March 24th, 2008
Fixing the unfairness of TCP congestion control
Exploiting Jacobson’s TCP algorithm
While Jacobson’s algorithm was suitable for the 1980s, cracks began to appear a decade later. By 1999, the first P2P (peer-to-peer) application called Swarmcast began to blatantly exploit Jacobson’s TCP congestion control mechanism. Using a technique called “parallel incremental downloading”, Swarmcast could grab a much larger share of the pie at the expense of others by exploiting the multi-stream and persistence loophole. These two loopholes would be used by every P2P application since.
Simply by opening up 10 to 100 TCP streams, P2P applications can grab 10 to 100 times more bandwidth than a traditional single-stream application under a congested Internet link. Since all networks have a bottleneck somewhere, a small percentage of Internet users utilizing P2P can hog the vast majority of resources at the expense of other users. The following diagram illustrates the multi-stream exploit in action where User A hogs more and more bandwidth over User B by opening more and more TCP streams. The large light green cutaway pipe represents a congested network link with finite capacity.
The other major loophole in Jacobson’s algorithm is the persistence advantage of P2P applications where P2P applications can get another order of magnitude advantage by continuously using the network 24×7. The diagram below shows what happens when an application like BitTorrent uses the network continuously. I wrote about this last month and presented a similar chart on Capitol Hill.
By combining these two loopholes, an application using 10 times as many TCP streams while being 10 times more persistent than other applications can get a 100 boost over other users when contending for network resources.
With millions of consumers on the Internet today with an insatiable appetite for multi-gigabyte videos, the Internet is facing its second congestion crisis. While the network isn’t completely melting down, it’s completely unfair because fewer than 10% of all Internet users using P2P hogs roughly 75% of all network traffic at the expense of all other Internet users. Even in a country like Japan which has the most per-user broadband capacity in the world, P2P applications have managed to turn Japan’s 100 Mbps per home fiber network in to a big traffic jam. The problem has gotten so severe in Japan that the nations ISPs in conjunction with their Government have agreed to ban P2P users who are trafficking copyrighted content.
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George Ou is Technical Director of ZDNet. See his full profile and disclosure of his industry affiliations.
