A Simple Protocol for Error Recovery

A couple of changes were made to make this new protocol work. (In the figure, the features added to make the protocol work are noted in bold letters.) First, this protocol requires that the sender of the packet number the packets. That way, when the receiver (Fred) notices the error, he can send a message back to the sender (Wilma) asking her to send the specific problem packet againin this case, packet 2. Wilma, of course, replies to Fred's request by resending packet 2.

Error recovery refers to the general process of not only noticing when errors occur in the transmission of a packet, but also requesting for retransmission of the packet until it is received successfully. By using packets instead of sending all the bits in the file at once, Fred and Wilma could recover by resending just part of the file.