[Search] [txt|pdf|bibtex] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06                                          
L2TPEXT Working Group                                  Andrew J. Valencia
Internet Draft                                                Tmima Koren
June 30, 2002
Expires January 2003

                  L2TP Header Compression ("L2TPHC")

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 groups may also distribute
   working documents as Internet-Drafts.

   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

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


   The Layer 2 Tunneling Protocol ("L2TP") [RFC2661] defines a
   mechanism for tunneling PPP sessions over arbitrary media. There
   exists a class of specific media applications for which protocol
   overhead may be optimized, and where such reduction results in
   improved operation. This document describes the solution space
   addressed, its underlying motivations, and the protocol modifications
   required. The enhancement to the L2TP protocol is called L2TP Header
   Compression, or "L2TPHC".

Valencia                  expires in 6 months                   [Page 1]

INTERNET DRAFT                                                June 2002

1. Introduction

   L2TP [RFC2661] defines a general purpose mechanism for tunneling PPP
   over various media. In most cases, the header overhead of the L2TP
   tunnel is negligible. However, when L2TP operates over bandwidth
   constrained networks such as dialup links or some classes of WAN
   backhauls, any savings of bytes transmitted results in a substantial
   efficiency gain. This effect is further amplified when streams of
   small IP packets dominate the traffic (thus increasing the header-
   to-payload ratio), as is common with multimedia and other types of
   real-time data traffic.

2. Simplifying Assumptions

   If several simplifying assumptions are met, it is possible to
   reduce the size of the L2TP encapsulation:

      - The tunnel will not operate through a NAT interface
      - The tunnel uses a single IP address for the life of the tunnel
      - The tunnel's host uses only one public IP network interface
      - There will be only one tunnel between the LAC and the LNS
      - There might be only one session within a tunnel
      - There might be only one protocol active on that session
      - Alignment is not required
      - Packet length is preserved by the IP header

   Each of these simplifying assumptions directly relates to an L2TP
   protocol header field's function. Because NAT functionality is not
   needed, the UDP header is not required. Because the endpoints will
   not change their source IP addresses (due to either changing IP
   addresses, moving among IP egress points, or switching to a distinct
   backup IP interface), the identity of the peer may be determined by
   its source IP address, rather than the Tunnel ID. If there is only
   one tunnel, it is trivial to determine the Tunnel ID. Because each
   byte is a measurable component of overhead, it is better to send
   fields on unaligned boundaries rather than ever pad. Because IP will
   preserve the packet length end-to-end, there is no need to
   communicate this in the header itself.

   In addition, several operational considerations permit further

      - There is no need to optimize control packet overhead
      - Version compatibility may be determined by control packets

Valencia                  expires in 6 months                   [Page 2]

INTERNET DRAFT                                                June 2002

   The first two bytes of an L2TP payload header determined the presence
   of further, optional, fields. It also contains a Version field, used
   to detect compatible version operation. Realistically, these may all
   be determined in advance of payload exchange.
   Thus, by choosing very reasonable simplifying assumptions, it is
   possible to minimize the L2TP fields in the header of a payload
   packet. The resulting protocol is a 0-byte "header" (i.e.,
   the header is absent). This would then be followed by a PPP frame
   (whose PPP encapsulation is also made optional), all encapsulated
   within a raw IP protocol header. These packets are exchanged in
   parallel with the regular UDP-based L2TP tunnel which provides all
   management and related functions.

3. Tunnel Establishment

   3.1 Negotiation

      L2TPHC is negotiated by an optional AVP "L2TPHC-No-Header"
      which is placed in the ICRQ/ICRP and OCRQ/OCRP session
      establishment messages. The effect of this AVP will never
      occur until L2TP reaches a state where the session within the
      tunnel is fully established (i.e., success indicated in the
      ICCN/OCCN). Additionally, each side intending to use L2TPHC MUST
      NOT do so unless it both sends and receives this AVP. Thus,
      unless both sides support L2TPHC, the optional AVP (in the ICRQ or
      OCRQ message) will be ignored by one side, and not sent (in the
      corresponding ICRP or OCRP) to the other side, and L2TP will
      operate in its regular mode. If this AVP is both sent and
      received, then payload is sent with no L2TPHC header at all--the
      PPP payload is carried directly within the IP packet. There can
      be only a single tunnel and a single session using L2TPHC between
             any two IP peers. Each individual PPP session MUST be identified
      by its IP source/destination pair.

      Another AVP, "L2TPHC-PPP-Protocol", may also be included
      in the control message. If both sent and received,
      its Value field indicates the PPP protocol number which
      will be the single value carried in the payload of all PPP
      packets, and the payload transmitted through L2TPHC will omit PPP
      HDLC flags and control, as well as the one or two byte protocol
      field. The receiving side would then have to recreate a suitable
      PPP header and insert it onto received payload.

      When L2TPHC is negotiated, all control messages are sent over
      the L2TP tunnel with an L2TP header. If a default PPP protocol
      number was also negotiated, all PPP packets with protocols
      different than the default must also be sent with an L2TP header.

Valencia                  expires in 6 months                   [Page 3]

INTERNET DRAFT                                                June 2002

   3.2 AVP Formats

   All AVP's MUST always be sent with the M, H, and "rsvd" bits all set
   to 0. All Attribute fields are 16-bit quantities in network byte
   order. The Vendor ID of each is 9, reflecting Cisco Systems, the
   initial developer of this extension. New Attribute values under
   Vendor ID 0 MUST be assigned when this document advances on the
   standards track.

       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
      |M|H| rsvd  |         6         |               9               |
      |               1               |

      L2TPHC-No-Header's Attribute is value 1.
      There is no Value field. When L2TPHC-No-Header
      is both sent and received, L2TPHC will directly encapsulate the
      PPP payload without any L2TPHC header byte.

       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
      |M|H| rsvd  |         8         |               9               |
      |               2               |          PPP Protocol         |

      L2TPHC-PPP-Protocol's Attribute is value 2. The Value field is
      any legal PPP value for an NCP protocol. Note that all PPP
      protocol values which can be sent in 8-bit format always have a
      corresponding 16-bit format, and this 16-bit format is always the
      one used in this AVP. This AVP can only follow an L2TPHC-No-
      Header AVP, and indicates that PPP traffic carried over L2TPHC
      will not only have no L2TPHC header, but will also have no PPP
      address, control, or protocol fields. If necessary, these fields
      will be reconstructed on the receiving L2TPHC peer side, with the
      protocol value being always set to the Value indicated by this AVP.

Valencia                  expires in 6 months                    [Page 4]

INTERNET DRAFT                                                 June 2002

4. Payload Exchange

   If the L2TPHC-No-Header AVP is sent to and received from
   the peer, PPP payload packets may be sent to the peer's IP address
   as raw IP packets, with the IP protocol number set to 115 (the
   IANA-assigned value for L2TP). Such payload may be sent any
   time it would have been legal to send such payload over the
   regular UDP-based L2TP tunnel. Similarly, payload over the UDP
   tunnel MUST always be accepted, even after payload has flowed using
   the header compressed raw IP packet format. The payload so exchanged
   is always associated with the tunnel on which the AVP was received,
   and with the single session within that tunnel.

   An L2TPHC packet is encoded as:

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ...+
      |        PPP packet...         ...|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ...+

   The PPP frame will consist of the usual PPP-over-HDLC address, con-
   trol, and protocol fields. However, if the L2TPHC-PPP-Protocol AVP
   has been sent and received, these fields are not present in the PPP
   payload, and must be re-inserted by the receiving side, using the
   protocol value indicated in the Value field of the
   L2TPHC-PPP-Protocol AVP.

5. Efficiency Considerations

   Some rough calculations will illustrate the environments in which
   L2TPHC may be beneficial. Overhead as a percentage of the carried
   traffic will be calculated for a typical packet size involved in bulk
   data transfer (700 bytes), and the canonical 64-byte "small IP
   packet". Percentages will be rounded to the nearest whole number.
   Overhead is tallied for an IP header of 20 bytes, a UDP header of 8
   bytes, an L2TP header of 8 bytes, and a PPP encapsulation of 4 bytes.

   The worst case is a 64-byte packet carried within a UDP L2TP header.
   The 64 bytes of payload is carried by an overall header of 40 bytes,
   resulting in an overhead of 63%. With the larger payload size of 700
   bytes, the header is amortized over many more bytes, reducing the
   overhead to 6%.

Valencia                  expires in 6 months                   [Page 5]

INTERNET DRAFT                                                June 2002

   With L2TPHC, the UDP and L2TP headers are absent, and the 4 bytes of
   PPP encapsulation have been deleted. Overall size is thus 20 bytes
   of IP header. The small packet now suffers an overhead of only 31%,
   and the larger packet 3%.

   Percentage overhead does not represent all the considerations
   involved in reducing overhead. Consider a modem connection operating
   at 14,400 bits per second, which translates to a per-byte real-time
   cost of 0.6 milliseconds (14400 divided by 8 bits, as async framing
   characters are not included in the modem-to-modem data transfer). A
   savings of 16 bytes per packet can also be viewed as a reduction of
   almost 10 milliseconds of latency per packet. While this latency is
   short enough to be unnoticeable by a human, it may impact real-time
   protocols such as streaming audio or video.

   Thus, L2TP Header Compression provides most of its benefits when car-
   rying streams of small packets. In environments such as downloading
   of graphic files, or where human interaction is intermingled with the
   short packets, the benefits of L2TP Header Compression will probably
   be undetectable.

6. Security Considerations

   Because L2TPHC has no security facilities, it is critical that its
   operation be reconciled with the security policy of its environment.
   Since L2TPHC may have no protocol header at all, it is trivial to
   spoof a source IP address and inject malicious packets into an ongo-
   ing session. There are several suitable techniques for controlling
   this exposure.

   In the simplest case, L2TPHC operates across a private network. For
   instance, a remote user may dial into a private NAS located on this
   network, and use L2TP (with or without L2TPHC) to cross an IP-only
   portion of this network to establish a multi-protocol session con-
   nected at a convenient point in the network. In this environment, no
   additional security may be required, and L2TPHC would operate trust-
   ing to the integrity of this private network.

   If the weak protection of a difficult-to-guess protocol header is
   deemed sufficient, expanded protocol overhead has clearly been deter-
   mined to be acceptable, and L2TP over UDP can be used without L2TPHC.

Valencia                  expires in 6 months                   [Page 6]

INTERNET DRAFT                                                June 2002

   If PPP encryption under ECP [RFC1968] is active, malicious PPP pack-
   ets are trivially detected and discarded as they are received on the
   raw IP port number. Similarly, if an IPsec session is protecting the
   IP packets themselves, malicious packets will also be discarded.
   Note that in both cases, an expanded header is implicit in these
   security facilities, which will greatly reduce the overhead
   efficiencies gained by L2TPHC.

7. IANA Considerations

   Two new AVP's are defined in this paper: L2TPHC-No-Header and
   L2TPHC-PPP-Protocol. The vendor ID of there AVP's should be 0.
   An Attribute needs to be assigned to each of the two new AVP's
   through IETF Consensus, as defined in [RFC2661] section 10.1

8. References

   [RFC2661]  M. Townsley, "Layer 2 Tunnel Protocol (L2TP)", RFC 2661,
   August 1999

   [RFC1968] G. Meyer, "PPP Encryption Control Protocol (ECP)",
   RFC 1968, June 1996

9. Acknowledgments

   Thanks to Gurdeep Singh Pall of Microsoft for identifying and
   describing scenarios in which L2TP header size become a concern.

   Thanks to Bill Palter of Redback Networks and W. Mark Townsley of
   Cisco Systems for help in reviewing this draft.

10. Authors' Addresses

   Andrew J. Valencia
   P.O. Box 2928
   Vashon, WA  98070

   Email: vandys@zendo.com

   Tmima Koren
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134-1706
   United States

   EMail: tmima@cisco.com

Valencia                  expires in 6 months                    [Page 7]