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Re: an idea about how to improve routing for interactive services

Roger Dingledine wrote:

On Mon, Jan 23, 2006 at 12:36:53PM +1000, glymr wrote:

These rules could be used to weight classes of ports, a node could keep a history of its uptime, and report its average uptime value accumulated over time to the directory. This would help for choosing interactive traffic routes, the longer the average uptime, the greater the chance of it being picked on interactive circuits.

Actually, we already do something like this. Nodes report their uptime, and we assume that a long uptime implies that it will stay up.

yes, that's not a good assumption to make however. average uptime is a more useful metric, when a system has been up for a long time it may be just about to go down. also, for irc users, a connection which can stay up for 8 hours or more is regarded as quite adequate by most.

Tor could automatically select it's preference for the different traffic classes according to these values. At this point, without an automated system to do this, it can be done by users (as I am doing) - by using a rate-limiting system (netlimiter) and allowing only a small set of interactive traffic types through (in my case, irc and silc) - since tor precludes the use of file transfers on these two protocols, I set the rate limiting between 2 and 4kb/s depending on whether I am downloading more or chatting more.

I'm not sure I understand this part. You are using an external tool to
traffic shape the stuff you send into Tor? Or you are using an external
tool to traffic shape the stuff that exits from your Tor server?

what i mean is, different ports, for different protocols, have different traffic characteristics. irc, silc, msn, aim/icq, jabber etc, these are all long lived and mostly low bandwidth connections, whereas web browsing is short lived and bursty. they should be treated differently.

regarding the shaper, yes, i'm shaping the bandwidth that comes out of my tor server so that it limits it, but because i have specified that it only allows exit with irc and silc, to a large degree that will reduce the amount of bursty high bandwidth connections it creates, once my node is up for the 'long uptime' period (i'm still nutting out some problems with a new installation, but i'm hoping i'll have over 24 hours uptime soon).

but unfortunately there is nothing yet in the protocol to stop my node being a part of a bursty, short-lived high-bandwidth circuit. being able to control this would be very useful. and that's what i'm talking about, having two classes of traffic in tor, so that nodes that have good uptime but low bandwidth can contribute to improving the interactive connection experience with tor.

However, I think it would be a worthwhile addition to the system by which Tor does its routing to use these rules in both the production of an uptime and bandwidth average, which is used by clients to select a pair of different circuit classes, interactive and high volume. High volume traffic usually is short lived, and interactive traffic is usually long lived. By specialising the circuits according to these rules one would find that interactivity is better promoted, and separated from volume.

Right, we do this too.

Check out the man page entry:

LongLivedPorts PORTS
 A list of ports for services that tend to have long-running connec-
 tions (e.g. chat and interactive shells). Circuits for streams that
 use  these ports will contain only high-uptime nodes, to reduce the
 chance that a node will go down  before  the  stream  is finished.
 (Default:  21,  22,  706, 1863, 5050, 5190, 5222, 5223, 6667, 8300,

In 0.1.0.x, "high-uptime" is defined as "claiming an uptime of at least 24
hours." In, high-uptime is defined as meeting or exceeding
the median uptime of running valid servers. On the current Tor network
this tends to vary between 2 to 3 days.

But this clearly does not totally solve the problem: long-term connections
over Tor do still break. Part of this is because the Tor network is very
young and still quite dynamic, so people are upgrading, restarting their
servers, and so on. There is also clearly a tension between scaling the
network (and thus being more flexible about the volunteers we can make
use of) and providing stable links.

Another nice approach would be to be able to move streams to a new
circuit if the current one dies. But this has its own problems:

What other approaches are there that might work?


I think that what i have described is the, at this point at least, best answer to the problem of improving long lived interactive connections.

One other point that might be worth mentioning is that these long lived connections would probably benefit, due to their long life and low ping, from having 4 or 5 hops instead of 3 to help reduce the traffic analysis problem, since it would be very easy to have a lot more people running these low capacity high uptime nodes, the extra traffic is insignificant.

Oh, and because these connections are very low bandwidth, it could be incorporated into the client to automatically relay traffic from known low bandwidth ports, if the client finds itself with a high uptime average.

Think about how important persistence is with interactive connections. SSH is a classic example... what happens if you are in the middle of some irritaingly long process and suddenly your connection pings out? I think that there should be a priority made in the tor architecture to promote this kind of use of tor because it's probably the most delicate, security wise. Consider the benefits for activists being able to use instant messaging without being monitored, for organising and such.