Proposal 160: Authorities vote for bandwidth offsets in consensus

[Roger Dingledine]

On Tue, May 12, 2009 at 10:05:11AM -0400, Nick Mathewson wrote:
> It makes more sense just to let bandwidth mean bandwidth.  If we want
> to have measured bandwidth count for more than reported bandwidth,
> let's have an optional flag on the vote line that looks like:
> 
>    w Bandwidth=X Measured=1
> 
> This way the median actually -is- the median.

Sounds like a fine plan.

> Suggested revision: If there are at least MIN_MEASURING (say, 3)
> voters that say "measured" about a given banwidth observation, then we
> take the median of their observations as the bandwidth of the router.
> Otherwise, we proceed as in the current dir-spec.txt for that router.

Ok.

> * We'd like to avoid having little changes in measured bandwidth
>   result in changes to the consensus, since we'd like to be able to
>   transfer consensus diffs.  Thus, let's round our votes to the
>   first N significant bits.
>   
>   In other words, if we've observed a bandwidth of 28789 bytes for a
>   node, that's  111 0000 0111 0101.   We round that down to 111 0000
>   0000 0000, and declare 26872.
> 
>   This is better than rounding to the nearest 1k, since a 1k change is
>   very significant for low values, and relatively frequent for high
>   values.

Ok, but another constraint here is that users want to look at the numbers
in the consensus and find them intuitive. Can we take the resulting
number (26872) and round it off to 26? That will take care of an unending
stream of users wondering why the heck it says 26872. As a side effect,
the status stanzas get smaller too.

--Roger