2. 本文约定
BCP 14 关键词 MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY 和 OPTIONAL 仅当全部大写出现时, 才按 RFC 2119 和 RFC 8174 解释.
BCP 14 关键词 MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY 和 OPTIONAL 仅当全部大写出现时, 才按 RFC 2119 和 RFC 8174 解释.