How does a router calculate the amount of data used per device?

Question

I'm using WiFi of the firm I work for and they only allow an employee to use 30 GBs of data in a month. Now, based on the answer to this question, I wonder if I encrypt my traffic, would the firm still be able to calculate the amount of data I use?

Answer 1

A router doesn't need to inspect the actual traffic being sent to know how much is being sent.

So no, encryption probably wouldn't help in this case. Presumably they are monitoring your total traffic, rather than trying to block downloading on specific files based on headers (as in the linked question). Encryption can hide what exactly you are doing, but they can still see how much encrypted traffic you are sending/receiving.

The only way I could see this working is if for some reason they were monitoring the HTTP response headers for the Content-Length and basing your total traffic solely on that. I would be very surprised if this were the case.

Answer 2

To draw an analogy, sending unencrypted packets is like sending postcards in the mail, while sending encrypted packets is like sending sealed envelopes instead. In both cases the sender and addressee are visible to the mailman (the router) but in one case the contents of the packet are not visible to him.

If the router is watching and counting packets you send, encrypting your data won't stop the tallying of the amount you send. The router knows who sent and who will receive all of the packets that pass through it; but with encrypted packets it doesn't know what was inside (the contents) of those packets.

In your specific situation, if you need to log into an account to use the company WiFi, your company is tying any data usage to that account. If you are not logging in (i.e. if you are using some sort of generic guest or visitor account) they would have to tie data usage to the specific device you are using.

Answer 3

The router will know the MAC address of each connected device.

The MAC address uniquely identifies the connected device to the router. Therefore, the amount of data sent to and from each device can be accounted for.

However, if you encrypt your traffic, the connection still needs to span across layer 2 to get from your device to the router and back again. That means, the router will still see the MAC address in cleartext.

Therefore, the only effect encryption would have is to slightly increase the amount of traffic due to the overheads of encryption and the initial handshake.

If you wish to reduce the amount of traffic, compression would be the way to go. For example, by using the Google Data Saver extension so that your plain HTTP traffic goes via Google's proxy which reduces the amount of data transmitted.