1 TCP. 2 Referencias Cerf, V., and R. Kahn, "A Protocol for Packet Network Intercommunication", IEEE Transactions on Communications, Vol. COM-22, No.

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TCP

2

Referencias

• Cerf, V., and R. Kahn, "A Protocol for Packet Network Intercommunication", IEEE Transactions on Communications, Vol. COM-22, No. 5, pp 637-648, May 1974.

• J. Postel, RFC-793 "Transmission Control Protocol" September 1981.

( Various errors and inconsistencies were detected)

• Clarifications and bug fixes in RFC 1122 October 1989

• Extensions in RFC 1323

• Peterson and Davie , pp 378-405

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Agenda

o Revisión Nivel de Transporteo El Protocolo TCP

Características TCP Connection setup Segmentos TCP Números de Secuencia TCP TCP Sliding Window Control de Flujo Timeouts y Retransmisiones (RTX)

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Recordar los modelos de capas….

1 Parte de la clase

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Nivel Transporte

Enri Lean

milagros.dc.uba.ar stone.ac.upc.es

Nivel de Red

Nivel de Enlace

Nivel de Aplicación

Nivel de Transporte

O.S. O.S.HeaderData HeaderData

HD

HD

HD

HD HD

HD

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Modelo OSI

Session

Network

Link

PhysicalPhysicalPhysical

Application

Presentation

Transport

Network

Link Link

Network

Transport

Session

Presentation

Application

Network

Link

Physical

Peer-layer communication

layer-to-layer communication

Router Router

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2

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6

7

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2

3

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5

6

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TCP : Características TCP es orientado a conexion

Manejo de la conexión : 3-way handshake usado para setup y 2-2 o 4 way handshake para la liberación

TCP provee un servicio de flujo de bytes (stream-of-bytes ) TCP es confiable ( estableciendo una suerte de “conexión lógica entre los

sockets”) Acknowledgements ACKs Checksums Números de secuencia para detectar datos perdidos o desordenados Datos perdidos o corruptos se RTX después de un timeout. Datos desordenados se podrían reordenar. Control de Flujo evita inundar al receptor .

TCP implementa mecanismos de control de congestión ( se le dedica una clase la semana próxima ).

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TCP : orientado a conexión

3-way handshake

(Activo)Cliente

(Pasivo)Server

Syn

Syn + Ack

Ack

4-way handshake

(Activo)Cliente

(Pasivo)Server

Fin

(Data +) Ack

Fin

Ack

Caso ideal , en el RFC 793 Se plantean varios escenarios

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TCP soporta un “stream de bytes”

By te 0

By te 1

By te 2

By te 3

By te 0

By te 1

By te 2

By te 3

Host A

Host B

By te 8 0

By te 8 0

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…dicho servicio se emula usando segmentos TCP

By te 0

By te 1

By te 2

By te 3

By te 0

By te 1

By te 2

By te 3

Host A

Host B

By te 8 0

TCP Data

TCP Data

By te 8 0

Un segmento se envía cuando:1. Segmento full (MSS bytes),2. No esta “full” , pero sucede

time out, 3. “Pushed” por la aplicación.

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Formato del Segmento TCP

IP HdrIP Data

TCP HdrTCP Data

Src port Dst port

Sequence #

Ack Sequence #

HLEN4

RSVD6

UR

GA

CK

PS

HR

ST

SYN

FIN

FlagsWindow Size

Checksum Urg Pointer

(TCP Options)

0 15 31

TCP Data

TCP Header y Data + dirección

IP

Los números de port Src/dst y la direcciones identifican unívocamente un “socket”

Veremos concepto de

pseudo header

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Números de SecuenciaHost A

Host B

TCP Data

TCP Data

TCP HDR

TCP HDR

ISN (initial sequence number)

Seq_Num = 1er byte

ACK Seq_Num = Próximo byte

esperado

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Initial Sequence Numbers

Connection Setup3-way handshake

(Active)Client

(Passive)Server

Syn +ISNA

Syn + Ack +ISNB

Ack

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TCP Sliding Window How much data can a TCP sender have

outstanding in the network? How much data should TCP retransmit when an

error occurs? Just selectively repeat the missing data?

How does the TCP sender avoid over-running the receiver’s buffers?

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TCP Sliding Window

Window Size

OutstandingUn-ack’d data

Data OK to send

Data not OK to send yet

Data ACK’d

Retransmission policy is “Go Back N”. Current window size is “advertised” by receiver (usually 4k – 8k Bytes when connection set-up).

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TCP Sliding Window

Host A

Host BACK

Window Size

Round-trip time

(1) RTT > Window size

ACK

Window Size

Round-trip time

(2) RTT = Window sizeACK

Window Size???

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Jacobson ( 1988)

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TCP: Retransmission and Timeouts

Host A

Host B

ACK

Round-trip time (RTT)

ACK

Retransmission TimeOut (RTO)

Estimated RTT

Data1 Data2

Guard

Band

TCP uses an adaptive retransmission timeout value:

CongestionChanges in Routing

RTT changes frequently

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TCP: Retransmission and Timeouts

Picking the RTO is important: Pick a values that’s too big and it will wait too long to

retransmit a packet, Pick a value too small, and it will unnecessarily retransmit

packets.

The original algorithm for picking RTO:1. EstimatedRTT = EstimatedRTT + (1 - ) SampleRTT2. RTO = 2 * EstimatedRTT

Characteristics of the original algorithm: Variance is assumed to be fixed. But in practice, variance increases as congestion

increases.

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TCP Timer ManagementOf the several timers TCP maintains the most important is the retransmission timer RTO, (also called timeout) . After each segment is sent, TCP starts a retransmission timer, if ACK arrives before timer expires, cancel timer. If timer expires first, consider segment lost.

How long should RTO be ?Typically some small multiple of RTT.

So how to measure RTT ?Measure time between segment sent and ACK receiver.

Unfortunately, in the Internet RTT are not constant, they a vary a lot.

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TCP Timer Management

(a) Probability density of ACK arrival times in the data link layer.

(b) Probability density of ACK arrival times for TCP.

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Retransmisión Adaptiva (Algoritmo Original)

• Mide SampleRTT para cada par segmento/ ACK• Calcula el promedio ponderado de RTT

– EstimatedRTT = x EstimatedRTT + x SampleRTT– donde + = 1 <= 0.9 <= 0.2

• Fijar timeout basado en EstimatedRTT– TimeOut = 2 x EstimatedRTT

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Algoritmo de Karn/Partridge

• No considerar RTT cuando se retransmite• Duplicar timeout luego de cada retransmisión

Sender Receiver

Original transmission

ACK

Sam

pleR

TT

Retransmission

Sender Receiver

Original transmission

ACK

Sam

pleR

TT

Retransmission

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Algoritmo de Jacobson/ Karels• Nueva forma de calcular el promedio de RTT• Diff = sampleRTT - EstRTT• EstRTT = EstRTT + ( x Diff)• Dev = Dev + ( |Diff| - Dev)

– donde es un factor entre 0 y 1 (Por ejemplo 1/8)

• Considerar varianza cuando fijamos el timeout• TimeOut = x EstRTT + x Dev

– donde = 1 y = 4

• Notas– Los algoritmos son tan buenos/malos como la granularidad del reloj

(500ms en Unix)– Un preciso mecanismo de timeout es importante para controlar la

congestión (más adelante)– Además de controlar congestión, la idea es no retransmitir cuando no es

necesario.

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RTO ( Ret. Timeout) exceptionsAssume a segment times out and is then retransmitted. An ACK for the segment arrives.

So for purposes for calculating M how do we decide if the ack is for the first send or the retransmission ?

We cannot. It might be for the first, but very delayed, or might be for the second. So we cannot use ACKs of retransmitted segments for calculating M (or updating RTT).

Rule: Don't use acks of retransmitted segments to update RTT. Instead, if segment times out, simply double RTO.

This is called the Karn's algorithm.

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Ejemplo de estimación de RTT:RTT: gaia.cs.umass.edu to fantasia.eurecom.fr

100

150

200

250

300

350

1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106

tiempo (segundos)

RTT

(mili

segu

ndos

)

MuestraRTT EstimadoRTT

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TCP

2 parte

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TCP

Repaso ( basado en el Peterson)

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Protocolos End-to-End

• Se apoyan en la capa Red, la cual es de mejor esfuerzo (best-effort)– descarta mensajes– re-ordena mensajes– puede entregar múltiples copias de un mensaje dado– limita los mensajes a algún tamaño finito– entrega mensajes después de un tiempo arbitrariamente largo

• Servicios comunes ofrecidos/deseados end-to-end– garantía de entrega de mensajes– entrega de mensajes en el mismo orden que son enviados– entrega de a lo más una copia de cada mensaje– soporte para mensajes arbitrariamente largos mensajes– soporte de sincronización– permitir al receptor controlar el flujo de datos del transmisor– soportar múltiples procesos de nivel aplicación en cada máquina

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Demultiplexor Simple (UDP: User Datagram Protocol)

• Servicio de entrega no confiable y no ordenado de datagramas

• Agrega multiplexión• No hay control de flujo• Hay puertos definidos en cada extremo

– servidor posee un puerto bien conocido– ver /etc/services en Unix (o Linux)

• Formato de encabezado

• Chequeo se suma opcional– psuedo header(IP) + UDP header + data

SrcPort DstPort

Checksum Length

Data

0 16 31

31

Version HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Contexto para encabezado UDP

SrcPort DstPort

Checksum Length

Data

IP

UDP

Pseudo encabezado

Largo de encabezado + datos

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TCP Generalidades• Orientado a conexión• flujo de byte

– app escriben bytes– TCP envía segmentos– app lee bytes

Application process

Writebytes

TCPSend buffer

Segment Segment Segment

Transmit segments

Application process

Readbytes

TCPReceive buffer

…

… …

• Full duplex• Control de flujo: evita que el Tx

rebalse al receptor• Control de congestión: evita que el Tx

sobrecargue la red

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Enlace de Datos Versus Transporte• Potencialmente conecta muchas máquinas diferentes

– requiere de establecimiento y término de conexión explícitos

• Potencialmente diferentes RTT– requiere mecanismos adaptivos para timeout

• Potencialmente largos retardos en la red– requiere estar preparado par el arribo de paquetes muy antiguos

• Potencialmente diferente capacidad en destino– requiere acomodar diferentes capacidades de nodos

• Potencialmente diferente capacidad de red– requiere estar preparado para congestión en la red

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Version HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Contexto Formato de Segmento

Options (variable)

Data

Checksum

SrcPort DstPort

HdrLen 0 Flags

UrgPtr

AdvertisedWindow

SequenceNum

Acknowledgment

IP

TCP

Pseudo encabezado

URG|ACK|PSH|RST|SYN|FIN

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Formato de Segmento

Options (variable)

Data

Checksum

SrcPort DstPort

HdrLen 0 Flags

UrgPtr

AdvertisedWindow

SequenceNum

Acknowledgment

0 4 10 16 31

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Formato de Segmento (cont)• Cada conexión es identificada por la 4-tupla:

– (SrcPort, SrcIPAddr, DsrPort, DstIPAddr)

• Ventana deslizante + control de flujo– acknowledgment, SequenceNum, AdvertisedWinow

• Flags– SYN, FIN, RESET, PUSH, URG, ACK

• Checksum– pseudo header(IP) + TCP header + data

Sender

Data (SequenceNum)

Acknowledgment +AdvertisedWindow

Receiver

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Establecimiento y Término de Conexión

Active participant(client)

Passive participant(server)

SYN, SequenceNum = x

SYN + ACK, SequenceNum = y,

ACK, Acknowledgment = y + 1

Acknowledgment = x + 1

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Diagrama de Estado de TransmisiónCLOSED

LISTEN

SYN_RCVD SYN_SENT

ESTABLISHED

CLOSE_WAIT

LAST_ACKCLOSING

TIME_WAIT

FIN_WAIT_2

FIN_WAIT_1

Passive open Close

Send/SYNSYN/SYN + ACK

SYN + ACK/ACK

SYN/SYN + ACK

ACK

Close/FIN

FIN/ACKClose/FIN

FIN/ACKACK + FIN/ACK Timeout after two segment lifetimes

FIN/ACK

ACK

ACK

ACK

Close/FIN

Close

CLOSED

Active open/SYN

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Revisión de Ventana Deslizante

• Lado TransmisorLastByteAcked < = LastByteSent

LastByteSent < = LastByteWritten

Se tiene en buffer los bytes entre LastByteAcked y LastByteWritten

Sending application

LastByteWritten

TCP

LastByteSentLastByteAcked

Receiving application

LastByteRead

TCP

LastByteRcvdNextByteExpected

• Lado ReceptorLastByteRead < NextByteExpected

NextByteExpected < = LastByteRcvd +1

Se tiene en buffer los bytes entre NextByteRead y LastByteRcvd

LastByteRead+1

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Control de Flujo• Tamaño del buffer de envío: MaxSendBuffer• Tamaño del buffer de recepción: MaxRcvBuffer• Lado receptor

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

• Lado Transmisor– LastByteSent - LastByteAcked < = AdvertisedWindow– EffectiveWindow = AdvertisedWindow - (LastByteSent - LastByteAcked)– LastByteWritten - LastByteAcked < = MaxSendBuffer– Bloquear Tx si (LastByteWritten - LastByteAcked) + y > MaxSenderBuffer, y

bytes que se desean escribir. • Siempre enviar ACK en respuesta a la llegada de segmentos de datos• Tx persiste enviando 1 byte cuando AdvertisedWindow = 0

Sending application

LastByteWrittenTCP

LastByteSentLastByteAcked

Receiving application

LastByteReadTCP

LastByteRcvdNextByteExpected

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• ¿Qué tan agresivamente el Tx explota la apertura de ventana?

• Soluciones en lado Receptor– Retardar los acuses de recibo

Síndrome de Ventana estúpida (Silly)

Sender Receiver

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Silly Window Syndrome

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Algoritmo de Nagle• ¿Qué tanto tiempo el Tx retarda la transmisión de datos?

– Demasiado largo: afecta aplicaciones interactivas

– Demasiado corto: Utilización de la red es pobre

– Estrategias: Basadas en timers v/s auto relojes

• Cuando la aplicación genera datos adicionales:– Si se llena un segmento (y la ventana está abierta): enviar

– Sino

• Si hay datos sin ack en Tx: dejar en buffer hasta llegada de ack

• sino: enviar datos

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Nagle's algorithmPurpose is to allow the sender TCP to make efficient use of the network, while still being responsive to the sender applications.

Idea:If application data comes in byte by byte, send first byte only. Then buffer all application data till until ACK for first byte comes in. If network is slow and application is fast, the second segment will contain a lot of data. Send second segment and buffer all data till ACK for second segment comes in.

This way the algorithm is clocking the sends to speed of the network and simultaneously preventing sending several one byte segments back to back.

An exception to this rule is to always send (not wait for ACK) if enough data for half the receiver window or MSS.

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Protección contra reapariciones de igual número de secuencia

• SequenceNum de 32 bits

Bandwidth Tiempo hasta tener problemaT1 (1.5 Mbps) 6.4 hoursEthernet (10 Mbps) 57 minutesT3 (45 Mbps) 13 minutesFDDI (100 Mbps) 6 minutesSTS-3 (155 Mbps) 4 minutesSTS-12 (622 Mbps) 55 secondsSTS-24 (1.2 Gbps) 28 seconds

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Mantención de la tubería llena

• AdvertisedWindow de 16 bits

Bandwidth Delay x Bandwidth ProductT1 (1.5 Mbps) 18KBEthernet (10 Mbps) 122KBT3 (45 Mbps) 549KBFDDI (100 Mbps) 1.2MBSTS-3 (155 Mbps) 1.8MBSTS-12 (622 Mbps) 7.4MBSTS-24 (1.2 Gbps) 14.8MB

Asumiendo RTT de 100 ms

64 KB

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Extensiones de TCP

• Son implementadas como opciones del encabezado

• Almacenar marcas de tiempo en segmentos de salida

• Extender espacio de secuencia con marca de tiempo de 32-bit (PAWS)

• Desplazar (escalar) ventana avisada. La idea es medir la ventana en unidades de 2, 4, 8 bytes.

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TCP OptionsSome TCP options are:

Maximum segment size (MSS): Specified what is the payload the sender is able to receive. (Default MSS = 536 bytes, i.e., Segment size = MSS + 20). SMSS/RMSS is Sender/Receiver MSS.

Window scale: The window size field allows for upto 2^16 bytes of data. But this might be inefficient for high bw x delay situations. This options TCP indicate a scaling factor.

Negative acknowledgement: Lets receiver user NAKs to get realize selective repeat rather than the normal go-back-N TCP behaviour.

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Recordemos MSS

Application Message

TCP dataTCP hdr

MSSTCP Segment

IP dataIP hdrIP Packet

Ethernet dataEthernet

Ethernet Frame

20 bytes

20 bytes

14 bytes 4 bytesMTU 1500 bytes

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Checksum

• Se calcula entre el TCP Header , Data y el pseudo header

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Window Management

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Initial Sequence Number• Select initial sequence numbers (ISN) to protect against

segments from prior connections (that may circulate in the network and arrive at a much later time)

• Select ISN to avoid overlap with sequence numbers of prior connections

• Use local clock to select ISN sequence number

• Time for clock to go through a full cycle should be greater than the maximum lifetime of a segment (MSL); Typically MSL=120 seconds

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TCP Connection Establishment

(a) TCP connection establishment in the normal case.(b) Call collision.

6-31

Initial sequence numbers are not 0. TCP uses a clock tick counter (at 4 usecs rate) to setup the initial sequence numbers. This scheme prevents delayed duplicates.

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Connection Establishment (cont)

Active participant(client)

Passive participant(server)

SYN, SequenceNum = x

ACK, Acknowledgment =y+1

Acknowledgment =x+1

SYN+ACK,

SequenceNum=y,

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TCP Connection Release • Graceful release:

– Each side of the connection released independently.

• Either side send TCP segment with FIN=1.• When FIN acknowledged, that direction is shut down for data.• Connection released when both sides shut down.

– 4 segments: 1 FIN and 1 ACK for each direction; 1st. ACK+2nd. FIN combined.

– Two-army problem , Timers , 2 MSL

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TCP Connection Management Modeling

TCP connection management finite state machine. The heavy solid line is the normal path for a client. The heavy dashed line is the normal path for a server. The light lines are unusual events. Each transition is labeled by the event causing it and the action resulting from it, separated by a slash.

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TCP Connection Management ModelingThe states used in the TCP connection management finite

state machine.

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Maximum Segment Size

• Maximum Segment Size– largest block of data that TCP sends to other end

• Each end can announce its MSS during connection establishment

• Default is 576 bytes including 20 bytes for IP header and 20 bytes for TCP header

• Ethernet implies MSS of 1460 bytes• IEEE 802.3 implies 1452

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Tear-down Packet Exchange

Sender ReceiverFIN

FIN-ACK

FIN

FIN-ACK

Data write

Data ack

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Connection Tear-down

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Detecting Half-open Connections

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TIME-WAIT Assassination