Question

Consider the SR protocol.

a. Draw the FSM diagram for the SR protocol assuming infinitely growing sequence numbers. Pay special attention to border cases (packet corruption, ACK loss, etc.)

b. The alternating bit protocol (“RDT 3.0”) may not operate correctly when the channel can reorder packets. Does the SR protocol with infinite sequence numbers work correctly in the presence of packet reordering? Justify your answer.

c. While in the GBN protocol the sender starts with packet sequence 1, the SR sender starts with packet sequence 0. Why?

Answer 1

a) FSM diagram for SR protocolFSM diagram for sender

FSM diagram for receiver

Description for sender FSM

FSM diagram for the sender depicts that the sender must wait to check that next sequence number is the packet to be sent before sending the packet

Sender wait when next sequence number is 1 to send it. Waits for retransmission until the time slot is over.

Wait till acknowledgement is received to determine what to send in next slot.

Description for receiver FSM.

It recieves from sender when rcvbase =1.

Recieves the packet n while it belongs to N, n€N.and send the acknowledgement of received packet.

b) Yes SR protocol will work correctly with infinite sequence number with packet reordering while ADP cannot.

ABP uses window of 1bit and uses FIFO(First In First Out) manner to retransmits lost or corrrupted messages. Since here no numberig is used like sequence numbers in SR protocol the reordering of packet is difficult and can't be done.

In SRP the packets that are unacknowledged are only resent no need to send the entire window. And it has a bufffer on receiver side.

The bufffer has the information about out of order packets and deliver them in order once the missed packets are received.

Unlike ABP these bufffer in receiver side helps in packet reordering in SRP protocol.

c) Sequence number for SRP starts with 0 and GBN with 1.

In the GBN (Go Back N) protocol reciever window size is 1 so GBN starts with sequence number 1. And in SRP protocol the sender and receiver window size is N.

In SRP only the lost or damaged packets are retransmitted and numbers as 0 to N-1 to acknowledge the packets with window size N. Acknowledgement type is individual.

In GBN all the frames are retransmitted till the last packet and reciever window size is 1. And acknowledgement type is cumulative.It starts with the sequence 1 to N.

GBN is easy to implement and SRP requires reciever window to sort the frames.