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XORs in The Air: Practical Wireless Network Coding

XORs in The Air: Practical Wireless Network Coding. Sachin Katti, Hariharan Rahul, Wenjun Hu, Dina Katabi, Muriel Medard, Jon Crowcroft MIT CSAIL & University of Cambridge. Daniel Courcy- daniel.courcy@hp.com. Introduction - COPE. New Architecture For Wireless Mesh Networks

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XORs in The Air: Practical Wireless Network Coding

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  1. XORs in The Air: Practical Wireless Network Coding Sachin Katti, Hariharan Rahul, Wenjun Hu, Dina Katabi, Muriel Medard, Jon CrowcroftMIT CSAIL & University of Cambridge Daniel Courcy- daniel.courcy@hp.com

  2. 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 Worcester Polytechnic Institute

  3. Review • Mesh Networks • Way to Route Data • Allows For Continuous Connections & Reconfigurations • Multicast • Transmission to Multiple Selected Recipients • Unicast • Transmission to Single Selected Recipient Worcester Polytechnic Institute

  4. COPE • Substantially Improves Throughput • ‘Coding Shim’ Between IP and MAC Layers • Finds Coding Opportunities • Benefits by Sending Multiple Packets in Single Transmission Worcester Polytechnic Institute

  5. COPE: Simple Example • Bob & Alice • Current Approach = 4 Transmissions • COPE = 3 Transmissions • Thus Allowing For Increased Throughput • Coding+MAC Worcester Polytechnic Institute

  6. 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 Worcester Polytechnic Institute

  7. Opportunistic Coding • Each Node Uses Knowledge of What It’s Neighbors Have (Which Packets) • This Data Allows For Source to Send XOR’ed 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 Worcester Polytechnic Institute

  8. 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) Worcester Polytechnic Institute

  9. COPE = 2 Key Principles (2) • COPE Employs Network Coding • Packets Are Mixed Before Transmission • Prior Work Is Mainly Theoretical • COPE Addresses: • Unicast Traffic • Dynamic & Bursty Flows • Other Practical Issues Regarding Implementation Worcester Polytechnic Institute

  10. 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 Worcester Polytechnic Institute

  11. 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 @ AP • w/o Hidden Terminals TCP Throughput Increases about 38% Worcester Polytechnic Institute

  12. 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 Worcester Polytechnic Institute

  13. 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 Worcester Polytechnic Institute

  14. 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 Worcester Polytechnic Institute

  15. COPE Overview • Before Getting Into The Details Know These Terms: Worcester Polytechnic Institute

  16. COPE Overview • 3 Main Techniques • Opportunistic Listening • Opportunistic Coding • Learning Neighbor State Worcester Polytechnic Institute

  17. COPE Overview:Opportunistic 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 Worcester Polytechnic Institute

  18. COPE Overview:Opportunistic 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 Worcester Polytechnic Institute

  19. COPE Overview:Opportunistic Coding • Always Seeking Largest N That Satisfies Above Rule Worcester Polytechnic Institute

  20. COPE Overview:Learning Neighbor State • Each Node Announces Its Stored Packets In Reception Reports • Sometimes Reports Don’t 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 Worcester Polytechnic Institute

  21. COPE’s Gains • How Beneficial is COPE? • Throughput Improvement Depends On: • Coding Opportunities • Traffic Patterns Worcester Polytechnic Institute

  22. COPE’s Gains:Coding Gain • Coding Gain: • # Transmissions w/o Coding to the Minimum # Transmissions w/ Coding • Remember Alice & Bob? • Coding Gain = 4/3 = 1.33 Worcester Polytechnic Institute

  23. COPE’s Gains:Coding Gain • Maximum Achievable Coding Gain? • For Arbitrary Topologies - Open Question • Authors Prove: • With Listening Certain Topologies Benefit Worcester Polytechnic Institute

  24. COPE’s Gain:Coding 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) Worcester Polytechnic Institute

  25. COPE’s Gain:Coding+MAC 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 Worcester Polytechnic Institute

  26. COPE’s Gain:Coding+MAC Gain • Authors Prove: Worcester Polytechnic Institute

  27. 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 Worcester Polytechnic Institute

  28. 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 @ 2M Packets (M=# Neighbors) • Packet Reordeing Bad (TCP Thinks Congestion) • Doesn’t Happen Much But If so They Are Put in Order Before Transport Layer Worcester Polytechnic Institute

  29. 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 Worcester Polytechnic Institute

  30. 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) Worcester Polytechnic Institute

  31. 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 Worcester Polytechnic Institute

  32. 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 Worcester Polytechnic Institute

  33. 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 Worcester Polytechnic Institute

  34. 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 ACK Immediately Upon Decoding via Header (or Control Packets which are also Used for Reception Reports) Worcester Polytechnic Institute

  35. 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 @ 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 Worcester Polytechnic Institute

  36. Implementation Details: Packet Format • COPE Inserts Variable Length Coding Header • Only Shaded Fields Below Required Worcester Polytechnic Institute

  37. 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 Worcester Polytechnic Institute

  38. 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 Worcester Polytechnic Institute

  39. Implementation Details: Control Flow Worcester Polytechnic Institute

  40. 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 -> Internet via Gateway • Throughput Improvement Between 5 - 70% • w/o Hidden Terminals TCP’s Gain Agrees With Expected Coding Gain Worcester Polytechnic Institute

  41. 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 @ 6 Mb/s Worcester Polytechnic Institute

  42. 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 • Djikstra’s 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 Worcester Polytechnic Institute

  43. Metrics • Network Throughput • End-to-End Throughput • Throughput Gain • Ratio of Measured Network Throughputs With and Without COPE • What Else Might Have Been Interesting? Worcester Polytechnic Institute

  44. 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) Worcester Polytechnic Institute

  45. COPE in Gadget Topologies • Above Results Show That w/ Congestion Control Results Lean Towards Coding Gain Rather than Coding+MAC • When Many Long-Lived Flows (TCP) Bottleneck Senders Backoff (to Avoid Drop) • This Leaves only Coding Gains Worcester Polytechnic Institute

  46. COPE in Gadget Topologies • Repeat of Above w/ UDP • Coding+MAC Gains (Better Than TCP) • Coding Allows Downstream Routers to Avoid Dropping Packets Already Having Consumed Bandwidth Worcester Polytechnic Institute

  47. 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 @ Bottlenecks Never Build Up • Therefore No Coding Gains (or Opportunities) • Would TCP Do Better w/o Collisions? Worcester Polytechnic Institute

  48. 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 Worcester Polytechnic Institute

  49. COPE in an Ad Hoc Network • UDP (Back to Large Scale Testbed) • (Again) Random Sender / Receiver • File Size Follows Internet Studies • 500 Experiments… Worcester Polytechnic Institute

  50. COPE in an Ad Hoc Network • Scare Coding Opp. At Low Demands • Demand Up / Congestion Up / Gain Up Worcester Polytechnic Institute

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