CS3201 Computer Networks
Tutorial 5
1. Suppose that the selective repeat protocol is used, and the sequence number space is
{0,1,2,3}. Suppose that the sender window size is 2. The sender sends packets with sequence
number {0, 1} at the beginning. The receiver receives these packets (with sequence number
{0, 1}) and therefore responds by sending ack 0 and ack 1, respectively. However, both acks
are lost.
a) After the receiver sends ack 0 and ack 1, what will be the receiver-base? What
packets are expected/acceptable at the receiver side?
b) After the timeout interval for packet 0, the sender retransmits packet 0. What will the
receiver do when it receives that packet?
The following two questions are designed to help you understand that, in Go-back-N, the sender
window size must be less than the size of the sequence number space:
2. Suppose that the Go-back-N protocol is used. The sequence number space is {0, 1, 2, 3}.
Suppose the sender window size is 4. The sender sends packets {0, 1, 2, 3}. The receivers
receive all these 4 packets in order and generate acks. Suppose that all acks are lost.
a) After the receiver sends out all acks, what will be the expected sequence number at
the receiver side?
b) After the timeout interval for packet 0, the sender retransmits packet 0. Suppose that
the receiver receives the packet. What will the receiver do?
3. Suppose that Go-back-N protocol is used. The sequence number space is {0, 1, 2, 3}.
Suppose the sender window size is 3. The sender sends packets {0, 1, 2,}. The receiver
receives all these 3 packets in order and generates acks. Suppose that all acks are lost.
a) After the receiver sends out all acks, what will be the expected sequence number at
the receiver side?
b) After the timeout interval for packet 0, the sender retransmits packet 0. Suppose that
the receiver receives the packet. What will the receiver do?
4. Suppose that Host A and Host B use a GBN protocol with window size N=3 and a longenough
range of sequence numbers. Assume Host A sends six application messages to Host
B and that all messages are correctly received except for the first acknowledgement and the
fifth data segment. Draw a timing diagram, showing data segments and the
acknowledgements sent along with the corresponding sequence and acknowledge numbers,
respectively. Assume that the first packet’s sequence number starts with 0.
5. Draw a timing diagram for problem 4 if the Selective-Repeat protocol is used.