Illustrated TCP/IP Illustrated TCP/IP
by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568   Pub Date: 11/01/98  

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Chapter 17
Requirements in RFCs

The first RFCs led to ambiguity in the protocol; not everyone reads and interprets alike. Therefore, most RFCs have the following to indicate precisely what should be implemented and what is optional:

MUST. This word or the adjective “REQUIRED” means that the item is an absolute requirement of this specification.
MUST NOT. This phrase means the item is an absolute prohibition of this specification.
SHOULD. This word or the adjective “RECOMMENDED” means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications should be understood and the case carefully weighed before choosing a different course.
SHOULD NOT. This phrase means that there may exist valid reasons in particular circumstances when the listed behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighted before implementing any behavior described with this label.
MAY. This word or the adjective “OPTIONAL” means that this item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because it enhances the product. Another vendor may omit the same item.

Requirements in RFCs

•  MUST—The word or adjective “REQUIRED” means that the item is an absolute requirement of this specification.
•  MUST NOT—This phrase means the item is an absolute prohibition of this specification.
•  SHOULD—The word or the adjective “RECOMMENDED” means that there may exist valid reason in particular circumstances to ignore this item, but the full implications should be understood and the case carefully weighed before choosing a different course.
•  SHOULD NOT—This phrase means that there may exist valid reasons in particular circumstances when the listed behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.
•  MAY—This word or the adjective “OPTIONAL” means that this item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because it enhances the product, while another vendor may omit the same item.


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