Unclogging the Arteries of the Internet

Have you ever noticed that, no matter what size hard drive sits in your computer, it is always filled to capacity? Besides making shareholders of Seagate happy, this fact exemplifies an important property of computing resources - innovation will create applications that can consume low-cost computing resources, independent of their total capacity. As hard drive capacities have grown, applications - music, video, email, video games - have found ways to consume them.

This same truth applies to bandwidth. The availability of low-cost flat-rate broadband access in many areas of the world has enabled many new applications. One of them, Peer-to-Peer (P2P) file sharing, was specifically engineered to take advantage of these low cost resources. P2P file sharing applications expand their bandwidth utilization to fill broadband access links to capacity. Indeed, in 2007, Bit Torrent traffic constituted a whopping third of all Internet usage.

What does P2P file sharing have to do with VoIP and SIP, though? Unfortunately, VoIP traffic shares the same pipes that carry BitTorrent traffic - pipes that are perpetually full. This results in packet loss for VoIP, and more importantly, increased latencies as router queues become continuously full from file sharing traffic. VoIP is not the only application affected, of course. Indeed, the increased usage of Internet capacity by file sharing applications has caused a feud of sorts to erupt between some ISPs, and Comcast in particular, and BitTorrent.

Fortunately, the IETF has stepped up to develop technical solutions that can allow for a more peaceful co-existence of file sharing applications and VoIP. This past August, at the 72nd meeting of the IETF, two new working groups were proposed, both of which discussed fascinating technologies for addressing the problem. Participants representing both BitTorrent and Comcast were in attendance.

The first proposed working group, called Alto, presented a technology allowing desktop applications, like BitTorrent, to obtain information about the network so they can work better. BitTorrent connects to other peers all over the Internet in order to download a file. Oftentimes, the peer is chosen poorly - a user in Philadelphia downloading a file may end up connecting to peers in China and Japan, even though a peer in nearby New York has the file. This wastes bandwidth on the ISP's inter-provider links, and causes the file to take longer to download. Using the proposed technology, the desktop application could submit a request to a server in the ISP network, asking it to provide information on which, amongst a set of possible peers, would be the "best" ones to connect to. The definition of "best" was a subject of much discussion in the meeting. Generally, it involves some kind of topological metric, so that a client can choose the closest peers.

The Alto technology helps VoIP both indirectly (by reducing consumption of network resources by P2P apps) and directly. Alto is not limited to P2P file sharing. A desktop softphone app, for example, could use the Alto technology to find the nearest conference bridge, or find the nearest NAT traversal server. Indeed, the latency sensitive nature of VoIP means that technologies like Alto - which help applications make smart choices about connectivity based on proximity - may be even more directly beneficial to it than to P2P applications.

The second proposed working group, called Tana, discussed a new transport protocol, akin to TCP and UDP, which would provide a "scavenger class" service. Scavenger class means that it would only utilize available network bandwidth; it would never push out other traffic. It only scavenges the leftover bandwidth that goes unused by other applications - like VoIP - that run in the network. The technology works by probing the network to determine the amount of traffic sitting in the router queues, and when the queues start to fill, it backs off and sends less data. This approach works particularly well with VoIP, which has the best performance when router queues are empty. Tana technology does not require changes to routers and switches, and is actually in active usage by some BitTorrent clients today. Widespread usage of the Tana technology could have a noticeable impact on making VoIP work better on the Internet.

The Alto and Tana meetings made this IETF one of the most exciting in recent memory. Both are very promising technologies which, by changing the behavior of one of the most significant sources of traffic on the entire Internet, can actually have a real impact on the performance of the Internet for all applications, including VoIP. It's something to keep an eye on. Jonathan Rosenberg is a Cisco Fellow.