Network Working Group                                     M. Christensen
Request for Comments: 4541                               Thrane & Thrane
Category: Informational                                       K. Kimball
                                                         Hewlett-Packard
                                                             F. Solensky
                                                                   Calix
                                                                May 2006

Considerations for Internet Group Management Protocol (IGMP)
and Multicast Listener Discovery (MLD) Snooping Switches

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2006).

Abstract

   This memo describes the recommendations for Internet Group Management
   Protocol (IGMP) and Multicast Listener Discovery (MLD) snooping
   switches.  These are based on best current practices for IGMPv2, with
   further considerations for IGMPv3- and MLDv2-snooping.  Additional
   areas of relevance, such as link layer topology changes and
   Ethernet-specific encapsulation issues, are also considered.

1. Introduction

   The IEEE bridge standard [BRIDGE] specifies how LAN packets are
   'bridged', or as is more commonly used today, switched between LAN
   segments.  The operation of a switch with respect to multicast
   packets can be summarized as follows.  When processing a packet whose
   destination MAC address is a multicast address, the switch will
   forward a copy of the packet into each of the remaining network
   interfaces that are in the forwarding state in accordance with
   [BRIDGE].  The spanning tree algorithm ensures that the application
   of this rule at every switch in the network will make the packet
   accessible to all nodes connected to the network.

   This behaviour works well for broadcast packets that are intended to
   be seen or processed by all connected nodes.  In the case of
   multicast packets, however, this approach could lead to less
   efficient use of network bandwidth, particularly when the packet is

   intended for only a small number of nodes.  Packets will be flooded
   into network segments where no node has any interest in receiving the
   packet.  While nodes will rarely incur any processing overhead to
   filter packets addressed to unrequested group addresses, they are
   unable to transmit new packets onto the shared media for the period
   of time that the multicast packet is flooded.  In general,
   significant bandwidth can be wasted by flooding.

   In recent years, a number of commercial vendors have introduced
   products described as "IGMP snooping switches" to the market.  These
   devices do not adhere to the conceptual model that provides the
   strict separation of functionality between different communications
   layers in the ISO model, and instead utilize information in the upper
   level protocol headers as factors to be considered in processing at
   the lower levels.  This is analogous to the manner in which a router
   can act as a firewall by looking into the transport protocol's header
   before allowing a packet to be forwarded to its destination address.

   In the case of IP multicast traffic, an IGMP snooping switch provides
   the benefit of conserving bandwidth on those segments of the network
   where no node has expressed interest in receiving packets addressed
   to the group address.  This is in contrast to normal switch behavior
   where multicast traffic is typically forwarded on all interfaces.

   Many switch datasheets state support for IGMP snooping, but no
   recommendations for this exist today.  It is the authors' hope that
   the information presented in this document will supply this
   foundation.

   The recommendations presented here are based on the following
   information sources: The IGMP specifications [RFC1112], [RFC2236] and
   [IGMPv3], vendor-supplied technical documents [CISCO], bug reports
   [MSOFT], discussions with people involved in the design of IGMP
   snooping switches, MAGMA mailing list discussions, and on replies by
   switch vendors to an implementation questionnaire.

   Interoperability issues that arise between different versions of IGMP
   are not the focus of this document.  Interested readers are directed
   to [IGMPv3] for a thorough description of problem areas.

   The suggestions in this document are based on IGMP, which applies
   only to IPv4.  For IPv6, Multicast Listener Discovery [MLD] must be
   used instead.  Because MLD is based on IGMP, we do not repeat the
   entire description and recommendations for MLD snooping switches.
   Instead, we point out the few cases where there are differences from
   IGMP.

   Note that the IGMP snooping function should apply only to IPv4
   multicasts.  Other multicast packets, such as IPv6, might be
   suppressed by IGMP snooping if additional care is not taken in the
   implementation as mentioned in the recommendations section.  It is
   desired not to restrict the flow of non-IPv4 multicasts other than to
   the degree which would happen as a result of regular bridging
   functions.  Likewise, MLD snooping switches are discouraged from
   using topological information learned from IPv6 traffic to alter the
   forwarding of IPv4 multicast packets.

2. IGMP Snooping Recommendations

   The following sections list the recommendations for an IGMP snooping
   switch.  The recommendation is stated and is supplemented by a
   description of a possible implementation approach.  All
   implementation discussions are examples only and there may well be
   other ways to achieve the same functionality.

2.1. Forwarding rules

The IGMP snooping functionality is separated into a control section
(IGMP forwarding) and a data section (Data forwarding).

2.1.1. IGMP Forwarding Rules

   1) A snooping switch should forward IGMP Membership Reports only to
      those ports where multicast routers are attached.

      Alternatively stated: a snooping switch should not forward IGMP
      Membership Reports to ports on which only hosts are attached.  An
      administrative control may be provided to override this
      restriction, allowing the report messages to be flooded to other
      ports.

      This is the main IGMP snooping functionality for the control path.

      Sending membership reports to other hosts can result, for IGMPv1
      and IGMPv2, in unintentionally preventing a host from joining a
      specific multicast group.

      When an IGMPv1 or IGMPv2 host receives a membership report for a
      group address that it intends to join, the host will suppress its
      own membership report for the same group.  This join or message
      suppression is a requirement for IGMPv1 and IGMPv2 hosts.
      However, if a switch does not receive a membership report from the
      host it will not forward multicast data to it.

      This is not a problem in an IGMPv3-only network because there is
      no suppression of IGMP Membership reports.

      The administrative control allows IGMP Membership Report messages
      to be processed by network monitoring equipment such as packet
      analyzers or port replicators.

      The switch supporting IGMP snooping must maintain a list of
      multicast routers and the ports on which they are attached.  This
      list can be constructed in any combination of the following ways:

      a) This list should be built by the snooping switch sending
         Multicast Router Solicitation messages as described in IGMP
         Multicast Router Discovery [MRDISC].  It may also snoop
         Multicast Router Advertisement messages sent by and to other
         nodes.

      b) The arrival port for IGMP Queries (sent by multicast routers)
         where the source address is not 0.0.0.0.

         The 0.0.0.0 address represents a special case where the switch
         is proxying IGMP Queries for faster network convergence, but is
         not itself the Querier.  The switch does not use its own IP
         address (even if it has one), because this would cause the
         Queries to be seen as coming from a newly elected Querier.  The
         0.0.0.0 address is used to indicate that the Query packets are
         NOT from a multicast router.

      c) Ports explicitly configured by management to be IGMP-forwarding
         ports, in addition to or instead of any of the above methods to
         detect router ports.

   2) IGMP networks may also include devices that implement "proxy-
      reporting", in which reports received from downstream hosts are
      summarized and used to build internal membership states.  Such
      proxy-reporting devices may use the all-zeros IP Source-Address
      when forwarding any summarized reports upstream.  For this reason,
      IGMP membership reports received by the snooping switch must not
      be rejected because the source IP address is set to 0.0.0.0.

   3) The switch that supports IGMP snooping must flood all unrecognized
      IGMP messages to all other ports and must not attempt to make use
      of any information beyond the end of the network layer header.

      In addition, earlier versions of IGMP should interpret IGMP fields
      as defined for their versions and must not alter these fields when
      forwarding the message.  When generating new messages, a given
      IGMP version should set fields to the appropriate values for its

      own version.  If any fields are reserved or otherwise undefined
      for a given IGMP version, the fields should be ignored when
      parsing the message and must be set to zeroes when new messages
      are generated by implementations of that IGMP version.  An
      exception may occur if the switch is performing a spoofing
      function, and is aware of the settings for new or reserved fields
      that would be required to correctly spoof for a different IGMP
      version.

      The reason to worry about these trivialities is that IGMPv3
      overloads the old IGMP query message using the same type number
      (0x11) but with an extended header.  Therefore there is a risk
      that IGMPv3 queries may be interpreted as older version queries
      by, for example, IGMPv2 snooping switches.  This has already been
      reported [IETF56] and is discussed in section 2.2.

   4) An IGMP snooping switch should be aware of link layer topology
      changes caused by Spanning Tree operation.  When a port is enabled
      or disabled by Spanning Tree, a General Query may be sent on all
      active non-router ports in order to reduce network convergence
      time.  Non-Querier switches should be aware of whether the Querier
      is in IGMPv3 mode.  If so, the switch should not spoof any General
      Queries unless it is able to send an IGMPv3 Query that adheres to
      the most recent information sent by the true Querier.  In no case
      should a switch introduce a spoofed IGMPv2 Query into an IGMPv3
      network, as this may create excessive network disruption.

      If the switch is not the Querier, it should use the 'all-zeros' IP
      Source Address in these proxy queries (even though some hosts may
      elect to not process queries with a 0.0.0.0 IP Source Address).
      When such proxy queries are received, they must not be included in
      the Querier election process.

   5) An IGMP snooping switch must not make use of information in IGMP
      packets where the IP or IGMP headers have checksum or integrity
      errors.  The switch should not flood such packets but if it does,
      it should also take some note of the event (i.e., increment a
      counter).  These errors and their processing are further discussed
      in [IGMPv3], [MLD] and [MLDv2].

   6) The snooping switch must not rely exclusively on the appearance of
      IGMP Group Leave announcements to determine when entries should be
      removed from the forwarding table.  It should implement a
      membership timeout mechanism such as the router-side functionality
      of the IGMP protocol as described in the IGMP and MLD
      specifications (See Normative Reference section for IGMPv1-3 and
      MLDv1-2) on all its non-router ports.  This timeout value should
      be configurable.

2.1.2. Data Forwarding Rules

   1) Packets with a destination IP address outside 224.0.0.X which are
      not IGMP should be forwarded according to group-based port
      membership tables and must also be forwarded on router ports.

      This is the main IGMP snooping functionality for the data path.
      One approach that an implementation could take would be to
      maintain separate membership and multicast router tables in
      software and then "merge" these tables into a forwarding cache.

   2) Packets with a destination IP (DIP) address in the 224.0.0.X range
      which are not IGMP must be forwarded on all ports.

      This recommendation is based on the fact that many host systems do
      not send Join IP multicast addresses in this range before sending
      or listening to IP multicast packets.  Furthermore, since the
      224.0.0.X address range is defined as link-local (not to be
      routed), it seems unnecessary to keep the state for each address
      in this range.  Additionally, some routers operate in the
      224.0.0.X address range without issuing IGMP Joins, and these
      applications would break if the switch were to prune them due to
      not having seen a Join Group message from the router.

   3) An unregistered packet is defined as an IPv4 multicast packet with
      a destination address which does not match any of the groups
      announced in earlier IGMP Membership Reports.

      If a switch receives an unregistered packet, it must forward that
      packet on all ports to which an IGMP router is attached.  A switch
      may default to forwarding unregistered packets on all ports.
      Switches that do not forward unregistered packets to all ports
      must include a configuration option to force the flooding of
      unregistered packets on specified ports.

      In an environment where IGMPv3 hosts are mixed with snooping
      switches that do not yet support IGMPv3, the switch's failure to
      flood unregistered streams could prevent v3 hosts from receiving
      their traffic.  Alternatively, in environments where the snooping
      switch supports all of the IGMP versions that are present,
      flooding unregistered streams may cause IGMP hosts to be
      overwhelmed by multicast traffic, even to the point of not
      receiving Queries and failing to issue new membership reports for
      their own groups.

      It is encouraged that snooping switches at least recognize and
      process IGMPv3 Join Reports, even if this processing is limited to
      the behavior for IGMPv2 Joins, i.e., is done without considering

      any additional "include source" or "exclude source" filtering.
      When IGMPv3 Joins are not recognized, a snooping switch may
      incorrectly prune off the unregistered data streams for the groups
      (as noted above); alternatively, it may fail to add in forwarding
      to any new IGMPv3 hosts if the group has previously been joined as
      IGMPv2 (because the data stream is seen as already having been
      registered).

   4) All non-IPv4 multicast packets should continue to be flooded out
      to all remaining ports in the forwarding state as per normal IEEE
      bridging operations.

      This recommendation is a result of the fact that groups made up of
      IPv4 hosts and IPv6 hosts are completely separate and distinct
      groups.  As a result, information gleaned from the topology
      between members of an IPv4 group would not be applicable when
      forming the topology between members of an IPv6 group.

   5) IGMP snooping switches may maintain forwarding tables based on
      either MAC addresses or IP addresses.  If a switch supports both
      types of forwarding tables then the default behavior should be to
      use IP addresses.  IP address based forwarding is preferred
      because the mapping between IP multicast addresses and link-layer
      multicast addresses is ambiguous.  In the case of Ethernet, there
      is a multiplicity of 1 Ethernet address to 32 IP addresses
      [RFC1112].

   6) Switches which rely on information in the IP header should verify
      that the IP header checksum is correct.  If the checksum fails,
      the information in the packet must not be incorporated into the
      forwarding table.  Further, the packet should be discarded.

   7) When IGMPv3 "include source" and "exclude source" membership
      reports are received on shared segments, the switch needs to
      forward the superset of all received membership reports on to the
      shared segment.  Forwarding of traffic from a particular source S
      to a group G must happen if at least one host on the shared
      segment reports an IGMPv3 membership of the type INCLUDE(G,
      Slist1) or EXCLUDE(G, Slist2), where S is an element of Slist1 and
      not an element of Slist2.

      The practical implementation of the (G,S1,S2,...) based data
      forwarding tables are not within the scope of this document.
      However, one possibility is to maintain two (G,S) forwarding
      lists: one for the INCLUDE filter where a match of a specific
      (G,S) is required before forwarding will happen, and one for the
      EXCLUDE filter where a match of a specific (G,S) will result in no
      forwarding.

2.2. IGMP Snooping-Related Problems

   A special problem arises in networks consisting of IGMPv3 routers as
   well as IGMPv2 and IGMPv3 hosts interconnected by an IGMPv2 snooping
   switch as recently reported [IETF56].  The router will continue to
   maintain IGMPv3 even in the presence of IGMPv2 hosts, and thus the
   network will not converge on IGMPv2.  But it is likely that the
   IGMPv2 snooping switch will not recognize or process the IGMPv3
   membership reports.  Groups for these unrecognized reports will then
   either be flooded (with all of the problems that may create for hosts
   in a network with a heavy multicast load) or pruned by the snooping
   switch.

   Therefore, it is recommended that in such a network, the multicast
   router be configured to use IGMPv2.  If this is not possible, and if
   the snooping switch cannot recognize and process the IGMPv3
   membership reports, it is instead recommended that the switch's IGMP
   snooping functionality be disabled, as there is no clear solution to
   this problem.

3. IPv6 Considerations

   In order to avoid confusion, the previous discussions have been based
   on the IGMP protocol which only applies to IPv4 multicast.  In the
   case of IPv6, most of the above discussions are still valid with a
   few exceptions that we will describe here.

   The control and data forwarding rules in the IGMP section can, with a
   few considerations, also be applied to MLD.  This means that the
   basic functionality of intercepting MLD packets, and building
   membership lists and multicast router lists, is the same as for IGMP.

   In IPv6, the data forwarding rules are more straight forward because
   MLD is mandated for addresses with scope 2 (link-scope) or greater.
   The only exception is the address FF02::1 which is the all hosts
   link-scope address for which MLD messages are never sent.  Packets
   with the all hosts link-scope address should be forwarded on all
   ports.

   MLD messages are also not sent regarding groups with addresses in the
   range FF00::/15 (which encompasses both the reserved FF00::/16 and
   node-local FF01::/16 IPv6 address spaces).  These addresses should
   never appear in packets on the link.

   Equivalent to the IPv4 behaviors regarding the null IP Source
   address, MLD membership reports must not be rejected by an MLD
   snooping switch because of an unspecified IP source address (::).
   Additionally, if a non-Querier switch spoofs any General Queries (as

   addressed in Section 2.1 above, for Spanning Tree topology changes),
   the switch should use the null IP source address (::) when sending
   said queries.  When such proxy queries are received, they must not be
   included in the Querier election process.

   The three major differences between IPv4 and IPv6 in relation to
   multicast are:

   -  The IPv6 protocol for multicast group maintenance is called
      Multicast Listener Discovery [MLDv2].  MLDv2 uses ICMPv6 message
      types instead of IGMP message types.

   -  The RFCs [IPV6-ETHER] and [IPV6-FDDI] describe how 32 of the 128
      bit DIP addresses are used to form the 48 bit DMAC addresses for
      multicast groups, while [IPV6-TOKEN] describes the mapping for
      token ring DMAC addresses by using three low-order bits.  The
      specification [IPV6-1394] makes use of a 6 bit channel number.

   -  Multicast router discovery is accomplished using the Multicast
      Router Discovery Protocol (MRDISC) defined in [MRDISC].

   The IPv6 packet header does not include a checksum field.
   Nevertheless, the switch should detect other packet integrity issues
   such as address version and payload length consistencies if possible.
   When the snooping switch detects such an error, it must not include
   information from the corresponding packet in the MLD forwarding
   table.  The forwarding code should instead drop the packet and take
   further reasonable actions as advocated above.

   The fact that MLDv2 is using ICMPv6 adds new requirements to a
   snooping switch because ICMPv6 has multiple uses aside from MLD.
   This means that it is no longer sufficient to detect that the next-
   header field of the IP header is ICMPv6 in order to identify packets
   relevant for MLD snooping.  A software-based implementation which
   treats all ICMPv6 packets as candidates for MLD snooping could easily
   fill its receive queue and bog down the CPU with irrelevant packets.
   This would prevent the snooping functionality from performing its
   intended purpose and the non-MLD packets destined for other hosts
   could be lost.

   A solution is either to require that the snooping switch looks
   further into the packets, or to be able to detect a multicast DMAC
   address in conjunction with ICMPv6.  The first solution is desirable
   when a configuration option allows the administrator to specify which
   ICMPv6 message types should trigger a CPU redirect and which should
   not.  The reason is that a hardcoding of message types is inflexible
   for the introduction of new message types.  The second solution
   introduces the risk that new protocols that use ICMPv6 and multicast

   DMAC addresses could be incorrectly identified as MLD.  It is
   suggested that solution one is preferred when the configuration
   option is provided.  If this is not the case, then the implementor
   should seriously consider making it available since Neighbor
   Discovery messages would be among those that fall into this false
   positive case and are vital for the operational integrity of IPv6
   networks.

   The mapping from IP multicast addresses to multicast DMAC addresses
   introduces a potentially enormous overlap.  The structure of an IPv6
   multicast address is shown in the figure below.  As a result, there
   are 2 ** (112 - 32), or more than 1.2e24 unique DIP addresses which
   map into a single DMAC address in Ethernet and FDDI.  This should be
   compared to 2**5 in the case of IPv4.

   Initial allocation of IPv6 multicast addresses, as described in
   [RFC3307], however, cover only the lower 32 bits of group ID.  While
   this reduces the problem of address ambiguity to group IDs with
   different flag and scope values for now, it should be noted that the
   allocation policy may change in the future.  Because of the potential
   overlap it is recommended that IPv6 address based forwarding is
   preferred to MAC address based forwarding.

      |   8    |  4 |  4 |             112 bits                  |
      +--------+----+----+---------------------------------------+
      |11111111|flgs|scop|             group ID                  |
      +--------+----+----+---------------------------------------+

4. IGMP Questionnaire

   As part of this work, the following questions were asked on the MAGMA
   discussion list and were sent to known switch vendors implementing
   IGMP snooping.  The individual contributions have been anonymized
   upon request and do not necessarily apply to all of the vendors'
   products.

   The questions were:

      Q1  Do your switches perform IGMP Join aggregation?  In other
          words, are IGMP joins intercepted, absorbed by the
          hardware/software so that only one Join is forwarded to the
          querier?

      Q2  Is multicast forwarding based on MAC addresses?  Would
          datagrams addressed to multicast IP addresses 224.1.2.3 and
          239.129.2.3 be forwarded on the same ports-groups?

      Q3  Is it possible to forward multicast datagrams based on IP
          addresses (not routed)?  In other words, could 224.1.2.3 and
          239.129.2.3 be forwarded on different port-groups with
          unaltered TTL?

      Q4  Are multicast datagrams within the range 224.0.0.1 to
          224.0.0.255 forwarded on all ports whether or not IGMP Joins
          have been sent?

      Q5  Are multicast frames within the MAC address range
          01:00:5E:00:00:01 to 01:00:5E:00:00:FF forwarded on all ports
          whether or not IGMP joins have been sent?

      Q6  Does your switch support forwarding to ports on which IP
          multicast routers are attached in addition to the ports where
          IGMP Joins have been received?

      Q7  Is your IGMP snooping functionality fully implemented in
          hardware?

      Q8  Is your IGMP snooping functionality partly software
          implemented?

      Q9  Can topology changes (for example spanning tree configuration
          changes) be detected by the IGMP snooping functionality so
          that for example new queries can be sent or tables can be
          updated to ensure robustness?

   The answers were:
         ---------------------------+-----------------------+
                                    |     Switch Vendor     |
         ---------------------------+---+---+---+---+---+---+
                                    | 1 | 2 | 3 | 4 | 5 | 6 |
         ---------------------------+---+---+---+---+---+---+
         Q1 Join aggregation        | x | x | x |   | x | x |
         Q2 Layer-2 forwarding      | x | x | x | x |(1)|   |
         Q3 Layer-3 forwarding      |(1)|   |(1)|   |(1)| x |
         Q4 224.0.0.X aware         |(1)| x |(1)|(2)| x | x |
         Q5 01:00:5e:00:00:XX aware | x | x | x |(2)| x | x |
         Q6 Mcast router list       | x | x | x | x | x | x |
         Q7 Hardware implemented    |   |   |   |   |   |   |
         Q8 Software assisted       | x | x | x | x | x | x |
         Q9 Topology change aware   | x | x | x | x |   |(2)|
         ---------------------------+---+---+---+---+---+---+
                   x  Means that the answer was Yes.
     (1) In some products (typically high-end) Yes; in others No.
     (2) Not at the time that the questionnaire was received
         but expected in the near future.

5. References

5.1. Normative References

   [BRIDGE]     IEEE Std. 802.1D-2004 IEEE Standard for Local and
                metropolitan area networks, Media Access Control (MAC)
                Bridges

   [IGMPv3]     Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
                Thyagarajan, "Internet Group Management Protocol,
                Version 3", RFC 3376, October 2002.

   [IPV6-1394]  Fujisawa, K. and A. Onoe, "Transmission of IPv6 Packets
                over IEEE 1394 Networks", RFC 3146, October 2001.

   [IPV6-ETHER] Crawford, M., "Transmission of IPv6 Packets over
                Ethernet Networks", RFC 2464, December 1998.

   [IPV6-FDDI]  Crawford, M., "Transmission of IPv6 Packets over FDDI
                Networks", RFC 2467, December 1998.

   [IPV6-TOKEN] Crawford, M., Narten, T., and S. Thomas, "Transmission
                of IPv6 Packets over Token Ring Networks", RFC 2470,
                December 1998.

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

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

   [MRDISC]     Haberman, B. and J. Martin, "Multicast Router
                Discovery", RFC 4286, December 2005.

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

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

   [RFC3307]    Haberman, B., "Allocation Guidelines for IPv6 Multicast
                Addresses", RFC 3307, August 2002.

5.2. Informative References

   [CISCO]      Cisco Tech Notes, "Multicast In a Campus Network: CGMP
                and IGMP snooping",
                http://www.cisco.com/warp/public/473/22.html

   [IETF56]     Briefing by Dave Thaler, Microsoft, presented to the
                MAGMA WG at the 56'th IETF meeting in San Francisco,
                http://www.ietf.org/proceedings/03mar/index.html

   [MSOFT]      Microsoft support article Q223136, "Some LAN Switches
                with IGMP Snooping Stop Forwarding Multicast Packets on
                RRAS Startup", http://support.microsoft.com/
                support/articles/Q223/1/36.ASP

6. Security Considerations

   Under normal network operation, the snooping switch is expected to
   improve overall network performance by limiting the scope of
   multicast flooding to a smaller portion of the local network.  In the
   event of forged IGMP messages, the benefits of using a snooping
   switch might be reduced or eliminated.

   Security considerations for IGMPv3 at the network layer of the
   protocol stack are described in [IGMPv3].  The introduction of IGMP
   snooping functionality does not alter the handling of multicast
   packets by the router as it does not make use of link layer
   information.

   There are, however, changes in the way that the IGMP snooping switch
   handles multicast packets within the local network.  In particular:

   -  A Query message with a forged source address which is less than
      that of the current Querier could cause snooping switches to
      forward subsequent Membership reports to the wrong network
      interface.  It is for this reason that IGMP Membership Reports
      should be sent to all multicast routers as well as the current
      Querier.

   -  It is possible for a host on the local network to generate
      Current-State Report Messages that would cause the switch to
      incorrectly believe that there is a multicast listener on the same
      network segment as the originator of the forged message.  This
      will cause unrequested multicast packets to be forwarded into the
      network segments between the source and the router.  If the router
      requires that all Multicast Report messages be authenticated as
      described in section 9.4 of [IGMPv3], it will discard the forged
      Report message from the host inside the network in the same way

      that it would discard one which originates from a remote location.
      It is worth noting that if the router accepts unauthenticated
      Report messages by virtue of them having arrived over a network
      interface associated with the internal network, investigating the
      affected network segments will quickly narrow the search for the
      source of the forged messages.

   -  As noted in [IGMPv3], there is little motivation for an attacker
      to forge a Membership report message since joining a group is
      generally an unprivileged operation.  The sender of the forged
      Membership report will be the only recipient of the multicast
      traffic to that group.  This is in contrast to a shared LAN
      segment (HUB) or network without snooping switches, where all
      other hosts on the same segment would be unable to transmit when
      the network segment is flooding the unwanted traffic.

   The worst case result for each attack would remove the performance
   improvements that the snooping functionality would otherwise provide.
   It would, however, be no worse than that experienced on a network
   with switches that do not perform multicast snooping.

7. Acknowledgements

   We would like to thank Martin Bak, Les Bell, Yiqun Cai, Ben Carter,
   Paul Congdon, Toerless Eckert, Bill Fenner, Brian Haberman, Edward
   Hilquist, Hugh Holbrook, Kevin Humphries, Isidor Kouvelas, Pekka
   Savola, Suzuki Shinsuke, Jaff Thomas, Rolland Vida, and Margaret
   Wasserman for comments and suggestions on this document.

   Furthermore, the following companies are acknowledged for their
   contributions: 3Com, Alcatel, Cisco Systems, Enterasys Networks,
   Hewlett-Packard, Vitesse Semiconductor Corporation, Thrane & Thrane.
   The ordering of these names do not necessarily correspond to the
   column numbers in the response table.

Authors' Addresses

   Morten Jagd Christensen
   Thrane & Thrane
   Lundtoftegaardsvej 93 D
   2800 Lyngby
   DENMARK

   EMail: mjc@tt.dk

   Karen Kimball
   Hewlett-Packard
   8000 Foothills Blvd.
   Roseville, CA 95747
   USA

   EMail: karen.kimball@hp.com

   Frank Solensky
   Calix
   43 Nanog Park
   Acton, MA 01720
   USA

   EMail: frank.solensky@calix.com

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