When a host initiates a TCP session, its initial sequence number is effectively random; it may be any value between 0 and 4,294,967,295 ( 2^32 - 1 ), inclusive .
The ISN (Initial Sequence Number) of a packet is not zero and assigned random value for the purpose SECURITY, preventing SPOOFING or HACKING or CONFLICTING with other data bytes transmitted over the data communication(TCP)!
BUT By default, the We take these starting sequence numbers as zero, regardless of what these values really are. The purpose of this is to make sequence and acknowledgement number analysis simpler. These values will be referred to as relative sequence numbers to the connection.
For sender side
It has a send window and the receiver side has a corresponding recieve window
Now as we are using Selective Repeat , this send window size= recieve window size Which is 32
Now minimum number of sequence number(frame numbers) required to communicate without any ambiguity is 64
So my sequence number space range from 0-63
200%32 = 8 (make set of 32, 0 to 31 )
192 will get 31
193 =0
194=1
195 = 2
196=3
197=4
198=5
199=6
200 = 7
The sequence number of the frame to be sent after sending 200 frames is 8
Reference :
http://www.lovemytool.com/blog/2010/08/practical-tcp-series-sequence-and-acknowledgement-numbers-by-chris-greer.html