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 1

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.

Network Architectures The OSI model’s seven layers: 3

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 متنوعة functions necessary for presenting the data package 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. 4

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. 5

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 ). 6

THE SEVEN OSI REFERENCE MODEL LAYERS Each layer defines a family of function distinct from those of the other layers.

An Exchange Using the OSI Model

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.

Physical Layer

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

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).

Mesh topology

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

Data Link Layer

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

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.

Data Link layer

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.

Network Layer Original source Final destination

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

Network Layer Example

Network Layer Example

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.

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.

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,…)

Transport Layer

Transport Layer Example

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

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.

Session Layer

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

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.

Presentation Layer

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….

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.

Application Layer

Summary of Layer Functions

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Network Architectures Chapter One - Introduction to Computer Networks And Data Communications Network Architectures The TCP/IP protocol suite (Internet model): 41

Network Architectures – TCP/IP Chapter One - Introduction to Computer Networks And Data Communications 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 42

Network Architectures Chapter One - Introduction to Computer Networks And Data Communications 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 43

Logical and physical connections

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 45

TCP/IP model in summary HTTP SNMP Network layer Network access layer

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

Protocol Layering: The Internet is build this way Internet Protocol (IP) provides a way to deliver packets to a destination SSH, FTP, HTTP, SMTP DNS, VoIP TCP UDP Internet Protocol

Internet (IP) addresses mmhh@dmu.ac.uk (email) http://www.apoptygma.eu.org (www) ftp://ftp.uk.debian.org (file transfer) telnet://towel.blinkenlights.nl (telnet) 144.32.100.24 148.122.211.110 195.224.53.39 62.250.7.101 These are the IP addresses of the above sites. IP addresses are 32 bits grouped into 4 octets. (Octet = 8 bits – a number from 0-255)

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

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 51 51

Protocol Layering: The Internet is build this way 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. SSH, FTP, HTTP, SMTP DNS, VoIP TCP UDP Internet Protocol

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. 53

Protocol Layering: The Internet is build this way 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 SSH, FTP, HTTP, SMTP DNS, VoIP TCP UDP Internet Protocol

Protocol Layering: The Internet is build this way SSH, FTP, HTTP, SMTP and many more applications use TCP connections to communicate data back and forth SSH, FTP, HTTP, SMTP DNS, VoIP TCP UDP Internet Protocol

Protocol Layering: The Internet is build this way 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 SSH, FTP, HTTP, SMTP DNS, VoIP TCP UDP Internet Protocol

Layers in the Example 58

Layers in the Example Send the datagram to 128.143.7.21 Send HTTP Request to neon Establish a connection to 128.143.71.21 at port 80 Open TCP connection to 128.143.71.21 port 80 IP datagram is a TCP segment for port 80 Send a datagram (which contains a connection request) to 128.143.71.21 Send IP data-gram to 128.143.71.21 Send IP datagram to 128.143.71.21 Frame is an IP datagram Frame is an IP datagram Send the datagram to 128.143.137.1 Send the datagram to 128.143.7.21 Send Ethernet frame to 00:e0:f9:23:a8:20 Send Ethernet frame to 00:20:af:03:98:28 59

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 60

Sending a packet from Argon to Neon 61

Sending a packet from Argon to Neon 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 Sending a packet from Argon to Neon 128.143.71.21 is on my local network. Therefore, I can send the packet directly. DNS: What is the IP address of “neon.tcpip-lab.edu”? 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? ARP: The MAC address of 128.143.137.1 is 00:e0:f9:23:a8:20 ARP: What is the MAC address of 128.143.71.21? ARP: The MAC address of 128.143.137.1 is 00:20:af:03:98:28 frame frame 62

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. 63

Chapter One - Introduction to Computer Networks And Data Communications 64