Initial request for document
GET /livingdocs/current_specs.pdf HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip
Initial response from server
HTTP/1.1 200 OK
Etag: “1234 … 000"
Content-Encoding: gzip
Next request for document
GET /livingdocs/current_specs.pdf HTTP/1.1
Host: www.example.com
If-none-match: “1234...