6MAN WG E. Nordmark Internet-Draft Cisco Systems, Inc. Expires: January 8, 2012 I. Gashinsky Yahoo! July 7, 2011 Neighbor Unreachability Detection is too impatient draft-nordmark-6man-impatient-nud-01.txt Abstract IPv6 Neighbor Discovery includes Neighbor Unreachability Detection. That function is very useful when a host has an alternative, for instance multiple default routers, since it allows the host to switch to the alternative in short time. This time is 3 seconds after the node starts probing. However, if there are no alternatives, this is far too impatient. This document proposes an approach where an implementation can choose the timeout behavior to be different based on whether or not there are alternatives. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on January 8, 2012. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect Nordmark & Gashinsky Expires January 8, 2012 [Page 1] Internet-Draft NUD is too impatient July 2011 to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Proposed Remedy . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5 4. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6.1. Normative References . . . . . . . . . . . . . . . . . . . 6 6.2. Informative References . . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6 Nordmark & Gashinsky Expires January 8, 2012 [Page 2] Internet-Draft NUD is too impatient July 2011 1. Introduction IPv6 Neighbor Discovery [RFC4861] includes Neighbor Unreachability Detection, which detects when a neighbor is no longer reachable. The timeouts specified are very short (three transmissions spaced one second apart). That can be appropriate when there are alternative paths the packet can be sent. For example, if a host has multiple default routers in its Default Router List, or if the host has a Neigbor Cache Entry (NCE) created by a Redirect message. The effect of NUD reporting a failure in those cases is that the host will try the alternative; the next router in the Default Router List, or discard the NCE which will also send using a different router. For that reason the timeouts where chosen to be short; this ensures that if a default router fails the host can use the next router in less than 45 seconds. However, where there is no alternative there are several benefits in making NUD try probing for a longer time. One of those benefits is to be more robust against transient failures, such as spanning tree recovergence and other layer 2 issues that can take many seconds to resolve. Marking the NCE as unreachable in that case causes additional multicast on the network. Assuming there are IP packets to send, the lack of an NCE will result in multicast Neighbor Solicitations every second instead of the unicast Neighbor Solicitations that NUD sends. As a result IPv6 is operationally more brittle than IPv4. For IPv4 there is no mandatory time limit on the retransmission behavior for ARP [RFC0826] which allows implementors to pick more robust schemes. The following constant values in [RFC4861] seem to have been made part of IPv6 conformance testing: MAX_MULTICAST_SOLICIT, MAX_UNICAST_SOLICIT, RETRANS_TIMER. While such strict conformance testing seems consistent with the the specificiation, it means that we need to update the standard if we want to allow IPv6 Neighbor Discovery to be as operationally robust as ARP. Additional motivations for making IPv6 Neighbor Discovery as robust as ARP are covered in [I-D.gashinsky-v6nd-enhance]. 2. Proposed Remedy We can clarify that the giving up after three packets spaced one second apart is only REQUIRED when there is an alternative, such as an additional default route or a redirect. Nordmark & Gashinsky Expires January 8, 2012 [Page 3] Internet-Draft NUD is too impatient July 2011 If implementations transmit more than MAX_*CAST_SOLICIT packets they MAY use binary exponential backoff of the retransmit timer. This is so that if we end up with implementations that try for a very long time we don't end up with a steady background level of retransmissions. However, even if there is no alternative, we still need to be able to handle the case when the link-layer address of the destination has changed. Thus at some point in time we need to switch to multicast Neighbor Solicitations. A possible way to describe a node behavior which captures all the cases is to introduce a new, optional, UNREACHABLE state in the conceptual model described in [RFC4861]. A NCE in the UNREACHABLE state retains the link-layer address, and IPv6 packets continue to be sent to that link-layer address. But the Neighbor Soliciations are multicast, using a timeout that follows a binary exponential backoff. In the places where RFC4861 says to to discard/delete the NCE after N probes (Section 7.3, 7.3.3 and Appendix C) we will instead transition to the UNREACHABLE state. If the Neighbor Cache Entry was created by a redirect, a node MAY delete the NCE instead of changing its state to UNREACHABLE. In any case, the node SHOULD NOT use an NCE created by a Redirect to send packets if that NCE is in unreachable state. Packets should be sent following the next-hop selection algorithm in section XXX which disregards NCEs that are not reachable. The default router selection in section 6.3.6 says to prefer default routers that are "known to be reachable". For the purposes of that section, if the NCE for the router is in UNREACHABLE state, it is not known to be reachable. Thus the particular text in section 6.3.6 which says "in any state other than INCOMPLETE" needs to be extended to say "in any state other than INCOMPLETE or UNREACHABLE". Apart from the use of multicast NS instead of unicast NS, and the binary exponential backoff of the timer, the UNREACHABLE state works the same as the current PROBE state. A node MAY garbage collect a Neighbor Cache Entry as any time as specified in RFC 4861. This does not change with the introduction of the UNREACHABLE state in the coneptual model. The UNREACHABLE state is conceptual and not a required part of this specification. A node merely needs to satisfy the externally observable behavior of this specificiation. Nordmark & Gashinsky Expires January 8, 2012 [Page 4] Internet-Draft NUD is too impatient July 2011 There is a non-obvious extension to the state machine description in Appendix C in RFC 4861 in the case for "NA, Solicited=1, Override=0. Different link-layer address than cached". There we need to add "UNREACHABLE" to the current list of "STALE, PROBE, Or DELAY". That is, the NCE would be unchanged. Note that there is no corresponding change necessary to the text in section 7.2.5 since it is phrased using "Otherwise" instead of explicitly listing the three states. The other state transitions described in Appendix C handle the introduction of the UNREACHABLE state without any change, since they are described using "not INCOMPLETE". There is also the more obvious change already described above. RFC 4861 has this: PROBE Retransmit timeout, Discard entry - N or more retransmissions. That needs to be replaced by: PROBE Retransmit timeout, Double timeout UNREACHABLE N or more Send multicast NS retransmissions. UNREACHABLE Retransmit timeout Double timeout UNREACHABLE Send multicast NS The binary exponential backoff SHOULD be clamped at some reasonable maximum retransmit timeout, such as 60 seconds. And if there is no IPv6 packets sent using the UNREACHABLE NCE, then it makes sense to stop the retransmits of the multicast NS until either the NCE is garbage collected, or there are IPv6 packets sent using the NCE. In essence the multicast NS and associated binary exponential backoff can be conditioned on the continued use of the NCE to send IPv6 packets to the recorded link-layer address. A node MAY unicast the first few Neighbor Soliciation messages while in UNREACHABLE state, but it MUST switch to multicast Neighbor Soliciations. Otherwise it would not detect a link-layer address change for the target. 3. Acknowledgements The comments from Thomas Narten and Philip Homburg have helped improve this draft. Nordmark & Gashinsky Expires January 8, 2012 [Page 5] Internet-Draft NUD is too impatient July 2011 4. Security Considerations Relaxing the retransmission behavior for NUD has no impact on security. In particular, it doesn't impact applying Secure Neighbor Discovery [RFC3971]. 5. IANA Considerations This are no IANA considerations for this document. 6. References 6.1. Normative References [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005. [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 4443, March 2006. [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. 6.2. Informative References [I-D.gashinsky-v6nd-enhance] Kumari, W., "Operational Neighbor Discovery Problems and Enhancements.", draft-gashinsky-v6nd-enhance-00 (work in progress), June 2011. [RFC0826] Plummer, D., "Ethernet Address Resolution Protocol: Or converting network protocol addresses to 48.bit Ethernet address for transmission on Ethernet hardware", STD 37, RFC 826, November 1982. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Nordmark & Gashinsky Expires January 8, 2012 [Page 6] Internet-Draft NUD is too impatient July 2011 Authors' Addresses Erik Nordmark Cisco Systems, Inc. 510 McCarthy Blvd. Milpitas, CA, 95035 USA Phone: +1 408 527 6625 Email: nordmark@cisco.com Igor Gashinsky Yahoo! 45 W 18th St New York, NY USA Email: igor@yahoo-inc.com Nordmark & Gashinsky Expires January 8, 2012 [Page 7]