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Ryerson University
Information Technology Management
ITM 301
Farid Shirazi

Final Exam Review: The OSI Model: -model that is used to describe network communication is called the OSI (Open System Interconnection) model -this model divides network communications into seven layers: Physical, Data Link, Network, Transport, Session, Presentation and Application -while performing those services, the protocols also interact with protocols in the layers directly above and below -protocols are rules by which computers communicate; it is simply a set of instructions written by a programmer to perform a function or group of functions -the OSI model is a representation of what happens between two nodes communicating on a network -OSI model is discussed from highest to lowest, beginning with Application layer where the flow of information is initiated Application Layer: -provides interface between software applications and a network for interpreting applications requests and requirements th -the 7 layer of the OSI model -does not include software programs such as Microsoft Word or Firefox, instead it facilitates communication between programs and lower-layer network services -services at this layer allow to enable the network to interpret a programs request and the program to interpret data sent from the network -For Example: • When you choose to open a Web page on Firefox, an application layer protocol called HTTP (Hypertext Transfer Protocol) formats and sends your request from your client browser to the server, it also then formats and sends the Web server’s response back to your client browser • Suppose you want to view the Library of Congress’s Website, you type in in Firefox and press enter  At this point, the Firefox’s API (application programming interface; a set of routines that make up part of the software) transfers your request to the HTTP protocol  HTTP prompts lower-layer protocols to establish a connection between your computer and the Web server  HTTP formats your request for the Web page and sends the request to the Web server one part of the request includes a command that begins with “GET” and tells the server what page you want to retrieve other part of the request indicates what version of HTTP you’re using, what types of graphics and what language your browser can accept, and what browser version your using  After receiving your computers HTTP request, the Web server for the website responds, its response includes the text and graphics that make you the web page and the content contained in the page, how many pages etc. if the web page is unavailable, the host will send an HTTP response containing an error message such as “Error 404- File Not Found”  After receiving the Web servers response, your workstation uses HTTP to interpret this response so that Firefox can present the web page in a format that you will recognize (with neatly arranged text and images) Presentation Layer: -allows hosts and applications to use a common language; performs data formatting, encrypting and compression -protocols accept Application layer data and format it so that one type of application and host can understand data from another type of application and host -this layer serves as a translator -Example: JPG, GIF, TIFF are methods of compressing and encoding graphics -Example: QuickTime and MPEG are two popular methods of compressing and encoding audio and video data -this layer also manages data encryption such as scrambling of passwords and decryption • For example: if you are going to look up your bank account status via the Internet, you are using a secure connection and the presentation layer protocol will encrypt your account data before it is transmitted  On your end layer, the presentation layer will decrypt the data as it is received Session Layer: -establishes, maintains, and terminates user connections -session layer coordinate and maintains communication between two nodes on the network, the term session refers to a connection for ongoing data exchange between two parties -session is often used in the context of a connection between a remote client and an access server between a Web browser client and a Web server -modern networks don’t make a use of session layer protocol for routine data exchange, such as web page retrieval or file sharing -applications that require precisely coordinated data exchanges such as videoconferencing communications do use session layer -session layer’s functions are establishing and keeping alive the communication link for the duration of the session, keeping the communication secure, synchronizing the dialogue between the two nodes, determining whether communications have been cut off if so then figuring out where to restart transmission and terminating communications -it also sets the terms of communication by deciding which node communicates first and how long -if connection is lost, session layer protocols will detect that and attempt to reconnect, if they cannot reconnect they will close the session and inform your client software that communication has been ended -session layer monitors the identification of session participants, ensuring that only the authorized nodes can access the session Transport Layer: -ensures accurate delivery of data through flow control, segmentation and reassembly, error correction and acknowledgment -accept data from the session layer and manage end to end delivery of data, which means to make sure that data is transferred from point A to point B in the correct sequence without any errors -without transport layer, data could not be verified or interpreted by its recipient -many different types of transport layer protocols exists, but the most modern (Internet) rely on only a few -transport layer protocol called TCP (transmission control protocol) takes care of reliably transmitting the HTTP protocols request from client to server -TCP is an example of a connection-oriented protocol, which ensures that data arrives exactly as it was sent Network Layer: Establishes network connections; translates network address into their physical counterparts and determines routing -translate network address into their physical counterparts and decide how to route data from the sender to the receiver -each node has 2 types of addresses (1) Network Address: follows a hierarchical addressing scheme and can be assigned through operating system software -they are hierarchy because they contain subsets of data, narrow down the location of a node just has your home address does (country, state, zip code, city, street, house number, person’s name) (2) Physical Address: Example, a computer running on a TCP/IP network might have a Network layer address of and a physical address of 0060972E87F3 -network layer protocol accepts the Transport layer segments and add logical addressing information in a network header -there are numerous Network layer protocols, but one of the most common that underlies most Internet traffic is the IP (Internet Protocol) • In the example of requesting a Web page, IP is the protocol that instructs the network where the HTTP request is coming from and where it should go Data Link Layer: -packages data in frames appropriate to network transmission method -protocol divided data they receive from the network layer into distinct frames that can be transmitted by the physical layer -Example to under this concept better: • You are in Ms. Jones large classroom, which is full of noisy students and you need to ask the teacher a question. In order to get your message across, you might say “Ms. Jones? Can you please this to me in detail …” In this example you are the sender (in a busy network) and you have addressed your recipient, Ms. Jones; just as the data link layer address another computer on the network -IEEE had divided the data link layer into two sub-layers • LLC (Logical Link Control) sub-layer: provides an interface to the network layer protocols, manage flow control and issues request for transmission for data that have suffered errors • MAC (Media Access Control) sub-layer: the lower layer of the data link layer manages access to the physical medium -the traditional physical addressing schemes, the OUI is six characters (24bits) long and the extension identifies is also 6 characters long -together the OUI and extension identifier form a whole physical address -physical address is frequently depicted as hexadecimal numbers separated by colons for example: 00:60:8C:oo:54:99, whereas the MAC address has a 49bit address Physical Layer: -managers signaling to and from physical network connections -protocols at the physical layer accept frames from the data link layer and generate signals as changes in voltage at the NIC -when receiving data, physical layer detects and accepts signals which they pass on to the Data Link Layer -it also sets the transmission rate and monitor data error rates -simple connectivity devices such as hubs and repeaters operate at the physical layer Characteristics of TCP/IP (Transmission Control Protocol/Internet Protocol): -it is not simply one protocol but rather a suite of specialized protocols which includes: TCP, IP, UDP, ARP and many others called “sub protocols” -most network administrators refer to the entire group as TCP/IP -the most recent versions of all other network operating systems also use TCP/IP as their default protocol -TCP/IP has become the standard: • It is open rather than proprietary: it is not owned by a company, which means you do not need to purchase a license to use it, it costs nothing and its code can be edited and modified by any programmer • It is flexible: can run virtually any platform and connect dissimilar operating systems and devices • It is routable: transmission carry network layer addressing information that can be interpreted by routers to determine the best path for directing data over a network The TCP/IP Model: -can be divided into 4 layers (roughly can be corresponded to the 7 layers of the OSI model) 1. Application Layer: application gain access to the network through this layer via protocols such as HTTP (hypertext transfer protocol), FTP (file transfer protocol), telnet, NTP (network time protocol), DHCP (dynamic host configuration protocol) and PING (packet internet groper) 2. Transport Layer: this model holds the transmission control protocol (TCP) and user datagram protocol (UDP) which provide flow control, error checking and sequencing 3. Internet Layer: this layer holds the internet protocol (IP) and internet control message protocol (ICMP), internet group management protocol (IGMP), and address resolution protocol (ARP), these protocol handle routing and address resolutions 4. Network Interface Layer (aka Link Layer): handles formatting of data and transmission to the network interface -TCP/IP model describes how protocols work after the protocols are widely in use The TCP/IP Core Protocols: -certain sub-protocols of the TCP/IP suite are called TCP/IP core protocols, they operate in the transport or network layers of the OSI model and provide basic services to protocols in other layers TCP (Transmission Control Protocol): -operates in the transport layer of the TCP/IP and OSI model which provides reliable data delivery -it is a connection oriented sub protocol which means that a connection must be established between communicating nodes before this protocol will transmit data -TCP ensures a connection has been made before it allows the message to continue -it also ensures reliable data delivery through sequencing and checksums -provides control to ensure that a node is not flooded with data UDP (User Datagram Protocol): -UDP (User Datagram Protocol), like TCP, belongs to the Transport layer of the TCP/IP and OSI models. Unlike TCP, however, UDP is a connectionless transport service. - In other words, UDP offers no assurance that packets will be received in the correct sequence. In fact, this protocol does not guarantee that the packets will be received at all. Furthermore, it provides no error checking or sequencing. -UDP’s lack of sophistication makes it more efficient than TCP. It can be useful in situations in which a great volume of data must be transferred quickly, such as live audio or video transmissions over the Internet. In these cases, TCP—with its acknowledgments, checksums, and flow-control mechanisms—would only add more over head to the transmission. -UDP is also more efficient for carrying messages that fit within one data packet. IP (Internet Protocol): -IP (Internet Protocol) belongs to the Internet layer of the TCP/IP model and the Network layer of the OSI model. It provides information about how and where data should be delivered, including the data’s source and destination addresses. -IP is the sub protocol that enables TCP/IP to internetwork—that is, to traverse more than one LAN segment and more than one type of network through a router. -Two versions of the IP protocol are used on networks today. • IPv4, which was introduced over 30 years ago, is still the standard on most networks. IPv4 is an unreliable, connectionless protocol, which means that it does not guarantee delivery of data. However, higher- level protocols of the TCP/IP suite, such as TCP, use IPv4 to ensure that data packets are delivered to the right addresses. • The newer version of IP, IPv6, also known as IP next generation, or IPng, was released in 1998. Most new applications, servers, clients, and network devices support IPv6. However, due to the cost of upgrading infrastructure, many organizations have hesitated to upgrade from IPv4. IGMP (Internet Group Management Protocol): -Another core TCP/IP protocol is IGMP (Internet Group Management Protocol or Internet Group Multicast Protocol). -IGMP operates at the Network layer of the OSI model and manages multicasting on networks running IPv4. Multicasting is a transmission method that allows one node to send data to a defined group of nodes. Whereas most data transmission occurs on a point-to-point basis, multicasting is a point-to-multipoint method. ARP (Address Resolution Protocol): -ARP (Address Resolution Protocol) is a Network layer protocol used with IPv4 that obtains the MAC (physical) address of a host, or node, and then creates a database that maps the MAC address to the host’s IP address. -If one node needs to know the MAC address of another node on the same network, the first node issues a broadcast message to the network, using ARP, that essentially says, “Will the computer with the IP address please send me its MAC address?” In the context of networking, a broadcast is a transmission that is simultaneously sent to all nodes on a particular network segment. The node that has the IP address then broadcasts a reply that contains the physical address of the destination host. -To make ARP more efficient, computers save recognized MAC-to-IP address mappings on their hard disks in a database known as an ARP table (also called an ARP cache). After a computer has saved this information, the next time it needs the MAC address for another device, it finds the address in its ARP table and does not need to broadcast another request. Although the precise format of ARP tables may vary from one operating system to another, the essential contents of the table and its purpose remain the same. ICMP (Internet Control Message Protocol): -Whereas IP helps direct data to its correct destination, ICMP (Internet Control Message Protocol) is a Network layer core protocol that reports on the success or failure of data delivery. -It can indicate when part of a network is congested, when data fails to reach its destination, and when data has been discarded because the allotted time for its delivery (it’s TTL) expired. -ICMP announces these transmission failures to the sender, but ICMP cannot correct any of the errors it detects; those functions are left to higher-layer protocols, such as TCP. However, ICMP’s announcements provide critical information for troubleshooting network problems. IPv6 relies on ICMPv6 (Internet Control Message Protocol version 6) to perform the functions that ICMP, IGMP, and ARP perform in IPv4. In other words, ICMPv6 detects and reports data transmission errors, discovers other nodes on a network, and manages multicasting. IPv4 Addressing: -Networks recognize two kinds of addresses: logical (or Network layer) and physical (or MAC, or hardware) addresses. Physical addresses are assigned to a device’s NIC at the factory by its manufacturer. -Logical addresses can be manually or automatically assigned and must follow rules set by the protocol standards. In the TCP/IP protocol suite, IP is the protocol responsible for logical addressing. -Each IP address is a unique 32-bit number, divided into fo
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