Ask HN: Books on Implementing Network Protocols

Hey everyone. I'm looking for some comprehensive and (preferably) readable material on implementing protocols. By this I mean, taking an RFC document and translating that into working code.

I'd also be interested in seeing open source implementations; but the problem with much of what I've already found is that the code is not written for educational purposes and thus does not implement things in a very straightforward (for me anyway) way, and the code gets bogged down in edge cases and unnecessary complexities for someone who just wants to get a sense of the protocol in its purest form.

Specifically, I'd like to write an SMTP client and server because I think it would be cool to get something like that working.

I've already gone through Advanced Programming in the UNIX Environment and it isn't exactly what I'm looking for.

Thank you so much!