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DIRAC: A Software-based DIRAC: A Software-based Wireless Router SystemWireless 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, CS7122003 Fall, CS712
22
Introduction Introduction [1/2][1/2]
RouterKey architectural component in the IP-based wired InternetMain functionalities
Data-plane packet forwarding Control-plane routing and management
Limitation of current router systemsLack of consideration for wireless link and host mobility
Wireless servicesSignificantly increased in recent yearsExamples
QoS support for delay-sensitive applications (ex. VoIP)Interactive multiplayer gamingMultimedia instant messaging, …
33
Introduction Introduction [2/2][2/2]
Supporting wireless services
Proposed protocolsAdaptive error control mechanisms
Wireless packet scheduling
Mobility-aware admission control, …
Fundamental principleAdapting 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
44
Target ScenarioTarget Scenario
Cell
AP
Subnet
EdgeRouter
Subnet
Subnet
AccessRouter
MN
BackboneNetwork
Packet-switched wireless data network based on 802.11b
What kinds of system architectures can improve performance?
55
Architecture AlternativesArchitecture AlternativesCurrent router system
Router: oblivious to wireless characteristicsAP: 802.11 MAC functionality only
Intelligent APAP: adaptive link-layer operationLimited support for network-layer functionsExpensive hardware cost
Ubiquitous routerAP routerHigh administration and deployment costNontrivial new service deploymentHard to coordinate among cells or subnets
66
Proposed ArchitectureProposed ArchitectureDIRAC (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 feedbackTo 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
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Router Core Router Core [1/2][1/2]
Control plane
Routing and management protocolsWhatever a user wants is possible
Control engineTo interact with link-layer for wireless services
Data plane
Scheduler + forwarderWhatever 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
88
Router Core Router Core [2/2][2/2]
Control Engine [revisited]
EventProcessorCalling appropriate callback functions registered by data/control plane components
StatisticsMonitorCentralized repository to maintain the latest channel quality information for each host
Periodically updated by RAs
ActionProcessorSending action_request message to the appropriate RA
RegistrationDBRegistry to store relationship b/w mobile host and AP
Each host is assigned a unique ID throughout the router system
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Router AgentRouter Agent
Lightweight messenger (not programmable interface for AP)
Router Core Router Agent AP
TasksMonitoring 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 frequencyTradeoff: state accuracy vs. communication overhead
Need not be high (50-100ms for walking-speed hosts)
1010
ImplementationImplementation
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 >
1111
Implementation:Implementation:
Two-interface RC configuration Two-interface RC configuration (wireless&wired)(wireless&wired)
< Two-interface RC configuration (wireless&wired) >
1212
Prototype Wireless ServicesPrototype 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
1313
Link-Layer Informed Fast Handover Link-Layer Informed Fast Handover [1/2][1/2]
Handover
BackboneNetwork
Data Loss
High Latency
1414
Link-Layer Informed Fast Handover Link-Layer Informed Fast Handover [2/2][2/2]
Fast handover
BackboneNetwork
Tunneling
Link-layer trigger
In DIRAC…
A component of RC registers a trigger
RA notifies link-layer handoff
It causes to execute the trigger
1515
FEC-Based Downlink Forwarding FEC-Based Downlink Forwarding [1/2][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)
Retransmittedcontinually to max
1616
FEC-Based Downlink Forwarding FEC-Based Downlink Forwarding [2/2][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 schedulingTo avoid transmitting consecutive packets to an error-prone flow
frames_lossframes_txmit
frames_errors= during [t, t + T]
n
k=1 - frames_loss
1717
Link-Layer Assisted Uplink PolicingLink-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 aggressiveBy checking Ractual/Rshare
Enforcer takes two shaping actionsRED-like dropping for aggressive flow
Temporary denial of access for persistent non-response flow
1818
System Evaluation System Evaluation [1/4][1/4]
System overhead
Processing time
Scalability
Operation Time(ns)
Action 498
Event 1712
Statistics report 32
Basic forwarding 1299
< # of AP > < # of MH / 1 AP (100 AP) >
not optimizedfuture work
1919
System Evaluation System Evaluation [2/4][2/4]
Link-layer informed fast handoverAverage end-to-end delay of UDP packets (50 packets/sec, 512 bytes)
2020
System Evaluation System Evaluation [3/4][3/4]
FEC-based downlink forwarding
# of received packets (10,000 packets, 65 packets/sec/node)
average quality
MN1 MN2 MN3
10.38 9903 9830 2647
12.38 9980 9940 7450
15.02 9980 9978 9581
20.17 9943 9903 9985
25.32 9925 9951 9984
35.53 9942 9936 9949
44.77 9928 9927 9954
average quality
MN1 MN2 MN3
10.38 4808 4807 3038
12.38 7559 7554 6558
15.02 9999 9999 9822
20.17 9999 9999 9988
25.32 9999 9999 9990
35.53 9999 9999 9996
44.77 9999 9999 9998< Packet received using retransmission > < Packet received using FEC >
2121
System Evaluation System Evaluation [4/4][4/4]
Link-layer assisted uplink policing
TCP throughput against aggressive UDP flowTCP: MP3 streaming (128kbps)
UDP: 3.6Mbps
< Throughput without policing > < Throughput with policing >
Aggressive flow
2222
ConclusionConclusionMotivation
To handle mobility and wireless link issues becomes important but conventional router is not fit wellMany protocols for wireless services are proposed but do not consider practical system framework
DIRACRC: core to take most complexity to perform routingRA: distributed simple and generic software module at each AP
In this paperPropose and implement DIRACBuild prototype systems using DIRACEvaluation of systems
