FAQ: In the congestion control model, what does it mean to evaluate r i at time t - tau i?

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(Contributed by Richard Murray, 2 July 2009)

Q: On page 79, line 14, the text says " is evaluated at time , representing the time required to receive additional acknowledgements. What does this mean?

A: The symbol represents the rate at which packets are cleared from the senders queue and it is equal to the window size divided by the round trip time . The window size is increased or decreased based on whether a packet arrived at the sender and the corresponding acknowledgement packet arrived back at the original sender, as described in equation (3.16):


The total time from when the packet was sent until the acknowledgement packet is received is the round trip time . Thus, the rate at which acknowledgements are received is the rate at which were sent ms ago. This is the meaning of .

Another point of clarification is that (from equation (3.16)) and so ( is a constant).