will therefore subsequently be echoed back in the DHCPv6 server's "Relay-reply" message.

When the DHCPv6 server issues the prefix delegation in a "Relay-reply" message via the AERO Server (acting as a DHCPv6 relay), the Server obtains the Client's link-layer address from the echoed CLLAO option and also obtains the Client's delegated prefix from the message. 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 AcceptTime and ForwardTime set to REACHABLE_TIME, since the Client will continue to send RS messages within REACHABLE_TIME seconds as long as it wishes to remain associated with this Server.

The Server also configures an IP forwarding table entry that lists the Client's AERO address as the next hop toward the delegated IP prefix with a lifetime derived from the DHCPv6 lease lifetime. The Server finally injects the Client's prefix as an IP route into the inter-Server/Relay routing system (see: Appendix A) then relays the DHCPv6 message to the Client while using fe80::ID 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 NS/RS messages from Clients on their AERO interfaces by returning an NA/RA message. The Server SHOULD NOT include PIOs in the RA messages it sends to Clients, since the Client will ignore any such options. When the Server receives an NS/RS message from the Client, it resets AcceptTime and ForwardTime to REACHABLE_TIME.

Servers ignore any RA messages they may receive from a Client, but they MAY examine RA messages received from other Servers for consistency verification purposes. Servers do not send NS messages for the purpose of updating Client neighbor cache timers, since Clients are responsible for refreshing neighbor cache state.

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, using IPv6 addressing as the example (while not shown, a corresponding example for IPv4 addressing can be easily constructed). The figure shows an AERO Server('A'), two AERO Clients ('B', 'C') and three ordinary IPv6 hosts ('D', 'E', 'F'):

                 .-(::::::::)
              .-(:::: IP ::::)-.   +-------------+
             (:: Internetwork ::)--|    Host F   |
              `-(::::::::::::)-'   +-------------+
                 `-(::::::)-'       2001:db8:2::1
                      |
               +--------------+
               | AERO Server A|
               | (D->B; E->C) |
               +--------------+
                   fe80::ID
                    L2(A)
                      |
    X-----+-----------+-----------+--------X
          |       AERO Link       |
         L2(B)                  L2(C)
  fe80::2001:db8:0:0      fe80::2001:db8:1:0         .-.
  +--------------+         +--------------+       ,-(  _)-.
  | AERO Client B|         | AERO Client C|    .-(_  IP   )-.
  | (default->A) |         | (default->A) |--(__    EUN      )
  +--------------+         +--------------+     `-(______)-'
  2001:DB8:0::/48           2001:DB8:1::/48           |
          |                                     2001:db8:1::1
         .-.                                   +-------------+
      ,-(  _)-.      2001:db8:0::1             |    Host E   |
   .-(_  IP   )-.   +-------------+            +-------------+
 (__    EUN      )--|    Host D   |
    `-(______)-'    +-------------+

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

                          .-(::::::::)
                       .-(:::: IP ::::)-.
                      (:: 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) |
  +--------------+                            +--------------+
        .-.                                         .-.
     ,-(  _)-.                                   ,-(  _)-.
  .-(_   IP  )-.                              .-(_   IP  )-.
 (__    EUN      )                           (__    EUN      )
    `-(______)-'                                `-(______)-'
         |                                           |
     +--------+                                  +--------+
     | Host A |                                  | Host G |
     +--------+                                  +--------+