I come to you to humbly request your assistance, on or off list. This not
an urgent technical matter, but something I'm rather fascinated by at the
moment.
While in China recently, I noticed that my Project Fi phone was accessing
Google. Not only Google, but Facebook, YouTube, Gmail, Twitter, and many
other normally perma-blocked websites. It's taken me a few days of sleep
deprived thinking to realize this, but I'm seeing the same or similar
26.x.x.x addresses across countries I've visited, including China, Spain,
Malaysia, and Hong Kong.
I'm not a cellular guy and I know even less about MVNO's, but I'm curious
if I'm inferring the technical operations of the network correctly. It
sounds like the local cellular companies are provisioning access upon
arrival, then packing up the packets and shipping them off at layer 2 or
below to Google, who's then handling the IP stack and up internet access.
I'm also assuming the Great Firewall then acts above these layers since
it's not blocking access on my phone.
If my inference is correct, I'd be curious to see if those responsible for
the Great Firewall are aware of this deal Google has with a Chinese
cellular provider and the technical specifics of how it works. Might we be
seeing a softening of Great Firewall policies for foreigners, or just
another soon to be inspected or blocked flow of traffic?
Anyway, I'd just love to hear from a knowledgeable engineer about how this
works.
If you've read this far, thanks for your time and have a great day!
I come to you to humbly request your assistance, on or off list. This not
an urgent technical matter, but something I'm rather fascinated by at the
moment.
While in China recently, I noticed that my Project Fi phone was accessing
Google. Not only Google, but Facebook, YouTube, Gmail, Twitter, and many
other normally perma-blocked websites. It's taken me a few days of sleep
deprived thinking to realize this, but I'm seeing the same or similar
26.x.x.x addresses across countries I've visited, including China, Spain,
Malaysia, and Hong Kong.
26/8 is T-Mobile using DOD space for their internal addressing.
Irrespective of where you are your connected to the Same APN and traffic from your UE is indeed tunneled through the PGW
I know the service/device uses VPN if you are using "wifi assist" to
connect to an open WAP -- it automatically tunnels the traffic so it can't
be read by nearby snoopers. Perhaps they employ a similar technology or are
using something like PPP to take all of the traffic back to one (or many)
"access servers" before sending it off to the Internet. I have no
experience whatsoever in cellular network operations, but I know many
providers employ similar methodologies to assist in meeting their CALEA
requirements.
When you roam onto another cellular network other than your home network,
your data is encapsulated and sent back to your home network before going
out to the internet. This is to provide a seamless experience for the
customer.
The network it rides on is the GRX/IPX which is a a worldwide MPLS network
that the GSMA specified to make the data roaming experience work. The
GRX/IPX also can carry voice and text back to the home network. Since it is
a separate network from the Internet, the Great Firewall was bypassed.
There are several GRX/IPX providers and they all peer with each other in
key locations which usually end up being in the same major Internet peering
locations. TATA, Syniverse, SAP, Telia, and many others run an IPX/GRX
network and Equinix has IPX/GRX peering exchanges.
My team mate was traveling to China with his Nexus 6 (with Project Fi
SIM-card) and was able to access Google services. The phone uses roaming
data to access Google and your phone gets IP assigned by your home mobile
network packet gateway (P-GW). There is no local data break-out.
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.
Similar to the SS7 phone network where call signaling data is done on a totally different path then the actual rtp path.
Carlos Alcantar
Race Communications / Race Team Member
1325 Howard Ave. #604, Burlingame, CA. 94010
Phone: +1 415 376 3314 / carlos@race.com / http://www.race.com
With Wi-Fi calling it gets a bit more simplified (no "transit" operators in
user plane) and may provide better privacy (only your home country will
monitor your calls, lol). The UE establishes IPsec tunnel over the Internet
to the home operator and uses it for native VoIP/messaging applications.
Part of the IPX spiel has been about encouraging local break-out to
improve the practical experience of the roamer. However, the excuse this
does not happen is the difficulty that brings to billing, despite all
the talk about Diametre signaling in IPX infrastructure...
You can imagine what my experience is like roaming in Honolulu when I
live in South Africa...