overview of mesh networking research @ msr
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Overview of Mesh Networking Research @ MSR. Jitendra Padhye Microsoft Research January 23, 2006. What are mesh networks?. Multi-hop wireless networks Mostly static nodes Unplanned node placement - PowerPoint PPT PresentationTRANSCRIPT
Overview of Mesh Networking Research @ MSR
Jitendra PadhyeMicrosoft Research
January 23, 2006
What are mesh networks?
• Multi-hop wireless networks
• Mostly static nodes
• Unplanned node placement
• Applications: Disaster relief, Backhaul for city-wide wireless networks, Meeting mesh, Neighborhood Meshes, internet connection sharing
• Many startups ….
Three main problems in mesh networking
• Capacity
• Capacity
• Capacity
Why is capacity a problem?
SourceMesh Router Destination
With a single radio, a node can not transmit and receive simultaneously.
A two-hop path has half the capacity of a one-hop path. Other interference patterns also possible.
Seminal Result by Gupta and Kumar (2000):Capacity = O(1/sqrt(n))
MSR’s research on Mesh Network Capacity
• Capacity estimation
• Capacity improvement using multiple radios and other techniques
• Feasibility study using realistic traffic
Mesh Network Capacity Estimation
• New framework for estimating capacity of multi-hop wireless networks– Gupta-Kumar result is asymptotic
– Our framework calculates optimal capacity of a given mesh network for given set of flows
MobiCom 2003 (Jain, Padhye, Padmanabhan and Qiu).
• Our framework requires knowledge of which links interfere with one another– Problem of “conflict graph” estimation
– N nodes O(N^2) links O(N^4) pairs!
– We developed an approximation technique that takes O(N^2) time IMC 2005 (Padhye, Agarwal, Padmanabhan, Qiu, Rao and Zill)
Key Insight: Multiple radios necessary to improve capacity
Improving capacity using Multiple Radios
• Select best radio to send each packet using locally available information– Multi-radio unification protocol
IEEE BroadNets 2004: Adya, Bahl, Padhye, Wolman and Zhou)
– Problem: sub-optimal in many cases
• Optimize entire path for a given flow– Take into account interference and link capacity along entire path
– Implemented in Mesh Connectivity Layer (MCL) MobiComm 2004: Padhye, Draves, Zill
• If second radio has very low bandwidth, can we use it to offload signaling?– Simulation-based study of separating control and data into different
frequency bands IEEE BroadNets 2005 (Kyasanur, Padhye, Bahl)
How do we know how much capacity is “enough”?
Feasibility study using realistic traffic
• Collect traffic traces from Microsoft’s wired network
• Replay on mesh testbed
• Study delay characteristics of replayed traffic
• Conclusions: – Factors such as specific card brands, placement of servers have
significant impact, routing metrics have less impact.
– 2-radio mesh network likely sufficient for supporting normal office traffic
– Some large delay spikes.
• MobiSys 2006 (Eriksson, Agarwal, Bahl, Padhye)
Ongoing work related to capacity:
• Capacity improvement using network coding
• Use of directional antennas to reduce interference
• Use of spectrum etiquettes and cognitive radios to improve spectrum utilization
Other challanges:
• Self-management– Network without administrator – is it possible?
– Engineering challenges such as automatic address assignment
• Security and Fairness– Freeloaders
– Information leakage by observing traffic
– Malicious nodes can disrupt routing
Backup slides
Mesh Connectivity Layer (MCL)Design & Implementation
Design ChoiceMulti-hop networking at layer 2.5
Framework– NDIS miniport – provides virtual adapter on virtual link– NDIS protocol – binds to physical adapters that provide next-hop
connectivity– Inserts a new L2.5 header
Why Layer 2.5?– Works over heterogeneous links (e.g. wireless, powerline)– Transparent to higher layer protocols.
• works equally well with IPv4 and IPv6– ARP etc. continue to work without any changes
Features– DSR-like routing with optimizations at virtual link layer
– Link Quality Source Routing (LQSR)– Incorporates 5 different link selection metrics:
– Hop count, RTT, Packet Pair, ETX, WCETT
Scope: Technical Problems we looked at
Range and Capacity– Off-the-shelf wireless hardware Is severely range limited – Throughput of 802.11 MAC degrades rapidly with the number of hopsOur Solution: multi-radio meshbox, directional ant., NLDP, Interference management, Capacity-cal
Routing– Network connectivity is highly dynamic– Classical single path & shortest path routing perform poorly in a dense network
Our Solution: LQSR & MR-LQSR, WCETT (ETX, PacketPair, RTT,..)
Security and Fairness– Mesh is susceptible to freeloaders and malicious users– Achieving “fairness” without topological and traffic information is difficult
Our Solution: “Windows certificate", greedy behavior detection, watchdog mechanism, intrusion detection
Self Management– End users are non-technical– A no-network operator model is challengingOur Solution: M3, watchdog mechanism, data cleaning, liar detection, on-line network simulation, beacon stuffing,
server placement
Spectrum Management– Tragedy of the commons– Exploit spectrum white space
Our Solution: Control channel, dual-frequency meshes, 700-900 MHz, Spectrum etiquettes
Impact of path length on throughput
Experimental Setup
• 23 node testbed
• One IEEE 802.11a radio per node (NetGear card)
• Randomly selected 100 sender-receiver pairs (out of 23x22 = 506)
• 3-minute TCP transfer, only one connection at a time
0
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6000
7000
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9000
10000
0 1 2 3 4 5 6
Byte-Averaged Path Length (Hops)
Th
rou
gh
pu
t (K
bp
s)
If a connection takes multiple paths over lifetime, lengths are byte-averaged
Total 506 points.Solution: Multi-Radio Meshes
Link Selection Metrics
Many metrics have been studied in literature– Hop count– Round trip time– Packet pair– Expected data transmission count incl. retransmission– Weighted cumulative expected transmission time– Signal strength stability– Energy related– Link error rate– Location related– …
The ones in red are implemented in MCL
Link Selection Metric for Single Radio: ETX
• Each node periodically broadcasts a probe
• The probe carries information about probes received from neighbors
• Each node can calculate loss rate on forward (Pf) and reverse (Pr) link to each neighbor
• Selects the path with least total ETX
Advantages– Explicitly takes loss rate into
account– Implicitly takes interference
between successive hops into account
– Low overhead
Disadvantages– PHY-layer loss rate of broadcast
probe packets is not the same as PHY-layer loss rate of data packets
Broadcast probe packets are smaller
Broadcast packets are sent at lower data rate
– Does not take data rate or link load into account)P(1*)P(1
1ETX
rf
Developed by De Couto et al @ MIT (2003)
Baseline comparison of Metrics Single Radio Mesh
Experimental Setup
• 23 node testbed
• One IEEE 802.11a radio per node (NetGear card)
• Randomly selected 100 sender-receiver pairs (out of 23x22 = 506)
• 3-minute TCP transfer, only one connection at a time
ETX performs the best
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600
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HOP ETX RTT PktPair
Med
ian
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rou
gh
pu
t (K
bp
s)
Median path length:HOP: 2, ETX: 3.01, RTT: 3.43, PktPair: 3.46
Link Selection Metric for Multiple Radios: WCETT
State-of-art metrics (shortest path, Packet Pair, RTT, ETX) do not leverage channel, range, data rate diversity
Multi-Radio Link Quality Source Routing (MR-LQSR)– Link metric: Expected Transmission Time (ETT)
Takes bandwidth and loss rate of the link into account
– Path metric: Weighted Cumulative ETTs (WCETT) Combine link ETTs of links along the path Takes channel diversity into account
– Incorporates into source routing
Developed by Draves, Padhye et al @ MSR(2004)
Expected Transmission Time (ETT)
Given:– Loss rate p– Bandwidth B– Mean packet size S– Min backoff window CWmin
Takes bandwidth and loss rate of the link into account
7i
0i
i1)(i
min
backoffxmit
backoffxmit
p21f(p)
p)2(1
f(p)CWET
p)B(1
SET
where,
ETETETT
WCETT = Combines link ETTs
Need to avoid unnecessarily long paths - bad for TCP performance - bad for global resources
All hops on a path on the same channel interfere– Add ETTs of hops that are on
the same channel
– Path throughput is dominated by the maximum of these sums
Given a n hop path, where each hop can be on any one of k channels, and two tuning parameters, a and b:
j channelon is i hopij
jkj1
n
1ii
ETTX
ba
Xmaxb*ETTa*WCETT
where
Select the path with min WCETT
Experimental Setup
• 23 node testbed
• Randomly selected 100 sender-receiver pairs (out of 23x22 = 506)
• 3-minute TCP transfer
• Two scenarios:– Baseline (Single radio):
802.11a NetGear cards
– Two radios 802.11a NetGear cards 802.11g Proxim cards
Median Throughput of 100 transfers
16011379
1155
2989.5
1508
844
0
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3500
WCETT ETX Shortest PathT
hro
ug
hp
ut
(Kb
ps)
Single Radio
Two Radios
WCETT utilizes 2nd radio betterthan ETX or shortest path
Baseline Comparison of Metrics Two Radio Mesh
Median path length:HOP: 2, ETX: 2.4, WCETT: 3
Path Length and ThroughputWhich metric is best?
Experimental Setup
• 23 node testbed
• Randomly selected 100 sender-receiver pairs (out of 23x22 = 506)
• 3-minute TCP transfer (transmit as many bytes as possible in 2 minutes, followed by 1 minute of silence)
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500
1000
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A C D E F
Testbed Configuration
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WCETT ETX HOP
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A C D E F
Testbed Configuration
Hop
Len
gth
WCETT ETX HOP
For 1 or 2 hop the choice of metric doesn’t matter
Comparison of MetricsWireless Office Scenario
4 4 3 3
89120
82
474
116 5
8
2
179
6
1
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10000
WCETT ETX HOP PKTPAIR RTT
Ad
dit
ion
al D
ela
y (
ms)
4 3 3
862 943
27 31 30
590
1
10
100
1000
10000
WCETT ETX HOP PKTPAIR RTT
Ad
dit
ion
al D
ela
y (
ms)
23 node indoor testbed. Two radios (both 802.11a) per node. 11 active clients, 4 servers.
Heavy Office Traffic1 hour, 308 sessions, 587.5 MB total
Light Office Traffic1 hour, 415 sessions, 19.72 MB total
Relatively light traffic means performance is okay for all metrics. WCETT does better under heavy load (worst case delay)
Management: Resiliency against Liars/Lossy Links
Problem• Identify nodes that report incorrect
information (liars)• Detect lossy links
Assume• Nodes monitor neighboring traffic, build
traffic reports and periodically share info.• Most nodes provide reliable information
Challenge Wireless links are error prone and unstable
Approach• Watchdogs• Find the smallest number of lying nodes to
explain inconsistency in traffic reports• Use the consistent information to estimate
link loss rates
Detect liars
0
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1
NL=1 NL=2 NL=5 NL=8 NL=10 NL=15 NL=20
Fra
ctio
n o
f ly
ing
no
des
id
enti
fied
coverage false positive
Detect lossy links
0
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1
NL=1 NL=2 NL=5 NL=8 NL=10 NL=15 NL=20
Fra
ctio
n o
f lo
ssy
links
id
enti
fied
coverage false positive
Simulation Results