XORs in The Air: Practical Wireless Network Coding

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XORs in The Air Practical Wireless Network Coding
Sachin Katti, Hariharan Rahul, Wenjun Hu, Dina
Katabi, Muriel Medard, Jon CrowcroftMIT CSAIL
University of Cambridge
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Introduction - COPE

• New Architecture For Wireless Mesh Networks
• Routers Forward Code (Mix) Packets
• Intelligent Mixing Improves Throughput
• Prior Work Theoretical Multicast
• This Study Practical Unicast

3
COPE

• Coding Shim Between IP and MAC Layers
• Finds Coding Opportunities
• IDEA Benefits by Sending Multiple Packets in
  Single Transmission

4
COPE Simple Example

• Bob Alice
• Current Approach 4 Transmissions
• COPE 3 Transmissions
• Thus Allowing For Increased Throughput
• CodingMAC

5
COPE Even Bigger Savings

• Previous Example Obvious Throughput Gains
• COPE Exploits Shared Nature of Wireless Medium
• Some Nodes Overhear Transmission
• Store These Packets for Short Time
• Sends Data Out Telling What It Has Heard
• This Data is Used For Opportunistic Coding

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Opportunistic Coding

• Each Node Uses Knowledge of What Its Neighbors
  Have (Which Packets)
• This Data Allows For Source to Send XORed
  Packets Intelligently
• In Other Words It Knows who Will Be Able to
  Decode The Encoded Packet
• Allows For More Than Two Flows
• Allows For Multiple Packets to be Coded

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COPE 2 Key Principles (1)

• COPE Embraces Broadcast Nature of Wireless
  Channel
• Typically Network Designers Use Point-to-Point
• Adaptations are Made To Use Forwarding and
  Routing Techniques Native to Wired Networking
• COPE Exploits Radio Broadcast (Does Not Attempt
  to Hide It)

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Why is COPE Different?

• 1st System Architecture For Wireless Network
  Coding
• Implementation 1st Deployment of Wireless
  Network Coding
• Performance Study with Results of Wireless
  Network Coding

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Summarized Findings

• Network Coding Can Improve Wireless Throughput
• When Congested with Mainly UDP Flows Throughput
  Gains 3 - 4x
• For Mesh Networks Connected to Internet via AP
  Gains Depend on Total Download / Upload Traffic _at_
  AP
• w/o Hidden Terminals TCP Throughput Increases
  about 38

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Background

• Famous Butterfly Example

• All Links Can Send One Message Per Unit of Time
• Sources Want to Hit Both Receivers
• Coding (Again) Increases Overall Throughput

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Background (cont.)

• Ahlswede et al. Pioneered Network Coding
• Routers That Mix Information Allow Communication
  to Achieve Multicast Capacity
• Li et al. Found For Multicast Linear Codes
  Sufficient to Achieve Max Capacity Bounds

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Background (cont.)

• All Previous Work Theoretical Multicast
• Some Unicast Scenarios Show Better Throughput for
  Those Scenarios
• This Paper Seeks to Bridge the Gap Between
  Network Coding, Practical Design
• Seeks to Provide Operational Protocol

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COPE Overview

• Before Getting Into The Details Know These Terms

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COPE Overview

• 3 Main Techniques
• Opportunistic Listening
• Opportunistic Coding
• Learning Neighbor State

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COPE OverviewOpportunistic Listening

• Wireless is a Broadcast Medium
• Many Chances for Nodes to Overhear
• COPE Sets All Nodes as Promiscuous
• They Store Overheard Packets for Time (T)
• Default T .5 Seconds
• Also Reception Reports Are Sent Out
• These Are Tacked Onto Normal Output
• Includes Seq. Number of Stored Packets

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COPE OverviewOpportunistic Coding

• Which Packets do we Combine to Achieve Maximum
  Throughput?
• Simple Answer Send as Many (Native Packets) as
  Possible While Ensuring Nexthop Has Enough Info
  to Decode

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COPE OverviewOpportunistic Coding

• Always Seeking Largest N That Satisfies Above Rule

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COPE OverviewLearning Neighbor State

• Each Node Announces Its Stored Packets In
  Reception Reports
• Sometimes Reports Dont Get Through
• Congestion or In Times of Light Traffic
• To Solve This Problem Intelligent Guess
• Estimation of Probability That Neighbor Has
  Packet Based On Delivery Probability

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COPEs Gains

• How Beneficial is COPE?
• Throughput Improvement Depends On
• Coding Opportunities
• Traffic Patterns

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COPEs GainsCoding Gain

• Coding Gain
• Transmissions w/o Coding to the Minimum
  Transmissions w/ Coding
• Remember Alice Bob?
• Coding Gain 4/3 1.33

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COPEs GainsCoding Gain

• Maximum Achievable Coding Gain?
• For Arbitrary Topologies - Open Question
• Authors Prove

• With Listening Certain Topologies Benefit

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COPEs GainCoding Gain

• Interesting to Note
• Previous Slide Talks About Theoretical Gain
• In Practice Gains Are Lower Due To
• Coding Opportunities
• Packet Header Overhead
• Medium Losses
• COPE Coding Gains Are Not Lost When Medium Is
  Fully Utilized (as Opposed to Opportunistic
  Routing)

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COPEs GainCodingMAC Gain

• Interaction Between Coding MAC
• Beneficial Results
• Example Bob Alice
• MAC Divides Bandwidth Between 3
• w/o Coding Router Sends 2 x More
• Makes Router a Bottleneck
• COPE Allows Routers Queue to Drain Fast
• Coding MAC Gain of Alice Bob 2

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COPEs GainCodingMAC Gain

• Authors Prove

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Making It Work - Packet Coding Algorithm

• Packets Are Never Delayed
• If There Is Nothing To Code With, Send Anyway
• Preference to XOR with Similar Lengths
• Small Packet XOR with Large Less Bandwidth
• If One Must XOR Different Lengths - Pad
• Never Code Packets to Same Nexthop

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Making It Work - Packet Coding Algorithm

• Searching For Appropriate Packets to Code is
  Efficient
• FIFO Output Queue
• De-queue, Small or Large?, Look at Appropriate
  Queues (Only Heads to Avoid Reordering)
• Worst Case - Looks _at_ 2M Packets (M Neighbors)
• Packet Reordeing Bad (TCP Thinks Congestion)
• Doesnt Happen Much But If so They Are Put in
  Order Before Transport Layer

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Making It Work - Packet Coding Algorithm

• Finally Relay Nodes All Estimate Probability That
  Neighbor Has Packet Prior to Sending

• PD Must Stay Higher Than Threshold G (G 0.8
  Default)
• If Equation Is Above G Each Nexthop Has
  Probability G of Being Able to Decode Next Packet

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Making It Work - Packet Decoding

• Fairly Simple
• Each Node Maintains a Packet Pool
• Searches Hash Table Keyed on Packet ID
• XORs Native Packets with Coded Packets
• Gets Packet Meant For It (Node)

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Making It Work -Pseudo-Broadcast

• 802.11 Has Two MAC Modes
• Unicast
• Broadcast
• Unicast
• Packets Are Ack-ed
• Exponential Backoff
• Multicast
• Un-Reliable
• No Backoff

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Making It Work -Pseudo-Broadcast

• Pseudo-Broadcast
• Piggy Backs Unicast
• Link-Layer Destination Set to One Intended Node
• XOR Header Added
• Other Nodes Can Overhear Transmission
• If Receiving Node is Nexthop - Continue
• Else Store Packet in Buffer
• More Reliable Than Pure Broadcast
• Packets Have Several Tries To Get To Destination
• Snooping Nodes Get More Chances to Update Their
  Buffers

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Making It Work - Hop-by-Hop ACKs and
Retransmissions

• (Again) Encoded Packets Require All Nexthops to
  Acknowledge Receipt of Native Packet
• Packets Headed Many Places Only Link Layer
  Designated Hop Returns Synchronous ACK
• COPE May Guess Node Has Enough Info to Decode
  When it Really Does Not

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Making It Work - Hop-by-Hop ACKs and
Retransmissions

• When a Node Sends an Encoded Packet It Schedules
  a Retransmission Event For Each Encoded Native
  Packet
• If Any Packet is Not Ack-ed within Some Threshold
  (Time) That Native Packet is Encoded and Re-Sent
  Later
• Nexthops Receive Packets and schedule an ACK Upon
  Decoding via Header (or Control Packets which are
  also Used for Reception Reports) cumulative ACKs

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Making It Work - TCP Packet Reordering

• Asynchronous ACKs Can Cause Packet Reordering
• TCP May See This As Congestion
• COPE Has Ordering Agent
• For Each TCP Flow Ending _at_ Host
• Maintains Packet Buffer
• Records Last TCP Sequence Number
• Will Not Pass On Packets to Transport Layer Until
  No Hole Exists or Timer Times Out

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Implementation Details Packet Format

• Variable Length Coding Header
• Only Shaded Fields Below Required

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Implementation Details Packet Format

• First Block Metadata For Decoding
• ENCODED_NUM Encoded
• For Each Packet PKT_ID (Dest. IP Seq. )
• MAC of Nexthop (For Each Native Packet)
• Reception Reports
• REPORT_NUM of Reports
• SRC_IP Source of Reported Packets
• Last_PKT Last Packet Heard From Source
• Bit Map of Recently Heard Packets

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Implementation Details Packet Format

• Asynchronous ACKs
• Cumulative ACKs on Per Neighbor Basis
• Local Sequence Numbers Established
• ACK Headers Start With of ACKs
• Each ACK Starts with MAC of Neighbor
• Next Each ACK Has Pointer to End of Cumulative
  ACKs
• Finally, Bit Map Shows Missing Packets

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Implementation Details Control Flow
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Experimental Results

• 20 Node Wireless Testbed
• Following Slides Will Show
• When Many Random UDP Flows
• Throughput 3 - 4x Increase
• Traffic Does Not Use Congestion Control
• Throughput Improves - Exceeding Coding Gain
• Mesh Network -gt Internet via Gateway
• Throughput Improvement Between 5 - 70
• w/o Hidden Terminals TCPs Gain Agrees With
  Expected Coding Gain

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Experimental Results Testbed

• 20 Node Wireless Network
• Two Floors Connected by Open Lounge
• Offices, Passages, Etc.
• Paths Btw 1 6 Hops
• Loss Rate Btw 0 - 30
• 802.11a _at_ 6 Mb/s

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Experimental Results Testbed

• Nodes Ran Linux / Used Click Toolkit
• Runs as User Space Daemon
• Applications Interact With Daemon as Normally
  Would With Any Network Device
• Testbed Used Srcr Routing Protocol
• Djikstras Shortest Path Algorithm
• Each Node Had 802.11 Card w/ Omni-Directional
  Antenna
• 802.11 Ad Hoc Mode w/ RTS / CTS Disabled
• udpgen ttcp Used to Generate Traffic
• Long-live Flows Attempt to Match Internet
  Traffic

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Metrics

• Network Throughput
• End-to-End Throughput
• Throughput Gain
• Ratio of Measured Network Throughputs With and
  Without COPE
• What Else Might Have Been Interesting?

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COPE in Gadget Topologies

• Toy Topologies
• Very Small Loss Rate No Hidden Terminals (40
  Different Runs)
• Long-Lived TCP Flows
• Close To Expected (Minus Overhead)

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COPE in Gadget Topologies

• Above Results Show That w/ Congestion Control
  Results Lean Towards Coding Gain Rather than
  CodingMAC
• When Many Long-Lived Flows (TCP) Bottleneck
  Senders Backoff (to Avoid Drop)
• This Leaves only Coding Gains

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COPE in Gadget Topologies

• Repeat of Above w/ UDP
• CodingMAC Gains (Better Than TCP)
• Coding Allows Downstream Routers to Avoid
  Dropping Packets Already Having Consumed
  Bandwidth

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COPE in an Ad Hoc Network

• Here it is 20 Node Wireless Testbed
• TCP
• TCP Flows Arrive w/ Poisson Process
• Pick Sender Receiver Randomly
• Traffic Models Internet
• No Significant Improvement (2-3)
• Hidden Terminals are Culprit
• Many Retransmissions
• Queues _at_ Bottlenecks Never Build Up
• Therefore No Coding Gains (or Opportunities)
• Would TCP Do Better w/o Collisions?

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COPE in an Ad Hoc Network

• Compressed Topology
• Within Carrier Sense Range
• Artificially Impose Original Loss Rates
• Hidden Terminals No More
• At Peak 38 Gain Over No Coding

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COPE in an Ad Hoc Network

• UDP (Back to Large Scale Testbed)
• (Again) Random Sender / Receiver
• File Size Follows Internet Studies
• 500 Experiments

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COPE in an Ad Hoc Network

• Scare Coding Opp. At Low Demands
• Demand Up / Congestion Up / Gain Up

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COPE in a Mesh Access Network

• Growing Interest In Accessing Internet Via
  Multi-hop Network With One (or More) Gateways
• Nodes Divided Into 4 Sets (1 is Gateway)
• UDP Flows (Of Course)
• Fluctuate Upload / Download Traffic
• Gain Goes Up as Upload Traffic Up

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COPE in Mesh Access Network
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Fairness

• Channel From Source to Bottleneck Matters
• Capture Effect (If Alices Channel is Bad Then
  Bob Might Push More Traffic)
• If Alice Moves Slowly Away?
• Coding Opp. Down, Throughput Down, Fairness Down
• w/o Coding Throughput Goes Up
• Coding Aligns Fairness Efficiency

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Fairness
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Conclusions

• Coding is an Old Theme
• COPE Has Potential to Largely Increase Network
  Throughput
• COPE Assists Many Random UDP Flows Best
• No Congestion Control Is a Good Thing
• No Hidden Terminals Is Good As Well (Even for
  TCP)
• Mesh Networks Connected to Internet via AP - COPE
  Shows Gains From 5 - 70
• Many Extensions - Sensor Networks? Cellular?