Re: X-* header fields (Was: Getting 2822 to Draft)
2004-01-04 11:12:17
On 1/4/04 at 2:54 AM -0500, Bruce Lilly wrote:
Pete Resnick wrote:
The only difference then between 822 and 2822 in this respect is
that 822 gives publication guidelines for extensions where 2822
does not. This has absolutely *no* effect on implementations of the
protocol.
There is a minor effect. A user-defined field (per 822 definition)
can be recognized as such by examining the first two octets of the
field name, which is quite efficient.
But *why* would an implementation care about the publication
guidelines for the field? That's the *only* thing "X-" tells you.
"X-" serves to differentiate user-defined fields from non-defined
fields (i.e. those field names for which there is no IETF published
definition or which the implementation does not recognize).
Again, what use is there in making that distinction? Do you think the
fact that "X-Priority" and "X-Face" start with "X-" means that you
shouldn't support them in your implementation or they don't have
well-defined syntax? What about "X-Sender"? Is "List-ID" a
non-defined field if your implementation doesn't recognize it? What
useful purpose is there in differentiating fields which start with
"X-"?
In order to be at all useful to implementors, registration would
have to be contingent upon the existence of a stable, formal, public
definition of the proposed field's syntax (with ABNF) and semantics.
Nonsense. It is useful to an implementation (especially an
implementation that generates fields) to know that a field exists for
a particular purpose even if its syntax and semantics have not
undergone extensive review and comment or are still under
development. It is by implementation that fields get stable and then
they can be documented as standards.
It's difficult to see how such problems are either unique to X- or
are more of a hindrance to interoperability than for other fields.
As an example, consider "Status" which was in "private" use (I
believe) by BSD "mailx" decades ago, and which is currently in use
by several other MUAs, and which *does* leak out; there is also a
formal definition of a "Status" field -- incompatible with the
private usage -- defined as one of the delivery status notification
fields (RFC 3464). So neither leakage nor incompatibility seem to be
unique to X- fields. Note that if BSD mailx' author(s) had used
X-Status for private use, there would be no conflict with the formal
DSN Status field.
First of all, the "Status" field of DSN is not defined to appear in a
top-level header of a [2]822 message, so there is no
"incompatibility" between the two. But let's talk about the leakage:
Yes, the "Status" field leaks. Now, what can be done about that?
Well, it could be documented in an RFC so that everyone could know
what it means. And if turned out useful, it could be a standardized
top-level header field. But what if mailx's author had instead used
"X-Status"? In that case, it's DOA, because by definition it could
not be documented in a standard way. So there is no way for an
implementor to figure out what "X-Status" means other than by word of
mouth. That invites incompatibility. So what exactly would that "X-"
have gained you?
There is one very good reason to use X- for private or experimental
use, viz. interoperability. Use of X- as a field name prefix for
private or experimental use guarantees that there will be no
conflict with a formal field name. Use of other names can lead to
conflicts
No it doesn't. Some "X-" field names are now just as formal as some
non-"X-" field names. And I can come up with a list of non-"X-" field
names for which it would be *much* safer to have two incompatible
implemenations than some "X-" field names. The only thing the "X-"
guarantees you is long, ongoing conflicts. Using a non-"X-" field
name means that you can document one use as *the* standard way to do
something.
An issue not mentioned above is migration of X- fields to a formal
definition following successful experimental use. That obviously
entails a name change. However name changes are not uncommon, e.g.
refer to the MIXER RFCs, which define a number of fields whose names
have changed. That merely means that parsers need to recognize one
name as a synonym for another.
But what about generators? Because there will be parsers out there
that will only interpret the "X-" form of the field, the generators
must continue to send the "X-" form. Furthermore, updating some
parsers is non-trivial.
pr
--
Pete Resnick <http://www.qualcomm.com/~presnick/>
QUALCOMM Incorporated - Direct phone: (858)651-4478, Fax: (858)651-1102
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- Re: Getting 2822 to Draft, (continued)
- Re: Getting 2822 to Draft, Al Costanzo
- X-* header fields (Was: Getting 2822 to Draft), Pete Resnick
- Re: X-* header fields (Was: Getting 2822 to Draft), Arnt Gulbrandsen
- Re: X-* header fields (Was: Getting 2822 to Draft), ned+ietf-822
- Re: X-* header fields (Was: Getting 2822 to Draft), Charles Lindsey
- Re: X-* header fields (Was: Getting 2822 to Draft), ned+ietf-822
- Re: X-* header fields (Was: Getting 2822 to Draft), Bruce Lilly
- Re: X-* header fields (Was: Getting 2822 to Draft),
Pete Resnick <=
- Re: X-* header fields (Was: Getting 2822 to Draft), Keith Moore
- Re: X-* header fields, Kai Henningsen
- Re: X-* header fields, Keith Moore
- Re: X-* header fields, Russ Allbery
- Re: X-* header fields, Keith Moore
- Re: X-* header fields, ned+ietf-822
- Re: X-* header fields (Was: Getting 2822 to Draft), Bruce Lilly
- Re: X-* header fields (Was: Getting 2822 to Draft), Paul Smith
- Re: X-* header fields, Kai Henningsen
- Re: X-* header fields, Bruce Lilly
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