Re: Multipath (was: Re: Re-chartering for extension work)

[Tommy Pauly <[e-mail]>]

> On Dec 20, 2019, at 5:30 AM, Olivier Bonaventure <[e-mail]> wrote:
> 
> Ryan,
> 
>>> On Wed, Dec 18, 2019 at 10:40 AM Olivier Bonaventure <[e-mail]> wrote:
>>> [snip]
>>> 
>>> A second use case is for mobile devices that need to preserve connectivity while switching from one wireless network to another. When such a device detects another wireless network, it needs to bet whether migrating the connection to the new network would improve performance. With a multipath transport, there is no need to bet as the transport will automatically select the best performing network. Apple’s deployment of Multipath TCP is a good example of the benefits of such a multipath strategy.
>> 
>> FWIW, QUIC connection migration was designed to do explicitly this and has been very successful in Google's deployment. I don't think this is a good justification for multipath QUIC.
>> 
> 
> Connection migration works well when there is a clear indication that one path is clearly broken. Unfortunately, there are many practical situations where paths are partially broken or experience high packet losses and this is where multipath really brings benefits.
> 
> Olivier

This depends on how migration is implemented, I expect. If an implementation only sends probes or migrates when it encounters significant loss or delay (or a loss of connectivity) on one path, then it can be too slow to react. The same goes for very conservative modes of MPTCP. 

However, with path probing, a QUIC migration implementation can track and monitor multiple paths, and can switch between them quickly if it wants. It can do this for any reason, either indication of slight degradation of loss, or just because the probes on one path seem to return fastest at the moment.

Fundamentally, the capabilities that “full” multipath enable on top of migration are:
- Bandwidth aggregation
- Separated congestion control
- The ability to split traffic (such as on a stream basis) across paths to take advantage of different link characteristics

These are all good to enable, but these are not the core benefits that devices right now are gaining from multipath, as much as the ability to migrate with agility. We should absolutely work on multipath, but it may be good to see how shipping migration (the first half of multipath) goes first. 

Best,
Tommy

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