手机公司如何检测您是否共享3G / 4G互联网连接？

Question

I just spoke with a phone company in Denmark, that offer "Free 3G/4G", on the phone, but they have a limitation if you create a hotspot and share the connection, this limitation is on 50GB.

How do they detect if your mobile device is sharing the connection? Do they intercept some special headers that the computer/other phones send over the connection?

Answer 1

MAC addresses are used at a lower layer than IP and relate to each hop or leg of an end to end communication so they don't really help with this issue.

Detecting tethered devices is quite a complicated task and there are special solutions to do exactly this. They tend to look at multiple things to try to determine if other devices are using the mobiles connection, even if the devices are spoofing or manipulating headers etc.

Examples of things a solution will look at:

• number of simultaneous sessions
• HTTP user-agent headers
• device type
• device screen size
• TCP Timestamp
• TCP Source Port
• TCP Sequence Number
• Application-based correlation to TCP flows
• Node-pair correlation to TCP flows

See here for an example solution and more details of the above (this is just one example solution):

• https://www.sandvine.com/downloads/general/sandvine-technology-showcases/policy-control-for-connected-and-tethered-devices.pdf

Answer 2

We put the question to Three, which not too surprisingly declined to answer. However, if you think about it logically, it should be fairly simple to detect tethering. Every device with a network connection (including Wi-Fi) has a unique hardware identifier called a MAC address. Assuming that the operator can trace the final destination of the data packets, it should be able to determine that the final MAC doesn't match your phone's. source

Although the post is about UK operators, it should apply in Denmark.