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Network Working Group                                        Tom Worster
Internet Draft
Expiration Date: February 2003
                                                           Yakov Rekhter
                                                  Juniper Networks, Inc.

                                                   Eric C. Rosen, editor
                                                     Cisco Systems, Inc.

                                                             August 2002


                    Encapsulating MPLS in IP or GRE


                  draft-rosen-mpls-in-ip-or-gre-00.txt

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
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
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Abstract

   In various applications of MPLS, label stacks with multiple entries
   are used.  In some cases, it is possible to replace the top label of
   the stack with an IP-based encapsulation, thereby enabling the
   application to run over networks which do not have MPLS enabled in
   their core routers.  This draft specifies two IP-based
   encapsulations, MPLS-in-IP, and MPLS-in-GRE.  Each of these is
   applicable in some circumstances.





Worster, et al.                                                 [Page 1]

Internet Draft    draft-rosen-mpls-in-ip-or-gre-00.txt       August 2002




Table of Contents

    1      Motivation  .............................................   2
    2      Encapsulation in IP  ....................................   3
    3      Encapsulation in GRE  ...................................   4
    4      Common Procedures  ......................................   4
    4.1    Fragmentation, Reassembly, and MTU  .....................   4
    4.2    TTL  ....................................................   5
    4.3    EXP and DSCP fields  ....................................   5
    5      Applicability  ..........................................   5
    6      Security Considerations  ................................   6
    7      Acknowledgments  ........................................   6
    8      References  .............................................   6
    9      Author Information  .....................................   7






1. Motivation

   In many applications of MPLS, packets traversing an MPLS backbone
   carry label stacks with more than one label.  As described in
   [RFC3031], section 3.15, each label represents a Label Switched Path
   (LSP).  For each such LSP, there is a Label Switching Router (LSR)
   which is the "LSP Ingress", and an LSR which is the "LSP Egress".  If
   LSRs A and B are the Ingress and Egress, respectively, of the LSP
   corresponding to a packet's top label, then A and B are adjacent LSRs
   on the LSP corresponding to the packet's second label (i.e., the
   label immediately beneath the top label)

   The purpose (or one of the purposes) of the top label is to get the
   packet delivered from A to B, so that B can further process the
   packet based on the second label.  In this sense, the top label
   serves as an encapsulation header for the rest of the packet.  In
   some cases the top label can be replaced, without loss of
   functionality, by other sorts of encapsulation headers.  For example,
   the top label could be replaced by an IP header or a GRE header.  As
   the encapsulated packet would still be an MPLS packet, the result is
   an MPLS-in-IP or MPLS-in-GRE encapsulation.

   With these encapsulations, it is possible for two LSRs that are
   adjacent on an LSP to be separated by an IP network, even if that IP
   network does not provide MPLS.




Worster, et al.                                                 [Page 2]

Internet Draft    draft-rosen-mpls-in-ip-or-gre-00.txt       August 2002


2. Encapsulation in IP

   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 [RFC791] and [RFC2460] 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
                [RFC3032].

          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 packet,

     - Y indicates an MPLS multicast packet.  (The use of the MPLS-in-IP
       encapsulation for MPLS multicast packets is for further study.)

   Following the IP header is an MPLS packet, as specified in [RFC3032].
   This encapsulation causes MPLS packets to be sent through "IP
   tunnels".  When a packet is received by the tunnel's receive
   endpoint, the receive endpoint decapsulates the MPLS packet by
   removing the IP header.  The packet is then processed as a received
   MPLS packet whose "incoming label" [RFC3031] is the topmost packet of
   the decapsulated packet.



Worster, et al.                                                 [Page 3]

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3. Encapsulation in GRE

   The MPLS-in-GRE encapsulation encapsulates an MPLS packet in GRE
   [RFC2784].  The packet then consists of an IP header followed by a
   GRE header followed by an MPLS label stack as specified in [RFC3032].
   The protocol type field in the GRE header MUST be set to the
   Ethertype value for MPLS Unicast (0x8847) or Multicast (0x8848).  The
   optional GRE checksum, key [RFC2890] and sequence number [RFC2890]
   fields MUST NOT be used.

   This encapsulation causes MPLS packets to be sent through "GRE
   tunnels". When a packet is received by the tunnel's receive endpoint,
   the receive endpoint decapsulates the MPLS packet by removing the IP
   header and the GRE header.  The packet is then processed as a
   received MPLS packet whose "incoming label" [RFC3031] is the topmost
   packet of the decapsulated packet.


4. Common Procedures

   Certain procedures are common to both the MPLS-in-IP and the MPLS-
   in-GRE encapsulations.  In the following, the encapsulator, whose
   address appears in the IP source address field of the encapsulating
   IP header,  is known as the "tunnel head".  The decapsulator, whose
   address appears in the IP destination address field of the
   decapsulating IP header, is known as the "tunnel tail".


4.1. Fragmentation, Reassembly, and MTU

   If an MPLS-in-IP or MPLS-in-GRE packet were to get fragmented (due to
   "ordinary" IP fragmentation), it would have to be be reassembled by
   the tunnel tail before the contained MPLS packet be decapsulated.  To
   avoid the need for the tunnel tail to perform reassembly, the tunnel
   head MUST set the Don't Fragment flag of the encapsulating IPv4
   header.

   The tunnel head SHOULD perform Path MTU Discovery [RFC1191] over each
   MPLS-in-IP and MPLS-in-GRE tunnel.

   The tunnel head MUST maintain a Tunnel MTU value for each MPLS-in-IP
   or MPLS-in-GRE tunnel. This is the minimum of (a) an administratively
   configured value, and, if known, (b) the discovered Path MTU value
   minus the encapsulation overhead.

   If the tunnel head receives, for encapsulation, an MPLS packet whose
   size exceeds the Tunnel MTU, that packet MUST be discarded.




Worster, et al.                                                 [Page 4]

Internet Draft    draft-rosen-mpls-in-ip-or-gre-00.txt       August 2002


   In some cases, the tunnel head receives, for encapsulation, an IP
   packet, which it first encapsulates in MPLS and then encapsulates in
   MPLS-in-IP or MPLS-in-GRE.  If the source of the IP packet is
   reachable from the tunnel head, and if the result of this
   encapsulation would be a packet whose size exceeds the Tunnel MTU,
   then the tunnel head SHOULD use the Tunnel MTU value for the purposes
   of fragmentation and PMTU discovery outside the tunnel.


4.2. TTL

   The tunnel head MAY place the TTL from the MPLS label stack into the
   encapsulating IP header.  The tunnel tail MAY place the TTL from the
   encapsulating IP header into the MPLS header, but only if that does
   not cause the TTL value in the MPLS header to become smaller.

   Whether such modifications are made, and the details of how they are
   made, will depend on the configuration of the tunnel tail and the
   tunnel head.


4.3. EXP and DSCP fields

   The tunnel head MAY consider the EXP field of the encapsulated MPLS
   packet when setting the DSCP field of the encapsulating IP header.
   The tunnel tail MAY modify the EXP field of the encapsulated MPLS
   packet, based on consideration of the DSCP field of the encapsulating
   IP header.

   Whether such modifications are made, and the details of how they are
   made, will depend on the configuration of the tunnel tail and the
   tunnel head.


5. Applicability

   The MPLS-in-IP encapsulation is the more efficient, and would
   generally be regarded as preferable, other things being equal.  There
   are however some situations in which the MPLS-in-GRE encapsulation
   may be used:

     - Two routers are "adjacent" over a GRE tunnel that exists for some
       reason that is outside the scope of this document, and those two
       routers need to send MPLS packets over that adjacency.  As all
       packets sent over this adjacency must have a GRE encapsulation,
       the MPLS-in-GRE encapsulation is more efficient than the
       alternative, which would be an MPLS-in-IP encapsulation which is
       then encapsulated in GRE.



Worster, et al.                                                 [Page 5]

Internet Draft    draft-rosen-mpls-in-ip-or-gre-00.txt       August 2002


     - Implementation considerations may dictate the use of MPLS-in-GRE.
       For example, some hardware device might only be able to handle
       GRE encapsulations in its fastpath.


6. Security Considerations

   MPLS-in-IP or MPLS-in-GRE tunnels may be secured using IPsec.  If
   they are not secured using IPsec, then some other method should be
   used to ensure that packets are decapsulated and forwarded by the
   tunnel tail only if those packets were encapsulated by the tunnel
   head.  This can be done by address filtering at the boundaries of an
   administrative domain.  When the tunnel head and the tunnel tail are
   not in the same domain, this may become difficult, and it can even
   become impossible if the packets must traverse the public Internet.


7. Acknowledgments

   This draft is a combination of two previous drafts:

     - draft-worster-mpls-in-ip, by Tom Worster, Paul Doolan, Yasuhiro
       Katsube, Tom K. Johnson, Andrew G. Malis, and Rick Wilder

     - draft-rekhter-mpls-over-gre, by Yakov Rekhter, Daniel Tappan, and
       Eric Rosen

   The current authors wish to thank all these authors for their
   contribution.


8. References

   [RFC7915] "Internet Protocol," J. Postel, Sep 1981

   [RFC2460]"Internet Protocol, Version 6 (IPv6) Specification," S.
   Deering and R. Hinden, RFC 2460,Dec 1998

   [RFC1191] "Path MTU Discovery", J.C. Mogul, S.E. Deering, November
   1990

   [RFC2784] "Generic Routing Encapsulation (GRE)", D. Farinacci, T. Li,
   S. Hanks, D. Meyer, P. Traina, March 2000

   [RFC2890] "Key and Sequence Number Extensions to GRE", G. Dommety,
   August 2000

   [RFC3031] "Multiprotocol Label Switching Architecture", E. Rosen, A.



Worster, et al.                                                 [Page 6]

Internet Draft    draft-rosen-mpls-in-ip-or-gre-00.txt       August 2002


   Viswanathan, R. Callon, January 2001

   [RFC3032] "MPLS Label Stack Encoding",  E. Rosen, D. Tappan, G.
   Fedorkow, Y. Rekhter, D. Farinacci, T. Li, A. Conta. January 2001


9. Author Information


      Tom Worster
      Email: fsb@thefsb.org



      Yakov Rekhter
      Juniper Networks, Inc.
      1194 N. Mathilda Ave.
      Sunnyvale, CA 94089
      Email: yakov@juniper.net



      Eric Rosen
      Cisco Systems, Inc.
      250 Apollo Drive
      Chelmsford, MA, 01824
      e-mail: erosen@cisco.com
























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