6MAN WG                                                      E. Nordmark
Internet-Draft                                       Cisco Systems, Inc.
Updates: 4861 (if approved)                                 I. Gashinsky
Intended status: Standards Track                                  Yahoo!
Expires: January 2, April 4, 2013                                          Yahoo!
                                                                Jul                                          Oct 2012

           Neighbor Unreachability Detection is too impatient
                  draft-ietf-6man-impatient-nud-02.txt
                  draft-ietf-6man-impatient-nud-03.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 by default.  However, if there are no
   alternatives, this is far too impatient.  This document specifies
   relaxed rules for Neighbor Discovery retransmissions that allows an
   implementation to choose different timeout behavior based on whether
   or not there are alternatives.  This document updates RFC 4861.

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
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   This Internet-Draft will expire on January 2, April 4, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Definition Of Terms . . . . . . . . . . . . . . . . . . . . . . 4
   3.  Protocol Updates  . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Example Algorithm . . . . . . . . . . . . . . . . . . . . . . . 6
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

1.  Introduction

   IPv6 Neighbor Discovery [RFC4861] includes Neighbor Unreachability
   Detection (NUD), which detects when a neighbor is no longer
   reachable.The
   reachable.  The timeouts specified are very short (by default three
   transmissions spaced one second apart).  That can be appropriate when
   there are alternative paths over which the packets can be sent.  For
   example, if a host has multiple default routers in its Default Router
   List, or if the host has a Neighbor 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 in [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 (taking into account a default
   ReachableTime of 30 seconds and the time spent in the DELAY state).

   However, when 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
   reconvergence 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 Neighbor Discovery is operationally more brittle
   than IPv4 ARP.  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 [RFC4861], it means that we
   need to update the standard if we want to allow IPv6 Neighbor
   Discovery to be as robust as ARP.

   This document updates RFC 4861 to relax the retransmission rules.

   Additional motivations for making IPv6 Neighbor Discovery more robust
   in the face of degenerate conditions are covered in [RFC6583].

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.  Protocol Updates

   Giving up after three packets spaced one second apart is only
   REQUIRED when there is an alternative, such as an additional default
   route
   router or a redirect.

   If implementations transmit more than MAX_*CAST_SOLICIT packets it
   SHOULD use (binary) exponential backoff of the retransmit timer.
   This is to avoid any significant load due to a steady background
   level of retransmissions from implementations that try for a long
   time.

   Even if there is no alternative, the protocol needs to be able to
   handle the case when the link-layer address of the destination has
   changed by switching to multicast Neighbor Solicitations at some
   point in time.

   In order to capture all the cases above this document introduces a
   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 Solicitations 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 5.2 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) 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 at any time as
   specified in RFC 4861.  This does not change with the introduction of
   the UNREACHABLE state in the conceptual model.

   The UNREACHABLE state is conceptual and not a required part of this
   specification.  A  Just as for [RFC4861] a node merely needs to satisfy
   the externally observable behavior of this 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. seconds (MAX_RETRANS_TIMER
   below).  If there is no IPv6
   packets packet 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.  The multicast NS and associated binary exponential
   backoff can be applied on the condition of the continued use of the
   NCE to send IPv6 packets to the recorded link-layer address.

   A node MAY unicast the first few Neighbor Solicitation messages while
   in UNREACHABLE state, but it MUST switch to multicast Neighbor
   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, then 60, then 60 and switch to UNREACHABLE after 3 transmissions, which
   represents: the three first
   transmissions.  Thus relative to the time of the first transmissions
   the retransmissions would occur at 1 second, 4 seconds, 13 seconds,
   40 seconds, then 100 seconds, and finally 160 seconds.  At 4 seconds
   from the first transmission the NCE would be marked UNREACHABLE.
   That recommended behavior corresponds to:

      MAX_UNICAST_SOLICIT=5

      RETRANS_TIMER=1 (default)

      MAX_RETRANS_TIMER=60

      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 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, Philip Homburg, and Joel Jaeggli Jaeggli, and
   Hemant Singh have helped improve this draft.

6.  Security Considerations

   Relaxing the retransmission behavior for NUD is belived believed to have no
   impact on security.  In particular, it doesn't impact the application
   Secure Neighbor Discovery [RFC3971].

7.  IANA Considerations

   This are no IANA considerations for this document.

8.  References

8.1.  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.

8.2.  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.

   [RFC6583]  Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
              Neighbor Discovery Problems", RFC 6583, March 2012.

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