Netflix, Akamai, and video delivery performance

Dan Rayburn’s blg post about the Akamai/Netflix relationship seems to have set off something of a firestorm, and I’ve been deluged by inquiries from Gartner Invest clients about it.

I do not want to add fuel to the fire by speculating on anything, and I have access to confidential information that prevents me from stating some facts as I know them, so for blog purposes I will stick to making some general comments about Akamai’s delivery performance for long-tail video content.

From the independent third-party testing that I’ve seen, Akamai delivers good large-file video performance, but their performance is not always superior to other major CDNs (excluding greater reach, i.e., they’re going to solidly trounce a rival who doesn’t have footprint in a given international locale, for instance). Actual performance depends on a number of factors, including the specifics of a customer’s deal with Akamai and the way that the service is configured. The testing location also matters. The bottom line is that it’s very competitive performance but it’s not, say, head and shoulders above competitors.

Akamai has, for the last few years, had a specific large-file delivery service designed to cache just the beginning of a file at the very edge, with the remainder delivered from the middle tier to the edge server, thus eliminating the obvious cache storage issues involved in, say, caching entire movies, while still preserving decent cache hit ratios. However, this has been made mostly irrelevant in video delivery by the rise in popularity of adaptive streaming techniques — if you’re thinking about Akamai’s capabilities in the Netflix or similar contexts, you should think of this as an adaptive streaming thing and not a large file delivery thing.

In adaptive streaming (pioneered by Move Networks), a video is chopped up into lots of very short chunks, each just a few seconds long, and usually delivered via HTTP. The end-consumer’s video player takes care of assembly these chunks. Based on the delivery performance of each chunk, the video player decides whether it wants to upshift or downshift the bitrate / quality of the video in real time, thus determining what the URL of the next video chunk is. This technique can also be used to switch sources, allowing, for instance, the CDN to be changed in real time based on performance, as is offered by the Conviva service. Because the video player software in adaptive streaming is generally instrumented to pay attention to performance, there’s also the possibility that it may send back performance information to the content owner, thus enabling them to get a better understanding of what the typical watcher is experiencing. Using an adaptive technique, your goal is generally to get the end-user the most “upshift” possible (i.e., sustain the highest bitrate possible), and if you can, have it delivered via the least expensive source.

When adaptive streaming is in use, that means that the first chunk of videos is now just a small object, easily cached on just about any CDN. Obviously, cache hit ratios still matter, and you will generally get higher cache hit ratios on a megaPOP approach (like Limelight) than you will with a high distributed approach (like Akamai), although that starts to get more complex when you add in server-side pre-fetching, deliberately serving content off the middle tier, and the like. So now your performance starts to boil down to footprint, cache hit ratio, algorithms for TCP/IP protocol optimization, server and network performance — how quickly and seamlessly can you deliver lots and lots of small objects? Third-party testing generally shows that Akamai benchmarks very well when it comes to small object delivery — but again, specific relative CDN performance for a specific customer is always unique.

In the end, it comes down to price/performance ratios. Netflix has clearly decided that they believe Level 3 and Limelight deliver better value for some significant percentage of their traffic, at this particular instant in time. Given the incredibly fierce competition for high-volume deals, multi-vendor sourcing, and the continuing fall in CDN prices, don’t think of this as an alteration in the market, or anything particularly long-term for the fate of the vendors in question.

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Posted on November 9, 2010, in Infrastructure and tagged , . Bookmark the permalink. 1 Comment.

  1. I feel so much happier now I unsdertnad all this. Thanks!


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