Explicit Congestion Notification (ECN) and Controlled Delay (CoDel)

[Sebastian Kummer]

Hi folks,

At the last WWDC, Stuart Cheshire talked about Explicit Congestion Notification (ECN -> RFC 3168) and Controlled Delay (CoDel) and how they used it to improve their screen sharing application. In short, they use this existing TCP/IP feature to overcome buffer bloat without experiencing packet loss. Additionally they can reduce latency in overused networks. 

You can watch the talk or download the slides at https://developer.apple.com/videos/wwdc/2015/?id=719 (interesting part starting at about 29:00)

After a quick scan of the WebRTC stack, I found that the usrsctplib has some code to handle an ECN echo, but I was not yet able to find out if the WebRTC itself does something with this information (e.g. reducing screen resolution). However, (in my understanding) if that already would be the case, the LEAKY_BUCKET flag then should not make such a difference like it actually does, because we would net experience buffer bloat / packet loss.

Am I missing something here? 

[Stefan Holmer]

Webrtc uses RTP for screencasting (handled as regular video). We currently don't use ECN when doing bandwidth estimation and congestion control for RTP streams.

What you are referring to is the webrtc DataChannels (sctp), and the leaky bucket/pacer of webrtc doesn't affect DataChannels.

I have not yet been able to look at the link you sent since it requires a mac, but I will take a look when I get a chance.

Thanks,

/Stefan

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[Sebastian Kummer]

Hi Stefan,

Thanks for your clarifications.

To watch the video on non-apple devices, you can download it here:

- HD Video:    http://devstreaming.apple.com/videos/wwdc/2015/719ui2k57m/719/719_hd_your_app_and_next_generation_networks.mp4?dl=1

- PDF Slides: http://devstreaming.apple.com/videos/wwdc/2015/719ui2k57m/719/719_your_app_and_next_generation_networks.pdf?dl=1

[Justin Uberti]

It's unclear whether ECN info is exposed to userland, or used at all for non-TCP flows. Do you know if there is an API in iOS9 for this?

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[Harald Alvestrand]

Ecn has been in linux kernels for years. Used to default off, think it may have changed recently. Dont rrmrmber abt Windows support. Codel is å routeer feature.

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[Sebastian Kummer]

I went through the talk once again and it became clear that there is no need to request ECN support in the application itself - ECN will just be enabled by default in iOS 9 and OSX 10.11 (http://wan.poly.edu/pam2015/papers/4.pdf predicts a minimal risk). Hopefully, other platforms will follow that priciple if Apple's experiments succeeds.

Unfortunately, it seems like they are only speaking about TCP, but not about other protocols like SCTP or UDP (-> https://tools.ietf.org/html/rfc6679)

[Justin Uberti]

That's what I would expect. That said, enabling ECN for TCP is a significant step forward in the journey of getting ECN working everywhere.

On Tue, Jun 23, 2015 at 5:32 PM, Sebastian Kummer <sebastian...@gmail.com> wrote:

I went through the talk once again and it became clear that there is no need to request ECN support in the application itself - ECN will just be enabled by default in iOS 9 and OSX 10.11 (http://wan.poly.edu/pam2015/papers/4.pdf predicts a minimal risk). Hopefully, other platforms will follow that priciple if Apple's experiments succeeds.

Unfortunately, it seems like they are only speaking about TCP, but not about other protocols like SCTP or UDP (-> https://tools.ietf.org/html/rfc6679)

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[Sebastian Kummer]

By the way: Microsoft enabled ECN by default in Windows Server 2012 - see https://technet.microsoft.com/en-us/library/hh997028.aspx

Seems like they are ready too.