Network Working Group                                     F. Le Faucheur
Request for Comments: 5549                                      E. Rosen
Category: Standards Track                                  Cisco Systems
                                                                May 2009

Advertising IPv4 Network Layer Reachability Information
with an IPv6 Next Hop

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) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   Multiprotocol BGP (MP-BGP) specifies that the set of network-layer
   protocols to which the address carried in the Next Hop field may
   belong is determined by the Address Family Identifier (AFI) and the
   Subsequent Address Family Identifier (SAFI).  The current AFI/SAFI
   definitions for the IPv4 address family only have provisions for
   advertising a Next Hop address that belongs to the IPv4 protocol when
   advertising IPv4 Network Layer Reachability Information (NLRI) or
   VPN-IPv4 NLRI.  This document specifies the extensions necessary to
   allow advertising IPv4 NLRI or VPN-IPv4 NLRI with a Next Hop address
   that belongs to the IPv6 protocol.  This comprises an extension of
   the AFI/SAFI definitions to allow the address of the Next Hop for
   IPv4 NLRI or VPN-IPv4 NLRI to also belong to the IPv6 protocol, the
   encoding of the Next Hop in order to determine which of the protocols
   the address actually belongs to, and a new BGP Capability allowing
   MP-BGP Peers to dynamically discover whether they can exchange IPv4
   NLRI and VPN-IPv4 NLRI with an IPv6 Next Hop.

   1. Introduction ....................................................2
   2. Requirements Language ...........................................4
   3. Extension of AFI/SAFI Definitions for the IPv4 Address Family ...4
   4. Use of BGP Capability Advertisement .............................5
   5. Operations ......................................................7
   6. Usage Examples ..................................................7
      6.1. IPv4 over IPv6 Core ........................................7
      6.2. IPv4 VPN over IPv6 Core ....................................8
   7. IANA Considerations .............................................8
   8. Security Considerations .........................................8
   9. Acknowledgments .................................................9
   10. References .....................................................9
      10.1. Normative References ......................................9
      10.2. Informative References ....................................9

1. Introduction

   Multiprotocol BGP (MP-BGP) [RFC4760] specifies that the set of
   network-layer protocols to which the address carried in the Next Hop
   field may belong is determined by the Address Family Identifier (AFI)
   and the Subsequent Address Family Identifier (SAFI).  A number of
   existing AFI/SAFIs allow the Next Hop address to belong to a
   different address family than the Network Layer Reachability
   Information (NLRI).  For example, the AFI/SAFI <25/65> used (as per
   [L2VPN-SIG]) in order to perform L2VPN auto-discovery, allows
   advertising NLRI that contains the identifier of a Virtual Private
   LAN Service (VPLS) instance or that identifies a particular pool of
   attachment circuits at a given Provider Edge (PE), while the Next Hop
   field contains the loopback address of a PE.  Similarly, the AFI/SAFI
   <1/132> (defined in [RFC4684]) in order to advertise Route Target
   (RT) membership information, allows advertising NLRI that contains
   such RT membership information, while the Next Hop field contains the
   address of the advertising router.

   Furthermore, a number of these existing AFI/SAFIs allow the Next Hop
   to belong to either the IPv4 Network Layer Protocol or the IPv6
   Network Layer Protocol, and specify the encoding of the Next Hop
   information in order to determine which of the protocols the address
   actually belongs to.  For example, [RFC4684] allows the Next Hop
   address to be either IPv4 or IPv6 and states that the Next Hop field
   address shall be interpreted as an IPv4 address whenever the length
   of Next Hop address is 4 octets, and as an IPv6 address whenever the
   length of the Next Hop address is 16 octets.

   There are situations such as those described in [RFC4925] and in
   [MESH-FMWK] where carriers (or large enterprise networks acting as

   carrier for their internal resources) may be required to establish
   connectivity between 'islands' of networks of one address family type
   across a transit core of a differing address family type.  This
   includes both the case of IPv6 islands across an IPv4 core and the
   case of IPv4 islands across an IPv6 core.  Where Multiprotocol BGP
   (MP-BGP) is used to advertise the corresponding reachability
   information, this translates into the requirement for a BGP speaker
   to advertise Network Layer Reachability Information (NLRI) of a given
   address family via a Next Hop of a different address family (i.e.,
   IPv6 NLRI with IPv4 Next Hop and IPv4 NLRI with IPv6 Next Hop).

   The current AFI/SAFI definitions for the IPv6 address family assume
   that the Next Hop address belongs to the IPv6 address family type.
   Specifically, as per [RFC2545] and [RFC3107], when the <AFI/SAFI> is
   <2/1>, <2/2>, or <2/4>, the Next Hop address is assumed to be of IPv6
   type.  As per [RFC4659], when the <AFI/SAFI> is <2/128>, the Next Hop
   address is assumed to be of IPv6-VPN type.

   However, [RFC4798] and [RFC4659] specify how an IPv4 address can be
   encoded inside the Next Hop IPv6 address field when IPv6 NLRI needs
   to be advertised with an IPv4 Next Hop.  [RFC4798] defines how the
   IPv4-mapped IPv6 address format specified in the IPv6 addressing
   architecture ([RFC4291]) can be used for that purpose when the <AFI/
   SAFI> is <2/1>, <2/2>, or <2/4>.  [RFC4659] defines how the IPv4-
   mapped IPv6 address format as well as a null Route Distinguisher can
   be used for that purpose when the <AFI/SAFI> is <2/128>.  Thus, there
   are existing solutions for the advertisement of IPv6 NLRI with an
   IPv4 Next Hop.

   Similarly, the current AFI/SAFI definitions for advertisement of IPv4
   NLRI or VPN-IPv4 NLRI assume that the Next Hop address belongs to the
   IPv4 address family type.  Specifically, as per [RFC4760] and
   [RFC3107], when the <AFI/SAFI> is <1/1>, <1/2>, or <1/4>, the Next
   Hop address is assumed to be of IPv4 type.  As per [RFC4364], when
   the <AFI/SAFI> is <1/128>, the Next Hop address is assumed to be of
   VPN-IPv4 type.  There is clearly no generally applicable method for
   encoding an IPv6 address inside the IPv4 address field of the Next
   Hop.  Hence, there is currently no specified solution for advertising
   IPv4 or VPN-IPv4 NLRI with an IPv6 Next Hop.

   This document specifies the extensions necessary to do so.  This
   comprises an extension of the AFI/SAFI definitions to allow the
   address of the Next Hop for IPv4 NLRI or VPN-IPv4 NLRI to belong to
   either the IPv4 or the IPv6 protocol, the encoding of the Next Hop
   information in order to determine which of the protocols the address
   actually belongs to, and a new BGP Capability allowing MP-BGP peers
   to dynamically discover whether they can exchange IPv4 NLRI and VPN-
   IPv4 NLRI with an IPv6 Next Hop.  The new BGP Capability allows

   gradual deployment of the new functionality of advertising IPv4
   reachability via an IPv6 Next Hop, without any flag day nor any risk
   of traffic black-holing.

2. Requirements Language

   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 [RFC2119].

3. Extension of AFI/SAFI Definitions for the IPv4 Address Family

   As mentioned earlier, MP-BGP specifies that the set of network-layer
   protocols to which the address carried in the Next Hop field may
   belong is determined by the Address Family Identifier (AFI) and the
   Subsequent Address Family Identifier (SAFI).  The following current
   AFI/SAFI definitions for the IPv4 NLRI or VPN-IPv4 NLRI (<1/1>,
   <1/2>, <1/4>, and <1/128>) only have provisions for advertising a
   Next Hop address that belongs to the IPv4 protocol.  This document
   extends the definition of the AFI/SAFI for advertisement of IPv4 NLRI
   and VPN-IPv4 NLRI to extend the set of network-layer protocols to
   which the Next Hop address can belong, to include IPv6 in addition to
   IPv4.

   Specifically, this document allows advertising with [RFC4760] of an
   MP_REACH_NLRI with:

   o  AFI = 1

   o  SAFI = 1, 2, 4, or 128

   o  Length of Next Hop Address = 16 or 32

   o  Next Hop Address = IPv6 address of next hop (potentially followed
      by the link-local IPv6 address of the next hop).  This field is to
      be constructed as per Section 3 of [RFC2545].

   o  NLRI= NLRI as per current AFI/SAFI definition

   This is in addition to the current mode of operation allowing
   advertisement of NLRI for <AFI/SAFI> of <1/1>, <1/2> and <1/4> with a
   next hop address of IPv4 type and advertisement of NLRI for <AFI/
   SAFI> of <1/128> with a next hop address of VPN-IPv4 type.

   The BGP speaker receiving the advertisement MUST use the Length of
   Next Hop Address field to determine which network-layer protocol the
   next hop address belongs to.  When the Length of Next Hop Address
   field is equal to 16 or 32, the next hop address is of type IPv6.

   Note that this method of using the Length of the Next Hop Address
   field to determine which network-layer protocol the next hop address
   belongs to (out of the set of protocols allowed by the AFI/SAFI
   definition) is the same as used in [RFC4684] and [L2VPN-SIG].

4. Use of BGP Capability Advertisement

   [RFC5492] defines a mechanism to allow two BGP speakers to discover
   if a particular capability is supported by their BGP peer and thus
   whether it can be used with that peer.  This document defines a new
   capability that can be advertised using [RFC5492] and that is
   referred to as the Extended Next Hop Encoding capability.  This
   capability allows BGP speakers to discover whether, for a given NLRI
   <AFI/SAFI>, a peer supports advertisement with a next hop whose
   network protocol is determined by the value of the Length of Next Hop
   Address field, as specified in Section 3.

   A BGP speaker that wishes to advertise to a BGP peer an IPv6 Next Hop
   for IPv4 NLRI or for VPN-IPv4 NLRI as per this specification MUST use
   the Capability Advertisement procedures defined in [RFC5492] with the
   Extended Next Hop Encoding Capability to establish whether its peer
   supports this for the NLRI AFI/SAFI pair(s) of interest.  The fields
   in the Capabilities Optional Parameter MUST be set as follows:

   o  The Capability Code field MUST be set to 5 (which indicates the
      Extended Next Hop Encoding capability).

   o  The Capability Length field is set to a variable value that is the
      length of the Capability Value field (which follows).

   o  The Capability Value field has the following format:

         +-----------------------------------------------------+
         | NLRI AFI - 1 (2 octets)                             |
         +-----------------------------------------------------+
         | NLRI SAFI - 1 (2 octets)                            |
         +-----------------------------------------------------+
         | Nexthop AFI - 1 (2 octets)                          |
         +-----------------------------------------------------+
         | .....                                               |
         +-----------------------------------------------------+
         | NLRI AFI - N (2 octets)                             |
         +-----------------------------------------------------+
         | NLRI SAFI - N (2 octets)                            |
         +-----------------------------------------------------+
         | Nexthop AFI - N (2 octets)                          |
         +-----------------------------------------------------+

      where:

      *  each triple <NLRI AFI, NLRI SAFI, Nexthop AFI> indicates that
         NLRI of <NLRI AFI / NLRI SAFI> may be advertised with a Next
         Hop address belonging to the network-layer protocol of Nexthop
         AFI.

      *  the AFI and SAFI values are defined in the Address Family
         Identifier and Subsequent Address Family Identifier registries
         maintained by IANA.

   Since this document only concerns itself with the advertisement of
   IPv4 NLRI and VPN-IPv4 NLRI with an IPv6 Next Hop, this specification
   only allows the following values in the Capability Value field of the
   Extended Next Hop Encoding capability:

   o  NLRI AFI = 1 (IPv4)

   o  NLRI SAFI = 1, 2, 4, or 128

   o  Nexthop AFI = 2 (IPv6)

   This specification does not propose that the Extended Next Hop
   Encoding capability be used with any other combinations of <NLRI AFI,
   NLRI SAFI, Nexthop AFI>.  In particular, this specification does not
   propose that the Extended Next Hop Encoding capability be used for
   NLRI AFI/SAFIs whose definition already allows use of both IPv4 and
   IPv6 next hops (e.g., AFI/SAFI = <1/132> as defined in [RFC4684]).
   Similarly, it does not propose that the Extended Next Hop Encoding
   capability be used for NLRI AFI/SAFIs for which there is already a
   solution for advertising a next hop of a different address family
   (e.g., AFI/SAFI = <2/1>, <2/2>, or <2/4> with IPv4 Next Hop as per
   [RFC4798] and AFI/SAFI = <2/128> with IPv4 Next Hop as per
   [RFC4659]).

   It is expected that if new AFI/SAFIs are defined in the future, their
   definition will have provisions (where appropriate) for both IPv4 and
   IPv6 Next Hops from the onset, with determination based on Length of
   Next Hop Address field.  Thus, new AFI/SAFIs are not expected to make
   use of the Extended Next Hop Encoding capability.

   A BGP speaker MUST only advertise to a BGP peer the IPv4 or VPN-IPv4
   NLRI with an IPv6 Next Hop if the BGP speaker has first ascertained
   via BGP Capability Advertisement that the BGP peer supports the
   Extended Next Hop Encoding capability for the relevant AFI/SAFI pair.

   The Extended Next Hop Encoding capability provides information about
   next hop encoding for a given AFI/SAFI, assuming that AFI/SAFI is

   allowed.  It does not influence whether that AFI/SAFI is indeed
   allowed.  Whether a AFI/SAFI can be used between the BGP peers is
   purely determined through the Multiprotocol Extensions capability
   defined in [RFC4760].

   The Extended Next Hop Encoding capability MAY be dynamically updated
   through the use of the Dynamic Capability capability and associated
   mechanisms defined in [DYN-CAP].

5. Operations

   By default, if a particular BGP session is running over IPvx (where
   IPvx is IPv4 or IPv6), and if the BGP speaker sending an update is
   putting its own address in as the next hop, then the next hop address
   SHOULD be specified as an IPvx address, using the encoding rules
   specified in the AFI/SAFI definition of the NLRI being updated.  This
   default behavior may be overridden by policy.

   When a next hop address needs to be passed along unchanged (e.g., as
   a Route Reflector (RR) would do), its encoding MUST NOT be changed.
   If a particular RR client cannot handle that encoding (as determined
   by the BGP Capability Advertisement), then the NLRI in question
   cannot be distributed to that client.  For sound routing in certain
   scenarios, this will require that all the RR clients be able to
   handle whatever encodings any of them may generate.

6. Usage Examples

6.1. IPv4 over IPv6 Core

   The extensions defined in this document may be used as discussed in
   [MESH-FMWK] for the interconnection of IPV4 islands over an IPv6
   backbone.  In this application, Address Family Border Routers (AFBRs;
   as defined in [RFC4925]) advertise IPv4 NLRI in the MP_REACH_NLRI
   along with an IPv6 Next Hop.

   The MP_REACH_NLRI is encoded with:

   o  AFI = 1

   o  SAFI = 1

   o  Length of Next Hop Network Address = 16 (or 32)

   o  Network Address of Next Hop = IPv6 address of Next Hop

   o  NLRI = IPv4 routes

   During BGP Capability Advertisement, the PE routers would include the
   following fields in the Capabilities Optional Parameter:

   o  Capability Code set to "Extended Next Hop Encoding"

   o  Capability Value containing <NLRI AFI=1, NLRI SAFI=1, Nexthop
      AFI=2>

6.2. IPv4 VPN over IPv6 Core

   The extensions defined in this document may be used for support of
   IPV4 VPNs over an IPv6 backbone.  In this application, PE routers
   would advertise VPN-IPv4 NLRI in the MP_REACH_NLRI along with an IPv6
   Next Hop.

   The MP_REACH_NLRI is encoded with:

   o  AFI = 1

   o  SAFI = 128

   o  Length of Next Hop Network Address = 16 (or 32)

   o  Network Address of Next Hop = IPv6 address of Next Hop

   o  NLRI = IPv4-VPN routes

   During BGP Capability Advertisement, the PE routers would include the
   following fields in the Capabilities Optional Parameter:

   o  Capability Code set to "Extended Next Hop Encoding"

   o  Capability Value containing <NLRI AFI=1, NLRI SAFI=128, Nexthop
      AFI=2>

7. IANA Considerations

   This document defines, in Section 4, a new Capability Code to
   indicate the Extended Next Hop Encoding capability in the [RFC5492]
   Capabilities Optional Parameter.  The value for this new Capability
   Code is 5, which is in the range set aside for allocation using the
   "IETF Review" policy defined in [RFC5226].

8. Security Considerations

   This document does not raise any additional security issues beyond
   those of BGP-4 and the Multiprotocol extensions for BGP-4.  The same
   security mechanisms are applicable.

   Although not expected to be the typical case, the IPv6 address used
   as the BGP Next Hop Address could be an IPv4-mapped IPv6 address (as
   defined in [RFC4291]).  Configuration of the security mechanisms
   potentially deployed by the network operator (such as security checks
   on next hop address) need to keep this case in mind also.

9. Acknowledgments

   The authors would like to thank Yakov Rekhter, Pranav Mehta, and John
   Scudder for their contributions to the approach defined in this
   document.

10. References

10.1. Normative References

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2545]    Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
                Extensions for IPv6 Inter-Domain Routing", RFC 2545,
                March 1999.

   [RFC3107]    Rekhter, Y. and E. Rosen, "Carrying Label Information in
                BGP-4", RFC 3107, May 2001.

   [RFC4291]    Hinden, R. and S. Deering, "IP Version 6 Addressing
                Architecture", RFC 4291, February 2006.

   [RFC4364]    Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
                Networks (VPNs)", RFC 4364, February 2006.

   [RFC4760]    Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
                "Multiprotocol Extensions for BGP-4", RFC 4760,
                January 2007.

   [RFC5492]    Scudder, J. and R. Chandra, "Capabilities Advertisement
                with BGP-4", RFC 5492, February 2009.

10.2. Informative References

   [DYN-CAP]    Chen, E. and S. Sangli, "Dynamic Capability for BGP-4",
                Work in Progress, November 2006.

   [L2VPN-SIG]  Rosen, E., "Provisioning, Autodiscovery, and Signaling
                in L2VPNs", Work in Progress, May 2006.

   [MESH-FMWK]  Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
                Framework", Work in Progress, February 2009.

   [RFC4659]    De Clercq, J., Ooms, D., Carugi, M., and F. Le Faucheur,
                "BGP-MPLS IP Virtual Private Network (VPN) Extension for
                IPv6 VPN", RFC 4659, September 2006.

   [RFC4684]    Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
                R., Patel, K., and J. Guichard, "Constrained Route
                Distribution for Border Gateway Protocol/MultiProtocol
                Label Switching (BGP/MPLS) Internet Protocol (IP)
                Virtual Private Networks (VPNs)", RFC 4684,
                November 2006.

   [RFC4798]    De Clercq, J., Ooms, D., Prevost, S., and F. Le
                Faucheur, "Connecting IPv6 Islands over IPv4 MPLS Using
                IPv6 Provider Edge Routers (6PE)", RFC 4798,
                February 2007.

   [RFC4925]    Li, X., Dawkins, S., Ward, D., and A. Durand, "Softwire
                Problem Statement", RFC 4925, July 2007.

   [RFC5226]    Narten, T. and H. Alvestrand, "Guidelines for Writing an
                IANA Considerations Section in RFCs", BCP 26, RFC 5226,
                May 2008.

Authors' Addresses

   Francois Le Faucheur
   Cisco Systems
   Greenside, 400 Avenue de Roumanille
   Sophia Antipolis  06410
   France

   EMail: flefauch@cisco.com

   Eric Rosen
   Cisco Systems
   1414 Massachusetts Avenue
   Boxborough, MA  01719
   USA

   EMail: erosen@cisco.com