Host A and B are directly connected using a 1000 Mbps link that many other hosts

Question

Host A and B are directly connected using a 1000 Mbps link that many other hosts use to transport data. Host A is sending Host B an enormous file over the 1000 Mbps link. Host A can send at a rate of 100 Mbps, but host B can receive data at a maximum rate of 50 Mbps. After a few seconds, Host A receives two ACK’s with duplicate values from Host B. The value for the rwnd field in the first ACK is 4380 and 5840 in the second ACK.

A- What can you deduce about the reason Host A is receiving duplicate ACK’s?

B- Why did the rwnd field increase?

Explanation / Answer

Answer:

A- The receive buffer will begin to fill up, due to the fact that Host A will be sending data into it faster than Host B can remove the data from it. Once the buffer has completely filled, Host B will send a message to Host A to stop sending data until Host B can remove data from the buffer. Host B will then send a TCP segment to Host A, informing it to continue sending data. The buffer will fill up again and this process will repeat until all of the data has been sent from Host A to Host B.

B- Rwnd is used to give the sender an idea of how kuch free space is available in the buffer. Thus as the nunber of unacknowledged data varies , rwnd size also changes.

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