INTERNET DRAFT J. De Clercq, D. Ooms
<draft-ooms-v6ops-bgp-tunnel-03.txt> Alcatel
S. Prevost
BTexact
F. Le Faucheur
Cisco
April, 2004
Expires October, 2004
Connecting IPv6 Islands over IPv4 MPLS
using IPv6 Provider Edge Routers (6PE)
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document explains how to interconnect IPv6 islands over a
Multi-Protocol 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
Multi-Protocol 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.
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1. Introduction
There are several approaches for providing IPv6 connectivity over an
MPLS core network [ISPSCEN]: (i) having the MPLS networks deploy
native IPv6 support, (ii) require that MPLS networks support setting
up IPv6 LSPs and set up IPv6 connectivity by using either these or
configured tunneling, (iii) use only configured tunneling over IPv4
LSPs, or (iv) use the IPv6 Provider Edge (PE) approach.
This document specifies operations of the 6PE approach for
interconnection of IPv6 islands over an IPv4 MPLS cloud. The approach
requires the edge routers that are connected to IPv6 islands to be
Dual Stack MP-BGP-speaking routers 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 don't require any explicit tunnel
configuration.
Throughout this document, the terminology of [IPV6] and [VPN] is
used.
In this document an 'IPv6 island' is an IPv6-upgraded network. A
typical example of an island would be a customer's IPv6 site
connected via its IPv6 Customer Edge (CE) router to one (or more)
Dual Stack Provider Edge (PE) router(s) of a Service Provider. These
Provider Edge routers are connected to an MPLS core network.
+--------+
|site A CE---+ +-----------------+
+--------+ | | | +--------+
PE-+ IPv4 MPLS core +-PE---CE site C |
+--------+ | | | +--------+
|site B CE---+ +-----------------+
+--------+
IPv6 islands IPv4 cloud IPv6 island
<-------------><---------------------><-------------->
The interconnection method described in this document typically
applies to an ISP that has an MPLS network and 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
configured IPv6-over-IPv4 tunnelling. With the 6PE approach described
here, the provider only has to upgrade some Provider Edge (PE)
routers to Dual Stack MP-BGP routers (6PE routers) while leaving the
IPv4 MPLS core routers untouched. These 6PE routers provide
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connectivity to IPv6 islands. They may also provide other services
simultaneously (IPv4 connectivity, IPv4 L3VPN services, L2VPN
services, etc.)
The ISP must obtain IPv6 connectivity to its peers and upstreams
using means outside of the scope of this memo, and have its 6PE
routers readvertise it over the 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 methods 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.
The scenario is also described in [ISPSCEN].
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 [KEYWRD].
2. Protocol Overview
Each IPv6 site is connected to at least one Dual Stack MP-BGP-
speaking 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 have at least one IPv4 address on the IPv4 side and at least one
IPv6 address on the IPv6 side. The IPv4 address MUST be routable in
the IPv4 cloud.
The 6PE routers that are attached to IPv6 islands need to insert a
route (normally a /32 IPv4 address prefix) providing reachability to
themselves (i.e. to their "IPv4 address") into the IGP routing tables
of the IPv4 backbone. This enables MPLS, at each node in the backbone
network, to assign an MPLS label corresponding to the route to each
6PE router. 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 will be injected in the IPv4 cloud.
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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).
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 [MP-BGP]:
The 6PE routers MUST exchange the IPv6 prefixes over MP-BGP
sessions running over IPv4. In doing so, the 6PE routers convey
their IPv4 address as the BGP Next Hop for the advertised IPv6
prefixes. Since MP-BGP assumes that the BGP Next Hop is of the
same address family as the NLRI, the IPv4 address needs to be
embedded in an IPv6 format. The IPv4-mapped IPv6 address is
defined in [V6ADDR] as an "address type used to represent the
addresses of IPv4 nodes as IPv6 addresses" which precisely fits
the above purpose. Therefore, the IPv4 address of the egress 6PE
router MUST be encoded as an IPv4-mapped IPv6 address in the BGP
Next Hop field.
(2) Tunneling IPv6 packets from Ingress 6PE router to Egress 6PE
router via MPLS LSPs:
The Ingress 6PE router MUST tunnel IPv6 data over the IPv4 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.
The MP-BGP AFI used in step (1) above MUST be IPv6 (value 2). The
MP-BGP SAFI is discussed below in the tunneling section.
As required by BGP specification, PE routers must form a full peering
mesh unless Route Reflectors are used.
3. Tunneling over MPLS LSPs
In this approach, the IPv4-mapped IPv6 addresses allow a 6PE router
that has to forward a packet to automatically determine the IPv4
endpoint of the tunnel by looking at the MP-BGP routing information.
Note that even when the number of peers is high, the number of
tunnels is not a scalability concern from an operational viewpoint
since those tunnels are set up automatically.
The IPv4 MPLS LSPs can be established using any existing technique
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(LDP, RSVP-TE, ...).
When tunneling IPv6 packets over the IPv4 MPLS backbone, rather than
successively prepend an IPv4 header and then perform label imposition
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 corresponds to the IPv4 LSP
starting on the ingress 6PE Router and ending on the egress 6PE
Router.
While this approach could operate in some situations using a single
level of labels, there are significant advantages in using a second
level of labels which are bound to IPv6 prefixes via MP-BGP
advertisements in accordance with [LABEL].
For instance, use of a second level label allows Penultimate Hop
Popping (PHP) on the Label Switch Router (LSR) upstream of the egress
6PE router without any IPv6 capabilities/upgrade on the penultimate
router even when the IPv6 packet is directly encapsulated in MPLS
(without an IPv4 header); since it still transmits MPLS packets even
after the PHP (instead of having to transmit IPv6 packets and
encapsulate them appropriately).
Also, an existing IPv4 LSP which is using "IPv4 Explicit NULL label"
over the last hop (say because that LSP is already used to transport
IPv4 traffic with the Pipe Diff-Serv Tunneling Model as defined in
[MPLS-DS]) could not be used to carry IPv6 with a single label since
the "IPv4 Explicit NULL label" can not be used to carry native IPv6
traffic (see [MPLS-STACK]), while it could be used to carry labeled
IPv6 traffic (see [EXP-NULL]).
Therefore, this approach MUST be used with a second label, advertised
with BGP in accordance with [LABEL].
The SAFI used in MP-BGP MUST be the "label" SAFI (4). The 'bottom
label' (i.e. the second label when no PHP is used, or the only
remaining label when PHP is used) indicates to the Egress 6PE Router
that the packet is an IPv6 packet. The bottom label advertised by the
Egress 6PE Router with MP-BGP MAY be an arbitrary label value and MAY
identify 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.
4. Crossing Multiple IPv4 Autonomous Systems
When the IPv6 islands are separated by multiple IPv4 Autonomous
Systems, two cases can be distinguished:
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1. The border routers between the IPv4 ASes are not 6PE routers, i.e
they are IPv4-only BGP routers. The 6PE routers of the IPv6 islands
from the different IPv4 ASes will be configured as multi-hop MP-EBGP
peers for the exchange of IPv6 reachability. Alternatively, where the
total number of such 6PE routers is high, IPv6 reachability across
ASes can be achieved via MP-BGP connection of Route Reflectors in
different ASes. Labeled IPv4 routes for the 6PE routers are exchanged
across ASes so that direct inter-AS LSPs can be used to tunnel
traffic across ASes from ingress 6PE router to egress 6PE router.
Note that the exchange of IPv6 routes can only start after BGP has
created IPv4 connectivity between the ASes.
2. The border routers between the IPv4 ASes are 6PE routers. Each of
these border 6PE routers will peer with the 6PE routers in its AS and
regular IPv6 routing will take place between the two ASes. No inter-
AS LSPs are used. There is effectively a separate mesh of LSPs
across the 6PE Routers of each AS.
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.
Acknowledgements
We wish to thank Gerard Gastaud and Eric Levy-Abegnoli who
contributed to this document, and we wish to thank Tri T. Nguyen who
initiated this document, but who unfortunately passed away much too
soon. We also thank Pekka Savola for his valuable comments and
suggestions.
Normative References
[IPV6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC2460.
[KEYWRD] S. Bradner, Key words for use in RFCs to Indicate Require-
ment Levels, RFC2119, March 1997.
[LABEL] Rekhter Y., E. Rosen, "Carrying Label Information in BGP-
4", RFC 3107, May 2001.
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[MP-BGP] T. Bates, R. Chandra, D. Katz, Y. Rekhter, "Multiprotocol
Extensions for BGP-4", RFC 2858.
[V6ADDR] Deering, S., and R. Hinden, "IP Version 6 Addressing Archi-
tecture", RFC 3513
Informative References
[ISPSCEN] Lind M., et al., "Scenarios and Analysis for Introducing
IPv6 into ISP Networks", draft-ietf-v6ops-isp-
scenarios-analysis, (work in progress)
[EXP-NULL] Rosen, E., et al., "Removing a Restriction on the use of
MPLS Explicit NULL", draft-rosen-mpls-explicit-null-
01.txt, work in progress
[MPLS-DS] Le Faucheur et al., "MPLS Support for DiffServ", RFC
3270
[MPLS-STACK] Rosen, E., et al., "MPLS Label Stack Encoding", RFC 3032
[V6VPN] Nguyen T., Gastaud G., De Clercq J., Ooms D.,"BGP-MPLS
VPN extension for IPv6 VPN over an IPv4 infrastructure",
draft-ietf-l3vpn-bgp-ipv6 (work in progress).
[VPN] Rosen E., Rekhter Y., Brannon S., Chase C., De Clercq
J., Hitchin P., Marshall , Srinivasan V., "BGP/MPLS
VPNs", draft-ietf-l3vpn-rfc2547bis (work in progress).
Authors' Addresses
Dirk Ooms
Alcatel
Fr. Wellesplein 1, 2018 Antwerp, Belgium
E-mail: dirk.ooms@alcatel.be
Jeremy De Clercq
Alcatel
Fr. Wellesplein 1, 2018 Antwerp, Belgium
E-mail: jeremy.de_clercq@alcatel.be
Stuart Prevost
BTexact Technologies
Room 136 Polaris House, Adastral Park,
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Martlesham Heath, Ipswich, Suffolk IP5 3RE, England
E-mail: stuart.prevost@bt.com
Francois Le Faucheur
Cisco Systems
Domaine Green Side, 400, Avenue de Roumanille, Batiment T3
06 410 BIOT, SOPHIA ANTIPOLIS, FRANCE
E-mail: flefauch@cisco.com
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APPENDIX A
[RFC-editor note: remove before publication]
Changes
ngtrans history (draft-ietf-ngtrans-bgp-tunnel-0x.txt)
00->01: editorial changes
extended section 4
01->02: editorial changes
added tunnel-specific considerations
added case of multiple IPv4 domains between IPv6 islands
added discussion on v6[v4]addresses in appendix A
02->03: complete rewrite: it turned out that two interpretations
of the previous drafts existed, the two different
interpretations are described explicitly in this version
03->04: renaming of the two approaches
editorial changes
clearly indicate which part requires standards track
04->05: added 5.1.3 to clarify how DS-BGP routers agree on tunnel
type
v6ops history (draft-ooms-v6ops-bgp-tunnel-0x.txt)
05->00 individual submission: no changes. The document passed
ngtrans last call early 2002, but the transfer to the IESG
was postponed because of the reorg and closing down of
ngtrans.
00->01 no changes
01->02 according to v6ops mailing list discussion, the scope of
the document was restricted to the "MP-BGP over IPv4 using
LSPs" approach.
02->03 adopted various comments
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