INTERNET-DRAFT Iljitsch van Beijnum Jul 11, 2005 Shim6 Reachability Detection draft-ietf-shim6-reach-detect-00.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet Draft expires Jan 11, 2006. Copyright Notice Copyright (C) The Internet Society (2005). All Rights Reserved. Abstract This draft discusses the issues of detecting failures in a currently used address pair between two hosts and picking a new address pair to be used when a failure occurs. The input for these processes are ordered lists of local and remote addresses that are reasonably likely to work. (I.e., not include addresses that are known to be unreachable for local reasons.) These lists must be available at both ends of the communication, although the ordering may differ. Building these address lists from locally available information and synchronizing them with the remote end are outside the scope of this document. This text is for the most part based on discussions on the multi6 list, several multi6 design team lists and the shim6 list, with notable contributions from Erik Nordmark and Marcelo Bagnulo. Van Beijnum Expires January 11, 2006 [Page 1] Internet-Draft Shim6 Reachability Detection July 2005 Suggestions and additions are more than welcome. 1 Introduction The most widespread mechanisms to ensure reachability in current protocols are: - Acknowledgments. For instance, in TCP each segment received is acknowledged immediately or after a short delay. Lack of acknowledgments leads to retransmissions, and eventually, session timeouts. - Keepalives. In routing protocols it's customary to send keepalives at periodic intervals and look for either responses to local keepalives or for keepalives generated by the other side. If no keepalives or responses were received for some time the other side is declared unreachable. - Monitoring and probing. IPv6 Neighbor Unreachability Detection monitors the progress of higher layer protocols, and in the absence of progress, probes the other side (when on-link) or the next hop with a directed neighbor solicitation message. If there is no answer, the other side (on-link) or router is declared unreachable. None of these mechanisms seems like a good candidate to adopt for end-to-end reachability detection, either because they duplicate existing mechanisms or introduce unnecessary overhead. In addition, exploring the full set of communication options between two hosts that both have two or more addresses is an expensive operation as the number of combinations to be explored increases very quickly with the number of addresses. For instance, with two addresses on both sides, there are four possible address pairs. Since we can't assume that reachability in one direction automatically means reachability for the complement pair in the other direction, the total number of two-way combinations is eight. (Combinations = nA * nB * 2.) Although links almost always work in two directions, routing protocols and filters only work in one direction so unidirectional reachability can happen. Without additional mechanisms, the practice of ingress filtering by ISPs makes unidirectional connectivity likely. In order to reduce packet overhead, it makes sense to have different on-the-wire protocols for confirming existing reachability and full exploration of potential reachability. 2 Determining reachability for the current pair In discussions two models came up for determining whether the current address pair used in ongoing communication still works. Van Beijnum Expires January 11, 2006 [Page 2] Internet-Draft Shim6 Reachability Detection July 2005 The first model resembles IPv6 neighbor unreachability detection (NUD). The idea is that when transport protocols see forward progress, they inform the shim layer (positive feedback) and the shim layer doesn't take any action. However, in the absence of positive feedback and in the presence of outgoing traffic, the shim layer generates packets that probe reachability. When the correspondent receives a probe, it sends back an acknowledgment so the shim layer at the originating host knows the address pair is still functional. When there are no acknowledgments for several probes, a full reachability exploration is executed. The second model ensures that all communication is bidirectional. So when communication isn't bidirectional, there must be a failure and again, a full reachability exploration is executed. Although most protocols generate traffic in both directions most of the time, there are times when there is only legitimate traffic in one direction and not the other. The shim layer monitors incoming and outgoing packets, and when there are incoming packets but no regular outgoing data packets, the shim generates keepalive packets. So when there is outgoing traffic, there must be either regular incoming traffic, or keepalives generated by the other side. If not, there is probably a failure so the full reachability exploration procedure is executed. There are several different tradeoffs between the two models: - In the first model, the sending host detects the problem, in the second model, the receiving host detects the problem - In the first model, a host can detect problems in either direction - In the second model, a host can only detect problems in the receiving direction so it must depend on the correspondent to detect problems in the other direction - The first model generates traffic in both directions, possibly competing with payload traffic in the high-volume direction - The second model only generates traffic in the no-traffic direction, so there is never competition with payload traffic - In absence of upper layer protocol feedback, the first model always sends periodic probes - The second model doesn't require upper layer protocol feedback to suppress keepalives There have been some discussions about positive versus negative feedback. The first model doesn't have any use for negative feedback, but needs positive feedback to reduce overhead. The second model has little or no use for positive feedback, but may use negative feedback Van Beijnum Expires January 11, 2006 [Page 3] Internet-Draft Shim6 Reachability Detection July 2005 to detect failures faster. However, using negative feedback from upper layer protocols may prove challenging because upper layers can't be trusted to provide the right quality or quantity feedback ("feedback spamming"). 3 Address pair exploration In its essence, address pair exploration is very simple: just send probes using every possible address pair, wait for something to come back and possibly consider the round trip time. In practice, doing a full address pair exploration is very undesirable because of the large number of packets involved. This can be especially harmful when a lot of hosts on a link start doing this for many of their correspondents at the same time when there is a failure further upstream. At this time, we don't have a clear vision of what this protocol should look like, except that it should be conservative in the number of packets it transmits in average-case scenarios, and that it's vitally important to reject very bad paths or address pairs. Since the failures that have the largest potential to generate a lot of local address pair exploration are the ones where a link that's used for a lot of different sessions breaks, it makes sense to somehow generalize results for one correspondent into optimizations in the address exploration with another correspondent. A promising way to avoid bad paths would be to send out a first probe, wait for about a round trip for the old working path and then send another probe, and after that do an exponential backoff. If either the first or the second pair were reasonable choices, there is a workable solution within several round trips. 4 Granularity It has not been determined what the association/multiplexing granularity of shim6 will be: host-to-host, upper-layer-identity-to-upper-layer-identity (ULID) or session. By its nature, the reachability detection works on address or locator pairs. It would be highly inefficient if each session, or even each ULID pair, would do its own address pair exploration. On the other hand, it would also be undesirable force all sessions or ULID associations between two hosts to use the same address pairs. This probably means that when a failure is determined, all sessions or associations should act accordingly, but when reachability is determined, each session or association may react according to its own preferences. Van Beijnum Expires January 11, 2006 [Page 4] Internet-Draft Shim6 Reachability Detection July 2005 5 NAT and firewall considerations Since shim6 is chartered for IPv6 solutions only, and NAT compatibility is not expected, and by most people, not desired in IPv6, there is no requirement for this protocol to pass through Network Address Translation devices. However, the protocol may be applicable outside shim6, making NAT compatibility desirable. It is absolutely essential that the shim6 negotiations and the reachability detection packets are passed through filters or firewalls wherever application packets are passed through. If the shim6 negotiation and reachability detection packets are filtered out, shim6 can't be used. A more complex situation arises when the shim6 negotiation packets pass through a firewall, but the reachability detection packets are blocked. To avoid this complexity, it's highly desirable to make the shim6 negotiation and reachability detection part of the same protocol, so either both are allowed through or both are blocked. However, the same is true if this reachability detection mechanism is used in other protocols. This makes it desirable to define the reachability detection protocol such that it can be embedded in other protocols. Since firewalls are in wide use, it's important to consider whether a new protocol will be able to pass through most firewalls without requiring changes to the filter configuration. On the other hand, it may not be possible to come up with a protocol that would be allowed through a large percentage of all firewalls without changes, so extra effort in this area may produce limited results. Also, in the long run firewall configuration will presumably be changed, so any compromises would only have short term benefits but long term downsides. 6 Security considerations To avoid exposing information (even if it's just the fact that an address is reachable), hosts will probably want to limit themselves to taking part in reachability detection with known correspondents. This means that there must be identifying information and a nonce that is at least hard to guess but easy to check in all reachability detection packets. 4 Document and author information This document expires January, 2006. The latest version will always be available at http://www.muada.com/drafts/. Comments are welcome at: Iljitsch van Beijnum Email: iljitsch@muada.com Van Beijnum Expires January 11, 2006 [Page 5] Internet-Draft Shim6 Reachability Detection July 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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