| Network Working Group | F. Templin, Ed. |
| Internet-Draft | Boeing Research & Technology |
| Intended status: Informational | February 18, 2012 |
| Expires: August 19, 2012 |
Asymmetric Extended Route Optimization (AERO)
draft-templin-aero-08.txt
Nodes attached to common multi-access link types (e.g., multicast-capable, shared media, non-broadcast multiple access (NBMA), etc.) can exchange packets as neighbors on the link, but may not always be provisioned with sufficient routing information for optimal neighbor selection. Such nodes should therefore be able to discover a trusted intermediate router on the link that provides both forwarding services to reach off-link destinations and redirection services to inform the node of an on-link neighbor that is closer to the final destination. This redirection can provide a useful route optimization, since the triangular path from the ingress link neighbor, to the intermediate router, and finally to the egress link neighbor may be considerably longer than the direct path from ingress to egress. However, ordinary redirection may lead to operational issues on certain link types and/or in certain deployment scenarios. This document therefore introduces an Asymmetric Extended Route Optimization (AERO) capability that addresses the issues.
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Nodes attached to common multi-access link types (e.g., multicast-capable, shared media, non-broadcast multiple access (NBMA), etc.) can exchange packets as neighbors on the link, but may not always be provisioned with sufficient routing information for optimal neighbor selection. Such nodes should therefore be able to discover a trusted intermediate router on the link that provides both default forwarding services to reach off-link destinations and redirection services to inform the node of an on-link neighbor that is closer to the final destination.
+--------------+
| Router A |
| (D->C) |
+--------------+
|
X--------+--------+--------+------X
| |
+----------+---+ +---+----------+
| Node B | | Router C |
| (default->A) | +-------+------+
+--------------+ .-.
,-( _)-.
.-(_ IPv6 )-.
(__ EUN )
`-(______)-'
+-------+------+
| Node D |
+--------------+
Figure 1 shows a classical multi-access link redirection scenario. In this figure, Node 'B' is provisioned with only a default route with Router 'A' as the next hop. Router 'A' in turn has a more-specific route that lists Router 'C' as the next hop neighbor on the link for Node 'D's attached network.
If Node 'B' has a packet to send to Node 'D', 'B' is obliged to send its initial packets via Router 'A'. Router 'A' then forwards the packet to Router 'C' and also returns a redirect message to inform 'B' that 'C' is in fact an on-link neighbor that is closer to the final destination 'D'. After receiving the redirect message, 'B' can place a more-specific route in its forwarding table so that future packets destined to 'D' can be sent directly via Router 'C', as shown in Figure 2.
+--------------+
| Router A |
| (D->C) |
+--------------+
|
X--------+--------+--------+------X
| |
+----------+---+ +---+----------+
| Node B | | Router C |
| (default->A) | +-------+------+
| (D->C) | .-.
+--------------+ ,-( _)-.
.-(_ IPv6 )-.
(__ EUN )
`-(______)-'
+-------+------+
| Node D |
+--------------+
For example, when an ingress link neighbor accepts an ordinary redirect message, it has no way of knowing whether the egress link neighbor is ready and willing to accept packets directly without involving an intermediate router. Likewise, the egress has no way of knowing that the ingress is authorized to forward packets from the claimed network layer (L3) source address. (This is especially important for very large links, since any node on the link can spoof the L3 source address with low probability of detection even if the link-layer (L2) source address cannot be spoofed.) Additionally, the ingress would have no way of knowing whether the direct path to the egress has failed, nor whether the final destination has moved away from the egress to some other network attachment point.
Therefore, a new approach is required that can enable redirection signaling from the egress to the ingress link node under the mediation of a trusted intermediate router. The mechanism is asymmetric (since only the forward direction from the ingress to the egress is optimized) and extended (since the redirection extends forward to the egress before reaching back to the ingress). This document therefore introduces an Asymmetric Extended Route Optimization (AERO) capability that addresses the issues.
While the AERO mechanisms were initially designed for the specific purpose of NBMA tunnel virtual interfaces (e.g., see: [RFC2529][RFC5214][RFC5569][I-D.templin-intarea-vet]) they can also be applied to any multiple access link types that support redirection. The AERO techniques are discussed herein with reference to IPv6 [RFC2460][RFC4861], however they can also be applied to any other network layer protocol (e.g., IPv4 [RFC0791][RFC0792][RFC2131]) that provides a redirection service (details of operation for other network layer protocols are out of scope.)
The terminology in the normative references apply; the following terms are defined within the scope of this document:
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [RFC2119].
The route optimization mechanism must satisfy the following requirements:
The following sections specify an Asymmetric Extended Route Optimization (AERO) capability that fulfills the requirements specified in Section 3.
In many AERO link use case scenarios (e.g., small enterprise networks, small and stable MANETs, etc.), routers can engage in a classical dynamic routing protocol (e.g., OSPF, RIP, IS-IS, etc.) so that routing/forwarding tables can be populated and standard forwarding between routers can be used. In other scenarios (e.g., large enterprise/ISP networks, cellular service provider networks, dynamic MANETs, etc.), this might be impractical due to routing protocol control message scaling issues.
When a classical dynamic routing protocol cannot be used, the mechanisms specified in this section can provide a useful on-demand route discovery capability. When both classical dynamic routing protocols and the AERO mechanism are active on the same link, routes discovered by the dynamic routing protocol should take precedence over those discovered by AERO.
The following sections discuss characteristics of nodes attached to links over which AERO can be used:
Intermediate AERO routers configure their AERO link interfaces as advertising router interfaces (see: [RFC4861], Section 6.2.2), and therefore may send Router Advertisement (RA) messages that include non-zero Router Lifetimes.
Edge AERO routers configure their AERO link interfaces as non-advertising router interfaces.
AERO hosts configure their AERO link interfaces as simple host interfaces.
AERO hosts send Router Solicitation (RS) messages to obtain RA messages from an intermediate AERO router. When the RA contains Prefix Information Options with non-link-local prefixes, the host autoconfigures L3 addresses from the prefixes using Stateless Address Autoconfiguration (SLAAC) [RFC4861][RFC4862]. When managed L3 address delegation services are available, the host can also (or instead) acquire L3 addresses taken from prefixes aggregated by the intermediate router through the use of stateful mechanisms, e.g., DHCPv6 [RFC3315], administrative configuration, etc.
After the host receives L3 addresses, it assigns them to its AERO interface and forwards any of its outbound packets via the intermediate router as a default router. The host may subsequently engage in the AERO route optimization procedure as specified in Section 4.4.
Edge AERO routers send RS messages to obtain RA messages from an intermediate AERO router, i.e., they act as "hosts" on their non-advertising AERO link router interfaces for the purpose of default router discovery. Edge routers can then acquire managed prefix delegations aggregated by an intermediate router through the use of, e.g., DHCPv6 Prefix Delegation [RFC3633], administrative configuration, etc.
After the edge router acquires prefixes, it can sub-delegate them to nodes and links within its attached End User Networks (EUNs), then can forward any outbound packets coming from its EUNs via the intermediate router. The edge router may subsequently engage in the AERO route optimization procedure as specified in Section 4.4.
Intermediate AERO routers respond to RS messages from AERO hosts and edge routers by returning an RA message. Intermediate routers may further configure a DHCP relay or server function on their AERO links and/or provide an administrative interface for delegation of L3 addresses and prefixes. (In any case, however, each intermediate router must be made aware of the L3 address/prefix delegations associated with the AERO edge routers and hosts that it serves.)
When the intermediate router completes a stateful L3 address or prefix delegation transaction (e.g., as a DHCPv6 relay/server, etc.), it establishes forwarding table entries that list the L2 address of the client AERO node as the L2 address of the next hop toward the delegated L3 addresses/prefixes.
When the intermediate router forwards a packet out the same AERO interface it arrived on, it initiates an AERO route optimization procedure as specified in Section 4.4.
Figure 3 depicts the AERO reference operational scenario. The figure shows an intermediate AERO router ('A'), two edge AERO routers ('B', 'D'), an AERO host ('F'), and three ordinary IPv6 hosts ('C', 'E', 'G'):
.-(::::::::)
.-(::: IPv6 :::)-. +-------------+
(:::: Internet ::::)--| Host G |
`-(::::::::::::)-' +-------------+
`-(::::::)-' 2001:db8:3::1
|
+--------------+ +--------------+
| Intermediate | | AERO Host F |
| AERO Router A| | (default->A) |
| (C->B; E->D) | +--------------+
+--------------+ 2001:db8:2:1
L3(A) L3(F)
L3(A) L2(F)
| |
X-----+-----------+-----------+-----------+---X
| AERO Link |
L2(B) L2(D)
L3(B) L3(D)
+--------------+ +--------------+ .-.
| AERO Edge | | AERO Edge | ,-( _)-.
| Router B | | Router 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 Figure 3, intermediate AERO router 'A' connects to the AERO link and also connects to the IPv6 Internet, either directly or via other IPv6 routers (not shown). 'A' configures an AERO link interface with a link-local L3 address L3(A) and with L2 address L2(A). 'A' next arranges to add L2(A) to a published list of valid intermediate routers for the link. Finally, 'A' is further provisioned with routing information listing node 'B' as the next-hop AERO router toward the EUN associated with node 'C', and listing node 'D' as the next-hop AERO router toward the EUN associated with node 'E'.
AERO edge router 'B' connects to one or more IPv6 EUNs and also connects to the AERO link via an interface with link-local L3 address L3(B) and with L2 address L2(B). 'B' next configures a default route with next-hop L3 address L3(A) via the AERO interface, then receives the L3 prefix 2001:db8:0::/48 through a stateful prefix delegation exchange that also establishes routing information in intermediate router 'A'. 'B' finally sub-delegates the L3 prefix to links and/or routers within its attached EUNs, where IPv6 host 'C' autoconfigures the L3 address 2001:db8:0::1.
AERO edge router 'D' connects to the AERO link via an interface with link-local L3 address L3(D) and with L2 address L2(D). 'D' next configures a default route with next-hop L3 address L3(A) via the AERO interface, then receives the L3 prefix 2001:db8:1::/48 through a stateful prefix delegation exchange in the same fashion as for router 'B'. 'D' finally sub-delegates the L3 prefix to links and/or routers within its attached EUNs, where IPv6 host 'E' autoconfigures L3 address 2001:db8:1::1.
Host 'F' connects to the AERO link via an interface with link-local L3 address L3(F) and with L2 address L2(F). 'F' next configures a default route with next-hop L3 address L3(A) via the AERO interface, then receives the L3 address 2001:db8:2::1 from a stateful address configuration exchange that also establishes routing information in intermediate router 'A'. When 'F' receives the L3 address, it assigns the address to the AERO interface.
Finally, IPv6 host 'G' connects to an IPv6 network outside of the AERO link domain. 'G' configures its IPv6 interface in a manner specific to its attached IPv6 link, and autoconfigures the L3 address 2001:db8:3::1.
In these arrangements, intermediate router 'A' must maintain state that associate the delegated L3 addresses/prefixes with the link-local L3 addresses of the correct edge routers and/or hosts on the AERO link. The routers and hosts must maintain at least a default route that points to 'A', and can discover more-specific routes either via a proactive dynamic routing protocol or via the AERO mechanisms specified in Section 4.4.
Section 4.3 describes the AERO reference operational scenario. We now discuss the operation and protocol details of AERO with respect to this reference scenario.
With reference to Figure 3, when source host 'C' sends a packet with source L3 address 'C' and destination L3 address 'E', the packet is first forwarded over 'C's attached EUN to the ingress AERO node 'B'. 'B' then forwards the packet over the AERO interface to the AERO link intermediate router 'A', which then forwards the packet to the egress AERO node 'D', where the packet is finally forwarded to destination host 'E'. When intermediate router 'A' forwards the packet back out on its advertising AERO interface, it must arrange to redirect 'B' toward 'D' as a better next hop node on the AERO link that is closer to the final destination. However, this redirection process should only occur if there is assurance that both 'B' and 'D' are willing participants.
Consider a first alternative in which intermediate router 'A' informs ingress AERO node 'B' only and does not inform egress AERO node 'D' (i.e., "classic redirection"). In that case, 'D' has no way of knowing that 'B' is authorized to forward packets from their claimed source L3 addresses, and may simply elect to drop the packets. Also, 'B' has no way of knowing whether 'D' is willing to accept its packets, nor whether 'D' is even reachable via a direct path that does not involve 'A'. Finally, 'B' has no way of knowing whether the final destination has moved away from 'D'.
Consider also a second alternative in which intermediate router 'A' informs both ingress AERO node 'B' and egress AERO node 'D' separately via independent redirection messages (i.e., "augmented redirection"). In that case, several conditions can occur that could result in communication failures. First, if 'B' receives the redirection message but 'D' does not, subsequent packets sent by 'B' could be dropped due to filtering since 'D' would not have neighbor state to verify their source L3 addresses. Second, if 'D' receives the redirection message but 'B' does not, subsequent packets sent in the reverse direction by 'D' would be lost. Finally, timing issues surrounding the establishment and garbage collection of neighbor state at 'B' and 'D' could yield unpredictable behavior. For example, unless the timing were carefully coordinated through some form of synchronization loop, there would invariably be instances in which one node has the correct neighbor state and the other node does not resulting in non-deterministic packet loss.
Since neither of these alternatives can satisfy the requirements listed in Section 3, a new redirection technique is needed. In this new method (i.e., "AERO redirection"), when intermediate router 'A' forwards a packet from ingress AERO node 'B' out the same AERO interface toward egress AERO node 'D', 'A' first sends a "Predirect" message forward to 'D' to inform it that 'B' is authorized to originate packets using source L3 address 'C'. After 'D' receives the Predirect, it creates neighbor state for 'B' and sends a Redirect message back to 'B' via 'A' as a trusted intermediary. When 'B' receives the Redirect, it both creates neighbor state for 'D' and knows that 'D' will accept the packets it sends with source L3 address 'C'. This process addresses the issues inherent to the classical and augmented redirection approaches; the following subsections therefore specify the AERO redirection steps necessary to support the reference operational scenario.
Each AERO node maintains a per AERO interface conceptual neighbor cache that includes an entry for each neighbor it communicates with on the AERO link the same as for any IPv6 interface (see: [RFC4861]).
Each AERO interface neighbor cache entry further maintains two lists of (src, dst) prefix pairs. The AERO node adds a prefix pair to the ACCEPT list if it has been informed by a trusted intermediate router that it is safe to accept packets from the neighbor using L3 source and destination addresses covered by the prefix pair. The AERO node adds a prefix pair to the FORWARD list if it has been informed by a trusted intermediate router that it is permitted to forward packets to the neighbor using L3 addresses covered by the prefix pair.
When the node adds a prefix pair to a neighbor cache entry ACCEPT list, it also sets an expiration timer for the prefix pair to ACCEPT_TIME seconds. When the node adds a prefix pair to a neighbor cache entry FORWARD list, it sets an expiration timer for the prefix pair to FORWARD_TIME seconds.
It is RECOMMENDED that FORWARD_TIME be set to the default constant value 30 seconds to match the default REACHABLE_TIME value specified for IPv6 neighbor discovery [RFC4861]. It is further RECOMMENDED that ACCEPT_TIME be set to the default constant value 40 seconds to allow a 10 second window so that the AERO redirection procedure can converge before the ACCEPT_TIME timer decrements below FORWARD_TIME.
Different values for FORWARD_TIME and ACCEPT_TIME 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. ACCEPT_TIME SHOULD further be set to a value that is sufficiently longer than FORWARD time to allow the AERO redirection procedure to converge.
AERO nodes MUST employ a data origin authentication check for the packets they receive on an AERO interface. In particular, the node considers the L3 source address correct for the L2 source address if:
When the AERO node receives a packet on an AERO interface, it processed the packet further if it satisfies one of these data origin authentication conditions; otherwise it drops the packet.
Note that on links in which L2 address spoofing is possible, AERO nodes may be obliged to require the use of digital signatures. In that case, only the third of the above conditions can be accepted in order to ensure adequate data origin authentication.
When an intermediate AERO router forwards a packet out the same AERO interface that it arrived on, the router sends a Predirect message forward toward the egress AERO node instead of sending a Redirect message back to the ingress AERO node.
The Predirect format is the same as the ICMPv6 Redirect message format depicted in Section 4.5 of [RFC4861], and is identified by three new bits known as the "AERO bits" taken from the Reserved field as shown in Figure 4:
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) | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |A|P|R| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Target Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Destination Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options ... +-+-+-+-+-+-+-+-+-+-+-+-
Where the new AERO bits are defined as:
In the reference operational scenario, when intermediate router 'A' forwards a packet sent by ingress node 'B' toward egress node 'D', it also sends a Predirect message forward toward 'D', subject to rate limiting (see Section 8.2 of [RFC4861]). The intermediate router ('A') prepares the Predirect message in a similar fashion as for an ordinary IPv6 Redirect message as follows:
The intermediate router ('A') then sends the Predirect message forward to the egress node ('D').
When the egress node ('D') receives an AERO Predirect message (i.e., a Redirect message with A=1; P=1), it accepts the message only if it satisfies the data origin authentication requirements specified in Section 4.4.3. (In particular, the egress node ('D') only accepts the message if it originated from a trusted intermediate router ('A') unless and until additional authenticating state has been established.) Next, the egress node ('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 egress node ('D') receives a Predirect it creates a neighbor cache entry (if necessary) that stores the Target address of the Predirect message (i.e., the link-local L3 address of the ingress node ('B')). The egress node ('D') then records the prefix found in the Predirect message RIO along with its own prefix that matches the L3 destination address in the packet header found in the RHO with the neighbor cache entry as an acceptable (src, dst) prefix pair. The egress node ('D') then adds the prefix pair to the ACCEPT list, and sets/resets an expiration timer for the prefix pair to ACCEPT_TIME seconds. If the timer later expires, the egress node ('D') deletes the prefix pair.
After processing the Predirect message, the egress node ('D') prepares a Redirect message response as follows:
After the egress node ('D') prepares the Redirect message, it sends the message to the intermediate router ('A').
When the intermediate router ('A') receives an AERO Redirect message (i.e., one with A=1; P=0; R=0), it accepts the message only if it satisfies the data origin authentication requirements specified in Section 4.4.3. Next, the intermediate router ('A') validates the message according to the ICMPv6 Redirect message validation rules in Section 8.1 of [RFC4861]. The intermediate router ('A') then "relays" the Redirect message back to the ingress node ('B') as follows.
In the reference operational scenario, the intermediate router ('A') receives the Redirect message from the egress node ('D') and prepares to relay the message to the ingress node ('B'). The intermediate router ('A') then verifies that the RIO encodes an L3 address/prefix that the egress node ('D') is authorized to use, and discards the message if verification fails. Otherwise, the intermediate router ('A') changes the L2 source address of the message to 'L2(A)', changes the L3 source address of the message to the link-local L3 address 'L3(A)', and changes the L2 destination address to 'L2(B)' . The intermediate router ('A') finally sets the AERO R bit to 1 and relays the Redirect message to the ingress node ('B') without decrementing the hopcount.
This relaying procedure therefore requires the intermediate router ('A') to examine the R bit before relaying a Redirect message in order to avoid a free-running loop due to the non-decrementing hopcount. In particular, the intermediate route discards any AERO Redirect message it receives with R==1.
When the ingress node ('B') receives an AERO Redirect message (i.e., one with A=1; P=0), it accepts the message only if it satisfies the data origin authentication requirements specified in Section 4.4.3. (In particular, the ingress node ('B') only accepts the message if it originated from a trusted intermediate router ('A') unless and until additional authenticating state has been established.) Next, the ingress node ('B') validates the message according to the ICMPv6 Redirect message validation rules in Section 8.1 of [RFC4861]. The ingress node ('B') then processes the message as follows.
In the reference operational scenario, when the ingress node ('B') receives the (relayed) Redirect message it creates a neighbor cache entry (if necessary) that stores the Target address of the Redirect message (i.e., the link-local L3 address of the egress node 'L3(D)'). The ingress node ('B') then records the (src, dst) prefix pair associated with the triggering packet in the neighbor cache entry FORWARD list, i.e., it records its prefix that matches the redirected packet's L3 source address and the prefix listed in the RIO as the prefix pair. The ingress node ('B') then sets/resets an expiration timer for the prefix pair to FORWARD_TIME seconds. If the timer later expires, the ingress node ('B') deletes the entry.
Now, the ingress node ('B') has a neighbor cache FORWARD list entry for the prefix pair, and the egress node ('D') has a neighbor cache ACCEPT list entry for the prefix pair. Therefore, the ingress node ('B') may forward ordinary network-layer data packets with L3 source and destination addresses that match the prefix pair directly to the egress node ('D') without involving the intermediate router ('A'). Note that the ingress node must have a way of informing the network layer of a route that associates the destination prefix with this neighbor cache entry. The manner of establishing such a route (and deleting it when it is no longer necessary) is left to the implementation.
To enable packet forwarding in the reverse direction, a separate AERO redirection operation is required which is the mirror-image of the forward operation described above, i.e., the forward and reverse AERO operations are asymmetric.
In order to prevent prefix pairs from expiring while data packets are actively flowing, the ingress node ('B') can periodically send Predirect keepalive messages directly to the egress node ('D') to solicit Redirect messages. Absent specific administrative configuration, it is RECOMMENDED that the ingress node ('B') send no more than 10 Predirect keepalive messages during each FORWARD_TIME interval.
In the reference operational scenario, when the ingress node ('B') wishes to refresh the FORWARD timer for a specific prefix pair, it can send a Predirect keepalive message directly to the egress node ('D') prepared as follows:
When the egress node ('D') receives the Predirect message, it accepts the message only if it satisfies the Predirect message validation rules given in Section 4.4.4. The egress node ('D') then resets its ACCEPT timer for the prefix pair that matches the originating packet's L3 source and destination addresses to ACCEPT_TIME seconds, and sends a Redirect message directly to the ingress node ('B') prepared as follows:
When the ingress node ('B') receives the Redirect message, it accepts the message only if it satisfies the redirect message validation rules given in Section 4.4.6. The ingress node ('B') then resets its FORWARD timer for the prefix pair that matches the originating packet's L3 source and destination addresses to FORWARD_TIME seconds.
When the ingress node ('B') receives a Redirect message informing it of a direct path to a new egress node ('D'), there is a question in point as to whether the new egress node ('D') can be reached directly without involving an intermediate router ('A'). On some AERO links, it may be reasonable for the ingress node ('B') to (optimistically) assume that reachability is transitive, and to immediately begin forwarding data packets to the egress node ('D') without testing reachability.
On AERO links in which an optimistic assumption of transitive reachability may be unreasonable, however, the ingress node ('B') can defer the redirection until it tests the direct path to the egress node ('D') by sending a Predirect message to elicit a Redirect as specified in Section 4.4.8. If the ingress node ('B') is unable to elicit a Redirect message after a small number of attempts, it should consider the direct path to the egress node ('D') as unusable.
In either case, the ingress node ('B') can process any link errors corresponding to the data packets sent directly to the egress node ('D') as a hint that the direct path has either failed or has become intermittent.
Again with reference to Figure 3, egress node 'D' can configure both a non-advertising router interface on a provider AERO link and advertising router interfaces on its connected EUN links. When node 'E' in one of the egress node's connected EUNs moves to a different network point of attachment, however, the EUN node ('E') can release its L3 address/prefix delegations that were registered with the egress node ('D') and re-establish them via a different router.
When the EUN node ('E') releases its L3 address/prefix delegations, the egress node ('D') marks the forwarding table entries that cover the L3 addresses/prefixes as "departed". When egress node ('D') receives packets from ingress node 'B' with L3 source and destination addresses that match a prefix pair on the ACCEPT list, it forwards them to the last-known L2 address of the EUN node ('E') as a means for avoiding mobility-related packet loss during routing changes. The egress node ('D') also returns a NULL Redirect message to inform the ingress node ('B') of the departure. The Redirect message is prepared as follows:
Eventually, any such correspondent AERO nodes will receive a NULL Redirect message and will cease to use the egress node ('D') as a next hop. They will then revert to sending packets destined to the EUN node ('E') via a trusted intermediate router and may subsequently receive new Redirect messages to discover that the EUN node ('E' ) is now associated with a new AERO edge router.
Note that any packets forwarded by the egress node ('D') via a departed forwarding table entry may be lost if the (mobile) EUN node ('E') moves off-link with respect to its previous EUN point of attachment. This should not be a problem for large links (e.g., large cellular network deployments, large ISP networks, etc.) in which all/most mobility events are intra-link.
When an AERO node configures one or more FORWARD/ACCEPT list prefix pair entries, and the prefixes associated with the pair are somehow re-configured or renumbered, the stale FORWARD/ACCEPT list information must be deleted.
When an ingress node ('B') re-configures it's L3 source prefix in such a way that the ACCEPT list entry in the egress node ('D') would no longer be valid (e.g., the prefix length of the source prefix changes), the ingress node ('B') simply deletes the prefix pair form its FORWARD list and allows subsequent packets covered by the prefix pair to again flow through an intermediate router ('A').
When the egress node ('D') re-configures it's L3 destination prefix in such a way that the FORWARD list entry in the ingress node ('B') would no longer be valid, the egress node ('D') sends a NULL Redirect message to the ingress node ('B') the same as described for Mobility Considerations when it receives either a Predirect message or a data packet (subject to rate limiting) from the ingress node ('B') .
If a legacy host or router receives an AERO Redirect or Predirect message, it will process the message as if it were an ordinary Redirect. This will cause no harmful effects, since the legacy system will safely ignore the AERO bits in the Reserved field, and will also ignore any RIOs that are included. The link-local L3 addresses encoded in the Redirect message Target and Destination addresses will also not cause the legacy node to create incorrect forwarding state. The mechanism therefore causes no harm to legacy systems, and supports natural incremental deployment.
This document defines new bits taken from the ICMPv6 Redirect message header Reserved field. There is currently no registration procedure for such bits, so there are no IANA considerations for this document.
AERO link security is dependent on a trust basis between edge nodes and intermediate routers. In particular, edge nodes must only engage in the AERO mechanism when it is facilitated by a trusted intermediate router.
AERO links must be protected against spoofing attacks in which an attacker on the link pretends to be a trusted neighbor. This is often possible on links that provide L2 securing mechanisms (e.g., WiFi networks) and on links that provide physical security (e.g., enterprise network LANs). In other instances, sufficient assurances against on-link spoofing attacks are possible if the source can digitally sign its messages. In that case, the destination can use the data origin authentication checks specified in Section 4.4.3 to verify the signature.
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, Joel Halpern, Lee Howard,
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC2460] | Deering, S.E. and R.M. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. |
| [RFC4861] | Narten, T., Nordmark, E., Simpson, W. and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. |
| [RFC4862] | Thomson, S., Narten, T. and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, September 2007. |
| [RFC4191] | Draves, R. and D. Thaler, "Default Router Preferences and More-Specific Routes", RFC 4191, November 2005. |
Figure 3 depicts a reference AERO operational scenario with a single intermediate router on the AERO link. In order to support scaling to larger numbers of nodes, the AERO link can deploy multiple intermediate routers, e.g., as shown in Figure 5
+--------------+ +--------------+
| Intermediate | +--------------+ | Intermediate |
| Router C | | Core Router D| | Router E |
| (default->D) | | (A->C; G->E) | | (default->D) |
| (A->B) | +--------------+ | (G->F) |
+-------+------+ +------+-------+
| |
X---+---+--------------------------------------+---+---X
| AERO Link |
+-----+--------+ +--------+-----+
| AERO Node B | | AERO Node F |
| (default->C) | | (default->E) |
+--------------+ +--------------+
.-. .-.
,-( _)-. ,-( _)-.
.-(_ IPv6 )-. .-(_ IPv6 )-.
(__ EUN A ) (__ EUN G )
`-(______)-' `-(______)-'
| |
+--------+ +--------+
| Host A | | Host G |
+--------+ +--------+
When ingress AERO node 'B' forwards a packet from host 'A' toward host 'G', it sends the packet to intermediate router 'C' in absence of more-specific forwarding information. Intermediate router 'C' in turn generates a "pseudo Predirect" message that is through some means conveyed through core router 'D' to intermediate router 'E'. When 'E' receives the pseudo Predirect, it sends an actual Predirect message to egress AERO node 'F'.
After processing the Predirect, egress node 'F' sends a Redirect message to intermediate router 'E'. Intermediate router 'E' in turn generates a "pseudo Redirect" that is through some means conveyed through core router 'D' to intermediate router 'C'. When 'C' receives the pseudo Redirect, it sends an actual Redirect message to ingress node 'B', thus completing the AERO redirection.
The interworkings between intermediate and core routers (including the conveyance of pseudo Predirects and Redirects) must be carefully coordinated in a manner outside the scope of this document. In particular, the intermediate and core routers must ensure that no routing loops are formed. See [I-D.templin-ironbis] for an architectural discussion of coordinations between intermediate and core routers.