Roger Dingledine arma at mit.edu Mon Jul 23 18:58:54 UTC 2012 [snip]
At the same time, much of our performance improvement comes from better load balancing -- that is, concentrating traffic on the relays that can handle it better. The result though is a direct tradeoff with relay diversity: on today's network, clients choose one of the fastest 5 exit relays around 25-30% of the time, and 80% of their choices come from a pool of 40-50 relays.
[snip]
From what I see on the TorStatus pages (torstatus.all.de, blutmagie.de) about a third of the roughly 3000 relays listed are at or below 64KB/sec of demonstrated bandwidth. No doubt some of these are soon-to-be-high-bandwidth servers that are just ramping up, and some are nodes having transitory networking problems. It seems reasonable to assume, though, that most of these low-bandwidth nodes are intentionally low-bandwidth, perhaps on the basis of the Tor doc stating a 20KB/sec minimum.
With "80% of their choices come from a pool of 40-50 relays" that leaves a 20% chance for the remaining 2950 nodes. A case for low-bandwidth nodes can be made as a means to dissuade anticipated routing (due to pool size), but it seems from the stats quoted above that there is little chance that 2000+ of these 3000 nodes will ever carry Tor traffic, and thus can be ignored for purposes of traffic analysis.
Is there any justification for a low-bandwidth Tor node? And if so, what is the practical minimum bandwidth needed to actually see any traffic?