Project Fi and the Great Firewall

Jean-Francois Mezei jfmezei_nanog at
Sun Nov 15 20:20:07 UTC 2015

On 2015-11-14 23:59, Yucong Sun wrote:
> This is what roaming data means, Your data packet is simply trunked to
> your original operator to process.  So you will be having a US ip on
> the web.

Based on my understanding, the phone establishes a local IP aconnection
with equipment associated with an antenna and gets an IP a from it. It
then establishes a tunnel to the APN operated by your carrier and the
tunnel gets the IP address that your apps see/use.

The IP address your apps see/use is given by your home carrier and all
packlets flow through your home carrier's APN before going to the
internet and you use your home carrier's DNS.

Where I am unclear is what happens when you move from tower to tower.
Whether your local IP changes and the tunnel is transparently moved to
the new local IP, of whether the local IP address moves with you and
routing tables are changed.

Some phones have "debug" modes that will show both the local (local
antenna)  and the public IP address (from APN) in use.

As your traffic flows out of China, it passes through the "great wall of
routers" as traffic between you and your carrier's APN, not between you
and some banned site you are trying to access.

They'd have to do DPI and possibly decrypt tunnel traffic to catch where
you are trying to connect and block those.

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