Please check this book "Computer Networking : A Top-Down Approach" By Kurose & Ross. It is the best textbook on computer networking, bar none. The latest edition also covers modern networking concepts, for example Software Defined Networks (SDN).

[1]https://www.amazon.com/Computer-Networking-Top-Down-Approach...

The Kurose/Ross textbook is also a favourite of mine.

Consider checking out https://book.systemsapproach.org/index.html as a free alternative as well.

The authors develop the book, as well as a few micro-books on topics like SDN, 5G, TCP congestion control, on github: https://github.com/SystemsApproach

They also have a blog with regular updates: https://www.systemsapproach.org/blog

And if you're curious about the motivation behind the project, they have a whitepaper: Open Source Networking Education: A Systems Approach - https://www.systemsapproach.org/uploads/4/7/7/4/47748257/sig...

Some pointers that might be useful:

https://beej.us/guide/bgnet/

https://github.com/srsLTE/srsLTE

https://www.gnuradio.org/grcon/grcon17/presentations/open_fi...

https://en.wikipedia.org/wiki/Asterisk_(PBX)

Don’t be afraid to study the source code if you’re curious about the inner workings of a system. Best of luck!

HPBN (High-Performance Browser Networking) is excellent. I can vouch for the print edition I bought a couple years ago, and the companion website gets updates: https://hpbn.co

Also, today thanks to this thread I discovered https://book.systemsapproach.org and can already recommend it.