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IETF RFC 5549
Advertising IPv4 Network Layer Reachability Information with an IPv6 Next Hop
Last modified on Tuesday, May 5th, 2009
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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.
Le Faucheur & Rosen Standards Track PAGE 1 
RFC 5549 v4 NLRI with v6 NH May 2009
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) [RFC 4760] 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 [RFC 4684]) 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, [RFC 4684] 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 [RFC 4925] and in
[MESH-FMWK] where carriers (or large enterprise networks acting as
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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 [RFC 2545] and [RFC 3107], 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 [RFC 4659], when the <AFI/SAFI> is <2/128>, the Next Hop
address is assumed to be of IPv6-VPN type.
However, [RFC 4798] and [RFC 4659] 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. [RFC 4798] defines how the
IPv4-mapped IPv6 address format specified in the IPv6 addressing
architecture ([RFC 4291]) can be used for that purpose when the <AFI/
SAFI> is <2/1>, <2/2>, or <2/4>. [RFC 4659] 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 [RFC 4760] and
[RFC 3107], 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 [RFC 4364], 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
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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 in RFC 2119 [RFC 2119].
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 [RFC 4760] 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 [RFC 2545].
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.
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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 [RFC 4684] and [L2VPN-SIG].
4. Use of BGP Capability Advertisement
[RFC 5492] 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 [RFC 5492] 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 [RFC 5492] 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) |
+-----------------------------------------------------+
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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 [RFC 4684]).
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
[RFC 4798] and AFI/SAFI = <2/128> with IPv4 Next Hop as per
[RFC 4659]).
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
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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 [RFC 4760].
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 [RFC 4925]) 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
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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, in Section 4, a new Capability Code to
indicate the Extended Next Hop Encoding capability in the [RFC 5492]
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 [RFC 5226].
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 [RFC 4291]). 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
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2545] Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
Extensions for IPv6 Inter-Domain Routing", RFC 2545,
March 1999.
[RFC 3107] Rekhter, Y. and E. Rosen, "Carrying Label Information in
BGP-4", RFC 3107, May 2001.
[RFC 4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC 4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC 4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
January 2007.
[RFC 5492] 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.
[RFC 4659] 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.
[RFC 4684] 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.
[RFC 4798] 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.
[RFC 4925] Li, X., Dawkins, S., Ward, D., and A. Durand, "Softwire
Problem Statement", RFC 4925, July 2007.
[RFC 5226] 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
Le Faucheur & Rosen Standards Track PAGE 10
Advertising IPv4 Network Layer Reachability Information with an IPv6 Next Hop
RFC TOTAL SIZE: 23637 bytes
PUBLICATION DATE: Tuesday, May 5th, 2009
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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