Network Working Group S. Bradner
Internet-Draft Harvard University
August 1996
Key words for use in RFCs to Indicate Requirement Levels
<draft-bradner-key-words-02.txt>
Status of this Memo
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Abstract
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. Note that the force of these words is
modified by the requirement level of the document in which they are
used.
1. MUST This word, or the adjectives "REQUIRED" or "SHALL", means that
the definition is an absolute requirement of the specification.
2. MUST NOT This phrase, or the phrase "SHALL NOT", means that the
definition is an absolute prohibition of the specification.
3. SHOULD This word, or the adjective "RECOMMENDED", means that there
may exist valid reasons in particular circumstances to ignore a
particular item, but the full implications must be understood and
carefully weighed before choosing a different course.
4. SHOULD NOT This phrase means that there may exist valid reasons in
particular circumstances when the particular behavior is acceptable
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Internet-Draft RFC Key Words August 1996
or even useful, but the full implications should be understood and
the case carefully weighed before implementing any behavior described
with this label.
5. MAY This word, or the adjective "OPTIONAL", means that an item is
truly optional. One vendor may choose to include the item because a
particular marketplace requires it or because the vendor feels that
it enhances the product while another vendor may omit the same item.
An implementation which does not include a particular option MUST be
prepared to interoperate with another implementation which does
include the option, though perhaps with reduced functionality. In the
same vein an implementation which does include a particular option
MUST be prepared to interoperate with another implementation which
does not include the option.(except, of course, for the feature the
option provides)
6. Guidance in the use of these Imperatives
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
interoperability.
6. Security Considerations
These terms are frequently used to specify options or behavior in a
way that can effect security risks. Careful consideration should be
taken to understand the security implications of any use of these
imperatives.
7. Acknowledgments
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, and Robert Elz.
8. Author's Address
Scott Bradner
Harvard University
1350 Mass. Ave.
Cambridge, MA 02138
phone - +1 617 495 3864
email - sob@harvard.edu
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