Review Questions

Review Question 1Why an application developer might choose to run an application over UDP rather than TCP

An application developer may not want its application to use TCPs congestion control, which can throttle the applications sending rate at times of congestion. Often, designers of IP telephony and IP videoconference applications choose to run their applications over UDP because they want to avoid TCPs congestion control. Also, some applications do not need the reliable data transfer provided by TCP.

Review Question 2Is it possible for an application to enjoy reliable data transfer even when the application runs over UDP? If so, how?

Yes. The application developer can put reliable data transfer into the application layer protocol.

Review Question 3 Consider transferring a file of L bytes from Host A to Host B. Assume an MSS of 1460 bytes.(a) What is the maximum value of L such that TCP sequence numbers are not exhausted?(b) For the L you obtain in (a), find how long does it take to transmit the file. Assume that a total of 66 bytes of transport, network, and data link header are added to each segment before the resulting packet is sent out over a 10 Mbps link. Ignore flow control and congestion control

There are possible sequence numbers.a) The sequence number does not increment by one with each segment. Rather, is increments by the number of bytes of data sent. So the size of the MSS is irrelevant -- the maximum size file that can be sent from A to B is simply the number of bytes equal to .b) The number of segments is . Now 66 bytes of header get added to each segment giving a total of 194,156,028 bytes of header. The total number of bytes transmitted is bits.Thus it would take 3,591 seconds = 59 minutes to transmit the file over a 10~Mbps link

Review Question 4For RTT estimation, Why do you think TCP avoids measurement of SampleRTT for the retransmitted segments

What happens if TCP measure SampleRTT for a retransmitted segment. Suppose the source sends packet P1, the timer for P1 expires, and the source then sends P2, a new copy of the same packet. Further suppose the source measures SampleRTT for P2 (the retransmitted packet). Finally suppose that shortly after transmitting P2 an acknowledgment for P1 arrives. The source will mistakenly take this acknowledgment as an acknowledgment for P2 and calculate an incorrect value of SampleRTT

Review Question 5TCP waits until it has received three duplicate ACKs before performing a fast retransmit. Why?

Suppose packets n, n+1, and n+2 are sent, and that packet n is received and ACKed. If packets n+1 and n+2 are reordered along the end-to-end-path (i.e., are received in the order n+2, n+1) then the receipt of packet n+2 will generate a duplicate ack for n and would trigger a retransmission under a policy of waiting only for second duplicate ACK for retransmission. By waiting for a triple duplicate ACK, it must be the case that two packets after packet are correctly received, while n+1 was not received. The designers of the triple duplicate ACK scheme probably felt that waiting for two packets (rather than 1) was the right tradeoff between triggering a quick retransmission when needed, but not retransmitting prematurely in the face of packet reordering.

Review Question 6To achieve a TCP throughput of 10GPS, find the tolerable segment loss probability

Lets assume 1500-byte packets and a 100 ms round-trip time. From the TCP throughput equation, we have10 Gbps = 1.22 * (1500*8 bits) / (.1 sec * srqt(L)), orsqrt(L) = 14640 bits / (10^^9 bits) = 0.00001464, or L = 2.14 * 10^^-10

Review Question 7If suppose the TCP sender sends a large amount of data and then goes idle at time A and becomes active again at B. What are the adv./disadv. of TCP using the Congwin and threshold values from A , at B.

An advantage of using the earlier values of CongWin and Threshold at B is that TCP would not have to go through slow start and congestion avoidance to ramp up to the throughput value obtained at A. A disadvantage of using these values is that they may be no longer accurate. In particular, if the path has become more congested between A and B, the sender will send a large windows worth of segments into an already (more) congested path.

Review Question 8Suppose TCP increased its congestion window by two rather than by one for each received acknowledgement during slow start. First window consists of one, second three and so on. Find the latency

K- number of windows that cover the object

Q is the number of times the server would idle for an object of infinite size.

Review Question 9Let RTT=1sec and O = 100kbytes. Prepare a chart that compares minimum latency with that of slow start for R = 28kbps, 100kbps, 1Mbps and 10Mbps.

RO/RPMinlatencyLatency withslow start28 Kbps29.25 s331.25 sec33.18 sec100 Kbps8.19 s510.19 sec13.86 sec1 Mbps819 msec72.81 sec9.26 sec10 Mbps82 msec72 sec9 sec

Review Question 10Consider sending an object of size O=100kbytes from server to client. Let S=536 bytes and RTT=100msec. Suppose TCP use static window size with window size W. Find the minimum possible latency and the window size that achieves this minimum value for transmission rates of 28kbps, 100kbps, 1Mbps and 10Mbps.

The minimum latency is 2RTT+O/R. The minimum W that achieves this latency is

Rmin latencyW28 Kbps28.77 sec2100 Kbps8.2 sec41 Mbps1 sec2510 Mbps0.28 sec235

Review Question 11The idea of doubling of the timeout interval can be considered to be a mechanism for congestion control. Justify.Does TCP need to employ window based congestion control

If TCP were a stop-and-wait protocol, then the doubling of the time out interval would suffice as a congestion control mechanism. However, TCP uses pipelining (and is therefore not a stop-and-wait protocol), which allows the sender to have multiple outstanding unacknowledged segments. The doubling of the timeout interval does not prevent a TCP sender from sending a large number of first-time-transmitted packets into the network, even when the end-to-end path is highly congested. Therefore a congestion-control mechanism is needed to stem the flow of data received from the application above when there are signs of network congestion