Connection oriented — vs — Connectionless
When we discussed the practical model, we encountered the terms connection oriented and connectionless several times. It’s important to know which protocols are connection oriented and which are connectionless.
If a protocol receives real‑time feedback from the other side, we call that protocol connection oriented.
Feedback is a reply about received packets. A direct reply is the clearest example of feedback. Most application protocols are themselves connection oriented.
Calling someone on the phone is clearly connection oriented. You hear the other person. Even meaningful silences are a form of feedback. The question “are you still there” adds nothing to the conversation itself — it’s about the connection.
TCP, a transport protocol, provides this kind of keep‑alive.
TCP sets up connections,
keeps them alive,
tears them down when they’re no longer needed,
or automatically after 300 seconds of silence.
TCP is connection oriented.
TCP is therefore used for application protocols with a complex dialogue.
Sending someone a letter by post is connectionless. You neither hear nor see the other side. The other side doesn’t have to reply, and if they don’t, you will never know — even after successful delivery — whether your letter actually arrived.
UDP, like TCP, is a transport protocol, but it does not concern itself with any form of feedback or keep‑alive. Simple application protocols that require little feedback work efficiently with UDP.
UDP is connectionless.
IP is the network protocol, but it only concerns itself with forwarding a packet to every next hop: From SOURCE to ROUTER, from ROUTER to ROUTER, … and from ROUTER to final destination.
IP is connectionless.
Sometimes IP does receive feedback from a router along the way, but through a different protocol: ICMP. ICMP is also used for PINGing the other side.