Transmission of IPv6 Packets over AERO Links

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

AERO hosts configure their AERO interfaces as host interfaces, while AERO routers configure their AERO interfaces as (non-advertising) router interfaces. End system applications on AERO hosts bind directly to the AERO interface, while applications on AERO routers (or IPv6 hosts served by an AERO router) bind to end user network (EUN) interfaces beyond which their packets may be forwarded over an AERO interface. AERO interfaces use IPv6-in-IPv6 encapsulation to exchange tunneled packets with AERO neighbors attached to an underlying IPv6 network, and use IPv6-in-IPv4 encapsulation to exchange tunneled packets with AERO neighbors attached to an underlying IPv4 network. AERO interfaces can also use IPsec encapsulation (either IPv6-in-IPv6 or IPv6-in-IPv4) in environments where strong authentication and confidentiality are required. When NAT traversal and/or filtering middlebox traversal is necessary, a UDP header is further inserted between the outer IP encapsulation header and the inner packet. AERO interfaces configure a Maximum Transmission Unit (MTU) that is the larger of 1500 bytes and the MTU of the underlying interface minus the encapsulation overhead (where the largest possible sizes are 64KB minus encapsulation overhead over IPv4, and 4GB minus encapsulation overhead over IPv6). AERO interfaces maintain a neighbor cache and use a variation of standard unicast IPv6 ND messaging. AERO interfaces use Neighbor Solicitation (NS), Neighbor Advertisement (NA) and Redirect messages the same as for any IPv6 link. They do not use Router Solicitation (RS) and Router Advertisement (RA) messages for several reasons. First, default router discovery is supported through other means more appropriate for AERO links as described below. Second, discovery of more-specific routes is through the receipt of NS, NA and Redirect messages. Finally, AERO nodes receive IPv6 prefix delegations via DHCPv6; hence, there is no need for RA-based prefix discovery. AERO Neighbor Solicitation (NS) and Neighbor Advertisement (NA) messages do not include Source/Target Link Layer Address Options (S/TLLAO). Instead, AERO nodes determine the link-layer addresses of neighbors by examining the encapsulation source address of any NS/NA messages they receive and ignore any S/TLLAOs included in these messages. This is vital to the operation of AERO links for which neighbors are separated by Network Address Translators (NATs) - either IPv4 or IPv6. AERO Redirect messages include a TLLAO the same as for any IPv6 link. The TLLAO includes the link-layer address of the target node, including both the IP address and the UDP source port number used by the target when it sends UDP-encapsulated packets over the AERO interface (the TLLAO instead encodes the value 0 when the target does not use UDP encapsulation). TLLAOs for target nodes that use an IPv6 underlying address include the full 16 bytes of the IPv6 address as shown in , while TLLAOs for target nodes that use an IPv4 underlying address include only the 4 bytes of the IPv4 address as shown in .

Finally, nodes on AERO interfaces use a simple data origin authentication for encapsulated packets they receive from other nodes. In particular, AERO Clients accept encapsulated packets with a link-layer source address belonging to their current AERO Server. AERO nodes also 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 route 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. (An exception is that NS, NA and Redirect messages may include a different link-layer address than the one currently in the neighbor cache, and the new link-layer address updates the neighbor cache entry.)

AERO interfaces encapsulate IPv6 packets according to whether they are entering the AERO interface for the first time or if they are being forwarded out the same AERO interface that they arrived on. This latter form of encapsulation is known as "re-encapsulation". AERO interfaces encapsulate packets per the specifications in ,, except that the interface copies the "TTL/Hop Limit", "Type of Service/Traffic Class" and "Congestion Experienced" values in the inner network layer header into the corresponding fields in the outer IP 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 outer IP header into the corresponding fields in the new outer IP header (i.e., the values are transferred between outer headers and *not* copied from the inner network layer header). When UDP encapsulation is used, the AERO interface inserts a UDP header between the inner header and outer 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 destination port to 8060 (i.e., the IANA-registered port number for AERO), sets the UDP checksum field to zero (see: ) and sets the UDP length field to the length of the inner packet plus 8 bytes for the UDP header itself. The AERO interface next sets the outer IP protocol number to the appropriate value for the first protocol layer within the encapsulation (e.g., IPv6, IPv6 Fragment Header, UDP, etc.). When IPv6 is used as the outer IP protocol, the ITE then sets the flow label value in the outer IPv6 header the same as described in . When IPv4 is used as the outer IP protocol, the AERO interface sets the DF bit as discussed in Section 3.10. AERO interfaces decapsulate packets destined either to the localhost or to a destination reached via an interface other than the receiving AERO interface per the specifications in ,,. When the encapsulated packet includes a UDP header, the AERO interfaces examines the first octet of data following the UDP header to determine the inner header type. If the most significant four bits of the first octet encode the value '0110', the inner header is an IPv6 header. Otherwise, the interface considers the first octet as an IP protocol type that encodes the value '44' for IPv6 fragment header, the value '50' for Encapsulating Security Payload, the value '51' for Authentication Header etc. (If the first octet encodes the value '0', the interface instead discards the packet, since this is the value reserved for experimentation under ,). During the decapsulation, the AERO interface records the UDP source port in the neighbor cache entry for this neighbor then discards the UDP header.

AERO Relays relay packets between nodes connected to the same AERO link and also forward packets between the AERO link and the native IPv6 network. The relaying process entails re-encapsulation of IPv6 packets that were received from a first AERO node and are to be forwarded without modification to a second AERO node. AERO Servers configure their AERO link interfaces as router interfaces, and provide default routing services to AERO Clients. An AERO Server may also act as an AERO Relay. AERO Clients are provisioned with IPv6 Prefix Delegations either through a DHCPv6 Prefix Delegation exchange with an AERO Server over the AERO link or via a static delegation obtained through an out-of-band exchange with an AERO link prefix delegation authority. Each AERO Client receives at least a /64 prefix delegation, and may receive even shorter prefixes. AERO Clients that act as routers configure their AERO link interfaces as router interfaces, i.e., even if the AERO Client otherwise displays the outward characteristics of an ordinary host (for example, the Client may internally configure both an AERO interface and (internal virtual) EUN interfaces). AERO Clients that act as routers sub-delegate portions of their received prefix delegations to links on EUNs. AERO Clients that act as ordinary hosts configure their AERO link interfaces as host interfaces and assign one or more IPv6 addresses taken from their received prefix delegations to their AERO interfaces but DO NOT assign the delegated prefix itself to the AERO interface. Instead, the host assigns the delegated prefix to a "black hole" route so that unused portions of the prefix are nullified.

An AERO address is an IPv6 link-local address assigned to an AERO interface and with an IPv6 prefix embedded within the interface identifier. The AERO address is formatted as: fe80::[IPv6 prefix] Each AERO Client configures an AERO address based on the delegated prefix it has received from the AERO link prefix delegation authority. The address begins with the prefix fe80::/64 and includes in its interface identifier the base /64 prefix taken from the Client's delegated IPv6 prefix. The base prefix is determined by masking the delegated prefix with the prefix length. For example, if an AERO Client has received the prefix delegation: 2001:db8:1000:2000::/56 it would construct its AERO address as: fe80::2001:db8:1000:2000 An AERO Client may receive multiple non-contiguous IPv6 prefix delegations, in which case it would configure multiple AERO addresses - one for each prefix. Each AERO Server configures the special AERO address fe80::1 to support the operation of IPv6 Neighbor Discovery over the AERO link; the address therefore has the properties of an IPv6 Anycast address. While all Servers configure the same AERO address and therefore cannot be distinguished from one another at the network layer, Clients can still distinguish Servers at the link layer by examining the Servers' link-layer addresses. Nodes that are configured as pure AERO Relays (i.e., and that do not also act as Servers) do not configure an IPv6 address of any kind on their AERO interfaces. The Relay's AERO interface is therefore used purely for transit and does not participate in IPv6 ND message exchanges.

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

B; E->D) | +--------------+ fe80::1 L2(A) | X-----+-----------+-----------+--------X | AERO Link | L2(B) L2(D) fe80::2001:db8:0:0 fe80::2001:db8:1:0 .-. +--------------+ +--------------+ ,-( _)-. | AERO Client B| | AERO Client D| .-(_ IPv6 )-. | (default->A) | | (default->A) |--(__ EUN ) +--------------+ +--------------+ `-(______)-' 2001:DB8:0::/48 2001:DB8:1::/48 | | 2001:db8:1::1 .-. +-------------+ ,-( _)-. 2001:db8:0::1 | Host E | .-(_ IPv6 )-. +-------------+ +-------------+ (__ EUN )--| Host C | `-(______)-' +-------------+ ]]> In , AERO Server ('A') connects to the AERO link and connects to the IPv6 Internet, either directly or via other IPv6 routers (not shown). Server ('A') assigns the address fe80::1 to its AERO interface with link-layer address L2(A). Server ('A') next arranges to add L2(A) to a published list of valid Servers for the AERO link. AERO Client ('B') assigns the address fe80::2001:db8:0:0 to its AERO interface with link-layer address L2(B). Client ('B') configures a default route via the AERO interface with next-hop network-layer address fe80::1 and link-layer address L2(A), then sub-delegates the prefix 2001:db8:0::/48 to its attached EUNs. IPv6 host ('C') connects to the EUN, and configures the network-layer address 2001:db8:0::1. AERO Client ('D') assigns the address fe80::2001:db8:1:0 to its AERO interface with link-layer address L2(D). Client ('D') configures a default route via the AERO interface with next-hop network-layer address fe80::1 and link-layer address L2(A), then sub-delegates the network-layer prefix 2001:db8:1::/48 to its attached EUNs. IPv6 host ('E') connects to the EUN, and configures the network-layer address 2001:db8:1::1. Finally, IPv6 host ('F') connects to an IPv6 network outside of the AERO link domain. Host ('F') configures its IPv6 interface in a manner specific to its attached IPv6 link, and assigns the network-layer address 2001:db8:3::1 to its IPv6 link interface.

AERO Clients observe the IPv6 router requirements defined in . AERO Clients first discover the link-layer address of an AERO Server 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 the IPv6 link-local address fe80::1 and the discovered address as the link-layer address. The Client further creates a default route with the link-local address fe80::1 as the next hop. Next, the Client acts as a requesting router to obtain IPv6 prefixes through DHCPv6 Prefix Delegation via the Server. After the Client acquires prefixes, it sub-delegates them to nodes and links within its attached EUNs. It also assigns the link-local AERO address(es) taken from its delegated prefix(es) to the AERO interface (see: Section 3.3). After configuring a default route and obtaining prefixes, the Client sends periodic NS messages to the server to obtain new NAs in order to keep neighbor cache entries alive. The Client can also forward IPv6 packets destined to networks beyond its local EUNs via the Server as an IPv6 default router. The Server may in turn return a Redirect message informing the Client of a neighbor on the AERO link that is topologically closer to the final destination as specified in .

AERO Servers observe the IPv6 router requirements defined in . They further configure a DHCPv6 relay/server function on their AERO links. When the Server delegates prefixes, it also establishes forwarding table and neighbor cache entries that list the AERO address of the Client as the next hop toward the delegated IPv6 prefixes (where the AERO address is constructed as specified in Section 3.3). Servers respond to NS messages from Clients on their AERO interfaces by returning an NA message. When the Server receives an 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 NS message as the neighbor's link-layer address. 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 .

After an AERO node has received a prefix delegation, it creates an AERO address as specified in Section 3.3. It can then send NS messages to elicit NA messages from other AERO nodes. When the AERO node sends NS/NA messages, however, it must also include the length of the prefix corresponding to the AERO address. AERO NS/NA messages therefore include an 8-bit "Prefix Length" field take from the low-order 8 bits of the Reserved field as shown in and .

When an AERO node sends an NS/NA message, it MUST use its AERO address as the IPv6 source address and MUST include its AERO address prefix length in the Prefix Length field. When an AERO node receives an NS/NA message, it accepts the message if the Prefix Length applied to the source address is correct for the neighbor; otherwise, it ignores the message.

describes the AERO reference operational scenario. We now discuss the operation and protocol details of AERO Redirection with respect to this reference scenario.

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

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

AERO Redirect/Predirect messages use the same format as for ICMPv6 Redirect messages depicted in Section 4.5 of , but also include a new "Prefix Length" field taken from the low-order 8 bits of the Redirect message Reserved field. The Redirect/Predirect messages are formatted as shown in :

When an AERO Server forwards a packet out the same AERO interface that it arrived on, the Server sends a Predirect message forward toward the AERO Client nearest the destination instead of sending a Redirect message back to AERO Client nearest the source. In the reference operational scenario, when Server ('A') forwards a packet sent by Client ('B') toward Client ('D'), it also sends a Predirect message forward toward Client ('D'), subject to rate limiting (see Section 8.2 of ). Server ('A') prepares the Predirect message as follows: the link-layer source address is set to 'L2(A)' (i.e., the underlying address of Server ('A')). the link-layer destination address is set to 'L2(D)' (i.e., the underlying address of Client ('D')). the network-layer source address is set to fe80::1 (i.e., the AERO address of Server ('A')). the network-layer destination address is set to fe80::2001:db8:1:0 (i.e., the AERO address of Client ('D')). the Type is set to 137. the Code is set to 1 to indicate "Predirect". the Prefix Length is set to the length of the prefix to be applied to Target address. the Target Address is set to fe80::2001:db8:0::0 (i.e., the AERO address of Client ('B')). the Destination Address is set to the IPv6 source address of the packet that triggered the Predirection event. the message includes a TLLAO set to 'L2(B)' (i.e., the underlying address of Client ('B')). the message includes a Redirected Header Option (RHO) that contains the originating packet truncated to ensure that at least the network-layer header is included but the size of the message does not exceed 1280 bytes. Server ('A') then sends the message forward to Client ('D').

When Client ('D') receives a Predirect message, it accepts the message only if it has a link-layer source address of the Server, i.e. 'L2(A)'. Client ('D') further accepts the message only if it is willing to serve as a redirection target. Next, Client ('D') validates the message according to the ICMPv6 Redirect message validation rules in Section 8.1 of . In the reference operational scenario, when the Client ('D') receives a valid Predirect message, it either creates or updates a neighbor cache entry that stores the Target Address of the message as the network-layer address of Client ('B') and stores the link-layer address found in the TLLAO as the link-layer address of Client ('B'). Client ('D') then applies the Prefix Length to the Interface Identifier portion of the Target Address and records the resulting IPv6 prefix in its IPv6 forwarding table. After processing the message, Client ('D') prepares a Redirect message response as follows: the link-layer source address is set to 'L2(D)' (i.e., the link-layer address of Client ('D')). the link-layer destination address is set to 'L2(A)' (i.e., the link-layer address of Server ('A')). the network-layer source address is set to 'L3(D)' (i.e., the AERO address of Client ('D')). the network-layer destination address is set to 'L3(B)' (i.e., the AERO address of Client ('B')). the Type is set to 137. the Code is set to 0 to indicate "Redirect". the Prefix Length is set to the length of the prefix to be applied to the Target and Destination address. the Target Address is set to fe80::2001:db8:1::1 (i.e., the AERO address of Client ('D')). the Destination Address is set to the IPv6 destination address of the packet that triggered the Redirection event. the message includes a TLLAO set to 'L2(D)' (i.e., the underlying address of Client ('D')). the message includes as much of the RHO copied from the corresponding AERO Predirect message as possible such that at least the network-layer header is included but the size of the message does not exceed 1280 bytes. After Client ('D') prepares the Redirect message, it sends the message to Server ('A').

When Server ('A') receives a Redirect message, it accepts the message only if it has a neighbor cache entry that associates the message's link-layer source address with the network-layer source address. Next, Server ('A') validates the message according to the ICMPv6 Redirect message validation rules in Section 8.1 of . Following validation, Server ('A') re-encapsulates the Redirect then relays the re-encapsulated Redirect on to Client ('B') as follows. In the reference operational scenario, Server ('A') receives the Redirect message from Client ('D') and prepares to re-encapsulate and forward the message to Client ('B'). Server ('A') first verifies that Client ('D') is authorized to use the Prefix Length in the Redirect message when applied to the AERO address in the network-layer source of the Redirect message, and discards the message if verification fails. Otherwise, Server ('A') re-encapsulates the message by changing the link-layer source address of the message to 'L2(A)', changing the network-layer source address of the message to fe80::1, and changing the link-layer destination address to 'L2(B)' . Server ('A') finally relays the re-encapsulated message to the ingress node ('B') without decrementing the network-layer IPv6 header Hop Limit field. While not shown in , AERO Relays relay Redirect and Predirect messages in exactly this same fashion described above. See in Appendix A for an extension of the reference operational scenario that includes Relays.

When Client ('B') receives the Redirect message, it accepts the message only if it has a link-layer source address of the Server, i.e. 'L2(A)'. Next, Client ('B') validates the message according to the ICMPv6 Redirect message validation rules in Section 8.1 of . Following validation, Client ('B') then processes the message as follows. In the reference operational scenario, when Client ('B') receives the Redirect message, it either creates or updates a neighbor cache entry that stores the Target Address of the message as the network-layer address of Client ('D') and stores the link-layer address found in the TLLAO as the link-layer address of Client ('D'). Client ('B') then applies the Prefix Length to the Interface Identifier portion of the Target Address and records the resulting IPv6 prefix in its IPv6 forwarding table. Now, Client ('B') has an IPv6 forwarding table entry for Client('D')'s prefix, and Client ('D') has an IPv6 forwarding table entry for Client ('B')'s prefix. Thereafter, the clients may exchange ordinary network-layer data packets directly without forwarding through Server ('A').

When a source Client discovers a target neighbor (either through redirection or some other means) it MUST test the direct path to the target by sending an initial NS message to elicit a solicited NA response. While testing the path, the Client SHOULD continue sending packets via the Server until target reachability has been confirmed. The Client MUST thereafter follow the Neighbor Unreachability Detection (NUD) procedures in Section 7.3 of , and can resume sending packets via the Server at any time the direct path appears to be failing. If the Client is unable to elicit an NA response after MAX_RETRY attempts, it SHOULD consider the direct path unusable for forwarding purposes but still viable for ingress filtering purposes. If reachability is confirmed, the Client SHOULD thereafter process any link-layer errors as a hint that the direct path to the target has either failed or has become intermittent.

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 such as IPsec 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 MUST use the following MTU mitigations to accommodate larger packets. First, the node sets the AERO interface MTU to the larger of 1500 bytes and the underlying interface MTU minus the encapsulation overhead. The node also optionallly caches per-neighbor MTU values in the underlying IP path MTU discovery cache initialized to the underlying interface MTU. The node then admits 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. (For IPv4, the node sets the DF bit to 0 for packets no larger than 1280 bytes and sets the DF but 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 larger than 1280 bytes minus the encapsulation overhead but no larger than 1500 bytes, if the outer protocol is IPv6 the node uses outer IPv6 fragmentation to fragment the packet into two pieces where the first fragment contains at least 1024 bytes of the fragmented inner packet then admits the fragments into the tunnel. If the outer protocol is IPv4, the node instead admits the packet into the tunnel with DF set to 0 subject to rate limiting to ensure that any fragmentation resulting in the path does not result in reassembly errors . For both IPv4 and IPv6, the node also sends a 1500 byte probe message to the neighbor, subject to rate limiting. To construct a probe, the node prepares an ICMPv6 Neighbor Solicitation (NS) message with trailing null padding added. The node then encapsulates the NS in the outer encapsulation headers with length set to 1500 plus the encapsulation overhead, sets DF to 1 (for IPv4) and sends the padded NS message to the neighbor. If the neighbor returns an NA message, the node may then send whole packets within this size range and (for IPv4) relax the rate limiting requirement. In addition to these MTU mitigations, AERO nodes rewrite the TCP Maximum Segment Size (MSS) value in any TCP connection handshakes they originate over the AERO interface . The nodes perform this "MSS clamping" by rewriting the MSS to a size that is no larger than 1500 bytes minus the length of the TCP and IPv6 headers minus the encapsulation overhead minus the length of any additional encapsulations (e.g., IPsec) expected on the path. By writing a reduced value in the TCP MSS, the AERO Client ensures that the resulting TCP session will use packet sizes small enough to avoid fragmentation. The communicating endpoints can subsequently probe for larger packet sizes using Packetization Layer Path MTU Discovery (PLMPMTUD) , which searches for successful packet sizes larger than the original MSS. Other protocol types that do not include an MSS exchange in their connection establishment (e.g., UDP) will still see a 1500 byte minimum MTU even if a small amount of fragmentation and reassembly are required.

When a Client needs to change its link-layer address (e.g., due to a mobility event, due to a change in underlying network interface, etc.), it sends an immediate NA message forward to any of its correspondents (including the Server and any other Clients) which then discover the new link-layer address. If two Clients change their link-layer addresses simultaneously, the NA messages may be lost. In that case, the Clients follow the same NUD procedures specified in Section 3.8.

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 source Client has no means for reaching the target directly (since they connect to underlying networks of different IP protocol versions) and so must ignore any Redirects and continue to send packets via the Server.

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

In some AERO link scenarios, there may be no Server on the link and/or no need for Clients to use a Server as an intermediary trust anchor. In that case, Clients can establish neighbor cache entries and IPv6 routes by performing direct NS/NA exchanges, and some other form of trust basis must be applied so that each Client can verify that the prospective neighbor is authorized to use its claimed prefix. When there is no Server on the link, Clients must arrange to receive prefix delegations and publish the delegations via a secure prefix discovery service through some means outside the scope of this document.

AERO nodes that connect to an IPv4 underlying network can configure a NAT64 function to support any IPv6 nodes on their attached EUNs. AERO nodes can use the Default Address Selection Policy with DHCPv6 option the same as on any IPv6 link.