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

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Last updated 2011-11-14
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6MAN WG                                                      E. Nordmark
Internet-Draft                                       Cisco Systems, Inc.
Expires: May 17, 2012                                       I. Gashinsky
                                                                  Yahoo!
                                                       November 14, 2011

           Neighbor Unreachability Detection is too impatient
                  draft-ietf-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

   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 May 17, 2012.

Copyright Notice

   Copyright (c) 2011 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

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   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 . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Proposed Remedy . . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6

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1.  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
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