What do those XXX blocks mean

From Stevens TCP/IP Illustrated Vol 2:

We will see the comment /* XXX */ throughout Net/3. It is a warning to the
reader that the code is obscure, contains nonobvious side effects, or is
quick solution to a more difficult problem.

The second volume of that series might be one of the best networking books ever written. Not because it is a good tome to learn networking from, it is instead a guide into the heart of a real system. It is close enough today to use as a starting point for finding out where things are and a step to finding out why they are.

It is where I go when I want to find out how my current machines get bytes from an application to packets on the wire.