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... report their multicast group memberships to any immediately-
neighboring multicast routers. This memo describes only the use of
IGMP between hosts ...
... routers to determine group membership.
Routers that are members of multicast groups are expected to behave
as hosts ...
... multicast groups are expected to behave
as hosts as well as routers, and may even respond to their own
queries. IGMP ...
... of 2. All IGMP messages described in this document are sent with IP
TTL 1, and contain the IP Router Alert option [RFC2113] in their IP
header ...
...
Varying this setting allows IGMPv2 routers to tune the "leave
latency" (the time between the moment the last host ...
... primary interface on an attached network"; if a router has multiple
physical interfaces on a single network ...
... of their attached physical networks. A multicast router keeps a list
of multicast group memberships for each attached network ...
... network, not a list of all of the members. With respect to
each of its attached networks, a multicast router may assume one of
two roles: Querier ...
... multicast router hears a
Query message from a router with a lower IP address, it MUST become a
Non-Querier ...
... router has not heard a Query
message from another router for [Other Querier Present Interval], it
resumes the role ...
... send a General Query on each attached network for which this router
is the Querier, to solicit membership information. On startup, a
...
... is the Querier, to solicit membership information. On startup, a
router SHOULD send [Startup Query Count] General Queries spaced
...
...
When a router receives a Report, it adds the group being reported to
the list of multicast group ...
... group before this timer has
expired, the router assumes that the group has no local members and
that it need not forward remotely-originated multicasts ...
... group, it SHOULD
send a Leave Group message to the all-routers multicast group
(224.0.0.2). If it was not the last host ...
... send a Leave Group message when it leaves a group. Routers SHOULD
accept a Leave Group message addressed to the group ...
... version of this
standard. Leave Group messages are addressed to the all-routers
group because other group members ...
... received after the response time of the last query expires, the
routers assume that the group has no local members, as above. Any
Querier ...
... Querier to non-Querier transition is ignored during this time; the
same router keeps sending the Group-Specific Queries.
...
... host may be placed on a subnet where the Querier router has
not yet been upgraded to IGMPv2. The following requirements ...
...
The IGMPv1 router will send General Queries with the Max
Response Time set to 0. This MUST be interpreted as a value of
...
... query was heard in the
last [Version 1 Router Present Timeout] seconds, and MUST NOT be
based upon the type of the last Query heard. This state ...
... IGMPv2 router may be placed on a subnet where at least one router
on the subnet has not yet been upgraded to IGMPv2 ...
... is no reliable way of dynamically determining whether IGMPv1
routers are present on a network. Implementations MAY provide a
way for system administrators ...
... system administrators to enable the use of IGMPv1 on
their routers; in the absence of explicit configuration, the
configuration MUST default to IGMPv2. When in IGMPv1 ...
... IGMPv2. When in IGMPv1 mode,
routers MUST send Periodic Queries with a Max Response Time of
0, and MUST ignore Leave Group ...
... hosts present for a particular group, a
router MUST ignore any Leave Group messages that it receives for
that group ...
... host to report is
cleared, this action MAY be skipped. The Leave Message is sent to
the ALL-ROUTERS group (224.0.0.2).
...
... Router State Diagram ...
...
Router behavior is more formally specified by the state transition
diagrams below.
...
...
The following three events can cause the router to change states:
...
... an IGMP Membership Query is received from a router on the same
network with a lower IP address ...
...
In addition, to keep track of which groups have members, a router may
be in one of four possible states with respect to any single IP
multicast group ...
... "Checking Membership" state, when the router has received a
Leave Group message but has not yet heard a Membership Report for
...
... "v2 report received" occurs when the router receives a Version 2
Membership Report for the group ...
... "v1 report received" occurs when the router receives a Version 1
Membership report for the group ...
... Query Interval] *
[Last Member Query Count] if this router is a Querier, or the [Max
Response Time] in the packet * [Last Member Query ...
... Most of these timers are configurable. If non-default settings
are used, they MUST be consistent among all routers on a single
link. Note that parentheses are used to group ...
... The Group Membership Interval is the amount of time that must pass
before a multicast router decides there are no more members of a
group on a network ...
... The Other Querier Present Interval is the length of time that must
pass before a multicast router decides that there is no longer
another multicast router which should be the querier ...
... pass before a multicast router decides that there is no longer
another multicast router which should be the querier. This value
MUST be ((the Robustness Variable ...
... Group-Specific Queries
sent before the router assumes there are no local members. Default:
the Robustness Variable.
...
... Membership Report The group being reported
Leave Message ALL-ROUTERS (224.0.0.2)
Note: in older (i.e., non-standard and now obsolete) versions ...
... hosts send Leave Messages to the group being left. A
router SHOULD accept Leave Messages addressed to the group being
left in the interests of backwards compatibility ...
... hosts.
In all cases, however, hosts MUST send to the ALL-ROUTERS address
to be compliant with this specification.
...
... forger. If the forger then sends no more Query messages, other
routers' Other Querier Present timer will time out and one will
...
... Report messages:
A forged Report message may cause multicast routers to think there
are members of a group on a subnet ...
... subnet will be ignored.
- Ignore Report messages without Router Alert options [RFC2113],
and require that routers ...
... Router Alert options [RFC2113],
and require that routers not forward Report messages. (The
requirement is not a requirement ...
... requirement of generalized filtering in the
forwarding path, since the packets already have Router Alert
options in them). This solution breaks backwards compatibility
with implementations of earlier versions ...
... with implementations of earlier versions of this specification
which did not require Router Alert.
A forged Version 1 ...
... state for a particular group, meaning that the
router will ignore Leave messages. This can cause traffic to flow
...
... Queries for the group in question. This causes extra
processing on each router and on each member of the group, but can
not cause loss of desired traffic ...
... subnet will be ignored.
- Ignore Leave messages without Router Alert options [RFC2113],
and require that routers ...
... Router Alert options [RFC2113],
and require that routers not forward Leave messages. (The
requirement is not a requirement ...
... requirement of generalized filtering in the
forwarding path, since the packets already have Router Alert
options in them). This solution breaks backwards compatibility
with implementations of earlier versions ...
... with implementations of earlier versions of this specification
which did not require Router Alert.
...
... IGMP Type is assigned to Version 2 Membership Report messages,
so a router may tell the difference between an IGMPv1 and IGMPv2 host ...
... IGMPv2. However, this means that
care must be taken when an IGMPv2 router is trying to coexist with an
IGMPv1 router ...
... router is trying to coexist with an
IGMPv1 router that uses a different querier election mechanism. In
particular, it means that an IGMPv2 ...
... querier election mechanism. In
particular, it means that an IGMPv2 router must be able to act as an
IGMPv1 router ...
... router must be able to act as an
IGMPv1 router on a particular network if configured to do so. The
actions required include:
...