two-way handshake and three-way handshake

Question

I was reading a book about computer network written by Tanenbaum specifically about handshaking. In there he explains two-way handshake is not enough, considering this case:

A wants to transfer money to B, so A sends a SYN to B, and then B sends an ACK to A. Connection is established and then A can send his money and then drop the connection after it's done. If there is a delayed duplicate SYN from A to B, B will send its ACK again and A will be transferring its money again.

That is one of the weakness of two-way handshake based on the book written by Tanenbaum if I understands it right. The book says three-way handshake can solve this problem.

With a delayed duplicate SYN from A, B sends an ACK and SYN which get rejected by A. This is where I don't get it, it's as if "Hey B why do you send me a SYN and ACK? Oh I know, this is from a delayed SYN, I should just drop it.". Why not, in two-way handshake, A doesn't know that the ACK is made by a delayed duplicate SYN?

Thanks.

Answer 1

To establish a connection, the three-way (or 3-step) handshake occurs:

1. SYN: The active open is performed by the client sending a SYN to the server. The client sets the segment's sequence number to a random value A.

2. SYN-ACK: In response, the server replies with a SYN-ACK. The acknowledgment number is set to one more than the received sequence number i.e. A+1, and the sequence number that the server chooses for the packet is another random number, B.

3. ACK: Finally, the client sends an ACK back to the server. The sequence number is set to the received acknowledgement value i.e. A+1, and the acknowledgement number is set to one more than the received sequence number i.e. B+1.

At this point, both the client and server have received an acknowledgment of the connection. The steps 1, 2 establish the connection parameter (sequence number) for one direction and it is acknowledged. The steps 2, 3 establish the connection parameter (sequence number) for the other direction and it is acknowledged. With these, a full-duplex communication is established.

Answer 2

According to Kurose and Ross's "Computer Networking: A top-down approach", 6th Edition, p. 232,

The first two segments carry no payload, that is, no application-layer data; the third of these segments may carry a payload. Because three segments are sent between the two hosts, this connection-establishment procedure is often referred to as a three-way handshake

In other words, A does not need to wait for the three-way handshake to complete before sending data. Only B needs to wait for the three-way handshake to complete.

And why does B need to wait? As S. Richmond says, B needs to know that A has received its sequence number before it starts sending data.

Answer 3

The three-way handshake is necessary because both parties need to synchronize their segment sequence numbers used during their transmission.

So, they(in turn) send a SYN segment with a sequence number set to a value n, which then is acknowledged by the other party via a ACK segment with a sequence number set to n+1.

Suppose that client does not send ACK(case of 2 way handshake). Now there might exist a case where seq number of client is not synchronized, but the server will assume that it is synchronized. This could cause a problem.