Re: p2: Considerations for new headers

Julian Reschke <julian.reschke@gmx.de> Thu, 25 April 2013 14:44 UTC

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Date: Thu, 25 Apr 2013 16:41:46 +0200
From: Julian Reschke <julian.reschke@gmx.de>
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To: Mark Nottingham <mnot@mnot.net>
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Subject: Re: p2: Considerations for new headers
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On 2013-04-24 10:06, Julian Reschke wrote:
> On 2013-04-24 10:03, Mark Nottingham wrote:
>> We should consider adding the following to the laundry list of
>> considerations in p2 8.3.1:
>>
>> * Whether the field should be stored by origin servers that understand
>> it upon a PUT request.

<http://trac.tools.ietf.org/wg/httpbis/trac/changeset/2227>

>> Furthermore, I think we should change:
>>
>> * How the header field might interact with caching (see [Part6]).
>>
>> to:
>>
>> * When the header is used in requests and affects response selection
>> [ref], it is good practice to advise listing that header in the Vary
>> response header [ref].

<http://trac.tools.ietf.org/wg/httpbis/trac/changeset/2227> (slightly 
rephrased).

>> Finally, we should add (near the top of the section):
>>
>> """
>> New header fields cannot change the semantics of a message in an
>> incompatible fashion. That is, it is not possible to require
>> recipients to understand a header field through its mere presence.
>> However, new methods and status codes can require the presence of
>> headers in their definitions, in the scope of the message they occur
>> within.
>> """
>>
>> Make sense?
> ...

I think the consequences of the first sentence are not totally clear.

- you can set a new header field on a message, but you can not rely on 
the recipient looking at it (because it's "must ignore")

- you could require the presence of a new header field on a request 
using a new method, or on a response using a new status code

...but then, you could require it in other cases as well (think a new 
auth scheme, a successful upgrade, an applied Preference...).

Not sure how to explain this better...

Best regards, Julian