the router; they typically do not cause transitions in state at the
router. Membership Reports that are sent in response to changes in
interface state require the router to take some action in response to
the received report (see Section 6.4).
The inability to distinguish between the two types of reports would
force a router to treat all Membership Reports as potential changes
in state and could result in increased processing at the router as
well as an increase in IGMP traffic on the network.
A.2 Host Suppression
In IGMPv1 and IGMPv2, a host would cancel sending a pending
membership reports if a similar report was observed from another
member on the network. In IGMPv3, this suppression of host
membership reports has been removed. The following points explain
the reasons behind this decision.
1. Routers may want to track per-host membership status on an
interface. This allows routers to implement fast leaves (e.g.,
for layered multicast congestion control schemes) as well as track
membership status for possible accounting purposes.
2. Membership Report suppression does not work well on bridged LANs.
Many bridges and Layer2/Layer3 switches that implement IGMP
snooping do not forward IGMP messages across LAN segments in order
to prevent membership report suppression. Removing membership
report suppression eases the job of these IGMP snooping devices.
3. By eliminating membership report suppression, hosts have fewer
messages to process; this leads to a simpler state machine
implementation.
4. In IGMPv3, a single membership report now bundles multiple
multicast group records to decrease the number of packets sent.
In comparison, the previous versions of IGMP required that each
multicast group be reported in a separate message.
A.3 Switching Router Filter Modes from EXCLUDE to INCLUDE
If there exist hosts in both EXCLUDE and INCLUDE modes for a single
multicast group in a network, the router must be in EXCLUDE mode as
well (see section 6.2.1). In EXCLUDE mode, a router forwards traffic
from all sources unless that source exists in the exclusion source
list. If all hosts in EXCLUDE mode cease to exist, it would be
desirable for the router to switch back to INCLUDE mode seamlessly
without interrupting the flow of traffic to existing receivers.
One of the ways to accomplish this is for routers to keep track of
all sources desired by hosts that are in INCLUDE mode even though the
router itself is in EXCLUDE mode. If the group timer now expires in
EXCLUDE mode, it implies that there are no hosts in EXCLUDE mode on
the network (otherwise a membership report from that host would have
refreshed the group timer). The router can then switch to INCLUDE
mode seamlessly with the list of sources currently being forwarded in
its source list.
Appendix B. Summary of Changes from IGMPv2
While the main additional feature of IGMPv3 is the addition of source
filtering, the following is a summary of other changes from RFC2236.
o State is maintained as Group + List-of-Sources, not simply Group as
in IGMPv2.
o Interoperability with IGMPv1 and IGMPv2 systems is defined as
operations on the IGMPv3 state.
o The IP Service Interface has changed to allow specification of
source-lists.
o The Querier includes its Robustness Variable and Query Interval in
Query packets to allow synchronization of these variables on non-
Queriers.
o The Max Response Time in Query messages has an exponential range,
changing the maximum from 25.5 seconds to about 53 minutes, for use
on links with huge numbers of systems.
o Hosts retransmit state-change messages for increased robustness.
o Additional data sections are defined to allow later extensions.
o Report packets are sent to 224.0.0.22, to assist layer-2 switches
in "snooping".
o Report packets can contain multiple group records, to allow
reporting of full current state using fewer packets.
o Hosts no longer perform suppression, to simplify implementations
and permit explicit membership tracking.
o New Suppress Router-Side Processing (S) flag in Query messages
fixes robustness issues which were also present in IGMPv2.
Authors' Addresses
Brad Cain
Cereva Networks
Steve Deering
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA 95134-1706
Phone: +1-408-527-8213
EMail: deering@cisco.com
Bill Fenner
AT&T Labs - Research
75 Willow Rd.
Menlo Park, CA 94025
Phone: +1-650-330-7893
EMail: fenner@research.att.com
Isidor Kouvelas
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA 95134-1706
Phone: +1-408-525-0727
EMail: kouvelas@cisco.com
Ajit Thyagarajan
Ericsson IP Infrastructure
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