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# TCP and Acknowledgements

Question: Consider a TCP sender and receiver.

Assume bytes 0 .. 99 have been sent, received, and acknowledged (the sender has received ack's for them). Assume the sender sends the following segments (of 20 bytes each): 100, 120, 140, 160, 180, 200.

Assume these segments are received in the following order:

140
160
120
100
180
200

Show me what ack is generated by the receive after receiving each of these segments. Will the sender retransmit a packet? Which one and when?

#### Solution Preview

Ack: The ack will be the next byte sequence number the receiver expects to receive from the sender. So if the segment is of 20 bytes long, then if the receiver receives a segment with sequence number of X, then the ack will be X+20. So,

140 --> ack: 160
160 -->ack: 180
120 -->ack: 140
100 --> ack: 120
180 ...

#### Solution Summary

In 240 words, this solution provides an explanation of how to consider a TCP sender and receiver sequence and understand the pattern of retransmission.

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