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Protocol Control Block for anyone else wondering https://www.oreilly.com/library/view/tcpip-illustrated/02016...

You still need to keep your unacknowledged data buffered somewhere. If the kernel isn't holding it then it's in userspace.

More specifically no PCB, for UDP itself.

It's not nil. For "connected" UDP sockets, it's smaller than TCP's, but not nil because, well, buffers. And for non-connected UDP sockets there's still buffers. The main thing is that you can have much less buffer space because you might always be willing to drop packets. Ultimately you can have much lower memory pressure from those buffers and the smaller PCBs.