[Docs] [txt|pdf] [Tracker] [WG] [Email] [Nits]

Versions: 00 01 02 03 04 05 06

IPng Working Group                                               R. Draves
Internet Draft                                          Microsoft Research
Document: draft-ietf-ipngwg-default-addr-select-00.txt    October 22, 1999
Category: Standards Track

                   Default 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.

Abstract

   This document describes two algorithms, for destination address
   ordering and for source address selection. The algorithms specify
   default behavior for all IPv6 implementations. They do not override
   choices made by applications or upper-layer protocols, nor do they
   preclude the development of more advanced mechanisms for address
   selection. The two algorithms share a common framework, including an
   optional mechanism for allowing administrators to provide policy
   that can override the default behavior.

1. Introduction

   The IPv6 addressing architecture [2] allows multiple unicast
   addresses to be assigned to interfaces. These addresses may have
   different reachability scopes (link-local, site-local, or global).
   These addresses may be "preferred" or "deprecated" [3]. In addition,
   multi-homing situations will result in more addresses per node. For
   example, a node may have multiple interfaces, some of them tunnels
   or virtual interfaces, or a site may have multiple ISP attachments.

   The end result is that IPv6 implementations will very often be faced
   with multiple possible source and destination addresses when
   initiating communication. It is desirable to have simple default
   algorithms, common across all implementations, for selecting source

Draves            Standards Track - Expires May 2000                 1
Default Address Selection for IPv6                    October 22, 1999


   and destination addresses so that developers and administrators can
   reason about and predict the behavior of their systems.

   This document specifies source address selection and destination
   address selection separately, but using a common framework so that
   together the two algorithms yield useful results. The algorithms
   attempt to choose source and destination addresses of appropriate
   scope and configuration status (preferred or deprecated).
   Furthermore, this document suggests a preferred method, longest
   matching prefix, for choosing among otherwise equivalent addresses
   in the absence of better information.

   The framework also has policy hooks to allow administrative override
   of the default behavior. For example, using these hooks an
   administrator can specify a preferred source prefix for use with a
   destination prefix, or prefer destination addresses with one prefix
   over addresses with another prefix. These hooks give an
   administrator flexibility in dealing with some multi-homing and
   transition scenarios, but they are certainly not a panacea.

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

1.1. 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 [4].

2. Framework

   Our framework for address selection derives from the most common
   implementation architecture, which separates the choice of
   destination address from the choice of source address. Consequently,
   the framework specifies two separate algorithms for these tasks. The
   algorithms are designed to work well together and they share a
   mechanism for administrative policy override.

   In this implementation architecture, applications use APIs [5] like
   getipnodebyname() and getaddrinfo() that return a list of addresses
   to the application. The application then passes a destination
   address to the IPv6 layer with connect() or sendto(). The
   application might just use the first address in the list, or it
   might loop over the list of addresses to find a working address. In
   any case, the IPv6 network layer is never in a position where it
   needs to choose a destination address from several alternatives. The
   application might also specify a source address with bind(), but
   often the source address is left unspecified. Therefore the IPv6
   layer does often choose a source address from several alternatives.

   As a consequence, we intend that implementations of
   getipnodebyname() and getaddrinfo() will use the destination address

Draves            Standards Track - Expires May 2000                 2
Default Address Selection for IPv6                    October 22, 1999


   ordering algorithm specified here to sort the list of addresses that
   they return. Separately, the IPv6 network layer will use the source
   address selection algorithm when an application or upper-layer has
   not specified a source address.

   The algorithms use several criteria in making their decisions. The
   combined effect is to prefer destination/source address pairs for
   which the two addresses are of equal scope or type, prefer smaller
   scopes over larger scopes for the destination address, prefer non-
   deprecated source addresses of sufficient scope to reach the
   destination, avoid the use of transitional addresses when native
   addresses are available, and all else being equal prefer address
   pairs having the longest possible common prefix.

   The framework optionally allows for the possibility of
   administrative configuration of policy that can override the default
   behavior of the algorithms. The policy override takes the form of a
   configurable table that provides precedence values and preferred
   source prefixes for destination prefixes. If an implementation is
   not configurable, or if an implementation has not been configured,
   then the default policy table specified in this document MUST be
   used.

2.1. Scope Comparisons

   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 address selection algorithms in the presence of
   multicast destination addresses requires the comparison of a unicast
   address scope with a multicast 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. For
   example, unicast site-local is equal to multicast site-local, which
   is smaller than multicast organization-local, which is smaller than
   unicast global, which is equal to multicast global.

   We write Scope(A) to mean the scope of address A. For example, if A
   is a link-local unicast address and B is a site-local multicast
   address, then Scope(A) < Scope(B).

   This mapping implicitly conflates unicast site boundaries and
   multicast site boundaries.

2.2. IPv4-Compatible Addresses and Other Format Prefixes

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



Draves            Standards Track - Expires May 2000                 3
Default Address Selection for IPv6                    October 22, 1999


   Similarly, NSAP addresses, IPX addresses, or addresses with as-yet-
   undefined format prefixes should be treated as having global scope
   and "preferred" configuration status. Later standards may supercede
   this treatment.

   The loopback address should be treated as having link-local scope
   and "preferred" configuration status.

2.3. Policy Table

   The policy table is a longest-matching-prefix lookup table, like a
   routing table. Given an address A, a lookup in the policy table
   produces three values: a precedence value Precedence(A), a
   classification or label Label(A), and a second label
   MatchSrcLabel(A).

   The precedence value Precedence(A) is used for sorting destination
   addresses. If Precedence(A) > Precedence(B), we say that address A
   has higher precedence than address B, meaning that our algorithm
   will prefer to sort destination address A before destination address
   B.

   The labels Label(A) and MatchSrcLabel(A) allow for policies that
   prefer a particular source address prefix for use with a destination
   address prefix. The algorithms prefer to use a source address S with
   a destination address D if Label(S) = MatchSrcLabel(D).

   IPv6 implementations SHOULD support configurable address selection
   via a mechanism at least as powerful as the policy tables defined
   here. If an implementation is not configurable or has not been
   configured, then it MUST operate according to the algorithms
   specified here in conjunction with the following default policy
   table:

                Prefix    Precedence Label MatchSrcLabel
                fe80::/10     40       1         1
                fec0::/10     30       2         2
                ::/0          20       3         3
                2002::/16     10       4         4
                ::/96         10       5         5

   One effect of the default policy table is to prefer using native
   source addresses with native destination addresses, 6to4 source
   addresses with 6to4 destination addresses, and v4-compatible source
   addresses with v4-compatible destination addresses. Another effect
   of the default policy table is to prefer communication using native
   addresses to communication using either 6to4 or v4-compatible
   addresses, but not to express a preference for 6to4 addresses over
   v4-compatible addresses or vice-versa.





Draves            Standards Track - Expires May 2000                 4
Default Address Selection for IPv6                    October 22, 1999


2.4. Candidate Source Addresses

   Both the destination address ordering algorithm and the source
   address selection algorithm use the concept of a "candidate set" of
   potential source addresses for a given destination address.

   We write CandidateSrc(A) to denote the candidate set for the address
   A. In some cases the destination address A may be qualified with a
   scope-id or other information that will constrain the candidate set.
   We write PreferSrc(A) to denote the subset of preferred (non-
   deprecated) addresses in CandidateSrc(A)  We write MatchSrc(A) to
   denote the subset of addresses S in PreferSrc(A) for which Label(S)
   = MatchSrcLabel(A).

   The destination address ordering algorithm and the source address
   selection algorithm specify somewhat different definitions for
   CandidateSrc(A). This is because the two algorithms operate in
   different environments. The source address selection algorithm
   assumes that an outgoing interface for a packet has already been
   selected, while the destination address ordering algorithm does not
   assume that knowledge. Therefore the destination address ordering
   algorithm uses a broader or more-inclusive definition of
   CandidateSrc(A).

   In any case, anycast addresses, multicast addresses, and the
   unspecified address MUST NOT be included in a candidate set.

2.5. Common Prefix Length

   We define the common prefix length CommonPrefixLen(A, B) of two
   addresses A and B as the length of the longest prefix that the two
   addresses have in common. It ranges from 0 to 128.

   We define the maximum common prefix length MaxCommonPrefixLen(A, X)
   of an address A and a non-empty set of addresses X as the maximum of
   CommonPrefixLen(A, B) for addresses B in the set X.

3. Destination Address Ordering

   The destination address ordering algorithm takes a list of
   destination addresses and sorts the addresses to produce a new list.
   It is specified here in terms of the pair-wise comparison of
   addresses DA and DB, where DA appears before DB in the original
   list.

   The pair-wise comparison consists of four rules, which MUST be
   applied in order. If a rule determines a result, then the remaining
   rules are not relevant and MUST be ignored. Subsequent rules act as
   tie-breakers for earlier rules.

   Rule 1: If MatchSrc(DA) is non-empty and MatchSrc(DB) is empty, then
   sort DA before DB. Similarly, if MatchSrc(DA) is empty and
   MatchSrc(DB) is non-empty, then sort DB before DA.

Draves            Standards Track - Expires May 2000                 5
Default Address Selection for IPv6                    October 22, 1999


   Rule 2: If Precedence(A) > Precedence(B), then sort DA before DB.
   Similarly, if Precedence(B) > Precedence(A), then sort DB before DA.

   Rule 3: If MatchSrc(DA) and MatchSrc(DB) are both non-empty. If
   MaxCommonPrefixLen(DA, MatchSrc(DA)) > MaxCommonPrefixLen(DB,
   MatchSrc(DB)), then sort DA before DB. Similarly, if
   MaxCommonPrefixLen(DB, MatchSrc(DB)) > MaxCommonPrefixLen(DA,
   MatchSrc(DA)), then sort DB before DA.

   Rule 4: Sort DA before DB.

   The third and fourth rules MAY be superceded if the implementation
   has other means of sorting destination addresses. For example, if
   the implementation somehow knows which destination addresses will
   result in the "best" communications performance.

3.1. Candidate Source Addresses

   For the purposes of destination address ordering, the candidate set
   of source addresses CandidateSrc(D) for a destination address D
   SHOULD contain all and only the unicast addresses assigned to
   interfaces that might be used to send to the destination D.

   For example, if the address D is a link-local unicast address that
   is qualified with a scope-id value specifying a particular
   interface, then CandidateSrc(D) SHOULD contain all and only the
   unicast addresses assigned to that interface.

   For example, if the address D is a global scope unicast address,
   then CandidateSrc(D) MAY contain every unicast address assigned to
   all interfaces. However if the implementation wishes to consult a
   routing table and determine a likely outgoing interface, then
   CandidateSrc(D) MAY contain only unicast addresses assigned to that
   outgoing interface.

4. Source Address Selection

   The source address selection algorithm chooses a source address for
   use with a destination address D. It is specified here in terms of
   the pair-wise comparison of addresses SA and SB. The pair-wise
   comparison can be used to select an address from the set
   CandidateSrc(D).

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

   Rule 1: If SA is in MatchSrc(D) and SB is not, then choose SA.
   Similarly, if SB is in MatchSrc(D) and SA is not, then choose SB.



Draves            Standards Track - Expires May 2000                 6
Default Address Selection for IPv6                    October 22, 1999


   Rule 2: If SA is equal to D, then choose SA. Similarly, if SB is
   equal to D, then choose SB.

   Rule 3a: If Scope(SA) < Scope(SB). If Scope(SA) < Scope(D), then
   choose SB. Otherwise, if one of the source addresses is "preferred"
   and one of them is "deprecated", then choose the "preferred"
   address. Otherwise, choose SA.

   Rule 3b: Similarly, if Scope(SB) < Scope(SA). If Scope(SB) <
   Scope(D), then choose SA. Otherwise, if one of the source addresses
   is "preferred" and one of them is "deprecated", then choose the
   "preferred" address. Otherwise, choose SB.

   Rule 4: The addresses SA and SB 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 5: If Label(SA) = MatchSrcLabel(D) and Label(SB) <>
   MatchSrcLabel(D), then choose SA. Similarly, if Label(SA) <>
   MatchSrcLabel(D) and Label(SB) = MatchSrcLabel(D), then choose SB.
   (Note that this rule will apply only when both SA and SB are
   deprecated.)

   Rule 6: If CommonPrefixLen(SA, D) > CommonPrefixLen(SB, D), then
   choose SA. Similarly, if CommonPrefixLen(SB, D) >
   CommonPrefixLen(SA, D), then choose SB.

   The sixth 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.

4.1. Candidate Source Addresses

   For the purposes of source address selection, the candidate set of
   source addresses CandidateSrc(D) for a destination address D MUST
   contain all and only the unicast addresses assigned to the interface
   that will be used to send to the destination D.

5. Interactions with Routing

   All IPv6 nodes, including both hosts and routers, MUST conform to
   this specification.

   This specification of source address selection implies that routing
   (more precisely, selecting an outgoing interface on a node with
   multiple interfaces) is done before source address selection.
   However, implementations MAY use source address considerations as a
   tiebreaker when choosing among otherwise equivalent routes.

   For example, suppose a node has interfaces on two different links,
   with both links having a working default router. One of the
   interfaces has a preferred global address and the other interface

Draves            Standards Track - Expires May 2000                 7
Default Address Selection for IPv6                    October 22, 1999


   only has a deprecated global address. When sending to a global
   destination address, if there's no routing reason to prefer one
   interface over the other, then an implementation MAY preferentially
   choose the outgoing interface that will allow it to use the
   preferred global source address.

6. Interactions with Mobility

   TBD

7. Implementation Considerations

   The destination address ordering algorithm needs information about
   potential source addresses. One possible implementation strategy is
   for getipnodebyname() and getaddrinfo() to call down to the IPv6
   network layer with a list of destination addresses, sort the list in
   the network layer with full current knowledge of available source
   addresses, and return the sorted list to getipnodebyname() or
   getaddrinfo(). This is simple but it introduces overhead.

   Another implementation strategy is to call down to the network layer
   to retrieve source address information and then sort the list of
   addresses directly in the context of getipnodebyname() or
   getaddrinfo(). To reduce overhead in this approach, the source
   address information SHOULD be cached, amortizing the overhead of
   retrieving it across multiple calls to getipnodebyname() and
   getaddrinfo(). If an implementation uses cached and possibly stale
   source address information in its implementation of destination
   address ordering, then it MUST ensure that the source address
   information is no more than one second out of date.

8. Security Considerations

   This document has no direct impact on Internet infrastructure
   security.

References

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

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

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

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

   5  R. Gilligan, S. Thomson, J. Bound, W. Stevens, "Basic Socket
      Interface Extensions for IPv6", RFC 2553, March 1999.


Draves            Standards Track - Expires May 2000                 8


Acknowledgments

   The author would like to acknowledge the contributions of the IPng
   Working Group.

Author's Address

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

Revision History

Changes from draft-draves-ipngw-simple-srcaddr-01

   Added framework discussion.

   Added algorithm for destination address ordering.

   Added mechanism to allow the specification of administrative policy
   that can override the default behavior.

   Added section on routing interactions and TBD section on mobility
   interactions.

   Changed the candidate set definition for source address selection,
   so that only addresses assigned to the outgoing interface are
   allowed.

   Changed the loopback address treatment to link-local scope.

Changes from draft-draves-ipngw-simple-srcaddr-00

   Minor wording changes because DHCPv6 also supports "preferred" and
   "deprecated" addresses.

   Specified treatment of other format prefixes; now they are
   considered global scope, "preferred" addresses.

   Reiterated that anycast and multicast addresses are not allowed as
   source addresses.

   Recommended that source addresses be taken from the outgoing
   interface. Required this for multicast destinations. Added analogous
   requirements for link-local and site-local destinations.

   Specified treatment of the loopback address.

   Changed the second selection rule so that if both candidate source
   addresses have scope greater or equal than the destination address
   and only of them is preferred, the preferred address is chosen.


Draves            Standards Track - Expires May 2000                 9
Default Address Selection for IPv6                    October 22, 1999


   Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.



























Draves            Standards Track - Expires May 2000                10


Html markup produced by rfcmarkup 1.111, available from https://tools.ietf.org/tools/rfcmarkup/