host
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...
This memo specifies the extensions required of a host implementation
of the Internet Protocol (IP ...
... IP multicasting is defined as the transmission of an IP datagram to a
"host group", a set of zero or more hosts identified by a single IP ...
... multicast datagram is delivered to all
members of its destination host group with the same "best-efforts"
reliability ...
... and leave groups at any time. There is no restriction on the
location or number of members in a host group. A host may be a
...
... location or number of members in a host group. A host may be a
member of more than one group at a time. A host ...
... host may be a
member of more than one group at a time. A host need not be a member
of a group to send datagrams ...
... network multicast which reaches all immediately-neighboring
members of the destination host group. If the datagram has an IP ...
... multicast.
This memo specifies the extensions required of a host IP
implementation to support IP multicasting ...
... host" is any
internet host or gateway other than those acting as multicast
routers. The algorithms ...
... algorithms and protocols used within and between
multicast routers are transparent to hosts and will be specified in
separate documents. This memo also does not specify how local
network ...
... IP implementations
support IP multicasting. Level 0 hosts will, in general, be
unaffected by multicast activity. The only exception arises on some
...
... multicast activity. The only exception arises on some
types of local network, where the presence of level 1 or 2 hosts may
cause misdelivery of multicast IP datagrams ...
... cause misdelivery of multicast IP datagrams to level 0 hosts. Such
datagrams can easily be identified by the presence of a class ...
... IP
address in their destination address field; they should be quietly
discarded by hosts that do not support IP multicasting. Class D
...
... services,
such as resource location or status reporting, but it does not allow
a host to join any host groups ...
... groups, as well as send
IP datagrams to host groups. It requires implementation of the
Internet Group Management Protocol ...
... local network service interfaces within the host. All of the
following sections of this memo are applicable to level 2
...
... HOST GROUP ADDRESSES ...
... address all
multicast hosts on the directly connected network. There is no
multicast address ...
... multicast address (or any other IP address) for all hosts on the
total Internet. The addresses ...
... Appendix II contains some background discussion of several issues
related to host group addresses.
...
...
The multicast extensions to a host IP implementation are specified in
terms of the layered model illustrated below. In this model, ICMP ...
... terms of the layered model illustrated below. In this model, ICMP
and (for level 2 hosts) IGMP are considered to be implemented within
the IP ...
...
To support level 1 multicasting, a host IP implementation must
support the transmission of multicast ...
... IP datagrams. To support level
2 IP multicasting, a host must also support the reception of
multicast IP datagrams ...
... upper-layer protocol module
merely specifies an IP host group address, rather than an individual
IP address ...
... network.
Second, for hosts that may be attached to more than one network, the
service interface ...
... management.
Third (level 2 implementations only), for the case in which the host
is itself a member of a group to which a datagram ...
... upper-layer protocols that restrict the membership of a group to one
process per host (such as a routing protocol), or that handle
loopback ...
... sender. (Level 2 implementations only.)
A host group address should not be placed in the source address field
...
... Experimental Ethernet, all IP host group addresses may be mapped to a
single local broadcast ...
... address (at the cost of increased overhead on
all local hosts). For a point-to-point link joining two hosts (or a
...
... interface )
The JoinHostGroup operation requests that this host become a member
of the host group ...
... The JoinHostGroup operation requests that this host become a member
of the host group identified by "group-address ...
... group-address" on the given network
interface. The LeaveGroup operation requests that this host give up
its membership in the host group ...
... network
interface. The LeaveGroup operation requests that this host give up
its membership in the host group identified by "group-address ...
... network interface. The interface argument may be omitted on
hosts that may be attached to only one network. For hosts that may
...
... hosts that may be attached to only one network. For hosts that may
be attached to more than one network, the upper-layer ...
... group address or interface
identifier. JoinHostGroup may fail due to lack of local resources.
LeaveHostGroup may fail because the host does not belong to the given
group on the given interface ...
... IP datagrams, the IP module
must be extended to maintain a list of host group memberships
associated with each network interface ...
... groups is processed exactly the same way as
datagrams destined to one of the host's individual addresses.
...
... Incoming datagrams destined to groups to which the host does not
belong are discarded without generating any error report. On hosts ...
... host does not
belong are discarded without generating any error report. On hosts
attached to more than one network, if a datagram ...
... network interface, destined for a group to which the host belongs
only on a different interface, the datagram ...
... group.
The list of host group memberships is updated in response to
JoinHostGroup and LeaveHostGroup requests from upper-layer ...
... specified in Appendix I. IGMP is used to keep neighboring multicast
routers informed of the host group memberships present on a
particular local network ...
... network
interface at initialization time and must remain a member for as long
as the host is active.
...
...
(Datagrams addressed to the all-hosts group are recognized as a
special case by the multicast routers ...
... network, regardless of their time-to-live. Thus, the all-
hosts address may not be used as an internet-wide broadcast ...
... address.
For the purpose of IGMP, membership in the all-hosts group is really
necessary only while the host ...
... However, it is specified that 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 should be low
enough that overhead is negligible, and (3) the all-hosts address may
serve other routing ...
... group-address" is an IP host group address. The
JoinLocalGroup operation requests the local network ...
... and deliver up subsequently arriving packets destined to the given IP
host group address. The LeaveLocalGroup operation requests the local
network ...
... network module to stop delivering up packets destined to the given IP
host group address. The local network module is expected to map the
...
... multicast addresses that correspond to the host's IP host group
addresses. It is highly desirable to take advantage of any address
...
... hardware interface may have,
so that the host receives only those packets that are destined to it.
Unfortunately, many current Ethernet interfaces ...
... IP hosts to
report their host group memberships to any immediately-neighboring
multicast routers ...
... multicast routers. IGMP is an asymmetric protocol and is specified
here from the point of view of a host, rather than a multicast
router. (IGMP may also be used, symmetrically or asymmetrically,
...
... IGMP is a integral part of IP. It is required to be
implemented by all hosts conforming to level 2 of the IP multicasting
specification. IGMP messages ...
... an IP protocol number of 2. All IGMP messages of concern to hosts
have the following format:
...
... is zeroed when sent, ignored when received.
In a Host Membership Report message, the group address field
holds the IP ...
... Query messages (hereinafter
called Queries) to discover which host groups have members on their
attached local networks ...
... Hosts respond to a Query by generating Host Membership Reports
(hereinafter called Reports), reporting each host group ...
... Query by generating Host Membership Reports
(hereinafter called Reports), reporting each host group to which they
belong on the network interface ...
... the total number of Reports transmitted, two techniques are used:
1. When a host receives a Query, rather than sending Reports
immediately, it starts ...
... timer expires, a
Report is generated for the corresponding host group. Thus,
Reports are spread out over a D second interval instead of
...
... group on
the same network can overhear the Report. If a host hears a
Report for a group to which it belongs on that network ...
... group to which it belongs on that network, the
host stops its own timer for that group and does not generate
...
... group present on the
network, by the member host whose delay timer expires first.
Note that the multicast routers ...
... datagrams, and therefore need not be addressed explicitly.
Further note that the routers need not know which hosts
belong to a group, only that at least one host ...
... hosts
belong to a group, only that at least one host belongs to a
group on a particular network ...
... rather allowed to continue running with its current value. Second, a
report delay timer is never set for a host's membership in the all-
hosts group ...
... timer is never set for a host's membership in the all-
hosts group (224.0.0.1), and that membership is never reported.
...
... group (224.0.0.1), and that membership is never reported.
If a host uses a pseudo-random number generator to compute the
reporting delays, one of the host ...
... host uses a pseudo-random number generator to compute the
reporting delays, one of the host's own individual IP address should
be used as part of the seed for the generator, to reduce the chance
...
... IP address should
be used as part of the seed for the generator, to reduce the chance
of multiple hosts generating the same sequence of delays.
A host ...
... hosts generating the same sequence of delays.
A host should confirm that a received Report has the same IP host
...
... IGMP group address
field, to ensure that the host's own Report is not cancelled by an
erroneous received Report. A host should quietly discard any IGMP ...
... field, to ensure that the host's own Report is not cancelled by an
erroneous received Report. A host should quietly discard any IGMP
message of type other than Host ...
... host should quietly discard any IGMP
message of type other than Host Membership Query or Host Membership
...
... (no more than once a minute) so as to keep the IGMP overhead on hosts
and networks very low. However, when a multicast router ...
... up its knowledge of local memberships.
When a host joins a new group, it should immediately transmit a
Report for that group ...
... IGMP behavior is more formally specified by the state transition
diagram below. A host may be in one of three possible states, with
respect to any single IP host ...
... host may be in one of three possible states, with
respect to any single IP host group on any single network interface:
...
... memberships on all network interfaces; it requires no storage
in the host.
- Delaying Member state ...
... valid IGMP
Host Membership Report message. To be valid, the Report
message must be at least 8 octets ...
... group (address 224.0.0.1) is handled as a special case.
The host starts in Idle Member state for that group ...
... APPENDIX II. HOST GROUP ADDRESS ISSUES ...
... host group addresses to physical hosts may be
considered a generalization of the binding of IP unicast ...
... network
interface on a single IP network. An IP host group address is
dynamically bound to a set of local network interfaces ...
...
It is important to understand that an IP host group address is NOT
bound to a set of IP unicast ...
... addresses. The multicast routers do not
need to maintain a list of individual members of each host group.
For example, a multicast router ...
... associate only a single Ethernet multicast address with each host
group having local members, rather than a list of the members'
...
...
Host group addresses have been defined specifically for use in the
destination address field ...
... destination address field of datagrams. For example, a mobile
IP host might have a host group address as its only identity ...
... datagrams. For example, a mobile
IP host might have a host group address as its only identity, used as
...
... identity, used as
the source of datagrams it sends. Whenever the mobile host moved
from one network to another, it would join ...
... network and depart from the group on the old network. Other hosts
communicating with the mobile one would deal only with the group
address and would be unaware of, and unaffected by, the changing
...
... group
address and would be unaware of, and unaffected by, the changing
network location of the mobile host.
Host ...
... host.
Host group addresses cannot, however, be used to solve all problems
of internetwork logical addressing ...
... delivery to the "nearest"
or the "least loaded" network interface of a multi-homed host.
Furthermore, there are hazards in using group addresses in the source
address ...
... datagrams when the group actually contains more than
one host. For instance, the IP datagram reassembly algorithm relies
...
... IP datagram reassembly algorithm relies
on every host using a different source address. Also, errors in a
datagram ...
... sender. In
view of these hazards, this memo specifies the use of host group
addresses only in the IP destination address field ...
... complaint, thereby allowing other implementations to experiment with
logical addressing applications of host group addresses.
...
... Allocation of Transient Host Group Addresses ...
... group address are allocated.
It is anticipated that different portions of the IP transient host
group address space will be allocated using different techniques.
...
... addresses which are
then algorithmically mapped to a subset of the IP transient host
group addresses, similarly to the way that IP ...
... In general, a host cannot assume that datagrams sent to any host
group address will reach only the intended hosts ...
... host
group address will reach only the intended hosts, or that datagrams
received as a member of a transient host ...
... hosts, or that datagrams
received as a member of a transient host group are intended for the
recipient. Misdelivery must be detected at a level above IP ...
... authentication tokens. Information
transmitted to a host group address should be encrypted or governed
...
... multicast agents" in RFC-988(-> 1112std5 | 1054(-> 1112std5))) and onto the hosts. This new
distribution of responsibility is consistent with the lightweight,
soft-state ...
... IP unicast services) to be
used among hosts on a single network when no router is up or present
...
... consequence of this shift of responsibility:
- Private hosts groups and access keys have been eliminated.
The multicast routers ...
... multicast routers are no longer considered trustworthy
controllers of group membership; it is up to hosts and their
administrators to provide their own mechanisms to prevent
...
...
- The CreateHostGroup operation has been eliminated. The
responsibility for allocating transient host groups has been
moved from multicast routers ...
... groups has been
moved from multicast routers to the hosts. See Appendix II
for a brief discussion of some ways in which hosts ...
... hosts. See Appendix II
for a brief discussion of some ways in which hosts might do
their own transient group allocation.
...
... approval from a multicast router when changing membership. It
is also no longer possible for a host to have its membership
revoked by a multicast router.
...
... RFC-988(-> 1112std5 | 1054(-> 1112std5)), reflecting the changed roles of hosts and multicast
routers.
...
...
- The new IGMP requires that there be an "all-hosts" group.
There is no longer a need for an "all-multicast ...
... interfaces, has been more precisely specified.
- Hosts are no longer allowed to place an IP host group address ...