Firebug why can the 'receiving' value in the net tab be zero?

Question

I am using Firebug and YSlow to benchmark a server on my LAN and on occaision I get '0 ms' for 'receiving'. According to Firebug (http://getfirebug.com/wiki/index.php/Net_Panel) this is supposed to represent the time it takes to read the data over the network.

However my results seem to indicate this happens in no time at all, yet other times gives a more likely result of a few milliseconds:

Anyone have a clue why this might happen? It often happens even if I clear cache and do a full page reload with Ctrl+F5.

Answer 1

You might be seeing the "BFCache" results; these typically have a very small "waiting" time (<20ms) and 0 "receiving" time because they are coming directly from the browser rather than from a new network request. These are shown in a lighter gray color, like the spacer.gif entry in your screenshot.

See Firebug 1.6: Tracking also BFCache reads for a more detailed explanation. You can turn these on/off with the Net tab options. (I usually keep them off for clarity.)

Updated: I did some further testing with the same settings you mentioned (browser cache disabled, not showing BFCache results) and I do see the same results using this page as an example. I also checked the same with Google Chrome Network panel and it has similar results. It looks like the "0 receive time" is happening only for the smallest response sizes (under about 1KB) which probably fit within a single network packet. I suspect this is an edge case in the way net panel timing is measured. That one packet signals both the start and end of the data transfer, so that may be why it's reported as a 0.