[pgorczak]

I definitely used to think TCP was more “high-level” than it actually is. Yes it does much more than UDP but still, its job is to get a sequence of bytes from A to B. You can tune it for higher throughput or more sensitive flow control but anything concerning message passing, request/response, … is beyond the scope of TCP.

[dimman]

Sure, but from a "high level" or "sockets" perspective, especially as a beginner it shouldn't be something you need to care about. A bit simplified, the basic stuff you need to know is:

1) UDP uses packages/messages which may or may not reach its destination. If it reaches its destination the data is intact. Normally connectionless.

2) TCP is a stream protocol. There is no package/message boundary unless you create it yourself (my tip is to do a simple binary TLV (type length value) protocol using say a fixed 4 byte header). Requires a connection to be setup first.

3) Network byte order - really important to read about.

4) Nagles algorithm (TCP_NODELAY) and SO_KEEPALIVE - those are a couple of things to read about.

5) Start with the simple select() approach to handle the socket activity.

You can then go ahead and get more advanced by doing nonblocking I/O or do blocking I/O with each client in its own thread, figuring out pros and cons for your use case. You can add SSL/TLS on top of your TCP connection etc.

EDIT: The SO_KEEPALIVE part is perhaps least important thing to start reading about. I'm a bit biased due to NAT traversal problems as I wrote a secure remote access solution for a major company several years back, utilising STUN/TURN servers, public key authentication (basically certificate pinning), TLS etc.

[pgorczak]

Yes and even at 2) some subtleties start. You can set up a connection, send a chunk of bytes, and close it. If you reach a clean connection close, you can be sure that all your bytes have reached the other side. As soon as you start sending multiple logical messages over a persistent connection and an error occurs, you need to write application logic to figure out where to pick up again after you reconnect. Even if you want to know which parts of your stream have already reached the other side, you need to add logic for that. This “multiple transactions over a persistent connection” may sound really straightforward but it’s not built into TCP itself.