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.
* improve the resolution mechanisms for address collisions.
* minimize disruption in the case of a collision.
It is not sufficient to merely reduce RetransTimer in order to reduce
the handover delay, as values of RetransTimer long enough to
guarantee detection of a collision are too long to avoid disruption
of time-critical services.
Moore Standards Track [Page 3]
RFC 4429 Optimistic DAD April 2006
1.2. Definitions
Definitions of requirements keywords ('MUST NOT', 'SHOULD NOT',
'MAY', 'SHOULD', 'MUST') are in accordance with the IETF Best Current
Practice, RFC 2119 [RFC2119]
Address Resolution - Process defined by [RFC2461], section 7.2.
Neighbor Unreachability Detection (NUD) - Process defined by
[RFC2461], section 7.3.
Standard Node - A Standard Node is one that is compliant with
[RFC2461] and [RFC2462].
Optimistic Node (ON) - An Optimistic Node is one that is compliant
with the rules specified in this memo.
Link - A communication facility or medium over which nodes can
communicate at the link layer.
Neighbors - Nodes on the same link, which may therefore be competing
for the same IP addresses.
1.3. Address Types
Tentative address (as per [