Consider the Telnet example discussed in Slide 78 in Chapter 3. A few seconds after the user types the letter ‘C’, the user types the letter ‘R’. After typing the letter ‘R’, how many segments are sent, and what is put in the sequence number and acknowledgement fields of the segments?

3 Segments,

First segment : seq = 43, ack=80

Second Segment : seq = 80, ack=44

Third Segment: seq = 44, ack=81

Explanation:

Three segments will be sent as soon as the user types the letter ‘C’ and the he waits for a few seconds to type the letter ‘R’.

In our Telnet example, 42 and 79 for the client and server were the starting sequence number. Since the letter 'R' is typed and the starting seq is 42 is incremented by 1 byte. Therefore the sequence number in the first segment is 43 and the acknowlegement number is 80.

The second segment is sent from the server to the client. It echoes back the letter ‘C’. 43 is put in the acknowledgment field and tells the client that it has successfully received everything up through byte 42 and is now waiting for bytes 43 onward. This second segment has the sequence number 79 which is incremented by 1 byte. Therefore the sequence number in the second segment is 80 and the acknowlegement number is 44.

The third segment is sent from the client to the server. Its acknowledges the data it has received from the server. It has 80 in the acknowledgment number field because the client has received the stream of bytes up through byte sequence number 79 and it is now waiting for bytes 80 onward. This is also incremented by 1 byte. Therefore the sequence number in the second segment is 44 and the acknowlegement number is 81.