Question

TCP at Packet Level

1. In a TCP packet level data transfer, sender and receiver both have a window that can hold 4 packets. Sender has 5 packets to send numbered 0-4. ACK 2 is lost and PKT 3 is lost too. (Remember in TCP the ACK number is one greater than the number of the last packet received in order and that ACKs are cumulative) There are no other errors or losses in either direction. Draw a timeline diagram that shows how the sender and receiver manage to recover from the losses using retransmission wherever necessary and successfully transfer the 5 packets. Show the contents of the 2 buffers at relevant instants. Assume timeouts are 2RTT and packet transmission time is 0.1RTT. On the timeline diagram mark the time in terms of RTT each time a packet is transmitted/retransmitted by the sender and each time a cumulative ACK is issued / reissued by the receiver. How many RTTs are needed to complete the entire transfer? (6)

2. Redo the time calculations in part (a) if the bit rate of transmission is halved but at the same time, timeouts are reduced to 1.5RTT.

Answer 1

In the problem, it is given that the sender and receiver side having the size of window equals 4, which means it is using selective repeat sliding window protocol for the transmission of the packets.

The problem is solved, consider the image below for the same.

When sender sends packet0, packet1, packet2, and packet3 to the receiver, it gives back the ack1 (expecting next packet to be the packet1), ack2 which is lost, ack3.

The sender didn't receive the ack2 before the timeout (2RTT), it will re transmit the packet2, packet3 and packet4 next time. And in response, receiver sends the ack4.