Re: Multi-path QUIC Extension Experiments

[Robin MARX <[e-mail]>]

Hello Yunfei,

Thanks for your extended explanations on Multipath HoL-blocking and
especially:

> I think the stream dependencies you mentioned here is a great point. In
our implementation, we introduced a stream-priority based reinjection which
tries to address such dependency (There is a figure in the material that
Yanmei sent). But we haven't tried when each stream is limited to a single
path. In our case, streams are distributed on multiple paths. I would
definitely want to hear more about the application you are dealing with,
and maybe for wired transport, such a design is needed.

This is exactly what I was trying to explore in my previous mail. You're
basically intentionally causing (or perhaps risking?) HOL blocking because
you split a single stream over multiple paths.
As noted by Christian with the 'equal cost multipath', this can have
bandwidth usage benefits, but only if paths are usable/similar. If not, HOL
blocking might undo all the benefits you get from this setup (and using a
single path per stream would be better).
So my question was: where is the inflection point where you might decide to
switch modes? At which parameters is one better than the other?
I'd hoped you would have experimented with the fixed-path-per-stream setup
to get some insight into this.

In my mind, the idea of doing a purely transport-level multipath scheduler
(i.e., without taking into account application layer streams / data
dependencies / etc.)
has historically made some sense for TCP / for completely separated stacks,
as the transport didn't have that type of information available.
It is however utterly strange to me that this approach would continue for
QUIC (at least in endpoint multipath, not things like in-network
aggregators that have been discussed),
where we have clear splits between streams and (hopefully) already some
type of prioritization information for each stream.
For QUIC, I'd expect one-path-per-stream to be the default, with
multiple-paths-per-stream to be an edge case if you have a single,
high-traffic stream (which I do assume is your situation with a video
stream).

With best regards,
Robin




On Tue, 20 Jul 2021 at 09:15, Lars Eggert <[e-mail]> wrote:

> On 2021-7-20, at 1:19, Roberto Peon <fenix=[e-mail]> wrote:
> >
> > If we have to send data along a path in order to discover properties
> about that path, then sending less data on the path means discovering less
> about that path.
> >
> > The ideal would be to send *enough* data on any one path to maintain an
> understanding of its characteristics (including variance), and no more than
> that, and then to schedule the rest of the data to whichever path(s) are
> best at the moment.
>
> ^^^ This.
>
> Because the Internet has no explicit network-to-endpoint signaling, an
> endpoint must build its understanding of the properties of a path by
> exercising it, and specifically exercising it to a degree that causes
> queues to form (to obtain "under load" RTTs, see bufferbloat) and
> congestion loss to happen (to obtain an understanding of available path
> capacity.) Some people have called this "putting pressure on a path".
>
> There has been a long-standing assumption that if you exercised a path in
> the (recent) past you can probably assume that the properties haven't
> changed much if you want to start exercising it again. This is why
> heuristics like caching path properties (RTTs, etc.) are often of benefit -
> often, but not always, and maybe never in some scenarios (e.g.,
> overcommitted CGNs.)
>
> There has been some work on this in the past for MPTCP. For example, on
> mobile devices - which most often have multiple possible paths to a
> destination via WiFi and cellular - exercising multiple paths comes at a
> distinct increase in energy usage. So you need a heuristic to determine if
> the potential benefit of going multipath is worth the energy cost of
> probing multiple paths before you do so.
>
> Thanks,
> Lars
>
>

-- 

dr. Robin Marx
Postdoc researcher - Web protocols
Expertise centre for Digital Media

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