[tsvwg] Re: Robustness to packet reordering

[[e-mail]]

I agree with Christian. My recollection is that movement -> reaction is about 20 ms. Working interactivity (note a change, note own reaction) is more relevant for gaming and other interactive apps, than picture quality. 13ms isn't that small then.

Regards,

Ruediger

-----Ursprüngliche Nachricht-----
Von: Christian Huitema <[e-mail]> 
Gesendet: Mittwoch, 12. Februar 2025 21:08
An: [e-mail]; Sebastian Moeller <moeller0=[e-mail]>
Cc: [e-mail]; Ryan Hamilton <rch=[e-mail]>; Martin Thomson <[e-mail]>; Greg White <g.white=[e-mail]>; Ingemar Johansson S <ingemar.s.johansson=[e-mail]>; [e-mail]
Betreff: [tsvwg] Re: Robustness to packet reordering


On 2/11/2025 11:39 PM, [e-mail] wrote:
>> In a later post you assess that 13ms reordering window acceptable and in accordance with the above recommendations, so let me ask, what is the threshold for acceptable and non-acceptable (max) reordering-induced delays?
> My metric is that delay increase matter only when they are a significant fraction of the current message latency. 13ms probably under 25% of most Internet delays, if not lower. It’s also a small fraction (about 1/6) of the delay a human would notice (100ms). So its impact is small both relatively (25%) and as an absolute (1/6 of notiicible).


I think the modern latency numbers are much smaller. RTT of connections going though a CDN is typically lower than 35ms, and in many cases lower than 15ms. Adding 13ms to that is quite a big penalty.

The delay that humans can notice is largely depending of task and context. The often cited figure of 150ms relates to turn taking in natural audio conversation, when silence after saying a sentence is the main clue that the current speaker has stopped speaking. If the latency is greater than 150ms, most people notice it and the communication becomes awkward. There are other important figures, like a latency of 10ms for hand-eye coordination, which is very relevant in video-games . 
There are even stronger requirements for virtual reality, when even a few milliseconds between detecting an eye movement and "repainting" the screen . Adding 13 ms would be very noticeable in video games, and would be a complete non starter for remote virtual reality displays.

Then there is machine communication. Displaying a web page, for example, may well require series of transactions to fetch data from separate databases. compose images, etc. Adding 13ms to several of these transactions will have a cumulative effect, very noticeable by humans.

Modern Internet application do require low delays. Gratuitously adding delays "because we always did delay correction at L2" is a very bad idea.

-- Christian Huitema