TCP Flow control
(13%) Suppose we have a sender with a fixed window size of 80 bytes. At t = 0 ms, we know byte 100 was the last sent and acknowledged byte and can be purged from the buffer (with ack number 101), bytes 101 through 140 are outstanding bytes, sent but not acknowledged, and bytes 141 through 150 are written and can be sent immediately. The picture of this is similar to Figure 15.22 of the 4th edition on page 457, or chart 51 of slides, with different numbers as shown below.
First outstanding byte
Next byte to send to
Timer
200
201
260
261
300
30
Bytes that cannot be sent until the right edge moves to the right
Bytes that are acknowledged (Outstanding bytes can be purged from buffer) (sent but not acknowledged)
Bytes that can be sent (Usable window)
Send window size (advertised by the receiver)
a. Send window
(a) At time t = 5 ms, the sender writes another 30 bytes, where the bytes 151 through 160 are in the window and also marked as can be sent immediately. How would you mark the 20 bytes 161 through 180? Are they also bytes which "can be sent immediately," "sent and acknowledged," or something else? Explain!
(b) At time t = 10 ms, the sender tries to send another 20 bytes. Can the sender send another 20 bytes? If so, what are the byte numbers? If not, why, and what is the maximum number of bytes the sender can send now? Explain!
(c) At time t = 15 ms, the sender tries to send another 20 bytes. Can the sender send another 20 bytes? If so, what are the byte numbers? If not, why, and what is the maximum number of bytes the sender can send now? Explain!
