[e2e] NDDI & OpenFlow

L.Wood@surrey.ac.uk L.Wood at surrey.ac.uk
Thu Apr 28 17:07:35 PDT 2011

One way of looking at OpenFlow is that, as routers have developed, they have gone from being integrated, to having modular linecards plugged into a backplane/bus, to having line cards plugged into an internal 10.x network within the box, because Gigabit Ethernet can provide a nice fast, well-understood backplane without further custom engineering. (Cisco's catalyst 6000 series is one example of this.) The linecards do forwarding at (hopefully) line speed, but receive their forwarding tables over the internal Ethernet from the central processor where routing and routing tables exist, and also receive their traffic over the internal Ethernet. All in internal VLANs where the forwarding table information and control data can be prioritised.

OpenFlow can take that internal Ethernet connectivity within the router, and stretches it so that the linecards are in different places around an office or university campus. So your network is no longer a bunch of smartish routers doing different and slightly repeated and redundant things in a hopefully coordinated fashion, but a bunch of somewhat dumber linecards being coordinated in synchrony from a central point. Your campus is now inside your router, and your campus-wide control plane just got way faster and more predictable.

So far, so good; your traffic engineering and what-does-QoS-mean problems now exist within a single homogenous router, rather than a bunch of uncoordinated routers that have to be configured in situ etc. So the routing protocol being run across campus is suddenly consistent, instead of e.g. migrating piecemeal to OSPF, having part of campus generate and rebroadcast RIP without telling you because they're not upgrading their old kit, etc.

Where I have trouble with the OpenFlow story is where network researchers say 'okay, now we've built this, we'll also instrument it and use it for traffic for our research experiments, in separate traffic-engineered virtualized slices of the network'. It's a production networking environment enabling a business or university to function, where network researchers don't do support, and which and whose budget will pay for this stuff and deal with downtime mitigation, exactly? Even handwaving the technology implications away, it's an accounting difficulty. I suppose researchers could found a startup that will charge the university to maintain its network, while at the same time also getting research funding to do new, interesting, and exciting things to their paying customer's network. It's a win-win (well, if we don't look too closely at funding models) right up until the first major outage.

But with the recent formation of the Open Networking Foundation, we'll see multiple commercial suppliers providing ever-more-complex kit to support this site-as-router paradigm, with the usual subtle interoperability problems, a minimal common feature set and creative technical differentiation to market products and give unique selling points. Standards are good, but they can always be improved upon. At which point, we're back to a heterogenous multi-supplier network - but one with a lot more subtle interdependencies, requiring more support when things go wrong, as they do. Routing problems can now be trickier forwarding state and sync problems. But that's what support is for; you don't just install a network, you maintain one, and the support costs are a given.

Meanwhile, the networking researchers remain locked out of the commercial kit for the sanity of the university administration, which is fine, because they've decided they can't do anything exciting with it anyway. Instead, they're off describing the problems with the status quo and winning new funding to look at distributed, fault-tolerant networking where there is no central control point. After all, you don't conquer complexity, you just shuffle it around.

This should be interesting.


Plus ça change, plus c'est la même chose.

Lloyd Wood

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