TCP/IP Client (Ephemeral) Ports and Client/Server Application Port Use
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Ephemeral Port Number Assignment
Ephemeral port numbers are assigned as needed to processes by the TCP/IP software. Obviously, each client process running concurrently needs to use a unique ephemeral port number, so the TCP and UDP layers must keep track of which are in use. These port numbers are generally assigned in a pseudo-random manner from a reserved pool of numbers. I say pseudo-random because there is no specific meaning to an ephemeral port number assigned to a process, so a random one could be selected for each client process. However, since it is necessary to reuse the port numbers in this pool over time, many implementations use a set of rules to minimize the chance of confusion due to reuse.
Consider a client process that just used ephemeral port number 4,121 to send a request, received a reply, and then terminated. Suppose we immediately reallocate 4,121 to some other process. However, the server accessed by the prior user of port 4,121 for some reason sent an extra reply. It would go to the new process, creating confusion. To avoid this, it is wise to wait as long as possible before reusing port number 4,121 for another client process. Some implementations will therefore cycle through the port numbers in to ensure the maximum amount of time elapses between consecutive uses of the same ephemeral port number.
The range of port numbers that is used for ephemeral ports on a device also depends on the implementation. The classic ephemeral port range was established by the TCP/IP implementation in BSD (Berkeley Standard Distribution) UNIX, where it was defined as 1,024 to 4,999, providing 3,976 ephemeral ports. This seems like a very large number, and it is indeed usually more than enough for a typical client. However, the size of this number can be deceiving. Many applications use more than one process, and it is theoretically possible to run out of ephemeral port numbers on a very busy IP device. For this reason, most of the time the ephemeral port number range can be changed. The default range may be different for other operating systems.
Just as well-known and registered port numbers are used for server processes, ephemeral port numbers are for client processes only. This means that the use of a range of addresses from 1,024 to 4,999 does not conflict with the use of that same range for registered port numbers as seen in the previous topic.
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