Q1- One of the services provided by the transport layer is ERROR CONTROL , what
ID: 3765407 • Letter: Q
Question
Q1- One of the services provided by the transport layer is ERROR CONTROL, what does this ser-vice responsible for? (1 mark)
Q2- SCTP is one of the transport layer protocols which supports many services such as MULTI-HOMING, talk about SCTP and MULTIHOMING. (1 mark)
Q3- Consider Figure 1 shown below. Assuming TCP Reno is the protocol experiencing the behav-ior shown below; answer the following questions. (2 marks)
a. Identify the intervals of time when TCP slow start (SS) is operating. (0.4 marks)
b. Identify the intervals of time when TCP congestion avoidance (CA) is operating. (0.4 marks)
c. After the 16th transmission round (RTT), is segment loss detected by a triple duplicate ACK (3dupACKs) or by a timeout? (0.2 mark)
d. After the 22nd transmission round (RTT), is segment loss detected by a triple duplicate ACK (3dupACKs) or by a timeout? (0.2 mark)
e. What is the initial value of ssthresh (or threshold) at the first transmission round (RTT)? (0.2 mark)
f. What is the value of ssthresh (or threshold) at the 18th transmission round (RTT)? (0.2 mark)
g. Assuming a packet loss is detected after the 26th transmission round (RTT) by the receipt of a triple duplicate ACK (3dupACKs), what will be the values of the congestion window (cwnd) and ssthresh (or threshold)? (0.4 marks)
Note: When asked about intervals and transmission rounds consider answering using ‘x-axis’ and when asked about ssthresh and congestion window try answering using ‘y-axis’.
cwnd (in MSS) 40 35 30 25- 20 15 10- 5 0 2 4 6 8 10 12 14 16 18 20 22 24 26 gure 1. Congestion window as a function of round tip tme Figure 1. Congestion window as a function of round trip timeExplanation / Answer
Q 1.) One of the services provided by the transport layer is ERROR CONTROL, this service responsible for transmission of data from one device to another device. The end to end transfer of data from a transmitting application to a receiving application involves many steps, each subject to error. With the error control process, we can be confident that the transmitted and received data are identical. Data can be corrupted during transmission. For reliable communication, error must be detected and corrected.Error control is the process of detecting and correcting both the bit level and packet level errors.
Q.2 )
Stream Control Transmission Protocol (SCTP) is a transport-layer protocol, serving in a similar role to the popular protocols Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). It provides some of the same service features of both: it is message-oriented like UDP and ensures reliable, in-sequence transport of messages with congestion control like TCP.
Multihoming refers to a computer or device connected to more than one computer network. It can be used, for example, to increase the reliability of an Internet Protocol network, such as a user served by more than one Internet service provider.
Q.3)
a)
TCP slowstart is operating in the intervals [1,6] and [23,26]
b)
TCP congestion advoidance 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. If there were a timeout, the congestion window size would have dropped to 1.
d)
After the 22nd transmission round, segment loss is detected due to timeout, and hence the congestion window size is set to 1.
e)
The threshold is set to half the value of the congestion window when packet loss is detected. When loss is detected during transmission round 16, the congestion windows size is 42. Hence the threshold is 21 during the 18 th transmission round.
f)
During the 1 st transmission round, packet 1 is sent; packet 2 3 are sent in the 2 nd transmission round; packets 4 - 7 are sent in the 3 rd transmission round; packets 8 - 15 are sent in the 4 th transmission round; packets 16 - 31 are sent in the 5 th transmission round; packets 32 - 63 are sent in the 6 th transmi ssion round; packets 64 – 96 are sent in the 7 th transmission round. Thus packet 70 is sent in the 7 th transmission round.
g)
The congestion window and threshold will be set to half the current value of the congestion window when the loss occurred. Thus the new values of the threshold and window will be 4.
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