Towards a Robust Protocol Stack for Diverse Wireless Networks

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science

Towards a Robust Protocol Stack for Diverse Wireless Networks

Arun Venkataramani(in collaboration with Ming Li, Devesh Agrawal, Deepak Ganesan, Aruna

Balasubramanian, Brian Levine, Xiaozheng Tie at UMass Amherst)

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS A AMHERST • MHERST • Department of Computer Science Department of Computer Science 2

Wireless network landscape today

Diverse wireless networks coexistAdapt TCP/IP stack in different ways

WLAN Mesh MANET DTN

Little mobility High mobilitySome mobility

Cellular


Interconnecting diverse networks?

Vision: A simple robust protocol stack to interconnect diverse wireless networks.

DTNWLAN Mesh MANET


Cellular


Why does TCP/IP not suffice?

E2E route disruptions cause unavailability

DTNWLAN Mesh MANET


Cellular


Why does TCP/IP not suffice?

Gap between what you buy vs. get

DTNWLAN Mesh MANET


Cellular

Advertised capacity: 11Mbps


Principle: Design for high uncertainty

Observe what works at an extreme point in the design space--always-partitioned DTNs--for insights into a robust design.


Outline

Motivation

Block transport

Replication routing

Research challenges


E2E Feedback

1. E2E Transport

E2E rate control is error-prone

E2E retransmissions are wasteful

Rate ControlRate

Control

DestSource Congestion? Link loss?

Loss Rate

RTT


1. E2E Transport

E2E rate control is error-prone

E2E retransmissions are wasteful

Source DestRedundant

Transmissions

E2E Retransmissions

P


2. Packet as Unit of Control

Channel accessLink layer ARQ

Listen Backoff RTS/CTS


2. Packet as Unit of Control

Channel access Link layer ARQ

Timeout Backoff Timeout Backoff


3. Complex Cross-Layer Interaction

Transport

Link

Link-layer ARQ/backoff hurts TCP rate control

Highly Variable RTT

Link ARQ Link ARQ Link ARQ

Rate ControlRate

Control


Hop: Clean-Slate Transport Protocol

End-To-End

Packets

Complexity

Hop-by-Hop

Blocks

Minimize interaction

Block-switched networks: A new paradigm for wireless networks, Li et al. [NSDI 2009]


Hop Design

Reliable Block Transfer

ACK Withholding

Micro-block Prioritization

Virtual Retransmission

Backpressure

Per-hopMulti-hop


Reliable Per-Hop Block Transfer

B-SYN Request for B-ACK

B-ACK Packet bitmap

Mechanisms

Burst mode (TXOP)

Block ACK based ARQ

Benefits

Amortizes control overhead

CSMA TXOP


Hop Design


ACK Withholding



Backpressure

Per-hopMulti-hop


MechanismLeverages in-network cachingRe-transmits blocks only when unavailable in cache

BenefitsFewer transmissionsLow overheadSimple

Virtual Retransmission (VTX)

B-SYNB-ACK

A B

E

C

D

DATA


MechanismLeverages in-network cachingRe-transmits blocks only when unavailable in cache

BenefitsFewer transmissionsLow overheadSimple

Virtual Retransmission (VTX)

A B

E

D

B-SYNVTX

B-ACK

B-SYNVTX B-SYN with VTX flag set

VTX TimerE2E ACK

B-SYNVTX Timer


Hop Design


ACK Withholding



Backpressure

Per-hopMulti-hop


MechanismLimits outstanding blocks per-flow at forwarder

Backpressure

A B C D E

Source Dest

Slow

Limit on outstanding blocks=2

B-SYN

Hold B-ACK

B-SYNB-SYN


MechanismLimits outstanding blocks per-flow at forwarder

BenefitsImproves network utilization

Backpressure

A B C

D E

Source

Dest

F G

Dest

Aggregate goodput without backpressure: 6Mbps

20Mbps 10Mbps 20Mbps

20Mbps

20Mbps1Mbps


MechanismLimits #outstanding blocks per-flow at forwarder

BenefitsImproves network utilization

Backpressure

A B C

D E

Source

Dest

F G

Dest

Aggregate goodput with backpressure: 10Mbps

20Mbps 10Mbps 20Mbps

20Mbps

20Mbps1Mbps

Limit of Outstanding Blocks=1


Hop Design


ACK Withholding



Backpressure

Per-hopMulti-hop


Mechanism:

Receiver withholds all but one BACK

Benefit:Low overheadLess conservativeSimple

Ack Withholding

A C B

B-SYNB-ACK

DATA

B-SYN

B-ACKDATA

Withhold B-ACK


Hop Design


ACK Withholding



Backpressure

Per-hopMulti-hop



MechanismsSender piggybacks small blocks to B-SYNReceiver prioritizes small block’s B-ACK

BenefitsLow delay for small blocks

SSH FTP

B-SYN

B-ACKDATA

B-SYN64B 1MB

Sender SenderReceiver


20 nodes on the 2nd floor of UMass CS building

Apple Mac Mini1.8GHz, 2GB RAM, Atheros 802.11 a/b/g card

Testbed


Single-flow Single-hop Performance

Hop achieves significant gains over TCP

HopTCP

28x

1.6x

1.2x


Single-flow Multi-hop Performance

HopTCP

2.7x

2.3x

1.9x

Hop achieves significant gains over TCP


Graceful Degradation with Loss

Emulated link layer losses at the receiver

TCP breaks down at moderate loss rates

HopTCP


Scalability to High Load

30 concurrent flows

Hop is much fairer than TCP

Meangoodput

HopTCP

Hop-by-hop TCP

150x

20x

2x


Hop Performance Breakdown


Low Delay for Small Transfers

AP

1 small transfer competing with 4 large

Small transfer size (KB)

Hop lowers delay across all file sizes


Related WorkFixing E2E rate-control

Separating loss/congestion [Snoop, WTCP, Westwood+, ATCP, TCP-ELFN][Snoop, WTCP, Westwood+, ATCP, TCP-ELFN]

Network-assisted rate control [ATP, NRED, IFRC, WCP][ATP, NRED, IFRC, WCP]

Hop circumvents rate control

BackpressureATM, theoretical work [Tassiulas et al.][Tassiulas et al.]

Tree/chain sensor data aggregation [Fusion, Flush][Fusion, Flush]

Reliable point-to-point transport [RAIN, CXCC, Horizon][RAIN, CXCC, Horizon]

Hop reduces backpressure overhead using blocksBatching

Common optimization at link [802.11e/802.11n, WiLDNet, Kim08, [802.11e/802.11n, WiLDNet, Kim08,

CMAP]CMAP], transport [Delayed-ACK, DTN2.5][Delayed-ACK, DTN2.5], and network [ExOR][ExOR] layersHop leverages batching across layers


Outline

Motivation

Block transport

Replication routing

Research challenges


Replication routing

PX Z

Y

P

P

W

P

Replication reduces delay under topology uncertainty


Routing as resource allocation problem

RAPID Protocol (X,Y):

1. Control channel: Exchange metadata

2. Direct Delivery: Deliver packets destined to each other

3. Replication: Replicate in decreasing order of marginal utility

4. Termination: Until all packets replicated or nodes out of range

YX

Change in utility

Packet size

Problem: Which packets to replicate given limited bandwidth to optimize a specified metric?

DTN routing as a resource allocation problem, Balasubramanian et al. [SIGCOMM 2007]


Objective: Minimize average delay

Define U(i) = -(T + D) T = time since created, D = expected remaining time to deliver

Simplistic assumptionsuniform exponential meeting with mean ¸global view

Utility computation example

D = ¸

iX

D = ¸/2

iY

D =¸/3

iW

Z


Utility computation example (cont’d)

Replicate i

Metric: Min average delay

Deadline of i < TDeadline of j = T1 > T

Metric: Maximize #packets delivered within deadline

Replicate j

iX

j

Y j

W Z


Deployment on DieselNet


Results based on DieselNet traces


Outline

Motivation

Block transport

Replication routing

Research challenges


C1: When to replicate? Replication useful under

Topology uncertainty Delay optimization Light load scenarios

WLAN Mesh MANET DTN

Low uncertaintyForwarding suffices

High uncertaintyReplication useful

X1

X2

Xk

src dst

relays

Xi = r.v. for delay of path iForwarding delay = mini(E[Xi])Replication delay = E[mini(Xi)]

Claim: Delay benefit of replication is unbounded.

Towards a Robust Protocol Stack for Diverse Wireless Networks

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