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Dynamic Configuration of IPv4 Link-Local Addresses
RFC 3927

Network Working Group                                        S. Cheshire
Request for Comments: 3927                                Apple Computer
Category: Standards Track                                       B. Aboba
                                                   Microsoft Corporation
                                                              E. Guttman
                                                        Sun Microsystems
                                                                May 2005

           Dynamic Configuration of IPv4 Link-Local Addresses

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 (2005).

Abstract

   To participate in wide-area IP networking, a host needs to be
   configured with IP addresses for its interfaces, either manually by
   the user or automatically from a source on the network such as a
   Dynamic Host Configuration Protocol (DHCP) server.  Unfortunately,
   such address configuration information may not always be available.
   It is therefore beneficial for a host to be able to depend on a
   useful subset of IP networking functions even when no address
   configuration is available.  This document describes how a host may
   automatically configure an interface with an IPv4 address within the
   169.254/16 prefix that is valid for communication with other devices
   connected to the same physical (or logical) link.

   IPv4 Link-Local addresses are not suitable for communication with
   devices not directly connected to the same physical (or logical)
   link, and are only used where stable, routable addresses are not
   available (such as on ad hoc or isolated networks).  This document
   does not recommend that IPv4 Link-Local addresses and routable
   addresses be configured simultaneously on the same interface.

Cheshire, et al.            Standards Track                     [Page 1]
RFC 3927                    IPv4 Link-Local                     May 2005

Table of Contents

   1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .  3
       1.1.  Requirements. . . . . . . . . . . . . . . . . . . . . .  3
       1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . .  3
       1.3.  Applicability . . . . . . . . . . . . . . . . . . . . .  5
       1.4.  Application Layer Protocol Considerations . . . . . . .  6
       1.5.  Autoconfiguration Issues. . . . . . . . . . . . . . . .  7
       1.6.  Alternate Use Prohibition . . . . . . . . . . . . . . .  7
       1.7.  Multiple Interfaces . . . . . . . . . . . . . . . . . .  8
       1.8.  Communication with Routable Addresses . . . . . . . . .  8
       1.9.  When to configure an IPv4 Link-Local Address. . . . . .  8
   2.  Address Selection, Defense and Delivery . . . . . . . . . . .  9
       2.1.  Link-Local Address Selection. . . . . . . . . . . . . . 10
       2.2.  Claiming a Link-Local Address . . . . . . . . . . . . . 11
       2.3.  Shorter Timeouts. . . . . . . . . . . . . . . . . . . . 13
       2.4.  Announcing an Address . . . . . . . . . . . . . . . . . 13
       2.5.  Conflict Detection and Defense. . . . . . . . . . . . . 13
       2.6.  Address Usage and Forwarding Rules. . . . . . . . . . . 14
       2.7.  Link-Local Packets Are Not Forwarded. . . . . . . . . . 16
       2.8.  Link-Local Packets are Local. . . . . . . . . . . . . . 16
       2.9.  Higher-Layer Protocol Considerations. . . . . . . . . . 17
       2.10. Privacy Concerns. . . . . . . . . . . . . . . . . . . . 17
       2.11. Interaction between DHCPv4 and IPv4 Link-Local
             State Machines. . . . . . . . . . . . . . . . . . . . . 17
   3.  Considerations for Multiple Interfaces. . . . . . . . . . . . 18
       3.1.  Scoped Addresses. . . . . . . . . . . . . . . . . . . . 18
       3.2.  Address Ambiguity . . . . . . . . . . . . . . . . . . . 19
       3.3.  Interaction with Hosts with Routable Addresses. . . . . 20
       3.4.  Unintentional Autoimmune Response . . . . . . . . . . . 21
   4.  Healing of Network Partitions . . . . . . . . . . . . . . . . 22
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . 23
   6.  Application Programming Considerations. . . . . . . . . . . . 24
       6.1.  Address Changes, Failure and Recovery . . . . . . . . . 24
       6.2.  Limited Forwarding of Locators. . . . . . . . . . . . . 24
       6.3.  Address Ambiguity . . . . . . . . . . . . . . . . . . . 25
   7.  Router Considerations . . . . . . . . . . . . . . . . . . . . 25

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