3. (25pts) TCP congestion control
(a) (20pts) We use the following simplified rules for TCP congestion control.
The window size is measured in segments. Assume all segments are of the same size (MSS). The TCP connection
begins from slow start (SS) until the first loss event. Assume that cumulative acknowledgements are used, i.e.,
ACK(k+1) acknowledges that the previous k segments are successfully received in order and without error.
Assume there is a buffer at the receiver that keeps out of order packets.
Consider the TCP congestion window that evolves in time in Fig. 2.
(5pts) Provide a loss scenario for the loss event at t1 when the window size is 32(MSS). Note that after t1,
the congestion window drops to one MSS, and in slow start phase.
(5pts) Calculate the threshold value (threshold?) that separates Slow Start and AI between t1 and t2.
(10pts) For the loss event when the congestion window size is 28(MSS) at t2, assume that the type of loss
event is one loss that results in triple ACKs and then fast retransmit.
Calculate the window size at ``?", right after the loss event when the window size is 28.
Draw a (partial) packet-level timing diagram to illustrate what happens: Include (parts of) the
congestion window before t2 that triggers fast retransmit, and the congestion widow right after
the loss events. (No need to draw an entire timing diagram for all time.)
In your packet timing diagram, please label the congestion windows, segments sent and lost, when
applicable.
CWND(1)
32
28
AI
threshold?
1
t
t1
t2
Fig. 2: TCP congestion window. AI: Additive increase
