OpenPGP and usability
wk at gnupg.org
Sun Aug 12 12:51:32 CEST 2007
On Fri, 10 Aug 2007 18:34, seh at panix.com said:
> Was there some change in this prescription? If so, from where? I hadn't
> heard about "X-" falling from use.
The current standard, RFC2822 does not mention it anymore:
3.6.8. Optional fields
Fields may appear in messages that are otherwise unspecified in this
standard. They MUST conform to the syntax of an optional-field.
This is a field name, made up of the printable US-ASCII characters
except SP and colon, followed by a colon, followed by any text which
conforms to unstructured.
The field names of any optional-field MUST NOT be identical to any
field name specified elsewhere in this standard.
and the change notices say:
11. Extension header fields no longer specifically called out.
In contrast the old rfc822 has very detailed information about extension
A limited number of common fields have been defined in
this document. As network mail requirements dictate, addi-
tional fields may be standardized. To provide user-defined
fields with a measure of safety, in name selection, such
extension-fields will never have names that begin with the
Names of Extension-fields are registered with the Network
Information Center, SRI International, Menlo Park, California.
Individual users of network mail are free to define and
use additional header fields. Such fields must have names
which are not already used in the current specification or in
any definitions of extension-fields, and the overall syntax of
these user-defined-fields must conform to this specification's
rules for delimiting and folding fields. Due to the
extension-field publishing process, the name of a user-
defined-field may be pre-empted
Note: The prefatory string "X-" will never be used in the
names of Extension-fields. This provides user-defined
fields with a protected set of names.
My conclusion is that X- was never required by the standard and that
after the 19 years the IETF realized that there was no need for it.
There is an obvious advantage of not using "X-": If the use of such an
optional field later makes it into a standard, the software does not
need to be enhanced to cope with the then standardized field
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