BGP-4 for IPv6: Use of BGP-4 Multiprotocol Extensions for IPv6 Inter-Domain Routing

Network Working Group                                        P. Marques
Request for Comments: 2545                          cisco Systems, Inc.
Category: Standards Track                                     F. Dupont
                                                                  Inria
                                                             March 1999


  Use of BGP-4 Multiprotocol Extensions for IPv6 Inter-Domain Routing

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 (1999).  All Rights Reserved.

Abstract

   BGP-4 Multiprotocol Extensions [BGP-MP] defines the format of two BGP
   attributes (MP_REACH_NLRI and MP_UNREACH_NLRI) that can be used to
   announce and withdraw the announcement of reachability information.
   This document defines how compliant systems should make use of those
   attributes for the purpose of conveying IPv6 routing information.

1. Introduction

   The BGP-4 protocol [BGP-4] in particular, and path vector routing
   protocols in general, are mostly independent of the particular
   Address Family for which the protocol is being used.

   IPv6 falls under the generic category of protocols for which BGP-4 is
   suitable and, unless stated otherwise in this document, the BGP-4
   procedures to apply when using BGP-4 to carry IPv6 reachability
   information are those defined in [BGP-4] and in subsequent documents
   that extend or update the BGP-4 specification.

   In terms of routing information, the most significant difference
   between IPv6 and IPv4 (for which BGP was originally designed) is the
   fact that IPv6 introduces scoped unicast addresses and defines
   particular situations when a particular address scope must be used.
   This document concerns itself essentially with the necessary rules to
   accommodate IPv6 address scope requirements.

2. IPv6 Address Scopes

   IPv6 defines 3 unicast address scopes [ADDR-ARCH]: global, site-local
   and link-local. Site-local addresses are non-link-local address which
   are valid within the scope of a "site" and cannot be exported beyond
   it. As this document makes no assumption on the characteristics of a
   particular routing realm where BGP-4 is used, it makes no distinction
   between global and site-local addresses and refers to both as
   "global" or "non-link-local". Network administrators must however
   respect address scope restrictions and should be aware that the
   concepts of a BGP-4 routing domain and "site" are orthogonal notions
   and that they may or may not coincide in a given situation.

   Companion IPv6 specifications further define that only link-local
   address can be used when generating ICMP Redirect Messages [ND] and
   as next hop addresses in some routing protocols (eg. RIPng [RIP]).

   This restrictions does imply that an IPv6 router must have a link-
   local next hop address for all directly connected routes (routes for
   which the given router and the next hop router share a common subnet
   prefix).

   Link-local addresses are not, however, well suited to be used as next
   hop attributes in BGP-4 given the rules defined for this attribute in
   the protocol specification [BGP-4].

   For the above reasons, when BGP-4 is used to convey IPv6 reachability
   information it is sometimes necessary to announce a next hop
   attribute that consists of a global address and a link-local address.
   The following section describes the rules that should be followed
   when constructing the Network Address of Next Hop field of an
   MP_REACH_NLRI attribute.

3. Constructing the Next Hop field

   A BGP speaker shall advertise to its peer in the Network Address of
   Next Hop field the global IPv6 address of the next hop, potentially
   followed by the link-local IPv6 address of the next hop.

   The value of the Length of Next Hop Network Address field on a
   MP_REACH_NLRI attribute shall be set to 16, when only a global
   address is present, or 32 if a link-local address is also included in
   the Next Hop field.

   The link-local address shall be included in the Next Hop field if and
   only if the BGP speaker shares a common subnet with the entity
   identified by the global IPv6 address carried in the Network Address
   of Next Hop field and the peer the route is being advertised to.

   In all other cases a BGP speaker shall advertise to its peer in the
   Network Address field only the global IPv6 address of the next hop
   (the value of the Length of Network Address of Next Hop field shall
   be set to 16).

   As a consequence, a BGP speaker that advertises a route to an
   internal peer may modify the Network Address of Next Hop field by
   removing the link-local IPv6 address of the next hop.

4. Transport

   TCP connections, on top of which BGP-4 messages are exchanged, can be
   established either over IPv4 or IPv6. While BGP-4 itself is
   independent of the particular transport used it derives implicit
   configuration information from the address used to establish the
   peering session.  This information (the network address of a peer) is
   taken in account in the route dissemination procedure. Thus, when
   using TCP over IPv4 as a transport for IPv6 reachability information,
   additional explicit configuration of the peer's network address is
   required.

   Note that the information referred above is distinct from the BGP
   Identifier used in the BGP-4 tie breaking procedure. The BGP
   Identifier is a 32 bit unsigned integer exchanged between two peers
   at session establishment time, within an OPEN message. This is a
   system wide value determined at startup which must be unique in the
   network and should be derived from an IPv4 address regardless of the
   network protocol(s) a particular BGP-4 instance is configured to
   convey at a given moment.

   The use of TCP over IPv6 as transport protocol for IPv6 reachability
   information also has the advantage of providing explicit confirmation
   of IPv6 network reachability between two peers.

5. Security Considerations

   The extensions defined in this document allow BGP to propagate
   reachability information about IPv6 routes. As such, no new security
   issues are raised beyond those that already exist in BGP-4 and its
   use with IPv4.

6. Acknowledgments

   This document derives from the work on BGP-4 Multiprotocol Extensions
   by Tony Bates, Ravi Chandra, Dave Katz and Yakov Rekhter.

7. References

   [ADDR-ARCH] Hinden, R. and S. Deering, "IP Version 6 Addressing
               Architecture", RFC 2373, July 1998.

   [BGP-4]     Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
               (BGP-4)", RFC 1771, March 1995.

   [BGP-MP]    Bates, T., Chandra, R., Katz, D. and Y. Rekhter,
               "Multiprotocol Extensions for BGP-4", RFC 2283, February
               1998.

   [IPv6]      Deering, S. and R. Hinden, "Internet Protocol, Version 6
               (IPv6) Specification", RFC 2460, December 1998.

   [ND]        Narten, T., Nordmark, E. and W. Simpson, "Neighbor
               Discovery for IP Version 6 (IPv6)", RFC 2461, December
               1998.

   [RIP]       Malkin, G. and R. Minnear, "RIPng for IPv6",
               RFC 2080, January 1997.

8. Author Information

   Pedro R. Marques
   cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA 95134
   USA

   Phone: +1 408 527-5202
   Fax:   +1 408 526-4952
   EMail: roque@cisco.com


   Francis Dupont
   GIE DYADE
   INRIA Rocquencourt
   Domaine de Voluceau
   BP 105
   78153 Le Chesnay CEDEX
   FRANCE

   Phone: +33 1 39 63 52 13
   Fax:   +33 1 39 63 58 66
   EMail: Francis.Dupont@inria.fr

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

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