Andrew S. Tanenbaum Edition 5th Exercise 6 Question 19 (Page No. 608)

Question

Both UDP and TCP use port numbers to identify the destination entity when delivering
a message. Give two reasons why these protocols invented a new abstract ID (port
numbers), instead of using process IDs, which already existed when these protocols
were designed.

Answer 1

Here are three reasons.

First, process IDs are OS-specific. Using process IDs would have made these protocols OS-dependent.

Second, a single process may establish multiple channels of communications. A single process ID (per process) as the destination identifier cannot be used to distinguish between these channels.

Third, having processes listen on well-known ports is easy, but well-known process IDs are impossible.

Source: https://www.coursehero.com/file/pqras1/9-Both-UDP-and-TCP-use-port-numbers-to-identify-the-destination-entity-when/