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100 lines
4.2 KiB
HTML
100 lines
4.2 KiB
HTML
---
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title: Speed Up Slow Downloads on iOS with a Proxy or CDN
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extra: How to speed up downloads on iOS.
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kind: article
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section: technical
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created_at: 2013-01-13 12:39:00
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keywords:
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- ios
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- proxy
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- download
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- podcast
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- squid
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- launchd
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- cdn
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---
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Whilst developing the Radiopaedia iOS app I ran into the problem of very slow
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download speeds within the app. The app has packs of content avaiable for
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purchase. The download speed was fine in the simulator but on the device it
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was painfully slow. Profiling the code didn't reveal any issues either.
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Some searching revealed people compaining about YouTube videos downloading
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faster over 3G than Wi-Fi. There were a number of theories and proposed fixes
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for this including:
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* YouTube was sending higher bitrate video when on Wi-Fi
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* Checking Wi-Fi access point settings
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* Set access point to 802.11b and it goes faster
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At the time I read something that I can no longer find that said that the
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iOS Wi-Fi stack has problems when it has a low latency Wi-Fi
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connection that is communicating to a comparatively high latency
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destination, for example YouTube servers in the US from AU (it
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appears there's YouTube end-points in AU these days). From memory I think the
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article cited power management under these circumstance as the issue. Howver
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since I can't find the original article I might be making that up.
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With this in mind I tested hosting the content packs on an Australian server,
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instead of in Amazon S3. This showed a huge improvement in download speed. At
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the time Amazon's CloudFront CDN did not have an Australian presence so we
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switched to hosting the packs on RackSpace's CloudFiles, which uses the much
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more extensive Akamai CDN and ensured that users of the app would have the
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best possible download speeds.
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<div class="seperator"><hr class="left">✦<hr class="right"></div>
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Fast forward a year or so and I started managing podcasts on my phone instead of
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via iTunes. I noticed that some of the podcasts I was subscribed to were horrendously
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slow to download. A 100Mb file would take many hours to download, particularly
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ones hosted a long way away, such as in the UK or
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The Netherlands. I also noted that podcasts that used a CDN downloaded quickly.
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This seemed to be the same problem I had encountered with the Radiopaedia app.
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I wondered if telling the iPhone to use a proxy on the local
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network would work around the issue. since effectivly move
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the destination onto the local network as far as the phone
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is concerned (since it will just be talking to the proxy).
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So I set up [Squid][squid] on the Mac mini that's connected to my TV
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(using [Homebrew][homebrew])
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since it's always on and has a wired connection to my modem.
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I configured the iPhone to use the proxy an lo-and-behold the
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very same podcast downloads that were taking hours, now took
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minutes.
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[squid]: http://www.squid-cache.org/
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[homebrew]: http://mxcl.github.com/homebrew/
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To be sure I paused the download, changed the phone's settings
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to not use the proxy and restart the download, give it a few
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minutes and the estimate was hours.
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* Turn the proxy back on and minutes.
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You can see this in action in the screenshots below. The first is not
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using the proxy. I have let it download a third of the file to be well
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clear [TCP's slow start][slow-start] and it is estimating 57 minutes to
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download the remaining 176.1 Mb (53 kb/sec). The second screenshot (only
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a minutes later) shows the result after I paused the download, changed
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the proxy settings and then again let the download warm up a little. The
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estimated time to complete the remaining 143.8 Mb is now only 5 mins
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(491 kb/sec) -- a huge improvment.
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[slow-start]: https://en.wikipedia.org/wiki/Slow-start
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<figure>
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<img src="/images/2013/01/podcast-download-without-proxy.png" width="320" height="480" alt="Podcast download without proxy" />
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<figcaption>Podcast download without proxy, 53 kb/sec</figcaption>
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</figure>
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<figure>
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<img src="/images/2013/01/podcast-download-with-proxy.png" width="320" height="480" alt="Podcast download with proxy" />
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<figcaption>Podcast download with proxy, 491 kb/sec</figcaption>
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</figure>
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After the successful experiement I left the phone using the proxy and configured
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launchd to start squid on boot:
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<script src="https://gist.github.com/4521952.js"></script>
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