They explicitly don't go into what happens on the routers between sender and receiver. Congestion control is a huge part of TCP's design. Don't know how common

They explicitly don't go into what happens on the routers between sender and receiver. Congestion control is a huge part of TCP's design. Don't know how common it is these days, but that is another place where packets will just get dropped. TCP packets get dropped too, but detecting that, resending, and then throttling back is built into the protocol. With UDP you either don't care or you handle that differently higher in the stack.