RE: Implicitly opened streams and exposing stream IDs

[Mikkel Fahnøe Jørgensen <[e-mail]>]

Isn’t this between QUIC, the QUIC application API, and the application
protocol?

If you tell QUIC to feed streams in order, just like you it to feed data
inside a stream in order, then it can virtually open streams in order
stream towards the API without affecting the network.

QUIC is also permitted to deliver data out of order inside a stream AFAIR,
although this would not be the usual API behaviour.

In the end, specific broadly accepted app protocols like QUIC/HTTP will
drive the API design.

For other use cases such as some small brief message streams and some heavy
data loaded streams, you might very well want to have the latest stream
arrive first, to process priority messages. Saying that QUIC can never, as
opposed to usually, expose the stream ID seems pointless. It is like saying
a QUIC api can never expose holes in a single streams data stream.

I don’t really see an issue on the wire, and I believe removing implicit
open takes a way some fear, uncertainty and doubt about stream resource
handling - at least I never felt completely comfortable with the stream
design until now.


Regards,
Mikkel


On 2 April 2018 at 14.06.04, Lucas Pardue ([e-mail]) wrote:

Hi Marten,

Would Stream headers fix this problem? I.e. anything that requires special
behaviour other than "bulk data" has some bytes of of magic at the start of
the stream.

Regards
Lucas
________________________________________
From: QUIC [[e-mail]] on behalf of Marten Seemann [
[e-mail]]
Sent: 02 April 2018 12:43
To: QUIC WG
Subject: Implicitly opened streams and exposing stream IDs

Recently, the implicit opening of streams (i.e. that when a receiver
receives a frame for stream N+4, it can assume that stream N was already
opened, but the packet might have been reordered) was removed from the
draft. I think this change has some consequences that we haven't discussed
so far.

For the purpose of writing some pseudocode, I'm assuming a QUIC API that
provides an AcceptStream() method, but the conclusions will be the same for
a callback-based API.

* If QUIC has implicit stream opening, AcceptStream() would return the
streams in order (and if a frame opening stream N+4 is received before
stream n is opened, AcceptStream() will first return stream N and then
stream N+4).
* Without implicit stream opening, AcceptStream() just returns whatever
stream is received first. Streams might be returned in arbitrary order, if
the peer doesn't open streams consecutively or if packets are reordered.

Now imagine an application protocol where the first unidirectional stream
opened by the client is a control stream, and all higher unidirectional
streams are data streams. The application on the server side needs to find
out which stream is the control stream, because it needs to be handled
separately.

With implicit stream opening, the server code would be:

control_stream = AcceptStream() // is guaranteed to open the first stream
// handle the control stream
while true:
stream = AcceptStream()
// handle the data stream

and without implicit stream opening:

while true:
stream = AcceptStream()
if stream.ID() == kControlStreamID:
// handle the control stream
else:
// handle the data stream

In this case, after accepting a stream, we first have to check the stream
ID, since there's no guarantee if the control stream will actually be
received first.

For this stream mapping, it seems like the removal of implicitly opened
streams implies that QUIC has to expose stream IDs to the application
layer. I'm not sure if this was intended when making the change, especially
since we're considering to change HQ such that it doesn't rely on QUIC
stream IDs any more.
We only manage to avoid the problem described here in our HTTP mapping,
because the HTTP control streams are unidirectional and the request streams
are bidirectional, and can therefore be told apart by their directionality.
However, as a general transport protocol, other applications built on top
of QUIC will have to find some way to deal with it.


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