mit roofnet robert morris daniel aguayo, john bicket, sanjit biswas, douglas de couto mit computer...
TRANSCRIPT
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MIT Roofnet
Robert MorrisDaniel Aguayo, John Bicket, Sanjit Biswas, Douglas De Couto
MIT Computer Science and Artificial Intelligence Laboratoryhttp://pdos.lcs.mit.edu/roofnet
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Talk Outline
1. Roofnet Overview2. Link-Quality-Aware Routing (ETX)3. Roofnet Performance and Status4. Opportunistic Routing (ExOR)
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The Roofnet Network54 nodes in students’ apartments
802.11 radios, antennas on roofs
Multi-hop routing to MIT’s campus net and the Internet
Gateways to DSL or campus net
2 km
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Existing Community Networks
• Goal: inexpensive sharing of Internet access
• Multi-hop mesh to extend reach
• Two+ directional antennas/node
• Use Internet routing protocols (OSPF)
photograph courtesy of BARWN.org
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Our Key Design Choice
Omni-directional antennas:Easy to installInexpensiveCan self-configureMore choice of neighborsBut no notion of a “link”…
Goal: enable much larger mesh networks
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Self-Installation Kits
Computer ($340)533 Mhz PC, hard disk,CDROM
802.11b card ($155)Engenius Prism 2.5, 200mW
Software (“free”)Our networking software
Omni Antenna ($65)8dBi, 20 degree vertical
Miscellaneous ($75)Chimney Mount, LightningArrestor, Wrench, etc.
50 ft. Cable ($40)Low loss (3dB/100ft)
Takes about 45 minutes to install
Total: $685
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Roofnet/Internet Connectivity
Internet
Roofnet Nodes5.x.x.x(172.16.x.x)
Wired GatewaysMIT campus net, orusers’ DSL
User’s LANLiving-room nets192.168.x.x
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Roofnet Node Software Structure
802.11 eth
Linux TCP/IP
Click
Kernel
User-space
sshd apachedhcpd
antenna
Living roomethernet
NAT
srcrr
ETX
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Talk Outline
1. Roofnet Overview2. Link-Quality-Aware Routing (ETX)3. Roofnet Performance and Status4. Opportunistic Routing (ExOR)
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Routing: Best Path from A to D?
E
A
B
D
C
• Internet approach: minimize the hop count• A-E-D
Radio connectivity
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Roofnet Throughput (Original)
Best possible routes
Minimumhop-count
routes
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Problem 1: Long Links Work Badly
100%
50% 50%
E
A
B
D
C100%100%
packet deliveryprobability
• Minimizing hop-count uses low-quality links• But S/N vs. BER specs suggest this isn’t important
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Roofnet Link Quality Distribution
Wide range of delivery ratios
Hard to say a link is either good or bad
Forward and reverse rates are often different
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One Link Over 24 Hours
• Cannot use Prism S/N ratio to predict link quality
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Problem 2: Asymmetric Links
“hello”E
A
B
D
C
datadata
acknowledgment
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Problem 3: Radios Share a Channel
E
A
B
D
C
• Nodes A, B, and C interfere• A-B-C-D: throughput is 1/3• A-E-D: throughput is 1/2
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Solution: ETX Metric
• Need to balance link quality, asymmetry, interference
• Idea: throughput 1 / (number of transmissions)– One transmission for each hop– One for each lost data packet (since 802.11 re-sends)– One for each lost acknowledgment
• ETX: “Expected Transmission Count”• Routing protocol chooses route w/ minimum
ETX
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Calculating Per-Link ETX
• ETX = 1 / P(delivery)• P(delivery) = P(data OK) * P(ACK OK)
• So, ETX = 1 / (df * dr)
• Each node periodically broadcasts a probe
• Neighbors measure df from probes
• Neighbors exchange df to get dr
• Problems: packet size, bit rate
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ETX Improves Roofnet Throughput
With ETX
WithoutETX
Best Possible
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SrcRR Routing Protocol
• Source routing, link-state database– DSDV isn’t stable when network is busy
• DSR-like queries to populate link-state database
• Keeping the source’s ETX values up to date:– Data packets accumulate latest per-hop ETX– Data packets carry random sample of nearby link ETXs– Ten 802.11 transmit failures: send link’s ETX back to source– One-way traffic: periodically send link’s ETX back to source
• Source only re-floods if half as good as original
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Talk Outline
1. Roofnet Overview2. Link-Quality-Aware Routing (ETX)3. Roofnet Performance and Status4. Opportunistic Routing (ExOR)
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End-to-end TCP Throughput
• Median about 1 mbit/s, max about 3.6 mbit/s
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End-to-end Ping Times
100-byte packets
1000-byte packets
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One-way Hop Counts
100-byte packets
1000-byte packets
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End-to-end Ping Loss Rates
100-byte packets
1000-byte packets
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Path Self-Interference?
A B C D
• A sends RTS, B sends CTS, A sends long packet• C heard the CTS and won’t send• What if D forwards a packet to E?• Will that repeatedly waste A’s entire transmission?
E
range
carrier sense range?
interference range?
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Token-Passing
• Goal: eliminate path self-interference• Send a token back and forth along active DSR
path• Holder can forward 10 packets, then forwards
token• Increases throughput a lot:
– Avoids interference loss?– Helps 802.11 firmware stay at high bit rates?
• Problems:– Doesn’t help much if two paths are active– Lost token? Duplicate token? Idling and re-creating the token?
• This is a major focus for us right now
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Other Current Roofnet Problems
1. Prism 2.5 MAC carrier sense ignores many packets– Declares “carrier” if signal strength above threshold– Threshold is too high, cannot be set lower– Result: two nodes transmit at the same time and interfere
2. Fix ETX to guess which links will run at 11 mbps
3. Prism 2.5 firmware is too timid about high rates
4. Only security problem: users running viruses/DDoS
5. We’ve lost one node to lightning
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Talk Outline
1. Roofnet Overview2. Link-Quality-Aware Routing (ETX)3. Roofnet Performance and Status4. Opportunistic Routing (ExOR)
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Routing: The Traditional View
src
A B
dst
C
• Measure all link qualities• Pick the best route• Forward data along that route’s links• This strategy is optimal for wired networks
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How Radios Actually Work
• Every packet is a radio broadcast…
src
A B
dst
C
3
3
3
1
1
1
2
2
4 4
4
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Assumptions
1. Many receivers hear every broadcast2. Gradual distance-vs-reception tradeoff3. Receiver losses are uncorrelated
src
A B
dst
C
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1. Multiple Receivers per Transmission
• Broadcast tests on rooftop network– Source sends
packets at max rate
– Receivers record delivery ratios
• Omni-directional antennas
• Multiple nodes in “radio range”
1km
S
100%75%50%25%0%
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2. Gradual Distance vs. Reception Tradeoff
• Wide spread of ranges, delivery ratios• Transmissions may “get lucky” and travel long distances
Distance (meters)
Deliv
ery
Rati
o
Same Source
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3. Receiver Losses are Uncorrelated
• Two 50% links don’t lose the same 50% of packets• Losses not due to common source of interference
Example Broadcast trace:
Receiver 1 (38%):
Receiver 2 (40%):
Receiver 3 (74%):
Receiver 4 (12%):
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Extremely Opportunistic Routing (ExOR) Design
Goals• Ensure only one receiver forwards the
packet• Receiver “closest” to the destination should
forward• Lost agreement messages may be common• Let’s not get eaten alive by overheads
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Who Received the Packet?
• Slotting prevents collisions (802.11 ACKs are synchronous)• Only 2% overhead per candidate, assuming 1500 byte frames
payload ACK
payload ACK1cand1
src dest
cand2 cand3src ACK2 ACK3
src cand1 cand2 cand3
src dest
Standard unicast 802.11 frame with ACK:
ExOR frame with slotted ACKs:
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Slotted ACK Example
• Packet to be forwarded by Node C• But if ACKs are lost, causes confusion
payloadD C BA
A D
ACK
C
ACK
B
A B C D
X
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Agreeing on the Best Candidate
A: Sends frame with (D, C, B) as candidate set
A B C D
C: Broadcasts ACK “C” in second slot (not rx’d by D)
D: Broadcasts ACK “D” in first slot (not rx’d by C, A)
B: Broadcasts ACK “D” in third slot
Node D is now responsible for forwarding the packet
XXX
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Putting it all Together
• ExOR Protocol in a nutshell:– Forwarder picks candidate set (using n2 matrix of loss
rates)– Forwarder broadcasts packet– Candidates send slotted ACKs– Single candidate responsible for forwarding
• Backup Duplicate Detection
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Protocol Simulation
• Methodology– Use Roofnet delivery ratios and topology– Every node has full matrix of inter-node loss rates– Loss rates constant over time
• Performance Measure: Total Transmissions– Simulator cannot compute throughput properly– Total transmission count is probably inverse of
throughput
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ExOR Outperforms Best Static Route
• Performance of all 402 possible routes (sorted)• Gains up to 2x on longer routes
Node Pair
Num
ber
of
Tra
nsm
issi
ons
Best Static Route
ExOR (4 ACKs)
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Transmission Distance
• Best static route: 5.94tx, ExOR: 3.3tx• ExOR moves packets farther using a variety of links
Distance (100m bins)
Num
ber
of
Tra
nsm
issi
ons
Static Route, Hops 2,4
ExOR
Static Route, Hops 1,3
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Roofnet Summary
• Roofnet provides useful broadband Internet access
• Wireless breaks standard routing assumptions
• Current areas of research:• Scheduling to avoid inter-hop
interference• ExOR
• Future areas of research:• Transmit power control• Routing-aware carrier sense
http://pdos.lcs.mit.edu/roofnet
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http://pdos.lcs.mit.edu/roofnet/
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Simulated ExOR Performance
Simulation of 50 nodes using loss rates from UCLA sensor networkReduces transmissions by nearly 2x over best predetermined path
Num
ber
of
Tra
nsm
issi
ons
Node Pair
Single Route
Opportunistic
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RSSI vs. Delivery Ratio