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Versions: 00 01 02 03 04 05 06 07 08 09 10                              
INTERNET DRAFT                                                 S. Biswas
IDMR Working Group                                               B. Cain
                                                         Nortel Networks
                                                             B. Haberman
                                                                     IBM
                                                             August 1999
                                                   Expires February 1999

                     IGMP Multicast Router Discovery
                   <draft-ietf-idmr-igmp-mrdisc-02.txt>

STATUS OF THIS MEMO

 This document is an Internet-Draft and is in  full  conformance  with
 all provisions of Section 10 of RFC2026.

 Internet-Drafts are working documents  of  the  Internet  Engineering
 Task  Force  (IETF),  its  areas,  and its working groups.  Note that
 other groups may  also  distribute  working  documents  as  Internet-
 Drafts.

 Internet-Drafts are draft documents valid for a maximum of six months
 and  may be updated, replaced, or obsoleted by other documents at any
 time.  It is inappropriate  to  use  Internet-  Drafts  as  reference
 material or to cite them other than as work in progress.

 The  list   of   current   Internet-Drafts   can   be   accessed   at
 http://www.ietf.org/ietf/1id-abstracts.txt

 The list of Internet-Draft Shadow  Directories  can  be  accessed  at
 http://www.ietf.org/shadow.html.


                                Abstract

   Companies have been proposing "IGMP snooping" type schemes for
   layer-2 bridging devices.  A method for discovering multicast capable
   routers is necessary for these schemes.  An IGMP query message is
   inadequate for discoverying multicast routers as one querier is
   elected.  In order to "discover" multicast routers, we introduce
   two new types of IGMP messages: Multicast Router Advertisement and
   Multicast Router Solicitation.  These two messages can be used by
   any device which listens to IGMP to discovery multicast routers.
   Multicast Router Solicitation messages may be used by any network
   device (e.g. layer-2 switch) to solicit discovery messages from
   multicast routers.

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1.   Introduction

    Multicast router discovery messages are useful for discovering
    multicast capable routers.  This capability is useful in a layer-2
    bridging domain with "IGMP snooping" type of schemes.  By listening
    to multicast router discovery messages, layer-2 devices can
    determine where to send multicast source data and IGMP Host
    Membership Reports [RFC1112] [IGMPv2].  Multicast source data and
    IGMP Host Membership Reports must be received by all multicast
    routers on a segment.  Using IGMP Host Membership Queries to
    discover multicast routers is not useful because of query
    suppression in IGMP.

    Unlike ICMP router discovery messages [RFC1256], multicast router
    discovery advertisements should not be listened to by hosts.
    Hosts need not know the identity of multicast routers.

    The use of the multicast router advertisement is not precluded
    from being used for other purposes.  Extensible options have been
    included in the advertisement message for future enhancements.

    The following are justifications for inventing another router
    discovery protocol:

       1. Using ICMP router discovery is not an appropriate solution
          for multicast router discovery because: 1.) It may confuse
          hosts listening to ICMP router advertisements; unicast and
          multicast topologies may not be congruent.  2.) It is
          desirable to have advertisements sent to a special link-
          local group address.  3.) There is no way to tell from a
          ICMP router advertisement if a router is running a multicast
          routing protocol.
       2. By making multicast router discovery messages extensible
          and sending messages to a special address, future
          enhancements can be made.
       3. By inventing a generic IP layer message, multiple types of
          messages per link layer are not needed (i.e. including this
          functionality as part of IP is better than inventing N
          discovery protocols for N layer-2 technologies).

   Although multicast router discovery messages could be sent as
   ICMP messages, IGMP was chosen because IGMP snooping switches
   already snoop IGMP messages and because the intended first use
   of these protocol messages is multicast specific.

1.1  Protocol Overview

    IGMP Multicast Router Discovery consists of three messages for
    discovering multicast routers.  The Multicast Router Advertisement
    is sent by routers to advertise IP multicast forwarding enabled
    on an interface.  The Multicast Router Solicitation is used by

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    routers to solicit Multicast Router Advertisements.  The Multicast
    Router Termination message is sent when a router terminates its
    multicast routing functions.

    Multicast routers send Multicast Router Advertisements (hereafter
    called advertisements) periodically on all interfaces on which
    multicast forwarding is enabled.

    Multicast Router Advertisements are also sent in response to
    Multicast Router Solicitations (hereafter called solicitations).
    These are sent to solicit a response of Multicast Router
    Advertisements from all multicast routers on a subnet.
    Solicitations are sent to the IGMP-MRDISC multicast group.

    Multicast Router Solicitations are sent whenever a router wishes
    to discover multicast routers on a directly attached subnet.

    Multicast Router Termination messages are sent when a router
    terminates its multicast routing functions.

    All IGMP Multicast Router Discovery messages are sent with an
    IP TLL of 1 and contain the IP Router Alert Option [RFC2113] in
    their IP header.  All IGMP Multicast Router Discovery messages
    are sent with to the IGMP-MRDISC multicast group (224.0.0.x).

    Other non-IP forwarding devices (e.g. layer-2 switches) may
    send Multicast Router Solicitations to solicit Multicast Router
    Advertisements.


2.   Multicast Router Advertisement

2.1  Overview

    Multicast Router Advertisements are sent periodically on all router
    interfaces on which multicast forwarding is enabled.  They are also
    sent in response to Multicast Router Solicitations.

    Router advertisements are sent upon expiration of a periodic
    timer, when a router starts up, and when a router interface (that
    has IP multicast forwarding enabled) initializes/restarts.
    Advertisements are sent as IGMP messages to the IGMP-MRDISC
    multicast address (224.0.0.x) and should be rate-limited.

    Router advertisements may contain any number of options.  Two
    options are defined in this document and MUST be supported by any
    implementation of IGMP multicast router discovery.  These options
    are described in Section 5.  Additional options may be defined as
    needed by future work.

2.2  IP Header Fields

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2.2.1  Source Address

    An IP address belonging to the interface from which this message is
    sent.  If multiple source addresses are configured on an interface,
    then the one chosen is implementation dependent.

2.2.2  Destination Address

    Router Advertisements are sent to the IGMP-MRDISC multicast
    address (224.0.0.x).

2.2.3  Time-to-Live

    The Time-to-Live field MUST be 1.

2.2.4  Protocol

    The protocol field is set to IGMP (2).

2.3  Multicast Router Advertisement Message Format

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      | Ad. Interval  |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Unused            |     Number of Options (N)     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Option [1] (TLV encoded)                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Option [...] (TLV encoded)              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Option [N] (TLV encoded)                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.3.1  Type Field

    The type field is set to 0x24.

2.3.2  Advertisement Interval

    This specifies the periodic time interval at which Multicast Router
    Advertisements are sent in units of seconds.  This value is set to
    the configured MaxAdvertisementInterval variable.

2.3.3  Checksum

    The 16-bit one's complement of the one's complement sum of the IGMP
    message, starting with the IGMP type.  For computing the checksum,
    the Checksum field is set to 0.

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2.3.4  Number of Options (N)

    The number of options contained in the router advertisement. If no
    options are sent this field MUST be set to 0.

2.3.5  Option[1..N]

    Options are encoded as TLV in the following manner:

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |    Length     |           Value               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    If the Number of Options field is not zero, all options MUST be
    examined by a receiver.  No strict ordering of options is enforced.

    Type: Set to option type being advertised

    Length: Length in bytes of Value field

    Value: Option dependent value

2.4  Sending Multicast Router Advertisements

    Router Advertisements are sent when the following events occur:

      1.  When the periodic advertisement interval timer expires.
          Note that it is not strictly periodic because the
          advertisement interval is a random number between
          MaxAdvertisementInterval and  MinAdvertisementInterval.
          (Default Value: 7-10 seconds).

      2.  After waiting for a random delay less than
          MaxInitialAdvertisementInterval when an interface first
          comes up, is (re)initialized, or IGMP Multicast Router
          Discovery is enabled.  A router may send up to a maximum of
          MaxInitialAdvertisements advertisements, waiting for a
          random delay less than MaxInitialAdvertisementInterval
          between each successive advertisement.

          This is to prevent an implosion of router advertisements.  An
          example of this occuring would be when many routers are
          powered on at the same time.

      3.  When a solicitation is received, a router advertisement is
          sent in response with a random delay less than
          MAX_RESPONSE_DELAY.  If a solicitation is received while
          an advertisement is pending (because of a recent
          solicitation), that solicitation will be ignored.

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    Whenever an advertisement is sent, the periodic advertisement
    interval timer may be reset.

2.5  Receiving Multicast Router Advertisements

    Upon receiving a router advertisement, routers will validate the
    message by the following criteria:

       1.  Verifying that the IGMP type is 0x24
       2.  Verifying the IGMP checksum
       3.  IP Destination Address = IGMP-MRDISC multicast address

    A router advertisement not meeting the validity requirements will
    be silently discarded. Routers MUST process all options, discarding
    options that are not recognized.

    If a router advertisement is not received for a particular neighbor
    within NeighborDeadInterval time interval, then the neigbor is
    considered to be unreachable.

2.6  Multicast Router Advertisement Configuration Variables

    A router that implements multicast router discovery MUST allow for
    the following variables to be configured by system management;
    default values are specified so as to make it unnecessary to
    configure any of these variables in many cases.

    For each interface the following configurable variables are
    defined:

2.6.1  MaxAdvertisementInterval

    The maximum time allowed between sending router advertisements from
    the interface, in seconds. Must be no less than 2 seconds and no
    greater than 180 seconds.

    Default: 20 seconds.

2.6.2  MinAdvertisementInterval

    The minimum time allowed between sending unsolicited router
    advertisements from the interface, in seconds. Must be no less
    than 3 seconds and no greater than MaxAdvertisementInterval.

    Default: 0.75 * MaxAdvertisementInterval

2.6.3  MaxInitialAdvertisementInterval

    The first router advertisement out of an interface is sent after
    waiting for a random interval less than this variable. This will
    prevent a flood of router advertisements when many routers start up
    at the same time.

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    Default: 2 seconds

2.6.4  MaxInitialAdvertisements

    The maximum number of router advertisements that will be sent
    on a subnet after a router boots.

    Default: 3

2.6.5  NeighborDeadInterval

    The maximum time allowed before declaring that a neighbor can
    can be declared "dead".  This variable is defined in seconds.
    In order for all routers to have a consistent state, it is
    necessary for the MaxAdvertisementInterval to be configured the
    same on all routers per subnet.

    Default: 3 * MaxAdvertisementInterval

3.   Multicast Router Solicitation

3.1  Overview

    Multicast Router Solitications are used to solicit Multicast Router
    Advertisements.  These messages are used when a router (or other
    device) wishes to discover multicast routers.  Upon receiving a
    solicitation on an interface with IP multicast forwarding enabled,
    router will respond with an advertisement.

    Router solicitations may be sent when a router starts up, when
    a router interface (re)initializes, or when IGMP Multicast Router
    Discovery is enabled.  Solicitations are sent as IGMP messages to
    the IGMP-MRDISC multicast address (224.0.0.x) and should be
    rate-limited.

3.2  IP Header Fields

3.2.1  Source Address

    An IP address belonging to the interface from which this message is
    sent.  If multiple source addresses are configured on an interface,
    then the one chosen is implementation dependent.

    If the solicitation is being sent from a device which does not have
    an IP address (i.e. non-managed layer-2 switch), then the source
    address should be set to all zeros.

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3.2.2  Destination Address

    Solicitation messages are sent to the IGMP-MRDISC multicast
    address (224.0.0.x).

3.2.3  Time-to-Live

    The time-to-live field MUST be 1.

3.2.4  Protocol

    The protocol field is set to IGMP (2).

3.3  Multicast Router Solicitation Message Format

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |   Reserved    |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.3.1  Type Field

    The type field is set to 0x25.

3.3.2  Reserved Field

    Sent as 0; ignored on reception.

3.3.3  Checksum

    The 16-bit one's complement of the one's complement sum of the IGMP
    message, starting with the IGMP type.  For computing the checksum,
    the Checksum field is set to 0.

3.4  Sending Multicast Router Solicitations

    Router solicitations are sent when the following events occur:

      1.  After waiting for a random delay less than
          SOLICITATION_INTERVAL when an interface first comes up, is
          (re)initialized, or IGMP Multicast Router Discovery is
          enabled.  A router may send up to a maximum of
          MAX_SOLICITATIONS, waiting for a random delay less than
          SOLICITATION_INTERVAL between each successive solicitation.
      2.  Optionally, for an implementation specific event.
          Solicitations MUST be rate-limited; no more than
          MAX_SOLICITATIONS MUST be sent in SOLICITATION_INTERVAL
          seconds.

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3.5  Receiving Multicast Router Solicitations

    Upon receiving a router solicitation, routers will validate the
    message by:

       1.  Verifying that the IGMP type is 0x25
       2.  Verifying the IGMP checksum
       3.  IP Destination Address = IGMP-MRDISC multicast address

   A router solicitation not meeting the validity requirements will be
   silently discarded.

   Solicitation message IP source addresses MUST NOT be used as part
   of the validity check.

3.6  Multicast Router Solicitation Configuration Variables

   There are no configurable variables with respect to router
   solicitations.

4.   Multicast Router Termination

4.1  Overview

    The Multicast Router Termination message is used to expedite the
    notification of a change in the status of a routers multicast
    forwarding functions.

4.2  IP Header Fields

4.2.1  Source Address

    An IP address belonging to the interface from which this message is
    sent.  If multiple source addresses are configured on an interface,
    then the one chosen is implementation dependent.

4.2.2  Destination Address

    Multicast Router Termination messages are sent to the IGMP-MRDISC
    multicast address (224.0.0.x).

4.2.3  Time-to-Live

    The Time-to-Live field MUST be 1.

4.2.4  Protocol

    The protocol field is set to IGMP (2).

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4.3  Multicast Router Termination Message Format

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |   Reserved    |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.3.1  Type Field

    The type field is set to 0x26.

4.3.2  Checksum

    The 16-bit one's complement of the one's complement sum of the IGMP
    message, starting with the IGMP type.  For computing the checksum,
    the Checksum field is set to 0.

4.4  Sending Multicast Router Termination Messages

    Multicast Router Termination messages are sent for three reasons :

        1.  Multicast forwarding is disabled on the interface
        2.  The interface is administratively disabled
        3.  The router is gracefully shutdown

4.5  Receiving Multicast Router Termination Messages

    Upon receiving a termination message, routers will validate the
    message by the following criteria:

       1.  Verifying that the IGMP type is 0x26
       2.  Verifying the IGMP checksum
       3.  IP Destination Address = IGMP-MRDISC multicast address

    A termination message not meeting the validity requirements will
    be silently discarded.

5.   Multicast Router Discovery Protocol Constants

     MAX_RESPONSE_DELAY                   2 seconds

     MAX_SOLICITATION_DELAY               1 second

     SOLICITATION_INTERVAL                3 seconds

     MAX_SOLICITATIONS                    3 transmissions

6.   Mandatory Advertisement Options

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6.1  Overview of Options

    The following options MUST be supported by an implementation of
    IGMP Multicast Router Disovery: Query Interval Advertisement
    Option and Robustness Variable Advertisement Option.  These options
    advertise specific IGMP variables and are sent in an advertisement
    depending on the version of IGMP enabled on an interface.  Although
    no requirements exist for multicast routers at this time, it is
    assumed that all multicast routers support the IGMP protocol.

6.2  Query Interval Advertisement Option

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Type=1      |  Length=2     |      IGMP Query Interval      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    If a multicast router has any version of IGMP [RFC1112] enabled on
    an interface on which IGMP Multicast Router Discovery is also
    enabled, it MUST send all advertisements with the Query Interval
    Advertisement Option.  This option contains the IGMP "Query
    Interval" configured on the interface on which advertisements are
    sent.

    This option is sent regardless of whether the router is currently
    the IGMP querier for the subnet.  This option is sent regardless of
    what version of IGMP the router is running.

    IGMP Query Interval field is equal (in seconds) to the configured
    IGMP "query interval" on the interface from which the advertisement
    was sent.

6.3  Robustness Variable Advertisement Option

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Type=2      |  Length=2     |     Robustness Variable       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    If a multicast router has IGMPv2 [IGMPv2] or IGMPv3 [IGMPv3]
    enabled on an interface on which IGMP Multicast Router Discovery
    is also enabled, it MUST send all advertisements with the
    Robustness Variable Advertisement Option.  This option contains
    the IGMP "Robustness Variable" configured on the interface on
    which advertisements are sent.

    This option is sent regardless of whether the router is currently
    the IGMP querier for the subnet.  This option may be omitted if
    IGMPv1 is enabled on the interface.

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    Robustness Variable is an integer which MUST not be zero [IGMPv2]
    and is equal to the IGMPv2 robustness variable.

7.   IPv6 Support

    The Multicast Router Discovery function for IPv6 is accomplished
    using the Neighbor Discovery Protocol for IPv6 [RFC2461] (hereafter
    called NDP).  Specifically, the Router Advertisement message
    contains new fields to support the discovery of multicast routers.
    For this reason, the timing mechanisms defined for NDP will be used
    instead of those defined in this document for IPv4 support.

7.1  Router Advertisement Message

    The Router Advertisement message contains two new fields to support
    the multicast router discovery mechanism.  The modified message
    format is :

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Cur Hop Limit |M|O|D|E| Rsrvd |       Router Lifetime         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Reachable Time                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Retrans Timer                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

    The two new fields are the 'D' and 'E' bits.  All other fields
    retain their defintions and functions as described in Section 4.2
    of the NDP specification [RFC2461].

7.1.1  Discovery (D) bit

    The 'D' bit is used by a router to indicate support for the
    Multicast Router Discovery protocol.  A value of '1' indicates that
    the router supports the discovery protocol.  A value of '0'
    indicates no support.  This allows for backwards compatibility of
    the Router Advertisement message.

7.1.2  Enabled (E) bit

    The 'E' bit indicates whether multicast routing is enabled on the
    router's interface.  A value of '1' indicates that multicast
    forwarding is enabled on the router's interface.  A value of '0'
    indicates that multicast forwarding is disabled.

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7.2  Router Solicitations

    An NDP Router Solicitation message can be sent to solicit a Router
    Advertisement message in order to determine the multicast
    forwarding state of a router.  The periodic transmission of
    solicitation messages is outlined in RFC 2461.

8.   Acknowledgements

    ICMP Router Discovery [RFC1256] was used as a general model for
    IGMP Multicast Router Discovery.

9.   References

[RFC1256]  Deering, S., "ICMP Router Discovery Messages", RFC 1256,
           September 1991.

[RFC1112]  Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
           August 1989.

[IGMPv2]   Fenner, W., "Internet Group Management Protocol, Version 2",
           Internet-Draft, November 1997.

[IGMPv3]   Cain, B., Deering, S., Thyagarajan, A., "Internet Group
           Management Protocol, Version 3", Internet-Draft, November
           1997.

[RFC2113]  Katz, D., "IP Router Alert Option," RFC 2113, April 1996.

[RFC2461]  Narten, T., Nordmark, E., and Simpson, W., "Neighbor
           Discovery IP Version 6 (IPv6)", RFC 2461, December 1998.

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10.   Authors' Addresses

    Shantam Biswas
    Nortel Networks
    600 Technology Park Drive
    Billerica, MA 01821
    EMail: sbiswas@baynetworks.com
    Phone: 1-978-916-8048

    Brad Cain
    Nortel Networks
    3 Federal Street
    Billerica, MA 01821
    EMail: bcain@baynetworks.com
    Phone: 1-978-916-1316

    Brian Haberman
    IBM Corporation
    800 Park Office Drive
    Research Triangle Park, NC  27709
    EMail: haberman@raleigh.ibm.com
    Phone: 1-919-254-2673

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