I can understand the confusion. Keep in mind that the window size, the sequence number and the number of segments sent are somewhat independent from each other. What do I mean?
Well, let’s say we have a window size of 21000 bytes. It is very unlikely that this will all be sent in one segment. It will definitely be split into several segments. The window size is “the number of bytes sent before an acknowledgement is required from the receiver.” These bytes can be sent in one or more segments. So, let’s take the following example:
Host A is sending a total of 100000 bytes (or 1 KB) of data to Host B. The window size is 7000 bytes. Let’s say the maximum segment size (which is affected by the Layer 2 and Layer 3 MTU configuration) is 1500 bytes. Host A will begin sending a series of segments with the following elements (SEQa is the sequence number sent by host A):
**Segment 1: SEQa = 1, Window Size = 7000, segment size = 1500 Bytes**
**Segment 2: SEQa = 1501, Window Size = 7000, , size = 1500 Bytes**
**Segment 3: SEQa = 3001, Window Size = 7000, , size = 1500 Bytes**
**Segment 4: SEQa = 4501, Window Size = 7000, , size = 1500 Bytes**
**Segment 5: SEQa = 6001, Window Size = 7000, , size = 1000 Bytes**
At this point, 7000 bytes have been sent. The TCP window has been exhausted. Host A stops sending and waits for a response. Assuming all went well, Host B send the following response:
**Acknowledgement from Host B to Host A: ACKb = 7001**
The ACKb number returned is the next expected byte, that is, byte 7001.
Once Host A successfully receives this acknowledgement, it continues with the next batch of segments:
**Segment 1: SEQa = 7001, Window Size = 7000, , segment size = 1500 Bytes**
**Segment 2: SEQa = 8501, Window Size = 7000, , size = 1500 Bytes**
**Segment 3: SEQa = 10001, Window Size = 7000, , size = 1500 Bytes**
**Segment 4: SEQa = 11501, Window Size = 7000, , size = 1500 Bytes**
**Segment 5: SEQa = 13001, Window Size = 7000, , size = 1000 Bytes**
Host B sends the following response
**Acknowledgement from Host B to Host A: ACKb = 14001**
… and so on…
So to specifically answer your question, yes sequence number is used to put the segments back in the proper order while window size is the number of bytes that must be sent before an acknowledgement must be received.
It just happens that in both my example and Rene’s example, the window size was very small, so only one segment was necessary to exhaust the window. That is why it seemed that the sequence number was directly related to the window size.
As for your second question:
One more thing is very confusing to me. In Rene’s tutorial, he is saying “H1 has setup a connection with H2 by using the 3 way handshake. We are sending 10 bytes of Data which means our “window size” is 10 bytes. The sequence number is 10”, but in your example even though when Left device is sending the first chunk of data of 10 bytes, still the sequence number is 1. I am not sure why. Would you please explain it to me?
When beginning a transfer, the SEQ number in the header refers to the first byte that is being sent in that specific segment. But you must also keep in mind that the first SEQ number is always generated randomly, so it can be anything between 0 and something over 4 billion (32 bits represent the SEQ number). It is possible that @ReneMolenaar is using a random number in that example and it just happened to be 10. This can be confusing, and I can ask Rene to look it over and see if it needs revising.
As for the field in the header that indicates the amount of data, there is no such field per se. However, this can be determined by examining the subsequent SEQ numbers in each received segment. Also, when the segments are received on the other end, at the end of the current window, the ACK number that is returned indicates the next expected byte, which in essence reveals the size of the last segment.
I hope this has been helpful!