Web application ephemeral port contention

Each client/server connection has a unique identifier (known as a quad) that includes an ephemeral port number that will later be reassigned. Each operating system has a range of numbers that it uses as ephemeral ports (for example, on Windows the range is 1024 through 4999).

The operating system allocates ephemeral ports when a new socket is set up.

If the range is relatively small and you are making several requests per page in parallel, you can run out of port numbers. At that point the ephemeral port numbers assigned by the operating system start colliding with ones already in use as they are recycled too quickly, and subsequent connections will be aborted.