This topic describes IP multicasting and how an application can
exploit multicasting using the TCP/IP socket APIs. IP multicasting
concepts in IPv4 and IPv6 protocols are very similar; however there
are some differences, such as the IP addresses used for multicasting
with each protocol. The topic that follows introduces the basic concepts
for IP multicasting with an emphasis on IPv4. However, most of the
concepts described here apply to IPv6 multicast applications as well.
A more detailed description of IPv6 multicast options follows in
the next topic.
IPv4 has three types of IP addresses: unicast, broadcast, and multicast.
When an IP datagram is sent to an individual IP address, it is called
a unicast IP datagram. The process of sending the datagram is called
unicasting. Unicasting is used when two IP nodes are communicating
with each other.
When an IP datagram is sent to all nodes on a specific network,
it is called broadcasting. Broadcasting support can be both limited
and directed.
Multicasting is used to send an IP datagram to a group of systems
identified by a class D address. The class D address is used as the
destination address. When an application program requests that it
receive datagrams with a particular class D destination IP address,
it is said to have joined a multicast group. Multicast datagrams (datagrams
with a class D destination address) are discarded by a host system
unless an application on that host has joined the matching multicast
group. The UDP application must bind in order to receive multicast
datagrams, after which the application can then receive an IP datagram.
The application can receive an IP datagram in two ways:
- The application must bind to the same port that is being used
by the sender of the multicast datagram.
- The application can bind to a unicast address, inaddr_any, or
to a class D address. However, if multiple applications need to receive
datagrams for the same multicast group, they should bind to the class
D address and set the SO_REUSEADDR socket option.
When a host is added to a group that group is referred to as a host group. A host group may span multiple networks.
Hosts may join and leave a host group as necessary and there is no
restriction to the number of hosts involved in a group. A host does
not have to belong to a group to send a message to that group. Any
hosts on an IP Internet can join a multicast group. The hosts need
not be on a single LAN and may be separated by routers. When an application
joins a group, it joins the multicast group on a specific interface.
Routers use this information to determine if multicast datagrams should
be forwarded from one interface to another.
Routers and hosts use a multicast routing protocol called Internet
Group Management Protocol (IGMP) to share information about multicast
groups. Through this protocol, hosts inform routers when they join
or leave a multicast group. Routers can query hosts about groups they
have joined and use this information in determining whether to forward
multicast datagrams. Some multicast group addresses are referred to
as permanent host groups. These addresses are assigned by the Internet
Assigned Numbers Authority group as well-known addresses similar to
the well-known TCP and UDP port numbers. For example, 224.0.0.1 means
all systems on this subnet, and 224.0.0.2 means all routers on this
subnet. For a review of The Internet Assigned Number RFC to familiarize
yourself with more of the well-known standard multicast address see
http://www.iana.org/assignments/multicast-addresses for IPv4 multicast address assignments and RFC 2375 for IPv6 multicast
address assignments.
Note: z/OS® (OMPROUTE) does not support a multicast routing protocol.