INTERNET DRAFT                           Paul Doolan, Ennovate Networks
Network Working Group                    Yasuhiro Katsube, Toshiba Corp
                              Tom K. Johnson, Litchfield Communications
                                       Andrew G. Malis, Vivace Networks
                                                   Rick Wilder, Masergy
                                         Tom Worster, Ennovate Networks
                                              Expires January 20th 2002

                        MPLS Label Stack Encapsulation in IP

    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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    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

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

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  Table of Contents

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

    2. MPLS-in-IP protocol specification ............................ 4

    3. Usage considerations ......................................... 5

    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|
         +----+        +----+----+        +----+----+        +----+
         +----+        +----+----+        +----+----+        +----+
          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:

      Signalled multi-protocol tunnelling
             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 multiplexing of
             multiple parallel protocol sessions, e.g. remote access, IP
             Virtual Private Networks with potentially overlapping
             addresses, or multi-hop voice-over-IP headers compression.

  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

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2. MPLS-in-IP protocol specification

  MPLS-in-IP messages have the following format:

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

       IP Header
             This field contains an IPv4 or an IPv6 datagram header
             as defined in [5] and [6] respectively. 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

       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.

  Allocation of an IP Protocol Number for MPLS unicast messages is
  required by MPLS-in-IP encapsulation.

  For the time being there appears to be no need to allocate an IP
  Protocol Number for MPLS multicast messages.

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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

  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

  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

4. Security Considerations

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


     [1]  E. Rosen et al, "Multiprotocol Label Switching
          Architecture," RFC 3031, Jan 2001.

     [2]  E. Rosen et al, "MPLS Label Stack Encoding," RFC 3032, Jan

     [3]  L. Andersson et al, "LDP Specification," RFC 3036, Jan

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     [4]  Y. Rekhter and E. Rosen, "Carrying Label Information in
          BGP-4," RFC 3107, May 2001.

     [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]  D. Farinacci et al, "Generic Routing Encapsulation (GRE),"
          RFC 2784, March 2000.

Authors' Addresses

   Paul Doolan
   Ennovate Networks, Inc.
   60 Codman Hill Road
   Boxborough, Mass, 01719
   Tel: +1 978 206 0529
   Fax: +1 978 263 1099

   Tom K. Johnson
   Tel: 860-945-1573
   Fax: 860-945-1578
   Litchfield Communications, Inc.
   76 Westbury Park Road
   Watertown, CT, 06795

   Yasuhiro Katsube
   Toshiba Corporation
   1, Toshiba-cho,
   Fuchu, Tokyo 183-8511
   Tel: +81 42 333 2884
   Fax: +81 42 340 8059

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

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   Rick Wilder
   Masergy, Inc.
   2901 Telestar Ct.
   Falls Church, VA 22042
   Tel: 571 217 5408

   Tom Worster  (contact for comments)
   Ennovate Networks, Inc.
   60 Codman Hill Road
   Boxborough, Mass, 01719
   A.I.M.: "the fsb"
   Tel: +1 978 206 0490
   Fax: +1 978 263 1099

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