CS 457, Fall 2004

Computer Networks

Homework 7 (Due Tuesday, Nov 30th, in class)

Please work on this part independently. Homework is subject to honor code. Please mark the single best answer for each of the following multiple choice questions:

1. TCP goes to slow start
(a) when the connection is first started
(b) upon timeouts
(c) in both (a) and (b)
2. The difference between fast retransmit and fast recovery is that
(a) the former is a way to avoid slow start whereas the latter is a way to reduce timeouts
(b) the former is a way to avoid performance loss due to timeouts whereas the latter is a way to avoid perfomance loss due to slow start
(c) the former algorithm runs on the source whereas the latter runs on the destination
3. The amount of traffic that a TCP client can have in transit (in the network) at the same time
(a) is no larger that the TCP congestion window size
(b) is exactly equal to the size of the TCP congestion window
(c) is at least the size of the TCP congestion window
4. A disadvantage of starting a new connection with a large initial window size is that
(a) a large burst of packets will be sent into the network
(b) the receiver will be unable to acknowledge all the packets
(c) the connection will time out
5. A router that experiences bursty input traffic
(a) will have a higher average queue size and a higher percentage of packet drops compared to a router that experiences the same amount of smooth input traffic
(b) will have a lower average queue size but a higher percentage of packet drops compared to a router that experiences the same amount of smooth input traffic
(c) will have a higher average queue size and a lower percentage of packet drops compared to a router that experiences the same amount of smooth input traffic
6. Linear increase, multiplicative decrease refers to
(a) using slow start at the beginning of a new connection then increasing the window linearly when the congestion threshold is exceeded
(b) increasing the congestion window linearly when there are no losses and dropping the window in half when a packet is presumed lost
(c) an algorithm for flow control to avoid receiver overflow
7. Karn's algorithm
(a) is a way to compute the flow control window of very slow senders
(b) is a way to combat the silly window syndrome
(c) is a way to avoid ambiguity in RTT measurements when packets are retransmitted
8. The retransmission timeout in TCP depends on
(a) the mean roun-trip time of the connection
(b) a measure of variability in connection round-trip times
(c) both (a) and (b)
9. You are opening a TCP session to send a file out to a friend. The file is 7 packets long. Assuming the congestion threshold is 10 packets, at least how many round trips do you expect your connection to last from the time the first SYN packet is sent to the time the last packet is acknowledged?
Answer: 4
10. In question 9, assume the application sends another file on the same TCP connection, right after the first file. What do expect will happen to the congestion window?
(a) It will be reset to 1 in between the two files
(b) TCP is oblivious to file boundaries. The window will continue to grow as the second file is sent
(c) TCP will divide the congestion window in half after the first file is sent, and then start sending the second file.