Strictly talking about code quality, I will nominate RCP100, which is a small, virtually unknown, now-abandoned routing software written in C [0]. I started programming with C way back in the 90s, and this is one of only two projects I can recall being immediately struck by the beauty of the code (Redis being the other). I know almost nothing about the author but he seems not to want to be known by name. You can browse the source on Github [1], which I uploaded myself, since you can only get a tarball from sourceforge. Anyway, as someone else mentions, C is usually a mess, but RCP100 struck me as beautiful.
Thanks for uploading RCP100. Your comment is a timely one. I wanted to learn how a router works and is built and was looking for a simpler implementation.
Can you recommend any resources from which I could learn more about network programming, so that I could understand RCP100 code better?
[0] http://rcp100.sourceforge.net/
[1] https://github.com/curtiszimmerman/rcp100