commit 28a8208232cac218b9bbbd95521ad7bd3c266b37 Author: Isis Lovecruft isis@torproject.org Date: Thu Aug 3 18:19:12 2017 +0000

prop271: Note Paul's concerns on guard sampling biases from Wilmington. --- proposals/271-another-guard-selection.txt | 17 +++++++++++++++++ 1 file changed, 17 insertions(+)

diff --git a/proposals/271-another-guard-selection.txt b/proposals/271-another-guard-selection.txt index f03a7c3..a0fba19 100644 --- a/proposals/271-another-guard-selection.txt +++ b/proposals/271-another-guard-selection.txt @@ -178,6 +178,23 @@ Implemented-In: 0.3.0.1-alpha if they have the "Guard" flag. Sampling is random but weighted by bandwidth.

+[Paul Syverson in a conversation at the Wilmington Meeting 2017 says that +we should look into how we're doing this sampling. Essentially, his +concern is that, since we are sampling by bandwidth at first (when we +choose the `sampled` set), then later there is another bias—when trying to +build circuits (and hence marking guards as confirmed) we select those +which completed a usable circuit first (and hence have the lowest +latency)—that this sort of "doubly skewed" selection may "snub" some +low-consensus-weight guards and leave them unused completely. Thus the +issue is primarily that we're not allocating network resources +efficiently. Mine and Nick's guard algorithm simulation code never +checked what percentage of possible guards the algorithm reasonably +allowed clients to use; this would be an interesting thing to check in +simulation at some point. If it does turn out to be a problem, Paul's +intuition for a fix is to select uniformly at random to obtain the +`sampled` set, then weight by bandwidth when trying to build circuits and +marking guards as confirmed. —isis] + Once a path is built and a circuit established using this guard, it is marked as confirmed. Until this point, guards are first sampled and then filtered based on information such as our current