[cabfpub] [Trans] What's the load on a CT log?

Rob Stradling rob.stradling at comodo.com
Thu Mar 13 20:27:05 UTC 2014

I'm not sure average load tells the whole story.

Won't there be a surge in audit traffic in the aftermath of a busy site 
installing a new cert?

On 13/03/14 16:06, Ben Laurie wrote:
> Several people have asked me this recently. Here's a nice way to estimate load.
> Let's assume a single log that takes all the load.
> Firstly, we see about 5,000 new certificates a day, so that's around
> 0.06 new certificates per second. Clearly a trivial load.
> Next is load from audit (i.e. from browsers that wish to validate SCTs
> accompanying certificates they see). Given some assumptions, we can
> calculate the load from audit.
> * Clients cache audit results.
> * There are approximately b = 2.5B browsers in the world
> (http://www.internetworldstats.com/stats.htm).
> * The average user visits w = 89 websites a month
> (http://www.creditloan.com/blog/how-the-world-spends-its-time-online/
> quoting a Nielsen report). Assume these are all TLS sites.
> * Assume a certificate lifetime of l = 12 months.
> So, each user sees w / l new certificates a month. Each new
> certificate needs to be audited, which means in practice, three web
> operations (fetch STH, fetch STH consistency proof, fetch SCT
> inclusion proof) - it might be a good idea to create a new API to do
> all three in one go.
> So, total average load is 3 * b * w / l ~ 20,000 web fetches per
> second. If we optimise the API we can get that down to 7,000 qps. Each
> query (in the optimised case) would be around 3 kB, which gives a
> bandwidth of around 150 kb/s.
> Monitors add extra load, but should only be at around the new
> certificate rate - i.e. ~ .06 * number of monitors fetches per second.
> IMO, this is achievable on a single machine (modulo reliability), with
> some care. Clearly not a vast farm, however its done.
> In practice, no one log would have to take this full load, this is a
> worst case analysis.
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Rob Stradling
Senior Research & Development Scientist
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