Filename: 148-uniform-client-end-reason.txt Title: Stream end reasons from the client side should be uniform Author: Roger Dingledine Created: 2-Jul-2008 Status: Closed Implemented-In: 0.2.1.9-alpha Overview When a stream closes before it's finished, the end relay cell that's sent includes an "end stream reason" to tell the other end why it closed. It's useful for the exit relay to send a reason to the client, so the client can choose a different circuit, inform the user, etc. But there's no reason to include it from the client to the exit relay, and in some cases it can even harm anonymity. We should pick a single reason for the client-to-exit-relay direction and always just send that. Motivation Back when I first deployed the Tor network, it was useful to have the Tor relays learn why a stream closed, so I could debug both ends of the stream at once. Now that streams have worked for many years, there's no need to continue telling the exit relay whether the client gave up on a stream because of "timeout" or "misc" or what. Then in Tor 0.2.0.28-rc, I fixed this bug: - Fix a bug where, when we were choosing the 'end stream reason' to put in our relay end cell that we send to the exit relay, Tor clients on Windows were sometimes sending the wrong 'reason'. The anonymity problem is that exit relays may be able to guess whether the client is running Windows, thus helping partition the anonymity set. Down the road we should stop sending reasons to exit relays, or otherwise prevent future versions of this bug. It turned out that non-Windows clients were choosing their reason correctly, whereas Windows clients were potentially looking at errno wrong and so always choosing 'misc'. I fixed that particular bug, but I think we should prevent future versions of the bug too. (We already fixed it so *circuit* end reasons don't get sent from the client to the exit relay. But we appear to be have skipped over stream end reasons thus far.) Design: One option would be to no longer include any 'reason' field in end relay cells. But that would introduce a partitioning attack ("users running the old version" vs "users running the new version"). Instead I suggest that clients all switch to sending the "misc" reason, like most of the Windows clients currently do and like the non-Windows clients already do sometimes.