Does TCP send a SYN/ACK on every packet or only on the first connection?

Solution 1:

It's kinda like:

+-------------------------------------------------------+
|     client           network            server        |
+-----------------+                +--------------------|
|    (connect)    | ---- SYN ----> |                    |
|                 | <-- SYN,ACK -- |     (accepted)     |
|   (connected)   | ---- ACK ----> |                    |
\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/

when client sends...
\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/
|                 |                |                    |
|     (send)      | ---- data ---> |                    |
|                 | <---- ACK ---- |  (data received)   |
\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/

when server sends...
\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/
|                 |                |                    |
|                 | <--- data ---- |       (send)       |
| (data received) | ---- ACK ----> |                    |
\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/

...and so on, til the connection is shut down or reset

SYN starts a connection; you'll usually only see it when the connection's being established. But all data being sent via TCP requires an ACK. Every byte sent must be accounted for, or it will be retransmitted (or the connection reset (closed), in severe cases).

Actual connections aren't usually exactly like the diagram above, though, for two reasons:

• ACKs can build up, so one ACK can acknowledge everything received up to that point. That means you can acknowledge two or more sends with one ACK.
• An ACK is simply a flag and field in a TCP header. Sending one requires at least a header's worth of bandwidth, plus whatever the lower layers tack on. But data segments already include all that...so if you're sending data, you can send an ACK at the same time for free.

Most TCP/IP stacks try to reduce the number of naked ACKs without unduly risking retransmission or a connection reset. So a conversation like this one is quite possible:

\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/
|                 |                |                    |
|                 | <--- data ---- |       (send)       |
| (data received) |                |                    |
|     (send)      | -- data,ACK -> |                    |
|                 |                |  (data received)   |
|                 | <- data,ACK -- |       (send)       |
| (data received) |                |                    |
|  (wait a bit)   | <--- data ---- |       (send)       |
| (data received) |                |                    |
|     (send)      | -- data,ACK -> |                    |
|                 |                |  (data received)   |
|     (send)      | ---- data ---> |   (wait a bit)     |
|                 |                |  (data received)   |
|                 | <- data,ACK -- |       (send)       |
| (data received) |                |                    |
|  (wait a bit)   |   (dead air)   |                    |
|                 | ---- ACK ----> |                    |
\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/

As for UDP, there's no built-in concept of SYN and ACK -- UDP is by nature "unreliable", and not connection-oriented, so the concepts don't apply as much. Your acknowledgement will usually just be the server's response. But some application-layer protocols built on top of UDP will have some protocol-specific way of acknowledging data sent and received.

Solution 2:

SYN is only at the beginning.

ACK is on subsequent segments in either direction. The ACK will also define a window size. If the window size is 100 for example, the sender can send 100 segments before it expects to receive an ACK. E.g If sender sends 100 segments but segment number 50 gets lost, then receiver will get 1-49 & 51 -100. Receiver will then ACK for 50 (next segment it expects) and set window size to 1. Sender will resend 1 segment with sequence number 50. Receiver will then ACK for 101 and set the window size back up to a higher number.

Both are actually fields in the TCP header and can be sent with data, though the SYN and the first ACK typically are data-less.

So neither of the scenarios you describe is quite correct. The first is actually closer to reality, but all the data packets after the SYN do have to include an ACK, and also an acknowledgement number field which identifies the number of the next packet expected.

The end of a session also involves handshakes with FIN flagged packets and ACKs relating to them.

The exchanged sequence numbers are used to identify lost packets and enable a retry mechanism, and also to reassemble the entire stream of packets in the correct order.

Also, if it's the first case, are there any benefits of UDP over TCP if you just keep the connection open over a long period of time?

With UDP you can't just keep the connection open over a long period of time. There is no connection.

This sequence of SYN/ACK/FIN flags is what makes a connection.

With UDP, there are no SYNs or ACKs, so communication is one-way, delivery is not guaranteed and order is not preserved. But it has less overhead, so it's useful when speed is more important than reliability, as for example in streaming media.

This is a bit simplified yet, but it's the best I can do at the moment.

There's much more on this in the wikipedia entry on TCP and of course in the RFCs.