Overview of Mesh Networking Research @ MSR

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					Overview of Mesh Networking Research

Jitendra Padhye
Microsoft 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?

                      Mesh Router           Destination

  With a single radio, a node can not transmit and receive

  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
   – 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
    – 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
    – 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 Choice
         Multi-hop networking at layer 2.5

     –    NDIS miniport – provides virtual adapter on virtual link
     –    NDIS protocol – binds to physical adapters that provide next-hop
     –    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

     –    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 hops
    Our Solution: multi-radio meshbox, directional ant., NLDP, Interference management, Capacity-cal

    – 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 challenging
    Our 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                       8000

    (NetGear card)                                        7000

                                      Throughput (Kbps)


• Randomly selected 100 sender-                           4000

  receiver pairs (out of 23x22 =                          3000

  506)                                                    1000

                                                                  0   1        2         3         4         5   6
• 3-minute TCP transfer, only one                                         Byte-Averaged Path Length (Hops)

  connection at a time
                                                    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              Advantages
  broadcasts a probe                     – Explicitly takes loss rate into
• The probe carries information          – Implicitly takes interference
  about probes received from               between successive hops into
  neighbors                                account
                                         – Low overhead
• Each node can calculate loss
  rate on forward (Pf) and reverse    Disadvantages
  (Pr) link to each neighbor             – PHY-layer loss rate of broadcast
                                           probe packets is not the same as
• Selects the path with least total        PHY-layer loss rate of data packets
  ETX                                         Broadcast probe packets are
                                              Broadcast packets are sent at
                    1                           lower data rate
   ETX                                  – Does not take data rate or link load
           (1  Pf) * (1  Pr)             into account

                                        Developed by De Couto et al @ MIT (2003)
Baseline comparison of Metrics
Single Radio Mesh

Experimental Setup                    Median path length:
                                                                         HOP: 2, ETX: 3.01, RTT: 3.43, PktPair: 3.46

• 23 node testbed

•   One IEEE 802.11a radio per node                               1400

                                       Median Throughput (Kbps)
    (NetGear card)                                                1200


• Randomly selected 100                                           800
  sender-receiver pairs (out of                                   600
  23x22 = 506)                                                    400

• 3-minute TCP transfer, only
  one connection at a time                                                HOP        ETX         RTT       PktPair

                                                                                ETX performs the best
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)
    –   Loss rate p
    –   Bandwidth B
    –   Mean packet size S
    –   Min backoff window CWmin

Takes bandwidth and loss rate of the link into account

               ETT  ETxmit  ETbackoff

                             S                          CWminf(p)
               ETxmit                    ETbackoff 
                          B(1  p)                       2(1  p)
                            i 7
               f(p)  1   2(i 1) p i
                            i 0
WCETT = Combines link ETTs

Need to avoid unnecessarily       Given a n hop path, where each hop
long paths                        can be on any one of k channels, and
                                  two tuning parameters, a and b:
   - bad for TCP performance
   - bad for global resources

                                   WCETT 
                                           a*  ETT  b* max    n

                                                                  i 1
                                                                         i          1 j k
All hops on a path on the same                                               a b
channel interfere                  where
 – Add ETTs of hops that are on    Xj             ETT             i
   the same channel                       hop i is on channel j

 – Path throughput is dominated
   by the maximum of these
   sums                           Select the path with min WCETT
Baseline Comparison of Metrics
Two Radio Mesh

Experimental Setup
                                                             Median path length:
                                                                            HOP: 2, ETX: 2.4, WCETT: 3
• 23 node testbed
                                                                       Median Throughput of 100 transfers

• Randomly selected 100

                                                                       2989.5                     Single Radio
  sender-receiver pairs (out of                       3000
                                                                                                  Two Radios
  23x22 = 506)

                                  Throughput (Kbps)

• 3-minute TCP transfer
                                                      1500                          1379
                                                      1000                                                        844

• Two scenarios:                                      500

   – Baseline (Single radio):                           0
                                                                 WCETT                ETX                Shortest Path
       802.11a NetGear cards

    – Two radios                                         WCETT utilizes 2nd radio better
        802.11a NetGear cards                           than ETX or shortest path
        802.11g Proxim cards
Path Length and Throughput
Which metric is best?
                                                                           WCETT   ETX   HOP

Experimental Setup                                            3.5


•   23 node testbed                                           2.5

                                                 Hop Length
•   Randomly selected 100 sender-                             1.5
    receiver pairs (out of 23x22 = 506)                         1

•   3-minute TCP transfer (transmit as                          0
    many bytes as possible in 2                                      A     C       D      E      F
                                                                           WCETT   ETX   HOP
    minutes, followed by 1 minute of                                     Testbed Configuration
    silence)                                                  4000

                                          Throughput (Kbps)

    For 1 or 2 hop the choice of                              1500

    metric doesn’t matter                                     1000
                                                                     A     C       D      E      F
                         Comparison of Metrics
                         Wireless Office Scenario

                                           23 node indoor testbed. Two radios (both 802.11a) per node.
                                                           11 active clients, 4 servers.

                                         Light Office Traffic                                                        Heavy Office Traffic
                                1 hour, 415 sessions, 19.72 MB total                                        1 hour, 308 sessions, 587.5 MB total
                        10000                                                                       10000
Additional Delay (ms)

                                                                            Additional Delay (ms)
                        1000                                                                        1000                862     943
                                                              474                                               590

                         100                 120
                                    89               82                                              100

                                                                                                                27      31      30

                          10        11
                                                              8                                       10
                                             6       5                6
                                    4        4       3        3                                                 4
                                                                      2                                                 3       3
                           1                                                                           1
                                 WCETT     ETX     HOP    PKTPAIR   RTT
                                                                                                             WCETT    ETX     HOP     PKTPAIR   RTT

                                         Relatively light traffic means performance is okay for all metrics.
                                         WCETT does better under heavy load (worst case delay)
Resiliency against Liars/Lossy Links

Problem                                            Simulation Results
•   Identify nodes that report incorrect
    information (liars)                                                                          Detect liars

•   Detect lossy links                                                        1

                                                   Fraction of lying nodes

Assume                                                                       0.6

•   Nodes monitor neighboring traffic, build                                 0.4

    traffic reports and periodically share info.                             0.2
•   Most nodes provide reliable information                                   0
                                                                                   NL=1   NL=2     NL=5     NL=8     NL=10 NL=15 NL=20

Challenge                                                                                         coverage      false positive

    Wireless links are error prone and unstable
                                                                                           Detect lossy links


                                                   Fraction of lossy links
•   Watchdogs

•   Find the smallest number of lying nodes to
    explain inconsistency in traffic reports
•   Use the consistent information to estimate                               0.2

    link loss rates                                                           0
                                                                                   NL=1   NL=2     NL=5   NL=8 NL=10 NL=15 NL=20

                                                                                                 coverage       false positive