How to measure nginx client's download speed rate - nginx

Question

I want to have an entry in my access log which shows client's download speed .
I know about limit_rate but clearly that is not what i want .
I have also searched in lua_module , but i couldn't find this variable .

Answer 1

http://wiki.nginx.org/HttpLogModule
$request_time, the time it took nginx to work on the request, in seconds with millisecond precision (just seconds for versions older than 0.5.19)
Maxim Dounin (nginx core dev):
$request_time is always time since start of the request (when first
bytes are read from client) till end of the request (when last bytes
are sent to client and logging happens).
$bytes_sent, the number of bytes transmitted to client
$body_bytes_sent, the number of bytes, transmitted to client minus the response headers.
With these variaves you get the amount you need

Answer 2

$request_time only indicates the time to read the client request and is independent from the server's processing time or response. So $request_length divided by $request_time would indicate the client's upload speed.
But the OP asked about the client's download speed, which means we need two pieces of information:
The number of bytes sent by the server (e.g. $bytes_sent or $bytes_body/Content-length assuming the header is relatively small);
The elapsed time between the first byte sent by the server and the last byte received by the client - which we don't immediately have from the NGINX server variables, since it's up to the TCP subsystem to complete the delivery and this is done asynchronously.
So we need some logic on the client side to calculate and report the latter value back to the application server: a do-it-yourself "speed test".
The NGINX $msec (current time in seconds with the milliseconds resolution) could be used to timestamp the start of the response with millisecond accuracy, perhaps by using a custom HTTP header field (e.g. X-My-Timestamp).
The X-My-Timestamp value roughly maps to the HTTP response header Date: field.
Further, the client could read its system clock upon the complete reception of the response. The delta between the latter and X-My-Timestamp (assuming UTC and NTP-synchronized clocks) would yield the elapsed time required to calculate the download speed.
The last step would be to have the client report the value(s) back to the server in order to take appropriate action.