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Re: X-* header fields

2004-01-21 16:50:55

On Sun, Jan 18, 2004 at 08:49:42AM -0800, ned+ietf-822(_at_)mrochek(_dot_)com 

Bruce Lilly wrote:

X- is used as an indicator for experimental or private-use tags in
many places
in Internet message protocols other than field names. For example:
The bottom line is that the success of x- fields in other places has anywhere
from disasterous at worst to mixed at best. And to a large extent the lack of
harm in many spaces has come from lack of use. So if you're looking for 
for keeping the X- distinction from the successful use of X- in other 
I'm afraid it is not forthcoming.

I don't quite understand why the X- headers or other similar X- stuff
must be successful. Aren't these simply meant for 'private', experimental
and test use? If I want to test out a new idea, if I want _my_ mailing
system A and _my_ mailing system B and _my friends_ mailing system C to
do something special, if I want to flag certain messages for special
handling in _my_ environment, then I'll use X- headers. All this may be
highly successful for me, but not for the world at large.

If some header-based feature or interoperation should work on a large
scale, not just in the environment I control, then I will need to define
and describe a non-X- header for it. The X- prefix only gives me
assurance that my experimental or internal stuff is not clashing with
any currently defined, widely used header, now or in the future.  The X-
header will not even guarantee that it will work through externally
controlled gateways, even though most of the time it will.

Anybody who uses X- headers or tags and expects large scale
interoperation is simply naive. I can easily make a rule that in my huge
private garden cars are to drive backwards. If I want them to do the
same on public roads, well, good luck.

I like the comparison between X- and 192.168.0.x. I can create a huge
internal network with 192.168.0.x IP addresses, I can even tunnel the
network over the public internet. This can all work perfectly well. But
I can not expect it to work outside of my private network, neither is it
accessible from the internet as is, nor is it guaranteed that it can
work with another private network using the same address range.

I thought RFC822 was quite clear about this:

  To  provide
  fields  with  a  measure  of  safety,  in name selection, such
  **extension-fields will never**
  have names  that
  **begin  with  the string "X-".**

  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.


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