Re: [TLS] Application-Layer Protocol Settings

Victor Vasiliev <> Tue, 21 July 2020 20:22 UTC

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From: Victor Vasiliev <>
Date: Tue, 21 Jul 2020 16:22:30 -0400
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To: David Benjamin <>
Cc: Lucas Pardue <>, "" <>, HTTP Working Group <>
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Subject: Re: [TLS] Application-Layer Protocol Settings
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How would this work with regular SETTINGS?  HTTP/2 and HTTP/3 have disjoint
setting number spaces, and it's unclear to me whether there's any
significant overlap between those.

On Tue, Jul 21, 2020 at 11:49 AM David Benjamin <>

> On Tue, Jul 21, 2020 at 8:22 AM Lucas Pardue <>
> wrote:
>> On Mon, Jul 20, 2020 at 10:42 PM David Benjamin <>
>> wrote:
>>> On Mon, Jul 20, 2020 at 5:00 PM Lucas Pardue <
>>> <>> wrote:
>>>> That makes sense but I guess I don't see the point in defining a new
>>>> thing that contains frames that are never sent on streams. That is, if
>>>> these are connection settings, just send the payload. Unframed extended
>>>> settings might get you there, if you can find a way to encapsulate
>>>> conventional settings inside them, then all the better.
>>> Could you elaborate on this a bit? I'm probably just failing to parse,
>>> but I'm not sure which alternative you're suggesting here. (Ah, the wonders
>>> of email.)
>>> David
>> I was trying to accommodate HTTP/2 and HTTP/3 in one breath, which is why
>> my intent was probably unclear. Basically, if ALPS relies on frames for
>> per-protocol settings then it has to accommodate the differences in frame
>> format between HTTP/2 and HTTP/3. In the examples from the ALPS and Client
>> Reliability proposals, the H2 frame needs to populate the frame header and
>> it pick stream 0, which doesn't exist until the connection is actually
>> made, so seems a bit kludgy. In H3, frames don't have the stream ID so you
>> avoid the problem above.
>> So my thought was to basically do away with the notion of
>> protocol-specific frames in ALPS, and instead define the a common payload
>> format that perhaps looks something like bishop-extended-settings [1], a
>> series of Type-Length-Value (but without any frame headers). This would
>> allow you to encode the old and new settings in a single format, rather
>> than needing to delineate things via frames.
>> [1] -
> Ah, gotcha. The thinking was the settings were ALPN-specific anyway, so we
> may as well define them however is more idiomatic for the protocol. This
> means we automatically can make existing H2 and H3 settings more reliable.
> Settings values can also be updated over the course of the connection, so
> using frames keeps continuity there. But, yeah, a separate key/value syntax
> would work too.
> (A small correction, the current Client Hint Reliability proposal allows
> ACCEPT_CH to be sent in application data too. Maybe the frontend realizes
> the origin's ACCEPT_CH preferences have changed and wants to notify
> existing connections. Though I don't consider this feature important. I
> doubt most folks, if anyone, will bother with this. Mostly that's how a
> SETTINGS or EXTENDED_SETTINGS value already would have worked, so I figured
> the semantics ought to be compatible in case EXTENDED_SETTINGS is revived.)
> David