December 1999
NI222 : NETWORKS AND THE INTERNET

QUESTION 3

Total Marks: 15 Marks

• Question 1
• Question 2
• Question 4
• Question 5
• Cover Page

SUGGESTED SOLUTIONS
Solutions and allocated marks are indicated in green.
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Explain briefly the role of the Transport layer in the ARPANET (Internet)four-layer architecture.

The purpose of the transport layer is to bridge the gap between applications and the network (1).The important task performed by this layer is establishing,managing and terminating TCP connection between two hosts (1).

What are the limitations of the TCP Protocol?

TCP data and control messages have the same message header structure which can lead to large overheads (1).The TCP checksum mechanism is very weak (1).It s hard to obtain the Quality of Service over TCP connection (1).

Discuss the cost –performance trade-offs between the upward and downward multiplexing at the transport layer.

Upward Multiplexing :Reduce network costs by having several transport layer connections share the same network layer connection (1).Limited bandwidth,hence transmission is not efficient (1).
Downward Multiplexing :High bandwidth as the transport layer connection can be spread over several network layer connections (1).But not cost effective,as more network connections are used (1).

Deadlock and Livelock are the problems encountered in setting up a TCP-style connection.For each,identify a protocol in which you might come across such problem and explain how with a suitable diagram.

Deadlock:Two-way Handshake protocol (1).Caused due to the reason that underlying network can lose or duplicate messages (1). Livelock:Naive Three-way Handshake protocol (1).Occurs when both hosts A and B send CR messages at about the same time and enters WACK state (1)
1 mark each for suitable diagram,up to max 2marks