ReliableByte-Stream(TCP) Outline - - PDF document

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Spring2002 CS461 1

ReliableByte-Stream(TCP)

Outline

ConnectionEstablishment/Termination SlidingWindowRevisited FlowControl AdaptiveTimeout

Spring2002 CS461 2

End-to-EndProtocols

• Underlyingbest-effortnetwork

– dropmessages – re-ordersmessages – deliversduplicatecopiesofagivenmessage – limitsmessagestosomefinitesize – deliversmessagesafteranarbitrarilylongdelay

• Commonend-to-endservices

– guaranteemessagedelivery – delivermessagesinthesameordertheyaresent – deliveratmostonecopyofeachmessage – supportarbitrarilylargemessages – supportsynchronization – allowthereceivertoflowcontrolthesender – supportmultipleapplicationprocessesoneachhost

Spring2002 CS461 3

SimpleDemultiplexor(UDP)

• Unreliableandunordereddatagramservice
• Addsmultiplexing
• Noflowcontrol
• Endpointsidentifiedbyports

– servershavewell-known ports – see/etc/services onUnix

• Headerformat
• Optionalchecksum

– psuedoheader+UDPheader+data

SrcPort DstPort Checksum Length Data 16 31

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Spring2002 CS461 4

TCPOverview

• Connection-oriented
• Byte-stream

– appwritesbytes – TCPsendssegments – appreadsbytes

Applicationprocess Write bytes TCP Sendbuffer Segment Segment Segment Transmitsegments Applicationprocess Read bytes TCP Receivebuffer … … …

• Fullduplex
• Flowcontrol:keepsender

fromoverrunningreceiver

• Congestioncontrol:keep

senderfromoverrunning network

Spring2002 CS461 5

DataLinkVersusTransport

• Potentiallyconnectsmanydifferenthosts

– needexplicitconnectionestablishmentandtermination

• PotentiallydifferentRTT

– needadaptivetimeoutmechanism

• Potentiallylongdelayinnetwork

– needtobepreparedforarrivalofveryoldpackets

• Potentiallydifferentcapacityatdestination

– needtoaccommodatedifferentnodecapacity

• Potentiallydifferentnetworkcapacity

– needtobepreparedfornetworkcongestion

Spring2002 CS461 6

SegmentFormat

Options(variable) Data Checksum SrcPort DstPort HdrLen Flags UrgPtr AdvertisedWindow SequenceNum Acknowledgment 4 10 16 31

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Spring2002 CS461 7

SegmentFormat(cont)

• Eachconnectionidentifiedwith4-tuple:

– (SrcPort,SrcIPAddr,DsrPort,DstIPAddr)

• Slidingwindow+flowcontrol

– acknowledgment,SequenceNum,AdvertisedWinow

• Flags

– SYN,FIN,RESET,PUSH,URG,ACK

• Checksum

– pseudoheader+TCPheader+data Sender Data (SequenceNum) Acknowledgment+ AdvertisedWindow Receiver

Spring2002 CS461 8

ConnectionEstablishmentand Termination

Activeparticipant (client) Passiveparticipant (server) SYN,SequenceNum=x S Y N

• +
• A

C K ,

• S

e q u e n c e N u m

• =
• y

, ACK,Acknowledgment=y +1 A c k n

• w

l e d g m e n t

• =
• x

+

• 1

Spring2002 CS461 9

StateTransitionDiagram

CLOSED LISTEN SYN_RCVD SYN_SENT ESTABLISHED CLOSE_WAIT LAST_ACK CLOSING TIME_WAIT FIN_WAIT_2 FIN_WAIT_1 Passiveopen Close Send/SYN SYN/SYN+ACK SYN+ACK/ACK SYN/SYN+ACK ACK Close/FIN FIN/ACK Close/FIN FIN/ACK ACK+FIN/ACK Timeoutaftertwo segmentlifetimes FIN/ACK ACK ACK ACK Close/FIN Close CLOSED Activeopen/SYN

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Spring2002 CS461 10

SlidingWindowRevisited

• Sendingside

– LastByteAcked <= LastByteSent – LastByteSent <= LastByteWritten – bufferbytesbetween LastByteAcked and LastByteWritten

Sendingapplication LastByteWritten TCP LastByteSent LastByteAcked Receivingapplication LastByteRead TCP LastByteRcvd NextByteExpected

• Receivingside

– LastByteRead < NextByteExpected – NextByteExpected <= LastByteRcvd+1 – bufferbytesbetween NextByteRead and LastByteRcvd

Spring2002 CS461 11

FlowControl

• Sendbuffersize:MaxSendBuffer
• Receivebuffersize:MaxRcvBuffer
• Receivingside

– LastByteRcvd - LastByteRead <=MaxRcvBuffer – AdvertisedWindow =MaxRcvBuffer - (NextByteExpected - NextByteRead)

• Sendingside

– LastByteSent - LastByteAcked <=AdvertisedWindow – EffectiveWindow =AdvertisedWindow - (LastByteSent - LastByteAcked) – LastByteWritten - LastByteAcked <=MaxSendBuffer – blocksenderif(LastByteWritten - LastByteAcked)+y > MaxSenderBuffer

• AlwayssendACKinresponsetoarrivingdatasegment
• PersistwhenAdvertisedWindow =0

Spring2002 CS461 12

SillyWindowSyndrome

• Howaggressivelydoessenderexploitopenwindow?
• Receiver-sidesolutions

– afteradvertisingzerowindow,waitforspaceequaltoa maximumsegmentsize(MSS) – delayedacknowledgements

Sender Receiver

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Spring2002 CS461 13

Nagle’sAlgorithm

• Howlongdoessenderdelaysendingdata?

– toolong:hurtsinteractiveapplications – tooshort:poornetworkutilization – strategies:timer-basedvsself-clocking

• Whenapplicationgeneratesadditionaldata

– iffillsamaxsegment(andwindowopen):sendit – else

• ifthereisunack’eddataintransit:bufferituntilACKarrives
• else:sendit

Spring2002 CS461 14

ProtectionAgainstWrapAround

• 32-bitSequenceNum

Bandwidth TimeUntilWrapAround T1(1.5Mbps) 6.4hours Ethernet(10Mbps) 57minutes T3(45Mbps) 13minutes FDDI(100Mbps) 6minutes STS-3(155Mbps) 4minutes STS-12(622Mbps) 55seconds STS-24(1.2Gbps) 28seconds

Spring2002 CS461 15

KeepingthePipeFull

• 16-bitAdvertisedWindow

Bandwidth DelayxBandwidthProduct T1(1.5Mbps) 18KB Ethernet(10Mbps) 122KB T3(45Mbps) 549KB FDDI(100Mbps) 1.2MB STS-3(155Mbps) 1.8MB STS-12(622Mbps) 7.4MB STS-24(1.2 Gbps) 14.8MB

assuming100msRTT

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Spring2002 CS461 16

TCPExtensions

• Implementedasheaderoptions
• Storetimestampinoutgoingsegments
• Extendsequencespacewith32-bittimestamp

(PAWS)

• Shift(scale)advertisedwindow

Spring2002 CS461 17

AdaptiveRetransmission (OriginalAlgorithm)

• MeasureSampleRTT foreachsegment/ACKpair
• ComputeweightedaverageofRTT

– EstRTT =α x EstRTT +β x SampleRTT – where α + β =1 − α between0.8and0.9 − β between0.1and0.2

• SettimeoutbasedonEstRTT

– TimeOut = 2 x EstRTT

Spring2002 CS461 18

Karn/PartridgeAlgorithm

• DonotsampleRTTwhenretransmitting
• Doubletimeoutaftereachretransmission

Sender Receiver O r i g i n a l

• t

r a n s m i s s i

• n

ACK SampleR TT R e t r a n s m i s s i

• n

Sender Receiver O r i g i n a l

• t

r a n s m i s s i

• n

ACK SampleR TT R e t r a n s m i s s i

• n

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Spring2002 CS461 19

Jacobson/KarelsAlgorithm

• NewCalculationsforaverageRTT
• Diff =SampleRTT - EstRTT
• EstRTT =EstRTT +(δ

δ δ δ x Diff)

• Dev=Dev+δ

δ δ δ(|Diff|- Dev)

– whereδ isafactorbetween0and1

• Considervariancewhensettingtimeoutvalue
• TimeOut =µ x EstRTT +φ x Dev

– whereµ =1andφ =4

• Notes

– algorithmonlyasgoodasgranularityofclock(500msonUnix) – accuratetimeoutmechanismimportanttocongestioncontrol(later)