IGMP is a session-layer protocol used to establish membership in a
multicast group. This datagram uses the IP protocol with a class D address,
i.e., those with "1110" as their high-order four bits (discussed
later in this section). The screen capture below shows a class D address
used in an IGMP group.
At the data link layer, the frame goes out as a multicast with the least
significant bit of the most significant byte set to a one
(01-00-5E-XX-XX-XX; further discussion is given later in this section). This
protocolÕs intention is not to carry data, but to establish membership. The
method of transport for data will traditionally be something like RTP or
RTSP (explained in another section).
Membership into a multicast group is dynamic, and members can enter and
leave the group at any time. A user (host) can be a member of one or more
groups simultaneously, and does not need to be a member of a group in order
to send datagrams to it. A "host" can be a user or a multicast
router.
There are different levels at which a host may support IGMP:
Level 0: No support for IP multicasting
At this level the host will not support IP multicasting. The data link
hardware in these hosts should discard these frames without using any CPU
cycles. Therefore, Level 0 hosts should be virtually unaffected by IP
multicast datagrams. This of course assumes that the network interface card
has this type of filtering capability. Otherwise, the CPU is called and
cycles are used. Many of todayÕs Pentium computers can handle thousands of
broadcasts per second with no visible drop in application performance.
Level 1: Support for sending, but not receiving, multicast IP
datagrams
At this level a host can take part in some of the multicast-based services,
such as resource location or status reporting. A host does not, however,
join any groups.
Level 2: Full support for IP multicasting
At this level a host can join and leave host groups, as well as send IP
multicast datagrams. IGMP is fully implemented here.
Packet formats are described using 32 bits per row in the following
format:
For a Level 2 host to join a multicast group, an IGMP datagram is
multicasted out onto the data link topology. This is done by sending a
packet to the "all-hosts" group address of 224.0.0.1, on every
interface where the host wishes to participate in IP multicast. The TTL of
the IP datagram is set to 1; this prevents the router from forwarding the
datagram. If the host does not receive a response, a second datagram may be
sent out with the TTL set to 2. The TTL could continue to increment the TTL
for each time out. This incrementing of the TTL will depend on the
implementation within the application. Default should always be a TTL of 1.
Version: This document discusses version 1.
Type: There are two types of IGMP messages of concern to hosts:
1 = Host Membership Query
2 = Host Membership Report
Unused: Unused field, zeroed when sent, ignored when received.
Checksum: The checksum is the 16-bit ones complement of the ones
complement sum of the eight-octet IGMP message. For computing the checksum,
the checksum field is zeroed.
Group Address:
In a Host Membership Query message, the group address field is zeroed when
sent, ignored when received. In a Host Membership Report message, the group
address field holds the IP host group address of the group being reported.
Queries:
Multicast routers (switches) send Host Membership Query messages, or
"Queries," to discover which hosts have members on their attached
local networks (described below). These queries are addressed to the
all-hosts group (address 224.0.0.1), and carry an IP time-to-live of 1.
Reports:
Hosts respond to a query by generating Host Membership Reports, which report
each host group to which they belong on the network interface where the
query was received. To avoid a storm of reports a technique described below
is used.
In the above screen capture, the IGMP report represents the multicast
group 233.211.183.14.
Once the multicast group is joined, the host will receive IGMP queries
from the router or switch (depending on implementation) about every 60
seconds. These IGMP queries are heard by all devices on the data link layer
broadcast domain. Upon receiving the IGMP query each level 2 host will start
a time called "D" second interval. This "D" time is
randomly generated (in part based on the hostÕs IP address) each time the
IGMP query is heard. The purpose of this is for one host to respond to the
query. Once a host hears that another host responded to the query, it will
stop the timer and not respond. This mechanism prevents a flood of IGMP
responses. If the router or switch does not hear a IGMP query from the data
link within the time-out period, the router or switch will assume that no
hosts want to receive the multicast and will stop forwarding the IP
multicast datagrams. The router does not need to know which hosts belong to
a group, only that a host exists which wants to participate in the multicast
group.
The class D IP address used in IGMP can be in the range 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 is used by IP multicast computers as shown below with the
illustrated command within WindowsNT 4.0.
The address 224.0.0.1 is used for query messages and is assigned to the
permanent group of all IP hosts (including gateways). All hosts must join
the 224.0.0.1 group in order to participate in IGMP.
A host group may be permanent or transient. A permanent group has a
well-known, administratively assigned IP address. It is this address, and
not the membership, that becomes permanent. Permanent groups could be
instituted for applications such as a radio stations being multicast over
the network. The address 224.0.0.1 is also an example of a permanent group.
Those IP addresses not permanent can be given out to transient groups for
dynamic creation. A transient group only exists so long as it has members.
The destination class D IP address will end up as a remote destination in
the local or remote look up within the IP stack. Therefore gateway
configuration is required. Ideally, if the end station is configured with a
natural mask and with no default gateway as in many proxy-arp routed
environments, the default gateway on the end stations should be made to be
themselves.
| PC IP Address: |
172.17.8.23 |
| Class B Natural Mask: |
255.255.0.0 |
| Default Gateway: |
172.17.8.23 |
Most IP stacks support this configuration, however there are exceptions.
As long as proxy-ARP is running on the router (and in most companies it is)
this configuration will work fine.
At the data link layer the frames hit the wire as a multicast, as
specified earlier, in the format 01-00-5E-XX-XX-XX. The trailing three
octets (XX-XX-XX) actually become the last three octets of the four-octet
class D IP address displayed in hexadecimal. For example: The first figure
below demonstrates the math to convert to a multicast address. The second
figure below shows an actual trace where this address was used in a video
conference.
In order for a host to participate in a multicast it must also be able to
perform two other operations:
JoinHostGroup (group address, interface)
LeaveHostGroup (group address, interface)
A JoinHostGroup datagram is sent by a host who wants to participate in
the multicast (class D address) identified in the frame. The LeaveHostGroup
is used to remove a host from a multicast group.
The following diagrams illustrate a multicast session:
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