ECE/CS 438  Communication Networks
Fall 2008

Homework 5
Assigned on October 17, 2008
Due by class time on October 24, 2008

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(1) Consider the stop-and-wait protocol on a wired link between nodes
	A and B. Suppose that the propagation delay in each direction
	is 10 ms. The transmission rate is 10 Mbps.
	Assume that date packet size is 1000 bytes, and Ack is 40 bytes.

	(i) What is the maximum data throughput (in packets/second) achievable
		using the stop-and-wait protocol, if none of the packets
		are lost ?

	(ii) What is the maximum data throughput (in packets/second) achievable
		using the stop-and-wait protocol, if data packets are lost
		with probability 0.1, but Ack packets are never lost ?

(2) Suggest a transport protocol that delivers packets in order, but not necessarily
	reliably. That is, the application at the receiver should not receive
	packet i before packet j if i>j, but it is acceptable if the
	application does not receive all the packets. A trivial protocol
	that achieves this goal is one that discards all packets (thus not
	delivering any packets to the receiver!). Your protocol should attempt
	to achieve better throughput than this trivial protocol.

	You may present your solution using state diagrams.


(3) Note that there is an error in a transition out of the state
	ESTABLISHED in the slides for TCP. Please see the textbook for
	the correct transitions.

    (i) Why is the TIME_WAIT state included in the TCP state-transition diagram?

    (ii) Is it possible for both the endpoints of a TCP connection to enter
	 the TIME_WAIT state? Explain briefly. 

(4) A TCP sender performs a retransmission ("fast retransmit") on receipt
	of three duplicate acknowledgements (dupacks). Does a receipt of three
	dupacks always imply a packet loss? Explain briefly.


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Suggested exercises

You do not need to turn in solutions for the questions below. The solutions
to these suggested exercises are provided in the textbook.

* Chapter 2, Problem 25  -- note that SWS denotes Send Window Size
	and RWS denotes Receive Window Size.

* Chapter 2, Problem 32 

* Chapter 4, Problem 46

* Chapter 5, Problem 10

* Chapter 5, Problem 13

* Chapter 5, Problem 27