tcp: transmission control protocol

Applied Network Research Group Department of Computer Engineering, Kasetsart University

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TCP: Transmission Control ProtocolPart I : Protocol basics

Surasak Sanguanpong

[email protected]

http://www.cpe.ku.ac.th/~nguanLast updated: July 30, 1999


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Agenda

� Services provided by TCP� TCP format� How TCP reliability is achieved� Sliding window� TCP Connection� TCP State


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TCP encapsulation

� with Ethernet frame

Ethernet hdr IP header TCP header data

segment


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TCP & UDP Services

� TCP : Transmission Control Protocol� RFC 793� connection-oriented service� full duplex� reliable service by adding more overhead to manage

acknowledgment, flow control, timer


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TCP: Transmission Control Protocol

� TCP performs typical transport layer functions:� passed data to relevant application-level services� mux and demux data from applications to and from IP

layer� error recovery� flow control data stream (avoid buffer overflow)


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TCP properties

� byte stream with full duplex transferring� adaptive to LAN/WAN� congestion avoidance and control


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TCP data stream

� TCP provides a full duplex service that simultaneousmanages two streams of data

� stream of octets passed between sender/receiver

applicationsend

receive

applicationreceivesend


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Ports� Port - a 16 bit address allocated for the most common

application layer services� UDP and TCP use port addressing to deliver info to

applications� Servers are known by ports number

z FTP 20, TELNET 23, SMTP 25, HTTP 80

� Port numbers are generally allocated by� 0 --not used� 1-255 --Reserved ports for well-known services� 256-1023 --Other reserved ports� 1024-65535 --user-defined server ports

� Unix store general used ports in /etc/services

Applications

Transport

Network access

1 2 3 4( ) ( ) ( ) ( )


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Sockets

� socket : a pair of the IP address and the portnumber

IP address is unique to a node, the port is unique on a node

the socket gives a unique identification ofan application layer services

IP address is unique to a node, the port is unique on a node

the socket gives a unique identification ofan application layer services

<158.108.33.3, 3000>

IP address port number


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Socket address

� A connection is identified by the socket address at its toends� client socket: 158.108.33.3,3000; 158.108.2.71,21� server socket: 158.108.2.71,21; 158.108.33.3,3000;

port : 3000 port : 21connection

client server

IP : 158.108.33.3 IP : 158.108.2.71


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Socket multiple connection

� server’s unique socket address can be accessedsimultaneously by clients

port : 3000

port : 21connection

clientserverIP : 158.108.33.2

IP : 158.108.2.71

port : 3120

clientIP : 158.108.33.3


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Transmission Control Protocol

� TCP passed block of data to IP, consisting of the TCP headerand application layer data, called segment

� adding reliability in TCP is achieved by� Error detection and correction (due to segments corrupted)� Flow control (prevent a transmitter overrunning a receiver owing a

resource limitations)� Resequencing (IP can deliver datagrams in any order)� Removing duplicate segments (due to error-recovery mechanisms

used by TCP)


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How TCP handles reliability

� Using sequence numbers to identify data� positive acknowledgments of data received in the correct

sequence� retransmission of segments which have not been

acknowledged within a (variable) time limit� Let’s see these mechanisms in TCP header


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TCP header

source port:16 destination port:16 sequence number:32 acknowledgment number:32data offs:4 resv:6 flag:6 window size:16 checksum :16 urgent pointer:16 options and padding

0 15 16 31


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TCP header details (I)

� source, destination port:16,16 - identify applications at endsof the connection

� sequence:32 - indicates 1st data octet in this segment� acknowledgment:32 - next expected sequence number,

valid only when the ACK bit (reside in flag) is set


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TCP header details (II)

� data offset:4 - 32 bit words offset tells thereceiver where user data begins

� reserved:6 -not used� flag:6

z URG : validity of urgent pointer fieldz ACK : validity of acknowledge fieldz PSH : push request (pass segment to appl layer immediately)z RST : reset the connectionz SYN : initial synchronizationz FIN : sender at end of byte stream


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TCP header details (III)

� window:16 - advertise amount of buffer space this node hasallocated

� checksum:16 - 16 bits 1’s complement of pseudo header,TCP header and data

� urgent pointer:16 - byte position of data that should beprocessed first

� options - variable length option e.g. MSS (max segmentsize) tells destination node


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Sliding window principle

� send and wait for acknowledgment� no ACK within a certain time, retransmit the packet� use for flow control :

� prevent sender from overloading receiver with data, e.g.high-performance server to slow PC

� congestion inside network, e.g. router performance, slow linkspeed

� How to provide flow control?� set the appropriate size of sliding window size


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Sliding window flow control

� Receiver “ advertises” it’s windows size inacknowledgments

� Sender will adjusts its allowed to send pointer as receiver’sadvertisement

no! no! I can load only 200 kg.


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Sliding window: small window size

SENDER RECEIVER

Send 1 Receive 1Ack 2

Send 2 Receive 2Ack 3

� 1 byte window size utilizes efficiency of channel in half(half-duplex transmission)

� why not send many packets and get back cumulative ACK?

window size =1


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Sliding window: larger window size

SENDERRECEIVER

Send 1Send 2Send 3

Receive 1Receive 2Receive 3

Receive ACK 4

� A larger window size allows more data to be transmitted pendingacknowledgment

� Window size specifies how many bytes the receiver is willing toaccept

window size =3

Send ACK 4Send 4Send 5Send 6

Receive 4Receive 5Receive 6

Receive ACK 4 Send ACK 4


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Sequence number in segment

� Data continuously sent more than segment� need not to wait for acknowledgment every segment

data bytes 500-599

data bytes 600-699

data bytes 700-799

host A host B

ACK 800

I received all up to 799!


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Retransmit a loss segment

data bytes 500-599

data bytes 600-699

data bytes 700-799

host A host B

ACK 700

data bytes 700-799


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Error recovery (I)

� receiver has to send ACK with sequence number� sender reset timer when receives ACK

segment #i,start timer

host A host B

cancel timer

ACK


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Error recovery (II)

� on time out, sender will retransmit the segment� this mechanism is used for error recovery

segment #i, start timer

host A host B

cancel timer

ACK

timer expires, resend #i


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Sliding window buffer� sender groups its packet to be transmitted with window indication

1000… 1099 1100… 1199 ....999 1200…

sent and ACK

sent and not ACKed

can send ASAP can’t send now

SndWnd

SndUna SndNxt SndUna+SndWnd

offered windows


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1300…

....999

Sliding window example� movement of the right and left edges of the window

1000… 1099 1100… 1199 1200…

SndUna, SndNxt SndUna+SndWnd

1000… 1099 1100… 1199....999 1200…

SndUna SndUna+SndWndSndNxt

....999

1000… 1099 1100... 1199 1200…

SndUna SndUna+SndWndSndNxt,

....999 1000... 1099 1100… 1199 1200 1299

SndUna SndUna+SndWndSndNxt

initial

send 100 bytes

more 100 bytes

ACK 100 bytes


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TCP in actions

� before data could be transferred, a connection must beopened� servers do passive open (listen)� clients do active open (connect)

� when it finished, the connection is closed� TCP has general 3 phases

� connection setup phase� data phase� connection close phase


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TCP connection establishment

(1) send SYN (seq=x)

host A host B

(3) send ACK (ack=y+1)

(2) send SYN (seq =y, ack=x+1)

� TCP uses 3-way handshake to establish a connection� exchange the sequence number� ensures that both ends are ready and sync sequence number

connection is setup!


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Transfer phase

� simple example with terminal connection such as Telnet.Host echoes back each received character

SEQ=92, ACK=109DATA=“w”

host A host B

SEQ=93, ACK=110DATA=“…”

SEQ=109, ACK=93 DATA=“w”

Host echoes back “w”


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TCP Connection close

� use FIN flag to close connection

SEQ=x, FIN

host A host B

ACK=y+1

ACK=x+1 SEQ=Y

inform application

application response withSEQ=y, FIN


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Open/Close mechanisms

� Half open - one end has closed, aborted without the knowledgeof the other end

� host may be crashed, power off� no detection if no data transfer� reset segment (RST bit) is sent when detected

� Half close - one end of connection terminated its output, but stillreceiving data from the other end

� Simultaneous open- both end perform an active open to eachother

� Simultaneous close - both end perform an active open to eachother


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TCP state diagram : open CLOSED

start

appl passive opens;send nothing

LISTEN

passive open

appl active opens;

send SYN SYN_SENT

active open

SYN_RCVD

got SYN; s

end SYN,A

CK

ESTABLISHED

got SYN, A

CK;

send A

CK

got ACK;send nothing

got SYN;

send SYN, ACK

simultaneous open

got RST

appl closes;send FIN

appl closes;

or time-out

(normal) client transition(normal) server transition


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TCP state diagram : close

ESTABLISHED

get FIN, ACK;

send: ACK

appl close;

send FINappl closes;

send FIN

FIN_WAIT_1 CLOSING

FIN_WAIT_2 TIME_WAIT


got FIN;

send ACK


got FIN;send ACK

2MSL time-out

CLOSE_WAIT

LAST_ACK

appl closes;send FIN

got FIN;send ACK


back to CLOSED

active close

passive close

simultaneous close

(normal) client transition(normal) server transition

tcp: transmission control protocol

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