TCP-Sequence No

Question

Answer 1

In a TCP mechanism we have 3 phases :

a) Connection establishment phase which consumes one sequence no during the 3- way handshaking process.Here actually this thing is done by SYN segment and ACK segment.The SYN segment consumes one sequence no while ACK segment does not.

b) Data transfer phase : Here the sequence no is given for each byte that is transferred..This is one of the essential features of TCP.

c) Connection termination phase

So in the question we are given initial sequence numbers which means sequence numbers of hosts before connection establishment..So we have to consider both connection establishment and data transfer phase.

We know if a sequence number is s  at sender's site the acknowledgement no at receiver's site is (s+1) , since acknowledgement means it is expecting next byte number from the sender which is (s+1).

So considering this fact ,

Sequence no of final byte by the host H1  = Initial sequence no of H1 + 1(for connection establishment phase by the SYN segment) + Data transmitted - 1(since we are calculating offset and 602 is included in sequence no)

                                                             = 902

So acknowledgement no of host H2         = sequence no of final byte of H1 + 1

                                                            = 902 + 1

                                                            = 903

Now H2 also behaves as data transmits.So it has also sequence no associated with it.

So sequence no of host H2                    =  1540  +  1 + 1000 - 1

                                                           =   2540

Hence C should be the correct option.

Comments

No doubt the TCP header sequence no field contains the initial sequence no..But here we are asked about the sequence no of last byte being transferred by the host H2..

Here I mention that the word "byte" should have been used in the question instead of packet.

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@Habibkhan Ok Thanks, Brother.  The last Line makes everything clear about the question

"  Here I mention that the word "byte" should have been used in the question instead of packet. "

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Is the sequence number alloted to each byte of payload only or is it (payload+header)?