Re: [quicwg/base-drafts] Default settings in HTTP (#2038)

Mike Bishop <> Mon, 26 November 2018 19:33 UTC

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Subject: Re: [quicwg/base-drafts] Default settings in HTTP (#2038)
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@LPardue, I think the key point here is in the text added to the extensions section:  "SETTINGS does not provide a mechanism to identify when the choice takes effect."

There are really three types of extensions:
- **Advisory, purely-optional frames / streams:**  Send speculatively; if the peer doesn't understand, they'll ignore them.  Once you see SETTINGS, if it's not supported, you can stop sending to save bytes.
- **Non-optional semantic-changing frames / streams:**  Send only once you've seen SETTINGS, but okay to use as soon as you see SETTINGS -- you know the peer will handle them on receipt.  Need to be willing to handle "surprise" arrival of these frames / streams before SETTINGS, unless they're frames on control streams.
- **Breaking changes to interpretation of existing frames/streams:**  Here's the sticky one.  There's no coordination point to know when the change to existing elements takes effect, so the peer could interpret what you send under the old or new scheme unless the extension itself provides a way to identify this.

I think this really only harms the last category, and the easiest solution seems to be jumping into the previous category for most extensions I can envision.  For example, redefining DATA to be a reference to a unidirectional stream instead of carrying payload would fall in the third bucket and be hard.  But if you instead defined a new EXTERNAL_DATA frame, you've moved into the second bucket.

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