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IPng Working Group                                            R. Draves
Internet Draft                                       Microsoft Research
Document: draft-draves-ipngwg-simple-srcaddr-00.txt      April 28, 1999
Category: Standards Track


                Simple Source Address Selection for IPv6


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026 [1].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.




1. Abstract

   This document describes a simple algorithm by which IPv6
   implementations can choose an appropriate source address to use for
   communication with a specified destination address.


2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC-2119 [2].


3. Introduction

   The IPv6 addressing architecture [3] allows multiple addresses to be
   assigned to interfaces. These addresses may have different
   reachability scopes (link-local, site-local, or global).
   Furthermore, addresses assigned via IPv6's auto-configuration
   mechanisms [4] may be "preferred" or "deprecated".

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   On occasion, an IPv6 implementation must choose from a set of
   available addresses an appropriate source address to use for a given
   destination address. This document specifies a simple set of rules
   for choosing a source address of appropriate scope and configuration
   status (preferred or deprecated). Furthermore, this document
   suggests a preferred method, longest matching prefix, for choosing
   among otherwise equivalent source addresses in the absence of better
   information.

   This document does not address the more general problem of choosing
   the "best" destination address / source address pair for
   communication with another node, given a set of possible destination
   addresses and a set of possible source addresses.

   This document does not specify a "strong host" or "weak host" model
   for source address selection [5, section 3.3.4.2]. It merely assumes
   that the implementation has a set of candidate source addresses from
   which one must be chosen. If the implementation uses the strong host
   model, this MAY be the set of addresses assigned to the outgoing
   interface that will be used for the destination address. If the
   implementation uses the weak host model, this MAY be the set of all
   addresses assigned to the node's interfaces.

   The rules specified in this document MUST NOT be construed to
   override an application's explicit choice of source address.

4. Source Address Selection

   This document specifies a pair-wise source address selection
   algorithm. Given a destination address and a pair of possible source
   addresses SA and SB (SA not equal to SB), it chooses a source
   address. Obviously, any pair-wise algorithm may be extended to
   select an address from a set of candidate source addresses.

   When comparing scopes, we say that link-local scope is smaller than
   site-local scope, which is smaller than global scope.

   Addresses that are manually configured (or otherwise not auto-
   configured according to [4]), we treat as having "preferred"
   configuration status.

   The algorithm consists of four rules, which MUST be applied in
   order. If a rule chooses a source address, then the remaining rules
   are not relevant and MUST be ignored. Subsequent rules act as tie-
   breakers for earlier rules. If the four rules fail to choose a
   source address, some unspecified tie-breaker MUST be used.

   Rule 1: If one of the source addresses is equal to the destination
   address, an implementation MUST choose that source address.

   Rule 2: If the source addresses SA and SB have different scope, then
   an implementation MUST choose the source address as follows. Without
   loss of generality, assume that SA has smaller scope than SB. If the

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   destination address scope is smaller than or equal to SA's scope,
   then choose SA. Otherwise choose SB.

   Rule 3: The two source addresses have the same scope. If one of the
   source addresses is "preferred" and one of them is "deprecated", an
   implementation MUST choose the one that is preferred.

   Rule 4: The two source addresses have the same scope and the same
   configuration status (both preferred or both deprecated). If one of
   the source addresses has a longer prefix matching the destination
   address, an implementation SHOULD choose the source address with the
   longer matching prefix.

   The fourth rule MAY be superceded if the implementation has other
   means of choosing among source addresses. For example, if the
   implementation somehow knows which source address will result in the
   "best" communications performance.

5. Multicast Destination Addresses

   Multicast destination addresses have a 4-bit scope field that
   controls the propagation of the multicast packet. The IPv6
   addressing architecture defines scope field values for node-local
   (0x1), link-local (0x2), site-local (0x5), organization-local (0x8),
   and global (0xE) scopes.

   Application of the source address selection algorithm to a multicast
   destination address requires the comparison of a unicast source
   address scope with a multicast destination address scope. We map
   unicast link-local to multicast link-local, unicast site-local to
   multicast site-local, and unicast global scope to multicast global
   scope. This mapping implicitly conflates unicast site boundaries and
   multicast site boundaries.


6. IPv4-Compatible Addresses

   For the purposes of this document, IPv4-compatible addresses have
   global scope and "preferred" configuration status.


7. Other Format Prefixes

   This document does not specify source address selection in the
   presence of NSAP addresses, IPX addresses, or addresses with as-yet-
   undefined format prefixes.


5. Security Considerations

   This document has no direct impact on Internet infrastructure
   security.


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6. References


   1  S. Bradner, "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.

   2  S. Bradner, "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.

   3  R. Hinden, S. Deering, "IP Version 6 Addressing Architecture",
      RFC 2373, July 1998.

   4  S. Thompson, T. Narten, "IPv6 Stateless Address
      Autoconfiguration", RFC 2462 , December 1998.

   5  R. Braden, editor, "Requirements for Internet Hosts -
      Communication Layers", RFC 1122 , October 1989.


7. Acknowledgments

   The author would like to acknowledge the contributions of the IPng
   Working Group, and in particular, Robert Elz and Jack McCann.


8. Author's Address

   Richard Draves
   Microsoft Research
   One Microsoft Way
   Redmond, WA 98052
   Email: richdr@microsoft.com





















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