Re: Proposal: drop QPACK encoder stream framing

[Martin Thomson <[e-mail]>]

I think that this is a reasonable request and I'd support this.  It
saves an encoder from having to buffer and it even saves a few octets.
The complexity increase (the increased need to delay acknowledgments)
is manageable and even encourages good practice - the current design
encourages a little bit of laziness because you can acknowledge at the
end of each block and have a reasonable expectation of it being
approximately correct... most of the time.
On Thu, Jun 7, 2018 at 5:11 PM Dmitri Tikhonov
<[e-mail]> wrote:
>
> Abstract
> --------
>
> The encoder block specified in Section 3.3 of draft-ietf-quic-qpack-00
> is unnecessary.  For reasons of efficiency, I propose that the framing
> be removed: let the encoder instructions be written and interpreted as
> a stream instead.
>
> Background
> ----------
>
> I brought this up during the first day (June 6) of the Kista Interim.
> Alan explained that there are two reasons for framing the instructions
> in this way on the encoder stream:
>
>     1. Historically, some (or most) HPACK decoders have struggled with
>        input that breaks in the middle of an instruction, as they do
>        not maintain state.
>
>     2. The blocks can be used as a clue to send Table State Synchronize
>        instructions on the decoder stream (Section 3.4.1).
>
> The downside to having length-prefixed blocks is that it makes it
> difficult to optimize encoding by writing to the outgoing packets
> directly.  One has to buffer the block, write the block length first,
> and then copy the block.
>
> Additionally, there is a potential for a deadlock (issue #1420).  Recall
> that QPACK owes its current form to the fact that the WG decided to
> prevent deadlocks at the compression protocol level, and not via advisory
> notes to the implementers.
>
> Reasons Behind the Block
> ------------------------
>
> First, let's look at the historical HPACK chokers.
>
> The HPACK decoder only processes HEADER blocks.  There is only one stream
> and thus only one HEADER block can be processed at a time.
>
> The QPACK decoder has to process two inputs: a) the encoder stream
> and b) HEADER blocks.  There is only one encoder stream.  This stream
> modifies the decoder state: the dynamic table.  The HEADER blocks do not
> modify the state.
>
> The HEADER blocks already come with a prefix length (HQ framing), and so
> QPACK implementations can choose to wait until the whole HEADER block is
> available.  It is processing of the encoder stream that is under
> consideration.
>
> With regards to the Table Synch signaling, the draft states that
>
>  "                                                         A decoder MAY
>  " coalesce multiple synchronization updates into a single update.
>
> This effectively means that the encoder already should not expect to get
> a one-to-one mapping between its Encoder Blocks and Table Synchs.
>
> Working with the Encoder Stream
> -------------------------------
>
> The dynamic table instruction stream can be thought of as its own state.
> Already, the QPACK decoder must be able to pause reading from streams when
> a blocked HEADERS block arrives.  Similar techniques can be used to resume
> reading from the encoder stream.  It should not be difficult to resume
> decoding a single instruction!  While breaking with the HPACK precedent
> (the hold-my-hand-I-need-buffer-size chokers), it keeps in line with the
> Seattle hum that the new compression mechanism is free from its HPACK
> shackles.
>
> Conclusion
> ----------
>
> Prefixing arbitrary sets of encoder instructions with a length denies
> zero-copy optimization opportunities.  At the cost of some slight
> complexity increase at the decoder, this framing mechanism can be
> dropped.  The proposed change is limited to just this -- technically
> superfluous -- piece.
>
>   - Dmitri.
>
> P.S.  An added bonus is some saved framing bytes on the encoder stream.
>       I believe the "compression performance above everything else" is
>       also the Working Group's consensus (Melbourne).
>