【rfc2119】Key words for use in RFCs to Indicate Requirement Levels

前面四个还比较好理解。

1. MUST This word, or the terms "REQUIRED" or "SHALL", mean that the

definition is an absolute requirement of the specification.

绝对要要求的

MUST== REQUIRED==SHALL

2. MUST NOT This phrase, or the phrase "SHALL NOT", mean that the

definition is an absolute prohibition of the specification.

MUST NOT == SHALL NOT

绝对要禁止的

3. SHOULD This word, or the adjective "RECOMMENDED", mean 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.

SHOULD == RECOMMENDED

是说，在特定的情形下，存在一些有根据的原因，可以忽略一些条目，

但是在做选择的时候，要全面的理解含义，小心的权衡。

4. SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that
there may exist valid reasons in particular circumstances when the
particular 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.

SHOULD NOT == NOT RECOMMENDED ，是说可能存在某些特定情形下合理的原因使得这种特定的行为是可以被接受或者甚至是有用的。

5. MAY This word, or the adjective "OPTIONAL", mean 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.)

MAY == OPTIONAL 意思是这个条目是可选的。

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.

7. Security Considerations
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.

8. 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, Neal McBurnett, and Robert Elz.