Re: question about QPACK decode framing

Dmitri Tikhonov <dtikhonov@litespeedtech.com> Thu, 01 November 2018 15:43 UTC

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Date: Thu, 1 Nov 2018 11:43:14 -0400
From: Dmitri Tikhonov <dtikhonov@litespeedtech.com>
To: Brian Swander <briansw@microsoft.com>
Cc: Matthew Cox <macox@microsoft.com>, "quic@ietf.org" <quic@ietf.org>
Subject: Re: question about QPACK decode framing
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On Thu, Nov 01, 2018 at 03:27:09PM +0000, Brian Swander wrote:
> From our read of the current drafts:
>
> It seems that in the drafts that the HEADERS frame is only for the
> compressed headers being sent on a real stream.  The encoder and decoder
> streams for QPACK are just the raw encodings as described in the QPACK
> spec without extra framing.  If so, then on the decoding on the decoder
> stream, there is no way to know (a priori) that we have a complete
> set of headers, since there is no overall header length available.

QPACK uses two unidirectional control streams: one called the
encoder stream and the other, the decoder stream.  Each side
opens both, so there are four streams altogether.

The encoder stream carries instructions that modify the dynamic
table.  The header sets are placed into header blocks which are
carried in HQ HEADERS frame on the bidirectional request/response
streams.  The encoder stream does *not* carry header sets.

With regards to framing: the encoder stream does not have framing.
For discussion for the rationale for the lack of encoder stream
framing, see this thread:

  https://www.ietf.org/mail-archive/web/quic/current/msg04229.html

  - Dmitri.