What to do at IP number change?

Jon McLachlan mcla0181 at umn.edu
Wed Jan 9 04:32:23 UTC 2008

Scott Bennett wrote:
>      On Tue, 08 Jan 2008 14:15:05 -0600 Jon McLachlan <mcla0181 at umn.edu>
> wrote:
>> dr._no at cool.ms wrote:
>>> Another point is that without a tor server my home would be vulnerable to traffic 
>>> analysis and a further point is that a tor server is more safe than only a client.
>> I think this depends largely on what type of traffic analysis we're 
>> talking about.  Traffic analysis, just looking at traffic, almost always 
>> divulges some level of information.  For example, if a local passive 
>> adversary simply watched a Tor Relay that was suspected to also contain 
>> a Tor Client, then one could imagine a simple traffic analysis as follows:
>> 1)  Establish running totals of all incoming and outgoing traffic from 
>> the machine.
>> 2)  Then, closely monitory when it is the case that the outgoing traffic 
>> level "spikes" or when the incoming traffic level "spikes" as they could 
>> indicate that a Tor Client was using the relay as an entry point.  How 
>> much it "spikes" could fingerprint a website ... or even be a 
>> maliciously modulated signal from an evil server might you might have 
>> connected to via your tunnel.
>> This exploits the behavior of a basic Tor Relay, in which everything 
>> that enters a relay must [immediately] leave that relay.  This traffic 
>> alone would generate what appears equal/average incoming and outgoing 
>> msgs.  Any spikes in the entering / leaving traffic is therefore 
>> probably not from the Tor Relay itself, but, from something else.  (or 
>> course, this ignorse dir service lookups, bridges, and prly a few other 
>> things).
>      Almost.  If you have an asymmetric broadband service and are not
> specifying BandwidthRate or BandwidthBurst in torrc, then your tor server
> is likely to top out around the transmission rate limit of your Internet
> connection.  At this point, only input spikes would be visible.  When data
> come in faster than they can go out, the cells just stay in an output queue
> until they can be sent.  If this goes on over an extended period of time,
> it will have at least a partial smoothing effect upon the inflow as well
> due to TCP source quench packets being returned.
>      Note also that spikes may occur for several other obvious reasons,
> e.g., a new stream on a circuit going through your tor server that is used
> for FTP, downloading large files (e.g., music or video or CD/DVD image
> files), or NNTP batch transfers.  Spikes often have nothing to do with a
> local client.
Yeah, I guess I was making synchronized assumptions, haha :).  But, I am 
having trouble imagining circuit based Tor relay traffic causing 
"spikes" in the differences between total incoming traffic and total 
outgoing traffic - since, if there were large discrepancies between 
these two totals, it would more or less be a kind of measure on your own 
relay's lag.  In a low-latnecy system, one of the centric goals is to 
minimize this - and in Tor, I believe several design decisions were 
based around this.  So, even if things are async, it seems likely that 
(large, consistent) discrepancies between total incoming/outgoing 
traffic would prly be due to "other" local traffic that either 
originates or exits form the relay.  But yes, I was also assuming a 
strong local passive adversary that is able to distinguish between 
circuit relay traffic and all other (ignored) network traffic.
>> Sounds like an interesting research project.
>                                   Scott Bennett, Comm. ASMELG, CFIAG
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