MIME charset tags are registered with the IANA (http://www.iana.org/numbers.htm). The charset registry is a flat-file text database of records. Each record contains a charset name, reference citations, a unique MIB number, a source description, and a list of aliases. A name or alias may be flagged "preferred MIME name."

Here is the record for US-ASCII: 

Name: ANSI_X3.4-1968 [RFC1345, KXS2]^
MIBenum: 3
Source: ECMA registry
Alias: iso-ir-6
Alias: ANSI_X3.4-1986
Alias: ISO_646.irv:1991
Alias: ASCII
Alias: ISO646-US
Alias: US-ASCII (preferred MIME name)
Alias: us
Alias: IBM367
Alias: cp367
Alias: csASCII

The procedure for registering a charset with the IANA is documented in RFC 2978 (http://www.ietf.org/rfc/rfc2978.txt).

 

Hypertext Transfer Protocol (HTTP)