[Bloat] The Dark Problem with AQM in the Internet?

[Rich Brown]

Hi Jerry,

> AQM is a great solution for bufferbloat.  End of story.  But if you want to track down which device in the network intentionally dropped a packet (when many devices in the network path will be running AQM), how are you going to do that?  Or how do youpropose to do that?

Yes, but... I want to understand why you are looking to know which device dropped the packet. What would you do with the information?

The great beauty of fq_codel is that it discards packets that have dwelt too long in a queue by actually *measuring* how long they've been in the queue. 

If the drops happen in your local gateway/home router, then it's interesting to you as the "operator" of that device. If the drops happen elsewhere (perhaps some enlightened ISP has installed fq_codel, PIE, or some other zoomy queue discipline) then they're doing the right thing as well - they're managing their traffic as well as they can. But once the data leaves your gateway router, you can't make any further predictions.

The SQM/AQM efforts of CeroWrt/fq_codel are designed to give near optimal performance of the *local* gateway, to make it adapt to the remainder of the (black box) network. It might make sense to instrument the CeroWrt/OpenWrt code to track the number of fq_codel drops to come up with a sense of what's 'normal'. And if you need to know exactly what's happening, then tcpdump/wireshark are your friends. 

Maybe I'm missing the point of your note, but I'm not sure there's anything you can do beyond your gateway. In the broader network, operators are continually watching their traffic and drop rates, and adjusting/reconfiguring their networks to adapt. But in general, it's impossible for you to have any sway/influence on their operations, so I'm not sure what you would do if you could know that the third router in traceroute was dropping...

Best regards,

Rich
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