MUST, SHOULD 의 차이 + ??

권고안을 보다 아주 기본적으로 필요하기 정리한다.
RFC 2119 에 잘 나와있네.ㅋ

1. MUST   This word, or the terms "REQUIRED" or "SHALL", mean that the
  definition is an absolute requirement of the specification.

2. MUST NOT   This phrase, or the phrase "SHALL NOT", mean that the
  definition is an absolute prohibition of the specification.

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.

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.


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.)