[Docs] [txt|pdf] [draft-ietf-l2tpex...] [Diff1] [Diff2]

EXPERIMENTAL

Network Working Group                                         G. Bourdon
Request for Comments: 4045                                France Telecom
Category: Experimental                                        April 2005


             Extensions to Support Efficient Carrying of
        Multicast Traffic in Layer-2 Tunneling Protocol (L2TP)

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   The Layer Two Tunneling Protocol (L2TP) provides a method for
   tunneling PPP packets.  This document describes an extension to L2TP,
   to make efficient use of L2TP tunnels within the context of deploying
   multicast services whose data will have to be conveyed by these
   tunnels.

Table of Contents

   1.  Introduction..................................................  2
       1.1.  Conventions Used in This Document.......................  3
       1.2.  Terminology.............................................  3
   2.  Motivation for a Session-Based Solution.......................  4
   3.  Control Connection Establishment..............................  5
       3.1.  Negotiation Phase.......................................  5
       3.2.  Multicast Capability AVP (SCCRQ, SCCRP).................  5
   4.  L2TP Multicast Session Establishment Decision.................  6
       4.1.  Multicast States in LNS.................................  6
       4.2.  Group State Determination...............................  8
       4.3.  Triggering..............................................  9
       4.4.  Multicast Traffic Sent from Group Members............... 10
   5.  L2TP Multicast Session Opening Process........................ 11
       5.1.  Multicast-Session-Request (MSRQ)........................ 11
       5.2.  Multicast-Session-Response (MSRP)....................... 12
       5.3.  Multicast-Session-Establishment (MSE)................... 12
   6.  Session Maintenance and Management............................ 13
       6.1.  Multicast-Session-Information (MSI)..................... 13
       6.2.  Outgoing Sessions List Updates.......................... 14



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             6.2.1.  New Outgoing Sessions AVP (MSI)................. 15
             6.2.2.  New Outgoing Sessions Acknowledgement AVP (MSI). 15
             6.2.3.  Withdraw Outgoing Sessions AVP (MSI)............ 17
       6.3.  Multicast Packets Priority AVP (MSI).................... 17
             6.3.1.  Global Configuration............................ 18
             6.3.2.  Individual Configuration........................ 19
             6.3.3.  Priority........................................ 19
   7.  Multicast Session Teardown.................................... 19
       7.1.  Operations.............................................. 20
       7.2.  Multicast-Session-End-Notify (MSEN)..................... 20
       7.3.  Result Codes............................................ 21
   8.  Traffic Merging............................................... 22
   9.  IANA Considerations........................................... 22
   10. Security Considerations....................................... 23
   11. References.................................................... 23
       11.1. Normative References.................................... 23
       11.2. Informative References.................................. 24
   12. Acknowledgements.............................................. 24
   Appendix A.  Examples of Group States Determination............... 25
   Author's Address.................................................. 27
   Full Copyright Statement.......................................... 28

1.  Introduction

   The deployment of IP multicast-based services may have to deal with
   L2TP tunnel engineering.  The forwarding of multicast data within
   L2TP sessions may impact the throughput of L2TP tunnels because the
   same traffic may be sent multiple times within the same L2TP tunnel,
   but in different sessions.  This proposal aims to reduce the impact
   by applying the replication mechanism of multicast traffic only when
   necessary.

   The solution described herein provides a mechanism for transmitting
   multicast data only once for all the L2TP sessions that have been
   established in a tunnel, each multicast flow having a dedicated L2TP
   session.

   Within the context of deploying IP multicast-based services, it is
   assumed that the routers of the IP network that embed a L2TP Network
   Server (LNS) capability may be involved in the forwarding of
   multicast data, toward users who access the network through an L2TP
   tunnel.  The LNS is in charge of replicating the multicast data for
   each L2TP session that a receiver who has requested a multicast flow
   uses.  In the solution described here, an LNS is able to send
   multicast data only once and to let the L2TP Access Concentrator
   (LAC) perform the traffic replication.  By doing so, it is expected
   to spare transmission resources in the core network that supports




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   L2TP tunnels.  This multicast extension to L2TP is designed so that
   it does not affect the behavior of L2TP equipment under normal
   conditions.

   A solution whereby multicast data is carried only once in a L2TP
   tunnel is of interest to service providers, as edge devices are
   aggregating more and more users.  This is particularly true for
   operators who are deploying xDSL (Digital Subscriber Line) services
   and cable infrastructures.  Therefore, L2TP tunnels that may be
   supported by the network will have to carry multiple redundant
   multicast data more often.  The solution described in this document
   applies to downstream traffic exclusively; i.e., data coming from the
   LNS toward end-users connected to the LAC.  This downstream multicast
   traffic is not framed by the LNS but by the LAC, thus ensuring
   compatibility for all users in a common tunnel, whatever the framing
   scheme.

1.1.  Conventions Used in This Document

   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].

1.2.  Terminology

   Unicast session

      This term refers to the definition of "Session" as it is described
      in the terminology section of [RFC2661].

   Multicast session

      This term refers to a connection between the LAC and the LNS.
      Additional Control Messages and Attribute-Value-Pairs (AVPs) are
      defined in this document to open and maintain this connection for
      the particular purpose of multicast traffic transportation.  This
      connection between the LAC and the LNS is intended to convey
      multicast traffic only.

   Session

      This term is used when there is no need to dissociate multicast
      from unicast sessions, and thus it designates both.








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   M-IGP

      Designates a Multicast Interior Gateway Protocol.

   Multicast flow

      Designates datagrams sent to a group from a set of sources for
      which multicast reception is desired.

   GMP

      Group Management Protocol, such as:

         - IGMPv1 ([RFC1112])
         - IGMPv2 ([RFC2236])
         - MLD ([RFC2710], [RFC3590])

   SFGMP

      Source Filtering Group Management Protocol, such as:

         - IGMPv3 ([RFC3376])
         - MLDv2 ([RFC3810])

2.  Motivation for a Session-Based Solution

   Multicast data have to be seen as a singular flow that may be
   conveyed into all the L2TP sessions that have been established in a
   tunnel.  This means that a given L2TP session can be dedicated for
   the forwarding of a multicast flow that will be forwarded to multiple
   receivers, including those that can be reached by one or several of
   these L2TP sessions.  A session carrying IP multicast data is
   independent from the underlying framing scheme and is therefore
   compatible with any new framing scheme that may be supported by the
   L2TP protocol.

   Using a single L2TP session per multicast flow is motivated by the
   following arguments:

   -  The administrator of the LNS is presumably in charge of the IP
      multicast-based services and the related engineering aspects.  As
      such, he must be capable of filtering multicast traffic on a
      multicast source basis, on a multicast group basis, and on a user
      basis (users who access the network using an L2TP session that
      terminates in this LNS).






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   -  Having an L2TP session dedicated for a multicast flow makes it
      possible to enforce specific policies for multicast traffic.  For
      instance, it is possible to change the priority treatment for
      multicast packets against unicast packets.

   -  It is not always acceptable or possible to have multicast
      forwarding performed within the network between the LAC and the
      LNS.  Having the multicast traffic conveyed within an L2TP tunnel
      ensures a multicast service between the LNS and end-users,
      alleviating the need for activating multicast capabilities in the
      underlying network.

3.  Control Connection Establishment

3.1.  Negotiation Phase

   The multicast extension capability is negotiated between the LAC and
   the LNS during the control connection establishment phase.  However,
   establishment procedures defined in [RFC2661] remain unchanged.  An
   LAC indicates its multicast extension capability by using a new AVP,
   the "Multicast Capability" AVP.  There is no explicit acknowledgement
   sent by the LNS during the control connection establishment phase.
   Instead, the LNS is allowed to use multicast extension messages to
   open and maintain multicast sessions (see Section 5).

3.2.  Multicast Capability AVP (SCCRQ, SCCRP)

   In order to inform the LNS that an LAC has the ability to handle
   multicast sessions, the LAC sends a Multicast Capability AVP during
   the control connection establishment phase.  This AVP is either sent
   in a SCCRQ or a SCCRP control message by the LAC towards the LNS.

   Upon receipt of the Multicast Capability AVP, a LNS may adopt two
   distinct behaviors:

   1) The LNS does not implement the L2TP multicast extension: any
      multicast-related information (including the Multicast Capability
      AVP) will be silently ignored by the LNS.

   2) The LNS implements L2TP multicast extensions and therefore
      supports the Multicast Capability AVP: the LNS is allowed to send
      L2TP specific commands for conveying multicast traffic toward the
      LAC.

   The multicast capability exclusively refers to the tunnel for which
   the AVP has been received during the control connection establishment
   phase.  It SHOULD be possible for an LNS administrator to shut down




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   L2TP multicast extension features towards one or a set of LAC(s).  In
   this case, the LNS behavior is similar to that in 1).

   The AVP has the following format:

      Vendor ID = 0
      Attribute = 80 (16 bits)

       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|0|0|0|0|      Length       |          Vendor ID            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              80               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The M-bit MUST be set to 0, the AVP MAY be hidden (H-bit set to 0 or
   1).

   The length of this AVP is 6 octets.

4.  L2TP Multicast Session Establishment Decision

4.1.  Multicast States in LNS

   The router that embeds the LNS feature MUST support at least one
   Group Management Protocol (GMP), such as:

      - IGMPv1
      - IGMPv2
      - MLD

   or a Source Filtering Group Management Protocol (SFGMP), such as:

      - IGMPv3
      - MLDv2

   The LAC does not have any group management activity: GMP or SFGMP
   processing is performed by the LNS.  The LAC is a layer-2 equipment,
   and is not supposed to track GMP or SFGMP messages between the
   receivers and the LNS in this context.  The LNS MUST always be at the
   origin of the creation of a multicast L2TP session dedicated for the
   forwarding of IP multicast datagrams destined to a multicast group.
   The LNS acts as a GMP or SFGMP Querier for every logical interface
   associated to an L2TP session.






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   As a multicast router, the equipment that embeds the LNS function
   will keep state per group per attached network (i.e., per L2TP
   session).  The LNS-capable equipment activating multicast extensions
   for L2TP will have to classify and analyze GMP and SFGMP states in
   order to create L2TP multicast sessions within the appropriate L2TP
   tunnels.  This is performed in three steps:

   1) The LNS has to compute group states for each L2TP tunnel, by using
      group states recorded for each L2TP session of the tunnel.  Group
      state determination for L2TP tunnels is discussed in Section 4.2.
      For each L2TP tunnel, the result of this computation will issue a
      list of states of the form (group, filter-mode, source-list):

      -  group: Denotes the multicast group.
      -  filter-mode: Either INCLUDE or EXCLUDE, as defined in
         [RFC3376].
      -  source-list: List of IP unicast addresses from which multicast
         reception is desired or not, depending on the filter-mode.

   2) According to each group state, the LNS will create one or multiple
      replication contexts, depending on the filter-mode for the
      considered group and the local policy configured in the LNS.

      For groups in INCLUDE mode, the LNS SHOULD implement two different
      policies:

      -  One session per (source, group) pair: the LNS creates one
         replication context per (source, group) pair.
      or
      -  One session per group: the LNS creates one replication context
         per (source-list, group) pair.

      For groups in EXCLUDE mode, the LNS will create one replication
      context per (list of sources excluded by *all* the receivers,
      group).  The list of sources represents the intersection of the
      sets, not the union.

   3) For each replication context, the LNS will create one L2TP
      multicast session (if threshold conditions are met; see Section
      4.3) and its associated Outgoing Session List (OSL).  The OSL
      lists L2TP sessions that requested the multicast flow
      corresponding to the group and the associated source-filtering
      properties.  There is one OSL per replication context; i.e., per
      L2TP multicast session.

   For a group member running an SFGMP, it is therefore possible to
   receive multicast traffic from sources that have been explicitly
   excluded in its SFGMP membership report if other group members in the



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   same L2TP tunnel wish to receive packets from these sources.  This
   behavior is comparable to the case where group members are connected
   to the same multi-access network.  When a group is in EXCLUDE mode or
   in INCLUDE mode with a policy allowing one session per (group,
   source-list), sharing the same L2TP tunnel is equivalent to being
   connected to the same multi-access network in terms of multicast
   traffic received.  For groups in INCLUDE mode with a policy allowing
   one L2TP multicast session per (source, group), the behavior is
   slightly improved because it prevents group members from receiving
   traffic from non-requested sources.  On the other hand, this policy
   potentially increases the number of L2TP multicast sessions to
   establish and maintain.  Examples are provided in Appendix A.

   In order for the LAC to forward the multicast traffic received
   through the L2TP multicast session to group members, the LNS sends
   the OSL to the LAC for the related multicast session (see Section 6).

4.2.  Group State Determination

   Source Filtering Group Management Protocols require querier routers
   to keep a filter-mode per group per attached network, to condense the
   total desired reception state of a group to a minimum set so that all
   systems' memberships are satisfied.

   Within the context of L2TP, each L2TP session has to be considered an
   attached network by GMP and SFGMP protocols.  When the L2TP multicast
   extension is activated, each L2TP Control Connection has to be
   considered a pseudo attached network, as well, in order to condense
   group membership reports for every L2TP session in the tunnel.

   Therefore, a list of group states is maintained for each L2TP Control
   Connection into which the membership information of each of its L2TP
   sessions is merged.  This list of group states is a set of membership
   records of the form (group, filter-mode, source-list).

   Each group state represents the result of a merging process applied
   to subscriptions on L2TP sessions of a Control Connection for a
   considered group.  This merging process is performed in three steps:

      1) Conversion of any GMP subscription into SFGMP subscription
         (IGMPv1/v2 to IGMPv3, MLDv1 to MLDv2);

      2) Removal of subscription timers and, if filter-mode is EXCLUDE,
         sources with source timer > 0;

      3) Then, resulting subscriptions are merged by using merging rules
         described in SFGMP specifications ([RFC3376], Section 3.2,
         [RFC3810], Section 4.2).



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   This process is also described in [PROXY].  Examples of group state
   determination are provided in Appendix A.

4.3.  Triggering

   The rules to be enforced by the LNS whereby it is decided when to
   open a dedicated L2TP multicast session for a multicast group SHOULD
   be configurable by the LNS administrator.  This would typically
   happen whenever a threshold of MULTICAST_SESSION_THRESHOLD
   receivers/sessions referenced in a replication context is reached.
   This threshold value SHOULD be valued at 2 by default, as it is worth
   opening a dedicated L2TP multicast session for two group members
   sharing the same desired reception state (which means that two L2TP
   unicast sessions are concerned).  In this case, the OSL will
   reference two distinct L2TP sessions.

   The actual receipt by the LNS of multicast traffic requested by end-
   users can also be taken into account to decide whether the associated
   L2TP multicast session has to be opened.

   Whenever an OSL gets empty, the LNS MUST stop sending multicast
   traffic over the corresponding L2TP multicast session.  Then the L2TP
   multicast session MUST be torn down as described in Section 7.

   Filter-mode changes for a group can also trigger the opening or the
   termination of L2TP multicast sessions in the following ways:

   a) From INCLUDE Mode to EXCLUDE Mode

   When a group state filter-mode switches from INCLUDE to EXCLUDE, only
   one replication context (and its associated L2TP multicast session)
   issued from this group state can exist (see Section 4.1).  The LNS
   SHOULD keep one replication context previously created for this group
   state and it has to update it with:

      -  a new source-list that has to be excluded from forwarding
      -  a new OSL

   The LNS MUST send an OSL update to the LAC to reflect L2TP session
   list changes (section 6.2), whenever appropriate.  The unused L2TP
   multicast sessions that correspond to previously created replication
   contexts for the group SHOULD be terminated, either actively or
   passively by emptying their corresponding OSLs.

   The remaining L2TP multicast session MAY also be terminated if the
   number of receivers is below a predefined threshold (see Section 7).
   To limit the duration of temporary packet loss or duplicates to
   receivers, the LNS has to minimize delay between OSL updates messages



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   sent to the LAC.  Therefore, one can assume that terminating a
   multicast session passively gives the smoothest transition.

   b) From EXCLUDE Mode to INCLUDE Mode

   When a group state filter-mode switches from EXCLUDE to INCLUDE,
   multiple replication contexts issued by this group state may be
   created (see Section 4.1).  The LNS SHOULD keep the replication
   context previously created for this group state and it has to update
   it accordingly with the following information:

      -  a new list of sources that has to be forwarded.  This list has
         only one record if there is one replication context per (group,
         source)
      -  a new OSL

   The LNS MUST send an OSL update to the LAC to reflect L2TP session
   list changes, whenever appropriate.  If the LNS is configured to
   create one replication context per (group, source), L2TP multicast
   sessions will be opened in addition to the existing one, depending on
   the number of sources for the group.

   If new L2TP multicast sessions have to be opened, the LNS SHOULD wait
   until these multicast sessions are established before updating the
   OSL of the original multicast session.  To limit the duration of
   temporary packet loss or duplicates to receivers, the LNS has to
   minimize delay between OSL updates messages sent to the LAC.

4.4.  Multicast Traffic Sent from Group Members

   The present document proposes a solution to enhance the forwarding of
   downstream multicast traffic exclusively; i.e., data coming from the
   LNS toward end-users connected to the LAC.  If a group member that
   uses an L2TP session is also a multicast source for traffic conveyed
   in a multicast session, datagrams may be sent back to the source.  To
   prevent this behavior, two options can be used in the LNS:

      1) Disable the multicast packets' forwarding capability, for those
         multicast datagrams sent by users connected to the network by
         means of an L2TP tunnel.  Protocols using well-known multicast
         addresses MUST NOT be impacted.

      2) Exclude from the OSL the L2TP session used by a group member
         that sends packets matching the replication context of this
         OSL.  Therefore, the corresponding multicast flow is sent by
         the LNS over the user L2TP unicast session, using standard
         multicast forwarding rules.




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5.  L2TP Multicast Session Opening Process

   The opening of an L2TP multicast session is initiated by the LNS.  A
   three-message exchange is used to set up the session.  The following
   is a typical sequence of events:

      LAC              LNS
      ---              ---
                       (multicast session
                       triggering)

                       <- MSRQ
      MSRP ->

      (Ready to
       replicate)

      MSE  ->
                       <- ZLB ACK

   The ZLB ACK is sent if there are no further messages waiting in the
   queue for that peer.

5.1.  Multicast-Session-Request (MSRQ)

   Multicast-Session-Request (MSRQ) is a control message sent by the LNS
   to the LAC to indicate that a multicast session can be created.  The
   LNS initiates this message according to the rules in Section 4.3.  It
   is the first in a three-message exchange used for establishing a
   multicast session within an L2TP tunnel.

   A LNS MUST NOT send a MSRQ control message if the remote LAC did not
   open the L2TP tunnel with the Multicast Capability AVP.  The LAC MUST
   ignore MSRQ control messages sent in an L2TP tunnel, if the L2TP
   tunnel was not opened with control messages including a Multicast
   Capability AVP.

   The following AVPs MUST be present in MSRQ:

      Message Type
      Assigned Session ID

   The following AVPs MAY be present in MSRQ:

      Random Vector
      Maximum BPS





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   The Maximum BPS value is set by the LNS administrator.  However, this
   value should be chosen in accordance with the line capabilities of
   the end-users.  The Maximum BPS value SHOULD NOT be higher than the
   highest speed connection for all end-users within the L2TP tunnel.

   The associated Message Type AVP is encoded with the following values:

      Vendor ID = 0
      Attribute Type = 0
      Attribute Value = 23 (16 bits)

   The M-bit MUST be set to 0, and the H-bit MUST be set to 0.

5.2.  Multicast-Session-Response (MSRP)

   Multicast-Session-Response (MSRP) is a control message sent by the
   LAC to the LNS in response to a received MSRQ message.  It is the
   second in a three-message exchange used for establishing a multicast
   session within an L2TP tunnel.

   MSRP is used to indicate that the MSRQ was successful and that the
   LAC will attempt to reserve appropriate resources to perform
   multicast replication for unicast sessions managed in the pertaining
   control connection.

   The following AVPs MUST be present in MSRP:

      Message Type
      Assigned Session ID

   The following AVP MAY be present in MSRP:

      Random Vector

   The associated Message Type AVP is encoded with the following values:

      Vendor ID = 0
      Attribute Type = 0
      Attribute Value = 24 (16 bits)

   The M-bit MUST be set to 0, and the H-bit MUST be set to 0.

5.3.  Multicast-Session-Establishment (MSE)

   Multicast-Session-Establishment (MSE) is a control message sent by
   the LAC to the LNS to indicate that the LAC is ready to receive
   necessary multicast information (Section 6) for the group using the




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   newly created multicast session.  It is the third message in the
   three-message sequence used for establishing a multicast session
   within an L2TP tunnel.

   The following AVP MUST be present in MSE:

      Message Type

   The following AVP MAY be present in MSE:

      Sequencing Required

   Sequencing will occur only from the LNS to the LAC, as a multicast
   session is only used to forward multicast traffic downstream.

   The associated Message Type AVP is encoded with the following values:

      Vendor ID = 0
      Attribute Type = 0
      Attribute Value = 25 (16 bits)

   The M-bit MUST be set to 0, and the H-bit MUST be set to 0.

6.  Session Maintenance and Management

   Once the multicast session is established, the LAC has to be informed
   of the L2TP unicast sessions interested in receiving the traffic from
   the newly created multicast session, and a related optional priority
   parameter, defined in Section 6.3.  To achieve this, a new control
   message type is defined: Multicast-Session-Information (MSI).

6.1.  Multicast-Session-Information (MSI)

   Multicast-Session-Information (MSI) control messages carry AVPs to
   keep the OSL synchronized between the LNS and the LAC, and to set the
   optional priority parameter for multicast traffic versus unicast
   traffic.  MSI may be extended to update the multicast session with
   additional parameters, as needed.

   Each MSI message is specific to a particular multicast session.
   Therefore, the control message MUST use the assigned session ID
   associated with the multicast session (assigned by the LAC), except
   for the case mentioned in 6.3.2.








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   The associated Message Type AVP is encoded with the following values:

      Vendor ID = 0
      Attribute Type = 0
      Attribute Value = 26 (16 bits)

   The M-bit MUST be set to 0, and the H-bit MUST be set to 0.

   The following AVP MUST be present in MSI:

      Message Type

   The following AVPs MAY be present in MSI:

      Random Vector
      New Outgoing Sessions
      New Outgoing Sessions Acknowledgement
      Withdraw Outgoing Sessions
      Multicast Packets Priority

   New Outgoing Sessions, New Outgoing Sessions Acknowledgement,
   Withdraw Outgoing Sessions, and Multicast Packets Priority are new
   AVPs defined in sections 6.2 and 6.3.

6.2.  Outgoing Sessions List Updates

   Whenever a change occurs in the Outgoing Sessions List, the LNS MUST
   inform the LAC of that change.  The OSL is built upon subscription
   reports recorded by GMP or SFGMP processes running in the LNS
   (Section 4.1).

   The LAC maintains an OSL as a local table transmitted by the LNS.  As
   for the LNS, the LAC has to maintain an OSL for each L2TP multicast
   session within an L2TP tunnel.  To update the LAC OSL, the LNS sends
   a New Outgoing Sessions AVP for additional sessions, or sends a
   Withdraw Outgoing Sessions AVP to remove sessions.  All sessions
   mentioned in these AVPs MUST be added or removed by the LAC from the
   relevant OSL.  The Outgoing Sessions List is identified by the tunnel
   ID and the multicast session ID to which the updating AVP refers.  To
   update the OSL, the following AVPs are used:

      Additional session(s): New Outgoing Sessions AVP
      Session(s) removal: Withdraw Outgoing Sessions AVP

   These new AVPs MUST be sent in an MSI message.






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6.2.1.  New Outgoing Sessions AVP (MSI)

   The New Outgoing Sessions AVP can only be carried within an MSI
   message type.  This AVP piggybacks every Session ID to which the
   multicast traffic has to be forwarded.

   The AVP has the following format:

      Vendor ID = 0
      Attribute = 81 (16 bits)

       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|0|0|0|0|      Length       |          Vendor ID            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              81               |         Session ID 0          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              ...              |         Session ID N          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   There can be from 1 to N Session IDs present in the New Outgoing
   Sessions AVP (considering the maximum value of the Length field).
   This AVP must be placed in an MSI message and sent after the
   establishment of the multicast session to indicate the initial
   outgoing sessions to the LAC, and must be sent at any time when one
   or more outgoing sessions appear during the multicast session
   lifetime.  Upon receipt of this AVP, the LAC sends a New Outgoing
   Sessions Acknowledgment AVP to the LNS to notify that the LAC is
   ready to replicate the multicast traffic toward the indicated
   sessions.

   Usage of this AVP is incremental; only new outgoing sessions have to
   be listed in the AVP.

   The M-bit MUST be set to 1, and the AVP MAY be hidden (H-bit set to 0
   or 1).

6.2.2.  New Outgoing Sessions Acknowledgement AVP (MSI)

   The New Outgoing Sessions Acknowledgement AVP can only be carried
   within an MSI message type.  This AVP informs the LNS that the LAC is
   ready to replicate traffic for every Session ID listed in the AVP.








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   The AVP has the following format:

      Vendor ID = 0
      Attribute = 82 (16 bits)

       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|0|0|0|0|      Length       |          Vendor ID            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              82               |         Session ID 0          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              ...              |         Session ID N          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This AVP must be placed in an MSI message and sent by the LAC toward
   the LNS to acknowledge the receipt of a New Outgoing Sessions list
   received in a New Outgoing Sessions AVP from the LNS.

   An LNS is allowed to send multicast traffic within the L2TP multicast
   session as soon as a New Outgoing Sessions Acknowledgement AVP is
   received for the corresponding L2TP multicast session.

   An LNS is allowed to stop sending packets of the corresponding
   multicast flow within L2TP unicast sessions only if it receives an
   MSI message with the New Outgoing Session Acknowledgement AVP, and
   only for the unicast Session IDs mentioned in this AVP.  The
   multicast traffic can then be conveyed in L2TP unicast sessions when
   the L2TP multicast session goes down.  From this standpoint, packets
   related to this multicast flow SHOULD NOT be conveyed within the L2TP
   unicast sessions mentioned in the AVP in order to avoid the
   duplication of multicast packets.

   There can be from 1 to N Session IDs present in the New Outgoing
   Sessions Acknowledgement AVP (considering the maximum value of the
   Length field).  Session IDs mentioned in this AVP that have not been
   listed in a previous New Outgoing Sessions AVP should be ignored.
   Non-acknowledged Session IDs MAY be listed in forthcoming New
   Outgoing Sessions AVPs, but multicast traffic MUST be sent to logical
   interfaces associated to these Session IDs as long as these Session
   IDs are not acknowledged for replication by the LAC.

   The M-bit MUST be set to 1, and the AVP MAY be hidden (H-bit set to 0
   or 1).







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6.2.3.  Withdraw Outgoing Sessions AVP (MSI)

   The Withdraw Outgoing Sessions AVP is sent whenever there is one or
   more withdrawn subscriptions for the corresponding multicast flow
   (designated by the session ID on which the MSI is sent).

   The LAC can stop forwarding packets to Session IDs mentioned in the
   AVP for the corresponding multicast flow as soon as it receives the
   MSI message embedding this Withdraw Target Session AVP.

   The AVP has the following format:

      Vendor ID = 0
      Attribute = 83 (16 bits)

       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|0|0|0|0|      Length       |          Vendor ID            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              83               |         Session ID 0          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              ...              |         Session ID N          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   There can be from 1 to N Session IDs present in the Withdraw Outgoing
   Sessions AVP (considering the value of the Length field).  The M-bit
   MUST be set to 1, and the AVP MAY be hidden (H-bit set to 0 or 1).

6.3.  Multicast Packets Priority AVP (MSI)

   The Multicast Packets Priority AVP is an optional AVP intended to
   indicate to the LAC how to process multicast traffic against unicast
   traffic.  Even though the LAC behavior is partially described here,
   the nature of the traffic (layer-2 frames for unicast traffic and
   pure IP packets for multicast traffic) is not a criteria for
   enforcing a traffic prioritization policy.  Traffic processing for
   the provisioning of a uniformly framed traffic for the final user is
   described is section 8.

   Three different behaviors can be adopted:

   1) Best effort: the traffic is forwarded from the LAC to the end-user
      in the order in which it comes from the LNS, whatever the type of
      traffic.






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   2) Unicast traffic priority: traffic coming down the L2TP unicast
      session has priority over traffic coming down the L2TP multicast
      session.

   3) Multicast traffic priority: traffic coming down the L2TP multicast
      session has priority over traffic coming down the L2TP unicast
      session.

   The priority is encoded as a 16-bit quantity, which can take the
   following values:

      0: Best effort (default)
      1: Unicast traffic priority
      2: Multicast traffic priority

   The AVP has the following format:

      Vendor ID = 0
      Attribute = 84 (16 bits)

       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|0|0|0|0|      Length       |          Vendor ID            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              84               |        Priority Value         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Note that the multicast traffic rate can reach up to Maximum BPS (as
   indicated in MSRQ).  This rate can exceed the maximum rate allowed
   for a particular end-user.  This means that even with a priority
   value of 0, the end-user may receive multicast traffic only; unicast
   packets might be dropped because the multicast flow overwhelms the
   LAC forwarding buffer(s).

   The default Priority Value is 0.  The M-bit MUST be set to 0, and the
   AVP MAY be hidden (H-bit set to 0 or 1).

   There are two ways of using this AVP: global configuration and
   individual configuration.

6.3.1.  Global Configuration

   The Multicast Priority Packet AVP is sent for all L2TP unicast
   sessions concerned with a specific multicast flow represented by an
   L2TP multicast session.  In this case, the AVP is sent in an L2TP MSI
   control message for the corresponding multicast session ID (Session
   ID = L2TP session for the corresponding multicast group).  The



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   priority value applies to all L2TP unicast sessions to which the
   multicast group designated by the L2TP multicast session is intended,
   as soon as this AVP is received.

6.3.2.  Individual Configuration

   The Multicast Priority Packet AVP is sent for a specific L2TP unicast
   session that SHALL adopt a specific behavior for both unicast and
   multicast traffics.  In this case, the AVP is sent in an L2TP MSI
   control message for the L2TP unicast session (Session ID = L2TP
   session for the concerned user).  The priority value applies to the
   targeted session only and does not affect the other sessions.  Note
   that in this case, all multicast packets carried in L2TP multicast
   sessions are treated the same way by the LAC for the concerned user.

   This is the only case in which an MSI control message can be sent for
   an L2TP unicast session.

6.3.3.  Priority

   It is the responsibility of the network administrator to decide which
   behavior to adopt between global or individual configurations, if the
   AVP is sent twice (one for a multicast group and one for a specific
   end-user).  By default, only the individual configurations SHOULD be
   taken into consideration in that case.

   Support of the Multicast Packets Priority AVP is optional and SHOULD
   be configurable by the LAC administrator, if it is relevant.

7.  Multicast Session Teardown

   An L2TP multicast session should be torn down whenever there are no
   longer any users interested in receiving the corresponding multicast
   traffic.  A multicast session becomes useless once the related OSL
   has fewer than a predefined number of entries, this number being
   defined by a threshold.

   Multicast session flapping may occur when the number of OSL entries
   oscillates around the threshold, if the same value is used to trigger
   the creation or deletion of an L2TP multicast session.  To avoid this
   behavior, two methods can be used:

   -  The threshold value that is used to determine whether the L2TP
      multicast session has to be torn down is lower than the
      MULTICAST_SESSION_THRESHOLD value;






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   -  The MULTICAST_SESSION_THRESHOLD value is used to determine whether
      the L2TP multicast session has to be torn down.  A multicast
      session SHOULD be killed after a period of
      MULTICAST_SESSION_HOLDTIME seconds if the corresponding OSL
      maintains fewer than a MULTICAST_SESSION_THRESHOLD number of
      entries.  The MULTICAST_SESSION_HOLDTIME value is 10 seconds by
      default and SHOULD be configurable by either the LAC or the LNS
      administrator.

   The multicast session can be torn down for multiple reasons,
   including specific criteria not described here (which can be vendor
   specific).

   A multicast session teardown can be initiated by either the LAC or
   the LNS.  However, multicast session teardown MUST be initiated by
   the LNS if the termination decision is motivated by the number of
   users interested in receiving the traffic corresponding to a
   multicast flow.

7.1.  Operations

   The actual termination of a multicast session is initiated with a new
   Multicast-Session-End-Notify (MSEN) control message, sent either by
   the LAC or by the LNS.

   The following is an example of a control message exchange that
   terminates a multicast session:

      LAC or LNS      LAC or LNS
      ----------      ----------
                      (multicast session
                      termination)

                      <- MSEN
                      (Clean up)
      ZLB ACK ->
      (Clean up)

7.2.  Multicast-Session-End-Notify (MSEN)

   The Multicast-Session-End-Notify (MSEN) is an L2TP control message
   sent by either the LAC or the LNS to request the termination of a
   specific multicast session within the tunnel.  Its purpose is to give
   the peer the relevant termination information, including the reason
   why the termination occurred.  The peer MUST clean up any associated
   resources and does not acknowledge the MSEN message.





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   As defined in [RFC2661], termination of a control connection will
   terminate all sessions managed within, including multicast sessions
   if there are any.

   The MSEN message carries a Result Code AVP with an optional Error
   Code.

   The following AVPs MUST be present in an MSEN message:

      Message Type
      Result Code
      Assigned Session ID

   The associated Message Type AVP is encoded with the following values:

      Vendor ID = 0
      Attribute Type = 0
      Attribute Value = 27 (16 bits)

   The M-bit MUST be set to 0, and the H-bit MUST be set to 0.

7.3.  Result Codes

   The following values are the defined result codes for MSEN control
   messages:

      1 (16 bits) - No multicast traffic for the group
      2 (16 bits) - Session terminated for the reason indicated in
                    the error code
      3 (16 bits) - No more receivers
      4 (16 bits) - No more receivers (filter-mode change)

      o  The code 1 MAY be used when the LAC detects that no traffic is
         coming down the multicast session, or when the LNS doesn't
         receive multicast traffic to be conveyed over the L2TP
         multicast session during a certain period of time.

      o  The code 2 refers to General Error Codes maintained by the IANA
         for L2TP.

      o  The code 3 MAY be used by the LAC or the LNS when the OSL is
         empty.

      o  The code 4 MAY be used by the LNS when a multicast session is
         torn down because of a filter-mode change.  This result code
         SHOULD also be used when the OSL becomes empty after a filter-
         mode change (passive termination when filter-mode changes from
         INCLUDE to EXCLUDE; see Section 4.3).



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8.  Traffic Merging

   Both unicast and multicast traffics have to be merged by the LAC in
   order to forward properly framed data to the end-user.  Multicast
   packets are framed by the LAC and transmitted toward the proper end-
   user.  Methods used to achieve this function are not described here,
   since it is an implementation-specific issue.

   All frames conveyed from the LAC to the end-users have to follow the
   framing scheme applied for the considered peer to which the traffic
   is destined (e.g., the LAC is always aware of the PPP [RFC1661] link
   parameters, as described in [RFC2661], Section 6.14).  Note that
   using L2TP Multicast Extension features is not appropriate for end-
   users who have negotiated a sequenced layer-2 connection with the
   LNS.  While inserting PPP-encapsulated multicast packets in a
   session, the LAC cannot modify PPP sequencing performed by the LNS
   for each PPP session.

9.  IANA Considerations

   This document defines:

      - 5 new Message Type (Attribute Type 0) Values:
           o Multicast-Session-Request (MSRQ)      : 23
           o Multicast-Session-Response (MSRP)     : 24
           o Multicast-Session-Establishment (MSE) : 25
           o Multicast-Session-Information (MSI)   : 26
           o Multicast-Session-End-Notify (MSEN)   : 27

      - 5 new Control Message Attribute Value Pairs:
           o Multicast Capability                  : 80
           o New Outgoing Sessions                 : 81
           o New Outgoing Sessions Acknowledgement : 82
           o Withdraw Outgoing Sessions            : 83
           o Multicast Packets Priority            : 84

      - 4 Result Codes for the MSEN message:
           o No multicast traffic for the group    : 1
           o Session terminated for the reason indicated in the
             error code                            : 2
           o No more receivers                     : 3
           o No more receivers (filter-mode change): 4









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10.  Security Considerations

   It is possible for one receiver to make additional multicast traffic
   that has not been requested go down the link of another receiver.
   This can happen if a single replication context per group is used in
   INCLUDE mode with receivers having divergent source lists, and in
   EXCLUDE mode if a receiver has a source list not shared by another.
   This behavior can be encountered every time receivers are connected
   to a common multi-access network.

   The extension described in this document does not introduce any
   additional security issues as far as the activation of the L2TP
   protocol is concerned.

   Injecting appropriate control packets in the tunnel toward the LAC to
   modify Outgoing Session List and to flood end-users with unwanted
   multicast traffic is only possible if the control connection is
   hacked.  As for any reception of illegitimate L2TP control messages,
   the following apply:

      -  If the spoofed control message embeds consistent sequence
         numbers, next messages will appear out of synch, yielding the
         control connection to terminate.

      -  If sequence numbers are inconsistent with current control
         connection states, the spoofed control message will be queued
         or discarded, as described in [RFC2661], Section 5.8.

   The activation of the L2TP multicast capability on the LAC could make
   the equipment more sensitive to Denial of Service attacks if the
   control connection or the related LNS is hacked.  The LAC might also
   be sensitive to the burden generated by the additional replication
   work.

   As mentioned in [RFC2661], Section 9.2, securing L2TP requires that
   the underlying transport make encryption, integrity, and
   authentication services available for all L2TP traffic, including
   L2TP multicast traffic (control and data).

11.  References

11.1.  Normative References

   [RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5,
             RFC 1112, August 1989.

   [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
             RFC 1661, July 1994.



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

   [RFC2236] Fenner, W., "Internet Group Management Protocol, Version
             2", RFC 2236, November 1997.

   [RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G.,
             and B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC
             2661, August 1999.

   [RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
             Listener Discovery (MLD) for IPv6", RFC 2710, October 1999.

   [RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
             Thyagarajan, "Internet Group Management Protocol, Version
             3", RFC 3376, October 2002.

   [RFC3438] Townsley, W., "Layer Two Tunneling Protocol (L2TP) Internet
             Assigned Numbers Authority (IANA) Considerations Update",
             BCP 68, RFC 3438, December 2002.

   [RFC3590] Haberman, B., "Source Address Selection for the Multicast
             Listener Discovery (MLD) Protocol", RFC 3590, September
             2003.

   [RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
             Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

11.2.  Informative References

   [PROXY]   Fenner, B., He, H., Haberman, B., Sandick, H., "IGMP/MLD-
             based Multicast Forwarding ("IGMP/MLD Proxying")", Work in
             Progress.

12.  Acknowledgements

   Thanks to Christian Jacquenet for all the corrections done on this
   document and his precious advice, to Pierre Levis for his
   contribution about IGMP, to Francis Houllier for PPP considerations,
   and to Xavier Vinet for his input about thresholds.  Many thanks to
   W. Mark Townsley, Isidor Kouvelas, and Brian Haberman for their
   highly valuable input on protocol definition.









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Appendix A.  Examples of Group States Determination

   *Example 1:

   All users are managed in the same control connection.

      Users {1, 2, 3} subscribe to (Group G1, EXCLUDE {})
      Users {3, 4, 5} subscribe to (Group G2, EXCLUDE {})

   Group states for this L2TP tunnel will be:

      (G1, EXCLUDE, {})
      (G2, EXCLUDE, {})

   Therefore, two replication contexts will be created:

      -RC1:
      (*, G1) packets, Multicast Session MS1, OSL = 1, 2, 3
      -RC2:
      (*, G2) packets, Multicast Session MS2, OSL = 3, 4, 5


   *Example 2:

   All users are managed in the same control connection.

      Users {1, 2, 3} subscribe to (Group G1, INCLUDE {S1})
      Users {4, 5, 6} subscribe to (Group G1, INCLUDE {S1,S2})
      Users {7, 8, 9} subscribe to (Group G1, INCLUDE {S2})

   The group state for this L2TP tunnel will be:

      (G1, INCLUDE, {S1, S2)})

   If the LNS policy allows one replication context per (group, source),
   two replication contexts will be created:

      -RC1:
      (S1, G1) packets, Multicast Session MS1, OSL = 1, 2, 3, 4, 5, 6
      -RC2:
      (S2, G1) packets, Multicast Session MS2, OSL = 4, 5, 6, 7, 8, 9

   If the LNS policy allows one replication context per (group, source-
   list), one replication context will be created:

      -RC1:
      ({S1, S2}, G1) packets, Multicast Session MS1, OSL = [1..9]




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   *Example 3:

   All users are managed in the same control connection.

      Users {1, 2} subscribe to (Group G1, EXCLUDE {S1})
      User {3} subscribes to (Group G1, EXCLUDE {S1, S2})

   The group state for this L2TP tunnel will be:

      (G1, EXCLUDE, {S1})

   Therefore, one replication context will be created:

      -RC1:
      (*-{S1}, G1) packets, Multicast Session MS1, OSL = 1, 2, 3

   Next, user {4} subscribes to (Group G1, INCLUDE {S1}).  The group
   state for the L2TP tunnel is changed to:

      (G1, EXCLUDE, {})

   The replication context RC1 is changed to:

      -RC1: (*, G1) packets, Multicast Session MS1, OSL = 1, 2, 3, 4


   *Example 4:

   All users are managed in the same control connection.  The LNS policy
   allows one replication context per (group, source).

      Users {1, 2, 3} subscribe to (Group G1, INCLUDE {S1, S2})

   The group state for this L2TP tunnel will be:

      (G1, INCLUDE, {S1, S2)})

   Therefore, two replication contexts will be created:

      -RC1:
      (S1, G1) packets, Multicast Session MS1, OSL = 1, 2, 3
      -RC2:
      (S2, G1) packets, Multicast Session MS2, OSL = 1, 2, 3







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   Next, user {4} subscribes to (Group G1, EXCLUDE {}), equivalent to an
   IGMPv2 membership report.  The group state for the L2TP tunnel is
   changed to:

      (G1, EXCLUDE, {})

   The replication context RC1 is changed to:

      -RC1: (*, G1) packets, Multicast Session MS1, OSL = 1, 2, 3, 4

   The replication context RC2 is changed to:

      -RC2: no packets to forward, Multicast Session MS2, OSL = {}
      (Multicast Session MS2 will be deleted)

   When user {4} leaves G1, the group state for the L2TP tunnel goes
   back to:

      (G1, INCLUDE, {S1, S2})

   Replication contexts become:

      -RC1:
      (S1, G1) packets, Multicast Session MS1, OSL = 1, 2, 3
      -RC2:
      (S2, G1) packets, Multicast Session MS2, OSL = 1, 2, 3
      (Multicast Session MS2 is re-established)

Author's Address

   Gilles Bourdon
   France Telecom
   38-40, rue du General Leclerc
   92794 Issy les Moulineaux Cedex 9 - FRANCE

   Phone: +33 1 4529-4645
   EMail: gilles.bourdon@francetelecom.com














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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
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   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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   The IETF takes no position regarding the validity or scope of any
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   The IETF invites any interested party to bring to its attention any
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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