INTERNET DRAFT                                     J. De Clercq, D. Ooms
<draft-ooms-v6ops-bgp-tunnel-03.txt>                             Alcatel
                                                              S. Prevost
                                                          F. Le Faucheur
                                                             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.

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at

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   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

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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

   Throughout this document, the terminology of [IPV6] and [VPN] is

   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",
   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

      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

   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.


   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

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

   [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
   Fr. Wellesplein 1, 2018 Antwerp, Belgium

   Jeremy De Clercq
   Fr. Wellesplein 1, 2018 Antwerp, Belgium

   Stuart Prevost
   BTexact Technologies
   Room 136 Polaris House, Adastral Park,

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   Martlesham Heath, Ipswich, Suffolk IP5 3RE, England

   Francois Le Faucheur
   Cisco Systems
   Domaine Green Side, 400, Avenue de Roumanille, Batiment T3

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   [RFC-editor note: remove before publication]

   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

   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
   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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