6MAN WG E. Nordmark Internet-Draft Cisco Systems, Inc.
Expires: May 17, 2012Updates: 4861 (if approved) I. Gashinsky Expires: September 13, 2012 Yahoo! November 14, 2011March 12, 2012 Neighbor Unreachability Detection is too impatient draft-ietf-6man-impatient-nud-00.txtdraft-ietf-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.probing by default. However, if there are no alternatives, this is far too impatient. This document proposes an approach wherespecifies relaxed rules for Neighbor Discovery retransmissions that allows an implementation canto choose thedifferent timeout behavior to be differentbased 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 May 17,September 13, 2012. Copyright Notice Copyright (c) 20112012 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 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. Definition Of Terms . . . . . . . . . . . . . . . . . . . . . . 34 3. Proposed Remedy .Protocol Updates . . . . . . . . . . . . . . . . . . . . . . . 4 4. AcknowledgementsExample Algorithm . . . . . . . . . . . . . . . . . . . . . . . 6 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 6.7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 7.7 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 7.1.7 8.1. Normative References . . . . . . . . . . . . . . . . . . . 6 7.2.7 8.2. Informative References . . . . . . . . . . . . . . . . . . 68 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 68 1. Introduction IPv6 Neighbor Discovery [RFC4861] includes Neighbor Unreachability Detection,Detection (NUD), which detects when a neighbor is no longer reachable. Thereachable.The timeouts specified are very short (three(by default three transmissions spaced one second apart). That can be appropriate when there are alternative paths over which the packetpackets can be sent. For example, if a host has multiple default routers in its Default Router List, or if the host has a NeigborNeighbor 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 wherein [RFC4861] were chosen to be short; this ensures that if a default router fails the host can use the next router in less than 45 seconds.seconds (taking into account a default ReachableTime of 30 seconds and the time spent in the DELAY state). However, wherewhen 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 recovergencereconvergence 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, and RETRANS_TIMER. While such strict conformance testing seems consistent with the the specificiation,[RFC4861], it means that we need to update the standard if we want to allow IPv6 Neighbor Discovery to be as operationally robust as ARP. This document updates RFC 4861 to relax the retransmission rules. Additional motivations for making IPv6 Neighbor Discovery as robust as ARP are covered in [I-D.gashinsky-v6nd-enhance]. 2. Definition Of Terms The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 3. Proposed Remedy We can clarify that the givingProtocol Updates 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. If implementations transmit more than MAX_*CAST_SOLICIT packets they MAYit SHOULD use binary(binary) exponential backoff of the retransmit timer. This is so that if we end up withto avoid any significant load due to a steady background level of retransmissions from implementations that try for a verylong time we don't end up with a steady background level of retransmissions.time. However, even if there is no alternative, we still needthe protocol needs to be able to handle the case when the link-layer address of the destination has changed. Thuschanged by switching to multicast Neighbor Solicitations at some point in time we need to switch to multicast Neighbor Solicitations. A possible waytime. In order to describe a node behavior which capturescapture all the cases is to introduceabove this document introduces a new, optional,new 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 in the UNREACHABLE state the NUD Neighbor SoliciationsSolicitations are multicast, using a timeout that follows a binary(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 XXX5.2 in [RFC4861] 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(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 coneptualconceptual 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.specification. 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 SoliciationSolicitation messages while in UNREACHABLE state, but it MUST switch to multicast Neighbor Soliciations.Solicitations. Otherwise it would not detect a link-layer address change for the target. 4. Example Algorithm This section is NOT normative, but specifies a simple implementation which conforms with this document. The implementation is described using operator configurable values that allows it to be configured in a way to be compatible with the retransmission behavior in [RFC4861]. The operator can configure the values for MAX_*CAST_SOLICIT, RETRANS_TIMER, and the new BACKOFF_MULTIPLE and MARK_UNREACHABLE. This allows the implementation to be as simple as: next_retrans = ($BACKOFF_MULTIPLE^$solicit_attempt_num)*$RetransTimer + jittered value. After MARK_UNREACHABLE retransmissions the implementation would mark the NCE UNREACHABLE and switch to multicast NUD probes. The recommended behavior is to have 5 attempts, with timing spacing of 0 (initial request), 1 second later, 3 seconds later, then 9, and then 27, and switch to UNREACHABLE after 3 transmissions, which represents: MAX_UNICAST_SOLICIT=5 RETRANS_TIMER=1 (default) BACKOFF_MULTIPLE=3 MARK_UNREACHABLE=3 After 3 retransmissions the implementation would mark the NCE UNREACHABLE and switch to multicast NUD probes. Thus we enter UNREACHABLE, and try any available alternative, after 4 seconds compared to the current 2 seconds. That additional delay is small compared to the default 30 seconds ReachableTime. If BACKOFF_MULTIPLE=1, MARK_UNREACHABLE=3 and MAX_UNICAST_SOLICIT=3, you would get the same behavior as in [RFC4861]. An Implementation following this algorithm would, if the request was not answered at first due for example to a transitory condition, retry immediately, and then back off for progressively longer periods. This would allow for a reasonably fast resolution time when the transitory condition clears. Note that RetransTimer and ReachableTime are by default set from the protocol constants RETRANS_TIMER and REACHABLE_TIME, but are overridden by values advertised in Router Advertisements as specified in [RFC4861]. That remains the case even with the protocol updates specified in this document. The key values that the operator would configure are BACKOFF_MULTIPLE, MAX_UNICAST_SOLICIT and MAX_MULTICAST_SOLICIT. It would be useful to have a maximum value for ($BACKOFF_MULTIPLE^$solicit_attempt_num)*$RetransTimer so that the retransmissions are not too far apart. A value 60 seconds is consistent with DHCP. 5. Acknowledgements The comments from Thomas Narten and Philip Homburg have helped improve this draft. 5.6. Security Considerations Relaxing the retransmission behavior for NUD has no impact on security. In particular, it doesn't impact applying Secure Neighbor Discovery [RFC3971]. 6.7. IANA Considerations This are no IANA considerations for this document. 7.8. References 188.8.131.52. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005. [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. 184.108.40.206. 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. Authors' Addresses Erik Nordmark Cisco Systems, Inc. 510 McCarthy Blvd. Milpitas, CA, 95035 USA Phone: +1 408 527 6625 Email: email@example.com Igor Gashinsky Yahoo! 45 W 18th St New York, NY USA Email: firstname.lastname@example.org