3Com S7902E, Configuration Manual

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1-7
2) Host registration: Receiver hosts are allowed to join and leave multicast groups dynamically. This
mechanism is the basis for group membership management.
3) Multicast routing: A multicast distribution tree (namely a forwarding path tree for multicast data on
the network) is constructed for delivering multicast data from a multicast source to receivers.
4) Multicast applications: A software system that supports multicast applications, such as video
conferencing, must be installed on multicast sources and receiver hosts, and the TCP/IP stack
must support reception and transmission of multicast data.
Multicast Addresses
To allow communication between multicast sources and multicast group members, network-layer
multicast addresses, namely, multicast IP addresses must be provided. In addition, a technique must be
available to map multicast IP addresses to link-layer multicast MAC addresses.
IP multicast addresses
1) IPv4 multicast addresses
Internet Assigned Numbers Authority (IANA) assigned the Class D address space (224.0.0.0 to
239.255.255.255) for IPv4 multicast. The specific address blocks and usages are shown in Table 1-2 .
Table 1-2 Class D IP address blocks and description
Address block Description
224.0.0.0 to 224.0.0.255
Reserved permanent group addresses. The IP address
224.0.0.0 is reserved, and other IP addresses can be used
by routing protocols and for topology searching, protocol
maintenance, and so on. Common permanent group
addresses are listed in
Table 1-3 . A packet destined for an
address in this block will not be forwarded beyond the local
subnet regardless of the Time to Live (TTL) value in the IP
header.
224.0.1.0 to 238.255.255.255
Globally scoped group addresses. This block includes two
types of designated group addresses:
z
232.0.0.0/8: SSM group addresses, and
z
233.0.0.0/8: Glop group addresses.
239.0.0.0 to 239.255.255.255
Administratively scoped multicast addresses. These
addresses are considered to be locally rather than globally
unique, and can be reused in domains administered by
different organizations without causing conflicts. For details,
refer to RFC 2365.
z
The membership of a group is dynamic. Hosts can join or leave multicast groups at any time.
z
“Glop” is a mechanism for assigning multicast addresses between different autonomous systems
(ASs). By filling an AS number into the middle two bytes of 233.0.0.0, you get 255 multicast
addresses for that AS. For more information, refer to RFC 2770.