[tsimionescu]

Ports are very much a concept of the transport layer. They are a very useful concept, so they are used in all major transport-layer protocols, but they are not necessary in a theoretical sense (though a transport layer protocol without them would only allow a single stream of traffic between any two machines, or at least IPs). But TCP port 22 is a completely different thing than UDP port 22, and they are both completely different from SCTP port 22. To prevent confusion, IANA typically assigns the same port number for a protocol on both TCP and UDP (e.g. DNS over TCP uses TCP port 53, just like DNS over UDP uses UDP port 53; and QUIC uses UDP port 443, just like HTTPS uses TCP port 443).

When a machine receives a packet, after the Ethernet and IP layers have made sure the packet is addressed to this machine, the very next thing that happens is checking what transport layer implementation should receive the packet - and this is done based on the "transport" bits in the IP header. If the packet is, say, TCP, then the TCP layer starts reading its own header and finds out the port number, and proceeds from there.

[geokon]

> But TCP port 22 is a completely different thing than UDP port 22

OH woah woah. Okay that's the root of my confusion then. Nevermind. I thought the OS network stack exposes ports and packets come on to ports

Nevermind then :)

[adrianmonk]

You are partially right, though. The OS network stack does expose and handle ports if you use a protocol that has them.

Networks are built in layers. There's a physical layer underneath IP, then there's IP, and then there's TCP and UDP on top of IP.

The OS network stack has components that handle all of these layers. That's why it's called a stack.

Port numbers are part of the individual protocol (TCP or UDP) because there are a lot of things you can do with networking, and port numbers don't necessarily make sense with all of them.

For example, when you ping another computer, that uses ICMP, and there is no need for ports with ICMP. You're pinging the whole computer, not trying to connect with one of several applications running on it. So ports are not really needed.