Gzip is an HTTP filter which enables Envoy to compress dispatched data from an upstream service upon client request. Compression is useful in situations where large payloads need to be transmitted without compromising the response time.



The window bits is a number that tells the compressor how far ahead in the text the algorithm should be looking for repeated sequence of characters. Due to a known bug in the underlying zlib library, window bits with value eight does not work as expected. Therefore any number below that will be automatically set to 9. This issue might be solved in future releases of the library.

How it works

When gzip filter is enabled, request and response headers are inspected to determine whether or not the content should be compressed. The content is compressed and then sent to the client with the appropriate headers if either response and request allow.

By default compression will be skipped when:

  • A request does NOT contain accept-encoding header.
  • A request includes accept-encoding header, but it does not contain “gzip”.
  • A response contains a content-encoding header.
  • A Response contains a cache-control header whose value includes “no-transform”.
  • A response contains a transfer-encoding header whose value includes “gzip”.
  • A response does not contain a content-type value that matches one of the selected mime-types, which default to application/javascript, application/json, application/xhtml+xml, image/svg+xml, text/css, text/html, text/plain, text/xml.
  • Neither content-length nor transfer-encoding headers are present in the response.
  • Response size is smaller than 30 bytes (only applicable when transfer-encoding is not chuncked).

When compression is applied:

  • The content-length is removed from response headers.
  • Response headers contain “transfer-encoding: chunked” and “content-encoding: gzip”.
  • The “vary: accept-encoding” header is inserted on every response.