HomeDocsIETF standardsRFCRFC 5500 — 5599
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Network Working Group
Request for Comments: 5549
Category: Standards Track
F. Le Faucheur
E. Rosen
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 © 2009 IETF Trust and the persons identified as the document authors. All rights reserved.

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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.

Table of Contents

   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:

  • AFI = 1
   o  SAFI = 1, 2, 4, or 128
  • Length of Next Hop Address = 16 or 32
  • 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].
  • 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:

  • The Capability Code field MUST be set to 5 (which indicates the Extended Next Hop Encoding capability).
  • The Capability Length field is set to a variable value that is the length of the Capability Value field (which follows).
  • 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
  • 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:

  • AFI = 1
  • SAFI = 1
  • Length of Next Hop Network Address = 16 (or 32)
  • Network Address of Next Hop = IPv6 address of Next Hop
  • NLRI = IPv4 routes

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

  • Capability Code set to "Extended Next Hop Encoding"
  • 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:

  • AFI = 1
  • SAFI = 128
  • Length of Next Hop Network Address = 16 (or 32)
  • Network Address of Next Hop = IPv6 address of Next Hop
  • NLRI = IPv4-VPN routes

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

  • Capability Code set to "Extended Next Hop Encoding"
  • 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