Question

Why is three-way handshake needed? What is the problem if we send only two packets and consider the connection established? What will be the problem from application's point of view? Will the packets be delivered to the wrong application?

Answer 1

Problem regarding 2-way handshake
The only real problem with a 2-way handshake is that duplicate packets from a previous connection( which has been closed) between the two nodes might still be floating on the network. After a SYN has been sent to the responder, it might receive a duplicate packet of a previous connection and it would regard it as a packet from the current connection which would be undesirable.
Again spoofing is another issue of concern if a two way handshake is used.Suppose there is a node C which sends connection request to B saying that it is A.Now B sends an ACK to A which it rejects & asks B to close connection.Beteween these two events C can send a lot of packets which will be delievered to the application..

The first two figures show how a three way handshake deals with problems of duplicate/delayed connection requests and duplicate/delayed connection acknowledgements in the network.The third figure highlights the problem of spoofing associated with a two way handshake.

Some Conventions
1. The ACK contains 'x+1' if the sequence number received is 'x'.
2. If 'ISN' is the sequence number of the connection packet then 1st data packet has the seq number 'ISN+1'
3. Seq numbers are 32 bit.They are byte seq number(every byte has a seq number).With a packet 1st seq number and length of the packet is sent.
4. Acknowlegements are cummulative.
5. Acknowledgements have a seq number of their own but with a length 0.So the next data packet have the seq number same as ACK.

Connection Establish

• The sender sends a SYN packet with serquence numvber say 'x'.
• The receiver on receiving SYN packet responds with SYN packet with sequence number 'y' and ACK with seq number 'x+1'
• On receiving both SYN and ACK packet, the sender responds with ACK packet with seq number 'y+1'
• The receiver when receives ACK packet, initiates the connection.

Connection Release

• The initiator sends a FIN with the current sequence and acknowledgement number.
• The responder on receiving this informs the application program that it will receive no more data and sends an acknowledgement of the packet. The connection is now closed from one side.
• Now the responder will follow similar steps to close the connection from its side. Once this is done the connection will be fully closed.
  
  
  
  

source : https://www.cse.iitk.ac.in/users/dheeraj/cs425/lec14.html