3Com S7902E, Configuration Manual

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1-4
“Leave group” mechanism
In IGMPv1, when a host leaves a multicast group, it does not send any notification to the multicast
router. The multicast router relies on host response timeout to know whether a group no longer has
members. This adds to the leave latency.
In IGMPv2, on the other hand, when a host leaves a multicast group:
1) This host sends a Leave Group message (often referred to as leave message) to all routers (the
destination address is 224.0.0.2) on the local subnet.
2) Upon receiving the leave message, the querier sends a configurable number of group-specific
queries to the group being left. The destination address field and group address field of the
message are both filled with the address of the multicast group being queried.
3) One of the remaining members, if any on the subnet, of the group being queried should send a
membership report within the maximum response time set in the query messages.
4) If the querier receives a membership report for the group within the maximum response time, it will
maintain the memberships of the group; otherwise, the querier will assume that no hosts on the
subnet are still interested in multicast traffic to that group and will stop maintaining the
memberships of the group.
Enhancements in IGMPv3
Built upon and being compatible with IGMPv1 and IGMPv2, IGMPv3 provides hosts with enhanced
control capabilities and provides enhancements of query and report messages.
Enhancements in control capability of hosts
IGMPv3 has introduced source filtering modes (Include and Exclude), so that a host can specify a list of
sources it expect or does not expect multicast data from when it joins a multicast group:
z
If it expects multicast data from specific sources like S1, S2, …, it sends a report with the
Filter-Mode denoted as “Include Sources (S1, S2, …).
z
If it does not expect multicast data from specific sources like S1, S2, …, it sends a report with the
Filter-Mode denoted as “Exclude Sources (S1, S2, …).
As shown in Figure 1-2 , the network comprises two multicast sources, Source 1 (S1) and Source 2 (S2),
both of which can send multicast data to multicast group G. Host B is interested only in the multicast
data that Source 1 sends to G but not in the data from Source 2.
Figure 1-2 Flow paths of source-and-group-specific multicast traffic
Source 2
Receiver
Host A
Host B
Host C
Packets (S1,G)
Packets (S2,G)
Source 1