| Network Working Group | F. Templin, Ed. |
| Internet-Draft | Boeing Research & Technology |
| Intended status: Informational | January 27, 2012 |
| Expires: July 28, 2012 |
Asymmetric Extended Route Optimization (AERO)
draft-templin-aero-07.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: Classical Multi-Access Link Redirection
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 |
+--------------+
Figure 2: More-Specific Routes Following Redirection
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 source address. (This is especially important for very large links, since any node on the link can spoof the network-layer source address with low probability of detection even if the link-layer 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 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 addresses from the prefixes using Stateless Address Autoconfiguration (SLAAC) [RFC4861][RFC4862]. When managed address delegation services are available, the host can also (or instead) acquire 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 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 receive redirection messages from the intermediate router listing a better next hop.
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 receive redirection messages from the intermediate router listing a better next hop.
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 addresses and prefixes.
When the intermediate router completes a stateful address or prefix delegation transaction (e.g., as a DHCPv6 relay/server, etc.), it establishes forwarding table entries that list the link-layer address of the client AERO node as the link-layer address of the next hop toward the delegated 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 |
`-(______)-' +-------------+
Figure 3: AERO Reference Operational Scenario
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 network-layer address L3(A) and with link-layer address L2(A). 'A' next arranges to add the link-layer address 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 network-layer address L3(B) and with link-layer address L2(B). 'B' next configures a default IPv6 route with next-hop address L3(A) via the AERO interface, then receives the IPv6 prefix 2001:db8:0::/48 through a stateful prefix delegation exchange via 'A'. 'B' finally sub-delegates the prefix to its attached EUNs, where IPv6 host 'C' autoconfigures the address 2001:db8:0::1.
AERO edge router 'D' connects to the AERO link via an interface with addresses L3(D)/L2(D), configures a default IPv6 route with next-hop address L3(A) via the AERO interface, and receives a stateful prefix delegation of 2001:db8:1::/48 in the same fashion as for router 'B'. 'D' finally sub-delegates the prefix to its attached EUNs, where IPv6 host 'E' autoconfigures IPv6 address 2001:db8:1::1.
Host 'F' connects to the AERO link via an interface with addresses L3(F)/L2(F). 'F' next configures a default IPv6 route with next-hop address L3(A) via the AERO interface, then receives the IPv6 address 2001:db8:2::1 from a stateful address configuration exchange via 'A'. When 'F' receives the IPv6 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 IPv6 address 2001:db8:3::1.
In these arrangements, intermediate router 'A' must maintain routes that associate the delegated IPv6 addresses/prefixes with 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 address 'C' and destination address 'E', the packet is first forwarded over 'C's attached EUN to the AERO link ingress 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 AERO link egress 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 ingress node 'B' toward egress node '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 node 'B' only and does not inform egress 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 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 node 'B' and egress 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 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 node 'B' out the same AERO interface toward egress router 'D', 'A' first sends a "Predirect" message forward to 'D' to inform it that 'B' is authorized to originate packets using source 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 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 neighbor cache state could be maintained, for example, as ancillary information in the IPv6 conceptual neighbor cache [RFC4861].
Each neighbor cache entry maintains the state variables ACCEPT and FORWARD. The node sets ACCEPT to TRUE if it has been informed by a trusted intermediate router that the neighbor is permitted to forward packets; otherwise, it sets ACCEPT to FALSE. The node sets FORWARD to TRUE if it has been informed by a trusted intermediate router that it is permitted to forward packets to the neighbor; otherwise, it sets FORWARD to FALSE.
When the node sets ACCEPT to TRUE, it also sets an expiration timer to ACCEPT_TIME seconds, where ACCEPT_TIME is set to the default constant value of 40 seconds. When the node the node sets FORWARD to TRUE, it also sets an expiration timer to FORWARD_TIME seconds, where FORWARD_TIME is set to the default constant value of 30 seconds.
The value 30 is chosen for FORWARD_TIME to match the default REACHABLE_TIME value specified for IPv6 neighbor discovery [RFC4861]. The value 40 is chosen for ACCEPT_TIME to allow a 10 second window so that the AERO redirection procedure can converge before the target neighbor's ACCEPT_TIME timer decrements below FORWARD_TIME.
Different values for FORWARD_TIME and ACCEPT_TIME may be 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 must 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 network-layer source address correct for the link-layer 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 link-layer 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.
Using AERO redirection, when an intermediate router forwards a packet out the same AERO interface that it arrived on, the router sends a Predirect message forward toward the AERO link egress node instead of sending a Redirect message back to the ingress 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 ... +-+-+-+-+-+-+-+-+-+-+-+-
Figure 4: AERO-Specific ICMPv6 Redirect Message Format
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]). 'A' prepares the Predirect message in a similar fashion as for an ordinary IPv6 Redirect message as follows:
'A' then sends the Predirect message forward to 'D'.
When an AERO link egress node 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. Next, the node validates the message according to the ICMPv6 Redirect message validation rules in Section 8.1 of [RFC4861].
In the reference operational scenario, when egress router 'D' receives a Predirect it creates a neighbor cache entry (if necessary) that stores the source address of the Predirect message (i.e., 'L3(B)') and records the prefixes associated with the packet that triggered the Predirect with the neighbor cache entry. 'D' then sets ACCEPT=TRUE for the neighbor cache entry and sets/resets an expiration timer to ACCEPT_TIME seconds. If the timer later expires, 'D' sets ACCEPT=FALSE.
After processing the Predirect message and establishing the forwarding table entry, 'D' prepares a Redirect message in response to the Predirect as follows:
After 'D' prepares the Redirect message, it sends the message to 'A'.
When an intermediate router 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 router validates the message according to the ICMPv6 Redirect message validation rules in Section 8.1 of [RFC4861]. The router then "relays" the Redirect message back to the original ingress node as follows.
In the reference operational scenario, intermediate router 'A' receives the Redirect message from egress router 'D' and verifies that the RIOs encode addresses/prefixes that 'D' is authorized to use. Without decrementing the hopcount in the Redirect message, 'A' next changes the link-layer source address of the message to 'L2(A)' and changes the link-layer destination address to 'L2(B)'. 'A' finally sets the AERO R bit to 1 and relays the Redirect message to ingress node 'B'.
This relaying procedure therefore requires the intermediate AERO router 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 router discards any AERO Redirect message it receives with R==1.
When an ingress node 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. Next, the node validates the message according to the ICMPv6 Redirect message validation rules in Section 8.1 of [RFC4861]. The node then processes the message as follows.
In the reference operational scenario, when ingress node 'B' receives the (relayed) Redirect message it creates a neighbor cache entry (if necessary) that stores the source address of the Redirect message (i.e., 'L3(D)') and records the prefixes associated with the triggering packet with the neighbor cache entry. 'B' then sets FORWARD=TRUE for the neighbor cache entry and sets/resets an expiration timer to FORWARD_TIME seconds. If the timer later expires, 'D' sets FORWARD=FALSE.
Now, 'B' has a neighbor cache entry for 'D" with FORWARD==TRUE, and 'D' has a neighbor cache entry for 'B' with ACCEPT=TRUE. Therefore, 'B' may forward ordinary network-layer data packets with destination addresses covered by 'D's prefixes directly to 'D' without involving 'A'.
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. Following the reverse operation, 'B' and 'D' can exchange packets bidirectionally without involving 'A'.
In order to prevent neighbor cache entries from expiring while data packets are actively flowing, the ingress node can periodically send Predirect messages directly to the egress node (subject to rate limiting) to solicit Redirect messages. In the reference operational scenario, when 'B' forwards a packet to 'D' and wishes to update the corresponding forwarding timer, 'B' can also send a Predirect message directly to 'D' prepared as follows:
When 'D' receives the Predirect message, it accepts the message only if it satisfies the Predirect message validation rules given in Section 4.4.4. 'D' then resets its accept timer for node 'B' to ACCEPT_TIME seconds, and sends a Redirect message directly to 'B' prepared as follows:
When 'B' receives the Redirect message, it accepts the message only if it satisfies the redirect message validation rules given in Section 4.4.6. 'B' then resets its forwarding timer for node 'D' to FORWARD_TIME seconds.
When an ingress node receives a Redirect message informing it of a direct path to a new egress node, there is a question in point as to whether the new egress can be reached directly without involving an intermediate router. On some AERO links, it may be reasonable for the ingress to (optimistically) assume that reachability is transitive, and to immediately begin forwarding data packets to the egress without testing reachability.
On AERO links in which an optimistic assumption of transitive reachability may be unreasonable, however, the ingress can defer the redirection until it tests the direct path to the egress by sending a Predirect message to elicit a Redirect as specified in Section 4.4.7. If the ingress is unable to elicit a Redirect message after a small number of attempts, it should consider the direct path to the egress as unusable.
In either case, the ingress can process any link errors corresponding to the data packets sent directly to the egress as a hint that the direct path has either failed or has become intermittent.
Again with reference to Figure 3, AERO edge router '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 'D's connected EUNs moves to a different network point of attachment, however, 'E' can release its address/prefix delegations that were registered with 'D' and re-establish them via a different router.
When 'E' releases its address/prefix delegations, 'D' marks the forwarding table entries that cover the addresses/prefixes as "departed". When 'D' receives packets destined to an address covered by the departed forwarding table entries, it forwards them to the last-known EUN link-layer address of 'E' as a means for avoiding mobility-related packet loss during routing changes. 'D' also returns a NULL Redirect message to inform correspondent neighbor 'B' of the departure. The Redirect message is prepared as follows:
Eventually, any such correspondent neighbors will receive a NULL Redirect message and will cease to use 'D' as a next hop. They will then revert to sending packets destined to 'E' via a trusted intermediate router and may subsequently receive new Redirect messages to discover that 'E' is now associated with a new edge AERO router. Note that any packets forwarded by 'D' via a departed forwarding table entry may be lost if the mobile node 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.
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 values 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 |
+--------+ +--------+
Figure 5: Multiple Intermediate Routers
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, AERO 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 AERO 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.