Re: [Webtransport] Measuring the friction of Http3Transport

[Ian Swett <[e-mail]>]

I'm *very* concerned about any functionality that *requires* pooling.
Pooling is a client-specific optimization and pooling policy is subject to
change(within reason) at any time.  If applications are saying "it'd be
cool if we could share congestion control and loss recovery state between
H3 and WebTransport most of the time", then I can see there being
real(albeit small) benefits, just like there would be for putting
DataChannels and RTP into one QUIC connection for WebRTC.  If people are
intending to rely on pooling so an HTTP(hopefully H3) request can trigger a
WebTransport response, I think we're in a world of pain and
over-optimization.

This is more obvious in light of the fact that a WebTransport endpoint is
likely not a generic CDN endpoint, as has been clearly stated by Mark
Nottingham.

Bernard's comments above point to a tighter coupling between HTTP/3 and
WebTransport than I think is practical in production, unless I
misunderstood(which I may have, it's been a long day).

All of this makes me think we should punt the HTTP based WebTransport
options down the road a year or so and focus on QuicTransport initially.
It's not to say we won't do them, but they're clearly more complex
than QuicTransport and it'd be good to have real deployment experience
prior to starting so we know what the requirements are and where the
dragons are lurking.  Also, the W3C should design APIs that make it easy to
detect failure to establish a QUIC-based WebTransport, and this ensures
those are well exercised.  Maybe we DO require pooling under certain
conditions and describe those, but there are likely privacy concerns under
some circumstances and I'm not enough of an expert to know what would be
acceptable to all clients.

Some people in the WG may think we have to release a TCP based WebTransport
at the same time as a QUIC based one, but I don't think that's true and
even if we standardized them at the same time, it doesn't mean both will
achieve wide deployment at the same time.  As I said at the mic, I don't
see any reason YouTube would deploy a TCP based WebTransport given it will
have to maintain an HTTP/1.1 option for basically forever and they've spent
a good bit of effort to make sure that works as well as it can.  It's clear
to me there are compelling use cases right now for QuicTransport, but the
driving use cases for the fallback are not clear to me.  Consistency is
intellectually appealing, but doesn't drive deployments.

Ian

On Mon, Nov 16, 2020 at 6:36 PM Bernard Aboba <[e-mail]>
wrote:

> David said:
>
> "Where does this come from? A lot of QUIC servers are implementing QUIC
> datagrams, and once you have those the delta with h3 datagrams is
> negligible. Additionally, multiple h3 server implementers have indicated
> they want to support WebTransport. It's starting to feel like the pushback
> on HTTP3Transport is purely based on a perception of complexity from folks
> who haven't tried to implement it? If that's the case, then we should get
> some implementation experience to confirm or debunk that theory."
>
> [BA] The pooling use cases often involve an interaction between an HTTP/3
> request/response and WebTransport.  so the HTTP/3 Request doesn't just
> elicit an HTTP/3 Response.  It also causes something else to happen,
> something that isn't specified in the HTTP/3 protocol, such as eliciting a
> datagram flow or bi/unidirectional stream from the server.  This is what is
> contemplated in several of the "Live Event" use cases, where the HTTP/3
> Request/Response would be requesting the desired Event. The major advantage
> over HLS is avoiding Head-of-Line blocking, reducing video freezes and
> stalls.  So developers understand the value that support for datagrams and
> server-initiated streams can provide - the question is whether they'll have
> all the interoperable pieces in place to deploy it.
>
> Just because a generic HTTP/3 server supports QUIC and WebTransport,
> doesn't mean that it will support these use cases.  I've been asked whether
> better interoperability and wider ecosystem support could be achieved by
> extending HTTP/3 in a way more in keeping with the existing protocol design
> - such as by allowing an HTTP/3 Request to specify a resource to be
> returned in a flow of datagrams.
>
> The situation might be more tenable if the node.js framework had HTTP/3
> and QUIC modules on the horizon.
>
> On Mon, Nov 16, 2020 at 1:00 PM David Schinazi <[e-mail]>
> wrote:
>
>>
>>
>> On Mon, Nov 16, 2020 at 12:40 PM Bernard Aboba <[e-mail]>
>> wrote:
>>
>>> Luke said:
>>>
>>> "My primary concern is that Http3Transport requires non-generic
>>> modifications to the HTTP/3 layer mostly to support connection pooling. I'm
>>> worried that this will limit support for Http3Transport and complicates any
>>> HTTP/3 implementation that does support it."
>>>
>>> [BA] I have also heard this concern from developers. The concerns
>>> primarily arise from the pooling of HTTP/3 Request/Responses with
>>> WebTransport. This does enable some interesting use cases, such as a
>>> Request/Response (e.g. gRPC) that can result in a server sending datagrams
>>> or initiating bi-directional or uni-directional streams.  So there is
>>> perceived value, but the question is whether that value will be widely
>>> supported across the HTTP/3 ecosystem.
>>>
>>> At this point, most generic HTTP/3 servers are not expected to support
>>> HTTP/3 datagrams (not part of the HTTP/3 protocol) or Http3Transport
>>> (perceived as a niche protocol), so that pooling would only be possible
>>> when using a framework.  node.js will not have QUIC support for 12-18
>>> months (held up by OpenSSL) so even framework support is expected to be
>>> limited.
>>>
>>
>> Where does this come from? A lot of QUIC servers are implementing QUIC
>> datagrams, and once you have those the delta with h3 datagrams is
>> negligible. Additionally, multiple h3 server implementers have indicated
>> they want to support WebTransport. It's starting to feel like the pushback
>> on HTTP3Transport is purely based on a perception of complexity from folks
>> who haven't tried to implement it? If that's the case, then we should get
>> some implementation experience to confirm or debunk that theory.
>>
>>
>>> On Mon, Nov 16, 2020 at 11:37 AM David Schinazi <
>>> [e-mail]> wrote:
>>>
>>>>
>>>>
>>>> On Mon, Nov 16, 2020 at 11:34 AM Luke Curley <[e-mail]>
>>>> wrote:
>>>>
>>>>> We're on the same page that there's code in HTTP/3 that could be
>>>>> leveraged for QuicTransport in the form of the header parsing. I think
>>>>> there's a path to merge QuicTransport and Http3Transport by leveraging
>>>>> HTTP/3 for the handshake and QUIC for stream delivery.
>>>>>
>>>>> My primary concern is that Http3Transport requires non-generic
>>>>> modifications to the HTTP/3 layer mostly to support connection pooling. I'm
>>>>> worried that this will limit support for Http3Transport and complicates any
>>>>> HTTP/3 implementation that does support it.
>>>>>
>>>>
>>>> Hi Luke,
>>>>
>>>> I'm curious, could you elaborate on what these "non-generic
>>>> modifications to the HTTP/3 layer" are please?
>>>>
>>>> Thanks,
>>>> David
>>>>
>>>>
>>>>> On Mon, Nov 16, 2020 at 4:49 AM Lucas Pardue <
>>>>> [e-mail]> wrote:
>>>>>
>>>>>> Hi Luke,
>>>>>>
>>>>>> On Mon, 16 Nov 2020, 12:26 Luke Curley, <[e-mail]> wrote:
>>>>>>
>>>>>>> Just to clarify, when I say "QuicTransport is simpler" I really mean
>>>>>>> "QuicTransport has greater interoperability". You can take any QUIC
>>>>>>> implementation (+ datagram extension) and add a thin layer to support
>>>>>>> QuicTransport. It's a nice layering of protocols and it's something that I
>>>>>>> want to see in general as QUIC replaces TCP.
>>>>>>>
>>>>>>
>>>>>> Thanks for the clarification. I appreciate the sentiment. But given
>>>>>> the development of QUIC its kind of surprising that people would find it
>>>>>> easier to identify an implementation that supports QUIC and datagram but
>>>>>> does not support HTTP/3.
>>>>>>
>>>>>> Thr situation for a more clean-room implementation is different, I
>>>>>> agree. However, by the time you've done all the hard stuff with QUIC, a
>>>>>> very focused HTTP/3 layer ends up quite thin too. You can ignore dynamic
>>>>>> compression, server push and extensibility points.
>>>>>>
>>>>>> I also see the current design being the thin end of the wedge. The
>>>>>> proposal I linked talks about HTTP-like header parsing. So its "just" a
>>>>>> thin layer to do some parsing. Then folks will probably want some want to
>>>>>> tweak parameter, acontrol channel, a way to do graceful close, a sprinkle
>>>>>> of GREASE. To extrapolate forward, what would be the delta be between some
>>>>>> final product QuicTransport and HTTP/3. Too small a delta and we've wasted
>>>>>> years of effort.
>>>>>>
>>>>>> Cheers
>>>>>> Lucas
>>>>>>
>>>>>>
>>>>>>
>>>>>>> On Mon, Nov 16, 2020 at 3:16 AM Lucas Pardue <
>>>>>>> [e-mail]> wrote:
>>>>>>>
>>>>>>>> Hi folks,
>>>>>>>>
>>>>>>>> To follow up on one thread of discussion during IETF 109. To over
>>>>>>>> simplify what we've heard a few folks say, they prefer QuicTransport
>>>>>>>> because it simpler and they don't need pooling.
>>>>>>>>
>>>>>>>> I'd like to push more on this, what is the measurable complexity of
>>>>>>>> HTTP/3 over QUIC plus a new application mapping that needs to be defined?
>>>>>>>> Let's isolate pooling as a variable and ignore it.
>>>>>>>>
>>>>>>>> Victor mentioned his proposal for a unified header semantic across
>>>>>>>> all transports, this is
>>>>>>>> https://github.com/ietf-wg-webtrans/draft-ietf-webtrans-overview/pull/4/files.
>>>>>>>> Taking a closer look at it, I have some concerns that I added as comments.
>>>>>>>> HTTP has semantics for a reason. If QuicTransport is going to start
>>>>>>>> borrowing HTTP piecemeal, I really wonder how simple people might feel it
>>>>>>>> is. I think it's important to avoid something that looks like HTTP but
>>>>>>>> behaves differently. Is it really much of a stretch to just do HTTP/3 but
>>>>>>>> give it a different ALPN so that we avoid problems related to
>>>>>>>> cross-protocol pooling?
>>>>>>>>
>>>>>>>> Cheers,
>>>>>>>> Lucas
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