6MAN WG                                                      E. Nordmark
Internet-Draft                                       Cisco Systems, Inc.
Expires: November 24, 2011                                  May 23, 2011


           Neighbor Unreachability Detection is too impatient
                draft-nordmark-6man-impatient-nud-00.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

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   This Internet-Draft will expire on November 24, 2011.

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   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of



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   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.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     5.1.  Normative References  . . . . . . . . . . . . . . . . . . . 4
     5.2.  Informative References  . . . . . . . . . . . . . . . . . . 4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 4





































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.  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.


.  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.

   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



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   time we don't end up with a steady background level of
   retransmissions.


.  Security Considerations

   Relaxing the retransmission behavior for NUD has no impact on
   security.  In particular, it doesn't impact applying Secure Neighbor
   Discovery [RFC3971].


.  IANA Considerations

   This are no IANA considerations for this document.


.  References

5.1.  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.

5.2.  Informative References

   [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.











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Author's Address

   Erik Nordmark
   Cisco Systems, Inc.
   510 McCarthy Blvd.
   Milpitas, CA, 95035
   USA

   Phone: +1 408 527 6625
   Email: nordmark@cisco.com









































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