--- title: Speed Up Slow Downloads on iOS with a Proxy or CDN extra: How to speed up downloads on iOS. kind: article section: technical created_at: 2013-01-13 12:39:00 keywords: - ios - proxy - download - podcast - squid - launchd - cdn --- Whilst developing the Radiopaedia iOS app I ran into the problem of very slow download speeds within the app. The app has packs of content avaiable for purchase. The download speed was fine in the simulator but on the device it was painfully slow. Profiling the code didn't reveal any issues either. Some searching revealed people compaining about YouTube videos downloading faster over 3G than Wi-Fi. There were a number of theories and proposed fixes for this including: * YouTube was sending higher bitrate video when on Wi-Fi * Checking Wi-Fi access point settings * Set access point to 802.11b and it goes faster At the time I read something that I can no longer find that said that the iOS Wi-Fi stack has problems when it has a low latency Wi-Fi connection that is communicating to a comparatively high latency destination, for example YouTube servers in the US from AU (it appears there's YouTube end-points in AU these days). From memory I think the article cited power management under these circumstance as the issue. Howver since I can't find the original article I might be making that up. With this in mind I tested hosting the content packs on an Australian server, instead of in Amazon S3. This showed a huge improvement in download speed. At the time Amazon's CloudFront CDN did not have an Australian presence so we switched to hosting the packs on RackSpace's CloudFiles, which uses the much more extensive Akamai CDN and ensured that users of the app would have the best possible download speeds.

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Fast forward a year or so and I started managing podcasts on my phone instead of via iTunes. I noticed that some of the podcasts I was subscribed to were horrendously slow to download. A 100Mb file would take many hours to download, particularly ones hosted a long way away, such as in the UK or The Netherlands. I also noted that podcasts that used a CDN downloaded quickly. This seemed to be the same problem I had encountered with the Radiopaedia app. I wondered if telling the iPhone to use a proxy on the local network would work around the issue. since effectivly move the destination onto the local network as far as the phone is concerned (since it will just be talking to the proxy). So I set up [Squid][squid] on the Mac mini that's connected to my TV (using [Homebrew][homebrew]) since it's always on and has a wired connection to my modem. I configured the iPhone to use the proxy an lo-and-behold the very same podcast downloads that were taking hours, now took minutes. [squid]: http://www.squid-cache.org/ [homebrew]: http://mxcl.github.com/homebrew/ To be sure I paused the download, changed the phone's settings to not use the proxy and restart the download, give it a few minutes and the estimate was hours. * Turn the proxy back on and minutes. You can see this in action in the screenshots below. The first is not using the proxy. I have let it download a third of the file to be well clear [TCP's slow start][slow-start] and it is estimating 57 minutes to download the remaining 176.1 Mb (53 kb/sec). The second screenshot (only a minutes later) shows the result after I paused the download, changed the proxy settings and then again let the download warm up a little. The estimated time to complete the remaining 143.8 Mb is now only 5 mins (491 kb/sec) -- a huge improvment. [slow-start]: https://en.wikipedia.org/wiki/Slow-start

Podcast download without proxy, 53 kb/sec

After the successful experiement I left the phone using the proxy and configured launchd to start squid on boot: