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X-* header fields (Was: Getting 2822 to Draft)

2004-01-03 10:40:35

On 1/3/04 at 11:00 AM -0500, Al Costanzo wrote:

I don't think anyone would have a problem with a small note in the "Changes to previous standards" section which says, "Some people think X-* fields are bad, some think they are good, they were removed from this document." For a fuller discussion, I think Keith is right that we might consider a separate informational document.

If they are not part of this document it cannot replace/obsolete 822.

I'm sorry, but I think this is rubbish. X-* fields are perfectly legal in 2822 (they fall under "optional-field"); they simply are not given the special treatment of 822, that they won't ever be official published extensions to the standard. 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. I defy anyone to explain to me why this should have any effect on 2822bis obsoleting 822.

Now, let me use this opportunity to explain *why* I don't think X-* fields should be in the standard in the first place:

The idea behind X-* fields (what 822 called user-defined fields) was so that folks could use new field names without worrying that they would interfere with the standard fields. That way, you didn't have to go through the trouble of publishing an RFC just to use a field that was only going to be internal to your particular system. The downside was that someone else could come along and potentially use the same field name in a completely incompatible way, but that was the risk that you took.

But since then, two things have made it clear that such user-defined fields are unnecessary and problematic:

1. Unnecessary: We now have IANA registration services. We could very simply create an IANA registry of e-mail field names. Then, if you wanted to create a new field, you just fill out the registration form and you're not only guaranteed that you won't interfere with published standard fields, you're also guaranteed that nobody else would use the field you chose in an incompatible way. And you wouldn't have to have a name that started with "X-".

2. Problematic: Some folks thought it was nice to have "X-*" fields for completely private use. But history teaches us that inevitably "X-*" fields *will* leak out onto the rest of the Internet. As several people in this thread have pointed out, we now have some "X-*" fields that are in widespread use. Sometimes, people use them very interoperably and they have served a fabulous purpose. Sometimes, they don't get used interoperably. It would be great if we could publish some standards to make sure that they do get used interoperably. But guess what: We can't! It is forbidden by RFC 822 to publish any standard for a field name that beings with "X-". No matter how widespread the use, no matter how useful the field is, it can not be standardized *by definition*. That decreases interoperability on the Internet.

Now, personally, I would love the document to say, "There is no good reason to use fields that begin with 'X-' as defined by RFC 822; any name that does not interfere with a currently registered or published field name is fine, and using an 'X-' field is discouraged because they can't ever be standardized." But there was widespread non-consensus in the DRUMS Working Group when this was discussed. 2822 does say "Extension header fields no longer specifically called out." In 2822bis, I think it would be fine to clarify that and say "User-defined header fields (those starting with 'X-') no longer specifically discussed", and even have an informational reference to a document (or 2) on why it was pulled. But I see absolutely no reason that this should stop 2822bis from moving to full Standard and obsoleting 822.

Pete Resnick <>
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