Movatterモバイル変換

Network Working Group                                     M. ChristensenRequest for Comments: 4541                               Thrane & ThraneCategory: Informational                                       K. Kimball                                                         Hewlett-Packard                                                             F. Solensky                                                                   Calix                                                                May 2006Considerations for Internet Group Management Protocol (IGMP)and Multicast Listener Discovery (MLD) Snooping SwitchesStatus 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 isChristensen, et al.          Informational                      [Page 1]

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 2006   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.Christensen, et al.          Informational                      [Page 2]

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 2006   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.Christensen, et al.          Informational                      [Page 3]

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 2006      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 itsChristensen, et al.          Informational                      [Page 4]

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 2006      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 insection 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.Christensen, et al.          Informational                      [Page 5]

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 20062.1.2.  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 consideringChristensen, et al.          Informational                      [Page 6]

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 2006      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.Christensen, et al.          Informational                      [Page 7]

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 20062.2.  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 (asChristensen, et al.          Informational                      [Page 8]

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 2006   addressed inSection 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 multicastChristensen, et al.          Informational                      [Page 9]

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 2006   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?Christensen, et al.          Informational                     [Page 10]

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 2006      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.Christensen, et al.          Informational                     [Page 11]

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 20065.  References5.1.  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.Christensen, et al.          Informational                     [Page 12]

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 20065.2.  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.ASP6.  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 wayChristensen, et al.          Informational                     [Page 13]

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 2006      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.Christensen, et al.          Informational                     [Page 14]

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 2006Authors' 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.comChristensen, et al.          Informational                     [Page 15]

RFC 4541     IGMP and MLD Snooping Switches Considerations      May 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Christensen, et al.          Informational                     [Page 16]