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With plain HTTP, cookieless domains are an optimization to avoid unnecessarily sending cookie headers for page resources.

However, the SPDY protocol compresses HTTP headers and in some cases eliminates unnecessary headers. My question then is, does SPDY make cookieless domains irrelevant?

Furthermore, should the page source and all of its resources be hosted at the same domain in order to optimize a SPDY implementation?

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A good question, shame the only response gave no real experience/answer! – Codemonkey May 18 '14 at 10:44
See also stackoverflow.com/a/13948404/41906, which was answered by Ilya Grigorik who is a web performance specialist. – Clint Pachl May 19 '14 at 19:25
Similar SPDY question: serverfault.com/q/545849/12985 – Clint Pachl May 19 '14 at 19:32

In the requests towards the (non-SPDY) website I work for i've seen requests with up to 4-5 KiB of request headers. (we kicked the offending javascript that caused that)

Assuming the SPDY compression does at least a compression to leave at most 25% of a header (note that I have NOT tested SPDY yet!!!) then the resulting request header goes down to a size that fits within a single tcp packet (1500 bytes). This takes away a biggest part of the needless overhead.

On the other hand the server still needs to decompress and parse all of these headers.

Overall my expectation is that using SPDY will reduce the need for a cookieless domain.

If you have stateless requests I would still go for a separate domein for two reasons:

  1. If you do not need the cookies you're better of avoiding getting them.
  2. Not all current browsers support SPDY yet.
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