http://i3xi5qxvbrngh3g6o7czwjfxwjzigook7zxzjmgwg5b7xnjcn5hzciad.onion/proposals/278-directory-compression-scheme-negotiation.html
This currently
means that will not use the LZMA compression scheme with a "preset"
value higher than 6.
Additionally, when a client, that supports this proposals, makes a
request to a directory document with the ".z"-suffix it must send an
ordered set of supported compression schemes where the last elements
in the set contains compression schemes that are supported by all of
the currently available Tor nodes ("gzip", "deflate", "identity").