6MAN WG                                                      E. Nordmark
Internet-Draft                                           Arista Networks
Updates: 4861 (if approved)                                 I. Gashinsky
Intended status: Standards Track                                  Yahoo!
Expires: April 24, 2014                                 October 21, 2013

           Neighbor Unreachability Detection is too impatient


   IPv6 Neighbor Discovery includes Neighbor Unreachability Detection.
   That function is very useful when a host has an alternative neighbor,
   for instance when there are multiple default routers, since it allows
   the host to switch to the alternative neighbor in short time.  This
   time is 3 seconds after the node starts probing by default.  However,
   if there are no alternative neighbors, this is far too impatient.
   This document specifies relaxed rules for Neighbor Discovery
   retransmissions that allow an implementation to choose different
   timeout behavior based on whether or not there are alternative
   neighbors.  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 April 24, 2014.

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   Copyright (c) 2013 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

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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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.  Definition Of Terms . . . . . . . . . . . . . . . . . . . . . . 4
   3.  Protocol Updates  . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Example Algorithm . . . . . . . . . . . . . . . . . . . . . . . 6
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9

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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 for NUD are very short (by default
   three transmissions spaced one second apart).  These short timeouts
   can be appropriate when there are alternative neighbors to 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.  In those cases,
   when NUD fails, the host will try the alternative neighbor by redoing
   next-hop selection.  That implies picking the next router in the
   Default Router List or discarding the redirect, respectively.

   The timeouts specified in [RFC4861] were chosen to be short in order
   to optimize for the scenarios where alternative neighbors are

   However, when there is no alternative neighbor there are several
   benefits in making NUD try probing for a longer time.  One of those
   benefits is to make NUD 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 being sent (to the solicited-node multicast address)
   every second instead of the unicast Neighbor Solicitations that NUD

   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
   the standard needs to be updated 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

   Discarding the NCE after after three packets spaced one second apart
   is only needed when an alternative neighbor is available, such as an
   additional default router or a redirect.

   If an implementation transmits more than MAX_UNICAST_SOLICIT/
   MAX_MULTICAST_SOLICIT packets then it SHOULD use exponential backoff
   of the retransmit timer.  This is to avoid any significant load due
   to a steady background level of retransmissions from implementations
   that retransmit a large number of NSes before discarding the NCE.

   Even if there is no alternative neighbor, the protocol needs to be
   able to handle the case when the link-layer address of the neighbor/
   target 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
   (to the solicited-node multicast address), using a timeout that
   follows an 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) this document instead
   specifies a 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 [RFC4861], Section 5.2,
   which disregards NCEs that are not reachable.

   The default router selection in [RFC4861], 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

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   to be extended to say "in any state other than INCOMPLETE or

   Apart from the use of multicast NS instead of unicast NS, and the
   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 freedom to garbage collect does not
   change with the introduction of the UNREACHABLE state in the
   conceptual model.  An implementation MAY prefer garbage collecting
   UNREACHABLE NCEs over other NCEs.

   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 [RFC4861], Section 7.2.5, since it is
   phrased using "Otherwise" instead of explicitly listing the three

   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:

   State           Event                   Action             New state

   PROBE           Retransmit timeout,     Discard entry         -
                   N or more

   That needs to be replaced by:

   State           Event                   Action             New state

   PROBE           Retransmit timeout,     Increase timeout  UNREACHABLE
                   N retransmissions.      Send multicast NS

   UNREACHABLE     Retransmit timeout      Increase timeout  UNREACHABLE
                                           Send multicast NS

   The 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 is

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   RECOMMENDED 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 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 can unicast the first few Neighbor Solicitation messages even
   while in UNREACHABLE state, but it MUST switch to multicast Neighbor
   Solicitations within 60 seconds of the initial retransmission to be
   able to handle a link-layer address change for the target.  The
   example below shows such behavior.

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_UNICAST_SOLICIT,
   implementation to be as simple as:

   next_retrans = ($BACKOFF_MULTIPLE ^ $solicit_retrans_num) *
   $RetransTimer * $JitterFactor where solicit_retrans_num is zero for
   the first transmission, and JitterFactor is a random value between
   synchronization of transmissions from different hosts.

   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 behavior of this example algorithm 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 behavior corresponds to:


      RETRANS_TIMER=1 (default)

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   After 3 retransmissions the implementation would mark the NCE
   UNREACHABLE.  That results in trying an alternative neighbor, such as
   another default router or ignoring a redirect as specified in
   [RFC4861].  With the above values that would occur after 4 seconds
   after the first transmission compared to the 2 seconds using the
   fixed scheme in [RFC4861].  That additional delay is small compared
   to the default 30,000 milliseconds ReachableTime.

   After 5 transmissions, i.e., 40 seconds after the initial
   transmission, the example behavior is to switch to multicast NUD
   probes.  In the language of the state machine in [RFC4861] that
   corresponds to the action "Discard entry".  Thus any attempts to send
   future packets would result in sending multicast NS packets.  An
   implementation MAY retain the backoff value as it switches to
   multicast NUD probes.  The potential downside of deferring switching
   to multicast is that it would take longer for NUD to handle a change
   in a link-layer address i.e., the case when a host or a router
   changes their link-layer address while keeping the same IPv6 address.
   However, [RFC4861] says that a node MAY send unsolicited NS to handle
   that case, which is rather infrequent in operational networks.  In
   any case, the implementation needs to follow the "SHOULD" in section
   Section 3 to switch to multicast solutions within 60 seconds after
   the initial transmission.

   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

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   ($BACKOFF_MULTIPLE^$solicit_attempt_num)*$RetransTimer so that the
   retransmissions are not too far apart.  The above 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, Tina Tsou, Suresh Krishnan, and Murray Kucherawy 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].

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.

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Authors' Addresses

   Erik Nordmark
   Arista Networks
   Santa Clara, CA

   Email: nordmark@acm.org

   Igor Gashinsky
   45 W 18th St
   New York, NY

   Email: igor@yahoo-inc.com

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