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Network Working Group J. De Clercq
Request for Comments: 4798 Alcatel-Lucent
Category: Standards Track D. Ooms
OneSparrow
S. Prevost
BT
F. Le Faucheur
Cisco
February 2007
Connecting IPv6 Islands over IPv4 MPLS Using
IPv6 Provider Edge Routers (6PE)
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 IETF Trust (2007).
Abstract
This document explains how to interconnect IPv6 islands over a
Multiprotocol Label Switching (MPLS)-enabled IPv4 cloud. This
approach relies on IPv6 Provider Edge routers (6PE), which are Dual
Stack in order to connect to IPv6 islands and to the MPLS core, which
is only required to run IPv4 MPLS. The 6PE routers exchange the IPv6
reachability information transparently over the core using the
Multiprotocol Border Gateway Protocol (MP-BGP) over IPv4. In doing
so, the BGP Next Hop field is used to convey the IPv4 address of the
6PE router so that dynamically established IPv4-signaled MPLS Label
Switched Paths (LSPs) can be used without explicit tunnel
configuration.
De Clercq, et al. Standards Track [Page 1]
RFC 4798 6PE February 2007
Table of Contents
1. Introduction ....................................................2
1.1. Requirements Language ......................................4
2. Protocol Overview ...............................................4
3. Transport over IPv4-signaled LSPs and IPv6 Label Binding ........5
4. Crossing Multiple IPv4 Autonomous Systems .......................7
5. Security Considerations ........................................10
6. Acknowledgements ...............................................10
7. References .....................................................11
7.1. Normative References ......................................11
7.2. Informative References ....................................11
1. Introduction
There are several approaches for providing IPv6 connectivity over an
MPLS core network [RFC4029] including (i) requiring that MPLS
networks support setting up IPv6-signaled Label Switched Paths (LSPs)
and establish IPv6 connectivity by using those LSPs, (ii) use
configured tunneling over IPv4-signaled LSPs, or (iii) use the IPv6
Provider Edge (6PE) approach defined in this document.
The 6PE approach is required as an alternative to the use of standard
tunnels. It provides a solution for an MPLS environment where all
tunnels are established dynamically, thereby addressing environments
where the effort to configure and maintain explicitly configured
tunnels is not acceptable.
This document specifies operations of the 6PE approach for
interconnection of IPv6 islands over an IPv4 MPLS cloud. The
approach requires that the edge routers connected to IPv6 islands be
Dual Stack Multiprotocol-BGP-speaking routers [RFC4760], while the
core routers are only required to run IPv4 MPLS. The approach uses
MP-BGP over IPv4, relies on identification of the 6PE routers by
their IPv4 address, and uses IPv4-signaled MPLS LSPs that do not
require any explicit tunnel configuration.
Throughout this document, the terminology of [RFC2460] and [RFC4364]
is used.
In this document an 'IPv6 island' is a network running native IPv6 as
per [RFC2460]. A typical example of an IPv6 island would be a
customer's IPv6 site connected via its IPv6 Customer Edge (CE) router
to one (or more) Dual Stack Provider Edge router(s) of a Service
Provider. These IPv6 Provider Edge routers (6PE) are connected to an
IPv4 MPLS core network.
De Clercq, et al. Standards Track [Page 2]
RFC 4798 6PE February 2007
+--------+
|site A CE---+ +-----------------+
+--------+ | | | +--------+
6PE-+ IPv4 MPLS core +-6PE--CE site C |
+--------+ | | | +--------+
|site B CE---+ +-----------------+
+--------+
IPv6 islands IPv4 cloud IPv6 island
<-------------><---------------------><-------------->
Figure 1
The interconnection method described in this document typically
applies to an Internet Service Provider (ISP) that has an IPv4 MPLS
network, that is familiar with BGP (possibly already offering
BGP/MPLS VPN services), and that wants to offer IPv6 services to some
of its customers. However, the ISP may not (yet) want to upgrade its
network core to IPv6, nor use only IPv6-over-IPv4 tunneling. With
the 6PE approach described here, the provider only has to upgrade
some Provider Edge (PE) routers to Dual Stack operations so that they
behave as 6PE routers (and route reflectors if those are used for the
exchange of IPv6 reachability among 6PE routers) while leaving the
IPv4 MPLS core routers untouched. These 6PE routers provide
connectivity to IPv6 islands. They may also provide other services
simultaneously (IPv4 connectivity, IPv4 L3VPN services, L2VPN
services, etc.). Also with the 6PE approach, no tunnels need to be
explicitly configured, and no IPv4 headers need to be inserted in
front of the IPv6 packets between the customer and provider edge.
The ISP obtains IPv6 connectivity to its peers and upstreams using
means outside of the scope of this document, and its 6PE routers
readvertise it over the IPv4 MPLS core with MP-BGP.
The interface between the edge router of the IPv6 island (Customer
Edge (CE) router) and the 6PE router is a native IPv6 interface which
can be physical or logical. A routing protocol (IGP or EGP) may run
between the CE router and the 6PE router for the distribution of IPv6
reachability information. Alternatively, static routes and/or a
default route may be used on the 6PE router and the CE router to
control reachability. An IPv6 island may connect to the provider
network over more than one interface.
The 6PE approach described in this document can be used for customers
that already have an IPv4 service from the network provider and
additionally require an IPv6 service, as well as for customers that
require only IPv6 connectivity.
De Clercq, et al. Standards Track [Page 3]
RFC 4798 6PE February 2007
The scenario is also described in [RFC4029].
Note that the 6PE approach specified in this document provides global
IPv6 reachability. Support of IPv6 VPNs is not within the scope of
this document and is addressed in [RFC4659].
Deployment of the 6PE approach over an existing IPv4 MPLS cloud does
not require an introduction of new mechanisms in the core (other than
potentially those described at the end of Section 3 for dealing with
dynamic MTU discovery). Configuration and operations of the 6PE
approach have a lot of similarities with the configuration and
operations of an IPv4 VPN service ([RFC4364]) or IPv6 VPN service
([RFC4659]) over an IPv4 MPLS core because they all use MP-BGP to
distribute non-IPv4 reachability information for transport over an
IPv4 MPLS Core. However, the configuration and operations of the 6PE
approach is somewhat simpler, since it does not involve all the VPN
concepts such as Virtual Routing and Forwarding (VRFs) tables.
1.1. 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].
2. Protocol Overview
Each IPv6 site is connected to at least one Provider Edge router that
is located on the border of the IPv4 MPLS cloud. We call such a
router a 6PE router. The 6PE router MUST be dual stack IPv4 and
IPv6. The 6PE router MUST be configured with at least one IPv4
address on the IPv4 side and at least one IPv6 address on the IPv6
side. The configured IPv4 address needs to be routable in the IPv4
cloud, and there needs to be a label bound via an IPv4 label
distribution protocol to this IPv4 route.
As a result of this, every considered 6PE router knows which MPLS
label to use to send packets to any other 6PE router. Note that an
MPLS network offering BGP/MPLS IP VPN services already fulfills these
requirements.
No extra routes need to be injected in the IPv4 cloud.
We call the 6PE router receiving IPv6 packets from an IPv6 site an
ingress 6PE router (relative to these IPv6 packets). We call a 6PE
router forwarding IPv6 packets to an IPv6 site an egress 6PE router
(relative to these IPv6 packets).
De Clercq, et al. Standards Track [Page 4]
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Interconnecting IPv6 islands over an IPv4 MPLS cloud takes place
through the following steps:
1. Exchange IPv6 reachability information among 6PE routers with MP-
BGP [RFC2545]:
The 6PE routers MUST exchange the IPv6 prefixes over MP-BGP
sessions as per [RFC2545] running over IPv4. The MP-BGP Address
Family Identifier (AFI) used MUST be IPv6 (value 2). In doing so,
the 6PE routers convey their IPv4 address as the BGP Next Hop for
the advertised IPv6 prefixes. The IPv4 address of the egress 6PE
router MUST be encoded as an IPv4-mapped IPv6 address in the BGP
Next Hop field. This encoding is consistent with the definition
of an IPv4-mapped IPv6 address in [RFC4291] as an "address type
used to represent the address of IPv4 nodes as IPv6 addresses".
In addition, the 6PE MUST bind a label to the IPv6 prefix as per
[RFC3107]. The Subsequence Address Family Identifier (SAFI) used
in MP-BGP MUST be the "label" SAFI (value 4) as defined in
[RFC3107]. Rationale for this and label allocation policies are
discussed in Section 3.
2. Transport IPv6 packets from the ingress 6PE router to the egress
6PE router over IPv4-signaled LSPs:
The ingress 6PE router MUST forward IPv6 data over the IPv4-
signaled LSP towards the egress 6PE router identified by the IPv4
address advertised in the IPv4-mapped IPv6 address of the BGP Next
Hop for the corresponding IPv6 prefix.
As required by the BGP specification [RFC4271], PE routers form a
full peering mesh unless Route Reflectors are used.
3. Transport over IPv4-signaled LSPs and IPv6 Label Binding
In this approach, the IPv4-mapped IPv6 addresses allow a 6PE router
that has to forward an IPv6 packet to automatically determine the
IPv4-signaled LSP to use for a particular IPv6 destination by looking
at the MP-BGP routing information.
The IPv4-signaled LSPs can be established using any existing
technique for label setup [RFC3031] (LDP, RSVP-TE, etc.).
To ensure interoperability among systems that implement the 6PE
approach described in this document, all such systems MUST support
tunneling using IPv4-signaled MPLS LSPs established by LDP [RFC3036].
When tunneling IPv6 packets over the IPv4 MPLS backbone, rather than
successively prepend an IPv4 header and then perform label imposition
De Clercq, et al. Standards Track [Page 5]
RFC 4798 6PE February 2007
based on the IPv4 header, the ingress 6PE Router MUST directly
perform label imposition of the IPv6 header without prepending any
IPv4 header. The (outer) label imposed MUST correspond to the IPv4-
signaled LSP starting on the ingress 6PE Router and ending on the
egress 6PE Router.
While this approach could theoretically operate in some situations
using a single level of labels, there are significant advantages in
using a second level of labels that are bound to IPv6 prefixes via
MP-BGP advertisements in accordance with [RFC3107].
For instance, the use of a second level label allows Penultimate Hop
Popping (PHP) on the IPv4 Label Switch Router (LSR) upstream of the
egress 6PE router, without any IPv6 capabilities/upgrades on the
penultimate router; this is because it still transmits MPLS packets
even after the PHP (instead of having to transmit IPv6 packets and
encapsulate them appropriately).
Also, an existing IPv4-signaled LSP that is using "IPv4 Explicit NULL
label" over the last hop (e.g., because that LSP is already being
used to transport IPv4 traffic with the Pipe Diff-Serv Tunneling
Model as defined in [RFC3270]) could not be used to carry IPv6 with a
single label since the "IPv4 Explicit NULL label" cannot be used to
carry native IPv6 traffic (see [RFC3032]), while it could be used to
carry labeled IPv6 traffic (see [RFC4182]).
This is why a second label MUST be used with the 6PE approach.
The label bound by MP-BGP to the IPv6 prefix indicates to the egress
6PE Router that the packet is an IPv6 packet. This label advertised
by the egress 6PE Router with MP-BGP MAY be an arbitrary label value,
which identifies an IPv6 routing context or outgoing interface to
send the packet to, or MAY be the IPv6 Explicit Null Label. An
ingress 6PE Router MUST be able to accept any such advertised label.
[RFC2460] requires that every link in the IPv6 Internet have an MTU
of 1280 octets or larger. Therefore, on MPLS links that are used for
transport of IPv6, as per the 6PE approach, and that do not support
link-specific fragmentation and reassembly, the MTU must be
configured to at least 1280 octets plus the encapsulation overhead.
Some IPv6 hosts might be sending packets larger than the MTU
available in the IPv4 MPLS core and rely on Path MTU discovery to
learn about those links. To simplify MTU discovery operations, one
option is for the network administrator to engineer the MTU on the
core facing interfaces of the ingress 6PE consistent with the core
MTU. ICMP 'Packet Too Big' messages can then be sent back by the
ingress 6PE without the corresponding packets ever entering the MPLS
De Clercq, et al. Standards Track [Page 6]
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core. Otherwise, routers in the IPv4 MPLS network have the option to
generate an ICMP "Packet Too Big" message using mechanisms as
described in Section 2.3.2, "Tunneling Private Addresses through a
Public Backbone" of [RFC3032].
Note that in the above case, should a core router with an outgoing
link with an MTU smaller than 1280 receive an encapsulated IPv6
packet larger than 1280, then the mechanisms of [RFC3032] may result
in the "Packet Too Big" message never reaching the sender. This is
because, according to [RFC4443], the core router will build an ICMP
"Packet Too Big" message filled with the invoking packet up to 1280
bytes, and when forwarding downstream towards the egress PE as per
[RFC3032], the MTU of the outgoing link will cause the packet to be
dropped. This may cause significant operational problems; the
originator of the packets will notice that his data is not getting
through, without knowing why and where they are discarded. This
issue would only occur if the above recommendation (to configure MTU
on MPLS links of at least 1280 octets plus encapsulation overhead) is
not adhered to (perhaps by misconfiguration).
4. Crossing Multiple IPv4 Autonomous Systems
This section discusses the case where two IPv6 islands are connected
to different Autonomous Systems (ASes).
Like in the case of multi-AS backbone operations for IPv4 VPNs
described in Section 10 of [RFC4364], three main approaches can be
distinguished:
a. eBGP redistribution of IPv6 routes from AS to neighboring AS
This approach is the equivalent for exchange of IPv6 routes to
procedure (a) described in Section 10 of [RFC4364] for the
exchange of VPN-IPv4 routes.
In this approach, the 6PE routers use IBGP (according to [RFC2545]
and [RFC3107] and as described in this document for the single-AS
situation) to redistribute labeled IPv6 routes either to an
Autonomous System Border Router (ASBR) 6PE router, or to a route
reflector of which an ASBR 6PE router is a client. The ASBR then
uses eBGP to redistribute the (non-labeled) IPv6 routes to an ASBR
in another AS, which in turn distributes them to the 6PE routers
in that AS as described earlier in this specification, or perhaps
to another ASBR, which in turn distributes them etc.
De Clercq, et al. Standards Track [Page 7]
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There may be one, or multiple, ASBR interconnection(s) across any
two ASes. IPv6 needs to be activated on the inter-ASBR links and
each ASBR 6PE router has at least one IPv6 address on the
interface to that link.
No inter-AS LSPs are used. There is effectively a separate mesh
of LSPs across the 6PE routers within each AS.
In this approach, the ASBR exchanging IPv6 routes may peer over
IPv6 or IPv4. The exchange of IPv6 routes MUST be carried out as
per [RFC2545].
Note that the peering ASBR in the neighboring AS to which the IPv6
routes were distributed with eBGP, should in its turn redistribute
these routes to the 6PEs in its AS using IBGP and encoding its own
IPv4 address as the IPv4-mapped IPv6 BGP Next Hop.
b. eBGP redistribution of labeled IPv6 routes from AS to neighboring
AS
This approach is the equivalent for exchange of IPv6 routes to
procedure (b) described in Section 10 of [RFC4364] for the
exchange of VPN-IPv4 routes.
In this approach, the 6PE routers use IBGP (as described earlier
in this document for the single-AS situation) to redistribute
labeled IPv6 routes either to an Autonomous System Border Router
(ASBR) 6PE router, or to a route reflector of which an ASBR 6PE
router is a client. The ASBR then uses eBGP to redistribute the
labeled IPv6 routes to an ASBR in another AS, which in turn
distributes them to the 6PE routers in that AS as described
earlier in this specification, or perhaps to another ASBR, which
in turn distributes them, etc.
There may be one, or multiple, ASBR interconnection(s) across any
two ASes. IPv6 may or may not be activated on the inter-ASBR
links.
This approach requires that there be label switched paths
established across ASes. Hence the corresponding considerations
described for procedure (b) in Section 10 of [RFC4364] apply
equally to this approach for IPv6.
In this approach, the ASBR exchanging IPv6 routes may peer over
IPv4 or IPv6 (in which case IPv6 obviously needs to be activated
on the inter-ASBR link). When peering over IPv6, the exchange of
labeled IPv6 routes MUST be carried out as per [RFC2545] and
[RFC3107]. When peering over IPv4, the exchange of labeled IPv6
De Clercq, et al. Standards Track [Page 8]
RFC 4798 6PE February 2007
routes MUST be carried out as per [RFC2545] and [RFC3107] with
encoding of the IPv4 address of the ASBR as an IPv4-mapped IPv6
address in the BGP Next Hop field.
c. Multi-hop eBGP redistribution of labeled IPv6 routes between
source and destination ASes, with eBGP redistribution of labeled
IPv4 routes from AS to neighboring AS.
This approach is the equivalent for exchange of IPv6 routes to
procedure (c) described in Section 10 of [RFC4364] for exchange of
VPN-IPv4 routes.
In this approach, IPv6 routes are neither maintained nor
distributed by the ASBR routers. The ASBR routers need not be
dual stack, but may be IPv4/MPLS-only routers. An ASBR needs to
maintain labeled IPv4 /32 routes to the 6PE routers within its AS.
It uses eBGP to distribute these routes to other ASes. ASBRs in
any transit ASes will also have to use eBGP to pass along the
labeled IPv4 /32 routes. This results in the creation of an IPv4
label switched path from the ingress 6PE router to the egress 6PE
router. Now 6PE routers in different ASes can establish multi-hop
eBGP connections to each other over IPv4, and can exchange labeled
IPv6 routes (with an IPv4-mapped IPv6 BGP Next Hop) over those
connections.
IPv6 need not be activated on the inter-ASBR links.
The considerations described for procedure (c) in Section 10 of
[RFC4364] with respect to possible use of multi-hop eBGP
connections via route-reflectors in different ASes, as well as
with respect to the use of a third label in case the IPv4 /32
routes for the PE routers are NOT made known to the P routers,
apply equally to this approach for IPv6.
This approach requires that there be IPv4 label switched paths
established across the ASes leading from a packet's ingress 6PE
router to its egress 6PE router. Hence the considerations
described for procedure (c) in Section 10 of [RFC4364], with
respect to LSPs spanning multiple ASes, apply equally to this
approach for IPv6.
Note also that the exchange of IPv6 routes can only start after
BGP has created IPv4 connectivity between the ASes.
De Clercq, et al. Standards Track [Page 9]
RFC 4798 6PE February 2007
5. Security Considerations
The extensions defined in this document allow BGP to propagate
reachability information about IPv6 routes over an MPLS IPv4 core
network. As such, no new security issues are raised beyond those
that already exist in BGP-4 and use of MP-BGP for IPv6.
The security features of BGP and corresponding security policy
defined in the ISP domain are applicable.
For the inter-AS distribution of IPv6 routes according to case (a) of
Section 4 of this document, no new security issues are raised beyond
those that already exist in the use of eBGP for IPv6 [RFC2545].
For the inter-AS distribution of IPv6 routes according to case (b)
and (c) of Section 4 of this document, the procedures require that
there be label switched paths established across the AS boundaries.
Hence the appropriate trust relationships must exist between and
among the set of ASes along the path. Care must be taken to avoid
"label spoofing". To this end an ASBR 6PE SHOULD only accept labeled
packets from its peer ASBR 6PE if the topmost label is a label that
it has explicitly signaled to that peer ASBR 6PE.
Note that for the inter-AS distribution of IPv6 routes, according to
case (c) of Section 4 of this document, label spoofing may be more
difficult to prevent. Indeed, the MPLS label distributed with the
IPv6 routes via multi-hop eBGP is directly sent from the egress 6PE
to ingress 6PEs in another AS (or through route reflectors). This
label is advertised transparently through the AS boundaries. When
the egress 6PE that sent the labeled IPv6 routes receives a data
packet that has this particular label on top of its stack, it may not
be able to verify whether the label was pushed on the stack by an
ingress 6PE that is allowed to do so. As such, one AS may be
vulnerable to label spoofing in a different AS. The same issue
equally applies to the option (c) of Section 10 of [RFC4364]. Just
as it is the case for [RFC4364], addressing this particular security
issue is for further study.
6. Acknowledgements
We wish to thank Gerard Gastaud and Eric Levy-Abegnoli who
contributed to this document. We also wish to thank Tri T. Nguyen,
who initiated this document, but unfortunately passed away much too
soon. We also thank Pekka Savola for his valuable comments and
suggestions.
De Clercq, et al. Standards Track [Page 10]
RFC 4798 6PE February 2007
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2545] Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
Extensions for IPv6 Inter-Domain Routing", RFC 2545, March
1999.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC3036] Andersson, L., Doolan, P., Feldman, N., Fredette, A., and
B. Thomas, "LDP Specification", RFC 3036, January 2001.
[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.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760, January
2007.
7.2. Informative References
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
Protocol Label Switching (MPLS) Support of Differentiated
Services", RFC 3270, May 2002.
[RFC4029] Lind, M., Ksinant, V., Park, S., Baudot, A., and P.
Savola, "Scenarios and Analysis for Introducing IPv6 into
ISP Networks", RFC 4029, March 2005.
[RFC4182] Rosen, E., "Removing a Restriction on the use of MPLS
Explicit NULL", RFC 4182, September 2005.
De Clercq, et al. Standards Track [Page 11]
RFC 4798 6PE February 2007
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[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.
De Clercq, et al. Standards Track [Page 12]
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Authors' Addresses
Jeremy De Clercq
Alcatel-Lucent
Copernicuslaan 50
Antwerpen 2018
Belgium
EMail: jeremy.de_clercq@alcatel-lucent.be
Dirk Ooms
OneSparrow
Belegstraat 13
Antwerpen 2018
Belgium
EMail: dirk@onesparrow.com
Stuart Prevost
BT
Room 136 Polaris House, Adastral Park, Martlesham Heath
Ipswich Suffolk IP5 3RE
England
EMail: stuart.prevost@bt.com
Francois Le Faucheur
Cisco
Domaine Green Side, 400 Avenue de Roumanille
Biot, Sophia Antipolis 06410
France
EMail: flefauch@cisco.com
De Clercq, et al. Standards Track [Page 13]
RFC 4798 6PE February 2007
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De Clercq, et al. Standards Track [Page 14]