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Versions: 00 01 02 03 04                                                
MULTIMOB Group                                                 H. Asaeda
Internet-Draft                                           Keio University
Expires: May 15, 2008                                  November 12, 2007


          IGMP and MLD Extensions for Mobile Hosts and Routers
         draft-asaeda-multimob-igmp-mld-mobility-extensions-00

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2007).















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Internet-Draft    IGMP and MLD Extensions for Mobility     November 2007


Abstract

   This document describes the IGMP and MLD protocol extensions for
   mobile hosts and routers.  IGMP and MLD are necessary protocols for
   hosts to request join or leave multicast sessions.  While the regular
   IGMP and MLD protocols support communication between mobile hosts and
   routers over wireless networks, this document discusses the
   conditions how mobile hosts and routers use IGMP and MLD in their
   communication more effectively.










































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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 RFC 2119 [1].


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Configurations . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.1.  Tracking of Membership Status  . . . . . . . . . . . . . .  7
     2.2.  IGMP/MLD Query Coordination  . . . . . . . . . . . . . . .  7
       2.2.1.  Unicasting IGMP/MLD General Query  . . . . . . . . . .  7
       2.2.2.  Multicasting IGMP/MLD Group-Specific Query . . . . . .  8
       2.2.3.  Values in IGMP/MLD Query . . . . . . . . . . . . . . .  9
     2.3.  IGMP/MLD Querier Selection . . . . . . . . . . . . . . . . 10
   3.  Protocol Extensions  . . . . . . . . . . . . . . . . . . . . . 11
   4.  Implementations  . . . . . . . . . . . . . . . . . . . . . . . 13
     4.1.  Host-Side Implementation . . . . . . . . . . . . . . . . . 13
     4.2.  Router-Side Implementation . . . . . . . . . . . . . . . . 13
   5.  Interoperability . . . . . . . . . . . . . . . . . . . . . . . 14
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 16
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 18























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1.  Introduction

   The Internet Group Management Protocol (IGMP) for IPv4 and the
   Multicast Listener Discovery Protocol (MLD) for IPv6 are the
   necessary protocols for hosts to request to join or leave multicast
   sessions.  These protocols must be also supported by the upstream
   multicast routers that have the downstream multicast member hosts on
   the same LAN.  By definition, IGMP and MLD work on wireless networks;
   there is not necessary to change these protocol specs for wireless
   communication environments.  However, when mobile hosts or routers
   attached on a wireless link multicast IGMP/MLD messages, the
   transmitted IGMP/MLD messages are flooded to all mobile hosts and
   routers on the link, where a large amount of flooding data consumes
   battery power of each mobile host.  In addition, it takes higher
   costs for the upstream router to maintain a large number of IGMP/MLD
   messages, and in this situation, it takes longer time for the router
   to converge the membership state (i.e. existence of downstream member
   hosts), where the longer convergence negatively affects leave
   latency.

   To create the feasible condition to communicate mobile hosts and
   routers with IGMP/MLD, it is required to "ease processing cost or
   battery power consumption by eliminating transmission of a large
   number of IGMP/MLD messages via flooding" and "realize fast state
   convergence by successive monitoring whether downstream members exist
   or not".  The possible approach to fulfill these requirements is
   relevant; if the upstream router traces all downstream members by
   limiting the number of solicited membership reports (by periodical
   IGMP/MLD Query), the number of transmitted IGMP/MLD messages is
   effectively reduced, and the upstream router can immediately update
   the membership information and proceed the fast leave.

   The function that enables to trace all downstream members is
   supported by IGMPv3 [2] and MLDv2 [3].  In the previous version
   protocols, IGMPv1 [4], IGMPv2 [5], and MLDv1 [6], a host would cancel
   sending a pending membership reports requested by IGMP/MLD Query if a
   similar report was observed from another member on the network.  This
   specification effectively reduced a possibility of network congestion
   or message flooding, but precluded the function for an upstream
   router to track membership status.  On the other hand, in IGMPv3 and
   MLDv2, the membership report suppression mechanism has been removed,
   and therefore all downstream member hosts must send their membership
   reports to an upstream router and the router can keep track of
   membership status.

   If the report suppression mechanism is removed from the host-side
   protocols, the upstream router supporting IGMPv3/MLDv2 receives all
   membership reports from the downstream hosts.  One may deduce that



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   the router does not need to periodically send IGMPv3/MLDv2 Query
   messages to trace membership status.  However, IGMPv3/MLDv2 capable
   routers usually configure to send periodical IGMP/MLD Query messages
   to seek membership information to the downstream hosts, and disable
   the function that keeps track of membership status.  One of the
   reasons is that IGMP/MLD message is non-reliable and may be lost in
   the transmission, and therefore the router would need to confirm the
   membership by sending query messages.  The other reason is that, for
   keeping track of membership status, the router needs additional
   processing cost and a possibly large size of the memory to record all
   member information.  The requirement to keep the compatibility mode
   with older version IGMP/MLD is also the reason, because the router
   needs to support the downstream hosts that are not upgraded to the
   latest versions of IGMP/MLD and run the report suppression mechanism.

   There is one more important function in IGMPv3 and MLDv2.  IGMPv3 and
   MLDv2 provide the ability for hosts to report interest in receiving
   packets only from specific source addresses to upstream routers.
   With IGMPv3/MLDv2, the mobile host specifies the interesting
   multicast and source addresses with INCLUDE filter mode in the join
   request.  Upon reception, the upstream router establishes the
   shortest path tree toward the source without coordinating a shared
   tree.  This function is called the source filtering function and
   required to support Source-Specific Multicast (SSM) [7].  SSM is
   advantageous to multicast routing tree establishment and satisfies
   current and future needs.

   IGMPv3 and MLDv2 support another operation with EXCLUDE filter mode.
   When a mobile host specifies multicast and source addresses with
   EXCLUDE filter mode in the join request, an upstream router forwards
   the multicast packets sent from all sources *except* the specified
   sources.  In fact, this operation gives the complexity in the host-
   side procedure.  If any application running on a host requests an
   EXCLUDE filter mode operation, the host sets the interface state to
   EXCLUDE mode for the requested multicast address, and the source
   address list of the interface record is the intersection of the
   source address lists requested by all applications in EXCLUDE mode,
   minus the source addresses that appear in any application in INCLUDE
   mode.  The state transition that maintains the interface record is
   complex, and the implementation cost will be relatively high for
   mobile hosts.

   Furthermore, specifying non-interesting source addresses with EXCLUDE
   filter mode reduces the advantage of scalable routing tree
   establishment in an SSM manner, because an upstream router needs to
   refresh and re-generate some or all of the corresponding routing tree
   including the RPT whenever the router receives join request with
   EXCLUDE filter mode from the downstream hosts.  This manner increases



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   the tree maintenance cost to not only the upstream multicast routers
   but other routers existed on the routing paths.  While the mobile
   multicast communication does not prohibit a traditional (*,G) join
   request (which is a join request with EXCLUDE filter mode without
   specifying any source address), all other join requests with EXCLUDE
   filter mode should be eliminated from the mobile multicast
   communication.

   This document describes the IGMP and MLD protocol extensions for
   mobile hosts and routers, and discusses the conditions how mobile
   hosts and routers use IGMP and MLD in their communication over
   wireless networks effectively.  The selective solutions that provide
   tangible benefits to the mobile hosts and routers are given by
   "keeping track of membership status by eliminating a report
   suppression mechanism", "varying IGMP/MLD Query types and values to
   tune the number of responses", and "using a source filtering
   mechanism in a lightweight manner".  The proposed solutions do not
   require changing the IGMP and MLD protocols.  This condition is
   advantageous to the deployment.
































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2.  Configurations

2.1.  Tracking of Membership Status

   Mobile hosts use IGMP and MLD to request to join or leave multicast
   sessions.  When the upstream routers receive the IGMP/MLD reports,
   they recognize the membership status on the LAN.  To update the
   membership status, the routers send IGMP/MLD Query messages
   periodically as a soft-state approach does, and the member hosts
   reply IGMP/MLD report messages upon reception.

   IGMP/MLD Query is therefore necessary to obtain the up-to-date
   membership information, but a large number of the reply messages sent
   from all member hosts may cause network congestion or consume network
   bandwidth.  To escape from the trouble, a membership report
   suppression mechanism was proposed in the traditional IGMP and MLD
   [4][5][6].  By the report suppression mechanism, a host would cancel
   sending a pending membership reports requested by IGMP/MLD Query if a
   similar report was observed from another member on the network.
   However, the report suppression mechanism precluded the function for
   an upstream router to track membership status.  In IGMPv3 and MLDv2,
   it is hence decided that the membership report suppression mechanism
   has been removed, and all downstream member hosts must send their
   membership reports to an upstream router.

   By eliminating membership report suppression, an IGMPv3 or MLDv2
   capable upstream router could trace all downstream members and track
   per-host membership status on the interface.  This reduces the number
   of solicited membership reports by periodical IGMP/MLD Query, and
   finally the total number of transmitted IGMP/MLD messages can be
   drastically reduced.  This is beneficial especially to mobile hosts
   that do not have enough battery power, since flooding IGMP/MLD
   messages on a LAN makes all multicast members give significant
   attention and induces power consumption to the member hosts.  This
   also allows the upstream router to proceed fast leaves, because the
   router can immediately converge and update the membership
   information, ideally.

2.2.  IGMP/MLD Query Coordination

2.2.1.  Unicasting IGMP/MLD General Query

   IGMP and MLD are asymmetric and non-reliable protocols; multicast
   routers still need to solicited membership reports by periodical
   IGMP/MLD Query, in order to be robust in front of host or link
   failures and packet loss.  Moreover, it happens that mobile hosts may
   turn off or move from the wireless network to other wireless network
   managed by the different router without any notification (i.e. leave



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   report).  Therefore, even though multicast routers keep track of the
   interests of downstream member hosts attached on the same LAN, IGMP/
   MLD Query must be sent periodically.

   However, periodical message flooding using the all-hosts multicast
   address (i.e. 224.0.0.1 or ff02::1) as its IP destination address
   gives the unwilling situation to the mobile hosts.  When the mobile
   hosts are operating in a dormant mode and not communicating with
   others, they should not be woken up by IGMP/MLD General Query and
   keep sleeping for saving the battery power.  In this case, only the
   hosts that are receiving multicast contents should make the response
   to the router.

   IGMPv3 and MLDv2 specifications [2][3] say that a host MUST accept
   and procss any Query whose IP Destination Address field contains any
   of the addresses (unicast or multicast) assigned to the interface on
   which the Query arrives.  According to the scenario, it unicasts the
   message to tracked member hosts in the [Unicast Query Interval].  It
   is happened especially when a multicast router has a small number of
   mobile hosts that are listening different multicast sessions.  In
   this situation, the router multicasts IGMP/MLD General Query with
   longer [Query Interval] (described in Section 2.2.3).

   [TODO: Define [Unicast Query Interval] value.  The value could be
   same of the default [Query Interval]?]

2.2.2.  Multicasting IGMP/MLD Group-Specific Query

   In the standard protocols [2][3], IGMP/MLD Group-Specific Query is
   sent to verify there are no hosts that desire reception of the
   specified group or to rebuild the desired reception state for a
   particular group.  Group-Specific Queries are sent when a router
   receives a State-Change record indicating a host is leaving a group.

   In a dormant mode operation, IGMP/MLD Group-Specific Query can be
   also used to build and refresh the group membership state of hosts on
   attached networks.  When more than one mobile host join the multicast
   contents whose multicast address is same, IGMP/MLD Group-Specific
   Query can be sent to maintain the group membership state of mobile
   hosts on attached networks, instead of IGMP/MLD General Query.  Since
   IGMP/MLD Group-Specific Query specifies the corresponding multicast
   address (not the all-hosts multicast address) as its IP destination
   address, dormant mode hosts that do not join any multicast session
   are not woken up by the IGMP/MLD Group-Specific Query and only active
   group member hosts that have been receiving multicast contents would
   reply IGMP/MLD reports.  This manner contributes to reducing the
   number of transmitted IGMP/MLD messages.




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   The [Multicast Group-Query Interval] is the interval between Group-
   Specific Queries sent by the querier, i.e., the router that sends the
   Group-Specific Query.  This value is same of the default [Query
   Interval] value the regular IGMP and MLD define [2][3].

   [TODO: Define [Multicast Group-Query Interval].]

2.2.3.  Values in IGMP/MLD Query

   A multicast router operating in a dormant mode keeps track of the
   membership status and checks the membership status by transmitting
   unicast IGMP/MLD General Query or multicast IGMP/MLD Group-Specific
   Query.  Cooperating with these scenarios, the message interval
   between IGMP/MLD General Queries is set to longer than the default
   [Query Interval] value.

   The Query Interval is the interval between General Queries sent by
   the querier, and the default value is 125 seconds [2][3].  By varying
   the [Query Interval], multicast routers can tune the number of IGMP
   messages on the network; larger values cause IGMP Queries to be sent
   less often.

   [TODO: We will provide the appropriate [Query Interval] value that
   would fit in the mobile communication environment based on some
   experimental results.  In our current sense, this value should be
   larger than the default value the regular IGMP and MLD define.]

   The Query Response Interval is the Max Response Time (or Max Response
   Delay) used to calculate the Max Resp Code inserted into the periodic
   General Queries, and the default value is 10 seconds [2][3].  By
   varying the [Query Response Interval], multicast routers can tune the
   burstiness of IGMP messages on the network; larger values make the
   traffic less bursty, as host responses are spread out over a larger
   interval.

   [TODO: We will provide the appropriate [Query Response Interval]
   value that would fit in the mobile communication environment based on
   some experimental results.  In our current sense, this value should
   be less than the default value the regular IGMP and MLD define,
   because, while the larger Query Interval does not reduce the number
   of transmitted IGMP/MLD messages, it may cause slow leave latency.]

   Mobile hosts may receive a variety of Queries on different interfaces
   and of different kinds (e.g., General Queries, Group-Specific
   Queries, and Group-and-Source-Specific Queries), each of which may
   require its own delayed response.

   [TODO: The timer management for each queries may or should be



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   independent.  E.g. the timer value for General Query should be longer
   than the one of other queries.  We will investigate this issue.]

   To cover the possibility of unsolicited reports being missed by
   multicast routers, unsolicited reports are retransmitted [Robustness
   Variable] - 1 more times, at intervals chosen at random from the
   defined range [2][3].  The QRV (Querier's Robustness Variable) field
   in IGMP/MLD Query contains the [Robustness Variable] value used by
   the querier.  Routers adopt the QRV value from the most recently
   received Query as their own [Robustness Variable] value, whose range
   should be set between "1" to "7".  While the default [Robustness
   Variable] value defined in IGMPv3 [2] and MLDv2 [3] is "2", the
   [Robustness Variable] value announced by the querier must not be "0"
   and should not be "1".

   [TODO: We will propose the robustness values that would be adjusted
   according to the number of receivers.  In our current sense, this
   value should not be bigger than "2" especially when the [Query
   Response Interval] is set to less than its default value.]

2.3.  IGMP/MLD Querier Selection

   [TODO: Is there any condition or assumption in which multiple
   multicast routers exist in a single wireless link?  If there is the
   case, do we need to consider IGMP/MLD querier selection mechanism and
   the corresponding timer values or intervals?  The Querier's Query
   Interval Code (QQIC) field specifies the [Query Interval] used by the
   querier may be tuned.  The actual interval, called the Querier's
   Query Interval (QQI), is derived from QQIC.  Multicast routers that
   are not the current querier adopt the QQI value from the most
   recently received Query as their own [Query Interval] value.]




















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3.  Protocol Extensions

   IGMPv3 and MLDv2 enable all member hosts to send membership reports
   to the upstream routers.  Not only this function, IGMPv3 and MLDv2
   support a source filtering function.  An IGMPv3 or MLDv2 capable host
   can tell its upstream router which group it would like to join by
   specifying which sources it does (or does not) intend to receive
   multicast traffic from.  IGMPv3 and MLDv2 add the capability for a
   multicast router to also learn which sources are (and are not) of
   interest to neighboring hosts, for packets sent to any particular
   multicast address.  This source filtering function is required to
   invoke Source-Specific Multicast (SSM) [7].

   IGMPv3 and MLDv2 introduce antithetic filter modes, INCLUDE and
   EXCLUDE filter modes, to expand the source filtering function.  If a
   host wants to receive from specific sources, it sends an IGMPv3 or
   MLDv2 report with the filter mode set to INCLUDE.  If the host does
   not want to receive from some sources, it sends a report with the
   filter mode set to EXCLUDE.  A source list for the given sources
   shall be included in the report message.  INCLUDE and EXCLUDE filter
   modes are also defined in a multicast router to process the IGMPv3 or
   MLDv2 reports.  When a multicast router receives the report messages
   from its downstream hosts, it forwards the corresponding multicast
   traffic by managing requested group and source addresses.

   However, practical applications do not use EXCLUDE mode to block
   sources very often, because a user or application usually wants to
   specify desired source addresses, not undesired source addresses.  In
   addition, this scheme leads an implementation cost to mobile hosts
   and complex procedures to maintain coexisting situation of the
   interesting source address lists with INCLUDE filter mode or non-
   interesting source address lists with EXCLUDE filter mode.

   Recently, Lightweight-IGMPv3 (LW-IGMPv3) and Lightweight-MLDv2 (LW-
   MLDv2) [8] are proposed in the IETF MBONED working group.  These
   protocols are the simplified versions of IGMPv3 and MLDv2, and
   eliminate an EXCLUDE filter mode operation.  Not only are LW-IGMPv3
   and LW-MLDv2 fully compatible with the full version of these
   protocols (i.e., the standard IGMPv3 and MLDv2), but also the
   protocol operations made by hosts and routers are simplified in the
   lightweight manner, and complicated operations are effectively
   reduced.  LW-IGMPv3 and LW-MLDv2 give the opportunity to grow SSM
   use.

   In the lightweight protocols, EXCLUDE mode on the host part is
   preserved only for EXCLUDE (*,G) join/leave, which denotes a non-
   source-specific group report (known as the traditional (*,G) join/
   leave) and is equivalent to the group membership join/leave triggered



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   by IGMPv2/IGMPv1/MLDv1.

   The aim of LW-IGMPv3 and LW-MLDv2 is not only for contributing to the
   simpler implementation or reducing the memory size on a host.
   Another advantage is that it reduces the processing cost on upstream
   routers by eliminating the EXCLUDE filter mode operations.  If both
   INCLUDE and EXCLUDE filter mode operations are supported in the
   networks, the routers need to maintain all source addresses joined
   from their downstream hosts.  Even if a Shortest-Path Tree (SPT) is
   well coordinated, the routers need to refresh (and re-generate) some
   or all of the corresponding routing paths including the Rendezvous-
   Point Tree (RPT) whenever the downstream host requests EXLUDE filter
   mode join.  LW-IGMPv3 and LW-MLDv2 preclude the unwilling situation.
   Since there is no side-effect, this document recommend to adopt LW-
   IGMPv3 and LW-MLDv2 to mobile hosts and routers, or eliminate EXCLUDE
   filter mode operation from mobile hosts if IGMPv3 and MLDv2 are
   adopted to hosts.


































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4.  Implementations

4.1.  Host-Side Implementation

   Mobile hosts should implement IGMPv3 or LW-IGMPv3 for IPv4 multicast
   and MLDv2 or LW-MLDv2 for IPv6 multicast.  All of these protocols
   eliminate a membership report suppression mechanism, and make hosts
   work with the function multicast routers use to trace downstream
   member hosts.  These protocols also support SSM.  According to the
   protocol requirement aforementioned, however, this document
   recommends to implement LW-IGMPv3 for IPv4 and LW-MLDv2 for IPv6 [8]
   rather than the full version protocols.

4.2.  Router-Side Implementation

   To keep track of multicast membership status and cooperate with SSM
   capable mobile hosts, multicast routers must implement IGMPv3/
   LW-IGMPv3 or MLDv2/LW-MLDv2.  The protocol requirement aforementioned
   does not require modification of the IGMPv3/LW-IGMPv3 and MLDv2/
   LW-MLDv2 protocol specifications.  This condition is advantageous to
   the deployment.  However, regarding the router-side implementation,
   the function to trace downstream members requires the hardware
   requirement that would cost the router additional hardware resources,
   especially CPU and memory resources.

   [TODO: This document assumes that multicast routers are not tiny and
   non-powerful systems nor battery or power sensitive.  Our assumption
   is correct?]

   As well as the host-side implementation, the elimination of the
   EXCLUDE filter mode will greatly simplify the router behavior, e.g.
   the action on reception of reports and the setting of the timers.
   This document therefore recommends to implement LW-IGMPv3 for IPv4
   and LW-MLDv2 for IPv6 rather than their full version protocols.  The
   detailed operation being simplified is described in [8].
















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5.  Interoperability

   TBD.

   [TODO: We believe it would be currently feasible to assume the
   routers who take care of mobile hosts MUST be IGMPv3/MLDv2 capable
   (regardless whether the protocols are the full version or not).  What
   we should understand is whether there is the case that mobile hosts
   may not be IGMPv3/MLDv2 capable or not.]










































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6.  Security Considerations

   TBD.
















































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7.  References

7.1.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to indicate requirement
        levels", RFC 2119, March 1997.

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

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

   [4]  Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
        August 1989.

   [5]  Fenner, W., "Internet Group Management Protocol, Version 2",
        RFC 2373, July 1997.

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

   [7]  Holbrook, H. and B. Cain, "Source-Specific Multicast for IP",
        RFC 4607, August 2006.

7.2.  Informative References

   [8]  Liu, H., Cao, W., and H. Asaeda, "Lightweight IGMPv3 and MLDv2
        Protocols",
        draft-ietf-mboned-lightweight-igmpv3-mldv2-01.txt (work in
        progress), June 2007.



















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Author's Address

   Hitoshi Asaeda
   Keio University
   Graduate School of Media and Governance
   5322 Endo
   Fujisawa, Kanagawa  252-8520
   Japan

   Email: asaeda@wide.ad.jp









































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Internet-Draft    IGMP and MLD Extensions for Mobility     November 2007


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