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Internet Engineering Task Force     Internet-Draft Multicast Remnants WG
INTERNET-DRAFT                                                 W. Fenner
draft-ietf-idmr-membership-reports-05.txt                  July 11, 2000
                                                   Expires December 2000

             Domain Wide Multicast Group Membership Reports

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
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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[RFC2236] 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 [RFC2119].

Tunable timer values are named inside square brackets, e.g. [Robustness
Variable].  These values are described in section 8.

2.  Packet Format

DWR packets are sent as UDP packets (IP protocol #17).  The UDP
destination port is 644.  The UDP checksum SHOULD be calculated on
transmission.  However, packets without checksums MUST be accepted.
Received packets with incorrect checksums MUST be dropped.  The UDP
payload is as follows:

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  MBZ                          |   DWR Type    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Type-specific header ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


2.1.  MBZ: 24 bits

     Must be zeroed on transmission and ignored on reception.




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2.2.  DWR Type: 8 bits

     The following DWR types are defined:

     center; c | c l | l.  Type Name _ 0x00 Domain-Wide Query
     0x01 Domain-Wide Membership Report 0x02 Domain-Wide Leave 0x03 Non-
     authoritative Domain-Wide Leave


2.3.  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   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               MBZ                             |   Priority    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


2.3.1.  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.

2.3.2.  Query Interval

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

2.3.3.  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.

2.3.4.  Priority

     The configured priority of this border router for Querier Election
     purposes.  If no value is configured, the default value is 128.
     Lower values are better, i.e. more likely to be selected as the
     querier.




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2.3.5.  MBZ

     This field must be zeroed on transmission and ignored on reception.

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

2.4.  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.

     2.4.1.  Data Description

     2.4.1.1.  Group Address

          The group address being reported or queried.

     2.4.1.2.  Option Number

          The number of the option.







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     2.4.1.3.  MBZ

          Must be zeroed on transmission and ignored on reception.

     2.4.1.4.  I

          Ignore this packet or group for group membership purposes if
          the option is not recognized.

     2.4.1.5.  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.

     2.4.1.6.  Option Length

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

     2.4.1.7.  Option Data

          Option Length words of data.

     2.4.2.  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.

     2.4.2.1.  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.

     2.4.2.2.  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.



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     2.4.3.  Defined Options

          2.4.3.1.  Padding (option #0)

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

          2.4.3.2.  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.

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

          2.4.3.3.  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



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               carries a unicast address, it is the address to be
               unicasted to.

               If a unicast address is specified in the option, the
               option MUST be ignored if the packet is not authenticated
               using IPSEC[RFC2401] as described later in this document.

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



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     message for the group(s) mentioned in the Non-Authoritative Domain-
     Wide Leave message.

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[RFC2236]).  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



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     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.
     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 this
     router's last attempt to report this group was suppressed by
     reception of a Report.

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.




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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,
     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 which has a lower ("better") priority, or an equal priority
     and 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[RFC2401] relationship for the DWQ
     multicast group among the domain border routers so that Querier



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     Election can not be fouled by a forged packet.

6.2.  Send Periodic Queries

     The elected border router sends periodic Queries once every [Query
     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] *
     [Fast 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 [Querier's Robustness
     Variable] * [Querier's Response Interval] + [Round Trip Delay].  If
     this timeout occurs without receiving a Report for the listed
     groups, the group membership record is removed.  The Querier's
     Query Interval and Querier's Response Interval are the values
     carried in the Query packet.

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



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

7.  Use of IPSEC

     The use of IPSEC AH is recommended on Domain-Wide Query packets for
     two reasons:

1.   Prevention of forgery of Queries which can foil Querier election.
     This use requires all border routers to use IPSEC.

2.   In order to allow the use of the Unicast Replies option.  This use
     requires all border and interior routers to use IPSEC.

     In either configuration, security associations are configured as
     described in section 4.7 of [RFC2401].  Briefly, a single Security
     Association is manually configured in all required devices with a
     static key.  The number of devices (e.g. domain border routers)
     should be small enough for this to not be an undue burden.  When a
     secure multicast key distribution protocol exists in the IPSEC
     framework, this protocol may be used instead of manual
     configuration.

8.  List of timers and tunable values

8.1.  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

8.2.  Query Interval

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






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8.3.  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].

8.4.  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.

8.5.  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.

     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.

9.  Message destinations

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









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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)


10.  Acknowledgments

     The ideas of unicasting DWR replies and of electing a "designated
     reporter" came from a discussion on the IDMR mailing list with
     Jeffrey Zhang and Yunzhou Li of Bay Networks.

11.  Security Considerations

11.1.  Forged Packets

11.1.1.  Forged Packets from Outside the Domain

     Since all packets in this memo are sent to domain-scoped multicast
     groups, a scope boundary around the domain which drops domain-
     scoped packets from entering the domain from outside protects
     against forged packets from outside the domain.

11.1.2.  Forged Packets from Inside the Domain

     We consider the effects of a forged packet of each type.

11.1.2.1.  Forged Query

     A forged Query will cause interior routers to send Reports for some
     or all of their group memberships, increasing control traffic
     within the domain but not affecting the memberships learned by the
     border routers.

     Border routers perform Querier Elections on Query messages.  A
     forged General Query could cause the forger to be elected Querier,
     giving him some control over the group membership reporting in the
     domain.  For this reason, IPSEC SHOULD be used between the domain
     border routers to ensure that Querier election only occurs between
     known border routers.

11.1.2.2.  Forged Report

     A forged Report will cause interior routers to suppress their own
     Report messages for the group being reported.  However, the forged



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     message will be accepted by the border routers, so this
     accomplishes nothing.

     If interior routers keep state for all Reports heard, forged
     Reports can cause increased memory consumption on interior routers.
     However, keeping state for all Reports is optional, so interior
     routers that are running low on memory due to the amount of DWR-
     related state may simply release all of that state.

     Border routers must keep state for all Reports heard, so they are
     vulnerable to increased memory consumption.  If this is a worry,
     once again IPSEC relationships may be created between all of the
     interior routers and the border routers.  This is a much larger
     burden on administrators, however, so is only recommended if this
     is expected to be a problem.

11.1.2.3.  Forged Leave

     A forged Leave (Authoritative or Non-Authoritative) will cause
     interior routers to forget their state (if any) regarding the
     suppression status of a group.  This may cause increased control
     message traffic during the next Query interval.

     A forged Non-Authoritative leave will cause the border routers to
     issue group-specific Query messages to learn if there are remaining
     group members.  This causes increased control message traffic.

     A forged Authoritative Leave will cause a black hole until the next
     Query occurs; the border routers will accept the Leave and not
     perform any Queries.  Border routers SHOULD be configurable to
     ignore all Authoritative Leaves and interior routers SHOULD be
     configurable to only send Non-Authoritative Leaves, in order to be
     able to prevent this attack.

11.2.  Unicast Responses

     Sending a DWQ requesting a unicast response can cause many DWR's to
     be unicasted to the sender.  In order to prevent the use of this
     option as a "packet amplifier", any DWQ message using this option
     SHOULD be authenticated using IPSEC as described in this document.

12.  References

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





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

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

RFC2401        Kent, S. and R. Atkinson, ``Security Architecture for the
               Internet Protocol'', RFC 2401, November 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
   AT&T Labs - Research
   75 Willow Road
   Menlo Park, CA 94025
   Phone: +1 650 330 7893
   Email: fenner@research.att.com




























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