Network Working Group S. Bradner Request for Comments: 2119 Harvard University BCP: 14 March 1997 Category: Best Current Practice
This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Distribution of this memo is unlimited.
In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. Authors who follow these guidelines should incorporate this phrase near the beginning of their document:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
Note that the force of these words is modified by the requirement level of the document in which they are used.
Imperatives of the type defined in this memo must be used with care
and sparingly. In particular, they MUST only be used where it is
actually required for interoperation or to limit behavior which has
potential for causing harm (e.g., limiting retransmisssions) For
example, they must not be used to try to impose a particular method
on implementors where the method is not required for
These terms are frequently used to specify behavior with security
implications. The effects on security of not implementing a MUST or
SHOULD, or doing something the specification says MUST NOT or SHOULD
NOT be done may be very subtle. Document authors should take the time
to elaborate the security implications of not following
recommendations or requirements as most implementors will not have had the benefit of the experience and discussion that produced the specification.
The definitions of these terms are an amalgam of definitions taken from a number of RFCs. In addition, suggestions have been incorporated from a number of people including Robert Ullmann, Thomas Narten, Neal McBurnett, and Robert Elz.
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