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Researchers use Akamai to find local BitTorrent peers

Computer science researchers have created a Java-based Azureus BitTorrent plug …

The rise of P2P traffic as a percentage of total network use has left ISPs looking for ways to limit its impact on their capacity. Comcast, notably, has chosen to throttle some P2P traffic, but that solution is generally viewed as a temporary fix. Testing of the hardware used for throttling P2P traffic reveals that it performs poorly when challenged with obfuscated or encrypted P2P traffic. Other ISPs, such as Verizon, are considering working with P2P software makers to help keep P2P traffic on local networks, and thus less burdensome. But computer science researchers from Northwestern University have been experimenting with ways of keeping the traffic local that don't require the cooperation of ISPs; the researchers provided Ars with a publication detailing some of the results.

Systems such as those used by Verizon speed P2P transfers and reduce their burdens on ISPs by providing network topology and traffic information to P2P clients. Those clients can then use this information to identify local peers with clear paths, speeding the transfer and putting less strain on the ISPs' larger network. These systems, however, require the explicit cooperation of ISPs, which have to consistently update the network information.

The new method of identifying local peers seeks to accomplish the same thing without getting ISPs involved. It relies on the fact that commercial content distribution systems such as the one operated by Akamai obtain or generate the equivalent information in order to redirect users to a local copy of a popular web site or video. Thus, if a two computers get redirected to the same Akamai content cache, they're likely to have local access to each other.

The researchers have implemented this logic via a Java-based plugin called Ono that works with the Azureus BitTorrent client. When Ono starts up, it polls a number of Akamai caches and creates a table of the IP addresses of local Akamai servers (the system works with other caching systems; Akamai is simply the most popular). Ono continues to poll the servers, but, the longer the results remain unchanged, the longer it takes before polling again. The net result is that the traffic for location determination is minimal, 18Kb upstream and 36Kb downstream a day even in the worst case; most of that goes to local DNS servers.

Northwestern's Aqualab developed the Ono plug-in

That information is transferred to peers for comparison if they are using the Ono plug-in, and those peers that share a similar set of Akamai caches are favored for use. These don't get used exclusively, however, in order to ensure that new and potentially closer peers continue to be identified. Given that not everyone will be using Ono, peers that are obtained without Ono are subjected to reverse DNS lookups in order to increase the chances of identifying local peers.

The researchers also set up Ono so that it could gather traffic data, such as ping times, traceroutes, and transfer rates, and report it back in to the researchers. Using a 15-day period from last December, they found the system worked as intended. Over a third of the peers identified by Ono did not require going outside of the local ISP's network. The median distance of Ono-identified peers was six hops, while the median of total peers (including those picked by Ono) was nearly 14 hops, resulting in a latency of only six milliseconds for Ono and 530ms for everyone else.

Oddly, this large difference in the quality of peers did not always affect the download experience. Here, the policy of ISPs regarding network throttling played a significant role, so that Ono users in different networks could have vastly different experiences. The popularity of a download could also dilute the impact of Ono use, given that even local peers can get overloaded by requests. Another thing noted by the researchers was that not all caching networks gave equal performance; for example, information derived from the CNN cache outperformed that from many other popular websites.

Ono seems to make life easier for ISPs, but it still requires their cooperation, in that they will need to ensure that local traffic doesn't get throttled. If local and external bandwidth are throttled equally, then finding local peers does nobody any good, including the ISPs. It's also worth noting the possibility that Ono will ultimately make Akamai less cooperative if the company gets dragged into the arguments over P2P use. Given that P2P will likely be a source of tension between ISPs and users for the indefinite future, however, the Ono data certainly has the potential to inform future practices.

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