Transcript Research is communication

Architecting Citywide Ubiquitous Wi-Fi Access
Nishanth Sastry
Jon Crowcroft, Karen Sollins
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Architecting Citywide Ubiquitous Wi-Fi Access
I: What’s wrong with sharing Wi-Fi?
II: Tunneling based Architecture to
safely & securely share Wi-Fi
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Terminology
Host AP +
Firewall +
NAT
Host
Guest’s Home
Guest
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What’s wrong with sharing Wi-Fi? (1/2)
Malicious guests can ...
 be bandwidth hogs
 infect host computers


Use bandwidth
limiters &
firewalls
download illegal content
be part of DDoS botnet*
*Where each flow is too small to be detected
Nishanth Sastry
Hosts have to
trust guests to
be well-behaved
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What’s wrong with sharing Wi-Fi? (1⅜/2)
Then there are the freeloaders...
seeking better connectivity than their homes
And kids escaping parental control software
@ home
How do we induce hosts to share Wi-Fi?
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What’s wrong with sharing Wi-Fi? (1⅝/2)
Captive portals, commonly used
for logins at public hotspots
(e.g. cafés & Fon), are
essentially dynamic firewalls
& are susceptible to users
who sniff & spoof an
authenticated user’s address
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What’s wrong with sharing Wi-Fi? (2/2)
Hosts can be malicious too. e.g. Pharming
Guest has to trust
host router!
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How to safely share Wi-Fi?
Eliminate latent trust dependencies
Home
 takes on responsibility for guest’s traffic
 hides guest traffic from host by encrypting
 acts as trusted source for guest DNS/IP
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Tunneling removes dependencies
Guest
Host AP +
Firewall +
NAT
Trusted Services
Tunnel
VPN server
vpn-local IPGuest’s DHCP
NAT beyond tunnel
Host
Guest’s Home
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Tunnel setup: Co-operative
Host AP +
Firewall +
NAT
Guest
coop-local IP
Co-op distributes two registries:
Coop-local IP  Member ID
Mapping of members’ ISP assigned IP
Guest’s Home
STUN
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But, what about performance?
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Path length inflation
Intra-City Latency
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30—60ms [Lakshminarayanan IMC’03]
Guest downlink = home downlink+uplink!
 Asymmetric broadband  limited uplinks
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Median uplink bandwith = 212 Kbps [ibid]
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Sufficient for emergency response [LeMay earlier]
Performance comparable to p2p flows
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Scale and scope of the co-op
depends on:
 regional laws governing “legal” content
technical factors...
 end2end latency
 sizeof(coop-local IP space)
 AP memory for home & coop-local IP tables
Works for citywide co-ops (broadband members)
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Technical summary
5. vpn-local IP
Guest
1.coop-local IP
3.Tunnel
4. Guest’s Home
2. STUN
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Key features enabled by home

Accountability in IP tracebacks

Simultaneous access through multiple hosts
 crucial for access with weak signals
5. vpn-local IP
Guest
1.coop-local IP
3.Tunnel
4. Guest’s Home
2. STUN
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Two paths to adoption

I: Without ISP support: Will host’s ISP let it
share its connection?
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hinges on what “internet connection” is
mandate sharing! unlicensed spectrum is public good
II: With ISP support: offer business model

Think Comcast Voice citywide!
Co-op can benefit from ISP:
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increase uplink bandwidth for guest access
make better tunnels (e.g. MPLS VPNs)
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Mesh networks  dense deployment
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Co-op tunnels ≠Mobile IP tunnels
X
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Triangular routing not possible
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External node typically initiates contact
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Need to register “care-of address” precludes
highly mobile guests like cars
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Local IP addresses
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vpn-local/coop-local IPs are private IPs
vpn-local is local to guest-home pair
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can be reused by host & other guests
coop-local is local to guest-host pair
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can be reused on office VPNs of guest/host
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Dealing with NATs
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Restricted Cone or Symmetric NAT
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Punch holes separately to each member
NATs with deep packet inspection
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STUN/rendezvous server acts as relay
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