INTERNET DRAFT                              T Worster, Ennovate Networks
Network Working Group                            Y Katsube, Toshiba Corp
                                                A Malis, Vivace Networks
                                              R Wilder, Broadband Office

                                                    Expires Feb 4th 2001


                       MPLS Label Stack Encapsulation in IP

                        <draft-worster-mpls-in-ip-02.txt>


     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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Abstract

     Several useful applications of MPLS tunnels based on LSPs with
     second level labels between non adjacent LSRs have been
     identified: IP-VPNs and VoIP over MPLS are just two examples. This
     tunnelling technique can easily be extended to non-MPLS core
     networks.

     This Internet-Draft explains the motivation for encapsulating MPLS
     messages in IP and provides the protocol specification of the
     encapsulation.



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   Table of Contents

    1. Motivation ................................................... 2

    2. MPLS-in-IP protocol specification ............................ 3

    3. Usage considerations ......................................... 4

    4. Security Considerations ...................................... 5



1.  Motivation

   MPLS provides not only for the label based forwarding of datagrams
   by label switching routers (LSRs) but also, through the use of a
   second or higher level labels, for the labelled forwarding of
   messages between non-adjacent LSRs [1]. This capability may be
   used for general purpose tunnelling between non-adjacent LSRs.
   Using extended MPLS signalling (e.g. [3] or [4]) and the label
   stacking technique, a pair LSRs may establish tunnels on demand
   without disturbing the intervening LSRs. Figure 1 illustrates the
   "labelled tunnelling" technique.


      +----+                                              +----+
      |L2=a|                                              |L2=a|
      +----+        +----+----+        +----+----+        +----+
      |L1=x|--------|L1=x|L1=y|--------|L1=y|L1=z|--------|L1=z|
      +----+        +----+----+        +----+----+        +----+

       LSR-1            LSR-2              LSR-3           LSR-4
          Figure 1 - Labelled tunnelling over an MPLS network
                         using a label stack

   In this example, an LSP exists between LSR-1 and LSR-4 that is
   label switched through LSRs-2 and -3. This LSP has labels x, y and
   z on the respective data-links between the LSRs, as shown.
   Additionally, LSRs-1 and -4 are directly connected via an LSP with
   the label a. (The label having been distributed via an extended
   MPLS signalling session, such as LDP or BGP-4, between LSRs-1 and
   -4.) This LSP may be used as a "labelled tunnel."

   Examples of the utility of this kind of MPLS tunnelling include:

       Tunnelling of arbitrary protocols.
          By adding FEC types to MPLS signalling, MPLS can be used to


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             tunnel arbitrary protocols. Alternatively, consistent
             configuration of LSRs may be used to associate specific
             label spaces with specific protocols. For the tunnelling of
             vendor specific protocols the opaque FEC type together with
             LDP's vendor specific TLVs may be used to indicate the
             encapsulated protocol type.

       Tunnelling of multiple protocol sessions.
             Extended MPLS signalling allows the efficient establishment
             and tear-down of tunnels between a pair of LSRs. This
             facility has value in the support of certain protocol
             stacking techniques that require the separation of multiple
             parallel protocol sessions, such as in remote access or
             virtual private IP networks using potentially overlapping
             addresses.

   The MPLS-in-IP encapsulation specified in Section 2 allows the use
   of labelled tunnelling in those situations in which the
   intervening network nodes are not MPLS LSRs. Figure 2 contrasts
   this technique with the label stacking technique shown in Figure
   1. The inherent protocol layering hides the differences between
   labelled tunnelling over MPLS (Figure 1) and labelled tunnelling
   over IP (Figure 2) from the tunnelled protocol layer and layers
   above, and from the extended MPLS signalling session between LSR-1
   and LSR-2.


      +----+                                              +----+
      |L1=a|                                              |L1=a|
      +----+                                              +----+
      |MiIP|                                              |MiIP|
      +----+        +---------+        +---------+        +----+
      | IP |--------|    IP   |--------|    IP   |--------| IP |
      +----+        +---------+        +---------+        +----+

       LSR-1           Router             Router           LSR-2

           Figure 2 - Labelled tunnelling over an IP network using
                      MPLS-in-IP (MiIP) encapsulation

Thus an MPLS-in-IP encapsulation extends the applicability of
extended MPLS signalling and labelled tunnelling to use over non-MPLS
clouds.


2.  MPLS-in-IP protocol specification

   MPLS-in-IP messages have the following format:

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               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               |                                     |
               |             IP Header               |
               |                                     |
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               |                                     |
               |          MPLS Label Stack           |
               |                                     |
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               |                                     |
               |            Message Body             |
               |                                     |
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       IP Header
                This field contains an IPv4 or an IPv6 datagram header
                as defined in [5] and respectively [6]. The source and
                destination addresses are set to addresses of the
                encapsulating and decapsulating LSRs respectively.

       MPLS Label Stack
                This field contains an MPLS Label Stack as defined in
                [2].

       Message Body
                This field contains one MPLS message body.

   The Protocol Number field in an IPv4 header and the Next Header
   field in an IPv6 are set as follows:

       X        indicates an MPLS unicast message,

       Y        indicates an MPLS multicast message.


3.  Usage considerations

   MPLS-in-IP is useful when an MPLS tunnel is useful but where an
   MPLS network between the tunnel end-points is not available. It
   should be noted, however, that certain capabilities often connoted
   with MPLS are not available with MPLS-in-IP. Firstly, RSVP and CR-
   LDP cannot provide resource allocation (e.g. bandwidth allocation)
   for the tunnels since the signalling does not interact with the
   network between the tunnel endpoints. Other techniques applicable
   at the IP level, such as Diff-Serv or RSVP/Int-Serv, may be used
   in conjunction with MPLS-in-IP. Secondly, in MPLS-in-IP, RSVP and
   CR-LDP signalling cannot provide control of a source route for the
   tunnels.

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   LDP and BGP directly support sessions between non-adjacent nodes.
   If, however, RSVP is to be used for control of MPLS-in-IP tunnels,
   RSVP packets requiring router alert should be encapsulated using
   IP-in-IP and addressed to the remote tunnel end-point.

   The source and destination addresses in the IP Header of MPLS-in-
   IP messages may be any of the respective encapsulating and
   decapsulating LSRs' addresses. Usually the LSR Ids will be
   suitable.

   MPLS-in-IP encapsulation is not normally appropriate if an MPLS
   messages needs to be forwarded over a GRE tunnel [7]. In this case
   GRE encapsulation with the Protocol Type set to the corresponding
   ethertype (MPLS Unicast = 0x8847 and MPLS Multicast = 8848) is
   preferable.


4.  Security Considerations

   Particular security precautions applicable to MPLS LSRs and LERs
   are applicable also when MPLS-in-IP encapsulation is used.


References

     [1]  E. Rosen et al, "Multiprotocol Label Switching
              Architecture," Internet-Draft draft-ietf-mpls-arch-06,
              work in progress, Aug 1999.

     [2]  E. Rosen et al, "MPLS Label Stack Encoding," Internet-
              Draft draft-ietf-mpls-label-encaps-07, work in progress,
              Sep 1999.

     [3]  L. Andersson et al, "LDP Specification," Internet-Draft
              draft-ietf-mpls-ldp-08.txt, work in progress, Jun 2000.

     [4]  E. Rosen et al, "Carrying Label Information in BGP-4,"
              Internet Draft draft-ietf-mpls-bgp4-mpls-04, Jan 2000.

     [5]  J. Postel, "Internet Protocol," STD 5, RFC 791, Sep 1981.

     [6]  S. Deering and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification," RFC 2460, Dec 1998.

     [7]  S. Hanks et al, "Generic Routing Encapsulation (GRE)," RFC
              1701, October 1994.




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Authors' Addresses

   Tom Worster  (contact for comments)
   Ennovate Networks, Inc.
   60 Codman Hill Road
   Boxborough, Mass, 01719
   Email: tom@ennovatenetworks.com
   Tel: +1 978 206 0490
   Fax: +1 978 263 1099

   Yasuhiro Katsube
   Toshiba Corporation
   1, Toshiba-cho,
   Fuchu, Tokyo 183-8511
   Email: yasuhiro.katsube@toshiba.co.jp
   Tel: +81 42 333 2884
   Fax: +81 42 340 8059

   Andrew G. Malis
   Vivace Networks
   2730 Orchard Parkway
   San Jose, CA 95134
   Email: Andy.Malis@vivacenetworks.com
   Tel: +1 408 383 7223
   Fax: +1 408 904 4748

   Rick Wilder
   Broadband Office, Inc.
   2900 Telestar Ct.
   Falls Church, VA 22042
   Tel: +1 703 641 6111
   Fax: +1 703 641 ΒΆ
   Email: rwilder@bbo.com

















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