Comment by macksd | Hacker News Reader

Comment by macksd | Hacker News Reader

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I actually remember being a bit confused by this too. Hopefully I can help:

These refer to 2 different protocols at the same layer: UDP, and TCP. Below these protocols there are layers that route little messages around local networks (like Ethernet) and that route little messages across the Internet (like IP), and UDP and TCP are another layer of abstraction on top of that.

TCP adds information about the ordering of messages, it has a mechanism for acknowledging receipt of a message, it has logic for resending a message, etc. UDP does not include these concepts, so if a message gets lost somewhere in the network, an application using UDP might not even notice.

The "connection" in this case, really refers to the state about these messages that is kept. It's a virtual / logical connection, not a physical connection.

Ah this does make sense. I'm guessing that when the sending machine starts a new TCP connection to another machine, the reciever sees that it is a TCP message and starts a TCP "session" (I'm not sure that's correct terminology) which maintains required state on its side and sender maintains its own state too?

"Session" is a good way to think of it. If it's inactive for too long, it can be closed and forgotten, etc. But "connection" is the correct terminology. You just sometimes need to be clear that you're talking about a TCP connection and not a physical connection, or even the logical connection that makes IP packets routable. It's one more layer above that.

But yes - both sides maintain state about the connection. Senders will re-send packets if they go too long without being acknowledged. They will slow down large batches of sending if lots of messages are being dropped (essentially throttling in case the network is being overwhelmed and they're making it worse). And receivers will acknowledge packets as they're received and assemble messages back in the right order if packets arrive out-of-order, or bits in the middle are missing.

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