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Network Working Group                                     F. Le FaucheurRequest for Comments: 5549                                      E. RosenCategory: Standards Track                                  Cisco Systems                                                                May 2009Advertising IPv4 Network Layer Reachability Informationwith an IPv6 Next HopStatus 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 toBCP 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.Le Faucheur & Rosen         Standards Track                     [Page 1]

RFC 5549                   v4 NLRI with v6 NH                   May 2009Table of Contents1. Introduction ....................................................22. Requirements Language ...........................................43. Extension of AFI/SAFI Definitions for the IPv4 Address Family ...44. Use of BGP Capability Advertisement .............................55. Operations ......................................................76. Usage Examples ..................................................76.1. IPv4 over IPv6 Core ........................................76.2. IPv4 VPN over IPv6 Core ....................................87. IANA Considerations .............................................88. Security Considerations .........................................89. Acknowledgments .................................................910. References .....................................................910.1. Normative References ......................................910.2. Informative References ....................................91.  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 asLe Faucheur & Rosen         Standards Track                     [Page 2]

RFC 5549                   v4 NLRI with v6 NH                   May 2009   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 allowsLe Faucheur & Rosen         Standards Track                     [Page 3]

RFC 5549                   v4 NLRI with v6 NH                   May 2009   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 inRFC 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 perSection 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.Le Faucheur & Rosen         Standards Track                     [Page 4]

RFC 5549                   v4 NLRI with v6 NH                   May 2009   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 inSection 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)                          |         +-----------------------------------------------------+Le Faucheur & Rosen         Standards Track                     [Page 5]

RFC 5549                   v4 NLRI with v6 NH                   May 2009      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 isLe Faucheur & Rosen         Standards Track                     [Page 6]

RFC 5549                   v4 NLRI with v6 NH                   May 2009   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 Examples6.1.  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 routesLe Faucheur & Rosen         Standards Track                     [Page 7]

RFC 5549                   v4 NLRI with v6 NH                   May 2009   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, inSection 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.Le Faucheur & Rosen         Standards Track                     [Page 8]

RFC 5549                   v4 NLRI with v6 NH                   May 2009   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.  References10.1.  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.Le Faucheur & Rosen         Standards Track                     [Page 9]

RFC 5549                   v4 NLRI with v6 NH                   May 2009   [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.comLe Faucheur & Rosen         Standards Track                    [Page 10]