IPng Working Group R. Draves Internet Draft Microsoft Research Document: draft-draves-ipngwg-simple-srcaddr-01.txt June 25, 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 unicast addresses to be assigned to interfaces. These addresses may have different reachability scopes (link-local, site-local, or global). Furthermore, these addresses may be "preferred" or "deprecated" [4. Draves Standards Track - Expires January 2000 1 Simple Source Address Selection for IPv6 April 28, 1999 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. The rules specified in this document MUST NOT be construed to override an application or upper-layer's explicit choice of source address. 4. Source Address Selection This document specifies source address selection in two steps. First, it specifies a set of candidate source addresses for a given destination address. Second, it 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, the pair-wise algorithm may be extended to select an address from the set of candidate source addresses. 4.1 Multicast Scopes 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. 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. This mapping implicitly conflates unicast site boundaries and multicast site boundaries. 4.2 Candidate Source Addresses Draves Standards Track - Expires January 2000 2 Simple Source Address Selection for IPv6 April 28, 1999 It is RECOMMENDED that the candidate source addresses be the set of unicast addresses assigned to the interface that will be used to send to the destination. (The "outgoing" interface.) For multicast and link-local destination addresses, the set of candidate source addresses MUST only include addresses assigned to the outgoing interface. For site-local destination addresses, the set of candidate source addresses MUST only include addresses assigned to interfaces belonging to the same site as the outgoing interface. In any case, anycast and multicast addresses MUST NOT be included in the candidate set. 4.3 Pair-Wise Source Address Selection 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 SA's scope is smaller than the destination address scope, 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 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. IPv4-Compatible Addresses and Other Format Prefixes Draves Standards Track - Expires January 2000 3 Simple Source Address Selection for IPv6 April 28, 1999 For the purposes of this document, IPv4-compatible addresses have global scope and "preferred" configuration status. 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 node-local scope and "preferred" configuration status. 6. Security Considerations This document has no direct impact on Internet infrastructure security. 7. 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. 8. Acknowledgments The author would like to acknowledge the contributions of the IPng Working Group. 9. Author's Address Richard Draves Microsoft Research One Microsoft Way Redmond, WA 98052 Email: richdr@microsoft.com 10. Changes from 00 to 01 Minor wording changes because DHCPv6 also supports "preferred" and "deprecated" addresses. Draves Standards Track - Expires January 2000 4 Simple Source Address Selection for IPv6 April 28, 1999 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 January 2000 5 Simple Source Address Selection for IPv6 April 28, 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 January 2000 6