In high school, I took journalism and worked on the school newspaper. This means that I know the value of a good headline, that the headline may not reflect reality, and that the author of an article often does not write the headline. In fact, the headline may not even reflect the contents of the article. Still, I was surprised to see the headline to a recent Network World article, It’s Microsoft vs. the professors with competing data center architectures. The article invokes an image of one side or the other throwing down the gauntlet and declaring war. In fact, nothing could be further from the truth. If it were true, I would definitely feel worried about taking on Microsoft.
My group has been active in data center research. The article does a very nice job of describing the architecture of PortLand, our recent work on a layer 2 network fabric designed to scale to very large data centers. The article also describes recent work on VL2 from my colleagues at Microsoft Research. Both papers appeared at SIGCOMM 2009 and were presented back-to-back in the same session at the conference.
I will leave detailed comparison between the two approaches to the papers themselves. However, at a high level, both efforts start with a similar premise: the data center networking fabric, at the scale of 10k-100k ports, should be managed as a single network fabric. One desirable goal here is to manage the netwrok as a single layer 2 domain. However, conventional wisdom dictates that you cannot go beyond a few 100’s of ports for a layer 2 domain because of scalability and performance problems with traditional layer 2 protocols. I described one such scalability limitation, limited switch state for forwarding tables, in an earlier post. There are other challenges including spanning tree protocols, and broadcast overhead of ARP.
So the main takeaway is that we cannot scale a layer 2 network to target levels without changing some of the underlying protocols, at least a bit. With perfect hindsight, the key difference between PortLand and VL2 is one of philosophy. Both groups agree that the network should consist of unmodified switch hardware. However, we believe that the end hosts should also remain unmodified, instead implementing new functionality by modifying switch software. All switch hardware vendors export some API for programming switch forwarding tables and recently, systems such as OpenFlow export standard APIs for programming switch forwarding tables. In fact, we implemented our prototype of PortLand using OpenFlow with the goal of maintaining the boundary between system and network administration. VL2, on the other hand, prefers to leave the switch software unmodified and instead introduces its new functionality by modifying the end hosts themselves. This leads to different architectural techniques and different designs.
One of our overriding goals is to reduce management burden, so we further introduce a decentralized Location Discovery Protocol (LDP) to automatically assign hierarchical prefixes to switches and end hosts. These prefixes are the basis for compact forwarding tables in intermediate switches. Both VL2 and PortLand leverage a directory service to essentially find an efficient path between a source and destination without resorting to broadcast (as would be required by default with ARP).
I consider the VL2 paper to be excellent. I certainly learned a lot from reading the paper. Perhaps the ultimate complement I can give is that I plan to assign it to my class in the spring when I teach graduate computer networks again.
Still, it is true that one of the best things about research is that we live in a marketplace of ideas and hence there must be some implicit competition. We can only get better knowing that the folks at Microsoft are working on similar problems and certainly the “truth” as ascertained with 20/20 hindsight in 5-10 years will consist of some mixture of the competing techniques. That way, everyone can declare victory.