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March 2017

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Proposal 276: Report bandwidth with lower granularity in consensus documents
by Nick Mathewson 01 Mar '17

01 Mar '17

Filename: 276-lower-bw-granularity.txt Title: Report bandwidth with lower granularity in consensus documents Author: Nick Mathewson Created: 20-Feb-2017 Status: Open Target: 0.3.1.x-alpha 1. Overview This document proposes that, in order to limit the bandwidth needed for networkstatus diffs, we lower the granularity with which bandwidth is reported in consensus documents. Making this change will reduce the total compressed ed diff download volume by around 10%. 2. Motivation Consensus documents currently report bandwidth values as the median of the measured bandwidth values in the votes. (Or as the median of all votes' values if there are not enough measurements.) And when voting, in turn, authorities simply report whatever measured value they most recently encountered, clipped to 3 significant base-10 figures. This means that, from one consensus to the next, these weights very often and with little significance: A large fraction of bandwidth transitions are under 2% in magnitude. As we begin to use consensus diffs, each change will take space to transmit. So lowering the amount of changes will lower client bandwidth requirements significantly. 3. Proposal I propose that we round the bandwidth values as they are placed in the votes to two no more than significant digits. In addition, for values beginning with decimal "2" through "4", we should round the first two digits the nearest multiple of 2. For values beginning with decimal "5" though "9", we should round to the nearest multiple of 5. This change does not require a consensus method; it will take effect once enough authorities have upgraded. 4. Analysis The rounding proposed above will not round any value by more than 5%, so the overall impact on bandwidth balancing should be small. In order to assess the bandwidth savings of this approach, I smoothed the January 2017 consensus documents' Bandwidth fields, using scripts from [1]. I found that if clients download consensus diffs once an hour, they can expect 11-13% mean savings after xz or gz compression. For two-hour intervals, the savings is 8-10%; for three-hour or four-hour intervals, the savings only is 6-8%. After that point, we start seeing diminishing returns, with only 1-2% savings on a 72-hour interval's diff. [1] https://github.com/nmathewson/consensus-diff-analysis 5. Open questions: Is there a greedier smoothing algorithm that would produce better results? Is there any reason to think this amount of smoothing would not be save? Would a time-aware smoothing mechanism work better?

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