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TCP and Acknowledgements

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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?

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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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See Also This Related BrainMass Solution

The guide gives detailed solutions for calculations of sequence number, source port number, and destination port number by three sending orders: host A to host B, host A before host B and host A after host B.

1. Hosts A and B are communicating over a TCP connection, and Host B has already received from Host A all bytes up through (and including) byte 248. Suppose Host A then sends two segments to Host B back-to-back. The first and second segments contain 40 and 60 bytes of data, respectively. In the first segment, the sequence number is 249, the source port number is 503, and the destination port number is 80. Host B sends an acknowledgment (ACK) whenever it receives a segment from Host A.

a) In the second segment sent from Host A to Host B, what are the sequence number, source port number, and destination port number?
b) If the first segment arrives before the second segment, in the acknowledgement of the first arriving segment what are the acknowledgement sequence number, the source port number, and the destination port number?
c) If the second segment arrives before the first segment, in the acknowledgement of the first arriving segment what is the acknowledgement sequence number?

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