Optimistic Duplicate Address Detection (DAD) for IPv6
RFC 4429

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Network Working Group                                           N. Moore
Request for Comments: 4429                        Monash University CTIE
Category: Standards Track                                     April 2006

         Optimistic Duplicate Address Detection (DAD) for IPv6

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   Optimistic Duplicate Address Detection is an interoperable
   modification of the existing IPv6 Neighbor Discovery (RFC 2461) and
   Stateless Address Autoconfiguration (RFC 2462) processes.  The
   intention is to minimize address configuration delays in the
   successful case, to reduce disruption as far as possible in the
   failure case, and to remain interoperable with unmodified hosts and
   routers.

Moore                       Standards Track                     [Page 1]
RFC 4429                     Optimistic DAD                   April 2006

Table of Contents

   1. Introduction ....................................................3
      1.1. Problem Statement ..........................................3
      1.2. Definitions ................................................4
      1.3. Address Types ..............................................4
      1.4. Abbreviations ..............................................5
   2. Optimistic DAD Behaviors ........................................6
      2.1. Optimistic Addresses .......................................6
      2.2. Avoiding Disruption ........................................6
      2.3. Router Redirection .........................................7
      2.4. Contacting the Router ......................................7
   3. Modifications to RFC-Mandated Behavior ..........................8
      3.1. General ....................................................8
      3.2. Modifications to RFC 2461 Neighbor Discovery ...............8
      3.3. Modifications to RFC 2462 Stateless Address
           Autoconfiguration ..........................................9
   4. Protocol Operation .............................................10
      4.1. Simple Case ...............................................10
      4.2. Collision Case ............................................10
      4.3. Interoperation Cases ......................................11
      4.4. Pathological Cases ........................................11
   5. Security Considerations ........................................12
   Appendix A. Probability of Collision ..............................13
      A.1. The Birthday Paradox ......................................13
      A.2. Individual Moving Nodes ...................................14
   Normative References ..............................................15
   Informative References ............................................15
   Acknowledgements ..................................................16

Moore                       Standards Track                     [Page 2]
RFC 4429                     Optimistic DAD                   April 2006

1.  Introduction

   Optimistic Duplicate Address Detection (DAD) is a modification of the
   existing IPv6 Neighbor Discovery (ND) [RFC2461] and Stateless Address
   Autoconfiguration (SLAAC) [RFC2462] processes.  The intention is to
   minimize address configuration delays in the successful case, and to
   reduce disruption as far as possible in the failure case.

   Optimistic DAD is a useful optimization because in most cases DAD is
   far more likely to succeed than fail.  This is discussed further in
   Appendix A.  Disruption is minimized by limiting nodes' participation
   in Neighbor Discovery while their addresses are still Optimistic.

   It is not the intention of this memo to improve the security,
   reliability, or robustness of DAD beyond that of existing standards,
   but merely to provide a method to make it faster.

1.1.  Problem Statement

   The existing IPv6 address configuration mechanisms provide adequate
   collision detection mechanisms for the fixed hosts they were designed
   for.  However, a growing population of nodes need to maintain
   continuous network access despite frequently changing their network
   attachment.  Optimizations to the DAD process are required to provide
   these nodes with sufficiently fast address configuration.

   An optimized DAD method needs to:

   * provide interoperability with nodes using the current standards.

   * remove the RetransTimer delay during address configuration.

   * ensure that the probability of address collision is not increased.