group
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... datagram is delivered to all
members of its destination host group with the same "best-efforts"
reliability as regular unicast ...
... not guaranteed to arrive intact at all members of the destination
group or in the same order relative to other datagrams.
...
... hosts may join
and leave groups at any time. There is no restriction on the
location or number of members in a host group ...
... groups at any time. There is no restriction on the
location or number of members in a host group. A host may be a
member of more than one group ...
... group. A host may be a
member of more than one group at a time. A host need not be a member
of a group ...
... A host group may be permanent or transient. A permanent group has a
well-known, administratively assigned IP address ...
... IP address. It is the address,
not the membership of the group, that is permanent; at any time a
permanent group may have any number of members, even zero. Those IP
multicast ...
... not the membership of the group, that is permanent; at any time a
permanent group may have any number of members, even zero. Those IP
multicast addresses that are not reserved for permanent groups ...
... group may have any number of members, even zero. Those IP
multicast addresses that are not reserved for permanent groups are
available for dynamic assignment to transient groups ...
... groups are
available for dynamic assignment to transient groups which exist only
as long as they have members.
...
... multicast which reaches all immediately-neighboring
members of the destination host group. If the datagram has an IP
...
... networks that have members of the destination group. On those other
member networks that are reachable within the IP ...
... host to join any host groups. An IP implementation may be upgraded
from level 0 to level 1 very easily and with little new code. Only
...
... IP datagrams to host groups. It requires implementation of the
Internet Group Management Protocol (IGMP ...
... host groups. It requires implementation of the
Internet Group Management Protocol (IGMP) and extension of the IP and
...
... HOST GROUP ADDRESSES ...
... In Internet standard "dotted decimal" notation, host group addresses
range from 224.0.0.0 to 239.255.255.255. The address ...
... range from 224.0.0.0 to 239.255.255.255. The address 224.0.0.0 is
guaranteed not to be assigned to any group, and 224.0.0.1 is assigned
to the permanent group of all IP ...
... guaranteed not to be assigned to any group, and 224.0.0.1 is assigned
to the permanent group of all IP hosts (including gateways ...
... merely specifies an IP host group address, rather than an individual
IP address, as the destination ...
... Third (level 2 implementations only), for the case in which the host
is itself a member of a group to which a datagram is being sent, the
service interface ...
... performance optimization for
upper-layer protocols that restrict the membership of a group to one
process per host (such as a routing protocol ...
... routing protocol), or that handle
loopback of group communication at a higher layer (such as a
multicast ...
... If the sending host is itself a member of the destination group on
the outgoing interface, a copy of the outgoing datagram ...
...
A host group address must never be placed in the source address field
or anywhere in a source route ...
... IP
datagrams is a procedure for mapping IP host group addresses to
Ethernet multicast addresses ...
... group address,
more than one host group address may map to the same Ethernet
multicast address ...
... destination
IP address. However, before any datagrams destined to a particular
group can be received, an upper-layer protocol must ask the IP module
...
... host become a member
of the host group identified by "group-address" on the given network
interface ...
... of the host group identified by "group-address" on the given network
interface. The LeaveGroup operation requests that this host ...
... its membership in the host group identified by "group-address" on the
given network interface ...
...
It is permissible to join the same group on more than one interface,
in which case duplicate multicast ...
... also permissible for more than one upper-layer protocol to request
membership in the same group.
...
... Both operations should return immediately (i.e., they are non-
blocking operations), indicating success or failure. Either
operation may fail due to an invalid group address or interface
identifier. JoinHostGroup may fail due to lack of local resources.
LeaveHostGroup may fail because the host ...
... LeaveHostGroup may fail because the host does not belong to the given
group on the given interface. LeaveHostGroup may succeed, but the
membership persist, if more than one upper-layer ...
... membership persist, if more than one upper-layer protocol has
requested membership in the same group.
...
... IP module
must be extended to maintain a list of host group memberships
associated with each network interface. An incoming datagram ...
... network interface. An incoming datagram
destined to one of those groups is processed exactly the same way as
datagrams destined to one of the host ...
...
Incoming datagrams destined to groups to which the host does not
belong are discarded without generating any error ...
... datagram
arrives via one interface, destined for a group to which the host
belongs only on a different interface ...
... datagram with an IP host group address in its source address field is
quietly discarded. An ICMP error message ...
...
The list of host group memberships is updated in response to
JoinHostGroup and LeaveHostGroup requests from upper-layer protocols.
...
... mechanism to handle multiple requests to join and leave the same
group. On the first request to join and the last request to leave a
group ...
... group. On the first request to join and the last request to leave a
group on a given interface, the local network module for that
...
... IGMP is used to keep neighboring multicast
routers informed of the host group memberships present on a
particular local network. To support IGMP ...
... (Datagrams addressed to the all-hosts group are recognized as a
special case by the multicast routers and are never forwarded beyond
...
... For the purpose of IGMP, membership in the all-hosts group is really
necessary only while the host belongs to at least one other group ...
... group is really
necessary only while the host belongs to at least one other group.
However, it is specified that the host shall remain a member of the
...
... host shall remain a member of the
all-hosts group at all times because (1) it is simpler, (2) the
frequency of reception of unnecessary IGMP queries ...
... address" is an IP host group address. The
JoinLocalGroup operation requests the local network module to accept
...
... IP
host group address. The LeaveLocalGroup operation requests the local
network module to stop delivering up packets destined to the given IP ...
... host's IP host group
addresses. It is highly desirable to take advantage of any address
filtering ...
... APPENDIX I. INTERNET GROUP MANAGEMENT PROTOCOL (IGMP) ...
... hosts to
report their host group memberships to any immediately-neighboring
multicast routers. IGMP ...
... Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
... In a Host Membership Query message, the group address field
is zeroed when sent, ignored when received.
...
... called Queries) to discover which host groups have members on their
attached local networks. Queries ...
... Queries are addressed to the all-hosts
group (address 224.0.0.1), and carry an IP time-to-live ...
... Host Membership Reports
(hereinafter called Reports), reporting each host group to which they
belong on the network interface from which the Query ...
... starts a report delay timer for each of its
group memberships on the network interface of the incoming
Query ...
... timer expires, a
Report is generated for the corresponding host group. Thus,
Reports are spread out over a D second interval instead of
all occurring at once.
...
... destination address equal to the
host group address being reported, and with an IP
time-to-live ...
... IP
time-to-live of 1, so that other members of the same group on
the same network can overhear the Report. If a host ...
... network can overhear the Report. If a host hears a
Report for a group to which it belongs on that network, the
host ...
... host stops its own timer for that group and does not generate
a Report for that group. Thus, in the normal case, only one
...
... timer for that group and does not generate
a Report for that group. Thus, in the normal case, only one
Report will be generated for each group present on the
...
... a Report for that group. Thus, in the normal case, only one
Report will be generated for each group present on the
network, by the member host ...
... routers need not know which hosts
belong to a group, only that at least one host belongs to a
group ...
... There are two exceptions to the behavior described above. First, if
a report delay timer is already running for a group membership when a
Query is received, that timer ...
... host's membership in the all-
hosts group (224.0.0.1), and that membership is never reported.
...
... IP destination field and its IGMP group address
field, to ensure that the host's own Report is not cancelled by an
...
... knowledge of memberships present on a particular network. If no
Reports are received for a particular group after some number of
Queries, the routers ...
... Queries, the routers assume that that group has no local members and
that they need not forward remotely-originated multicasts for that
...
... that they need not forward remotely-originated multicasts for that
group onto the local network. Queries are normally sent infrequently
...
...
When a host joins a new group, it should immediately transmit a
Report for that group, rather than waiting for a Query ...
... host joins a new group, it should immediately transmit a
Report for that group, rather than waiting for a Query, in case it is
the first member of that group ...
... group, rather than waiting for a Query, in case it is
the first member of that group on the network. To cover the
possibility of the initial Report being lost or damaged, it is
...
... simple way to accomplish this is to act as if a Query had been
received for that group only, setting the group's random report delay
timer ...
... Query had been
received for that group only, setting the group's random report delay
timer. The state transition diagram ...
... Non-Member state, when the host does not belong to the group
on the interface. This is the initial state ...
... Delaying Member state, when the host belongs to the group on
the interface and has a report delay timer ...
... Idle Member state, when the host belongs to the group on the
interface and does not have a report delay timer ...
... group" occurs when the host decides to join the group on
the interface. It may occur only in the Non-Member state ...
... "leave group" occurs when the host decides to leave the group
on the interface. It may occur only in the Delaying Member
...
... destination field and its IGMP group address field. A Report
applies only to the membership in the group identified by the
...
... IGMP group address field. A Report
applies only to the membership in the group identified by the
Report, on the interface from which the Report is received.
...
... timer expired" occurs when the report delay timer for the
group on the interface expires. It may occur only in the
Delaying Member state ...
... "start timer" for the group on the interface, using a random
delay value between 0 and D seconds.
...
... host starts in Idle Member state for that group on every
interface, never transitions to another state ...
... APPENDIX II. HOST GROUP ADDRESS ISSUES ...
... Group Address Binding ...
... IP network. An IP host group address is
dynamically bound to a set of local network interfaces on a set of IP
networks ...
... It is important to understand that an IP host group address is NOT
bound to a set of IP unicast addresses ...
... multicast routers do not
need to maintain a list of individual members of each host group.
For example, a multicast router attached to an Ethernet ...
... multicast address with each host
group having local members, rather than a list of the members'
individual IP or Ethernet ...
... Allocation of Transient Host Group Addresses ...
...
This memo does not specify how transient group address are allocated.
It is anticipated that different portions of the IP transient host ...
... IP transient host
group address space will be allocated using different techniques.
For example, there may be a number of servers that can be contacted
to acquire a new transient group address ...
... group address space will be allocated using different techniques.
For example, there may be a number of servers that can be contacted
to acquire a new transient group address. Some higher-level
protocols (such as VMTP, specified in RFC-1045exp ...
... VMTP, specified in RFC-1045exp) may generate higher-
level transient "process group" or "entity group" addresses ...
... level transient "process group" or "entity group" addresses which are
then algorithmically mapped to a subset of the IP ...
... group addresses, similarly to the way that IP host group addresses
are mapped to Ethernet multicast addresses ...
... are mapped to Ethernet multicast addresses. A portion of the IP
group address space may be set aside for random allocation by
applications that can tolerate occasional collisions with other
...
... datagrams sent to any host
group address will reach only the intended hosts, or that datagrams
...
... datagrams
received as a member of a transient host group are intended for the
recipient. Misdelivery must be detected at a level above IP, using
...
... tokens. Information
transmitted to a host group address should be encrypted or governed
by administrative routing ...