tcpip 3-way handshake

Question

Why is data not transferred during the 3rd part of TCP 3-way handshake? e.g.

(A to B)SYN

(B to A)ACK+SYN

(A to B) ACK.... why cant data be transferred along with this ACK?

Answer 1

I've always believed it was to keep the session establishment phase separate from the data transfer phase so that no real data is transferred until both ends of the session have agreed on the sequence numbers and session options, especially since packets arriving may be from a totally different, previous, session that just happens to have the same endpoints.

However, on further investigation, I'm not entirely certain that transmitting data with the handshake packets is disallowed. The section on TCP connection establishment in my Internetworking with TCP/IP¹ book contains the following snippet:

Because of the protocol design, it is possible to send data along with the initial sequence numbers in the handshake segments. In such cases, the TCP software must hold the data until the handshake completes. Once a connection has been established, the TCP software can release data being held and deliver it to a waiting application program quickly.

Since it's certainly possible to construct a TCP packet with SYN (or ACK) and data, this may well be allowed. I've never seen it happen in the wild but, then again, I've never seen a hairy-eared dwarf lemur in the wild either, though I'm assured they exist.

It may be that it's the sockets software that prevents data going out before the session is fully established but TCP appears to consider it valid. It appears you can send data with a SYN-ACK packet (phase 2 of the connection establishment) since you have the other end's sequence number and options. Likewise, sending data with the phase 3 ACK packet appears to be possible as well.

The reason the TCP software holds on to the data until the handshake is fully complete is probably due to the reason mentioned above - only once both ends have agreed on the sequence numbers can you be sure that the data is not from a previous session.

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¹Internetworking with TCP/IP Volume 1 Principles, Protocols and Architecture, 3rd edition, Douglas E. Comer, ISBN 0-13-216987-8.