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Versions: 00                                                            
Network Working Group                                   W. Mark Townsley
Internet-Draft                                             cisco Systems
<draft-townsley-mpls-over-l2tpv3-00.txt>                       Ted Seely
December 2004                                                     Sprint
                                                        Jeffery S. Young

    Encapsulation of MPLS over Layer 2 Tunneling Protocol Version 3

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   The Layer 2 Tunneling Protocol, Version 3, (L2TPv3) defines a
   protocol for tunneling a variety of payload types over IP networks.
   This document defines how to carry an MPLS label or label stack and
   its payload over L2TPv3. This enables an application which
   traditionally requires an MPLS-enabled core network to utilize an
   L2TPv3 encapsulation over an IP network instead.

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   Status of this Memo..........................................    1

   1. Introduction..............................................    2

   2. MPLS over L2TPv3 Encoding.................................    2

   3. Assigning the L2TPv3 Session ID and Cookie................    4

   4. Applicability.............................................    4

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

   6. IANA Considerations.......................................    6

   7. Acknowledgments...........................................    6

   8. References................................................    6
      8.1 Normative References..................................    6
      8.2 Informative References................................    6

   9. Contacts..................................................    6

Specification of Requirements

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.  The key
   "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document
   are to be interpreted as described in [RFC2119].

1. Introduction

   This document defines how to encapsulate an MPLS label or label stack
   and its payload over L2TPv3. After defining the MPLS over L2TPv3
   encapsulation procedure, other MPLS over IP encapsulation options
   including IP, GRE and IPsec are discussed in context with MPLS over
   L2TPv3 in an Applicability section. This document only describes
   encapsulation and does not concern itself with all possible MPLS-
   based applications which may be enabled over L2TPv3.

2. MPLS over L2TPv3 Encoding

   MPLS over L2TPv3 allows tunneling of an MPLS stack [RFC3032] over an
   IP network utilizing the L2TPv3 encapsulation defined in [RFC3931].

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         |        IP         |
         |      L2TPv3       |
         | MPLS Label Stack  |

   Figure 2.1 MPLS Stack over L2TPv3/IP

   The L2TPv3 encapsulation carrying a single MPLS label is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |                          Session ID                           |
   |               Cookie (optional, maximum 64 bits)...
                                 ...                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Label
   |                Label                  | Exp |S|       TTL     | Stack
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entry

             Figure 2.2 MPLS label over L2TPv3 encapsulation

   Session ID

      The L2TPv3 Session ID is a 32-bit identifier field locally
      selected as a lookup key for the context of an L2TP Session.  An
      L2TP Session contains necessary context for processing a received
      L2TP packet. At a minimum, such context contains whether the
      Cookie (see description below) is present, the value it was
      assigned, as well as what type of tunneled encapsulation follows
      (i.e., Frame Relay, Ethernet, MPLS, etc).


      The L2TPv3 Cookie field contains a variable length (maximum 64
      bits) randomly assigned value.  It is intended to provide an
      additional level of guarantee that a data packet has been directed
      to the proper L2TP session by the Session ID.  While the Session
      ID may be encoded and assigned any value (perhaps optimizing for
      local lookup capabilities, redirection in a distributed forwarding
      architecture, etc.), the Cookie MUST be selected as a
      cryptographically random value [RFC1750], with the added
      restriction that it not be the same as a recently used value for a
      given Session ID.  A well-chosen Cookie will prevent inadvertent

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      misdirection of a stray packet containing a recently reused
      Session ID, a Session ID that is subject to packet corruption, and
      protection against some specific malicious packet insertion
      attacks as described in more detail in Section 4.2 of this

   Label Stack Entry

      An MPLS label as defined in [RFC3032].

   The optional L2-Specific-Sublayer defined in [RFC3931] is generally
   not present for MPLS over L2TPv3.

   Generic IP encapsulation procedures such as MTU considerations,
   handling of TTL, EXP and DSCP bits, etc. are the same as the "Common
   Procedures" for IP encapsulation of MPLS defined in Section 5 of
   [MPLS-IP-GRE] and are not reiterated here.

3. Assigning the L2TPv3 Session ID and Cookie

   Much like an MPLS label, the L2TPv3 Session ID and Cookie must be
   selected and exchanged between participating nodes before L2TPv3 can
   operate. These values may be configured manually, or distributed via
   a signaling protocol. This document concerns itself only with the
   encapsulation of MPLS over L2TPv3, thus the particular method of
   assigning the Session ID and Cookie is out of scope.

4. Applicability

   The methods defined [MPLS-IP-GRE], [MPLS-IPSEC] and this document all
   describe methods for carrying MPLS over an IP network. Cases where
   MPLS over L2TPv3 may be applicable compared to other alternatives are
   discussed in this section.

   It is generally simpler to have one's border routers refuse to accept
   an MPLS packet than to configure a router to refuse to accept certain
   MPLS packets carried in IP or GRE to or from certain IP sources or
   destinations. Thus, the use of IP or GRE to carry MPLS labels
   increases the opportunity for MPLS label spoofing attacks. When ACLs
   fail or are not available, L2TPv3 provides an additional level of
   protection against packet spoofing before allowing a packet to enter
   a VPN (much like IPsec provides an additional level of protection at
   a PE rather than relying on ACL filters). Checking the value of the
   L2TPv3 Cookie is similar to any sort of ACL which inspects the
   contents of a packet header, except that we give ourselves the luxury

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   of "seeding" the L2TPv3 header with a very difficult to spoof value.
   MPLS over L2TPv3 may be favorable compared to [MPLS-IP-GRE], if:

      Two routers are "adjacent" over an L2TPv3 tunnel that exists for
      some reason outside the scope of this document, and those two
      routers need to send MPLS packets over that adjacency.

      Implementation considerations dictate the use of MPLS over L2TPv3.
      For example, a hardware device may be able to take advantage of
      the L2TPv3 encapsulation for faster processing.

      Packet spoofing is of a concern, and the L2TPv3 Cookie may be used
      as a method for validating the source of the L2TPv3 packet before
      continuing to process it.

   (The first two of the above applicability statements were adopted
   from [MPLS-IP-GRE])

   In summary, L2TPv3 can provide a balance between the limited security
   against IP spoofing attacks offered by [MPLS-IP-GRE] vs. the greater
   security and associated operational and processing overhead offered
   by [MPLS-IPSEC].  Further, MPLS over L2TPv3 may be faster in some
   hardware, particularly if it is already optimized to classify
   incoming L2TPv3 packets carrying IP framed in a variety of ways. For
   example, IP encapsulated by HDLC or Frame Relay over L2TPv3 may be
   considered not that far removed from IP encapsulated by MPLS over

5. Security Considerations

   The L2TPv3 Cookie does not provide protection via encryption.
   However, when used with a sufficiently random 64-bit value which is
   kept secret from a hacker, the L2TPv3 Cookie may be used as a simple
   yet effective packet source authentication check which is quite
   resistent to brute force packet spoofing attacks. It also alleviates
   the need to rely solely on filter lists based on a list of valid
   source IP addresses, and thwarts attacks which could benefit by
   spoofing a permitted source IP address.

   L2TPv3 tunnels may also be secured using IPsec.  When using IPsec,
   the tunnel head and the tunnel tail should be treated as the
   endpoints of a Security Association.  The MPLS over L2TPv3
   encapsulated packets should be considered as originating at the
   tunnel head and as being destined for the tunnel tail; IPsec
   transport mode should thus be used. Key distribution may be done
   either manually or automatically.

   Security is also discussed as part of the applicability discussion in

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   section 4 of this document.

6. IANA Considerations

   There are no IANA considerations for this document.

7. Acknowledgments

   Thanks to Robert Raszuk, Clarence Filsfils and Eric Rosen for their
   review of this document. Some text was adopted from [MPLS-IP-GRE].

8. References

8.1 Normative References

   [RFC3931]     J. Lau, M. Townsley, I. Goyret, "Layer Two Tunneling
                 Protocol (Version 3)", work in progress,
                 draft-ietf-l2tpext-l2tp-base-15.txt, December 2004.

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [MPLS-IP-GRE] T. Worster, Y. Rekhter, E. Rosen, "Encapsulating
                 MPLS in IP or Generic Routing Encapsulation (GRE)",
                 work in progress, draft-ietf-mpls-in-ip-or-gre-08.txt,
                 June 2004.

8.2 Informative References

   [RFC2547]     E. Rosen, Y. Rekhter, "BGP/MPLS VPNs", RFC 2547, March 1999.

   [RFC3032]     R. Rosen, et. al., "MPLS Label Stack Encoding," RFC 3032,
                 January 2001.

   [MPLS-IPSEC]  E. Rosen, J. De Clercq, O/ Paridaens, Y. T'Joens,
                 C. Sargor, "Use of PE-PE IPsec in RFC2547 VPNs",
                 work in progress, draft-ietf-l3vpn-ipsec-2547-01.txt,
                 August 2003.

9. Contacts

   W. Mark Townsley
   cisco Systems

   Ted Seely

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   Jeffrey S. Young

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