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DIRAC: A Software-based Wireless Router System

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  • 1. DIRAC: A Software-based Wireless Router System Petros Zerfos, Gary Zhong, Jerry Cheng, Haiyun Luo, Songwu Lu (UCLA) Jefferfy Jia-Ru Li (HIQ) MobiCom 2003 Hyunku Jeong (Computer Architecture Laboratory) 2003 Fall, CS712
  • 2. Introduction [1/2]
    • Router
      • Key architectural component in the IP-based wired Internet
      • Main functionalities
        • Data-plane packet forwarding
        • Control-plane routing and management
      • Limitation of current router systems
        • Lack of consideration for wireless link and host mobility
    • Wireless services
      • Significantly increased in recent years
      • Examples
        • QoS support for delay-sensitive applications (ex. VoIP)
        • Interactive multiplayer gaming
        • Multimedia instant messaging, …
  • 3. Introduction [2/2]
    • Supporting wireless services
      • Proposed protocols
        • Adaptive error control mechanisms
        • Wireless packet scheduling
        • Mobility-aware admission control, …
      • Fundamental principle
        • Adapting to wireless channel dynamics and also being mobility aware
        • Interaction with underlying link-layer is required
        • Also, decisions among subnets have to be coordinated
    • Subject of this paper
      • System framework is missing to facilitate real implementation
      • To develop a router system providing such a system framework
  • 4. Target Scenario
    • Packet-switched wireless data network based on 802.11b
      • What kinds of system architectures can improve performance?
    Cell AP Subnet Edge Router Subnet Subnet Access Router MN Backbone Network
  • 5. Architecture Alternatives
    • Current router system
      • Router: oblivious to wireless characteristics
      • AP: 802.11 MAC functionality only
    • Intelligent AP
      • AP: adaptive link-layer operation
      • Limited support for network-layer functions
      • Expensive hardware cost
    • Ubiquitous router
      • AP  router
      • High administration and deployment cost
      • Nontrivial new service deployment
      • Hard to coordinate among cells or subnets
  • 6. Proposed Architecture
    • DIRAC (DIstributed Router ArChitecture for wireless networks_
      • Software-based wireless router systems
      • To balance b/w performance, complexity, cost and manageability
    • Idea
      • Router Core (RC): regular router, but using link-layer feedback
        • To take most complexity to perform routing
      • Router Agent (RA): messenger b/w RC and AP (device driver)
        • Distributed simple and generic software module at each AP
    RC RA RA RA Actions Statistics Events RC  RA Lightweight communication protocol over UDP
  • 7. Router Core [1/2]
    • Control plane
      • Routing and management protocols
        • Whatever a user wants is possible
      • Control engine
        • To interact with link-layer for wireless services
    • Data plane
      • Scheduler + forwarder
        • Whatever a user wants is possible
      • Providing asymmetric operations for uplink and downlink flows
        • Downlink service: proactive, fine-grained with time (msec)
        • Uplink service: reactive, coarse-grained with time (sec)
        • By inherited wireless network constraint and traffic characteristics
  • 8. Router Core [2/2]
    • Control Engine [revisited]
      • EventProcessor
        • Calling appropriate callback functions registered by data/control plane components
      • StatisticsMonitor
        • Centralized repository to maintain the latest channel quality information for each host
        • Periodically updated by RAs
      • ActionProcessor
        • Sending action_request message to the appropriate RA
      • RegistrationDB
        • Registry to store relationship b/w mobile host and AP
        • Each host is assigned a unique ID throughout the router system
  • 9. Router Agent
    • Lightweight messenger (not programmable interface for AP)
      • Router Core  Router Agent  AP
      • Tasks
        • Monitoring the state and channel quality for each mobile host
        • Sending appropriate messages to RC when events occurs in the cell
        • Intercepting msgs. sent by RC, requesting action msgs. from the AP
    • Link-layer statistics
      • Collected for each mobile host by RA periodically
      • Collection frequency
        • Tradeoff: state accuracy vs. communication overhead
        • Need not be high (50-100ms for walking-speed hosts)
  • 10. Implementation
    • Router Core
      • Commodity PC + Linux + Click
      • Click: extensible and modular software router framework (Click element configuration diagram is in next page …)
    • Router Agent
      • Embedded system + Linux + PCMCIA 802.11b + OpenAP Platform
      • Use ioctl() system call to collect link-layer statistics
    < Router Agent >
  • 11. Implementation: Two-interface RC configuration (wireless&wired) < Two-interface RC configuration (wireless&wired) >
  • 12. Prototype Wireless Services
    • To demonstrate the practicability of DIRAC
      • Selecting protocols not feasible in current router systems
      • Spanning control/data planes, and both uplink and downlink
    • Three target protocols
      • Link-layer informed fast handover
      • Channel-adaptive FEC-based packet forwarding
      • Link-layer assisted uplink policing
  • 13. Link-Layer Informed Fast Handover [1/2]
    • Handover
    Backbone Network Data Loss High Latency
  • 14. Link-Layer Informed Fast Handover [2/2]
    • Fast handover
    Backbone Network Link-layer trigger
    • In DIRAC…
      • A component of RC registers a trigger
      • RA notifies link-layer handoff
      • It causes to execute the trigger
    Tunneling
  • 15. FEC-Based Downlink Forwarding [1/2]
    • Head-of-Line (HoL) blocking problem
    • FEC (Forward Error Correction) based forwarding
      • Transmitting same data several times
      • To compensate for lack of retransmission
      • (n,k)-FEC (or k/n FEC)
        • k: number of application packets
        • n: total number of packets (including n – k packets)
    Retransmitted continually to max
  • 16. FEC-Based Downlink Forwarding [2/2]
    • In DIRAC…
      • RA reports frame lost due to error to RC per each host periodically (T)
      • RC computes n and k
      • RC forwarding packets by Deficit Round Robin scheduling
        • To avoid transmitting consecutive packets to an error-prone flow
    frames_loss frames_txmit frames_errors = during [t, t + T] n k = 1 - frames_loss
  • 17. Link-Layer Assisted Uplink Policing
    • Aggressive uplink flows
      • Consuming wireless bandwidth, because all packets are dropped at router in link-layer unaware scheme
    • In DIRAC…
      • Arbitrator in RC decides which uplink flows are aggressive
        • By checking R actual /R share
      • Enforcer takes two shaping actions
        • RED-like dropping for aggressive flow
        • Temporary denial of access for persistent non-response flow
  • 18. System Evaluation [1/4]
    • System overhead
      • Processing time
      • Scalability
    < # of AP > < # of MH / 1 AP (100 AP) > 1299 Basic forwarding 32 Statistics report 1712 Event 498 Action Time(ns) Operation not optimized future work
  • 19. System Evaluation [2/4]
    • Link-layer informed fast handover
      • Average end-to-end delay of UDP packets (50 packets/sec, 512 bytes)
  • 20. System Evaluation [3/4]
    • FEC-based downlink forwarding
      • # of received packets (10,000 packets, 65 packets/sec/node)
    < Packet received using retransmission > < Packet received using FEC > 9954 9927 9928 44.77 9949 9936 9942 35.53 9984 9951 9925 25.32 9985 9903 9943 20.17 9581 9978 9980 15.02 7450 9940 9980 12.38 2647 9830 9903 10.38 MN3 MN2 MN1 average quality 9998 9999 9999 44.77 9996 9999 9999 35.53 9990 9999 9999 25.32 9988 9999 9999 20.17 9822 9999 9999 15.02 6558 7554 7559 12.38 3038 4807 4808 10.38 MN3 MN2 MN1 average quality
  • 21. System Evaluation [4/4]
    • Link-layer assisted uplink policing
      • TCP throughput against aggressive UDP flow
        • TCP: MP3 streaming (128kbps)
        • UDP: 3.6Mbps
    < Throughput without policing > < Throughput with policing > Aggressive flow
  • 22. Conclusion
    • Motivation
      • To handle mobility and wireless link issues becomes important but conventional router is not fit well
      • Many protocols for wireless services are proposed but do not consider practical system framework
    • DIRAC
      • RC: core to take most complexity to perform routing
      • RA: distributed simple and generic software module at each AP
    • In this paper
      • Propose and implement DIRAC
      • Build prototype systems using DIRAC
      • Evaluation of systems