This document describes a simple and efficient method for random selection of a client port number, such that the possibility of an attacker guessing the exact value is reduced. While this is not a replacement for cryptographic methods, the described port number randomization algorithms provide improved security/obfuscation with very little effort and without any key management overhead. The mechanisms described in this document are a local modification that may be incrementally deployed, and that does not violate the specifications of any of the transport protocols that may benefit from it, such as TCP, UDP, SCTP, DCCP, and RTP.
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Source: https://packetstormsecurity.com/files/61586/draft-ietf-tsvwg-port-randomization-00.txt.html