





Network Working Group                                        M. Townsley
Request for Comments: 4817                                  C. Pignataro
Category: Standards Track                                     S. Wainner
                                                           Cisco Systems
                                                                T. Seely
                                                           Sprint Nextel
                                                                J. Young
                                                              March 2007


    Encapsulation of MPLS over Layer 2 Tunneling Protocol Version 3

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   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 stack and its payload
   over the L2TPv3 data encapsulation.  This enables an application that
   traditionally requires an MPLS-enabled core network, to utilize an
   L2TPv3 encapsulation over an IP network instead.



















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Table of Contents

   1. Introduction ....................................................2
      1.1. Specification of Requirements ..............................2
   2. MPLS over L2TPv3 Encoding .......................................2
   3. Assigning the L2TPv3 Session ID and Cookie ......................4
   4. Applicability ...................................................4
   5. Congestion Considerations .......................................6
   6. Security Considerations .........................................6
      6.1. In the Absence of IPsec ....................................7
      6.2. Context Validation .........................................7
      6.3. Securing the Tunnel with IPsec .............................8
   7. Acknowledgements ................................................9
   8. References .....................................................10
      8.1. Normative References ......................................10
      8.2. Informative References ....................................10

1.  Introduction

   This document defines how to encapsulate an MPLS label stack and its
   payload inside the L2TPv3 tunnel payload.  After defining the MPLS
   over L2TPv3 encapsulation procedure, other MPLS over IP encapsulation
   options, including IP, Generic Routing Encapsulation (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 that may be
   enabled over L2TPv3.

1.1.  Specification of Requirements

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

2.  MPLS over L2TPv3 Encoding

   MPLS over L2TPv3 allows tunneling of an MPLS stack [RFC3032] and its
   payload over an IP network, utilizing the L2TPv3 encapsulation
   defined in [RFC3931].  The MPLS Label Stack and payload are carried
   in their entirety following IP (either IPv4 or IPv6) and L2TPv3
   headers.








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                          +-+-+-+-+-+-+-+-+-+-+
                          |        IP         |
                          +-+-+-+-+-+-+-+-+-+-+
                          |      L2TPv3       |
                          +-+-+-+-+-+-+-+-+-+-+
                          | MPLS Label Stack  |
                          +-+-+-+-+-+-+-+-+-+-+
                          |    MPLS Payload   |
                          +-+-+-+-+-+-+-+-+-+-+

                 Figure 2.1 MPLS Packet over L2TPv3/IP

   The L2TPv3 encapsulation carrying a single MPLS label stack entry 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 over L2TPv3 encapsulation

   When encapsulating MPLS over L2TPv3, the L2TPv3 L2-Specific Sublayer
   MAY be present.  It is generally not present; hence, it is not
   included in Figure 2.2.  The L2TPv3 Session ID MUST be present.  The
   Cookie MAY be present.

   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, the presence and type of an L2TPv3 L2-Specific Sublayer,
      as well as what type of tunneled encapsulation follows (i.e.,
      Frame Relay, Ethernet, MPLS, etc.)







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   Cookie

      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 [RFC4086], 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
      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 of this
      document.

   Label Stack Entry

      An MPLS label stack entry 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 fragmentation and MTU
   considerations, handling of Time to Live (TTL), EXP, and
   Differentiated Services Code Point (DSCP) bits, etc. are the same as
   the "Common Procedures" for IP encapsulation of MPLS defined in
   Section 5 of [RFC4023] 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 (if present) is out of scope.

4.  Applicability

   The methods defined in [RFC4023], [MPLS-IPSEC], and this document all
   describe methods for carrying MPLS over an IP network.  Cases where
   MPLS over L2TPv3 is comparable to other alternatives are discussed in
   this section.





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   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 packets
   increases the likelihood that an MPLS label-spoofing attack will
   succeed.  L2TPv3 provides an additional level of protection against
   packet spoofing before allowing a packet to enter a Virtual Private
   Network (VPN) (much like IPsec provides an additional level of
   protection at a Provider Edge (PE) router rather than relying on
   Access Control List (ACL) filters).  Checking the value of the L2TPv3
   Cookie is similar to any sort of ACL that inspects the contents of a
   packet header, except that we give ourselves the luxury of "seeding"
   the L2TPv3 header with a value that is very difficult to spoof.

   MPLS over L2TPv3 may be advantageous compared to [RFC4023], if:

      Two routers are already "adjacent" over an L2TPv3 tunnel
      established for some other reason, and wish to exchange 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 or distributed processing.

      Packet spoofing and insertion, service integrity and resource
      protection are of concern, especially given the fact that an IP
      tunnel potentially exposes the router to rogue or inappropriate IP
      packets from unknown or untrusted sources.  IP Access Control
      Lists (ACLs) and numbering methods may be used to protect the PE
      routers from rogue IP sources, but may be subject to error and
      cumbersome to maintain at all edge points at all times.  The
      L2TPv3 Cookie provides a simple means of validating the source of
      an L2TPv3 packet before allowing processing to continue.  This
      validation offers an additional level of protection over and above
      IP ACLs, and a validation that the Session ID was not corrupted in
      transit or suffered an internal lookup error upon receipt and
      processing.  If the Cookie value is assigned and distributed
      automatically, it is less subject to operator error, and if
      selected in a cryptographically random nature, less subject to
      blind guesses than source IP addresses (in the event that a hacker
      can insert packets within a core network).

   (The first two of the above applicability statements were adopted
   from [RFC4023].)

   In summary, L2TPv3 can provide a balance between the limited security
   against IP spoofing attacks offered by [RFC4023] vs. the greater
   security and associated operational and processing overhead offered



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   by [MPLS-IPSEC].  Further, MPLS over L2TPv3 may be faster in some
   hardware, particularly if that hardware is already optimized to
   classify incoming L2TPv3 packets carrying IP framed in a variety of
   ways.  For example, IP encapsulated by High-Level Data Link Control
   (HDLC) or Frame Relay over L2TPv3 may be equivalent in complexity to
   IP encapsulated by MPLS over L2TPv3.

5.  Congestion Considerations

   This document specifies an encapsulation method for MPLS inside the
   L2TPv3 tunnel payload.  MPLS can carry a number of different
   protocols as payloads.  When an MPLS/L2TPv3 flow carries IP-based
   traffic, the aggregate traffic is assumed to be TCP friendly due to
   the congestion control mechanisms used by the payload traffic.
   Packet loss will trigger the necessary reduction in offered load, and
   no additional congestion avoidance action is necessary.

   When an MPLS/L2TPv3 flow carries payload traffic that is not known to
   be TCP friendly and the flow runs across an unprovisioned path that
   could potentially become congested, the application that uses the
   encapsulation specified in this document MUST employ additional
   mechanisms to ensure that the offered load is reduced appropriately
   during periods of congestion.  The MPLS/L2TPv3 flow should not exceed
   the average bandwidth that a TCP flow across the same network path
   and experiencing the same network conditions would achieve, measured
   on a reasonable timescale.  This is not necessary in the case of an
   unprovisioned path through an over-provisioned network, where the
   potential for congestion is avoided through the over-provisioning of
   the network.

   The comparison to TCP cannot be specified exactly but is intended as
   an "order-of-magnitude" comparison in timescale and throughput.  The
   timescale on which TCP throughput is measured is the roundtrip time
   of the connection.  In essence, this requirement states that it is
   not acceptable to deploy an application using the encapsulation
   specified in this document on the best-effort Internet, which
   consumes bandwidth arbitrarily and does not compete fairly with TCP
   within an order of magnitude.  One method of determining an
   acceptable bandwidth is described in [RFC3448].  Other methods exist,
   but are outside the scope of this document.

6.  Security Considerations

   There are three main concerns when transporting MPLS-labeled traffic
   between PEs using IP tunnels.  The first is the possibility that a
   third party may insert packets into a packet stream between two PEs.
   The second is that a third party might view the packet stream between
   two PEs.  The third is that a third party may alter packets in a



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   stream between two PEs.  The security requirements of the
   applications whose traffic is being sent through the tunnel
   characterizes how significant these issues are.  Operators may use
   multiple methods to mitigate the risk, including access lists,
   authentication, encryption, and context validation.  Operators should
   consider the cost to mitigate the risk.

   Security is also discussed as part of the applicability discussion in
   Section 4 of this document.

6.1.  In the Absence of IPsec

   If the tunnels are not secured with IPsec, then some other method
   should be used to ensure that packets are decapsulated and processed
   by the tunnel tail only if those packets were encapsulated by the
   tunnel head.  If the tunnel lies entirely within a single
   administrative domain, address filtering at the boundaries can be
   used to ensure that no packet with the IP source address of a tunnel
   endpoint or with the IP destination address of a tunnel endpoint can
   enter the domain from outside.

   However, when the tunnel head and the tunnel tail are not in the same
   administrative domain, this may become difficult, and filtering based
   on the destination address can even become impossible if the packets
   must traverse the public Internet.

   Sometimes, only source address filtering (but not destination address
   filtering) is done at the boundaries of an administrative domain.  If
   this is the case, the filtering does not provide effective protection
   at all unless the decapsulator of MPLS over L2TPv3 validates the IP
   source address of the packet.

   Additionally, the "Data Packet Spoofing" considerations in Section
   8.2 of [RFC3931] and the "Context Validation" considerations in
   Section 6.2 of this document apply.  Those two sections highlight the
   benefits of the L2TPv3 Cookie.

6.2.  Context Validation

   The L2TPv3 Cookie does not provide protection via encryption.
   However, when used with a sufficiently random, 64-bit value that is
   kept secret from an off-path attacker, the L2TPv3 Cookie may be used
   as a simple yet effective packet source authentication check which is
   quite resistant 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 that could benefit by
   spoofing a permitted source IP address.  The L2TPv3 Cookie provides a
   means of validating the currently assigned Session ID on the packet



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   flow, providing context protection, and may be deemed complimentary
   to securing the tunnel utilizing IPsec.  In the absence of
   cryptographic security on the data plane (such as that provided by
   IPsec), the L2TPv3 Cookie provides a simple method of validating the
   Session ID lookup performed on each L2TPv3 packet.  If the Cookie is
   sufficiently random and remains unknown to an attacker (that is, the
   attacker has no way to predict Cookie values or monitor traffic
   between PEs), then the Cookie provides an additional measure of
   protection against malicious spoofed packets inserted at the PE over
   and above that of typical IP address and port ACLs.

6.3.  Securing the Tunnel with IPsec

   L2TPv3 tunnels may also be secured using IPsec, as specified in
   Section 4.1.3 of [RFC3931].  IPSec may provide authentication,
   privacy protection, integrity checking, and replay protection.  These
   functions may be deemed necessary by the operator.  When using IPsec,
   the tunnel head and the tunnel tail should be treated as the
   endpoints of a Security Association.  A single IP address of the
   tunnel head is used as the source IP address, and a single IP address
   of the tunnel tail is used as the destination IP address.  The means
   by which each node knows the proper address of the other is outside
   the scope of this document.  However, if a control protocol is used
   to set up the tunnels, such control protocol MUST have an
   authentication mechanism, and this MUST be used when the tunnel is
   set up.  If the tunnel is set up automatically as the result of, for
   example, information distributed by BGP, then the use of BGP's
   Message Digest 5 (MD5)-based authentication mechanism can serve this
   purpose.

   The MPLS over L2TPv3 encapsulated packets should be considered as
   originating at the tunnel head and being destined for the tunnel
   tail; IPsec transport mode SHOULD thus be used.

   Note that the tunnel tail and the tunnel head are Label Switched Path
   (LSP) adjacencies (for label distribution adjacencies, see
   [RFC3031]), which means that the topmost label of any packet sent
   through the tunnel must be one that was distributed by the tunnel
   tail to the tunnel head.  The tunnel tail MUST know precisely which
   labels it has distributed to the tunnel heads of IPsec-secured
   tunnels.  Labels in this set MUST NOT be distributed by the tunnel
   tail to any LSP adjacencies other than those that are tunnel heads of
   IPsec-secured tunnels.  If an MPLS packet is received without an
   IPsec encapsulation, and if its topmost label is in this set, then
   the packet MUST be discarded.






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   Securing L2TPv3 using IPsec MUST provide authentication and
   integrity.  (Note that the authentication and integrity provided will
   apply to the entire MPLS packet, including the MPLS label stack.)

   Consequently, the implementation MUST support Encapsulating Security
   Payload (ESP) with null encryption.  ESP with encryption MAY be
   supported if a source requires confidentiality.  If ESP is used, the
   tunnel tail MUST check that the source IP address of any packet
   received on a given Security Association (SA) is the one expected.

   Key distribution may be done either manually or automatically by
   means of the Internet Key Exchange (IKE) Protocol [RFC2409] or IKEv2
   [RFC4306].  Manual keying MUST be supported.  If automatic keying is
   implemented, IKE in main mode with preshared keys MUST be supported.
   A particular application may escalate this requirement and request
   implementation of automatic keying.  Manual key distribution is much
   simpler, but also less scalable, than automatic key distribution.  If
   replay protection is regarded as necessary for a particular tunnel,
   automatic key distribution should be configured.

7.  Acknowledgements

   Thanks to Robert Raszuk, Clarence Filsfils, and Eric Rosen for their
   review of this document.  Some text was adopted from [RFC4023].



























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8.  References

8.1.  Normative References

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

   [RFC3032]     Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
                 Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
                 Encoding", RFC 3032, January 2001.

   [RFC3931]     Lau, J., Townsley, M., and I. Goyret, "Layer Two
                 Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931,
                 March 2005.

   [RFC4023]     Worster, T., Rekhter, Y., and E. Rosen, "Encapsulating
                 MPLS in IP or Generic Routing Encapsulation (GRE)", RFC
                 4023, March 2005.

   [RFC4086]     Eastlake, D., 3rd, Schiller, J., and S. Crocker,
                 "Randomness Requirements for Security", BCP 106, RFC
                 4086, June 2005.

8.2.  Informative References

   [MPLS-IPSEC]  Rosen, E., De Clercq, J., Paridaens, O., T'Joens, Y.,
                 and C. Sargor, "Architecture for the Use of PE-PE IPsec
                 Tunnels in BGP/MPLS IP VPNs", Work in Progress, August
                 2005.

   [RFC3031]     Rosen, E., Viswanathan, A., and R. Callon,
                 "Multiprotocol Label Switching Architecture", RFC 3031,
                 January 2001.

   [RFC2409]     Harkins, D. and D. Carrel, "The Internet Key Exchange
                 (IKE)", RFC 2409, November 1998.

   [RFC4306]     Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
                 RFC 4306, December 2005.

   [RFC3448]     Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
                 Friendly Rate Control (TFRC): Protocol Specification",
                 RFC 3448, January 2003.








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Authors' Addresses

   W. Mark Townsley
   Cisco Systems
   USA

   Phone: +1-919-392-3741
   EMail: mark@townsley.net


   Carlos Pignataro
   Cisco Systems
   7025 Kit Creek Road
   PO Box 14987
   Research Triangle Park, NC 27709
   USA

   Phone: +1-919-392-7428
   EMail: cpignata@cisco.com


   Scott Wainner
   Cisco Systems
   13600 Dulles Technology Drive
   Herndon, VA 20171
   USA

   EMail: swainner@cisco.com


   Ted Seely
   Sprint Nextel
   12502 Sunrise Valley Drive
   Reston, VA 20196
   USA

   Phone: +1-703-689-6425
   EMail: tseely@sprint.net


   Jeff Young

   EMail: young@jsyoung.net








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Full Copyright Statement

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