I have for long been wanting to be able to check which programs download and upload data at any given point in time, and also determine their totals for a period of time (e.g., day, week, etc).
This as a matter of security. Sometimes, I see things that look strange, but I cannot determine what is going on.
For example, today my modem shows that is has uploaded over 550Mb of data for the day, which is just plain ridiculous. As I write this, I am running a virus scan in the background, but I would love to be able to open a program and see what applications are sending all of that data, where. I don't have anything in my DropBox folder that would warrant that kind of data "spend" and have not sent any files in other ways today. So what is sending all that data?

I remember from many years ago someone speaking about a "packet sniffer" but it's way beyond my field of experience.

Anyone have an idea for a program that I can install that can monitor all the data coming in and going out, and tell me:
-Which applications sent/received the data
-what the address was of the server to/from which the data came?