Consider a network where the sliding window protocol is in use with SWS = RWS =

Question

Consider a network where the sliding window protocol is in use with SWS = RWS = 2
frames and a one-way delay of 100ms (i.e. for a frame sent at time t, it arrives at t + 100ms). Assume that when multiple frames are all able to be sent according to the window size, that they are sent 20 ms apart (i.e., frame 1 starts at time t, and frame 2 starts at time t + 20 ms).

What would be a reasonable timeout value for this link? Why not something smaller? How about larger?

Now, assume that by sending a frame every 5 ms we can saturate the link. In this case, determine the smallest SWS and RWS that maximizes throughput (i.e., keeps the link fully utilized).

Explanation / Answer

The assumption for minimum timeout value is “the processing time between receiving a data frame and preparing the acknowledgement is zero”. Hence minimum timeout value for the given network is 200ms. If we assume that ACKNOWLEDGEMENTs can only be sent 20ms apart (like data frames), then 220ms would also be appropriate, as the receiver may not be able to send its acknowledgement due to being in the middle of sending a different ACKNOWLEDGEMENT or DATA frame. The bandwidth delay product for this link is 5 frames - thus a reasonable window size to “keep the pipe full” would be 10 - 5 frames to send for the forward delay, and 5 frames to send for the ACKNOWLEDGEMENT's delay.

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