This is still the header! Main site

If Sockets had Paths

This is post no. 91 for Kev Quirk's #100DaysToOffload challenge. The point is to write many things, not to write good ones. Please adjust quality expectations accordingly :)

After all these posts about how TCP ports and their numbers are stupid... here is how it could have looked like.

Imagine if we just took the example of UNIX domain sockets, and extended them to work over The Internet.

As in: instead of, you would connect to

How does TCP work currently?

It's all happening over IP. (It's TCP/IP for a reason, after all.) Here is how a TCP header looks like: a TCP packet header; from Wikipedia, linked on click

All this is sitting in an IP packet, with the following header: an IP packet header; from Wikipedia, linked on click

(... you can find a nice combined depiction here.)

As you can see, to address a port, you need a 4-byte IP address (... for IPv6 at least) and a 2-byte port number. Which... has all kinds of problems:

We can't really just replace port numbers with arbitrary-length strings though: stuffing an entire string into each packet would be really wasteful.

The alternative

However... no one said that establishing a connection should be as cheap as sending packets through it!

Namely... what if there was two kinds of packets:

That way you could also have connections that don't necessarily break if one side changes IP addresses: if you get a packet from a different IP, you just update the address of your counterpart!

... at least you do so after ensuring that you're still talking to the same endpoint. Which probably needs some cryptography.

... so when will we get here?

Probably never. Since replacing already working protocols with new (even possibly better) ones is pretty much impossible.

Or... actually...

... have you ever looked at QUIC?

The protocol that does HTTP over UDP?

Because it's supposed to be lower latency and resistant to IP changes?

(Here is some more details of what's going on with it.)

Put together with HTTP/3 (... which is HTTP over QUIC, basically), it's an alternative stack that:

So, as it looks like, we managed to invent named-by-strings "sockets", except it's over UDP (which has its own port numbers), and per-application (instead of handling said strings on the OS level).

Also, it's probably a lot more complicated than as if it was implemented about 30 years ago. But... it does look like we did get there already!

... comments welcome, either in email or on the (eventual) Mastodon post on Fosstodon.