I want to log the number of bytes being transferred from my iPhone app to server so that I could put a watch on the bandwidth being eaten up and could take necessary actions.
How can I do this?
In Instruments there is Network Activity Monitor that does what you need. You can always correlate your findings with network usage numbers in your Settings.app on actual device. Settings->General->Usage
Update: Second approach:
On the picture below is traffic used for my app in 2 minutes.