Consider the following network fragment: v has three attached neighbors, u, w, a
ID: 3825123 • Letter: C
Question
Consider the following network fragment: v has three attached neighbors, u, w, and x. u is connected only to v. w has a minimum-cost path to destination y of 7, and x has a minimum-cost path to y of 6. The complete paths from w and x to y are not shown. All link costs in the network have strictly positive integer values. recover packets lost due to varying channel conditions (e.g. fading, collisions, etc..), Typically, the MAC-layer will try retransmit a packet N times before giving up. (a) How does the local MAC-layer retransmission help enhance the throughput performance of the end-to-end TCP connection? (b) If local retransmission is beneficial. it seems that one should choose a large N to increase the chances of successful repair. Give two reasons or scenarios where it is "not" desirable to have a large N.Explanation / Answer
Q.1 How does MAC layer retransmission help enhance the throughput performance of end-to-end TCP connection?
Ans: TCP is a reliable delivery protocol which uses GO-BACK-N protocol and timer based retransmissions. Time out interval is calculated on the basis of end-to-end delay.
Typically, a TCP segment is fragmented at the intermediate nodes between source and destination because of MTU(maximum transmission unit) of different links joining source and destination.
at the destination, these fragments are combined to form the original segment. even if one fragment is missing, the entire segment is assumed to be lost/damaged and is discarded. this loss of segment event causes the sender to assume congestion in network and makes the sender reduce the size of congestion/transmission window, which results in low network bandwidth utilization. also sender retransmits the entire segment, resulting in unnecessary packet delay.
data link layer has the job moving a fragment reliably between two nodes across the link. for every fragment sent, it expects an ack. and retransmits fragment in the wake of bit errors/lost fragment .
retransmissions by data link layer helps to avoid the retransmission of entire TCP segment(as mentioned above) and hence, it certainly improves TCP throughput.
Q.2 If local retransmission is beneficial, it seems that one should choose a large N to increase the chances of successful repair. Give two reasons/ Scenarios where it is not desirable to have large N.
Ans.
1. In a lossy environment, retransmissions result in decrease in packet throughput. hence in a network with multimedia users using multimedia application which are latency sensitive and loss tolerant, retransmissions result in poor performance of application.
2. Too many retransmissions at link layer causes the TCP timer to expire, resulting in end to end retransmission and reduction in size of congestion window which further causes low utilization of network bandwidth.
hence, a limit should be set on number of MAC retransmissions as too less number of retransmissions cause end to end retransmission which is expensive and too many retransmissions cause unnecessary delay.
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