Re: Multi-path QUIC Extension Experiments

[Spencer Dawkins at IETF <[e-mail]>]

Hi, Christian,

On Tue, Sep 21, 2021 at 1:31 PM Christian Huitema <[e-mail]>
wrote:

> On 9/21/2021 9:37 AM, Spencer Dawkins at IETF wrote:
>
> > I have been maintaining a collection of various goals for path selection
> > (most recently, in
> >
> https://datatracker.ietf.org/doc/html/draft-dawkins-quic-multipath-selection-01#section-3
> ).
> > It's not a short list.
> >
> > If we're also including user preferences that would include knowing the
> > difference between cellular connections that are metered, cellular
> > connections that are unmetered but being throttled, and cellular
> > connections that are unmetered and unthrottled, and cross-matching them
> > with wifi connections that are likely to work better than cellular
> > connections vs. wifi connections that are only intermittently performing
> > well, that's not a small amount of complexity.
>
> This leads to the problem of asymmetric knowledge between client and
> server. On the client, applications can use system APIs and user
> preferences to identify which connections are "metered, cellular
> connections that are unmetered but being throttled, and cellular
> connections that are unmetered and unthrottled." But the server side
> does not know that. Yet in many scenarios most of the traffic volume
> originates from the server, and the scheduling choices of the server may
> cause increased bills or exhaustion of quotas for the client. Which
> implies that there should be some way for the client to signal how the
> server shall consider connection.
>

Yes, very much so. My point was that this is complex, and your point about
asymmetric knowledge is well-taken. The idea that "clients" (UEs) know
things that "servers" (in this case, UPFs) don't know is also popping up in
3GPP ATSSS discussions (for example, in clause 5.32.8 of 3GPP TS 23.501
V17.1.1):

*UE-assistance indicator: *This indicator may be provided only when the
Steering Mode is Load-Balancing. When provided by the network, it indicates
that (a) the UE may decide how to distribute the UL traffic of the matching
SDF based on the UE's internal state (e.g. when the UE is in the special
internal state, e.g. lower battery level), and that (b) the UE may inform
the UPF how it decided to distribute the UL traffic of the matching SDF. In
the normal cases, although with this indicator provided, the UE shall
distribute the UL traffic as indicated by the network.



> I think this "knowledge sharing" is independent from "application
> preferences". The work at Ali Baba shows that there is a lot of benefits
> in incorporating application states in scheduling decisions. For
> example, if the video streaming application notices that the video
> buffers are emptying too fast, it might tell the server to start using
> more bandwidth, even if that means using metered connections. But that
> application state, "video buffers are emptying", is not the same as the
> network state, "path 3 is on a metered connection".
>

I like this (especially because what you're talking about is meaningful to
applications and independent of changes in underlying network technologies
along a path).


> I don't think that we will be able to fully standardize the way
> applications pass their preferences. That's why in draft-liu the
> "QOE_CONTROL_SIGNALS" is mostly used to pass application-defined
> signals. But we might be able to have a reasonable set of codes (or
> priorities) explaining the "state of a path", and use that in the
> "PATH_STATUS" frame.


You and I have shared the hope that we would be able to describe building
blocks that applications can use, rather than define new codes every time
someone comes up with a new way of distributing packets across multiple
paths, for a while now -
https://datatracker.ietf.org/doc/html/draft-dawkins-quic-what-to-do-with-multipath-03#section-2.4
was based on a discussion we had, and that was submitted last January.

I hope the QUIC working group's return to multipath discussions gives us a
chance to make progress on that.

Best,

Spencer