[tsvwg] Re: Robustness to packet reordering

[Christian Huitema <[e-mail]>]

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