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Versions: 00 01                                                         
PPP Working Group                                      Andrew J. Valencia
Request for Comments: DRAFT                                 Cisco Systems
Category: Internet Draft
Title: draft-ietf-pppext-l2tphc-01.txt
Date: December 1997

                  L2TP Header Compression (``L2TPHC'')

Status of this Memo

   This document is an Internet-Draft.  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.  Internet-Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet-
   Drafts as reference material or to cite them other than as a
   ``working draft'' or ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   1id-abstracts.txt listing contained in the Internet-Drafts Shadow
   Directories on ds.internic.net, nic.nordu.net, ftp.nisc.sri.com, or


   The Layer 2 Tunneling Protocol (``L2TP'') defines a mechanism for
   tunneling PPP sessions over arbitrary media.  There exists a class of
   specific media and 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''.

1. Introduction

   L2TP [1] defines a general purpose mechanism for tunneling PPP over
   various media.  By design, it insulates L2TP operation from the
   details of the media over which it operates.  A significant
   application of L2TP has emerged, known as ``voluntary tunneling''
   [2].  In this environment, the L2TP tunnel runs from the dial-up
   client itself, through a public IP infrastructure, and then
   terminating at the target LNS.  Because this mode of operation
   results in the L2TP header traversing the slow, high-latency dial-up
   link, each byte of tunnel overhead can have a measurable impact on
   the operation of the carried protocols.

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2. Simplifying Assumptions

   Fortunately, several simplifying assumptions may be made in the
   voluntary tunneling environment:

      - The client will not operate through a NAT interface
      - The client will not roam (i.e., change its IP address)
      - The client has only one public IP network interface
      - There will be only one tunnel between the client and its LNS
      - There will be only one session within this tunnel
      - 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 client will not
   change its source IP address (due to either roaming or switching to a
   distinct backup IP interface), the identity of the client may be
   determined by its source IP address, rather than the Tunnel ID.
   Because there is only one session within the tunnel, it is trivial to
   determine the Session 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

   In addition, several operational considerations permit further

      - There is no need to optimize control packet overhead
      - Version compatibility may be determined by control packets
      - Rate pacing may be determined outside the main payload exchange

   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.  Similarly, the
   optional rate pacing of L2TP could determined outside of the core
   payload packet path, or the Priority bit facility could be used

   Thus, by choosing very reasonable simplifying assumptions, it is
   possible to minimize the L2TP fields from the header of a payload
   packet.  The resulting protocol is a one octet mandatory header,
   followed by 0, 1, or 2 additional octets, followed by PPP frames, 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

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      L2TPHC is negotiated by an optional AVP ``L2TPHC-Proto'' which is
      placed in the SCCRQ/SCCRP tunnel establishment messages.  The
      effect of this AVP will never occur until L2TP reaches a state
      where payload data may be forwarded within the session in the
      tunnel.  Additionally, each side intending to use L2TPHC MUST NOT
      do so until it both sends and receives this AVP.  Thus, unless
      both sides support L2TPHC, the optional AVP will be ignored by one
      side, and not sent to the other side, and L2TP will operate in its
      regular mode.

      Further sessions within an L2TPHC tunnel MUST NOT be initiated.
      However, L2TPHC permits multiple tunnels if a second AVP,
      indicating a special Tunnel ID, is included immediately following
      L2TPHC-Proto AVP in the SCCRQ/SCCRP exchange.  This optional AVP,
      ``L2TPHC-Tunnel'', is ignored unless it is both sent and received.
      In this case, the Value indicates the octet value which will be
      included as the Tunnel ID within the L2TPHC header.

      Once the tunnel associated with a given L2TPHC context has been
      terminated, the L2TPHC context is considered free, and may be used
      in future L2TP connections.  Because all control passes over the
      parallel L2TP session corresponding to the L2TPHC one, the L2TP
      tunnel terminates, and the L2TPHC tunnel is implicitly terminated.

   3.2 AVP Format

      The AVP L2TPHC-Proto is encoded as Vendor ID 9, Attribute is the
      16-bit quantity 0 (the ID 9 reflects Cisco Systems, the initial
      developer of this specification, and it SHOULD be changed to 0 and
      an official Attribute value chosen if this specification advances
      on a standards track).  The Value is a single octet, encoding the
      IP protocol number to use for the exchange of payload.  Unless and
      until an official protocol number is allocated, the value 251 is
      recommended.  The AVP is marked optional, permitting
      interoperability with peers not implementing L2TPHC.

       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
      |0|0|0|0|           7           |               9               |
      |               0               |       251     |

      The L2TPHC-Tunnel AVP is also marked optional.  It MUST NOT be
      present except when immediately following an L2TPHC-Proto AVP.
      The Attribute is the 16-bit value 1, encoded in network byte
      order.  The single octet Value is a Tunnel ID to be used in the
      L2TPHC encapsulation.  If this AVP is both sent and received, up
      to 256 parallel tunnels may be supported between the peers, and
      all L2TPHC packets MUST include the T bit, and the Tunnel ID
      specified by the peer MUST be used as the Tunnel ID in all packets
      sent to that peer for a given tunnel.

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       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
      |0|0|0|0|           7           |               9               |
      |               1               |  Tunnel ID    |

4. Payload Exchange

   If the L2TPHC 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 as indicated from the peer.  Note that it
   is legal for each peer to have specified a different protocol number;
   traffic sent is always to the number indicated in the peer's AVP.
   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

   Each L2TPHC payload packet is encoded as:

       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
      |F|I|P|0|0|0|0|0|      Nr       |      Ns       |   Tunnel ID   |
      | PPP packet... |

   The Nr/Ns fields MUST be present if the F bit is 1, otherwise they
   MUST be omitted.  Their use is identical to that of the fields of the
   same name in L2TP payload packets, except that their numerical range
   is only 8 bits, rather than 16 in L2TP.  A regular L2TP tunnel MUST
   be used in any situation where the speed of a link and the latency of
   the path result in more packets outstanding than can be accounted
   with a byte numbering space.

   The I bit MUST be set, and the Tunnel ID MUST be present, if the
   L2TPHC-Tunnel AVP was both sent and received during tunnel setup.
   Otherwise I must be sent 0, and the Tunnel ID octet omitted from the

   The P bit is the Priority bit, and serves the same function as the
   bit of the same name in an L2TP packet.  Priority packets MUST enjoy
   priority over traffic queued on both the UDP tunnel as well as the
   corresponding L2TPHC raw IP tunnel.

   Therefore, an L2TPHC packet will have an L2TPHC header of at least
   one octet, with up to three more octets as indicated by the flags in
   this first octet.

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   Since packet flow over this raw IP tunnel is distinct from the UDP
   based tunnel, it is possible that an asymmetry in the path (for
   instance, the unintentional presence of a NAT device) may disrupt one
   but not the other.  It is recommended that at least during the time
   immediately following establishment of the session, that LCP echoes
   be used in tandem with the L2TP keepalive function so that
   connectivity of both paths may be verified.

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, and an L2TP header of 8 bytes (4 bytes of rate pacing with the
   Nr/Ns fields will probably be avoided in favor of the more compact
   though less comprehensive Priority header bit).

   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 36 bytes,
   resulting in an overhead of 56%.  With the larger payload size of 700
   bytes, the header is amortized over many more bytes, reducing the
   overhead to 5%.

   With L2TPHC, the UDP is absent and the L2TPHC header is 1 byte for
   the most compact case.  Overall size is thus one byte of L2TPHC and
   20 bytes of IP header.  The small packet now suffers an overhead of
   only 32%, and the larger packet 3%.

   Percentage overhead does not represent all the considerations
   involved in reducing overhead.  The average modem connection is still
   only 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).
   Thus, 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
   carrying 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 has no protocol header at all, it is trivial to spoof a
   source IP address and inject malicious packets into an ongoing
   session.  There are several suitable techniques for controlling this

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   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
   connected at a convenient point in the network.  In this environment,
   no additional security may be required, and L2TPHC would operate
   trusting 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
   determined to be acceptable, and L2TP over UDP can be used without

   If PPP encryption under ECP [3] is active, malicious PPP packets 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.  For instance, an MD5 AH IPsec header will add 32
   bytes  to the packet.  The 16 bytes saved by L2TPHC quickly
   approaches statistical insignificance.

7. Acknowledgments

   Thanks to Gurdeep Singh Pall of Microsoft for identifying and
   describing scenarios in which L2TP header size become a concern, and
   for help in designing the L2TPHC header.

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

8. Contacts

   Andrew J. Valencia
   Cisco Systems
   170 West Tasman Drive
   San Jose CA 95134-1706

9. References

   [1] A. Valencia, ``Layer 2 Tunnel Protocol (L2TP)'', Internet Draft,
      October 1997

   [2] G. Zorn, ``RADIUS Attributes for Tunnel Protocol Support'', Internet
      draft, July 1997

   [3] G. Meyer, ``PPP Encryption Control Protocol (ECP)'', RFC 1968

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