IDMR Working Group M. Christensen
Internet Draft Vitesse
June 2001 F. Solensky
Expiration Date: December 2001 Gotham Networks
IGMP and MLD snooping switches
<draft-ietf-idmr-snoop-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [RFC2026].
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Abstract
This memo describes the interoperability problems and issues that can
arise when a mixed deployment of IGMPv3 and IGMPv2 capable hosts and
routers are interconnected by a switch with IGMP snooping
capabilities. The memo also covers MLDv2 for IPv6. It is intended as
an accompanying document to the IGMPv3 and MLDv2 specifications.
The memo contains a brief IGMP walk through followed by a description
of the IGMP snooping functionality. Specific examples are given
which are all based on Ethernet as the link layer protocol. MLDv2 for
IPv6 is discussed. Finally recommendations are given for the
behavior of IGMP snooping switches.
The purpose of this document is twofold:
- We want to summarize IGMP snooping induced problems and best cur-
rent solutions. We hope that a description of IGMP snooping will
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be of aid to the IETF when standardizing new protocols and behav-
iors within this scope.
- We also hope to bring this work to the attention of switch ven-
dors, typically active within the IEEE community but perhaps not
within IETF, in order to minimize protocol interoperability
problems in the future.
1. Introduction
In recent years, a number of commercial vendors have introduced prod-
ucts 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 utilizes information in the upper
level protocol headers as factors to be considered in the 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 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.
The discussions in this document are based on IGMP which applies to
IPv4 only. For IPv6 we must use MLD in stead. Because MLD is based on
IGMP with only a few differences these discussions also apply to
IPv6.
2. IGMP snooping overview
For a full description of IGMP we refer to [IGMPv3], however, IGMP
operation is summarized in the following:
* Hosts send IGMP Membership Report messages to inform neighboring
routers that they wish to join a specific IP multicast group.
* IGMPv3 Membership Reports may be qualified with a list of allowed
or forbidden source addresses.
* Routers periodically send IGMP Query messages to hosts in order
to maintain group membership state information. These queries
can be either general or group specific queries.
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* Hosts respond to queries with Membership Reports.
* Hosts running either IGMPv2 or IGMPv3 may also send a Leave Group
message to routers to withdraw from the group.
A traditional Ethernet network may be separated into different net-
work segments to prevent placing too many devices onto the same
shared media. These segments are connected by bridges and switches.
When a packet with a broadcast or multicast destination address is
received, the switch will forward a copy into each of the remaining
network segments in accordance with the IEEE MAC bridge standard
[BRIDGE]. Eventually, the packet is made accessible to all nodes
connected to the network.
This approach works well for broadcast packets that are intended to
be seen or processed by all connected nodes. In the case of multi-
cast 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 pack-
ets 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.
The problem of wasting bandwidth is even worse when the LAN segment
is not shared, for example in Full Duplex links. Full Duplex is
standard today for most switches operating at 1Gbps, and it will be
standard for 10Gbps ethernet too. In this case the wasted bandwidth
is proportional to the number of attached nodes.
Allowing switches to snoop IGMP packets is a creative effort to solve
this problem. The switch uses the information in the IGMP packets as
they are being forwarded throughout the network to determine which
segments should receive packets directed to the group address.
IGMP snooping is being implemented slightly different by different
switch vendors. We will not address specific implementations here as
documentation is not widely available. For details of one implementa-
tion we refer to [CISCO].
In the following we will describe problems in relation to IGMP snoop-
ing with the following constraints, which we believe are the most
common cases.
1. Group membership is based on multicast MAC addresses only.
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2. Forwarding is based on a 'list' of member ports for each sup-
ported multicast group.
3. The switch is equipped with a CPU for maintaining group member-
ship information.
Constraint 3 above is not a strict requirement as IGMP snooping could
be accomplished entirely in hardware. However, when sending IGMP
datagrams all is done to ensure that the packets are not routed. For
example the TTL is set to 1 and the IP header contains the router
alert option. This is a hint to developers that there is probably a
need to send this packet to the CPU.
IGMP snooping switches build forwarding lists by listening for (and
in some cases intercepting) IGMP messages. Although the software
processing the IGMP messages may maintain state information based on
the full IP group addresses, the forwarding tables are typically
mapped to link layer addresses. An example of such a forwarding
table is shown in Figure 1.
Multicast MAC address | Member ports
-------------------------------------
01-00-5e-00-00-01 | 2, 7
01-00-5e-01-02-03 | 1, 2, 3, 7
01-00-5e-23-e2-05 | 1, 4
-------------------------------------
Figure 1.
Because only the least significant 23 bits of the IP address are
mapped to Ethernet addresses [RFC1112], there is a loss of informa-
tion when forwarding solely on the destination MAC address. This
means that for example 224.0.0.123 and 239.128.0.123 and similar IP
multicast addresses all map to MAC address 01-00-5e-00-00-7b (for
Ethernet). As a consequence, IGMP snooping switches may collapse IP
multicast group memberships into a single Ethernet multicast member-
ship group.
Finally, it should be mentioned that in addition to building and
maintaining lists of multicast group memberships the snooping switch
should also maintain a list of multicast routers. When forwarding
multicast packets they should be forwarded on ports which have joined
using IGMP but also on ports on which multicast routers are attached.
The reason for this is that in IGMP there is only one active querier.
This means that all other routers on the network are suppressed and
thus not detectable by the switch.
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2.1. Problems in older networks
The drawback of using IGMP snooping switches to make the flooding of
multicast traffic more efficient is that the underlying link layer
topology is required to remain very stable. This is especially true
in IGMP versions 1 and 2 where group members do not transmit Member-
ship Report messages after having overheard a report from another
group member.
This problem can be demonstrated with an example. In the topology
illustrated in figure 2, a topology loop exists between four IGMP
snooping switches labeled A, B, C and D.
- The spanning tree algorithm would detect this loop and disable
one of the links; for example, the link connecting ports B3 and
C1.
- Host H1 transmits a group Membership Report which will be flooded
throughout the network.
- When switch A hears the report, it determines that packets
addressed to the group should be forwarded to port A3.
- Router R hears the Join message and starts forwarding packets
with the multicast destination address into the network. Host H1
is now part of the group.
- The link between D2 and C2 is broken. The spanning tree algo-
rithm reactivates the blocked link B3-C1.
- If switch A relies solely on the exchange of IGMP messages to
alter its forwarding behavior, host H1 will be unable to receive
packets forwarded to the group address until router R sends out
another Membership Query.
One possible approach to work around this limitation would be for the
switch to keep track of which nodes belong to the group, altering the
forwarding tables whenever a member becomes visible through a different
port. When switch A sees that host H1 has moved from port A3 to A2, the
group membership table would be updated. This does not work, however,
when more than one node joins the same group address when at least one
of them has not yet been upgraded to IGMPv3: if hosts H1 and H2 were to
join the group at approximately the same time, they would both start off
random timers for the transmission of their first Membership Reports.
If host H2 selects a longer interval than H1, it will hear H1's report
message and cancel the one it was about to send. Switch A, therefore,
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+------+ B2
B1 |Snoop |----- - - - +------+
-----|Switch| | Host |
/ | B |----- / | H1 |
+--------+ A2 / +------+ B3 X C1 +------+ +--+---+
A1 | Snoop |----- / -----|Snoop | |
--+----| Switch | |Switch|-----+----
| | A |----- -----| C | C3
+-+-+ +--------+ A3 \ +------+ D2 / C2 +------+
| R | \ D1 |Snoop |-----
++-++ -----|Switch|
| | | D |---------+------ - - -
+------+ D3 |
+--+---+
| Host |
| H2 |
+------+
Figure 2
never learns that node H2 has joined the group. When the switch learns
that H1 is now accessible through port A2, it has no way of knowing that
it should continue forwarding group packets to port A3 as well.
Two recommendations can be made based on the above discussion:
- The switch should play an active role when detecting a topology
change; The spanning tree root bridge (which is also a snooping
switch) should initiate the transmission of a IGMP General Query,
for example through signalling the CPU. This will help to reduce
the join latency otherwise introduced.
- IGMP Membership Reports should not be flooded because this will
lead to Join suppression.
2.2. IGMPv2 snooping and 224.0.0.X
Special attention should be brought to the IP address range from
224.0.0.1 through 224.0.0.255 which is reserved for routing protocols
and other low-level topology discovery or maintenance protocols
[IANA]. Examples of reserved multicast addresses are:
Multicast routers are discouraged from routing packets when a desti-
nation address falls within this range, regardless of the TTL value.
The router will be the originator or consumer of these messages so it
has less of a motivation to maintain forwarding path information for
these addresses. As a result, it becomes less critical for the
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224.0.0.2 All Routers on this Subnet
224.0.0.4 DVMRP
224.0.0.5 (M)OSPF routers
224.0.0.6 (M)OSPF DRs
224.0.0.9 RIP2 Routers
224.0.0.13 PIM Routers
224.0.0.22 IGMPv3 Membership Reports
router to send out periodic Query messages for these groups. If the
router chooses not to, the group would be unable to recover from
topology changes as described above. Note that the only difference
between the 'all hosts' address (224.0.0.1) and the remainder of this
range is that the router has no discretion in the former case: it
MUST NOT send Queries.
To avoid this situation, IGMP snooping switches should be less con-
servative when forwarding packets to these addresses and flood them
to all ports.
As an example of this, it is reported in [MSOFT] that a number of
switches can be misconfigured to perform IGMP snooping and forwarding
for all IP multicast groups:
Figure 3 illustrates the scenario where two routers R1 and R2 are
communicating using for example 224.0.0.6. The routers never send
IGMP Joins for this address. The switch floods the (unknown) multi-
cast traffic on all ports.
Now the server SVR is started and it sends an IGMP Join for
224.0.0.6, which is snooped by the switch. The switch then generates
a Membership Query on all ports to determine which ports have devices
attached that also belong to this group.
The routers R1 and R2 do not respond and the switch builds a forward-
ing port list with only SVR in it. Now R1 and R2 are not able to
communicate using this address.
+----+ +----------+
--| R1 |-----| |
+----+ | Snooping | +-----+
| |----| SVR |
+----+ | switch | +-----+
--| R2 |-----| |
+----+ +----------+
Figure 3.
There are two possible fixes to this problem: One is to require that
all routers (also being hosts) which use IP multicast respond to IGMP
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queries in the range 224.0.0.X. This seems unnecessary as discussed
above because of the inherent link local scope of these messages.
Another solution to this problem, which is also discussed above, is
that the switch is configured to forward all packets for a range of
IP multicast addresses to all ports (flooding).
It is suggested that all multicast packets in the range 224.0.0.1
through 224.0.0.255 are forwarded on all ports. This of course
requires an examination of the network layer header. Note that these
are IP adress ranges and that mapping these to MAC address range
01-00-5e-00-00-X is subject to problems discussed in the previous
sections.
2.3. IGMPv3 and IGMPv2 coexistence
IGMPv3 and IGMPv2 are designed to interoperate with older versions of
IGMP. Both hosts and routers are capable of falling back to an ear-
lier version when receiving older IGMP messages, thus enabling a
mixed deployment and migration to new versions. While this works fine
in a network of hosts and routers an IGMP snooping switch introduces
problems.
In figure 4 where hosts H1 and H2 are connected to an IGMP snooping
switch on ports P1 and P2 respectively, consider the following
sequence of communication:
- Router R sends an IGMPv3 Query
- Host H1 sends an IGMPv2 Report since it has only implemented v2.
R notices this and switches to IGMPv2 mode. The report is not
received by H2 because of the snooping functionality.
- Switch S puts H1's port P1 in the forwarding table.
- Host H2 sends an IGMPv3 Report in response to R's Query.
- Switch S fails to add H2's port P2 to the forwarding table
because it doesn't support IGMPv3.
- H2 does not receive any traffic before R sends its next Query
which will put H2 in IGMPv2 mode.
This introduces a Join latency for host H2, which apparently cannot be
avoided. The latency is potentially of the order of minutes. It is
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possible however to reduce this latency by tuning the Query Interval
which defaults to 125 seconds.
When operating in a mixed deployment mode it is suggested that initially
the Query Interval is set to "a low value" until the compatibility modes
have stabilized both host and routers on the same IGMP version. After
stabilization the Query Interval could be increased to its original
value.
2.4. Source Specific Joins
Even for IGMPv3 snooping capable switches there can be limitations
caused by link layer based forwarding. This is illustrated in figure
4.
Assume that host H1 sends a Join(S1, G) to R and that host H2 sends a
Join(S2, G) to R.
The switch adds both hosts to the forwarding list for group G.
Frames originating from sources S1 and S2 for the same multicast
address G are routed via R. These are sent from R with the router's
MAC address as source.
The switch is unable to distinguish the two different types of flow
and forwards both flows to both hosts. This effectively disables the
Join source functionality in this network configuration.
+----+ P1+----------+
| H1 |-----| |
+----+ | Snooping | +---+ (S1, G)
| |----| R |--- and
+----+ | switch | +---+ (S2, G)
| H2 |-----| |
+----+ P2+----------+
Figure 4.
This is a problem caused by layer 2 based forwarding of a layer 3
flow in conjunction with the difference between the link layer and
the network layer information.
The example above means that host implementations cannot rely on the
router to perform all source address filtering. Therefore they must
still filter out packets that do not match the source address
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criterion specified in the Join messages. While this might be seen
as an inconvience, this is no different than the case where the
router is directly connected to both hosts on a shared LAN and no
snooping switch is present.
An complete solution would be for the switch to further qualify the
search process by including the source IP address when determining
which ports should forward the packet.
Similar problems occur with the attempt to exclude sources.
3. Snooping Requirements
Note that in the following we provide suggestions for good/best prac-
tices when designing IGMP snooping devices. Keywords as MUST, SHOULD,
MUST NOT etc. are suggestions only.
1) All IGMP packets (IP packets with IP-PROTO = 2) SHOULD be redi-
rected to the CPU for IGMP snooping processing and table management.
This allows for the most flexible IGMP snooping solution.
2) The switch that provides support for IGMP snooping MUST forward
all unrecognized IGMP messages and MUST NOT attempt to make use of
any information beyond the end of the network layer header. In par-
ticular, messages where any reserved fields are non-zero MUST NOT be
subject to "normal" snooping since this could indicate an incompati-
ble change to the message format.
3) Packets with a destination IP address in the 224.0.0.X range which
are *not* IGMP SHOULD be forwarded on all ports.
4) Packets with a destination IP address outside 224.0.0.X which are
*not* IGMP SHOULD be forwarded according to port membership tables
and MUST also be forwarded on router ports.
5) If a switch receives a *non* IGMP multicast packet without having
first processed Membership Reports for the group address, it MUST
forward the packet on all ports. In other words, the switch must
allow for the possibility that connected hosts and routers have been
upgraded to support future versions or extensions of IGMP that the
switch does not yet recognize. A switch MAY have a configuration
option that suppresses this operation, but default behavior MUST be
to allow flooding of unregistered packets.
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6) A snooping switch SHOULD forward IGMP Membership Reports on router
"ports" only.
7) The switch supporting IGMP snooping MUST maintain a list of multi-
cast routers. This list SHOULD be built using IGMP Multicast Router
Discovery [MRDISC] which is currently going through IETF Last Call.
IGMP snooping switches MAY build this list based on the arrival port
for packets destined to 224.0.0.X, when
- The packets are IGMP Queries or
- The packets are *not* IGMP or
- The ports are configured (by management) as having multicast
routers attached
8) IGMP snooping switches MAY maintain forwarding tables based on either
MAC addresses or IP addresses. If a switch supports both types of for-
warding tables then the default behavior SHOULD be to use IP addresses.
9) Switches which rely on information in the IP header MAY verify that
the IP header checksum is correct.
10) IGMP snooping switches SHOULD inform the CPU (or hardware) when a
link layer topology change has been detected. Following a topology
change the switch SHOULD initiate the transmission of a General Query on
all ports in order to reduce Join latency.
4. IPv6 Considerations
In order to avoid confusion, the previous discussions have been based
on IGMPv3 functionality which only applies to IPv4 multicast. In the
case of IPv6 most of the above discussions are still valid with a few
exceptions which we will describe here.
In IPv6 the protocol for multicast group maintenance is called Multi-
cast Listener Discovery (MLDv2). IPv6 is not widely deployed today
and neither is IPv6 multicast. However, it is anticipated that at
some time IPv6 switches capable of MLD snooping will appear.
The three main differences between IGMPv3 and MLDv2 are
- MLDv2 uses ICMPv6 message types instead of IGMP message types.
- The ethernet encapsulation is a mapping of 32bits of the 128bit
DIP addresses into 48bit DMAC addresses [IPENCAPS].
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- Multicast router discovery is done using Neighbor Discovery Pro-
tocol (NDP) for IPv6. NDP uses ICMPv6 message types.
A minor difference which applies to the requirements section is that in
IPv6 there is no checksum in the IP header. This is the reason that the
checksum validation requirement is listed as a MAY.
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 redirect packets to the CPU. If this
was the case the CPU queue assigned for MLD would potentially be filled
with non-MLD related packets. Furthermore ICMPv6 packets destined for
other hosts would not reach their destination.
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 only if it is configurable which message
types should trigger a CPU redirect and which should not. The reason is
that a hardcoding of message types is unflexible for the introduction of
new message types.
The second solution introduces the risk of new protocols, which are not
related to MLD but uses ICMPv6 and multicast DMAC addresses wrongly
being identified as MLD. We do not suggest a recommended solution in
this case.
The mapping from IP multicast addresses to multicast DMAC addresses
introduces a potentially enormous overlap. The structure of an IPv6 mul-
ticast address is shown in figure 5. Theoretically 2**80, two to the
power of 80 (128 - 8 - 4 - 4 - 32) unique DIP addresses could map to one
DMAC address. This should be compared to 2**5 in the case of IPv4.
Initial allocation of IPv6 multicast addresses, however, uses only the
lower 32 bits of group ID. This eliminates the address ambiguity for the
time being but it should be noted that the allocation policy may change
in the future.
| 8 | 4 | 4 | 112 bits |
+--------+----+----+---------------------------------------+
|11111111|flgs|scop| group ID |
+--------+----+----+---------------------------------------+
Figure 5
In the case of IPv6 forwarding can be made on the basis of DMAC
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addresses in the forseable future.
Finally we mention the reserved address range FF0X:0:0:0:0:0:X:X where X
is any value. This range is similar to 224.0.0.X for IPv4 and is
reserved to routing protocols and resource discovery [RFC2375]. In the
case of IPv6 it is suggested that packets in this range are forwarded on
all ports if they are not MLD packets.
5. Security Considerations
Security considerations for IGMPv3 are accounted for in [IGMPv3].
The introduction of IGMP snooping switches adds the following consid-
erations with regard to IP multicast.
The exclude source failure which could cause traffic from sources
that are 'black listed' to reach hosts that have requested otherwise.
This can also occur in certain network topologies without IGMP snoop-
ing.
It is possible to generate packets which make the switch wrongly
believe that there is a multicast router on the segment on which the
source is attached. This will potentially lead to excessive flooding
on that segment. The authentication methods discussed in [IGMPv3]
will also provide protection in this case.
IGMP snooping switches which rely on the IP header of a packet for
their operation and which do not validate the header checksum poten-
tially will forward packets on the wrong ports. Even though the IP
headers are protected by the ethernet checksum this is a potential
vulnerability.
Generally though, it is worth to stress that IP multicast must so far
be considered insecure until the work of for example the suggested
Multicast Security (MSEC) working group or similar is completed or at
least has matured.
6. References
[BRIDGE] IEEE 802.1D, "Media Access Control (MAC) Bridges"
[CISCO] Cisco Tech Notes, "Multicast In a Campus Network: CGMP
and IGMP snooping", http://www.cisco.com/warp/pub-
lic/473/22.html
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[IANA] Internet Assigned Numbers Authority, "Internet Multicast
Addresses", http://www.isi.edu/in-notes/iana/assign-
ments/multicast-addresses
[IGMPv3] Cain, B., "Internet Group Management Protocol, Version
3", draft-ietf-idmr-igmp-v3-06.txt, November 2000
[IPENCAPS]
Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC2464, December 1998.
[MLDv2] Vida, R., "Multicast Listener Discovery Version 2 (MLDv2)
for IPv6", draft-vida-mld-v2-00.txt, February 2001.
[MRDISC] Biswas, S. "IGMP Multicast Router Discovery", draft-ietf-
idmr-igmp-mrdisc-06.txt, May 2001.
[MSOFT] Microsoft support article Q223136, "Some LAN Switches
with IGMP Snooping Stop Forwarding Multicast Packets on
RRAS Startup", http://support.microsoft.com/sup-
port/kb/articles/Q223/1/36.ASP
[RFC1112] Deering, S., "Host Extensions for IP Multicasting", RFC
1112, August 1989.
[RFC2026] Bradner, S. "The Internet Standards Process -- Revision
3", RFC2026, October 1996.
[RFC2236] Fenner, W., "Internet Group Management Protocol, Version
2", RFC2236, November 1997.
[RFC2375] Hinden, R. "IPv6 Multicast Address Assignments", RFC2375,
July 1998.
Christensen, Solensky [Page 14]
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7. Acknowledgements
We would like to thank Bill Fenner, Yiqun Cai, Edward Hilquist and
Martin Bak for comments and suggestions on this document.
8. Author's Addresses:
Morten Jagd Christensen
Vitesse Semiconductor Corporation
Hoerkaer 16
2730 Herlev
DENMARK
email: mjc@vitesse.com
Frank Solensky
Gotham Networks
15 Discovery Way
Acton, MA 01720
USA
email: fsolensky@GothamNetworks.com
solensky@acm.org
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. IGMP snooping overview . . . . . . . . . . . . . . . . . . . . . 2
2.1 Problems in older networks . . . . . . . . . . . . . . . . . . . 5
2.2 IGMPv2 snooping and 224.0.0.X . . . . . . . . . . . . . . . . . 6
2.3 IGMPv3 and IGMPv2 coexistence . . . . . . . . . . . . . . . . . 8
2.4 Source Specific Joins . . . . . . . . . . . . . . . . . . . . . 9
3. Snooping Requirements . . . . . . . . . . . . . . . . . . . . . . 10
4. IPv6 Considerations . . . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . . . 13
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 15
8. Author's Addresses: . . . . . . . . . . . . . . . . . . . . . . . 15
Christensen, Solensky [Page 16]
