Neighbor Unreachability Detection Is Too Impatient
RFC 7048

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Internet Engineering Task Force (IETF)                       E. Nordmark
Request for Comments: 7048                               Arista Networks
Updates: 4861                                               I. Gashinsky
Category: Standards Track                                         Yahoo!
ISSN: 2070-1721                                             January 2014

           Neighbor Unreachability Detection Is Too Impatient

Abstract

   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 a short
   time.  By default, this time is 3 seconds after the node starts
   probing.  However, if there are no alternative neighbors, this
   timeout behavior 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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7048.

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RFC 7048                  NUD Is Too Impatient              January 2014

Copyright Notice

   Copyright (c) 2014 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
   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 ....................................................2
   2. Definition of Terms .............................................3
   3. Protocol Updates ................................................3
   4. Example Algorithm ...............................................6
   5. Acknowledgements ................................................7
   6. Security Considerations .........................................8
   7. References ......................................................8
      7.1. Normative References .......................................8
      7.2. Informative References .....................................8

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 the next-hop selection.  That implies picking the next router
   in the Default Router List or discarding the NCE created by a
   Redirect message, respectively.

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

   However, when there is no alternative neighbor, there are several
   benefits to making NUD probe for a longer time.  One benefit is to
   make NUD more robust against transient failures, such as spanning

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RFC 7048                  NUD Is Too Impatient              January 2014

   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