On Tue, 28 Apr 2015, Sebastian Moeller wrote:
I consider induced latencies of 30ms as a "green" band because that is
the outer limit of the range modern aqm technologies can achieve (fq
can get closer to 0). There was a lot of debate about 20ms being the
right figure for induced latency and/or jitter, a year or two back,
and we settled on 30ms for both, so that number is already a
compromise figure.
Ah, I think someone brought this up already, do we need to make
allowances for slow links? If a full packet traversal is already 16ms can we
really expect 30ms? And should we even care, I mean, a slow link is a slow
link and will have some drawbacks maybe we should just expose those instead of
rationalizing them away? On the other hand I tend to think that in the end it
is all about the cumulative performance of the link for most users, i.e. if
the link allows glitch-free voip while heavy up- and downloads go on, normal
users should not care one iota what the induced latency actually is (aqm or no
aqm as long as the link behaves well nothing needs changing)
It is highly likely that folk here are not aware of the extra-ordinary
amount of debate that went into deciding the ultimate ATM cell size
back in the day. The eu wanted 32 bytes, the US 48, both because that
was basically a good size for the local continental distance and echo
cancellation stuff, at the time.
In the case of voip, jitter is actually more important than latency.
Modern codecs and coding techniques can tolerate 30ms of jitter, just
barely, without sound artifacts. >60ms, boom, crackle, hiss.
Ah, and here is were I understand why my simplistic model from above
fails; induced latency will contribute significantly to jitter and hence is a
good proxy for link-suitability for real-time applications. So I agree using
the induced latency as measure to base the color bands from sounds like a good
approach.
Voice is actually remarkably tolerant of pure latency. While 60ms of jitter
makes a connection almost unusalbe, a few hundred ms of consistant latency
isn't a problem. IIRC (from my college days when ATM was the new, hot
technology) you have to get up to around a second of latency before
pure-consistant latency starts to break things.
Gaming and high frequency trading care about the minimum latency a LOT. but most
other things are far more sentitive to jitter than pure latency. [1]
The trouble with bufferbloat induced latency is that it is highly variable based
on exactly how much other data is in the queue, so under the wrong conditions,
all latency caused by buffering shows up as jitter.
David Lang
[1] pure latency will degrade the experience for many things, but usually in a
fairly graceful manner.
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