Question

Consider a scenario where Host A and Host B want to send messages to Host C. Hosts A and C are connected by a channel that can lose and corrupt (but not reorder) messages. Hosts B and C are connected by another channel (independent of the channel connecting A and C) with the same properties. The transport layer at Host C should alternately deliver M (M>1) consecutive messages received from A to its application layer and N (N>1) consecutive messages received from B to its application layer, i.e., M messages from A, N messages from B, M messages from A, N messages from B, and so on.

Design a NAK-free stop-and-wait protocol to achieve reliable data transfer from A and B to C, by giving FSM descriptions of the protocols at A and C. (The FSM for B is essentially the same as that for A except the number of messages, and hence you do not need to provide B’s FSM.) Notes: (1) If you use variables and procedure calls other than those defined in Chapter 3.4 of the textbook, clearly describe their functions. (2) You can use the following mechanisms: checksum, sequence numbers (you need a larger sequence number range than just 0 and 1), ACKs (but not NAKs), timer and sender retransmission. (3) Recall we do not consider buffering of received packets in a stop-and-wait RDT protocol.

Answer 1

Stop-and-wait like error-control protocol:

1. While transmitting a message through the channel,a message can be lost in Stop-and-wait like error-control protocol.
2. It is required to use the sequence numbers of the packets, to detect and re-transmit the lost packets.
3. In this protocol, it is also possible that the sender sends the lost messages repeatedly that are received by the receiver.

Flow of the Sender FSM:

The data in the sender FSM (Finite state machine) changes the states as follows:

1. Initialize the session and the sequence number should be started with 0.

2. Build the packet with the sequence number and data .

3. Start the timer and broadcast the packet with hosts in the channel.

4. Wait for the acknowledgement.

5. If acknowledgement is not received within sepecified time, then broadcast the packet to all the hosts in the channel.

Flow of the Receivers FSM:

The acknowledgement in the receiver FSM (Finite state machine) changes the states as follows:

1. Initialize the session and the acknowledgement number should be started with 0.
2. Wait for the uncorrupted data packet to be recieved from all hosts with sequence number same as the acknowledgement number.
3. If a packet’s sequence number received does not match with the acknowledgement number, then the acknowledgement is lost and the packet should be made with acknowledgement number 1.
4. If packet’s sequence number received matches with the acknowledgement number, then send the acknowledgement number to the sender.
5. Extract and process the data packet.