[twic]

It's defined in an RFC. The original RFC is from 1995, at which time RFCs were fairly readable, although still very detailed, documents, so that's worth a look:

https://tools.ietf.org/html/rfc4506.html

The backstory to XDR is that it fell out of Sun's development of NFS. Sun engineers realised that rather than defining a protocol for NFS from scratch, they could define a generic remote procedure call mechanism (SunRPC, which became ONC RPC), and then define NFS as an application protocol on top of that. An RPC mechanism needs a serialisation format, and XDR is that format.

I get the impression that this was part of a project build a whole new world of IDL-specified RPC-based protocols that would replace the janky, ad-hoc, mostly-textual protocol suite that existed at the time - telnet, rlogin, FTP, SMTP, etc.DCE and CORBA were rival attempts at around the same time. It was an interesting time.

Of course, what actually happened was HTTP. gRPC/HTTP2 is this idea finally coming back into fashion.