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Wesley Moore 2013-01-13 16:10:13 +11:00
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--- ---
title: Speed Up Slow Downloads on iOS with a Proxy or CDN title: Speed Up Slow Downloads on iOS with a Proxy or CDN
extra: How to speed up downloads on iOS. extra: Using a proxy or CDN can dramatically improve download speeds on an iOS device.
kind: article kind: article
section: technical section: technical
created_at: 2013-01-13 12:39:00 created_at: 2013-01-13 12:39:00
keywords: keywords:
- ios - ios
- proxy - proxy
- slow
- download - download
- podcast - podcast
- squid - squid
@ -14,71 +15,63 @@ keywords:
- cdn - cdn
--- ---
Whilst developing the Radiopaedia iOS app I ran into the problem of very slow Whilst developing the [Radiopaedia iOS app][radiopaedia-app] I ran
download speeds within the app. The app has packs of content avaiable for into the problem of very slow download speeds within the app. The app
purchase. The download speed was fine in the simulator but on the device it has packs of content available for purchase and download. The download
was painfully slow. Profiling the code didn't reveal any issues either. speed was fine in the simulator but on the device it was painfully
slow<sup>1</sup>. Both the Mac running the simulator and device were
connected to the same Wi-Fi network. Profiling the code didn't reveal
any issues either.
Some searching revealed people compaining about YouTube videos downloading [radiopaedia-app]: http://radiopaedia.org/articles/radiopaedia-ios-radiology-app
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 Some searching revealed people complaining about YouTube videos
* Checking Wi-Fi access point settings downloading faster over 3G than Wi-Fi. There were a number of theories
* Set access point to 802.11b and it goes faster and proposed fixes, including [people seeing faster rates when
downgrading their access point to 802.11b][80211b]<sup>2</sup> but I
found one particular article (that I can no longer find) that said the
power management in the iOS Wi-Fi stack interfered with download speeds
when communicating to a high latency destination over a high speed Wi-Fi
connection. For example YouTube servers in the US from AU<sup>3</sup>.
At the time I read something that I can no longer find that said that the [80211b]: http://web.archive.org/web/20120511020134/http://qelix.com/blog/2008/08/31/get-better-wifi-speeds-on-iphone-3g
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, With this in mind I tested hosting the content packs on an Australian
instead of in Amazon S3. This showed a huge improvement in download speed. At server, instead of in Amazon S3 accessed via CloudFront. This showed a huge
the time Amazon's CloudFront CDN did not have an Australian presence so we improvement in download speed. At the time CloudFront didn't
switched to hosting the packs on RackSpace's CloudFiles, which uses the much have an Australian presence so we switched to hosting the packs on
more extensive Akamai CDN and ensured that users of the app would have the RackSpace's CloudFiles, which uses the much more extensive Akamai CDN
best possible download speeds. and ensured that users of the app would have the best possible download
speeds no matter where they were.
<div class="seperator"><hr class="left">✦<hr class="right"></div> <div class="seperator"><hr class="left">✦<hr class="right"></div>
Fast forward a year or so and I started managing podcasts on my phone instead of Fast forward a year or so and I started managing podcasts on my phone
via iTunes. I noticed that some of the podcasts I was subscribed to were horrendously instead of via iTunes. I noticed that some of the podcasts I was
slow to download. A 100Mb file would take many hours to download, particularly subscribed to were horrendously slow to download. Some would take hours
ones hosted a long way away, such as in the UK or to download, particularly ones hosted a long way away, such as in the
The Netherlands. I also noted that podcasts that used a CDN downloaded quickly. 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. 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 I wondered if telling the iPhone to use a proxy on the local
network would work around the issue. since effectivly move network would work around the issue.
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 To test my theory I set up [Squid][squid]<sup>4</sup> on the Mac
(using [Homebrew][homebrew]) mini that's connected to my TV since it's always on and has a wired
since it's always on and has a wired connection to my modem. connection to my modem. I configured the iPhone to use the proxy and
I configured the iPhone to use the proxy an lo-and-behold the compared the speed with, and without the proxy in use.
very same podcast downloads that were taking hours, now took
minutes.
[squid]: http://www.squid-cache.org/ [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 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 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 clear of [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 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 a minute later) shows the result after I paused the download, changed
the proxy settings and then again let the download warm up a little. The 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 estimated time to complete the remaining 143.8 Mb is now only 5 mins
(491 kb/sec) -- a huge improvment. (491 kb/sec) -- a huge improvement.
[slow-start]: https://en.wikipedia.org/wiki/Slow-start [slow-start]: https://en.wikipedia.org/wiki/Slow-start
@ -92,9 +85,14 @@ estimated time to complete the remaining 143.8 Mb is now only 5 mins
<figcaption>Podcast download with proxy, 491 kb/sec</figcaption> <figcaption>Podcast download with proxy, 491 kb/sec</figcaption>
</figure> </figure>
After the successful experiement I left the phone using the proxy and configured After the successful experiment I left the phone using the proxy and configured
launchd to start squid on boot: launchd to start squid on boot:
<script src="https://gist.github.com/4521952.js"></script> <script src="https://gist.github.com/4521952.js"></script>
----
<sup>1</sup> Tested on iPhone 3GS and iPhone 4S.
<sup>2</sup> Original article linked from [Apple support discussion](https://discussions.apple.com/message/9564410#9564410).
<sup>3</sup> It appears there's YouTube end-points in AU these days.
<sup>4</sup> I installed squid using [Homebrew](http://mxcl.github.com/homebrew/).