Multicast over IP

In most situations Internet applications utilise unicast packets for transporting information directly from one point to point. Unicast is used for browsing the web with HTTP, fetching files with FTP or email delivery SMTP.

The internet protocol stack provides another method of distributing data to multiple destinations, multicast.

There are situations where the transmission of information by multicast is more appropriate.

A data packet may be sent to a multicast group address that the same packet will be delivered to multiple destination.

Multicast, conserves the bandwidth of a network as the transmission of a packet is needed, rather than sending packets individually addressed each node.

When a large number of clients wish to receive the same information, for example, a broadcast video stream, then multicast can be carried efficiently over the networks backbone.

The use of unicast stream to each recipient would be as many separate video streams through the backbone as clients.

Multicast only requires a single stream, this assumes that each customer must receive the same video simultaneously or within the a time frame less than their local caching capability.


  • Bandwidth conservation
  • * Broadcast data streams
  • * Constantly updated data streams
  • Heartbeat
  • * Logging information

Multicast Groups Addresses

IP multicast group has a Class D Address group address. IP multicast provides only open groups. it is not necessary to be a member of a group in order to send datagrams to the group.

Multicast address are IP addresses but instead of being for single hosts, and is written in the same way: A.B.C.D. Multicast addresses do not clash with host addresses because a portion of the IP address space is specifically reserved for multicast. The reserved range consists of addresses from to However multicast addresses from to are reserved for multicast routing information Application programs should use multicast addresses outside this range.

TTL - Time To Live

The IP multicast routing protocol uses the Time To Live (TTL) field of IP datagrams to decide how “far” from a sending host a given multicast packet should be forwarded. The default TTL for multicast datagrams is 1, which will result in multicast packets going only to other hosts on the local network.

As the values of the TTL field increase, routers will expand the number of hops they will forward a multicast packet. To provide meaningful scope control, multicast routers enforce the following “thresholds” on forwarding based on the TTL field:

0 restricted to the same host 1 restricted to the same subnet 32 restricted to the same site 64 restricted to the same region 128 restricted to the same continent 255 unrestricted

ip_multicast.txt · Last modified: 2009/09/24 13:55 by Martin Spamer