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Versions: 00 01                                                         
PPP Working Group                                         Richard Shea
INTERNET DRAFT                                         Nortel Networks
Category: Internet Draft
Title: draft-ietf-pppext-l2tpdwin-01.txt
Date: November 1998


                   L2TP Dynamic Data Window Adjustment


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 ftp.ietf.org, nic.nordu.net, ftp.nisc.sri.com, or
   munnari.oz.au.

Abstract

   The Layer Two Tunneling Protocol (L2TP) defines the specification
   of congestion window sizes for data sessions.  In addition, an LNS
   is given the capability to turn off sequence number processing for
   a data session, provided the LAC did not include the Sequencing
   Required AVP during session setup.  This document specifies a
   mechanism whereby an L2TP peer can dynamically change the maximum
   window size values being used for a data session, in order to
   provide the flexibility to operate with smaller window sizes when
   history-bound protocols are operating over a session, and larger
   window sizes when no history-bound protocols are operating over a
   session.

1. Introduction

   In the L2TP protocol sequence numbers are used for preserving
   packet order, detecting packet loss, rate pacing, and congestion


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   control.  An effective window size value for congestion control is
   influenced in opposite directions by two things.  First, in the
   case where protocols with history are being carried, the window
   size must be small enough so that forced packet loss is not
   excessive in the case of a real packet drop where resynchronization
   is necessary.  At the same time, the larger the delay between the
   LAC and LNS, the larger the window size should be so that the
   available bandwidth between the LAC and LNS is not underutilized
   due to rate pacing and congestion control.

   Unfortunately, the L2TP protocol specifies that the window sizes
   for a session are determined when the session is established, at a
   time before it is known whether or not protocol with history will
   be operating over the session.

   It is important for L2TP to provide the flexibility to maximize
   performance for the cases where history-bound protocols are
   operating over a data session for a tunnel which is operating over
   a lossy network and where no history-bound protocols are operating
   over a data session being tunneled over a high-delay path.  Because
   the knowledge of whether history-bound protocols will be operating
   over a data session is not known at the time of session setup, a
   mechanism for dynamically updating the data session window sizes is
   needed.

   It is also not possible in all cases for the LAC to detect when a
   history-bound protocol is being used or not.  A mechanism is also
   included so the LNS can inform the LAC as to whether or not
   history-bound protocols are being run over the link.

1.1 Conventions

   The following language conventions are used in the items of
   specification in this document:

      o   MUST, SHALL, or MANDATORY -- This item is an absolute
         requirement of the specification.

      o   SHOULD or RECOMMEND -- This item should generally be followed
         for all but exceptional circumstances.

      o   MAY or OPTIONAL -- This item is truly optional and may be
         followed or ignored according to the needs of the implementor.

1.2 Terminology

   This draft assumes the reader is knowledgable about terms found in
   [1].  In addition, the following terms are used in this document:


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   History

        Application information that is transferred between peers that
        spans the information conveyed in more than one datagram.

   History-Bound Protocol

        A protocol that uses a history during its operation.  The
        canonical example of such protocols in the context of PPP is
        compression protocols.  While compression protocols can be run
        in a mode where history is not kept between packets, there are
        some implementations that do not support such a mode; nor is
        such a mode always the most desirable mode of operation.

2. Protocol overview

   The current practice for L2TP is for data session maximum window
   sizes to be indicated at the time of session setup, and for these
   maximum window sizes to remain the same for the life of the
   session.

   This document describes an operational addition to L2TP to allow
   data session maximum window sizes to change during the life of a
   data session.

   There are several factors that an implementation can use when
   deciding on a value for its maximum receive window size:

      o  Rate Pacing  - Based on the access speed of the physical
      connection of the client to the LAC, the LAC may desire to rate
      pace the data to stay at the rate that the physical connection
      can handle.

      o  Congestion Control - Based on the load on the box or relative
      priority of the tunneled user identity.

      o  Operation of history-bound protocols on the link - In order
      to get reasonable performance on a link using history-bound
      protocols in the face of packet loss, the maximum window size
      should be kept small (4 packets or so).

   Furthermore, the detection of whether or not a history-bound
   protocol is running over the link is not always possible for an
   L2TP endpoint.  Specifically, a LAC implementation generally does
   not (and perhaps for performance reasons should not) inspect PPP
   traffic being forwarded between the LNS and the client being


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   tunneled.

   The additions to the protocol suggested here are therefore to:

      1.  Provide a method for the LNS to indicate to the LAC whether
      or not any history-bound protocols are operating over the link.

      2.  Provide a method for the LNS and LAC to communicate changes
      in their maximum receive window sizes to each other.

   To provide both of these mechanisms a new message is specified.
   When this message is sent from the LNS to the LAC it includes the
   current state of history-bound protocol operation and a new maximum
   receive window size.  When this new message is sent from the LAC to
   the LNS it includes a new maximum receive window size.

3. Protocol additions

   This document specifies that the protocol be extended with a new
   message type: Session-Update (SU).  This new message is used during
   the life of a session to communicate session update information for
   a data session.

   This document further specifies two AVPs that can optionally be
   included in the SU message.  For SU messages sent from the LNS to
   the LAC a new AVP indicating the current state of history protocol
   operation over the tunneled session can be included.  Both the LNS
   and the LAC can send the SU with the Receive Window Size AVP
   included to change the maximum receive window size for the data
   session.


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          L2TP Control Message Header        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Session-Update               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | History Operation State |
   |     (LNS->LAC only)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Receive Window Size     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+


   The format of the L2TP control message header is given in [1].

   The Message Type AVP for this message contains the value [TBD]
   indicating that this message is a Session-Update message.


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   History Operation State

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|0|0|0|0|0|         8         |               0               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            [TBD]              |               0           |V|H|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   History Operation State encodes the state of history-bound protocol
   operation using two bits.  The V (VJ) bit indicates whether or not
   VJ TCP/IP header compression [3] is operating over the link.  If
   the V bit is one (1) this indicates that VJ packets are being sent
   over the tunneled data session be the LNS.  This informs the LAC
   that upon detection of lost data packets, an indication should be
   sent over the physical connection to the client that a packet was
   lost (e.g. the LAC can force a CRC error on the physical connection
   to the client).  If the V bit is zero (0) this indicates that VJ
   packets are not being sent over the tunneled data session by the
   LNS.  The H (History) bit is set if any other history-bound
   protocol (other than VJ compression) is being run over the tunneled
   session.  Attribute is [TBD], indicating History Operation Status,
   and is marked mandatory.  This AVP MUST be present in an SU sent by
   the LNS.  This AVP MUST NOT be present in an SU sent by the LAC.


   Receive Window Size

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|0|0|0|0|0|         8         |               0               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              10               |              Size             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Receive Window Size AVP encodes the window size being advertised
   for this call.  Attribute is 10, indicating Receive Window Size,
   and is marked mandatory.  This AVP itself is optional.  The Size
   value indicates the number of received data packets the sender of
   the SU will buffer for this call, which is also the maximum number
   of data packets the receiver of the SU should send before waiting
   for an acknowledgment.

4. Protocol Operation


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   This extension MUST only be used if both L2TP peers have signaled
   support of this extension during tunnel establishment using the
   Extensions AVP defined in [4].

   When a session is first started it is not known if a history-bound
   protocol will be negotiated.  An implementation should therefore
   pick a maximum receive window size based on the assumption that a
   history-bound protocol will be negotiated.  If the tunnel is
   operating over a reliable medium this is not a factor.  If the
   tunnel is not operating over a reliable medium then an
   appropriately small window size should be chosen (recommend 4?).

   When an LNS detects a change in the state of operation of
   history-bound protocols over the tunneled session it MUST send an
   SU to the LAC.  This allows the LAC to make adjustments to its
   maximum receive window size, even if the LNS does not make a change
   itself.

   Upon reception of the SU with Receive Window Size AVP, the receiver
   of the SU MUST begin operating under the rules for Receive Window
   Size AVP values received during data session setup.

   If the value of the Receive Window Size AVP in the SU is greater
   than the value of the last Receive Window Size AVP received, there
   is no further action required by the receiver of the SU other than
   changing its maximum send window accordingly.

   If the value of the Receive Window Size AVP in the SU is less than
   the value of the last Receive Window Size AVP received, then the
   receveiver of the SU must take special action.  In this case, the
   receiver of the SU must change its maximum send window accordingly,
   consider any currently unacknowledged packets as acknowledged, and
   send an R Bit in the next data packet sent to the peer.  This
   prevents the data session from unnecessarily hanging when the
   window size is adjusted down.

   If for a particular data session a peer does not send the Receive
   Window AVP during session establishment, the Receive Window AVP
   MUST NOT be sent in a subsequent SU message.

5. Security Considerations

   Security is not addressed in this document.

References

   [1] A. Valencia, et al, "Layer Two Tunneling Protocol", Work In


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       Progress: draft-ietf-pppext-l2tp-12.txt, October 1998

   [2] D. Rand, "PPP Reliable Transmission". RFC 1663

   [3] V. Jacobson, "Compressing TCP/IP Headers for Low-Speed Serial
       Links", RFC 1144, February 1990

   [4] R. Shea, "Framework for L2TP Message Extensions", Work In
       Progress: draft-ietf-pppext-l2tpmsgext-00.txt, November 1998

Author's Address

   Richard Shea
   Nortel Networks
   125 Nagog Park
   Acton, Massachusetts 01720
   rshea@BayNetworks.com
































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INTERNET DRAFT                                           November 1998