Two hosts A and B are using TCP to send data to each other. Each segment may car

Question

Two hosts A and B are using TCP to send data to each other. Each segment may carry piggybacked ACKs in addition to data. The timing diagram on the next page shows a few of the TCP segments that are transmitted in each direction (there may have been earlier transmissions). The X denotes a lost segment. The receiver buffers out-of-sequence segments, and there is no window size constraint. For each segment, the Sequence number, the ACK sequence number, and the size of data carried in the segment are shown, except some of the numbers are missing. Fill in these missing quantities. Also show all the transmissions that occur in both directions after the Timeout at host A, assuming that no new data packets are sent after this in either direction and no more packets are lost.

Explanation / Answer

Following is the missing sequence.

Packet seq. ACK. SIZE

A1. 4800 1500. 1600

A2. 4801. 1500. 1600

A3. 4802. 1500. 1600

A4. 4803. 1501. 1600

A5. 4804. 1502. 1600

A6. 4805. 1503. 1600

B1. 1500. 4801. 500

B2. 1501. 4801. 500. (A3 received delayed A2)

B3. 1502. 4803. 500 (A2 A3 RECEIVED)

B4. 0. 4803. 0. (ACK PACKET)

B5 0. 4803 0. (ACK PACKET)

B4 and B5 are ACK PACKET hence size is 0 and has no sequence numbers.

After timeout host A will retransmit packet A4.

Possible sequence after timeout

A4. 4803. 1503. 1600

B6. 0. 4806. 0. (ACK for A4 - cumulative ACK till A6)

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