Header compression is a good feature with real world applications,
   and deflate with persistent context is a good approach to achieve
   it. A fixed dictionary is probably not very effective as it
   complicates the implementation while only providing initial value to
   the compression. As an example, this is how the average requests
   sizes compares between HTTP and SPDY in different modes, using the
   set of captured headers used to train the current SPDY 3 dictionary.

     HTTP                             821.1
     HTTP zlib compressed             543.5
     HTTP compressed with dictionary  497.0
     SPDY                             913.7
     SPDY zlib compressed             606.5
     SPDY compressed with dictionary  517.0

   I.e. Just putting the HTTP request in a SPDY stream (after removing
   disallowed headers) only differs by 20 bytes to SPDY binary header
   format with dictionary based zlib compression. The benefit of using
   a dictionary basically goes away entirely on the subsequent request.