Why would a browser make two separate requests for the same file?

Question

I'm debugging a program I wrote and noticed something strange. I set up an HTTP server on port 12345 that servers a simple OGG video file, and attempted to access it from Firefox.

Upon sniffing the network requests, I found these two requests were made:

GET /video.ogv HTTP/1.1
Host: 127.0.0.1:12345
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive


GET /video.ogv HTTP/1.1
Host: 127.0.0.1:12345
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
Range: bytes=8122368-

The video is almost 8 MB in size, so the fact that the second request specifics 8122368 bytes, which is 7932 KB, suggests it is requesting the very end of the file for some reason. Anyone have ideas?

Answer 1

Its actually requesting 8122368 bytes starting backwards from the end. Which is 7.74MB if I did my calcs correctly.

it might be something in how the buffering for that file type is done.

Answer 2

Some media format have meta data at the end of the file, and this data is usually required to allow proper seeking of the video.

Answer 3

In order to support seeking and playing back regions of the media that aren't yet downloaded, Gecko uses HTTP 1.1 byte-range requests to retrieve the media from the seek target position. So because Ogg files don't contain their duration, the initial download connection is terminated. Then there is a seek to the end of the Ogg file and read a bit of data to extract the time duration of the media. Info from here and here.