how to evaluate exotic wireless routing protocols? 1 dimitrios koutsonikolas 1, y. charlie hu 1,...
TRANSCRIPT
How to Evaluate Exotic Wireless Routing Protocols?
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Dimitrios Koutsonikolas1, Y. Charlie Hu1, Konstantina Papagiannaki2
1Purdue University , 2Intel Research, Pittsburgh
Evolution of Wireless Routing Protocols
• From the Ad Hoc Era to the Mesh Era– New design goals
• High throughput vs. connectivity
– New “exotic” optimization techniques– Cross – layer design
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1994 1996 1997 1998 2000 2003 2004 2005 2006 2007
DSDV
DSR
AODVTORA
Performance comparisons
ETX ETT
ExORROMER SOAR
COPE
MOREMC2
noCoCo
Ad Hoc Networking Era Mesh Networking Era
In This Talk…• Review the evolution of wireless
protocol design– Reveal challenges to evaluation
methodology of new routing protocols
• Discuss current practices– Weaknesses
• Suggest guidelines for fair and meaningful evaluation
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Ad Hoc Networking Era• Primary challenge
– Deal with route breaks due to host mobility
• Layering principle– Routing protocol discovers route– 802.11 unicast transmits packets to next
hop• ACK/RETX, exponential backoff
• Evaluation– PDR, control overhead, tradeoffs– Low constant offered load
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Mesh Networking Era• Static routers
– Mobility not a concern
• Commercial applications– Compete with other internet
technologies
• New research focus– High Throughput
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Towards High Throughput• Link-quality routing metrics
– Examples: ETX, ETT– Still follow layering principle
• “Exotic” optimization techniques– Examples: Opportunistic Routing,
Network Coding– Abandon layering principle
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Opportunistic Routing• First demonstrated in ExOR [SIGCOMM ‘05]• Packet broadcast at each hop, all neighbors
can receive it• Neighbor closest to destination rebroadcasts
– Coordination required
S B DC S DA
A
B
C
50%
50%
50%
0%
0%
0%
Intra-Flow Network Coding• First demonstrated in MORE [SIGCOMM ‘07]• Routers randomly mix packets• Benefits
– Remove need for coordination– FEC-style reliability, no ACK/RETX
S D
A
B
p1, p2
p1, p2
p1, p2
S D
A
B
p1, p2
γ*p1+ δ*p2
α*p1+ β*p2
Who forwards? Both forward
Coordination Required! No Coordination!
Inter-Flow Network Coding• First demonstrated in COPE [SIGCOMM
‘06]• Routers mix packets from different flows• Increase network capacity!• Implied evaluation methodology
– Subject network to congestion– Use network coding to eliminate congestion
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Alice Router Bob
1:p1 2:p2
4:p2 3:p1
Traditional Routing: 4 TX
Alice Router Bob
1:p1 2:p2
3:p1+p2
Network Coding: 3 TX
3:p1+p2
Implications of 802.11 Broadcast
• 802.11 broadcast has no ACK/RETX, no exponential backoff– No reliability– Nodes can send faster than in unicast
• Exotic techniques do not work well with TCP– Batching
• Consequence – Reliability and rate control are brought to
routing layer from lower or upper layers10
Evolution of Protocol Stack
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Physical Layer Physical Layer
MAC LayerMAC Layer
Network Layer
Network Sublayer 1
Transport LayerNetwork Sublayer 2
Network Sublayer 3
Application Layer
Application Layer
Medium Access
Hop-by-hop Reliability
Packet Forwarding
End-to-end Rate Control
End-to-end Reliability
Medium Access
Hop-by-hop Reliability
Hop-by-hop Rate Control
Network Coding
Packet Forwarding
End-to-end Reliability
End-to-end Rate Control
Traditional Network Stack
New Network Stack
Implications on Protocol Evaluation
• Evaluation becomes a much subtler task– Possible conflicts between new and old
mechanisms• Inter-flow network coding vs. rate control
• Current state– Diverse set of evaluation methodologies– Lack of clear guidelines
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Evaluation of Unreliable Protocols
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Practice 1: Making Both Protocols Reliable
• Evaluation of ExOR, comparison with Srcr– ExOR guarantees delivery of 90% of the file– Srcr offers no guarantee
• Methodology– Download a 1MB file– Send 1.1MB with ExOR to compensate for loss– Carry the whole file hop-by-hop with Srcr to
avoid collisions
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ProblemRemoves spatial reuse from traditional routing
Practice 2: No Rate Control – Varying the Sending Rate
• Evaluation of COPE, comparison with Srcr– COPE increases network capacity
• Methodology– UDP traffic– Vary offered load – Exceed nominal capacity (6Mbps)
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ProblemPDR drops quickly as network capacity is
exceeded
Practice 3: A Protocol With Rate Control Against a Protocol Without
Rate Control• Evaluation of SOAR, comparison
with Shortest Path (SP)– SOAR applies rate control– SP has no rate control
• Methodology– Saturate the network
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ProblemNot clear what fraction of gain comes from
opportunistic routing and what from rate control
Evaluation of Reliable Protocols
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Practice 5: A Reliable Against an Unreliable Protocol
• Evaluation of MORE, comparison with Srcr– MORE offers FEC-style e2e reliability– Srcr offers no reliability
• Methodology – UDP sent at maximum possible rate
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Problem•Srcr suffers losses due to congestion•Same amount of data sent by src, different amount delivered to dst
Practice 6: Running an Unreliable Protocol Under TCP
• Evaluation of noCoCo, comparison with COPE– noCoCo applies backpressure-based
congestion control/reliability– COPE has no congestion control, weak
reliability
• Methodology – Run COPE under TCP
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ProblemTCP performs poorly in multihop wireless networks Solution – Practice 7
Modify COPE to use noCoCo’s congestion control/reliability
Use (or No Use) of Autorate Adaptation
• Traditional routing uses 802.11 unicast– Exploits autorate adaptation
• Exotic optimization techniques rely on 802.11 broadcast– Operates on single rate
• Methodology – Evaluation of most exotic protocols disables
autorate adaptation for traditional routing• For “fair”comparison
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ProblemMethodology can be unfair to traditional routing
Recommendations for more consistent and meaningful
evaluation
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The Importance of Rate Control I Unreliable Protocols
• Traditional routing under UDP has no rate control– Packets dropped beyond
capacity– Throughput reduction
• Exotic protocols w/o rate control– Increase throughput, may increase capacity– Packets still dropped beyond (new) capacity
• Exotic protocols w/ rate control– Constant throughput beyond capacity– No need to increase offered load beyond capacity
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The Importance of Rate Control II Reliable Protocols
• FEC-style reliability provides no rate control
• PDR remains 100%, rate control still needed
• Exceeding capacity may lead to – Increased delays– Unfairness among flows
• Related recommendation– Evaluate with multiple flows
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Isolating the Benefit from Exotic Technique
• Evaluation should quantify the gain from new exotic optimization technique
• Tricky part– Adding an exotic technique may require
old techniques to move to the routing layer
• Recommendation– Old techniques should also be incorporated
into traditional routing24
Separating Rate Control from End-to-end Reliability
• Running traditional routing under TCP+ No modification to the protocol itself– TCP performs poorly in multihop wireless
networks– TCP provides both rate control and reliability
• If new protocol has only one mechanism, overkill to run old protocol under TCP
• Recommendation– Incorporate reliability/rate control mechanism
of new protocol to old protocol
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How to Incorporate Reliability To Traditional Routing
• Case 1: reliability component disjoint to exotic technique– Example: ARQ component in noCoCo– Method: add same component to
traditional routing
• Case 2: reliability component merged with exotic technique– Example: intra-flow NC in MORE– Method: add FEC to traditional routing?
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MAC Autorate Adaptation• Exotic protocols should try to
incorporate autorate adaptation– Not always feasible
• Recommendation– Enable autorate adaptation for
traditional routing– Show exotic protocol outperforms
traditional routing both with and without autorate adaptation
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Conclusions• Inconsistencies in evaluating wireless mesh
routing protocols
• Fundamental reason– No unified framework for understanding
interactions among• MAC• Congestion• Reliability • Interference• Network coding
• Real problem goes beyond how to evaluate exotic protocols
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Thank You!
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