存在哪些 GZip 额外字段子字段？

Question

RFC 1952（GZIP 文件格式规范）第 2.3.1.1 节内容如下：

2.3.1.1. Extra field

   If the FLG.FEXTRA bit is set, an "extra field" is present in
   the header, with total length XLEN bytes.  It consists of a
   series of subfields, each of the form:

      +---+---+---+---+==================================+
      |SI1|SI2|  LEN  |... LEN bytes of subfield data ...|
      +---+---+---+---+==================================+

   SI1 and SI2 provide a subfield ID, typically two ASCII letters
   with some mnemonic value.  Jean-Loup Gailly
   <email@hidden> is maintaining a registry of subfield
   IDs; please send him any subfield ID you wish to use.  Subfield
   IDs with SI2 = 0 are reserved for future use.  The following
   IDs are currently defined:

      SI1         SI2         Data
      ----------  ----------  ----
      0x41 ('A')  0x70 ('P')  Apollo file type information

   LEN gives the length of the subfield data, excluding the 4
   initial bytes.

AP除了RFC 中给出的之外，是否还存在任何子字段类型？网络搜索找不到列表；GZip 的维基百科页面、GNU 主页、gzip 源代码或 Stack Overflow 上都没有提及。

Answer 1

据我所知，没有维护这样的注册表。Jean-loup 不再使用 gzip。