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
Christensen, 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 its
Christensen, 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 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.
Christensen, et al. Informational [Page 5]RFC 4541 IGMP and MLD Snooping Switches Considerations May 2006
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
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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 2006
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
Christensen, et al. Informational [Page 8]
RFC 4541 IGMP and MLD Snooping Switches Considerations May 2006
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
Christensen, 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 2006
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.
Christensen, et al. Informational [Page 12]RFC 4541 IGMP and MLD Snooping Switches Considerations May 2006
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
Christensen, 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 2006
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
Christensen, et al. Informational [Page 15]
RFC 4541 IGMP and MLD Snooping Switches Considerations May 2006
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Christensen, et al. Informational [Page 16]
