This document analyzes how current TCP implementations process TCP urgent indications and how the behavior of some widely deployed middleboxes affects how end systems process urgent indications. This document updates the relevant specifications such that they accommodate current practice in processing TCP urgent indications, raises awareness about the reliability of TCP urgent indications in the Internet, and recommends against the use of urgent indications (but provides advice to applications that do).
You can download it from the following link: https://packetstormsecurity.com/files/download/97864/rfc6093.txt
Source: https://packetstormsecurity.com/files/97864/RFC6093-On-The-Implementation-Of-The-TCP-Urgent-Mechanism.html