MavEtJu's Distorted View of the World

A proper WAN Optimization analogy

Posted on 2011-03-29 18:00:00
Tags: Networking, Riverbed

Even after working for Riverbed Technology for two and a half years now, I still have to come up with a bullet-proof analogy of how WAN optimization works.

Consider a network from one side of this planet to the other side: A round trip time of 300 milliseconds for 20 million meters. Of that 300 milliseconds, you have two factors: The speed of light to get from A to B, and serialization delay which happens at every hop and is related to the bandwidth and the size of the packet. One is constant, the other one is variable.

To move data in a stream, the throughput is limited to the smallest bandwidth in the path. Although you can move data faster on other parts, it all has to go through this one.

As an example, say you have a stream of 300 Mb and a smallest bandwidth of 2 Mbps. Without any protocol overhead, this will take 20 minutes to go through there. With a fourty byte protocol overhead and an MTU size of 1500, this will take 20 minutes and 33 seconds.

Now with WAN optimization. It consists of three parts: Optimization on TCP level, which has been ignored for now. Latency optimization on application specific protocols, which has been ignored for now. And data optimization, where the data is either compressed or only referred to. If the same or similar data gets transfered via two WAN optimizers twice, the first time you would get a relative small reduction factor, depending on the compressability of the data, while the second time you would get a large reduction factor because the data patterns is alreayd known on both devices.

If that 300 Mb is split into segments of 1024 bytes, making it 300 000 segments, and each segment has a 64 byte label, you end up with only about 19 Mb worth of labels.

Transfering that 19 Mb through a 2 Mbps link will be take 80 seconds, about 15 times faster.

Now back to the topic: A good analogy for WAN optimization.

Is it faster than light? It feels like it, but the Round Trip Time of the WAN is still the same. And the speed the packets go via is still the same.

Is it a "wormhole"? Wormhole-based paths which are shorter than a non-wormhole-based paths. The path travelled travelled for optimized traffic still has the same distance.

Is it comparable with ships, where goods are stored in containers (labels) and then transported in large bulk carriers? If the speed limit of other ships was limited to the speed limit of the bulk carriers, then it would be a good start.

Is it a train analogy, where passengers are cramped into carriages and efficiently transported across the rail network? It could be, except that on the railroad network everything is put into train carriages and transported efficiently on it. Comparing it with the French TGV and the Japanese bullet trains does not work neither, because the speed of the packets is still the same while these trains are way fast.

So, the analogy needs to use the same speed limits on the transport mechanism, and needs to give the impression that the delivery gets faster without changing the distance.

The best thing I come up with is transport of goods via large trucks instead of via small delivery vans: Goods are shipped via small delivery vans to a distribution point, stored into a single large truck which then uses the same transport infrastructure as small fast vans would have used if they would have transported their payload. Instead of a long convoy of small vans, you get one truck towards the distribution point which there gets reloaded into numerous small vans. The only thing which does not make sense yet is that small delivery vans are often 2 x 4 x 1.5 meters and big trucks are 3 x rather long x rather high, which gives the impression that size still matters while this isn't the case on WAN optimized traffic...

That is the problem if you work with magic :-)

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