【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