1 Chapter 12 Transmission Control Protocol (TCP) Chapter 12 Transmission Control Protocol (TCP)...

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Chapter 12Chapter 12 Transmission Control Transmission Control Protocol (TCP) Protocol (TCP)

Mi-Jung Choi

Dept. of Computer Science and Engineering

mjchoi@postech.ac.kr

12.1 TCP SERVICES

12.2 TCP FEATURES

12.3 SEGMENT

12.4 A TCP CONNECTION

12.5 STATE TRANSITION DIAGRAM

12.6 FLOW CONTROL

12.7 ERROR CONTROL

12.8 CONGESTION CONTROL

12.9 TCP TIMERS

12.10 OPTIONS

12.12 TCP PACKAGE

Contents Contents

Be able to name and understand the services offered by TCP

Understand TCP’s flow and error control and congestion control

Be familiar with the fields in a TCP segment

Understand the phases in a connection-oriented connection

Understand the TCP transition state diagram

Be able to name and understand the timers used in TCP

Be familiar with the TCP options

ObjectivesObjectives

TCP/IP ProtocolsTCP/IP Protocols

To create a process-to-process communication (using port numbers)

To create a flow control mechanism at the transport level (using

sliding window)

To create a error control mechanism at the transport level (using

Ack packet, time-out, retransmission)

Sequence control mechanism

A connection oriented, reliable transport protocol

TCP/IP FunctionsTCP/IP Functions

12.1 TCP SERVICES12.1 TCP SERVICES

We explain the services offered by TCP to the processes at the We explain the services offered by TCP to the processes at the

application layerapplication layer

The topics discussed in this section include:The topics discussed in this section include:

Process-to-Process CommunicationProcess-to-Process Communication

Stream Delivery ServiceStream Delivery Service

Full-Duplex CommunicationFull-Duplex Communication

Connection-Oriented ServiceConnection-Oriented Service

Reliable ServiceReliable Service

For client/server communication

we must define the Local host Local client program Remote host Remote server program

Process-to-Process CommunicationProcess-to-Process Communication

PROCESS-TO-PROCESS COMMUNICATION (cont.)PROCESS-TO-PROCESS COMMUNICATION (cont.)

Port numbers :

~ mentioned in UDP chapter

PROCESS-TO-PROCESS COMMUNICATION (cont.)PROCESS-TO-PROCESS COMMUNICATION (cont.) Well-known port in TCP

Port Protocol Description

7 Echo Echoes a received datagram back to the sender

9 Discard Discards any datagram that is received

11 Users Active users

13 Daytime Returns the date and the time

17 Quote Returns a quote of the day

19 Chargen Returns a string of characters

20 FTP,data File transfer Protocol for data

21 FTP,control File transfer Protocol for control

23 TELNET Terminal Network

25 SMTP Simple Mail Transfer Protocol

53 DNS Domain Name Server

67 BOOTP Bootstrap protocol

79 Finger finger

80 HTTP Hypertext Transfer Protocol

111 RPC Remote Procedure Call

As we said in Chapter 11, in UNIX, the well-known ports are stored in a file called /etc/services. Each line in this file gives the name of the server and the well-known port number. We can use the grep utility to extract the line corresponding to the desired application. The following shows the ports for FTP.

$ grep ftp /etc/services

ftp-data 20/tcpftp-control 21/tcp

Example 1Example 1

PROCESS-TO-PROCESS COMMUNICATION (cont.)PROCESS-TO-PROCESS COMMUNICATION (cont.)

Socket addresses

~ a combination of IP address and port number

~ to make a connection for each end

~ to need a pair of socket addresses: client and server socket address

These four pieces of information are part of the IP header (IP

address) and TCP header (port number)

12.1 12.1 TCP SERVICES (cont.)TCP SERVICES (cont.)

Stream delivery service

Sending and receiving buffers

Segments

Full-Duplex service

piggybacking

Connection-Oriented service

A virtual connection (not physical connection)

Reliable service

Reply acknowledge packet

Sending TCP

~ receives data as a stream of bytes from application process using sending buffer

~ make data to appropriate segments and transfer to network

Receiving TCP

~ receives segments using receiving buffer~ reassemble segments to data and send data as a stream of bytes to

application process

Stream deliveryStream delivery

SSending and receiving buffersending and receiving buffers

The sending process and the receiving process may not produce

and consume data at the same speed, TCP needs buffers for storage

TCP segmentsTCP segments

IP layer, as a service provider for TCP, needs to send data in

packets, not as a stream of bytes.

TCP groups a number of bytes together into a packet called segment

UDP vs. TCP communicationUDP vs. TCP communication

응용 응용 응용 응용

TCP송신버퍼

TCP수신버퍼

segmentsegment segmentsegment

응용 응용 응용 응용

역다중화

datagramdatagram datagramdatagram

다중화

12.2 TCP FEATURES12.2 TCP FEATURES

To provide the services mentioned in the previous section, TCP has To provide the services mentioned in the previous section, TCP has

several features that are briefly summarized in this section. several features that are briefly summarized in this section.

Numbering System Numbering System

Flow ControlFlow Control

Error ControlError Control

Congestion ControlCongestion Control

Numbering systemNumbering system

TCP keeps track of the segment being transmitted or

received using sequence number and acknowledge number

These number is used for flow and error control

The bytes of data being transferred in each connection

are numbered by TCP

The numbering starts with a randomly generated number

(b/w 0 ~ 232-1)

Numbering systemNumbering system

When TCP receives bytes of data from the process and

stores them in sending buffer

After numbering the bytes, TCP assigns sequence

number to each segment that is being sent

The value of the sequence number field in a segment

defines the number of the first data byte contained in that

segment

The value of the acknowledgment field in a segment defines

the number of the next byte a party expects to receives.

The acknowledgment number is cumulative

Example 2Example 2

Suppose a TCP connection is transferring a file of 5000 bytes. The first byte is

numbered 10001. What are the sequence numbers for each segment if data is sent

in five segments, each carrying 1000 bytes?

Solution:

The following shows the sequence number for each segment:

Segment 1 ➡ Sequence Number: 10,001 (range: 10,001 to 11,000)

A packet in TCP is called a segmentA packet in TCP is called a segment

•FormatFormat•EncapsulationEncapsulation

12.3 SEGMENT12.3 SEGMENT

The unit of data transfer b/w 2 devices using TCP is a segment

12.3 SEGMENT (cont.)12.3 SEGMENT (cont.)

Segment

source port address : the port number of the application program in the sender’s host

destination port address : the port number of the application program in the receiver’s host

sequence number : the number of the 1st byte of data in this segment

acknowledgement number : the byte number that the receiver is expecting to receive from the other party

header length : 4 bytes

control :

window size: 16 bits

checksum : all segment including pseudo-header

urgent pointer :

option :

Control

Flag Description

----- --------------

URG Urgent pointer field is valid

ACK Acknowledgment field is valid

PSH Push the data

RST Connection must be reset

SYN Synchronize sequence numbers

FIN Terminate the connection

Control

URG: 긴급 포인터가 유효함

ACK: 확인 응답 번호가 유효함

PSH: 수신측은 데이터를 가능한 빨리 응용으로 보내야 함

RST: 연결을 재설정

SYN: 연결을 초기화히기 위해 순서 번호를 동기화

FIN: 송신측이 데이터 전송을 종료함

Pseudoheader added to the TCP datagramPseudoheader added to the TCP datagram

The inclusion of the checksum in TCP is mandatory

Encapsulation and decapsulationEncapsulation and decapsulation

12.4 TCP CONNECTION 12.4 TCP CONNECTION

TCP is connection-oriented. A connection-oriented transport protocol TCP is connection-oriented. A connection-oriented transport protocol

establishes a virtual path between the source and destination. All of the establishes a virtual path between the source and destination. All of the

segments belonging to a message are then sent over this virtual path. A segments belonging to a message are then sent over this virtual path. A

connection-oriented transmission requires three phases: connection connection-oriented transmission requires three phases: connection

establishment, data transfer, and connection termination.establishment, data transfer, and connection termination.

Connection EstablishmentConnection Establishment

Data TransferData Transfer

Connection TerminationConnection Termination

Connection ResetConnection Reset

12.4 TCP CONNECTION12.4 TCP CONNECTION

TCP: connection oriented

Establishment the VC b/w source TCP and destination

Connection establishment and termination

Connection establishment

3 단계 수행1) 호스트 A 는 호스트 B 에게 연결 설정 세그먼트를 전송 ( 초기화 정보 )

2) 호스트 B 는 호스트 A 에게 확인응답 세그먼트 와 초기화 정보 세그먼트 전송3) 호스트 A 는 호스트 B 에게 확인응답 세그먼트 전송

Connection termination

4 단계 수행 1) 호스트 A 는 연결 종료를 알리고 세그먼트 전송2) 호스트 B 는 A 의 요구에 대해 확인 응답하는 세그먼트 전송3) 호스트 B 는 연결 종료를 알리는 세그먼트 전송4) 호스트 A 는 B 의 요구에 대해 확인응답

Connection establishment using three-way handshakingConnection establishment using three-way handshaking

Connection establishment using three-way Connection establishment using three-way hhandshakingandshaking A SYN segment cannot carry data, but it consumes one sequence

number.

A SYN + ACK segment cannot carry data, but does consume one

sequence number.

An ACK segment, if carrying no data, consumes no sequence number.

Data transferData transfer

The FIN segment consumes one sequence number if it does not carry data

Connection termination using three-way handshakingConnection termination using three-way handshaking

The FIN + ACK segment consumes one sequence number if it does not carry data

Half CloseHalf Close

One end stops sending data while still receiving data.

Normally initiated by client.

It can occur when the server needs all data before processing can begin

Sorting example

The client, after sending all data to be sorted, can close the connection in the outbound direction.

However, the inbound direction must remain open to receive the sorted data.

Connection ResetConnection Reset

The TCP at end may deny a connection request, may abort a

connection, or may terminate an idle connection. All of these are

done with the RST (reset) flag.

Denying a connection

Aborting a connection

Terminating an idle connection

To keep track of all the different events happening during To keep track of all the different events happening during connection establishment, connection termination, and data connection establishment, connection termination, and data transfer, the TCP software is implemented as a finite state machine. transfer, the TCP software is implemented as a finite state machine.

•ScenariosScenarios

12.5 STATE TRANSITION DIAGRAM12.5 STATE TRANSITION DIAGRAM

12.512.5 STATE TRANSITION DIAGRAMSTATE TRANSITION DIAGRAM

finite state machine

At any moment, the machine is in one of the state

TCP states

State Description----- --------------CLOSED There is no connectionLISTEN The server is waiting for calls from the clientSYN-SENT A connection request is sent; waiting for acknowledgmentSYN-RCVD A connection request is receivedESTABLISHED Connection is establishedFIN-WAIT-1 The application has requested the closing of the connectionFIN-WAIT-2 The other side has accepted the closing of the connectionCLOSINGBoth sides have decided to close simultaneouslyTIME-WAIT Waiting for retransmitted segments to dieCLOSE-WAIT The server is waiting for the application to closeLAST-ACK The server is waiting for the last acknowledgment

STATE TRANSITION DIAGRAMSTATE TRANSITION DIAGRAM

SYN +ACK

client server

- state : ovals- Transition from one to another

state : directed line- values on line

input / output- Dotted line : server- Solid line : client - Thin line : unusual situation

Connection scenarioConnection scenario

The common value for MSL (Maximum Segment Lifetime) is between 30 seconds and 1 minute

Client states

Server states

STATE TRANSITION DIAGRAMSTATE TRANSITION DIAGRAM

Connection termination using three-way handshakeConnection termination using three-way handshake

Simultaneous OpenSimultaneous Open

Simultaneous CloseSimultaneous Close

Denying a ConnectionDenying a Connection

Aborting a connectionAborting a connection

TCP OPERATIONTCP OPERATION

Encapsulation and decapsulation

TCP OPERATION (cont.)TCP OPERATION (cont.)

Buffering

TCP OPERATION (cont.)TCP OPERATION (cont.)

Multiplexing and demultiplexing

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