Yes I realise I'm necroposting an old thread, but - exact same issue here, more than 6 months later. Hmm.
In my case I'm trying to update a BRAND NEW iPhone 6+ from the 8.1.2 that it came with - the 8.3 download is over 500 MB and I am now on the third attempt. Error 9006 or whatever, "unknown".
What's really annoying is the d/l speed is particularly pathetic - in the region of 40 kB/s or about 320 kb/s when the DSL line is capable of nearly 2,000 kb/s - and the file partially downloads by a substantial amount before failing: over 200 MB the first time and 300 MB the second so it ain't the anti-virus software (I'm doing it via a brand-new installation of iTunes on a WIndows 7 PC).
To make matters worse, I'm attempting this via TeamViewer remote control and each time the **** job fails the **** phone has to be unlocked using the **** passcode so I have to phone the user and ask her to type it in... **** YOU STEVE JOBS. Also we pay per MB of data in this country, and several attempts at a large d/l are not great on the budget.
Apple apparently has much to learn from Microsoft about making their server farms highly available and high throughput.
Edit: ha ha the word that sounds like a large water storage body or reservoir gets bleeped out on this forum. That's funny.