Hi,

It's accepted practice that when you support compression, Vary: Accept-Encoding http header is sent. From:
https://developers.google.com/speed/docs/best-practices/caching

"Set the Vary: Accept-Encoding response header. This instructs the proxies to cache two versions of the resource: one compressed, and one uncompressed. The correct version of the resource is delivered based on the client request header. This is a good choice for applications that are singly homed and depend on public proxies for user locality."

Apache does this:
http://httpd.apache.org/docs/2.2/mod/mod_deflate.html#proxies

The functionality can be enabled with the following patch:

*************** class GZipContentEncoding(OutputTransfor
*** 1670,1675 ****
--- 1670,1676 ----
("Content-Encoding" not in headers)
if self._gzipping:
headers["Content-Encoding"] = "gzip"

•         headers["Vary"] = "Accept-Encoding"
        self._gzip_value = BytesIO()
        self._gzip_file = gzip.GzipFile(mode="w", fileobj=self._gzip_value)
        chunk = self.transform_chunk(chunk, finishing)
  

I'm using tornado==2.2