Suppose we have two network entities, A and B$.$ B has a supply of data mes

sages that will be sent to A according to the following conventions. When A

gets a request from the layer above to get the next data $($D$)$ message from B$,$

A must send a request $($R$)$ message to B on the A$-$to$-$B channel. Only when B

receives an R message can it send a data $($D$)$ message back to A on the B$-$to

A channel. A should deliver exactly one copy of each D message to the layer

above. R messages can be lost $($but not corrupted$)$ in the A$-$to$-$B channel; D

messages, once sent, are always delivered correctly. The delay along both

channels is unknown and variable.

Design $($give an FSM description of$)$ a protocol that incorporates the appro

priate mechanisms to compensate for the loss$-$prone A$-$to$-$B channel and

implements message passing to the layer above at entity A$,$ as discussed

above. Use only those mechanisms that are absolutely necessary.

to Hosts B and C$.$ A is connected to B and C via a broadcast channel$$a

packet sent by A is carried by the channel to both B and C$.$ Suppose that

the broadcast channel connecting A$,$ B$,$ and C can independently lose and

corrupt packets $($and so$,$ for example, a packet sent from A might be cor

rectly received by B$,$ but not by C$).$ Design a stop$-$and$-$wait$-$like error$-$control

protocol for reliably transferring packets from A to B and C$,$ such that A will

not get new data from the upper layer until it knows that both B and C have

correctly received the current packet. Give FSM descriptions of A and C$.$

$($Hint: The FSM for B should be essentially the same as for C$.)$ Also, give a

description of the packet format$($s$)$ used.