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Improving TCP Performance Improving TCP Performance over Mobile Networksover Mobile Networks

HALA ELAARAGHALA ELAARAG Stetson UniversityStetson University

Speaker : AronSpeaker : Aron

ACM Computing Surveys 2002ACM Computing Surveys 2002

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OverviewOverview

Regular TCPRegular TCP The Problems ?The Problems ? Why regular TCP is not suitable?Why regular TCP is not suitable? The solutions to improve the performanceThe solutions to improve the performance

Link layerLink layer End to endEnd to end Split connectionSplit connection

ConclusionsConclusions

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Introduction Introduction

Mobile users would like to use the same applicationMobile users would like to use the same applications over the wireless link and with the same quality of s over the wireless link and with the same quality of service (QoS) they are getting over a wired link. service (QoS) they are getting over a wired link.

Objective: to improve the performance of TCP oObjective: to improve the performance of TCP over mobile wireless networks.ver mobile wireless networks.

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The behavior of regular TCPThe behavior of regular TCP

Congestion controlCongestion control Slow-startSlow-start Congestion avoidanceCongestion avoidance Fast RetransmitFast Retransmit

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Problems with wireless and mobile networksProblems with wireless and mobile networks

High bit error ratesHigh bit error rates DisconnectionsDisconnections Limited and variable bandwidthLimited and variable bandwidth Cell sizeCell size Power scarcityPower scarcity Dynamic network topologyDynamic network topology

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Why regular TCP is not suitable?Why regular TCP is not suitable?

TCP’s main problem is the delay caused by packet TCP’s main problem is the delay caused by packet losses due to congestion. losses due to congestion.

Wired links have low bit error rates (BER), as Wired links have low bit error rates (BER), as opposed to wireless links that suffer from high bit opposed to wireless links that suffer from high bit error rates. error rates.

If regular TCP is used on a mobile network, it can If regular TCP is used on a mobile network, it can severely degrade performance.severely degrade performance.

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Problems of mobile TCP implementProblems of mobile TCP implement

Non-congestion delayNon-congestion delay Serial timeoutsSerial timeouts Packet size variationPacket size variation

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The solutions to improve the performanceThe solutions to improve the performance Link layer protocolsLink layer protocols

RLPRLP AIRMAILAIRMAIL SnoopSnoop

End-to-end protocolsEnd-to-end protocols RenoReno New-RenoNew-Reno SACKSACK FRFR EBSNEBSN

Split-connectionSplit-connection MTCPMTCP I-TCPI-TCP M-TCPM-TCP WAPWAP

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Link layer protocolsLink layer protocols

Objective – Objective – Increase the quality of the lossy wireless linkIncrease the quality of the lossy wireless link Solve the problem at the link layerSolve the problem at the link layer Transport layer protocol is too slow to recover from lossTransport layer protocol is too slow to recover from loss

eses Congestion control mechanisms of transport layer are uCongestion control mechanisms of transport layer are u

nnecessarily triggered, throughput is decreasednnecessarily triggered, throughput is decreased

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Link layer protocolsLink layer protocols – RLP – RLP

Radio Link Protocol (RLP)Radio Link Protocol (RLP) Automatic Repeat reQuest Automatic Repeat reQuest (ARQ) for radio channels.(ARQ) for radio channels. Retransmit a packet when transmitter make sure it was Retransmit a packet when transmitter make sure it was

not received.not received. May Solve High bit error rates at link layerMay Solve High bit error rates at link layer

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Link layer protocolsLink layer protocols –AIRMAIL –AIRMAIL

Approaches to improve link layer protocol performaApproaches to improve link layer protocol performancence Automatic Repeat reQuest (ARQ)Automatic Repeat reQuest (ARQ) Forward Error Correction (FEC)Forward Error Correction (FEC) Mobility and handoff processing by window managementMobility and handoff processing by window management

and state transferand state transfer

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Link layer protocolsLink layer protocols – Snoop – Snoop

Snoop protocol Snoop protocol (TCP-aware link-layer schemes)(TCP-aware link-layer schemes)

Introduce a module--snoop agent at the base stationIntroduce a module--snoop agent at the base station monitors every packet that passes through the TCP monitors every packet that passes through the TCP

connection in both directionsconnection in both directions MMaintains a cache of TCP packets sent from the sender aintains a cache of TCP packets sent from the sender

that haven’t yet been acknowledged by the receiver.that haven’t yet been acknowledged by the receiver. If detect packet loss (use duplicate ACKs or local If detect packet loss (use duplicate ACKs or local

timeout), retransmit the packet if it is in the cache and timeout), retransmit the packet if it is in the cache and suppress the duplicate ACKssuppress the duplicate ACKs

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End-to-end protocolsEnd-to-end protocols – – RenoReno 、 、 New-RenoNew-Reno 、、 SACK TCSACK TCPP

Change Fast Retransmit to include Fast RecoveryChange Fast Retransmit to include Fast Recovery New-RenoNew-Reno 、、 SACK TCP improve the performance when multiple pacSACK TCP improve the performance when multiple pac

kets lost in the same windowkets lost in the same window Improvement of regular TCP in wireless is expected to be limitedImprovement of regular TCP in wireless is expected to be limited

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End-to-end protocolsEnd-to-end protocols – FR – FR

Fast retransmission schemeFast retransmission scheme Providing smooth hand-offs on networks that lose Providing smooth hand-offs on networks that lose

packet during handoff.packet during handoff. When mobile IP software signal hand-off complete, When mobile IP software signal hand-off complete,

mobile host signals fixed host to invoke retransmission mobile host signals fixed host to invoke retransmission scheme.scheme.

Focus on hand-offFocus on hand-off

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End-to-end protocolsEnd-to-end protocols – EBSN – EBSN Explicit Bad State NotificationExplicit Bad State Notification Base Station sends EBSN message to sender if packets Base Station sends EBSN message to sender if packets

cannot be transmitted successfullycannot be transmitted successfully Sender changes Sender changes TimeoutTimeout based on current RTT based on current RTT Timeout is reset to original on receipt of new ack.Timeout is reset to original on receipt of new ack. Eliminates unnecessary timeoutsEliminates unnecessary timeouts

EBSN

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Split Connection SchemesSplit Connection Schemes

Divide TCP connection into 2 connectionsDivide TCP connection into 2 connections Isolate wired network from wireless networkIsolate wired network from wireless network

TCP II TCP I

Wired LinkWireless Link

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Split-connectionSplit-connection – MTCP – MTCP

Protect the wired connection from the impact of Protect the wired connection from the impact of the erratic behavior of wireless connectionthe erratic behavior of wireless connection

Use session layer protocol at BS and MHUse session layer protocol at BS and MH Selective Repeat Protocol (SRP) Selective Repeat Protocol (SRP) – recover quickly packet – recover quickly packet

lossloss

TCP II TCP I

Wired LinkWireless Link

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Split-connectionSplit-connection – I-TCP – I-TCP

I-TCP (Indirect TCP)I-TCP (Indirect TCP) The idea is the same with MTCPThe idea is the same with MTCP

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Split-connection Split-connection – – M-TCPM-TCP

Three-level hierarchy architectureThree-level hierarchy architecture

High-Speed Network

SHSH SHSH

Mobile Support Station(MSS)

Cell

Mobile Host(MH)

Supervisor Host

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M-TCPM-TCP cont.cont.

End-to-end TCP connectionEnd-to-end TCP connection

SH

Fixed Host

(Sender)

Mobile Host

(Receiver)

SH-TCP M-TCP

M-TCPTCP

TCP connection is split at the SHTCP connection is split at the SH

The SH does not send an ack to FH unless SH has received The SH does not send an ack to FH unless SH has received

an ack from MHan ack from MH Maintains end-to-end semanticsMaintains end-to-end semantics

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M-TCPM-TCP cont.cont.

TCP Persist ModeTCP Persist Mode When a new ack is received with receiver’s advertised When a new ack is received with receiver’s advertised

window = 0, the sender enters persist modewindow = 0, the sender enters persist mode

Sender does not send any data in persist modeSender does not send any data in persist mode

When a positive window advertisement is received, When a positive window advertisement is received,

sender exits persist modesender exits persist mode

On exiting persist mode, RTO and congestion window On exiting persist mode, RTO and congestion window

are same as before the persist modeare same as before the persist mode

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M-TCPM-TCP cont.cont.

AdvantagesAdvantages Maintains the TCP end-to-end semanticsMaintains the TCP end-to-end semantics In case disconnection, avoids useless retransmission and slow startIn case disconnection, avoids useless retransmission and slow start Need not buffer at SHNeed not buffer at SH Efficient handoffEfficient handoff Adapt to dynamically changing bandwidth over starved linkAdapt to dynamically changing bandwidth over starved link

DisadvantagesDisadvantages SH does not act as proxySH does not act as proxy

Packet loss on wireless link is propagated to the senderPacket loss on wireless link is propagated to the sender Requires modifications to MH protocol software and new network Requires modifications to MH protocol software and new network

elements like the bandwidth management moduleelements like the bandwidth management module

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Split-connection Split-connection – – WAPWAP

Web Server

Content

CGIScriptsetc.

WML D

ecks

with W

ML-Script

WAP Gateway

WML Encoder

WMLScriptCompiler

Protocol Adapters

Client

WML

WML-Script

WTAI

Etc.

HTTPWSP/WTP

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Split-connection Split-connection – – WAP WAP cont.cont.

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Comparison of categoriesComparison of categories

  RLP Airmail Snoop FR EBSN MTCP I-TCP M-TCP

High BER √ √ √   √ √ √  

Bursty error   √ √   √ √    

Handoff   √ √ √     √ √

Long Disconnections             √ √

Frequent Disconnections             √ √

Bandwidth √ √           √

Cell size       √        

Power scarcity   √     √     √

Serial timeouts         √ √ √ √

Packet size variation         √ √    

End-to-end TCP semantics √ √ √ √ √     √

Compatability √ √ √     √ √ √

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ConclusionConclusion

Avoid erroneously triggering congestion control mAvoid erroneously triggering congestion control mechanisms on the fixed host.echanisms on the fixed host.

Avoid the serial timeout problem on the fixed host.Avoid the serial timeout problem on the fixed host. Be reliable, by solving the problems arising from tBe reliable, by solving the problems arising from t

he lossy wireless links and their bursty high BER.he lossy wireless links and their bursty high BER.

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Conclusions Conclusions cont.cont.

Can efficiently deal with handoff.Can efficiently deal with handoff. Can handle frequent and long disconnections of the Can handle frequent and long disconnections of the

mobile host.mobile host. Take into consideration the limited bandwidth and Take into consideration the limited bandwidth and

power scarcity of mobile hosts.power scarcity of mobile hosts.

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Conclusions Conclusions cont.cont.

Use a dynamic packet size depending on the Use a dynamic packet size depending on the dynamic bandwidth available for mobile hosts.dynamic bandwidth available for mobile hosts.

Preferably provide compatibility; that is, do not Preferably provide compatibility; that is, do not require any software on the fixed hosts.require any software on the fixed hosts.