Our application sends relatively large number of small requests (responses < 50 bytes) via HTTPS. I would like to reduce the server bandwidth usage. I want to tell how many bytes is used to establish the SSL connection, to see if it is worth to optimize this (maybe replace the SSL cert?).
Is there a command or tool I can use to measure the exact overhead of SSL of my website?
Unless you have a very low latency but low bandwidth connection the main performance problem is not the number of bytes transferred in the TLS handshake but the several round trips needed for the setup of the TCP connection and then the TLS handshake on top.
Thus is would be much better to change your application so that it uses the same TLS connection for multiple messages or at least to implement session resumption. Session resumption not only reduces the round trips needed but also the number of bytes sent within the handshake considerable.
And as suggested by the other answer wireshark is a useful tool to track such problems (like number of bytes transferred but also latency) and also to see the effects after optimization.
You should be able to use Wireshark to record the requests between client and server - You can then decrypt SSL traffic, if required, assuming you have the certificate, to better see what is happening in each packet.
You should stop as much other traffic on your machine before starting the capture, but if you can't clear it completely, you should be able to see a single TCP stream by right-clicking on a single packet in that stream and clicking Follow TCP Stream.
