DATA COMMUNICATION

1. short answer

a. How many bytes are there in the UDP packet header? How many in the TCP header?

b. Give two reasons you might prefer to implement an application using UDP, rather than TCP. Give two reasons you might prefer to implement an application using TCP.

c. Is the following statement true or false? With UDP Sockets, a server can easily determine the IP address of the client, from the data returned via a socket read. Answer the same question (true or false) for TCP sockets. Briefly explain your answers.

d. Briefly describe TCP's slowstart algorithm. What causes the TCP slowstart algorithm to end, and TCP congestion avoidance to begin?

e. Consider a TCP connection between hosts A and B. Suppose that the TCP segments from A to B have source port number x and destination port number y. What are the source and destination port numbers for the segments traveling from B to A?

f. It is said that a TCP connection "probes" the network path it uses for available bandwidth. What is meant by that? g. Explain the process of a three way handshake in TCP that initiates communications on the Internet.

h. The UDP server described in class needed only one socket, whereas the TCP server needed two sockets. Why? If the TCP server were to support N simultaneous connections, each from a different client host, how many sockets would the TCP server need? Express your answer as an equation.

i. How does the congestion window (cwnd) increase on the receipt of an ACK during slow-start? During congestion avoidance?

j. What controls when the transition between slow-start and congestion avoidance occurs?

k. In the basic TCP Tahoe, what happens to cwnd and the slow-start threshold when loss is detected?

2. Consider a 150 Mb/s link that is 800 km long, with a queue large enough to hold 5,000 packets. Assume

that packets arrive at the queue with an average rate of 40,000 packets per second and that the average packet length is 3,000 bits.

a. Approximately, what is the propagation delay for the link (be sure to include the units in your answer)?

b. What is the transmission time for an average length packet?

c. What is the traffic intensity?

d. What is the average number of packets in the queue?

e. What is the average number in the queue, if the average arrival rate is 80,000 packets per

second?

3. A user in Norfolk, connected to the internet via a 5 Mb/s connection, retrieves a 50 KB (B=bytes) web

page from a web server in California, where the page references 4 images of 300 KB each. Assume that the one way propagation delay is 20 ms.

a. Approximately how long does it take for the page (including images) to appear on the users screen, assuming persistent HTTP?

b. How long would it take using non-persistent HTTP (assume a single connection) ?