Wireless Mesh Networks XORs in the Air A. Zubow “XORs in The Air: Practical Wireless Network Coding”, Sachin Katti, Hariharan Rahul, Wenjun Hu, Dina Katabi, Muriel M´edard, Jon Crowcroft, http://piper.csail.mit.edu/papers/copesc.pdf

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 COPE = Inter-flow Network Coding MORE = Intra-flow Network Coding

COPE Substantially improves throughput ‘Coding Shim’ between IP and MAC Layers Finds coding opportunities Benefits by sending multiple packets in single transmission (wireless broadcast)

COPE: Simple Example Bob & Alice Current approach = 4 transmissions COPE = 3 transmissions Thus allowing for increased throughput Coding+MAC

COPE: Even Bigger Savings Previous example: obvious throughput gains COPE exploits shared nature of wireless medium (broadcast) Nodes overhear transmissions Store these packets for short time Sends data out telling what it has heard This data is used for opportunistic coding

Opportunistic Coding Each node uses knowledge of what it’s neighbors have (which packets) This information allows for source to send XOR’ed packets intelligently It knows who will be able to decode the encoded packet Allows for more than two flows Allows for multiple packets to be coded

COPE = 2 Key Principles COPE exploits broadcast nature of wireless channel COPE employs network coding Packets are mixed before transmission COPE addresses: Unicast traffic Dynamic & bursty flows Other practical issues regarding implementation

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%

Background (Theory) Famous Butterfly Example: All Links Can Send One Message Per Unit of Time Sources Want to Hit Both Receivers Coding (Again) Increases Overall Throughput

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

COPE Overview COPE terms:

COPE Overview 3 Main Techniques Opportunistic Listening Opportunistic Coding Learning Neighbor State

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

COPE Overview: Opportunistic Coding Q.: Which packets do we combine to achieve maximum throughput? A.: Send as many (native packets) as possible while ensuring nexthop has enough info to decode.

COPE Overview: Opportunistic Coding Always seeking largest N that satisfies above rule

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: educated guess Estimation of probability that neighbor has packet based on delivery probability

COPE’s Gains How Beneficial is COPE? Throughput improvement depends on: Coding opportunities Traffic patterns

COPE’s Gains: Coding Gain # transmissions w/o coding to the minimum # transmissions w/ Coding Remember Alice & Bob? Coding gain = 4/3 = 1.33

COPE’s Gains: Coding Gain Maximum Achievable Coding Gain? For arbitrary topologies - open question Authors prove: With listening certain topologies benefit

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.

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

COPE’s Gain: Coding+MAC Gain Authors Prove:

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 next hop

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 reordering bad (TCP thinks congestion) Doesn’t happen much but if so they are put in order before transport layer

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

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)

Making It Work - Pseudo-Broadcast 802.11 Has Two MAC Modes: Unicast Broadcast Packets Are Ack-ed Exponential Backoff Un-Reliable No Backoff

Making It Work - 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

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

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)

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

Implementation Details: Packet Format COPE inserts variable length coding header Only shaded fields below required

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 rackets Last_PKT: last packet heard from source Bit map of recently heard packets

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

Implementation Details: Control Flow

Experimental Results 20 Node wireless testbed Results: 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

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

Experimental Results: Testbed Nodes ran Linux / used Click toolkit 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

Metrics Network throughput Throughput gain E2E throughput Throughput gain Ratio of measured network throughputs with and without COPE What else might have been interesting?

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)

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

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

COPE in an Ad Hoc Network 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?

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

COPE in an Ad Hoc Network UDP (back to large scale testbed) Random sender / receiver File size follows Internet studies 500 experiments…

COPE in an Ad Hoc Network Scare coding opp. at low demands demand up / congestion up / gain up

COPE in an Ad Hoc Network Low demand - reports arrive to late Demand goes up - bottlenecks form - longer wait times - nodes get more reports Demands get higher - high loss rates of reception reports - guessing relied upon

COPE in an Ad Hoc Network @ peak gain point (5.6 Mb/s) On average 3 packets coded together Packets drained from bottlenecks faster Throughput gains 3 - 4 x

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 Fluctuate upload / download traffic Gain goes up as upload traffic up

COPE in Mesh Access Network

Fairness Channel from source to bottleneck matters Capture effect (If Alice’s 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

Fairness

Discussion Target: stationary wireless mesh networks Memory needed to store packets Need more than delay bandwidth product Need omni-directional antenna Current design does not consider power

Conclusions Coding is an old theme 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?

Resources Sachin Katti, Hariharan Rahul, Wenjun Hu, Dina Katabi, Muriel Medard, Jon Crowcroft, MIT CSAIL & University of Cambridge Daniel Courcy- daniel.courcy@hp.com, Worcester Polytechnic Institute