5. Broadcast Methods

A host's IP receiving layer must be modified to support broadcasting. In the absence of broadcasting, a host determines if it is the recipient of a datagram by matching the destination address against all of its IP addresses. With broadcasting, a host must compare the destination address not only against the host's addresses, but also against the possible broadcast addresses for that host.

The problem of how best to send a broadcast has been extensively discussed [1, 3, 4, 13, 14]. Since we assume that the problem has already been solved at the data link layer, an IP host wishing to send either a local broadcast or a directed broadcast need only specify the appropriate destination address and send the datagram as usual. Any sophisticated algorithms need only reside in gateways.

The problem of broadcasting to all hosts on a subnetted IP network is apparently somewhat harder. However, even in this case it turns out that the best known algorithms require no additional complexity in non-gateway hosts. A good broadcast method will meet these additional criteria:

• No modification of the IP datagram format.

• Reasonable efficiency in terms of the number of excess copies generated and the cost of paths chosen.

• Minimization of gateway modification, in both code and data space.

• High likelihood of delivery.

The algorithm that appears best is the Reverse Path Forwarding (RPF) method [4]. While RPF is suboptimal in cost and reliability, it is quite good, and is extremely simple to implement, requiring no additional data space in a gateway.