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Problem Statement for Default Address Selection in Multi-Prefix Environments: Operational Issues of RFC 3484 Default Rules
RFC 5220

Network Working Group                                       A. Matsumoto
Request for Comments: 5220                                   T. Fujisaki
Category: Informational                                              NTT
                                                               R. Hiromi
                                                           Intec Netcore
                                                             K. Kanayama
                                                           INTEC Systems
                                                               July 2008

    Problem Statement for Default Address Selection in Multi-Prefix
       Environments: Operational Issues of RFC 3484 Default Rules

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   A single physical link can have multiple prefixes assigned to it.  In
   that environment, end hosts might have multiple IP addresses and be
   required to use them selectively.  RFC 3484 defines default source
   and destination address selection rules and is implemented in a
   variety of OSs.  But, it has been too difficult to use operationally
   for several reasons.  In some environments where multiple prefixes
   are assigned on a single physical link, the host using the default
   address selection rules will experience some trouble in
   communication.  This document describes the possible problems that
   end hosts could encounter in an environment with multiple prefixes.

Matsumoto, et al.            Informational                      [Page 1]
RFC 5220                  Address Selection PS                 July 2008

Table of Contents

   1. Introduction ....................................................2
      1.1. Scope of This Document .....................................3
   2. Problem Statement ...............................................4
      2.1. Source Address Selection ...................................4
           2.1.1. Multiple Routers on a Single Interface ..............4
           2.1.2. Ingress Filtering Problem ...........................5
           2.1.3. Half-Closed Network Problem .........................6
           2.1.4. Combined Use of Global and ULA ......................7
           2.1.5. Site Renumbering ....................................8
           2.1.6. Multicast Source Address Selection ..................9
           2.1.7. Temporary Address Selection .........................9
      2.2. Destination Address Selection .............................10
           2.2.1. IPv4 or IPv6 Prioritization ........................10
           2.2.2. ULA and IPv4 Dual-Stack Environment ................11
           2.2.3. ULA or Global Prioritization .......................12
   3. Conclusion .....................................................13
   4. Security Considerations ........................................14
   5. Normative References ...........................................14

1.  Introduction

   In IPv6, a single physical link can have multiple prefixes assigned
   to it.  In such cases, an end host may have multiple IP addresses
   assigned to an interface on that link.  In the IPv4-IPv6 dual-stack
   environment or in a site connected to both a Unique Local Address
   (ULA) [RFC4193] and globally routable networks, an end host typically
   has multiple IP addresses.  These are examples of the networks that
   we focus on in this document.  In such an environment, an end host
   may encounter some communication troubles.

   Inappropriate source address selection at the end host causes
   unexpected asymmetric routing, filtering by a router, or discarding
   of packets because there is no route to the host.

   Considering a multi-prefix environment, destination address selection
   is also important for correct or better communication establishment.

   RFC 3484 [RFC3484] defines default source and destination address
   selection algorithms and is implemented in a variety of OSs.  But, it
   has been too difficult to use operationally for several reasons, such
   as lack of an autoconfiguration method.  There are some problematic
   cases where the hosts using the default address selection rules
   encounter communication troubles.

   This document describes the possibilities of incorrect address
   selection that lead to dropping packets and communication failure.

Matsumoto, et al.            Informational                      [Page 2]
RFC 5220                  Address Selection PS                 July 2008

1.1.  Scope of This Document

   As other mechanisms already exist, the multi-homing techniques for
   achieving redundancy are basically out of our scope.

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