头字段注册在“消息头”注册表中，保留在http://www.iana.org/assignments/message-headers/

    本文档定义了下列HTTP头字段，所以他们关联的注册项目已经被更新为常驻注册项如下（查看BCP90）：

    1.    +---------------------+----------+----------+-------------+
    2.    | Header Field Name   | Protocol | Status   | Reference   |
    3.    +---------------------+----------+----------+-------------+
    4.    | ETag                | http     | standard | Section 2.3 |
    5.    | If-Match            | http     | standard | Section 3.1 |
    6.    | If-Modified-Since   | http     | standard | Section 3.3 |
    7.    | If-None-Match       | http     | standard | Section 3.2 |
    8.    | If-Unmodified-Since | http     | standard | Section 3.4 |
    9.    | Last-Modified       | http     | standard | Section 2.2 |
    10.    +---------------------+----------+----------+-------------+

    变更控制者是“IETF (iesg@ietf.org) - 互联网工程任务组”。