Why does Web Server respond "304 Not Modified" when no Etag and no "If-Modified-Since" is in request header

I get back

304 Not Modified 

from the web server, but in Firebug, the request header doesn't have Etag and doesn't have If-Modified-Since, and I looked at all the header line and ther开发者_C百科e was no datetime info at all, so how does the server know the content is not modified?

The "ETag" header is sent by the server in the response - the client sends an "If-None-Match" back to the server with the ETag value if it already has it cached.