Lecture 12 - TCP Overview

8/7/2019 Lecture 12 - TCP Overview

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CS65-Computer Networks Dept. of EEE

S.Muralidharan 1

TRANSPORT LAYER

Duties

The transport layer is responsible for processThe transport layer is responsible for process--toto--processprocessdeliverydeliverythe delivery of a packet, part of a message, fromthe delivery of a packet, part of a message, from

one process to another.one process to another.

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Example 1

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Example 2

port addresses

physical address

network addresses

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Service Access Point contd

To implement process-to-process communication, client-server paradigm is easier.

Both client and server process have the same name. For eg. To get the day and time from remote machine, we need a

Daytime client process running on the local host and a Daytimeserver process running on a remote machine.

Like MAC address at Data link layer, IP address at Networklayer , Transport layer uses Port number to identify aprocess.

In the internet model, port numbers are 16-bit integers

between 0 & 65,535. The client program defines a port number chosen randomly

by the transport layer protocol running on it. This is calledEphemeral Port Number

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Service Access Point contd

Client and server will interact to using special packet toknow the port address, they want to communicate. But

requires more overhead.

To avoid this, internet uses universal port numbers for

servers called well-known port numbers.

Every client process knows the well-known port number of

corresponding server process.

Shortlived/Briefport number

well-knownport number

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Port Number

Internet Assigned Number Authority (IANA) has dividedports into three ranges : Well known, Registered and Dynamic.

Well-known : Ranges between 0 and 1023; assigned and controlledby IANA

Registered : Ranges between 1024 and 49151; not assigned andcontrolled by IANA but can be registered with IANA to preventduplication.

Dynamic : Ranges between 49152 and 65635; neither controlled norregistered; can be used by any process. These are aphemeral ports.

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IP addressing Vs Port addressing

IP defines the host to be

selected among the different

hosts in the world.

Port number defines one ofthe processes on this

particular host.

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Socket Address

This is the combination of IP address and port number

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Multiplexing & Demultiplexing

Transport entity perform multiplexing function with respect tothe network services that it uses.

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Multiplexing & Demultiplexing .Contd

Upward multiplexingUpward multiplexing single transport layer share multiple

higher layer

Downward multiplexingDownward multiplexing single transport layer share

multiple lower layer

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Connectionless & Connection Oriented

Connectionless service :Connectionless service :

Packets are sent from one party to another with no need forconnection establishment and connection release.

Packets are not numbered , they may be delayed or lost or arrive

out of sequence.

There is no acknowledgement Connection oriented service :Connection oriented service :

Connection is established before transferring data and at the endconnection is released.

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Reliable and Unreliable

Transport layer service can be reliable or unreliable. If thehigher layer (application layer) needs reliability, we use areliable transport layer by employing flow and error control.

Error is defined as a lost or duplicated network protocol dataunit.

If the error is caught and corrected by the network service ina fashion that is transparent to the transport entity, nodamage is done.

If the network service detects an error, cannot recover, andsignals the transport entities, this is known as a signaled

failure.

Those which are not corrected and for which the transportentity is not notified is called residual error.

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Reliable and Unreliable contd

ISO has defined three types of network services : Type A : network connections with acceptable residual error rate

and acceptable rate of signaled failures. (Reliable)

Type B : network connections with acceptable residual error rate

but unacceptable rate of signaled failure. (Unreliable)

Type C : network connections with residual error rate not

acceptable to the transport service user. (Unreliable)

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Position of UDP, TCP, and SCTP in TCP/IP suite

User Datagram Protocol (UDP) is connectionless and unreliable ;

Transmission Control Protocol (TCP) and Stream Control Transmission

Protocol (SCTP) are connection oriented and reliable.

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Transmission Control Protocol (TCP)

TCP is a connectionTCP is a connection--oriented protocoloriented protocol

It creates a virtual connection between two TCPs to send

data. In addition, TCP uses flow and error control

mechanisms at the transport level.

Services provided by TCP are

Process-to-process communication

Stream Delivery service

Sending and receiving buffer

Segments

Full duplex communication

Connection oriented service

Reliable service

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Well-known ports used by TCP

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Stream Delivery

TCP allows the sending process to deliver data as a streamof bytes and allows the receiving process to obtain data as a

stream of bytes.

TCP creates an environment in which the two processes

seem to be connected by an imaginary tube that carries

their data across the Internet.

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Sending & Receiving Buffers

Since the sending and receiving process may not be

working at the same speed, a circular array of 1-byte length

is used as buffer by both the stations.

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TCP Segments

Though Transport layer handle in the form of streams of

data, the service provider for Transport layer, ie. Network

layer (IP), handles the data in the form of packets.

Hence TCP groups a number of bytes together into a packet

called Segment and adds a header to it.

These segments are encapsulated by IP datagram and

transmitted.

Segments need not be of same size.

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Others services.

TCP offers Full Duplex Communication TCP offers Connection oriented service

TCP offers Reliable service

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Numbering System

Numbering is used forflow & error control

Byte Number : TCP numbers all the data bytes that are transmitted in a connection,

When TCP receives bytes of data from a process, it stores them in the

sending buffer and numbers them.

The numbering scheme start with a random number between 0 and 232

1. Sequence Number :

After numbering each bytes of data, TCP assigns a sequence number to

each segment that is being sent.

The sequence number for each segment is the number of the first byte

carried in that segment.

Acknowledgement Number : It defines the number of next byte that the receiving end expects.

Acknowledgement is cumulative.

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Example

Imagine a TCP connection is transferring a file of 6000 bytes.

The first byte is numbered 10010.

What are the sequence numbers for each segment if data are sent in five

segments with the first four segments carrying 1000 bytes and the last segment

carrying 2000 bytes?

The following shows the sequence number for each segment:

Segment 1 ==> sequence number: 10010 (range: 10,010 to 11,009)





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TCP Segment Format

TCP Segment consists of a 20-byte(without options) to 60-bytes(with options) header followed by data.

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Source Portand Destination Port Identifies points at whichupper-layer source and destination processes receive TCP services.

Sequence Number Usually specifies the number assigned tothe first byte of data in the current message. In the connection-establishment phase, this field also can be used to identify an initialsequence number to be used in an upcoming transmission.

Acknowledgment Number Contains the sequence number ofthe next byte of data the sender of the packet expects to receive.

HLEN Header Length - Indicates the number of 32-bit words inthe TCP header and thereby it indicate where the data field begins.

Reserved Remains reserved for future use.

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Flags (6 bits)

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Window Specifies the size of the senders receive window (that is,the buffer space available for incoming data).

Checksum used to indicate whether the segment was damaged intransit.

Urgent Pointer Points to the first urgent data byte in the packet.

Options Specifies various TCP options [Max. segment size,window scale upto 214 (i.e., 65,536 x 214 = 107,37,25,440 bytes) &Time stamp option]

Data Contains upper-layer information.

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TCP Header Overload

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Connection establishment using three-way handshaking

Connection establishment By Three way handshake

Between pairs of ports

One port can connect to multiple destinations

Server program informs its TCP that it is ready to accept any

connection. This is called Passive open. But a server cannot initiate

any connection.

A client program issues a request for an active open

Malicious attacker may send a large number of SYN segments to the

server, pretending that each of them is coming from a different client

by faking the source. Without knowing this server may startedallocating its resources, which sometimes lead to server crash. This

attack is called SYN flooding attack and to manage this situation

server may restrict the number of connections or avoid requests from

unwanted sources.

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Connection establishment using three-way handshaking

A SYN segmentcannot carry data,

but it consumes onesequence number

A SYN+ACK

segment cannotcarry data, but does

consume onesequence number

An ACK segment, ifcarrying no data,

consumes nosequence number.

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Data Transfer

Data transfer

Logical stream of octets

Octets numbered modulo 232

Flow control by credit allocation of number of octets

Data buffered at transmitter and receiver

To overcome buffering and requiring immediate attention, data istransferred with PSH flag set.

To draw the immediate attention by the receiving TCP, URG flag is

used.

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Data transfer

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Connection termination using three-way handshaking

Connection termination Graceful close

TCP users issues CLOSE primitive

Transport entity sets FIN flag on last segment sent

Abrupt termination by ABORT primitive

Entity abandons all attempts to send or receive data

RST segment transmitted

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Connection termination using three-way handshaking

FIN segment

consumes one

sequence number

if it does not carrydata

The FIN+ACKsegment

consumes onesequence

number if itdoes not carry

data

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Half-close

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TCP a state transition diagram

input/output

Client

Server

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Flow control

The amount of data a source can send before receiving

an ACK from the destination

Whether to send 1 byte of data and wait for ACK or send

all the bytes and wait for the ACK for the complete

message?

TCP gives a solution in between

Sliding window protocol

byte oriented

Already dealt along with Unit I

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USER DATAGRAM PROTOCOL(UDP)

It is the simplest form of transport protocol.

The datagram packet of this protocol has the followingfields

Source port address :Address of the application program that hascreated the message.

Destination port address : Address of the application program

that will receive the message Total length : The total length field defines the total length of the

user datagram in bytes

Check sum : The checksum is a 16-bit field used in errordetection

Data

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UDP Header

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Lecture 12 - TCP Overview