cmsc 417 Fall 2001
Exam 1

If you want your exam/project scores displayed on class page by last four digits of your SID, sign at right and give the last four digits of your SID:

Total points 30. Total time 75 mins. 3 problems over 4 pages. No book, no notes, no calculator.

1. [15 pts] 

This problem deals with TCP congestion control using the ``window-level'' model (in Note 5) and the scenario shown above. Assume the following:

Determine the following for a 20 KB file transfer: the transfer time in ms (from start to everything acked); the number of windows sent; and, for each window send, which 1 KB chunks are sent (answers for the first two window sends are shown below, assuming the file consists of chunks 0, 1, ..., 19):
  window send    file chunks sent
    1              0
    2              1, 2

Determine the time to transfer a huge file of size F KB (e.g., F > 106). Your answer is a function of F. Ignore initial and final ``transient'' effects.

2. [10 pts]  This problem deals with a variation of the sliding-window data transfer protocol in Note 4. Assume a TCP-like situation as follows:

Give the code for the receive data event at Sink. Specifically, indicate how rbuf, nr, rw, nc are updated upon reception of (D,data,len,cj).

Your code must not exceed 20 lines in total. Program elegance counts.

3. [5 pts]  Consider the sliding window data transfer protocol in Note 4 with the following changes:

Does this protocol correctly transfer data, i.e., from Source user to Sink user in sequence without any loss.

If you answer YES, give a brief justification. If you answer NO, give an evolution of the protocol that transfers data out-of-sequence, starting from na=ns=nr=0 and empty channels.