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

           Neighbor Unreachability Detection is too impatient


   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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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on April 4, 2013.

Copyright Notice

   Copyright (c) 2012 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
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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents

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   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 . . . . . . . . . . . . . . . . . . . . . . 4
   3.  Protocol Updates  . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Example Algorithm . . . . . . . . . . . . . . . . . . . . . . . 6
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   IPv6 Neighbor Discovery [RFC4861] includes Neighbor Unreachability
   Detection (NUD), which detects when a neighbor is no longer
   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].

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2.  Definition Of Terms

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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
   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

   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".

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   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 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.  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

   That needs to be replaced by:

   PROBE           Retransmit timeout,     Increase timeout  UNREACHABLE
                   N or more               Send multicast NS

   UNREACHABLE     Retransmit timeout      Increase timeout  UNREACHABLE
                                           Send multicast NS

   The (binary) exponential backoff SHOULD be clamped at some reasonable
   maximum retransmit timeout, such as 60 seconds (see MAX_RETRANS_TIMER
   below).  If there is no IPv6 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.

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   A node MAY unicast the first few Neighbor Solicitation messages even
   while in UNREACHABLE state, but it MUST switch to multicast Neighbor
   Solicitations sooner or later.  Otherwise it would not detect a link-
   layer address change for the target.  The example below shows such

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,
   MARK_UNREACHABLE.  This allows the implementation to be as simple as:

   next_retrans = ($BACKOFF_MULTIPLE^$solicit_attempt_num)*$RetransTimer
   + jittered value.

   After MARK_UNREACHABLE transmissions the implementation would mark
   the NCE UNREACHABLE and as result explore alternate next hops.  After
   MAX_UNICAST_SOLICIT the implementation would switch to multicast NUD

   The recommended behavior is to have 5 attempts, with timing spacing
   of 0 (initial request), 1 second later, 3 seconds after the first
   retransmission, then 9, then 27, and switch to UNREACHABLE after the
   first three transmissions.  Thus relative to the time of the first
   transmissions the retransmissions would occur at 1 second, 4 seconds,
   13 seconds, and finally 40 seconds.  At 4 seconds from the first
   transmission the NCE would be marked UNREACHABLE.  That recommended
   behavior corresponds to:


      RETRANS_TIMER=1 (default)




   After 3 retransmissions the implementation would mark the NCE
   UNREACHABLE.  That results in trying an alternative, such as another
   default router or ignoring a redirect as specified in [RFC4861].
   With the above recommended values that would occur after 4 seconds

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   after the first transmission compared to the 2 seconds using the
   fixed scheme in [RFC4861].  That additional delay is small compared
   to the default 30 seconds ReachableTime.

   After 5 transmissions, i.e., 40 seconds after the initial
   transmission, the recommended behavior is to switch to multicast NUD
   probes.  In the language of the state machine in [RFC4861] says that
   a node MAY send unsolicited NS to handle that case, which is rather
   infrequent in operational networks.

   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

   It is be useful to have a maximum value for
   ($BACKOFF_MULTIPLE^$solicit_attempt_num)*$RetransTimer so that the
   retransmissions are not too far apart.  The recommended value of 60
   seconds for this MAX_RETRANS_TIMER is consistent with DHCPv6.

5.  Acknowledgements

   The comments from Thomas Narten, Philip Homburg, Joel Jaeggli, Hemant
   Singh, and Tina Tsou have helped improve this draft.

6.  Security Considerations

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

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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

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

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   Igor Gashinsky
   45 W 18th St
   New York, NY

   Email: igor@yahoo-inc.com

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