| Network Working Group | F. L. Templin, Ed. |
| Internet-Draft | Boeing Research & Technology |
| Obsoletes: rfc6706 (if approved) | January 13, 2014 |
| Intended status: Standards Track | |
| Expires: July 17, 2014 |
Transmission of IPv6 Packets over AERO Links
draft-templin-aerolink-02.txt
This document specifies the operation of IPv6 over tunnel virtual Non-Broadcast, Multiple Access (NBMA) links using Automatic Extended Route Optimization (AERO). Nodes attached to AERO links can exchange packets via trusted intermediate routers on the link that provide forwarding services to reach off-link destinations and/or redirection services to inform the node of an on-link neighbor that is closer to the final destination. Operation of the IPv6 Neighbor Discovery (ND) protocol over AERO links is based on an IPv6 link local address format known as the AERO address.
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This document specifies the operation of IPv6 over tunnel virtual Non-Broadcast, Multiple Access (NBMA) links using Automatic Extended Route Optimization (AERO). Nodes attached to AERO links can exchange packets via trusted intermediate routers on the link that provide forwarding services to reach off-link destinations and/or redirection services to inform the node of an on-link neighbor that is closer to the final destination.
Nodes on AERO links use an IPv6 link-local address format known as the AERO Address. This address type has properties that statelessly link IPv6 Neighbor Discovery (ND) to IPv6 routing. The AERO link can be used for tunneling to neighboring nodes on either IPv6 or IPv4 networks, i.e., AERO views the IPv6 and IPv4 networks as equivalent links for tunneling. The remainder of this document presents the AERO specification.
The terminology in the normative references applies; the following terms are defined within the scope of this document:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
The following sections specify the operation of IPv6 over Automatic Extended Route Optimization (AERO) links:
All nodes connected to an AERO link configure their AERO interfaces as router interfaces (not host interfaces). End system applications therefore do not bind directly to the AERO interface, but rather 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 [RFC2473] to exchange tunneled packets with AERO neighbors attached to an underlying IPv6 network, and use IPv6-in-IPv4 encapsulation [RFC4213] to exchange tunneled packets with AERO neighbors attached to an underlying IPv4 network. AERO interfaces can also use IPsec encapsulation [RFC4301] (either IPv6-in-IPv6 or IPv6-in-IPv4) in environments where strong authentication and confidentiality are required.
AERO interfaces further use the Subnetwork Encapsulation and Adaptation Layer (SEAL) [I-D.templin-intarea-seal] and can therefore configure an unlimited Maximum Transmission Unit (MTU). This entails the insertion of a SEAL header (i.e., an IPv6 fragment header with the S bit set to 1) between the inner IPv6 header and the outer IP encapsulation header. When NAT traversal and/or filtering middlebox traversal is necessary, a UDP header is further inserted between the outer IP encapsulation header and the SEAL header. (Note that while [RFC6980] forbids fragmentation of IPv6 ND messages, the SEAL fragmentation header applies only to the outer tunnel encapsulation and not the inner IPv6 ND packet.)
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 links do not use any on-link prefixes other than link-local; hence, there is no need for 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 Figure 1, while TLLAOs for target nodes that use an IPv4 underlying address include only the 4 bytes of the IPv4 address as shown in Figure 2.
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 | UDP Source Port (or 0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-- --+
| |
+-- IPv6 Address --+
| |
+-- --+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: AERO TLLAO Format for IPv6
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 | UDP Source Port (or 0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: AERO TLLAO Format for IPv4
AERO Servers configure their AERO link interfaces as router interfaces, and provide default routing services to AERO Clients.
AERO Clients also 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 (virtual) EUN interfaces). 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 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.
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:
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:
it would construct its AERO address as:
An AERO Client may receive multiple discontiguous 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.
Figure 3 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'):
.-(::::::::)
.-(::: IPv6 :::)-. +-------------+
(:::: Internet ::::)--| Host F |
`-(::::::::::::)-' +-------------+
`-(::::::)-' 2001:db8:3::1
|
+--------------+
| AERO Server A|
| (C->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 |
`-(______)-' +-------------+
Figure 3: AERO Reference Operational Scenario
In Figure 3, 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 [RFC6434]. 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 name "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 [RFC3633] 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 Section 3.7.
AERO Servers observe the IPv6 router requirements defined in [RFC6434]. 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 Section 3.7.
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 a new 8-bit "Prefix Length" field take from the low-order 8 bits of the Reserved field as shown in Figure 4 and Figure 5.
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 (=135) | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Figure 4: AERO Neighbor Solicitation (NS) Message Format
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 (=136) | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| R|S|O| Reserved | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Figure 5: AERO Neighbor Advertisement (NA) Message Format
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.
Section 3.4 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 Figure 3, 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 forwards the packet to AERO Client ('D'), where the packet is finally forwarded to the IPv6 destination host ('E'). When Server ('A') 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 Figure 3, 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').
Using AERO redirection, Server ('A') then 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 Section 3.7.4. 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 Section 3.7.5. 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 processes the message and forwards it on to Client ('B') as specified in Section 3.7.6. 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 Section 3.7.7. 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 [RFC4861], 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 Figure 6:
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 6: AERO Redirect/Predirect Message Format
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 [RFC4861]). Server ('A') prepares the Predirect message as follows:
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 [RFC4861].
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:
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 [RFC4861]. Following validation, Server ('A') re-encapsulates the Redirect as discussed in [I-D.templin-intarea-seal], and 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 forward a corresponding Redirect message to Client ('B'). Server ('A') then 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 redirect 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 Figure 3, AERO Relays relay Redirect and Predirect messages in exactly this same fashion described above. See Figure 7 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 [RFC4861]. 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 [RFC4861], 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.
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 NS 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 NS/NA exchange(s) may fail. 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 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 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.
An early implementation is available at: http://linkupnetworks.com/seal/sealv2-1.0.tgz.
There are no IANA actions required for this document.
AERO link security considerations are the same as for standard IPv6 Neighbor Discovery [RFC4861] except that AERO improves on some aspects. In particular, AERO is dependent on a trust basis between AERO Clients and Servers, where the Clients only engage in the AERO mechanism when it is facilitated by a trust anchor.
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., WiFi networks) and links that provide physical security (e.g., enterprise network LANs) provide a first line of defense that is often sufficient. In other instances, securing mechanisms such as Secure Neighbor Discovery (SeND) [RFC3971] or IPsec [RFC4301] must be used.
Discussions both on the v6ops list and in private exchanges helped shape some of the concepts in this work. Individuals who contributed insights include Mikael Abrahamsson, Fred Baker, Stewart Bryant, Brian Carpenter, Brian Haberman, Joel Halpern, and Lee Howard. 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 Balaguruna Chidambaram, Jeff Holland, Cam Brodie, Yueli Yang, Wen Fang, Ed King, Mike Slane, Kent Shuey, Gen MacLean, and other members of the BR&T and BIT mobile networking teams.
Earlier works on NBMA tunneling approaches are found in [RFC2529][RFC5214][RFC5569].
Figure 3 depicts a reference AERO operational scenario with a single Server on the AERO link. In order to support scaling to larger numbers of nodes, the AERO link can deploy multiple Servers and Relays, e.g., as shown in Figure 7.
.-(::::::::)
.-(::: 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 |
+--------+ +--------+
Figure 7: AERO Server/Relay Interworking
When host ('A') sends a packet toward destination host ('G'), IPv6 forwarding directs the packet through the EUN to Client ('B'), which forwards the packet to Server ('C') in absence of more-specific forwarding information. Server ('C') forwards the packet, and it also generates an AERO Predirect message that is then forwarded through Relay ('D') to Server ('E'). When Server ('E') receives the message, it forwards the message to Client ('F').
After processing the AERO Predirect message, Client ('F') sends an AERO Redirect message to Server ('E'). Server ('E'), in turn, forwards the message through Relay ('D') to Server ('C'). When Server ('C') receives the message, it forwards the message to Client ('B') informing it that host 'G's EUN can be reached via Client ('F'), thus completing the AERO redirection.
The network layer routing information shared between Servers and Relays must be carefully coordinated in a manner outside the scope of this document. In particular, Relays require full topology information, while individual Servers only require partial topology information (i.e., they only need to know the EUN prefixes associated with their current set of Clients). See [IRON] for an architectural discussion of routing coordination between Relays and Servers.