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概述
1. Consider the following plot of TCP window size as a function of time.
Assuming TCP Reno is the protocol experiencing the behaviour shown above, answer the following questions. In all cases, you should provide a short discussion justifying your answer.
a) Identify the intervals of time when TCP slow start is operating.
b) Identify the intervals of time when TCP congestion avoidance is operating.
c) After the 16th transmission round, is segment loss detected by a triple duplicate ACK or by a timeout?
d) After the 22nd transmission round, is segment loss detected by a triple duplicate ACK or by a timeout?
e) What is the initial value of threshold at the first transmission round?
f) What is the value of of Threshold at the 18th transmission round?
g) What is the value of of Threshold at the 24th transmission round?
h) During what transmission round is the 70th segment sent?
i) Assuming a packet loss is detected after the 26th round by receipt of a triple duplicate ACK, what will be the values of the congestion window size and of Threshold?
a. TCP slow start is operating in the intervals (1,6) and (23,26).
b. TCP congestion avoidance is operating in the intervals (6,16) and (17,22).
c. After the 16th transmission round, packet loss is recognized by a triple duplicate ACK. Because CongWin will set to 1 after timeout event.
d. After the 22nd transmission round, segment loss is detected due to timeout. Because CongWin will set to 1/2 of its value after triple duplicate ACK.
e. The threshold is initially 32, since it is at this window size that slow tart stops and congestion avoidance begins.
f. When loss is detected during transmission round 16, the congestion windows size is 42. Hence the threshold is 21 during the 18th transmission round.
g. When timeout occurs, threshold set to of the previous max CongWin. Therefore, threshold is around 26/2=13 at the 24th transmission round.
h. During the 1st transmission round, packet 1 is sent; packet 2-3 are sent in the 2nd transmission round; packets 4-7 are sent in the 3rd transmission round; packets 8- 15 are sent in the 4th transmission round; packets15-31 are sent in the 5th transmission round; packets 32-63 are sent in the 6th transmission round; packets 64 – 96 are sent in the 7th transmission round. Thus packet 70 is sent in the 7th transmission round.
i. The congestion window and threshold will be set to half the current value of the congestion window 8 when the loss occurred. The new values of the threshold and window will be 4.
2. Consider the network setup as shown in the figure below. Suppose that the ISP instead assigns the router the address 126.13.89.67 and that the subnet address of the home network is 192.168/16.
j) Assign addresses to all interfaces in the home network.
k) Suppose each host has two ongoing TCP connections, all to port 80 at host 128.119.40.86. Provide the six corresponding entries in the NAT translation table.
a. I assign 192.168.0.1 192.168.0.2 192.168.0.3 to 3 home work interfaces. Actually, I can use any addresses from 192.168.0.0 to 192.168.255.255 without some special reserve addresses.
b.
| WAN side | LAN side |
| 126.13.89.67,5001 | 192.168.0.1,3345 |
| 126.13.89.67,5002 | 192.168.0.1,3354 |
| 126.13.89.67,5003 | 192.168.0.2,3325 |
| 126.13.89.67,5004 | 192.168.0.2,3356 |
| 126.13.89.67,5005 | 192.168.0.3,3345 |
| 126.13.89.67,5006 | 192.168.0.3,3357 |
3. Suppose an application generates chunks of 40 bytes of data every 20 ms, and each chunk gets encapsulated in a TCP segment and then an IP datagram. What percentage of each datagram will be overhead, and what percentage will be application data?
25% for TCP overhead, 25% for IP overhead and 50% for application data. Because each chunk contains 20 bytes IP header and 20 bytes TCP header with 40 bytes of date
4. Suppose two TCP connections are present over some bottleneck link of rate R bps. Both connections have a huge file to send (in the same direction over the bottleneck link). The transmissions of the files start at the same time. What is the transmission rate that TCP would like to give to each of the connections? Explain your answer briefly in a couple of sentences.
R/2 because TCP congestion control mechanisms is fair. It controls each TCP connections to share the bandwidth.
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