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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