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Is this how it works?

1. Client sends syn to server.
2. Server responds with syn ack.
3. Client responds with ack.

What would happen if the server just responded with syn or with just ack? What would happen if the server doesn't respond at all? What would happen if the client doesn't respond with ack?

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What exactly the question here? Have you read -> en.wikipedia.org/wiki/Transmission_Control_Protocol "TCP protocol operations may be divided into three phases. Connections must be properly established in a multi-step handshake process (connection establishment) before entering the data transfer phase. After data transmission is completed, the connection termination closes established virtual circuits and releases all allocated resources." – Vick Vega Mar 17 '11 at 6:00
@Vicka: The question is clear enough, in the last paragraph. – grawity Mar 17 '11 at 6:18
Possible duplicate of superuser.com/questions/267693/… – RedGrittyBrick Apr 7 '11 at 10:58
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1 Answer

up vote 7 down vote accepted

Is this how it works?

Yes.

What would happen if the server just responded with syn

Simultaneous initiation is described in RFC 793 section 3.4 and also covered by RFC 1122 section 4.2.2.10.

The procedure also works if two TCP simultaneously initiate the procedure. When simultaneous attempt occurs, each TCP receives a "SYN" segment which carries no acknowledgment after it has sent a "SYN".

or with just ack?

I'm not entirely sure, but I think a RST is sent back and the connection is terminated.

What would happen if the server doesn't respond at all?

What would happen if the client doesn't respond with ack?

When no response is received, the client remains in SYN-SENT (server - SYN-RECEIVED) state and eventually times out.

Overall, RFC 793 "Transmission Control Protocol" is an excellent resource.

See also RFC 1122 "Requirements for Internet Hosts -- Communication Layers".

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