Browse Theses

Broadcasting should be a standard addressing mode of all packet-switched computer networks. Further, when networks are interconnected to form an internet, a broadcast mechanism is also required.

Broadcasting is the delivery of a packet to all hosts in a network; unicasting is the delivery of a packet to one specific host. They are distinct forms of interprocess communication; functions that are simple to do with one are difficult to do using only the other.

A broadcast is used when you don't know whom specifically to address. There are two situations where this occurs: (1) when you are searching for some information but you don't know who to ask (for example, standing up in a theatre and saying "Is there a doctor in the house__ __"), and (2) when you possess some information of use to others but you don't know specifically who (for example, standing up in a theatre and yelling "Fire!").

A network should give its best efforts to deliver a copy of a broadcast packet to each host, but perfectly reliable delivery is not required. It is sufficient that most hosts receive a broadcast and that the same hosts not miss retransmissions. Just as with unicasting, higher-level protocols can be used to improve the reliability of the basic broadcast delivery mechanism, if required. Such an "unreliable" broadcast mechanism is straightforward to implement in all types of packet-switched networks.

In an internet composed of possibly thousands of networks and millions of hosts, a full internet-wide broadcast, the obvious internet analog to broadcasting in a single network, is seldom the right choice. A directed broadcast, delivery of a packet to all hosts on any single network in an internet, is simpler to implement, closer to what most users need, and sufficient to construct many forms of broadcast-based interprocess communication, including an internet-wide broadcast.