The Postel Principle Patch:

--- ietf/postels-principle
+++ ietf/postels-principle
  
- Be liberal about what you accept.

+ Be definite about what you accept.(*) 
+
+ Treat inputs as a language, accept it with a matching computational
+ power, generate its recognizer from its grammar.
+
+ Treat input-handling computational power as privilege, and reduce it
+ whenever possible.
+
+
+ (*) For the sake of your users, be definite about what you accept.
+ Being liberal worked best for simpler protocols and languages,
+ and is in fact limited to such languages; be sure to keep your
+ language regular or at most context free (no length fields).    
+ Being more liberal did not work so well for early IPv4 stacks: 
+ they were initially vulnerable to weak packet parser attacks, and 
+ ended up eliminating many options and features from normal use. 
+ Furthermore, presence of these options in traffic came to be regarded 
+ as a sign of suspicious or malicious activities, to be mitigated by 
+ traffic normalization or outright rejection. At current protocol 
+ complexities, being liberal actually means exposing the users of your 
+ software to intractable or malicious computations.