11 Network Architectures A reference model that describes the layers of hardware and software necessary to transmit data between two points or for multiple.

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Network Architectures

• A reference model that describes the layers of hardware and software necessary to transmit data between two points or for multiple devices / applications to interoperate

• Reference models are necessary to increase likelihood that different components from different manufacturers will converse

• Two models to learn: OSI model and TCP/IP protocol suite

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Network Architectures

• A network architecture, or communications model, places the appropriate network pieces in layers.

• The layers define a model for the functions or services that need to be performed.

• Each layer defines what services either the hardware or software (or both) provides.

• the layers work together to allow an application to send its data over a network of computers.

• The TCP/IP protocol suite is currently used on the Internet, while the OSI model is a theoretical model.

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Network Architectures• The OSI model’s seven layers:

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Network Architectures - OSI• Application layer – where the application using the

network resides. Common network applications include web browsing, e-mail, file transfers, and remote logins

• Presentation layer – performs a series of miscellaneous functions necessary for presenting the data متنوعةpackage properly to the sender or receiver

• Session layer – responsible for establishing sessions between users. manages a session by initiating the opening and closing of sessions between end-user application processes.

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Network Architectures - OSI• Transport layer – provides an end-to-end error-free network

connection. Makes sure the data arrives at the destination exactly as it left the source. – includes error control information in case one packet from a sequence of

packets does not arrive at the final destination, and packet sequencing information so that all the packets stay in the proper order.

– We say that the transport layer performs end-to-end error control and end-to-end flow control. This means the transport layer is not in use while the data packet is hopping from point to point within the network—it is used only at the two endpoints of the connection

• Network layer – responsible for creating, maintaining and ending network connections. Transfers a data packet from node to node within and between networks.

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Network Architectures - OSI– As Network layer sends the packet from node to node, it generates the

network addressing necessary for the system to recognize the next intended receiver.

• Data link layer – responsible for taking the data and transforming it into a frame with header, control and address information, and error detection code– The Data link layer might also perform flow control. In a large network

where the data hops from node to node as it makes its way across the network, flow control ensures that one node does not overwhelm the next node with too much data.

– operations are quite similar to some of the transport layer operations. The primary difference is that the transport layer might perform its operations only at the endpoints, while the network access layer performs its operations at every stop (node) along the path.

• Physical layer – handles the transmission of bits over a communications channel. Includes voltage levels, connectors, media choice, modulation techniques (digital or analog data is encoded or modulated onto a digital or analog signal ).

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THE SEVEN OSI REFERENCE MODEL LAYERS

Each layer defines a family of function distinct from those of the other layers.

8

An Exchange Using the OSI Model

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Physical Layer

• Coordinates the function required to transmit a bit stream over a physical medium.

• Defines the procedures and function that physical devices and interfaces have to perform for transmission to occur.

10

Physical Layer

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Physical layer concerns

• Physical characteristics of interface and media. The physical layer defines the characteristics of the interface between devices and the transmission medium. It also defines the type of the transmission media such as copper wires or fiber optic cables.

• Representation of bit.The physical layer defines the type of encoding (how 0s and 1s are changed to signals)

• Data rate: The number of bits send per second.• Synchronization of bits: The sender and the receiver clocks must be

synchronized.• Transmission mode.

The physical layer also defines the direction of transmission between two devices (the direction of signal flow). Simplex, Half duplex or Full duplex

12

Physical layer concerns

• Line configuration.The physical layer is concerned with the connection of devices to the medium. In a point-to-point configuration, two devices are connected through a dedicated link. In a multipoint configuration, a link is shared among several devices.

• Physical topology.The physical topology defines how devices are connected to make a network. Devices can be connected by using: – a mesh topology (every device is connected to every other device), – a star topology (devices are connected through a central device), – a ring topology (each device is connected to the next, forming a ring), – a bus topology (every device is on a common link), – or a hybrid topology (this is a combination of two or more topologies).

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Mesh topology

14

Data Link Layer

Transforms the physical layer, a raw transmission facility, to a reliable link and it responsible for node-to-node delivery.

15

Data Link Layer

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Data Link layer responsibilities

• Framing: The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

• Physical addressing: The data link layer adds a header to the frame to define the physical address of the sender (source address) and/or receiver (destination address) of the frame.– Ex: A media access control address (MAC address) is a 

unique identifier assigned to network interfaces for communications on the physical network segment.

– MAC addresses are most often assigned by the manufacturer of a network interface controller (NIC) and are stored in its hardware

– The standard format for MAC addresses is six groups of two hexadecimal digits. e.g. 01-23-45-67-89-ab   or   01:23:45:67:89:ab

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Data Link layer responsibilities

• Flow control: If the rate at which the data are absorbed by the receiver is less than the rate produced in the sender, the data link layer impose a flow control mechanism to prevent overwhelming the receiver.

• Error control: The data link layer adds reliability to the physical layer by adding mechanism to detect and retransmit damage or lost frames and prevent duplication of frames.

• Access control: When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

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Data Link layer

19

Network layer

• Responsible for the source-to-destination delivery of a packet possibly across multiple networks (links).

• The network layer ensures that each packet gets from its point of origin to its final destination.

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Network Layer

Original source Final destination

21

Network layer responsibilities

• Logical addressing:– The network layer adds a header to the packet

coming from the upper layer, includes the logical addresses of the sender and receiver.

• Routing:– When independent networks or link are connected

together to create an internetwork ( a network of networks) the connecting devices (router or gateways) route the packet to their final destination Using network layer logical addresses in each packet

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Network Layer Example

23

Network Layer Example

24

Transport layer

• Responsible for source-to-destination (end-to-end) delivery of the entire message.

• Ensure that the whole message arrives intact and in order.

• Transport layer, may create a connection between the two end port.

25

Transport layer responsibilities

• Service-point addressing: gets the entire message to the correct process on that computer, not only from one computer to the next but also from specific process on the other.

• Segmentation and reassembly: divides the message into segments, each segment containing a sequence number, these numbers enable the T.L to reassemble the message when it arrived correctly.

• Connection control: – Connectionless : each segment is an independent

packet. data is sent from one end point to another without prior connection. No guarantees that data will arrive in the proper sequence

– Connection-oriented: make a connection with the transport layer on the receiving machine before delivering the packets. it guarantees that data will arrive in the proper sequence.

26

Transport layer responsibilities

• Flow control: end-to-end flow control

• Error control: the sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, loss,…)

27

Transport Layer

28

29

Transport Layer Example

30

Session layer

Is the next dialog controller, it establishes, maintain, and synchronizes the interaction between communicating systems.

31

Session layer responsibilities

• Dialog control:The Session layer allows two systems to enter into dialog. Allows the communication between two processes to take place either in half-duplex (one way at a time) or full-duplex ( two ways at a time)

• Synchronization:

Session layer allows a process to add checkpoints

(synchronization points) into a stream of data.

32

Session Layer

33

Presentation layer

Is concerned with the syntax and semantics of the information exchanged between two systems.

34

Presentation layer responsibilities

• Translation:• before being transmitted, information in the form of characters and

numbers should be changed to bit streams.

Presentation layer is responsible for interoperability التوافقية between different encoding methods. As different computers use different encoded methods it translate

• Encryption:– A system must be able to assure privacy.– Encryption means, that the sender transforms the original

information to another form and sends the resulting message out over the network.

• Compression:Data compression reduce the number of bits to be transmitted.

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Presentation Layer

36

Application layer

The application layer enables the user, wither human or software, to access the network. It provides user interfaces and support for services, such as electronic mail, remote file access and transfer….

37

Application layer services

• Network Virtual Terminal.– Network virtual terminal is a software version of a physical

terminal and allows a user to log on a remote host.

• File transfer, Access, and management (FTAM)– This application allows a user to access files in a remote

computer, to retrieve file files from a remote computers.

• Mail Services.– This application provides the basis for e-mail forwarding and

storage.

• Directory services– This application provides distributed database sources and

access for global information about various objects and services.

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Application Layer

39

Summary of Layer Functions

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Network Architectures• The TCP/IP protocol suite (Internet model):

Chapter One - Introduction to ComputerNetworks And Data Communications

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Network Architectures – TCP/IP

• Application layer – equivalent to OSI’s application and presentation layers

• Transport layer – equivalent to OSI’s transport layer• Network (Internet or internetwork) layer – equivalent

to OSI’s network layer• Network access (data link/physical) layer –

equivalent to OSI’s data link and physical layers

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Network Architectures

• Logical and physical connections – A logical connection is one that exists only in the software, while a physical connection is one that exists in the hardware

• Note that in a network architecture, only the lowest layer contains the physical connection, while are higher layers contain logical connections

Chapter One - Introduction to ComputerNetworks And Data Communications

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Logical and physical connections

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The TCP/IP protocol suite in action

• Note the flow of data from user to web browser and back

• At each layer, information is either added or removed, depending on whether the data is leaving or arriving at a workstation

• The adding of information over pre-existing information is termed encapsulation

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TCP/IP model in summary

Network layer

Network access layer

SNMPHTTP

47

frequently used applications in application layer

• The TCP/IP application layer includes several frequently used applications:– Hypertext Transfer Protocol (HTTP) to allow Web browsers

and servers to send and receive World Wide Web pages– Simple Mail Transfer Protocol (SMTP) to allow users to send

and receive electronic mail– File Transfer Protocol (FTP) to transfer files from one

computer system to another– Telnet to allow a remote user to log in to another computer

system– Simple Network Management Protocol (SNMP) to allow the

numerous elements within a computer network to be managed from a single point

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Protocol Layering:The Internet is build this way

• Internet Protocol (IP) provides a way to deliver packets to a destination

TCP

Internet Protocol

SSH, FTP, HTTP, SMTP

UDP

DNS, VoIP

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Internet (IP) addresses

[email protected] (email)http://www.apoptygma.eu.org (www)ftp://ftp.uk.debian.org (file transfer)telnet://towel.blinkenlights.nl (telnet)144.32.100.24 148.122.211.110195.224.53.3962.250.7.101

These are the IP addressesof the above sites. IP addressesare 32 bits grouped into 4 octets.(Octet = 8 bits – a number from0-255)

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Three different kinds of addresses

– Host names (e.g., www.cnn.com)– IP addresses (e.g., 64.236.16.20)– MAC addresses (e.g., 00-15-C5-49-04-A9)– Ports: Each process that wants to communicate with another

process identifies itself to the TCP/IP protocol suite by one or more ports. A port is a 16-bit number, used by the host-to-host protocol to identify to which higher level protocol or application program (process) it must deliver incoming messages

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MAC Address vs. IP Address

• MAC addresses– Hard-coded in read-only memory when adaptor is built– Like a social security number– Flat name space of 48 bits (e.g., 00-0E-9B-6E-49-76)– Portable, and can stay the same as the host moves– Used to get packet between interfaces on same network

• IP addresses– Configured, or learned dynamically– Like a postal mailing address– Hierarchical name space of 32 bits (e.g., 12.178.66.9)– Not portable, and depends on where the host is

attached– Used to get a packet to destination IP subnet

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• TCP—Transmission Control Protocol, reliable connect-oriented transfer of a byte stream.

• TCP uses packets to maintain connections” across a network, and thus is layered above IP.

TCP

Internet Protocol

SSH, FTP, HTTP, SMTP

UDP

DNS, VoIP

Protocol Layering:The Internet is build this way

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Transmission Control Protocol

• TCP is connection-oriented.• HTTP is an application layer protocol which uses TCP as its

transport.• Each host has a very formal way of ensuring the accuracy of

the message it receives in a connection-oriented transport.• Being connection-oriented introduces a guarantee of reliability

in the connection.• Reliability – every byte of data is guaranteed to be received at

the other end.• Data can be lost in the intermediate network. TCP adds support

to detect errors or lost data and to trigger retransmission until the data is correctly and completely received.

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• UDP—User Datagram Protocol, best-effort connectionless transfer of individual messages.

• UDP just sends or receives raw packets with a best-effort approach, also layered above IP

• It adds no reliability, flow-control, or error recovery to IP. It simply serves as a multiplexer/demultiplexer for sending and receiving datagrams, using ports to direct the datagrams

TCP

Internet Protocol

SSH, FTP, HTTP, SMTP

UDP

DNS, VoIP

Protocol Layering:The Internet is build this way

55

56

• SSH, FTP, HTTP, SMTP and many more applications use TCP connections to communicate data back and forth

TCP

Internet Protocol

SSH, FTP, HTTP, SMTP

UDP

DNS, VoIP

Protocol Layering:The Internet is build this way

57

• DNS, VoIP, and many more applications use UDP packets to communicate data – DNS : A domain name system server translates a

human readable domain name (such as example.com) into a numerical IP address

TCP

Internet Protocol

SSH, FTP, HTTP, SMTP

UDP

DNS, VoIP

Protocol Layering:The Internet is build this way

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Layers in the Example

HTTP

TCP

IP

argon.tcpip-lab.edu

128.143.137.144

Ethernet Ethernet Ethernet

IP

HTTP

TCP

IP

neon.tcpip-lab.edu128.143.71.21

Ethernet

router71.tcpip-lab.edu

128.143.137.100:e0:f9:23:a8:20

router137.tcpip-lab.edu

128.143.71.1

HTTP protocol

TCP protocol

IP protocol

Ethernet

IP protocol

Ethernet

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Layers in the Example

HTTP

TCP

IP

argon.tcpip-lab.edu

128.143.137.144

Ethernet Ethernet Ethernet

IP

HTTP

TCP

IP

neon.tcpip-lab.edu128.143.71.21

Ethernet

router71.tcpip-lab.edu

128.143.137.100:e0:f9:23:a8:20

router137.tcpip-lab.edu128.143.71.1

Send HTTP Request to neon

Establish a connection to 128.143.71.21 at port 80Open TCP connection to

128.143.71.21 port 80

Send a datagram (which contains a connection request) to 128.143.71.21Send IP datagram to

128.143.71.21

Send the datagram to 128.143.137.1

Send Ethernet frame to 00:e0:f9:23:a8:20

Send Ethernet frame to 00:20:af:03:98:28

Send IP data-gram to 128.143.71.21

Send the datagram to 128.143.7.21

Frame is an IP datagram

Frame is an IP datagram

IP datagram is a TCP segment for port 80

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Layers and Services

• Service provided by TCP to HTTP:– reliable transmission of data over a logical connection

• Service provided by IP to TCP:– unreliable transmission of IP datagrams across an IP

network• Service provided by Ethernet to IP:

– transmission of a frame across an Ethernet segment

• Other services:– DNS: translation between domain names and IP addresses– ARP: Translation between IP addresses and MAC addresses

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neon.tcpip-lab.edu"Neon"

128.143.71.21

argon.tcpip-lab.edu"Argon"128.143.137.144

router137.tcpip-lab.edu"Router137"

128.143.137.1

router71.tcpip-lab.edu"Router71"128.143.71.1

Ethernet NetworkEthernet Network

Router

Sending a packet from Argon to Neon

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DNS: The IP address of “neon.tcpip-lab.edu” is 128.143.71.21

ARP: What is the MAC address of 128.143.137.1?

neon.tcpip-lab.edu"Neon"

128.143.71.21

argon.tcpip-lab.edu"Argon"128.143.137.144

router137.tcpip-lab.edu"Router137"

128.143.137.1

router71.tcpip-lab.edu"Router71"128.143.71.1

Ethernet NetworkEthernet Network

Router

Sending a packet from Argon to Neon

DNS: What is the IP address

of “neon.tcpip-lab.edu”?ARP: The MAC address of 128.143.137.1 is 00:e0:f9:23:a8:20

128.143.71.21 is not on my local network.Therefore, I need to send the packet to my

default gateway with address 128.143.137.1

frame

128.143.71.21 is on my local network.Therefore, I can send the packet directly.

ARP: The MAC address of 128.143.137.1 is 00:20:af:03:98:28

ARP: What is the MAC address of 128.143.71.21?

frame

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• DNS – Domain Name System

• DNS is the application that turns our web site addresses into Internet Protocol addresses.

• Like an operator, given a name it will return a phone number.

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Chapter One - Introduction to ComputerNetworks And Data Communications