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[liberationtech] Open-source GSM network update

Nathan of Guardian nathan at guardianproject.info
Mon Jun 20 20:41:10 PDT 2011


On 06/20/2011 08:26 PM, Moxie Marlinspike wrote:
> But based on the acrobatics we go through to get
> encrypted VoIP working acceptably over low-latency UDP links, I imagine
> that RTP over TCP alone would be mostly unusable, and that RTP over TCP
> over Tor would probably just be totally fucked.

It would sound like that moment where Neo enters the Matrix for the
first time (queue sound fx "mwwwwmwwaaaaaaaoaaawwwrrrrr."), or perhaps
slightly better than the first telephone call ever made. Regardless, I
think the last thing that Tor needs (at least for now) is a ton of
people making onion-routed telephone calls.

Fortunately, from my initial, limited experience with this system
(OpenBTS+USRP->Freeswitch->over BGAN, ->over Tor, etc), it seems like
the idea that you could drop a suitcase setup in somewhere, flip it on,
and tell everyone to go crazy making free phonecalls to Al Jazeera
hotlines is not a very feasible idea. Well, at least to sound slightly
more positive, the issue of scalability of voice applications,
particularly in these super dynamic contexts, needs to be seriously
investigated.

What could work though is SMS, perhaps MMS (if GPRS support existed), as
it is asynchronous (perfect for Tor), easier to scale on a limited
amount of hardware, cost effective (more on that later), and it all runs
over the SIP channel via TCP, if I am not mistaken. We even tested
TextSecure in this setup (sorry that wasn't mentioned earlier), and it
worked great, defeating any of the SIP/UDP keyword packet filtering we
attempted, for obvious reasons, though it was cool to watch the OTR
handshaking happen from the inside out.

Back to the pricing thing, it is said that typical SMS costs $1310/MB
(http://en.wikipedia.org/wiki/Text_messaging#Pricing_concerns), so
paying for the $5/MB cost of routing SMS over BGAN seems like an
insanely great deal.

Best,
 Nathan



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