[tor-dev] tor's definition of 'median'

[teor]

> On 14 Aug 2015, at 03:10 , nusenu <nusenu at openmailbox.org> wrote:
> 
>> Changing the code to return the mean of the two center elements from
>> even arrays would break all authority voting, and wouldn't actually be
>> useful.
> 
> Yes, that is what Sebastian said on IRC as well. Can you shed some light
> as to why it would break voting?

If the authorities supply different values in the consensus, voting breaks.

Authorities using the low-median would supply one value, and authorities using the mean-median would supply another value. (Authorities typically run different versions of tor, and don't upgrade all at once.)

Breaking changes like this are typically negotiated among the authorities using numbered consensus methods. Once enough authorities support a new consensus method, it is activated during voting.

Rather than creating a new consensus method to implement mean-median, it's much easier to patch the documentation to specify low-median. (And I see no significant gain in changing from low-median to mean-median.)

I'd rather see bandwidth measurements become more accurate, for more relays, more of the time, than change how their median is defined.

Tim

Tim Wilson-Brown (teor)

teor2345 at gmail dot com
pgp ABFED1AC
https://gist.github.com/teor2345/d033b8ce0a99adbc89c5

teor at blah dot im
OTR D5BE4EC2 255D7585 F3874930 DB130265 7C9EBBC7

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