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Versions: 00 01 02 03 04 05                                             
Internet Engineering Task Force        Inter-Domain Multicast Routing WG
INTERNET-DRAFT                                                 W. Fenner
draft-ietf-idmr-membership-reports-01.txt                 August 5, 1998
                                                   Expires December 1998

             Domain Wide Multicast Group Membership Reports

Status of this Memo

This document is an Internet Draft.  Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas, and
its Working Groups.  Note that other groups may also distribute working
documents as Internet Drafts.

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Internet Drafts may be updated, replaced, or obsoleted by other
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Distribution of this document is unlimited.

                                Abstract

     When running a multi-level multicast routing protocol, upper levels
     need to know about group memberships in lower levels in a
     protocol-independent fashion.  Domain Wide Multicast Group
     Membership Reports allow this information to be learned in a
     fashion similar to IGMP[Fenn97] at the domain level.

This document is a product of the IDMR working group within the Internet
Engineering Task Force.  Comments are solicited and should be addressed
to the working group's mailing list at idmr@cs.ucl.ac.uk and/or the
author.







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1.  Introduction

Domain-Wide Multicast Group Membership Reports (DWRs) are a group
membership protocol at the domain level.  When using a hierarchical
multicast routing protocol [Thya94,Estr98], the inter-domain protocol
needs to learn of group memberships inside domains.  Although some
intra-domain routing protocols can provide this information easily to
the domain border routers, some cannot.  DWRs specify a protocol-
independent manner to learn group membership inside a domain.

This document specifies the DWR protocol, as used by border routers and
interior routers.  It specifies a behavior that can be used with any
intra-domain protocol, along with optimizations for certain intra-domain
protocols, and a transition scheme so that all interior routers need not
be updated.

1.1.  Conventions

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 [Bradner97].

Tunable timer values are named inside square brackets, e.g. [Robustness
Variable].  These values are described in section 7.

2.  Packet Format

DWR packets are sent as IGMP packets (IP protocol #2) when using IPv4.
It's not yet clear what IPv6 type will be used, or if they should have
their own IP protocol number which can be the same for v4 and v6.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    IGMP type  |    DWR type   |          Checksum             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Type-specific header ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


2.1.  IGMP Type: 8 bits

     The IGMP type field is defined to be 0x21.






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2.2.  DWR Type: 8 bits

     The following DWR types are defined:

                 Type |                Name
                 _____|____________________________________
                 0x00 | Domain-Wide Query
                 0x01 | Domain-Wide Membership Report
                 0x02 | Domain-Wide Leave
                 0x03 | Non-authoritative Domain-Wide Leave

2.3.  Checksum

     IP-style checksum [Brad88] over the whole packet, zeroed for
     computing checksum.  This checksum MUST be computed when
     transmitting packets, and MUST be verified when receiving.
     Received packets with incorrect checksums MUST be dropped.

2.4.  Type-specific header

     The only type-specific header defined is for the Domain-Wide Query;
     its header contains:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Response Time         | Query Interval|  Robustness   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


2.4.1.  Response Time

     The time, in units of 10ms, allowed for responses to this query.
     Varying this field allows tuning the burstiness of DWR traffic at
     the cost of higher latencies.

2.4.2.  Query Interval

     The time, in units of 10 seconds, between periodic Query messages
     from this Querier.

2.4.3.  Robustness

     The Robustness variable, described later.  Along with the Query
     Interval, conveying this data in the Query allows exact calculation
     of Querier timeouts and allows interior routers to calculate the
     group membership lifetime.



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     There is no type-specific header for Report and Leave messages; the
     data field starts immediately after the checksum.

2.5.  Data

     The remainder of the packet consists of a series of groups and
     options.  Each field in the rest of the packet is either a group
     address:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Group Address                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     or an option header:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | option number |    MBZ    |S|I|          Option Length        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Option Data...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


     The two forms may be told apart because option numbers are assigned
     in the range [0,223], the first byte of an IPv4 group address is in
     the range [224,239], and the first byte of an IPv6 group address is
     255.

     2.5.1.  Data Description

     2.5.1.1.  Group Address

          The group address being reported or queried.

     2.5.1.2.  Option Number

          The number of the option.

     2.5.1.3.  MBZ

          Must be zeroed on transmission and ignored on reception.

     2.5.1.4.  I

          Ignore this packet or group for group membership purposes if



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          the option is not recognized.

     2.5.1.5.  S

          Ignore this packet or group for report suppression purposes
          (on Reports or Leaves) or for reply purposes (on Queries) if
          the option is not recognized.

     2.5.1.6.  Option Length

          The number of words, excluding the initial word, of option
          data following the option header.

     2.5.1.7.  Option Data

          Option Length words of data.

     2.5.2.  Option Processing

          Options which precede any group addresses are called Global
          Options.  Options which follow a group address are associated
          with that group address and are called Group Options.  There
          are two bits describing how to handle unsupported options.

     2.5.2.1.  The S bit

          The S bit is used when processing Queries, Reports and Leaves
          by interior routers.  Groups with options attached should be
          ignored as if they were not present if there are unrecognized
          Group Options with the S bit set.  Packets with Global Options
          should be ignored as if they were not received if there are
          unrecognized Global Options with the S bit set.

     2.5.2.2.  The I bit

          The I bit is used when processing Reports and Leaves by border
          routers.  Groups with options attached should be ignored as if
          they were not present if there are unrecognized Group Options
          with the I bit set.  Packets with Global Options should be
          ignored as if they were not received if there are unrecognized
          Global Options with the I bit set.

     2.5.3.  Defined Options

          2.5.3.1.  Padding (option #0)

               This option need not be handled specially by option
               parsers; it may be left as an unrecognized option.  The S



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               and I bits are both 0, so failing to recognize this
               option does not affect the processing of the packet.  The
               length field may be 0, meaning there are 0 additional
               words of data associated with the option.  A non-zero
               length field may be used with the padding option if
               additional padding is required.

               Routers MUST interpret the S and I bits of Padding
               options as though the option is not supported.

          2.5.3.2.  Group masks accepted/present (option #1)

               This option may be used as a global option on a Query, to
               indicate that all border routers understand the group
               mask option in Report and Leave messages.  This option
               MUST only be sent when the Querier knows that all border
               routers support it; in general this can only be by manual
               configuration.  In this use, the I and S bits are off.

               When the most recent Query message contained the Group
               masks accepted global option, a router may attach a group
               masks present option to any group in its Report or Leave
               messages.  This option contains the following data:

                0                   1                   2                   3
                0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               |       1       |    MBZ    |0|0|       1 for IPv4, 4 for IPv6  |
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               |                        Mask to go with group                  |
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               The data portion is a bitwise mask, to be applied to the
               group to create a group range. (XXX should it be a mask
               length?  Are there still proposed address allocation
               schemes which use noncontiguous masks?)

               This usage also has the S and I bits turned off.

          2.5.3.3.  Unicast-reply (option #2)

               This option has the S and I bits turned off.  If a query
               is received with this option, the reply should be
               unicasted to the source of the query.  If the option
               carries a unicast address, it is the address to be
               unicasted to.

               If a unicast address is specified in the option, the



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               option MUST be ignored if the packet is not fully
               authenticated using IPSEC[Atki95].

3.  Message Descriptions

3.1.  Domain-Wide Query

     A Domain-Wide Query is sent periodically by one of the domain's
     border routers.  The default period is 5 minutes, and MUST be
     configurable.  Domain-Wide Query messages are sent to the well-
     known Domain-Wide Query multicast group (224.0.255.254).  This
     group is in the range of addresses that are scoped to a single
     domain, 224.0.255.0 through 224.0.255.255.

     A Domain-Wide Query with no additional data is a request for
     knowledge of all multicast group memberships in the domain.  The
     Domain-Wide Query may be restricted by including groups or options
     in the data portion of the packet.  If group addresses or DWR
     options are specified in the packet, the Query is restricted to
     those groups or other options as specified in the packet.

3.2.  Domain-Wide Report

     A Domain-Wide Report is sent by a router in response to a Domain-
     Wide Query message, or in response to the appearance of a new group
     member in the domain.  The latter messages are called "Unsolicited"
     Domain-Wide Reports.

     A Domain-Wide Report message includes a list of group memberships
     and other options in the additional data portion of the packet.

3.3.  Domain-Wide Leave

     A Domain-Wide Leave is sent by a router when it knows that there
     are no more members in the domain of a group or groups.  The group
     or groups listed in the additional data portion of the packet are
     considered by the border routers to have no more members.

3.4.  Non-Authoritative Domain-Wide Leave

     A Non-Authoritative Domain-Wide Leave is sent by a router when it
     knows of no more members of the group but cannot be sure there are
     no more members in the domain.  Reception of a Domain-Wide Leave
     causes the elected border router to send a Domain-Wide Query
     message for the group(s) mentioned in the Non-Authoritative
     Domain-Wide Leave message.





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4.  Interior Router Behavior

4.1.  General Behavior

     This section describes the general behavior of interior routers, or
     of proxies running inside domains.  Some of these behaviors may be
     optimized when running multicast routing protocols with more
     centralized knowledge of group memberships inside the domain; these
     optimizations will be described later.

     If a router has not yet been upgraded to perform domain-wide
     reports, a proxy may be placed on each of its connected networks.
     This proxy must participate in the network's group membership
     protocol (e.g.  IGMPv2[Fenn97]).  For example, it may perform only
     the duties of a Non-Querier router in IGMPv2, which allow it to
     passively learn all of the group members on a network.  The proxy
     can then respond to Domain-Wide Query messages just as the interior
     router would.

4.1.1.  Reception of Queries

     All routers in the domain join the Domain-Wide Query well-known
     multicast group.  Upon reception of a Domain-Wide Query message, a
     router sets a timer to a value randomly chosen from the range (0,
     Response time] as specified in the packet.  The Data section of the
     Query should be saved to be used when the timer expires.

4.1.2.  Transmission of Reports

     Upon the expiration of a Domain-Wide Query timer, a router
     assembles a packet containing the list of group memberships known
     to this router via IGMP or other mechanism, excluding those that
     were suppressed by previous reports, and sends this message to the
     Domain-Wide Report well-known multicast group (224.0.255.253).  If
     the Domain-Wide Query contained a list of groups or options, the
     Report should be restricted to those groups in the list in the
     Query message.

4.1.3.  Reception of Reports

     All routers in the domain join the Domain-Wide Report well-known
     multicast group in order to perform suppression, as follows.  Upon
     reception of a Domain-Wide Report message, a router processes the
     list of groups in the message.  If the packet contains unrecognized
     global options, the packet should be dropped and not processed if
     any of the unrecognized options have their S bit set.  For each
     group, if the group has unrecognized options, the group should be
     skipped if any of the unrecognized options have their S bit set.



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     Otherwise, if the router's Domain-Wide Query timer is running, it
     SHOULD mark the group as having been suppressed and SHOULD NOT
     report it when the Domain-Wide Query timer expires.

     Routers MAY record the existence of this group membership in the
     domain to be used for future suppression.  This record MUST time
     out after [Query Interval] * [Robustness Variable], and MUST be
     canceled by reception of a Domain-Wide Leave or Non-Authoritative
     Domain-Wide Leave message mentioning this group.

4.1.4.  Reception of Leaves

     Upon the reception of a Domain-Wide Leave, a router should process
     the list of groups in the message.  For each group, if the group
     has unrecognized options, it should be skipped if any of the
     unrecognized options have their S bit set.  Otherwise, the router
     should remove its record of the existence of another group
     membership in the domain.

4.1.5.  Transmission of Leaves

     A router sends a Non-Authoritative Leave when it no longer knows of
     any members of the group.  This message MAY be suppressed if some
     other router was the last to report group membership with a DWR.

4.2.  Optimizations

     In explicit group membership protocols like PIM, CBT and MOSPF,
     there is a set of routers smaller than "all routers in the domain"
     which knows of group memberships in the domain.  This section
     describes the optimizations possible when running a protocol like
     this.

     In PIM and CBT, only RP's and Cores must participate.  MOSPF is a
     special case, in that all routers in the MOSPF domain know of all
     group memberships in the domain.  In this case, the DWR protocol
     may degenerate to a virtual protocol run entirely inside the
     elected border router.

4.2.1.  Reception of a Query Message

     Only participating interior routers must join the Domain-Wide Query
     well-known multicast group.

4.2.2.  Transmission of a Report Message

     Report messages contain all memberships that this router knows
     about (e.g. in MOSPF, it's all memberships in the domain; in PIM,



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     it's all groups that for which this router is the RP).

4.2.3.  Reception of a Report Message

     If there is no overlap of the groups being reported by each
     participant, the interior routers need not join the Domain-Wide
     Report well-known multicast group so will not receive Report
     messages.  E.g.  if R1 and R2 each handle one half of the multicast
     group address space, there is no need for either of them to join
     the Domain-Wide Report group.

4.2.4.  Reception of a Leave Message

     As with Reports, if there is no overlap, the interior routers need
     not join the DWR group so will not receive these messages.

4.2.5.  Transmission of a Leave Message

     If it is possible to know when the last member in the domain goes
     away, routers SHOULD send authoritative Domain-Wide Leave messages,
     instead of Non-Authoritative Domain-Wide Leave messages.

5.  Unsolicited messages

     When a new group member appears in the domain, a Report message
     SHOULD be transmitted (called an Unsolicited Report).  Interior
     routers MAY track the presence of group members inside the domain;
     a router which is doing this SHOULD suppress its unsolicited Report
     if it knows of another group member inside the domain.

6.  Border Router Behavior

6.1.  Querier Election

     All border routers should join the Domain-Wide Queries well-known
     multicast group, in order to perform Querier Election.  All routers
     start up thinking they are the elected Querier.  If a router hears
     a DWQ from a router with a lower IP address, it elects that router
     as Querier.  If a router has not heard a DWQ from the elected
     Querier in [Querier's Query Interval] * [Querier's Robustness
     Variable] + [Querier's Response Interval], it assumes the role of
     Querier.  It is recommended to have an IPSEC[Atki95] relationship
     among the domain border routers so that Querier Election can not be
     fouled by a forged packet.

6.2.  Send Periodic Queries

     The elected border router sends periodic Queries once every [Query



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     Interval].  These Queries include the router's Query Interval and
     Robustness Variable.  The Response Interval should be set to
     [Normal Response Interval].

6.3.  Reception of Non-Authoritative Leave

     Upon reception of a Non-Authoritative Leave, the elected Querier
     sets a group membership timeout timer to [Robustness Variable] *
     [Response Interval] + [Round Trip Delay], and sends a group-
     specific Query, listing all groups in the Non-Authoritative Leave
     message.  The Response Interval should be set to [Fast Response
     Interval].  Until a response is heard for each listed group, the
     Query should be retransmitted once every [Fast Response Interval]
     for a total of [Robustness Variable] transmissions.  The Querier
     MUST wait an additional [Round Trip Delay] after the final [Fast
     Response Interval] for reports before assuming that there are no
     members present in the domain.

6.4.  Reception of Group-Specific Queries

     Upon reception of a Group-Specific Query, non-Querier routers MUST
     set a group membership timeout timer to [Robustness Variable] *
     [Response Interval] + [Round Trip Delay].  If this timeout occurs
     without receiving a Report for the listed groups, the group
     membership record is removed.

6.5.  Reception of Reports

     Upon reception of a Domain-Wide Report message, all border routers
     set a group membership timer for each group mentioned in the
     Report.  This timer's value is set to [Querier's Query Interval] *
     [Querier's Robustness Variable] + [Querier's Response Interval] *
     2.  The Querier's Query Interval, Querier's Response Interval and
     Querier's Robustness Variable are remembered from the last General
     Query received from the Querier.  This timer is refreshed by
     reception of further messages.

6.6.  Request Unicast Replies

     If a Border Router wishes to reduce the amount of DWR multicast
     traffic in a domain, it may add the "Reply via Unicast" option to
     its DWQ's.  This has the advantage of reducing the amount of state
     kept inside the domain for forwarding packets destined to the DWR-
     reply multicast group, at the cost of eliminating suppression.  The
     border router must multicast DWR's summarizing the replies it got
     via unicast to the DWR-reply multicast group at the end of the
     response interval, in order to share membership information with
     all routers.  This summary MUST contain a global padding option



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     with its S bit set to 1, to prevent suppression of real reports.

7.  List of timers and tunable values

7.1.  Robustness Variable

     The Robustness Variable allows tuning for the expected packet loss
     in a domain.  If transmission inside a domain is expected to be
     lossy, the Robustness Variable may be increased, at the cost of
     increased latency in determining failures.  The DWR protocol is
     robust to ([Robustness Variable] - 1) packet losses.  The
     Robustness Variable MUST NOT be zero, and SHOULD NOT be one.
     Default value: 2

7.2.  Query Interval

     The Query Interval is the interval between General Queries sent by
     the domain-wide Querier.  Default value: 5 minutes

7.3.  Normal Response Interval

     The Normal Response Interval is the Response Time inserted into the
     periodic General Queries.  Default: 60 seconds

     By varying the [Normal Response Interval], an administrator may
     tune the burstiness of DWR messages in the domain; larger values
     make the traffic less bursty, as host responses are spread out over
     a larger interval.  The number of seconds represented by the
     [Normal Response Interval] must be less than the [Query Interval].

7.4.  Fast Response Interval

     The Fast Response Interval is the Response Time inserted into
     Group-Specific Queries in response to Non-Authoritative Leave
     messages, and is also the time between the Group-Specific Query
     messages.  Default: 1 second

     This value may be tuned to modify the "leave latency" of the
     domain.  A reduced value results in reduced time to detect the loss
     of the last member of a group.

7.5.  Round Trip Delay

     The Round Trip Delay is the worst-case round trip time through the
     domain.  This is used to ensure that group membership is not lost
     due to a small Fast Response Interval and a large round trip delay
     through the domain.  This value must be manually configured.
     Default: 100ms.



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     IGMPv2 ignores end-to-end message delay, assuming that this delay
     is negligible.  Although the DWR protocol is very similar to
     IGMPv2, the reality is that end-to-end round trip delays can be
     very different on LANs vs. in a routing domain.  On a LAN, the
     round trip delay is generally dwarfed by the IGMPv2 response
     interval.  Within a domain, the opposite may be true, so it's
     important for the protocol to acknowledge that.

8.  Message destinations

This information is provided elsewhere in the document, but is
summarized here for convenience.

Message Type                  Destination Group
------------                  -----------------
General Query                 Domain-wide Query group (224.0.255.254)
Group-specific Query          Domain-wide Query group (224.0.255.254)
Report                        Domain-wide Report group (224.0.255.253)
Leave                         Domain-wide Report group (224.0.255.253)
Non-Authoritative Leave       Domain-wide Report group (224.0.255.253)


9.  Miscellaneous issues

9.1.  Elect Reporters?

     In order to help with suppression in a domain, a Querier might
     choose to elect a "Designated Reporter" for a group for a certain
     duration.  The Designated Reporter is the only router which should
     send Reports for the designated group(s) for the designated time.
     All others should act as though they have been suppressed for the
     designated time.  The Querier should cancel a router's Designated
     Reporter status when that router sends a Leave message, or when it
     hasn't heard a reply from that router for <N> Query intervals.
     When canceling a router's Designated Reporter status, a Querier
     should send a Group-Specific Query for the group(s) in question and
     can optionally elect one of the responders as the new Designated
     Reporter.

     Designated Reporters are only required when using unicast replies;
     when using multicast replies, routers may keep track of the
     presence of other reporters in the natural course of handling
     suppression.

10.  Acknowledgments

     The ideas of unicasting DWR replies and of electing a "designated
     reporter" came from a discussion on the IDMR mailing list with



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     Jeffrey Zhang and Yunzhou Li of Bay Networks.

11.  Security Considerations

     Many same as IGMPv2[Fenn97]

11.1.  Unicast Responses

     Sending a DWQ requesting a unicast response can cause many DWR's to
     be unicasted to the sender.  Requiring IPSEC authentication on the
     DWQ only if it requests unicast to a different address may not be
     strong enough - for example, someone at the other end of a slow
     link may swamp the link by sending a DWQ.  (At the same time,
     someone at the other end of a slow link may swamp the link by
     sending a multicast ping...)

12.  References

Atki95         Atkinson, R., ``IP Authentication Header'', RFC 1826,
               August 1995.

Brad88         Braden, B., D. Borman, C. Partridge, "Computing the
               Internet Checksum", RFC 1071, ISI, September 1988.

Bradner97      Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", RFC 2119/BCP 14, Harvard University,
               March 1997.

Fenn97         Fenner, W.  ``Internet Group Management Protocol, Version
               2'', RFC2236, Xerox PARC, November 1997.

Estr98         Estrin, D., D. Meyer, D. Thaler, ``Border Gateway
               Multicast Protocol (BGMP): Protocol Specification'', Work
               In Progress, March 1998.

Thya95         Thyagarajan, A. and S. Deering, ``Hierarchical Distance-
               Vector Multicast Routing'', Proceedings of ACM Sigcomm,
               September 1995.

13.  Author's Address


   William C. Fenner
   Xerox PARC
   3333 Coyote Hill Road
   Palo Alto, CA 94304
   Phone: +1 650 812 4816
   Email: fenner@parc.xerox.com



Fenner                   Expires December 1998                 [Page 14]