[Cerowrt-devel] [aqm] chrome web page benchmarker fixed
dave.taht at gmail.com
Fri Apr 18 15:40:52 EDT 2014
> The specific thing I've been concerned about was not the probability of
> a dns loss, although as you note the consequences are huge -
> but the frequency and cost of a cache miss and the resulting fill.
> This is a very simple namebench test against the alexa top 1000:
> This is a more comprehensive one taken against my own recent web history file.
> Both of these were taken against the default SQM system in cerowrt
> against a cable modem, so you can
> pretty safely assume the ~20ms (middle) knee in the curve is basically
> based on physical
> RTT to the nearest upstream DNS server.
> And it's a benchmark so I don't generally believe in the relative hit
> ratios vs a vs "normal traffic", but do think the baseline RTT, and
> the knees in the curves in the cost of a miss and file are relevant.
To be utterly clear here, the ~20ms knee in this data is a variable dependent
on the RTT to the nearest upstream DNS server. Most - nearly all -
ISPs - have a hefty dns server in their co-location facility, but the
baseline physical RTT is dependent on the actual technology in use.
The actual RTT of an query is dependent on the outstanding queue
length in a single queue AQM, unless otherwise prioritized. If prioritized
(cerowrt's 3 band system does this for queries coming from the router)
I imagine the packet loss rate drops hugely, also.
To give an extreme example of the DNS rtt problem, dns lookups over
satellite links take
800+ms, and this is one reason why web proxy servers are so common
in such environments as the whole query is shipped to a local-to-the-internet
proxy server so as to avoid this rtt cost. This technique is of increasingly
limited value in an age of e2e encryption.
Also: recently we've seen increasing use of non-local or otherwise redirected
dns servers such as here
It would be a good research project for someone to catagorize typical
nearest-upstream DNS RTTs, the availability of local-to-site dns
servers, hit/miss ratios
in homes and small business, the cost of dnssec, etc.
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