Unit I,II, III Notes CNCC PDF

UNIT-I ▪ Computer Network Components: Computer network components are the major parts which are needed to install the software. Some important network components are NIC, switch, cable, hub, router, and modem. Depending on the type of network that we need to install, some network components c...

UNIT-I ▪ Computer Network Components: Computer network components are the major parts which are needed to install the software. Some important network components are NIC, switch, cable, hub, router, and modem. Depending on the type of network that we need to install, some network components can also be removed. For example, the wireless network does not require a cable. Following are the major components required to install a network: ▪ NIC: NIC stands for network interface card. NIC is a hardware component used to connect a computer with another computer onto a network It can support a transfer rate of 10,100 to 1000 Mb/s. The MAC address or physical address is encoded on the network card chip which is assigned by the IEEE to identify a network card uniquely. The MAC address is stored in the PROM (Programmable read-only memory). There are two types of NIC: 1. Wired NIC 2. Wireless NIC ▪ Wired NIC: The Wired NIC is present inside the motherboard. Cables and connectors are used with wired NIC to transfer data. ▪ Wireless NIC: The wireless NIC contains the antenna to obtain the connection over the wireless network. For example, laptop computer contains the wireless NIC. ▪ Hub: A Hub is a hardware device that divides the network connection among multiple devices. When computer requests for some information from a network, it first sends the request to the Hub through cable. Hub will broadcast this request to the entire network. All the devices will check whether the request belongs to them or not. If not, the request will be dropped. The process used by the Hub consumes more bandwidth and limits the amount of communication. Nowadays, the use of hub is obsolete, and it is replaced by more advanced computer network components such as Switches, Routers. ▪ Switch: A switch is a hardware device that connects multiple devices on a computer network. A Switch contains more advanced features than Hub. The Switch contains the updated table that decides where the data is transmitted or not. Switch delivers the message to the correct destination based on the physical address present in the incoming message. A Switch does not broadcast the message to the entire network like the Hub. It determines the device to which the message is to be transmitted. Therefore, we can say that switch provides a direct connection between the source and destination. It increases the speed of the network. ▪ Router: A router is a hardware device which is used to connect a LAN with an internet connection. It is used to receive, analyze and forward the incoming packets to another network. A router works in a Layer 3 (Network layer) of the OSI Reference model. A router forwards the packet based on the information available in the routing table. It determines the best path from the available paths for the transmission of the packet. ▪ Advantages of Router: Security: The information which is transmitted to the network will traverse the entire cable, but the only specified device which has been addressed can read the data. Reliability: If the server has stopped functioning, the network goes down, but no other networks are affected that are served by the router. Performance: Router enhances the overall performance of the network. Suppose there are 24 workstations in a network generates a same amount of traffic. This increases the traffic load on the network. Router splits the single network into two networks of 12 workstations each, reduces the traffic load by half. Network range ▪ Modem: A modem is a hardware device that allows the computer to connect to the internet over the existing telephone line. A modem is not integrated with the motherboard rather than it is installed on the PCI slot found on the motherboard. It stands for Modulator/Demodulator. It converts the digital data into an analog signal over the telephone lines. Based on the differences in speed and transmission rate, a modem can be classified in the following categories: Standard PC modem or Dial-up modem Cellular Modem Cable modem ▪ Cables and Connectors: Cable is a transmission media used for transmitting a signal. There are three types of cables used in transmission: Twisted pair cable  Coaxial cable Fibre-optic cable ▪ Computer Network Types: A computer network is a group of computers linked to each other that enables the computer to communicate with another computer and share their resources, data, and applications. A computer network can be categorized by their size. A computer network is mainly of four types: LAN(Local Area Network) PAN(Personal Area Network) MAN(Metropolitan Area Network) WAN(Wide Area Network) ▪ LAN (Local Area Network): Local Area Network is a group of computers connected to each other in a small area such as building, office. LAN is used for connecting two or more personal computers through a communication medium such as twisted pair, coaxial cable, etc. It is less costly as it is built with inexpensive hardware such as hubs, network adapters, and Ethernet cables. The data is transferred at an extremely faster rate in Local Area Network. Local Area Network provides higher security. ▪ PAN (Personal Area Network): Personal Area Network is a network arranged within an individual person, typically within a range of 10 meters. Personal Area Network is used for connecting the computer devices of personal use is known as Personal Area Network. Thomas Zimmerman was the first research scientist to bring the idea of the Personal Area Network. Personal Area Network covers an area of 30 feet. Personal computer devices that are used to develop the personal area network are the laptop, mobile phones, media player and play stations. There are two types of Personal Area Network: Wired Personal Area Network Wireless Personal Area Network ▪ Wireless Personal Area Network: Wireless Personal Area Network is developed by simply using wireless technologies such as Wi-Fi, Bluetooth. It is a low range network. ▪ Wired Personal Area Network: Wired Personal Area Network is created by using the USB. ▪ Examples of Personal Area Network: Body Area Network: Body Area Network is a network that moves with a person. For example, a mobile network moves with a person. Suppose a person establishes a network connection and then creates a connection with another device to share the information. Offline Network: An offline network can be created inside the home, so it is also known as a home network. A home network is designed to integrate the devices such as printers, computer, television but they are not connected to the internet. Small Home Office: It is used to connect a variety of devices to the internet and to a corporate network using a VPN ▪ MAN (Metropolitan Area Network): A metropolitan area network is a network that covers a larger geographic area by interconnecting a different LAN to form a larger network. Government agencies use MAN to connect to the citizens and private industries. In MAN, various LANs are connected to each other through a telephone exchange line. The most widely used protocols in MAN are RS-232, Frame Relay, ATM, ISDN, OC-3, ADSL, etc. It has a higher range than Local Area Network (LAN). ▪ Uses of Metropolitan Area Network: MAN is used in communication between the banks in a city. It can be used in an Airline Reservation. It can be used in a college within a city. It can also be used for communication in the military. ▪ WAN (Wide Area Network): A Wide Area Network is a network that extends over a large geographical area such as states or countries. A Wide Area Network is quite bigger network than the LAN. A Wide Area Network is not limited to a single location, but it spans over a large geographical area through a telephone line, fibre optic cable or satellite links. The internet is one of the biggest WAN in the world. A Wide Area Network is widely used in the field of Business, government, and education. ▪ Examples of Wide Area Network: Mobile Broadband: A 4G network is widely used across a region or country. Last mile: A telecom company is used to provide the internet services to the customers in hundreds of cities by connecting their home with fiber. Private network: A bank provides a private network that connects the 44 offices. This network is made by using the telephone leased line provided by the telecom company. ▪ Advantages of Wide Area Network: Following are the advantages of the Wide Area Network: Geographical area: A Wide Area Network provides a large geographical area. Suppose if the branch of our office is in a different city then we can connect with them through WAN. The internet provides a leased line through which we can connect with another branch. Centralized data: In case of WAN network, data is centralized. Therefore, we do not need to buy the emails, files or back up servers. Get updated files: Software companies work on the live server. Therefore, the programmers get the updated files within seconds. Exchange messages: In a WAN network, messages are transmitted fast. The web application like Facebook, Whatsapp, Skype allows you to communicate with friends. Sharing of software and resources: In WAN network, we can share the software and other resources like a hard drive, RAM. Global business: We can do the business over the internet globally. High bandwidth: If we use the leased lines for our company then this gives the high bandwidth. The high bandwidth increases the data transfer rate which in turn increases the productivity of our company. ▪ Disadvantages of Wide Area Network: The following are the disadvantages of the Wide Area Network: Security issue: A WAN network has more security issues as compared to LAN and MAN network as all the technologies are combined together that creates the security problem. Needs Firewall & antivirus software: The data is transferred on the internet which can be changed or hacked by the hackers, so the firewall needs to be used. Some people can inject the virus in our system so antivirus is needed to protect from such a virus. High Setup cost: An installation cost of the WAN network is high as it involves the purchasing of routers, switches. Troubleshooting problems: It covers a large area so fixing the problem is difficult. ▪ Internetwork: An internetwork is defined as two or more computer network LANs or WAN or computer network segments are connected using devices, and they are configured by a local addressing scheme. This process is known as internetworking. An interconnection between public, private, commercial, industrial, or government computer networks can also be defined as internetworking. An internetworking uses the internet protocol. The reference model used for internetworking is Open System Interconnection (OSI). ▪ Types of Internetwork: 1. Extranet: An extranet is a communication network based on the internet protocol such as Transmission Control protocol and internet protocol. It is used sed for information sharing. The access to the extranet is restricted to only those users who have login credentials. An extranet is the lowest level of internetworking. It can be categorized as MAN, WAN or other computer networks. An extranet cannot have a single LAN, at least it must have one connection to the external network. 2. Intranet: An intranet is a private network based on the internet protocol such as Transmission Control protocol and internet protocol. An intranet belongs to an organization which is only accessible by the organization's employee or members. The main aim of the intranet is to share the information and resources among the organization employees. An intranet provides the facility to work in groups and for teleconferences. ▪ Intranet advantages: Communication: It provides a cheap and easy communication. An employee of the organization can communicate with another employee through email, chat. Time-saving: Information on the intranet is shared in real time, so it is time-saving. time Collaboration: Collaboration is one of the most important advantages of the intranet. The information is distributed among the employees of the organization and can only be accessed by the authorized user. Platform independency: It is a neutral architecture as th thee computer can be connected to another device with different architecture. Cost effective: People can see the data and documents by using the browser and distributes the duplicate copies over the intranet. This leads to a reduction in the cost. ▪ What is Topology? Topology defines the structure of the network of how all the components are interconnected to each other. There are two types of topology: physical and logical topology. Physical topology is the geometric representation of all the nodes in a network. ▪ Bus Topology: The bus topology is designed in such a way that all the stations are connected through a single cable known as a backbone cable. Each node is either connected to the backbone cable by drop cable or directly connected to the backbone cable. When a node wants to send a message over the network, it puts a message over the network. All the stations available in the network will receive the message whether it has been addressed or not. The bus topology is mainly used in 802.3 (Ethernet) and 802.4 standard networks. The configuration of a bus topology is quite simpler as compared to other topologies. The backbone cable is considered as a "single lane" through which the message is broadcast to all the stations. The most common access method of the bus topologies is CSMA (Carrier Sense Multiple Access). ▪ CSMA: It is a media access control used to control the data flow so that data integrity is maintained, i.e., the packets do not get lost. There are two alternative ways of handling the problems that occur when two nodes send the messages simultaneously. CSMA CD: CSMA CD (Collision detection) is an access method used to detect the collision. Once the collision is detected, the sender will stop transmitting the data. Therefore, it works on "recovery after the collision". CSMA CA: CSMA CA (Collision Avoidance) is an access method used to avoid the collision by checking whether the transmission media is busy or not. If busy, then the sender waits until the media becomes idle. This technique effectively reduces the possibility of the collision. It does not work on "recovery after the collision". ▪ Advantages of Bus topology: Low-cost cable: In bus topology, nodes are directly connected to the cable without passing through a hub. Therefore, the initial cost of installation is low. Moderate data speeds: Coaxial or twisted pair cables are mainly used in bus-based networks that support upto 10 Mbps.  Familiar technology: Bus topology is a familiar technology as the installation and troubleshooting techniques are well known, and hardware components are easily available. Limited failure: A failure in one node will not have any effect on other nodes. ▪ Disadvantages of Bus topology: Extensive cabling: A bus topology is quite simpler, but still it requires a lot of cabling. Difficult troubleshooting: It requires specialized test equipment to determine the cable faults. If any fault occurs in the cable, then it would disrupt the communication for all the nodes. Signal interference: If two nodes send the messages simultaneously, then the signals of both the nodes collide with each other. Reconfiguration difficult: Adding new devices to the network would slow down the network. Attenuation: Attenuation is a loss of signal leads to communication issues. Repeaters are used to regenerate the signal. ▪ Ring Topology: Ring topology is like a bus topology, but with connected ends. The node that receives the message from the previous computer will retransmit to the next node. The data flows in one direction, i.e., it is unidirectional. The data flows in a single loop continuously known as an endless loop. It has no terminated ends, i.e., each node is connected to other node and having no termination point. The data in a ring topology flow in a clockwise direction. The most common access method of the ring topology is token passing. Token passing: It is a network access method in which token is passed from one node to another node. Token: It is a frame that circulates around the network. ▪ Working of Token passing: A token move around the network and it is passed from computer to computer until it reaches the destination. The sender modifies the token by putting the address along with the data. The data is passed from one device to another device until the destination address matches. Once the token received by the destination device, then it sends the acknowledgment to the sender. In a ring topology, a token is used as a carrier. ▪ Advantages of Ring topology: Network Management: Faulty devices can be removed from the network without bringing the network down. Product availability: Many hardware and software tools for network operation and monitoring are available. Cost: Twisted pair cabling is inexpensive and easily available. Therefore, the installation cost is very low. Reliable: It is a more reliable network because the communication system is not dependent on the single host computer. ▪ Disadvantages of Ring topology: Difficult troubleshooting: It requires specialized test equipment to determine the cable faults. If any fault occurs in the cable, then it would disrupt the communication for all the nodes. Failure: The breakdown in one station leads to the failure of the overall network. Reconfiguration difficult: Adding new devices to the network would slow down the network. Delay: Communication delay is directly proportional to the number of nodes. Adding new devices increases the communication delay. ▪ Star Topology: Star topology is an arrangement of the network in which every node is connected to the central hub, switch or a central computer.  The central computer is known as a server, and the peripheral devices attached to the server are known as clients. Coaxial cable or RJ-45 cables are used to connect the computers. Hubs or Switches are mainly used as connection devices in a physical star topology. Star topology is the most popular topology in network implementation. ▪ Advantages of Star topology: Efficient troubleshooting: Troubleshooting is quite efficient in a star topology as compared to bus topology. In a bus topology, the manager has to inspect the kilometers of cable. In a star topology, all the stations are connected to the centralized network. Therefore, the network administrator has to go to the single station to troubleshoot the problem. Network control: Complex network control features can be easily implemented in the star topology. Any changes made in the star topology are automatically accommodated. Limited failure: As each station is connected to the central hub with its own cable, therefore failure in one cable will not affect the entire network. Familiar technology: Star topology is a familiar technology as its tools are cost-effective. Easily expandable: It is easily expandable as new stations can be added to the open ports on the hub. Cost effective: Star topology networks are cost-effective as it uses inexpensive coaxial cable. High data speeds: It supports a bandwidth of approx 100Mbps. Ethernet 100BaseT is one of the most popular Star topology networks. ▪ Disadvantages of Star topology: A Central point of failure: If the central hub or switch goes down, then all the connected nodes will not be able to communicate with each other. Cable: Sometimes cable routing becomes difficult when a significant amount of routing is required. ▪ Tree topology: Tree topology combines the characteristics of bus topology and star topology.  A tree topology is a type of structure in which all the computers are connected with each other in hierarchical fashion. The top-most node in tree topology is known as a root node, and all other nodes are the descendants of the root node. There is only one path exists between two nodes for the data transmission. Thus, it forms a parent-child hierarchy. ▪ Advantages of Tree topology: Support for broadband transmission: Tree topology is mainly used to provide broadband transmission, i.e., signals are sent over long distances without being attenuated. Easily expandable: We can add the new device to the existing network. Therefore, we can say that tree topology is easily expandable. Easily manageable: In tree topology, the whole network is divided into segments known as star networks which can be easily managed and maintained. Error detection: Error detection and error correction are very easy in a tree topology. Limited failure: The breakdown in one station does not affect the entire network. Point-to-point wiring: It has point-to-point wiring for individual segments. ▪ Disadvantages of Tree topology: Difficult troubleshooting: If any fault occurs in the node, then it becomes difficult to troubleshoot the problem. High cost: Devices required for broadband transmission are very costly. Failure: A tree topology mainly relies on main bus cable and failure in main bus cable will damage the overall network. Reconfiguration difficult: If new devices are added, then it becomes difficult to reconfigure. ▪ Mesh topology: Mesh technology is an arrangement of the network in which computers are interconnected with each other through various redundant connections. There are multiple paths from one computer to another computer.  It does not contain the switch, hub or any central computer which acts as a central point of communication. The Internet is an example of the mesh topology. Mesh topology is mainly used for WAN implementations where communication failures are a critical concern. Mesh topology is mainly used for wireless networks. Mesh topology can be formed by using the formula: Number of cables = (n*(n-1))/2; Where n is the number of nodes that represents the network. ▪ Mesh topology is divided into two categories: Fully connected mesh topology Partially connected mesh topology Full Mesh Topology: In a full mesh topology, each computer is connected to all the computers available in the network. Partial Mesh Topology: In a partial mesh topology, not all but certain computers are connected to those computers with which they communicate frequently. ▪ Advantages of Mesh topology: Reliable: The mesh topology networks are very reliable as if any link breakdown will not affect the communication between connected computers. Fast Communication: Communication is very fast between the nodes. Easier Reconfiguration: Adding new devices would not disrupt the communication between other devices. ▪ Disadvantages of Mesh topology: Cost: A mesh topology contains a large number of connected devices such as a router and more transmission media than other topologies. Management: Mesh topology networks are very large and very difficult to maintain and manage. If the network is not monitored carefully, then the communication link failure goes undetected. Efficiency: In this topology, redundant connections are high that reduces the efficiency of the network. ▪ Hybrid Topology: The combination of various different topologies is known as Hybrid topology. A Hybrid topology is a connection between different links and nodes to transfer the data. When two or more different topologies are combined together is termed as Hybrid topology and if similar topologies are connected with each other will not result in Hybrid topology. For example, if there exist a ring topology in one branch of ICICI bank and bus topology in another branch of ICICI bank, connecting these two topologies will result in Hybrid topology. ▪ Advantages of Hybrid Topology: Reliable: If a fault occurs in any part of the network will not affect the functioning of the rest of the network. Scalable: Size of the network can be easily expanded by adding new devices without affecting the functionality of the existing network. Flexible: This topology is very flexible as it can be designed according to the requirements of the organization. Effective: Hybrid topology is very effective as it can be designed in such a way that the strength of the network is maximized and weakness of the network is minimized. ▪ Disadvantages of Hybrid topology: Complex design: The major drawback of the Hybrid topology is the design of the Hybrid network. It is very difficult to design the architecture of the Hybrid network. Costly Hub: The Hubs used in the Hybrid topology are very expensive as these hubs are different from usual Hubs used in other topologies. Costly infrastructure: The infrastructure cost is very high as a hybrid network requires a lot of cabling, network devices, etc. ▪ What Is the OSI Model: The Open Systems Interconnection (OSI) model describes seven layers that computer systems use to communicate over a network. It was the first standard model for network communications, adopted by all major computer and telecommunication companies in the early 1980s The modern Internet is not based on OSI, but on the simpler TCP/IP model. However, the OSI 7-layer model is still widely used, as it helps visualize and communicate how networks operate, and helps isolate and troubleshoot networking problems. OSI was introduced in 1983 by representatives of the major computer and telecom companies, and was adopted by ISO as an international standard in 1984. ▪ OSI Model Explained: The OSI 7 Layers: We’ll describe OSI layers “top down” from the application layer that directly serves the end user, down to the physical layer. 7. Application Layer: The application layer is used by end-user software such as web browsers and email clients. It provides protocols that allow software to send and receive information and present meaningful data to users. A few examples of application layer protocols are the Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Post Office Protocol (POP), Simple Mail Transfer Protocol (SMTP), and Domain Name System (DNS). 6. Presentation Layer: The presentation layer prepares data for the application layer. It defines how two devices should encode, encrypt, and compress data so it is received correctly on the other end. The presentation layer takes any data transmitted by the application layer and prepares it for transmission over the session layer. 5. Session Layer: The session layer creates communication channels, called sessions, between devices. It is responsible for opening sessions, ensuring they remain open and functional while data is being transferred, and closing them when communication ends. The session layer can also set checkpoints during a data transfer—if the session is interrupted, devices can resume data transfer from the last checkpoint. 4. Transport Layer: The transport layer takes data transferred in the session layer and breaks it into “segments” on the transmitting end. It is responsible for reassembling the segments on the receiving end, turning it back into data that can be used by the session layer. The transport layer carries out flow control, sending data at a rate that matches the connection speed of the receiving device, and error control, checking if data was received incorrectly and if not, requesting it again. 3. Network Layer: The network layer has two main functions. One is breaking up segments into network packets, and reassembling the packets on the receiving end. The other is routing packets by discovering the best path across a physical network. The network layer uses network addresses (typically Internet Protocol addresses) to route packets to a destination node. 2. Data Link Layer: The data link layer establishes and terminates a connection between two physically- connected nodes on a network. It breaks up packets into frames and sends them from source to destination. This layer is composed of two parts—Logical Link Control (LLC), which identifies network protocols, performs error checking and synchronizes frames, and Media Access Control (MAC) which uses MAC addresses to connect devices and define permissions to transmit and receive data. 1. Physical Layer: The physical layer is responsible for the physical cable or wireless connection between network nodes. It defines the connector, the electrical cable or wireless technology connecting the devices, and is responsible for transmission of the raw data, which is simply a series of 0s and 1s, while taking care of bit rate control. ▪ Advantages of OSI Model: ▪ The OSI model helps users and operators of computer networks: Determine the required hardware and software to build their network. Understand and communicate the process followed by components communicating across a network.  Perform troubleshooting, by identifying which network layer is causing an issue and focusing efforts on that layer. ▪ The OSI model helps network device manufacturers and networking software vendors: Create devices and software that can communicate with products from any other vendor, allowing open interoperability Define which parts of the network their products should work with. Communicate to users at which network layers their product operates – for example, only at the application layer, or across the stack. ▪ TCP/IP Model: The OSI Model we just looked at is just a reference/logical model. It was designed to describe the functions of the communication system by dividing the communication procedure into smaller and simpler components. But when we talk about the TCP/IP model, it was designed and developed by Department of Defense (DoD) in 1960s and is based on standard protocols. It stands for Transmission Control Protocol/Internet Protocol. The TCP/IP model is a concise version of the OSI model. It contains four layers, unlike seven layers in the OSI model. The layers are: Process/Application Layer Host-to-Host/Transport Layer Internet Layer Network Access/Link Layer The diagrammatic comparison of the TCP/IP and OSI model is as follows : ▪ OSI vs. TCP/IP Model:  The Transfer Control Protocol/Internet Protocol (TCP/IP) is older than the OSI model and was created by the US Department of Defense (DoD). A key difference between the models is that TCP/IP is simpler, collapsing several OSI layers into one: OSI layers 5, 6, 7 are combined into one Application Layer in TCP/IP OSI layers 1, 2 are combined into one Network Access Layer in TCP/IP – however TCP/IP does not take responsibility for sequencing and acknowledgement functions, leaving these to the underlying transport layer. Other important differences: TCP/IP is a functional model designed to solve specific communication problems, and which is based on specific, standard protocols. OSI is a generic, protocol-independent model intended to describe all forms of network communication. In TCP/IP, most applications use all the layers, while in OSI simple applications do not use all seven layers. Only layers 1, 2 and 3 are mandatory to enable any data communication. ▪ Difference between TCP/IP and OSI Model: TCP/IP OSI TCP refers to Transmission Control OSI refers to Open Systems Protocol. Interconnection. TCP/IP has 4 layers. OSI has 7 layers. TCP/IP is more reliable OSI is less reliable TCP/IP does not have very strict OSI has strict boundaries boundaries. TCP/IP follows a horizontal approach. OSI follows a vertical approach. TCP/IP uses both session and presentation OSI uses different session and layer in the application layer itself. presentation layers. TCP/IP developed protocols then model. OSI developed model then protocol. Transport layer in TCP/IP does not In OSI model, transport layer provides provide assurance delivery of packets. assurance delivery of packets. Connection less and connection oriented TCP/IP model network layer only both services are provided by network provides connection less services. layer in OSI model.  While in OSI model, Protocols are better Protocols cannot be replaced easily in covered and is easy to replace with the TCP/IP model. change in technology. 1. Network Access Layer: This layer corresponds to the combination of Data Link Layer and Physical Layer of the OSI model. It looks out for hardware addressing and the protocols present in this layer allows for the physical transmission of data. We just talked about ARP being a protocol of Internet layer, but there is a conflict about declaring it as a protocol of Internet Layer or Network access layer. It is described as residing in layer 3, being encapsulated by layer 2 protocols. 2. Internet Layer: This layer parallels the functions of OSI’s Network layer. It defines the protocols which are responsible for logical transmission of data over the entire network. The main protocols residing at this layer are: 1. IP: stands for Internet Protocol and it is responsible for delivering packets from the source host to the destination host by looking at the IP addresses in the packet headers. IP has 2 versions: IPv4 and IPv6. IPv4 is the one that most of the websites are using currently. But IPv6 is growing as the numbers of IPv4 addresses are limited in number when compared to the number of users. 2. ICMP: stands for Internet Control Message Protocol. It is encapsulated within IP datagrams and is responsible for providing hosts with information about network problems. 3. ARP: stands for Address Resolution Protocol. Its job is to find the hardware address of a host from a known IP address. ARP has several types: Reverse ARP, Proxy ARP, Gratuitous ARP and Inverse ARP. 3. Host-to-Host Layer: This layer is analogous to the transport layer of the OSI model. It is responsible for end- to-end communication and error-free delivery of data. It shields the upper-layer applications from the complexities of data. The two main protocols present in this layer are: 1. Transmission Control Protocol (TCP): It is known to provide reliable and error-free communication between end systems. It performs sequencing and segmentation of data. It also has acknowledgment feature and controls the flow of the data through flow control mechanism. It is a very effective protocol but has a lot of overhead due to such features. Increased overhead leads to increased cost. 2. User Datagram Protocol (UDP): On the other hand does not provide any such features. It is the go-to protocol if your application does not require reliable transport as it is very cost-effective. Unlike TCP, which is connection-oriented protocol, UDP is connectionless. 4. Application Layer: This layer performs the functions of top three layers of the OSI model: Application, Presentation and Session Layer. It is responsible for node-to-node communication and controls user-interface specifications. Some of the protocols present in this layer are: HTTP, HTTPS, FTP, TFTP, Telnet, SSH, SMTP, SNMP, NTP, DNS, DHCP, NFS, X Window, LPD. Have a look at Protocols in Application Layer for some information about these protocols. Protocols other than those present in the linked article are: 1. HTTP and HTTPS: HTTP stands for Hypertext transfer protocol. It is used by the World Wide Web to manage communications between web browsers and servers. HTTPS stands for HTTP-Secure. It is a combination of HTTP with SSL(Secure Socket Layer). It is efficient in cases where the browser need to fill out forms, sign in, authenticate and carry out bank transactions. 2. SSH: SSH stands for Secure Shell. It is terminal emulations software similar to Telnet. The reason SSH is more preferred is because of its ability to maintain the encrypted connection. It sets up a secure session over a TCP/IP connection. 3. NTP: NTP stands for Network Time Protocol. It is used to synchronize the clocks on our computer to one standard time source. It is very useful in situations like bank transactions. Assume the following situation without the presence of NTP. Suppose you carry out a transaction, where your computer reads the time at 2:30 PM while the server records it at 2:28 PM. The server can crash very badly if it’s out of sync.  UNIT-II ▪ Switching witching in Computer Networks: Circuit, Packet and Message: ▪ Switching in Computer Networks Networks: In broad networks, there can be various paths to send a message from sender to receiver. Switching in computer networks is used to select the best path for data transmission. For this purpose, different switching techniques are used. The switched network comprises a series of interlink nodes called switches. Switches are hard-wired wired software devices that are capable of creating temporary connections between two or more devices. There is a link to switch, but not to each other. The nodes are connected with each other through common devices and some nodes are used to route pa packages. ▪ Types of Switching Techniques in Computer Networks Networks: There are three types of switching techniques in computer networks, and each technique has a different purpose. Types of switching techniques in computer networks are as follows. Circuit Switching Packet Switching Message Switching. ▪ Circuit Switching: It is a type of switching in which we set a physical connection between sender and receiver. The connection is set up when the call is made from transmitter to receiver telephone. Once a call is set up, the dedicated path exits between both ends. The path will continue to exist until the call is disconnected.  The above diagram shows the functionality of circuit switching in computer networks. Every computer has a physical connection to a node, as you can see in the circuit switching diagram. Using nodes, devices can send a message from one end to another ▪ Advantages of Circuit Switching Switching: It provides a guaranteed data rate. No delay in the data flow. ▪ Disadvantages of Circuit Switching Switching: It requires more bandwidth. It takes a long time to establish a connection. It is not suitable for high traffic. ▪ Packet Switching: In packet switching, a message is broken into packets for transmission. Each packet has the source, destination,, and intermediate node address information. The entire message is divided into smaller pieces, called packets. Each packet travels independently and contains address information. These packets travel through the shortest path in a communication network. All the packets are reassembled at the receiving end to make a complete message. There are two types of packet switching in computer networks, as follows. Datagram Packet Switching Virtual Circuit Packet Switching The above diagram shows the concept of packet switching. The message is divided into four packets (i.e. 1, 2,, 3 and 4). These packets contain the addresses and information. By travelling through the shortest path, packets reach their destination. At receiving end, the packets are reassembled in the same order (which is 1234) to generate an entire message. ▪ Advantages of Packet Switching Switching: Bandwidth is reduced. If one link goes down, the remaining packets can be sent through another route. ▪ Message Switching: In message switching, the compl complete ete message is transferred from one end to another through nodes. There is no physical connection or link between sender and receiver. The message contains the destination address. Each node stores the message and then forward it to the next node as shown in the below diagram. In telegraphy, the text message is encoded using the morse code into a sequence of dots and dashes. Each dot or dash is communicated by transmitting a short and long pulse of electrical current. The following diagram shows the concep conceptt of message switching in computer networks. ▪ Advantages of Message Switching Switching: Reduces network traffic Network devices share the channel. ▪ Disadvantages: It does not establish a dedicated path between two communication paths. ▪ Difference between Circuit, Packet and Message Switching Switching: The following table shows a comparison between the three types of switching techniques in computer networks. Circuit Switching Packet Switching Message Switching There is a physical connection There is no physical connection There is no physical path between sender and receiver. between sender and receiver. between sender and receiver.  Circuit Switching Packet Switching Message Switching Packets are stored then All packets use the same path. Packets travel independently. forwarded. Congestion has occurred per Congestion has occurred per There is no congestion in minute. packet. message switching. It is not suitable for handling It is suitable for handling high It is not suitable for handling traffic. traffic. traffic. There is no wastage of There is also no wastage of The wastage of bandwidth is bandwidth. bandwidth. possible. The recording of the packet is The recording of the packet is The recording of the packet not possible. possible. is possible. The message is in the form of The message is in the form of The message is in the form packets. packets. of blocks. We can use it with a real-time We can use it in real-time We cannot use it in real-time application. applications. applications. ▪ Data Link Layer Design Issues: The data link layer in the OSI (Open System Interconnections) Model is in between the physical layer and the network layer. This layer converts the raw transmission facility provided by the physical layer to a reliable and error-free link. The main functions and the design issues of this layer are Providing services to the network layer Framing Error Control Flow Control ▪ Services to the Network Layer: In the OSI Model, each layer uses the services of the layer below it and provides services to the layer above it. The data link layer uses the services offered by the physical layer. The primary function of this layer is to provide a well defined service interface to network layer above it.  The types of services provided can be of three types − Unacknowledged connectionless service Acknowledged connectionless service Acknowledged connection - oriented service ▪ Framing: The data link layer encapsulates each data packet from the network layer into frames that are then transmitted. A frame has three parts, namely − Frame Header Payload field that contains the data packet from network layer Trailer ▪ Error Control: The data link layer ensures error free link for data transmission. The issues it caters to with respect to error control are: Dealing with transmission errors Sending acknowledgement frames in reliable connections Retransmitting lost frames Identifying duplicate frames and deleting them Controlling access to shared channels in case of broadcasting ▪ Flow Control: The data link layer regulates flow control so that a fast sender does not drown a slow receiver. When the sender sends frames at very high speeds, a slow receiver may not be able to handle it. There will be frame losses even if the transmission is error-free. The two common approaches for flow control are: Feedback based flow control Rate based flow control ▪ Error Detection and Correction in Data Link Layer: Data-link layer uses error control techniques to ensure that frames, i.e. bit streams of data, are transmitted from the source to the destination with a certain extent of accuracy. ▪ Errors: When bits are transmitted over the computer network, they are subject to get corrupted due to interference and network problems. The corrupted bits leads to spurious data being received by the destination and are called errors. ▪ Types of Errors: Errors can be of three types, namely single bit errors, multiple bit errors, and burst errors. Single bit error: In the received frame, only one bit has been corrupted, i.e. either changed from 0 to 1 or from 1 to 0  Multiple bits error: In the received frame, more than one bits are corrupted. Burst error: In the received frame, more than one consecutive bits are corrupted. ▪ Error Control: Error control can be done in two ways Error detection: Error detection involves checking whether any error has occurred or not. The number of error bits and the type of error does not matter. Error correction: Error correction involves ascertaining the exact number of bits that has been corrupted and the location of the corrupted bits. For both error detection and error correction, the sender needs to send some additional bits along with the data bits. The receiver performs necessary checks based upon the additional redundant bits. If it finds that the data is free from errors, it removes the redundant bits before passing the message to the upper layers. ▪ Error Detection Techniques: There are three main techniques for detecting errors in frames: Parity Check, Checksum and Cyclic Redundancy Check (CRC). ▪ Parity Check: The parity check is done by adding an extra bit, called parity bit to the data to make a number of 1s either even in case of even parity or odd in case of odd parity. While creating a frame, the sender counts the number of 1s in it and adds the parity bit in the following way  In case of even parity: If a number of 1s is even then parity bit value is 0. If the number of 1s is odd then parity bit value is 1. In case of odd parity: If a number of 1s is odd then parity bit value is 0. If a number of 1s is even then parity bit value is 1. On receiving a frame, the receiver counts the number of 1s in it. In case of even parity check, if the count of 1s is even, the frame is accepted, otherwise, it is rejected. A similar rule is adopted for odd parity check. The parity check is suitable for single bit error detection only. ▪ Checksum: In this error detection scheme, the following procedure is applied Data is divided into fixed sized frames or segments. The sender adds the segments using 1’s complement arithmetic to get the sum. It then complements the sum to get the checksum and sends it along with the data frames. The receiver adds the incoming segments along with the checksum using 1’s complement arithmetic to get the sum and then complements it. If the result is zero, the received frames are accepted; otherwise, they are discarded. ▪ Cyclic Redundancy Check (CRC): Cyclic Redundancy Check (CRC) involves binary division of the data bits being sent by a predetermined divisor agreed upon by the communicating system. The divisor is generated using polynomials. Here, the sender performs binary division of the data segment by the divisor. It then appends the remainder called CRC bits to the end of the data segment. This makes the resulting data unit exactly divisible by the divisor. The receiver divides the incoming data unit by the divisor. If there is no remainder, the data unit is assumed to be correct and is accepted. Otherwise, it is understood that the data is corrupted and is therefore rejected. ▪ Error Correction Techniques: Error correction techniques find out the exact number of bits that have been corrupted and as well as their locations. There are two principle ways Backward Error Correction (Retransmission): If the receiver detects an error in the incoming frame, it requests the sender to retransmit the frame. It is a relatively simple technique. But it can be efficiently used only where retransmitting is not expensive as in fiber optics and the time for retransmission is low relative to the requirements of the application. Forward Error Correction: If the receiver detects some error in the incoming frame, it executes error-correcting code that generates the actual frame. This saves bandwidth required for retransmission. It is inevitable in real-time systems. However, if there are too many errors, the frames need to be retransmitted. The four main error correction codes are Hamming Codes Binary Convolution Code Reed-Solomon Code Low-Density Parity-Check Code ▪ Elementary Data Link Protocols: Protocols in the data link layer are designed so that this layer can perform its basic functions: framing, error control and flow control. Framing is the process of dividing bit - streams from physical layer into data frames whose size ranges from a few hundred to a few thousand bytes. Error control mechanisms deals with transmission errors and retransmission of corrupted and lost frames. Flow control regulates speed of delivery and so that a fast sender does not drown a slow receiver. ▪ Types of Data Link Protocols: Data link protocols can be broadly divided into two categories, depending on whether the transmission channel is noiseless or noisy. ▪ Simplex Protocol: The Simplex protocol is hypothetical protocol designed for unidirectional data transmission over an ideal channel, i.e. a channel through which transmission can never go wrong. It has distinct procedures for sender and receiver. The sender simply sends all its data available onto the channel as soon as they are available its buffer. The receiver is assumed to process all incoming data instantly. It is hypothetical since it does not handle flow control or error control. ▪ Stop-and-Wait Protocol: Stop-and-Wait protocol is for noiseless channel too. It provides unidirectional data transmission without any error control facilities. However, it provides for flow control so that a fast sender does not drown a slow receiver. The receiver has a finite buffer size with finite processing speed. The sender can send a frame only when it has received indication from the receiver that it is available for further data processing. In this protocol we assume that data is transmitted in one direction only. No error occurs; the receiver can only process the received information at finite rate. These assumptions imply that the transmitter cannot send frames at rate faster than the receiver can process them. The main problem here is how to prevent the sender from flooding the receiver. The general solution for this problem is to have the receiver send some sort of feedback to sender, the process is as follows: Step1: The receiver send the acknowledgement frame back to the sender telling the sender that the last received frame has been processed and passed to the host. Step 2: Permission to send the next frame is granted. Step 3: The sender after sending the sent frame has to wait for an acknowledge frame from the receiver before sending another frame. This protocol is called Simplex Stop and wait protocol, the sender sends one frame and waits for feedback from the receiver. When the ACK arrives, the sender sends the next frame. The Simplex Stop and Wait Protocol is diagrammatically represented as follows ▪ Go-Back-N ARQ: Go-Back-N ARQ provides for sending multiple frames before receiving the acknowledgement for the first frame. It uses the concept of sliding window, and so is also called sliding window protocol. The frames are sequentially numbered and a finite number of frames are sent. If the acknowledgement of a frame is not received within the time period, all frames starting from that frame are retransmitted. ▪ Selective Repeat ARQ: This protocol also provides for sending multiple frames before receiving the acknowledgement for the first frame. However, here only the erroneous or lost frames are retransmitted, while the good frames are received and buffered. ▪ Simplex Protocol for Noisy Channel: Data transfer is only in one direction, consider separate sender and receiver, finite processing capacity and speed at the receiver, since it is a noisy channel, errors in data frames or acknowledgement frames are expected. Every frame has a unique sequence number. After a frame has been transmitted, the timer is started for a finite time. Before the timer expires, if the acknowledgement is not received , the frame gets retransmitted, when the acknowledgement gets corrupted or sent data frames gets damaged, how long the sender should wait to transmit the next frame is infinite. The Simplex Protocol for Noisy Channel is diagrammatically represented as follows: ▪ What is channel allocation in computer network? When there are more than one user who desire to access a shared network channel, an algorithm is deployed for channel allocation among the competing users. The network channel may be a single cable or optical fiber connecting multiple nodes, or a portion of the wireless spectrum. Channel allocation algorithms allocate the wired channels and bandwidths to the users, who may be base stations, access points or terminal equipment. ▪ Channel Allocation Schemes: Channel Allocation may be done using two schemes: Static Channel Allocation Dynamic Channel Allocation ▪ Static Channel Allocation: In static channel allocation scheme, a fixed portion of the frequency channel is allotted to each user. For N competing users, the bandwidth is divided into N channels using frequency division multiplexing (FDM), and each portion is assigned to one user. This scheme is also referred as fixed channel allocation or fixed channel assignment. In this allocation scheme, there is no interference between the users since each user is assigned a fixed channel. However, it is not suitable in case of a large number of users with variable bandwidth requirements. ▪ Dynamic Channel Allocation: In dynamic channel allocation scheme, frequency bands are not permanently assigned to the users. Instead channels are allotted to users dynamically as needed, from a central pool. The allocation is done considering a number of parameters so that transmission interference is minimized. This allocation scheme optimizes bandwidth usage and results are faster transmissions. Dynamic channel allocation is further divided into centralized and distributed allocation. ▪ Multiple Access Protocols in Computer Network: The Data Link Layer is responsible for transmission of data between two nodes. Its main functions are- Data Link Control Multiple Access Control ▪ Data Link control: The data link control is responsible for reliable transmission of message over transmission channel by using techniques like framing, error control and flow control. For Data link control refer to-Stop and Wait ARQ ▪ Multiple Access Control: If there is a dedicated link between the sender and the receiver then data link control layer is sufficient, however if there is no dedicated link present then multiple stations can access the channel simultaneously. Hence multiple access protocols are required to decrease collision and avoid crosstalk. For example, in a classroom full of students, when a teacher asks a question and all the students (or stations) start answering simultaneously (send data at same time) then a lot of chaos is created (data overlap or data lost) then it is the job of the teacher (multiple access protocols) to manage the students and make them answer one at a time. For example, suppose that there is a classroom full of students. When a teacher asks a question, all the students (small channels) in the class start answering the question at the same time (transferring the data simultaneously). All the students respond at the same time due to which data is overlap or data lost. Therefore it is the responsibility of a teacher (multiple access protocol) col) to manage the students and make them one answer. Following are the types of multiple access protocol that is subdivided into the different process as: A. Random Access Protocol: In this protocol, all the station has the equal priority to send the data over a channel. In random access protocol, one or more stations cannot depend on another station nor any station control another station. Depending on the channel's state (idle or bu busy), sy), each station transmits the data frame. However, if more than one station sends the data over a channel, there may be a collision or data conflict. Due to the collision, the data frame packets may be lost or changed. And hence, it does not receive by tthe receiver end. Following are the different methods of random random-access access protocols for broadcasting frames on the channel. Aloha CSMA CSMA/CD CSMA/CA ▪ ALOHA Random Access Protocol Protocol: It is designed for wireless LAN (Local Area Network) but can also be used in a shared medium to transmit data. Using this method, any station can transmit data across a network simultaneously when a data frameset is available for transmission. ▪ Aloha Rules: 1. Any station can transmit data to a channel at any time. 2. It does not require any carrier sensing. 3. Collision and data frames may be lost during the transmission of data through multiple stations. 4. Acknowledgment of the frames exists in Aloha. Hence, there is no collision detection. 5. It requires retransmission of data after som some random amount of time. ▪ Pure Aloha: Whenever data is available for sending over a channel at stations, we use Pure Aloha. In pure Aloha, when each station transmits data to a channel without checking whether the channel is idle or not, the chances of collision may occur, and the data frame ccanan be lost. When any station transmits the data frame to a channel, the pure Aloha waits for the receiver's acknowledgment. If it does not acknowledge the receiver end within the specified time, the station waits for a random amount of time, called the back off time (Tb). And the station may assume the frame has been lost or destroyed. Therefore, it retransmits the frame until all the data are successfully transmitted to the receiver. 1. The total vulnerable time of pure Aloha is 2 * Tfr. 2. Maximum throughput occurs curs when G = 1/ 2 that is 18.4%. 3. Successful transmission of data frame is S = G * e ^ - 2 G. As we can see in the figure above, there are four stations for accessing a shared channel and transmitting data frames. Some frames collide because most stations send their frames at the same time. Only two frames, frame 1.1 and frame 2.2, are successfully transmitted to the receiver end. At the same time, other frames are lost or destroyed. Whenever two frames fall on a shared channel simultaneously, collisions can occur, and both will suffer damage. If the new frame's first bit enters the channel before ffinishing inishing the last bit of the second frame. Both frames are completely finished, and both stations must retransmit the data frame. ▪ Slotted Aloha: The slotted Aloha is designed to overcome the pure Aloha's efficiency because pure Aloha has a very high possibility of frame hitting. In slotted Aloha, the shared channel is divided into a fixed time interval called slots. So that, if a station wants to send end a frame to a shared channel, the frame can only be sent at the beginning of the slot, and only one frame is allowed to be sent to each slot. And if the stations are unable to send data to the beginning of the slot, the station will have to wait until tthehe beginning of the slot for the next time. However, the possibility of a collision remains when trying to send a frame at the beginning of two or more station time slot. 1. Maximum throughput occurs in the slotted Aloha when G = 1 that is 37%. 2. The probabilityy of successfully transmitting the data frame in the slotted Aloha is S = G * e ^ - 2 G. 3. The total vulnerable time required in slotted Aloha is Tfr. ▪ CSMA (Carrier Sense Multiple Access)Access): It is a carrier sense multiple access based on media access protocol to sense the traffic on a channel (idle or busy) before transmitting the data. It means that if the channel is idle, the station can send data to the channel. Otherwise, it must wait until the channel becomes idle. Hence, it reduces the chances of a collision on a transmission medium. ▪ CSMA Access Modes: Persistent mode of CSMA that defines each node, first sense the 1-Persistent: In the 1-Persistent shared channel and if the channel is idle, it immediately sends the data. Else it must wait and keep track of the status of the channel to be idle and broadcast the frame unconditionally as soon as the channel is idle. Non-Persistent: It is the access mode of CSMA that defines before transmitting the data, each node must sense the channel, and if the channel is inactive, it immediately sends the data. Otherwise, the station must wait for a random time (not continuously), and when the channel is found to be idle, it transmits the frames. P-Persistent: It is the combination of 11-Persistent and Non-persistent persistent modes. modes The P- Persistent mode defines that each node senses the channel, and if the channel is inactive, it sends a frame with a P probability. If the data is not transmitted, it waits for a (q ( = 1-p probability)) random time and resumes the frame with the next ttime ime slot. O-Persistent: It is an O O-persistent persistent method that defines the superiority of the station before the transmission of the frame on the shared channel. If it is found that the channel is inactive, each station waits for its turn to retransmit the data data. ▪ CSMA/ CD: It is carriers sense multiple access/ collision detection network protocol to transmit data frames. The CSMA/CD protocol works with a medium access control layer. Therefore, it first senses the shared channel before broadcasting the frames, and if the channel is idle, it transmits a frame to check whether the transmission was successful. If the frame is successfully received, the station sends another frame. If any collision is detected in the CSMA/CD, the station sends a jam/ stop signal to the shared channel to terminate data transmission. After that, it waits for a random time before sending a frame to a channel. ▪ CSMA/CA: It is a carrier sense multiple access/collision avoidance network protocol for carrier transmission of data frames. It is a protocol that works with a medium access control layer. When a data frame is sent to a channel, it receives an acknowledgment to check whether the channel is clear. If the station receives only a single (own) acknowledgments, that means the data frame has been successfully transmitted to the receiver. But if it gets two signals (its own and one more in which the collision of frames), a collision of the frame occurs in the shared channel. Detects the collision of the frame when a sender receives an acknowledgment signal. Following are the methods used in the CSMA/ CA to avoid the collision: Interframe space: In this method, the station waits for the channel to become idle, and if it gets the channel is idle, it does not immediately send the data. Instead of this, it waits for some time, and this time period is called the Interframe space or IFS. However, the IFS time is often used to define the priority of the station. Contention window: In the Contention window, the total time is divided into different slots. When the station/ sender is ready to transmit the data frame, it chooses a random slot number of slots as wait time. If the channel is still busy, it does not restart the entire process, except that it restarts the timer only to send data packets when the channel is inactive. Acknowledgment: In the acknowledgment method, the sender station sends the data frame to the shared channel if the acknowledgment is not received ahead of time. B. Controlled Access Protocol: It is a method of reducing data frame collision on a shared channel. In the controlled access method, each station interacts and decides to send a data frame by a particular station approved by all other stations. It means that a single station cannot send the data frames unless all other stations are not approved. It has three types of controlled access: Reservation, Polling, and Token Passing. C. Channelization Protocols: It is a channelization protocol that allows the total usable bandwidth in a shared channel to be shared across multiple stations based on their time, distance and codes. It can access all the stations at the same time to send the data frames to the channel. Following are the various methods to access the channel based on their time, distance and codes: 1. DMA (Frequency Division Multiple Access) 2. TDMA (Time Division Multiple Access) 3. CDMA (Code Division Multiple Access) FDMA: It is a frequency division multiple access (FDMA) method used to divide the available bandwidth into equal bands so that multiple users can send data through a different frequency to the subchannel. Each station is reserved with a particular band to prevent the crosstalk between the channels and interferences of stations. ▪ TDMA: Time Division Multiple Access (TDMA) is a channel access method. It allows the same frequency bandwidth to be shared across multiple stations. And to avoid collisions in the shared channel, it divides the channel into different frequency slots that allocat allocatee stations to transmit the data frames. The same frequency bandwidth into the shared channel by dividing the signal into various time slots to transmit it. However, TDMA has an overhead of synchronization that specifies each station's time slot by adding ssynchronization bits to each slot. ▪ CDMA: The code division multiple access (CDMA) is a channel access method. In CDMA, all stations can simultaneously send the data over the same channel. It means tthat hat it allows each station to transmit the data frames with full frequency on the shared channel at all times. It does not require the division of bandwidth on a shared channel based on time slots. If multiple stations send data to a channel simultaneously simultaneously,, their data frames are separated by a unique code sequence. Each station has a different unique code for transmitting the data over a shared channel. For example, there are multiple users in a room that are continuously speaking. Data is received by the users if only two-person interact with each other using the same language. Similarly, in the network, if different stations communicate with each other simultaneously with different code language. ▪ Network Layer: Layer-3 in the OSI model is called Network layer. Network layer manages options pertaining to host and network addressing, managing sub-networks, and internetworking. Network layer takes the responsibility for routing packets from source to destination within or outside a subnet. Two different subnet may have different addressing schemes or non- compatible addressing types. Same with protocols, two different subnet may be operating on different protocols which are not compatible with each other. Network layer has the responsibility to route the packets from source to destination, mapping different addressing schemes and protocols. ▪ Layer-3 Functionalities: Devices which work on Network Layer mainly focus on routing. Routing may include various tasks aimed to achieve a single goal. These can be: Addressing devices and networks. Populating routing tables or static routes. Queuing incoming and outgoing data and then forwarding them according to quality of service constraints set for those packets. Internetworking between two different subnets. Delivering packets to destination with best efforts. Provides connection oriented and connection less mechanism. ▪ Network Layer Features: With its standard functionalities, Layer 3 can provide various features as: Quality of service management Load balancing and link management Security Interrelation of different protocols and subnets with different schema. Different logical network design over the physical network design. L3 VPN and tunnels can be used to provide end to end dedicated connectivity. Internet protocol is widely respected and deployed Network Layer protocol which helps to communicate end to end devices over the internet. It comes in two flavors. IPv4 which has ruled the world for decades but now is running out of address space. IPv6 is created to replace IPv4 and hopefully mitigates limitations of IPv4 too. ▪ Network Addressing: Layer 3 network addressing is one of the major tasks of Network Layer. Network Addresses are always logical i.e. these are software based addresses which can be changed by appropriate configurations. A network address always points to host / node / server or it can represent a whole network. Network address is always configured on network interface card and is generally mapped by system with the MAC address (hardware address or layer-2 address) of the machine for Layer-2 communication. There are different kinds of network addresses in existence: IP IPX AppleTalk We are discussing IP here as it is the only one we use in practice these days. IP addressing provides mechanism to differentiate between hosts and network. Because IP addresses are assigned in hierarchical manner, a host always resides under a specific network. The host which needs to communicate outside its subnet, needs to know destination network address, where the packet/data is to be sent. Hosts in different subnet need a mechanism to locate each other. This task can be done by DNS. DNS is a server which provides Layer-3 address of remote host mapped with its domain name or FQDN. When a host acquires the Layer-3 Address (IP Address) of the remote host, it forwards its entire packet to its gateway. A gateway is a router equipped with all the information which leads to route packets to the destination host. Routers take help of routing tables, which has the following information: Method to reach the network Routers upon receiving forwarding request, forwards packet to its next hop (adjacent router) towards the destination. The next router on the path follows the same thing and eventually the data packet reaches its destination. Network address can be of one of the following: Unicast (destined to one host)  Multicast (destined to group) Broadcast (destined to all) Anycast (destined to nearest one) A router never forwards broadcast traffic by default. Multicast traffic uses special treatment as it is most a video stream or audio with highest priority. Anycast is just similar to unicast, except that the packets are delivered to the nearest destination when multiple destinations are available. ▪ Network Layer Routing: When a device has multiple paths to reach a destination, it always selects one path by preferring it over others. This selection process is termed as Routing. Routing is done by special network devices called routers or it can be done by means of software processes. The software based routers have limited functionality and limited scope. A router is always configured with some default route. A default route tells the router where to forward a packet if there is no route found for specific destination. In case there are multiple path existing to reach the same destination, router can make decision based on the following information: Hop Count Bandwidth Metric Prefix-length Delay Routes can be statically configured or dynamically learnt. One route can be configured to be preferred over others. Unicast routing Most of the traffic on the internet and intranets known as unicast data or unicast traffic is sent with specified destination. Routing unicast data over the internet is called unicast routing. It is the simplest form of routing because the destination is already known. Hence the router just has to look up the routing table and forward the packet to next hop. ▪ Broadcast routing: By default, the broadcast packets are not routed and forwarded by the routers on any network. Routers create broadcast domains. But it can be configured to forward broadcasts in some special cases. A broadcast message is destined to all network devices. Broadcast routing can be done in two ways (algorithm): A router creates a data packet and then sends it to each host one by one. In this case, the router creates multiple copies of single data packet with different destination addresses. All packets are sent as unicast but because they are sent to all, it simulates as if router is broadcasting. This method consumes lots of bandwidth and router must destination address of each node. Secondly, when router receives a packet that is to be broadcasted, it simply floods those packets out of all interfaces. All routers are configured in the same way. This method is easy on router's CPU but may cause the problem of duplicate packets received from peer routers. Reverse path forwarding is a technique, in which router knows in advance about its predecessor from where it should receive broadcast. This technique is used to detect and discard duplicates. ▪ Multicast Routing: Multicast routing is special case of broadcast routing with significance difference and challenges. In broadcast routing, packets are sent to all nodes even if they do not want it. But in Multicast routing, the data is sent to only nodes which wants to receive the packets.  The router must know that there are nodes, which wish to receive multicast packets (or stream) then only it should forward. Multicast routing works spanning tree protocol to avoid looping. Multicast routing also uses reverse path Forwarding technique, to detect and discard duplicates and loops. Anycast Routing Anycast packet forwarding is a mechanism where multiple hosts can have same logical address. When a packet destined to this logical address is received, it is sent to the host which is nearest in routing topology.  Anycast routing is done with help of DNS server. Whenever an Anycast packet is received it is enquired with DNS to where to send it. DNS provides the IP address which is the nearest IP configured on it. ▪ Unicast Routing Protocols: There are two kinds of routing protocols available to route unicast packets: Distance Vector Routing Protocol: Distance Vector is simple routing protocol which takes routing decision on the number of hops between source and destination. A route with less number of hops is considered as the best route. Every router advertises its set best routes to other routers. Ultimately, all routers build up their network topology based on the advertisements of their peer routers, For example Routing Information Protocol (RIP). Link State Routing Protocol: Link State protocol is slightly complicated protocol than Distance Vector. It takes into account the states of links of all the routers in a network. This technique helps routes build a common graph of the entire network. All routers then calculate their best path for routing purposes. for example, Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (ISIS). ▪ Multicast Routing Protocols: Unicast routing protocols use graphs while Multicast routing protocols use trees, i.e. spanning tree to avoid loops. The optimal tree is called shortest path spanning tree. DVMRP: Distance Vector Multicast Routing Protocol MOSPF: Multicast Open Shortest Path First CBT: Core Based Tree PIM: Protocol independent Multicast Protocol Independent Multicast is commonly used now. It has two flavors: PIM Dense Mode: This mode uses source-based trees. It is used in dense environment such as LAN. PIM Sparse Mode: This mode uses shared trees. It is used in sparse environment such as WAN. ▪ Routing Algorithms: The routing algorithms are as follows: ▪ Flooding: Flooding is simplest method packet forwarding. When a packet is received, the routers send it to all the interfaces except the one on which it was received. This creates too much burden on the network and lots of duplicate packets wandering in the network. Time to Live (TTL) can be used to avoid infinite looping of packets. There exists another approach for flooding, which is called Selective Flooding to reduce the overhead on the network. In this method, the router does not flood out on all the interfaces, but selective ones. ▪ Shortest Path: Routing decision in networks, are mostly taken on the basis of cost between source and destination. Hop count plays major role here. Shortest path is a technique which uses various algorithms to decide a path with minimum number of hops. Common shortest path algorithms are: Dijkstra's algorithm Bellman Ford algorithm Floyd Warshall algorithm ▪ Network Layer Design Issues: The network layer or layer 3 of the OSI (Open Systems Interconnection) model is concerned delivery of data packets from the source to the destination across multiple hops or links. It is the lowest layer that is concerned with end-to-end transmission. The designers who are concerned with designing this layer needs to cater to certain issues. These issues encompass the services provided to the upper layers as well as internal design of the layer. The design issues can be elaborated under four heads: Store-and-Forward Packet Switching Services to Transport Layer Providing Connection Oriented Service Providing Connectionless Service ▪ Store-and-Forward Packet Switching: The network layer operates in an environment that uses store and forward packet switching. The node which has a packet to send, delivers it to the nearest router. The packet is stored in the router until it has fully arrived and its checksum is verified for error detection. Once, this is done, the packet is forwarded to the next router. Since, each router needs to store the entire packet before it can forward it to the next hop, the mechanism is called store-and- forward switching. ▪ Services to Transport Layer: The network layer provides service its immediate upper layer, namely transport layer, through the network-transport layer interface. The two types of services provided are: Connection-Oriented Service-In this service, a path is setup between the source and the destination, and all the data packets belonging to a message are routed along this path. Connectionless Service − In this service, each packet of the message is considered as an independent entity and is individually routed from the source to the destination. The objectives of the network layer while providing these services are: The services should not be dependent upon the router technology. The router configuration details should not be of a concern to the transport layer. A uniform addressing plan should be made available to the transport layer, whether the network is a LAN, MAN or WAN. ▪ Providing Connection Oriented Service: In connection − oriented services, a path or route called a virtual circuit is setup between the source and the destination nodes before the transmission starts. All the packets in the message are sent along this route. Each packet contains an identifier that denotes the virtual circuit to which it belongs to. When all the packets are transmitted, the virtual circuit is terminated and the connection is released. An example of connection − oriented service is Multi Protocol Label Switching (MPLS). ▪ Providing Connectionless Service: In connectionless service, since each packet is transmitted independently, each packet contains its routing information and is termed as datagram. The network using datagrams for transmission is called datagram networks or datagram subnets. No prior setup of routes are needed before transmitting a message. Each datagram belong to the message follows its own individual route from the source to the destination. An example of connectionless service is Internet Protocol or IP. ▪ Routing Algorithm in Computer Network: A routing algorithm is a procedure that lays down the route or path to transfer data packets from source to the destination. They help in directing Internet traffic efficiently. After a data packet leaves its source, it can choose among the many different paths to reach its destination. Routing algorithm mathematically computes the best path, i.e. “least – cost path” that the packet can be routed through. ▪ Types of Routing Algorithms: Routing algorithms can be broadly categorized into two types, adaptive and nonadaptive routing algorithms. They can be further categorized as shown in the following diagram − ▪ Adaptive Routing Algorithms: Adaptive routing algorithms, also known as dynamic routing algorithms, makes routing decisions dynamically depending on the network conditions. It constructs the routing table depending upon the network traffic and topology. They try to compute the optimized route depending upon the hop count, transit time and distance. The three popular types of adaptive routing algorithms are are: Centralized algorithm: It finds the least least-cost cost path between source and destination nodes by using global knowledge about the network. So, it is also known as global routing algorithm. Isolated algorithm: This algorithm procures the routing information by using local information ation instead of gathering information from other nodes. Distributed algorithm: This is a decentralized algorithm that computes the least-cost least path between source and destination iteratively in a distributed manner. ▪ Non-Adaptive Adaptive Routing Algorithms Algorithms: Non-adaptive adaptive Routing algorithms, also known as static routing algorithms, construct a static routing table to determine the path through which packets are to be sent. The static routing table is constructed based upon the routing information stored in the routers when the network is booted up. The two types of non – adaptive routing algorithms are are: Flooding: In flooding, when a data packet arrives at a router, it is sent to all the outgoing links except the one it has arrived on. Flooding may be uncontrolled, uncontrolle controlled or selective flooding. Random walks: This is a probabilistic algorithm where a data packet is sent by the router to any one of its neighbours randomly. ▪ Internetworking in Computer Network Network: In real world scenario, networks under same administration are generally scattered geographically. There may exist requirement of connecting two different networks of same kind as well as of different kinds. Routing between two networks is called internetworking. Networks can be considered different based on various parameters such as, Protocol, topology, Layer-22 network and addressing scheme. In internetworking, routers have knowledge of each other’s address and addresses beyond them. They can be statically configured go on different network or they can learn by using internetworking routing protocol.  Routing protocols which are used within an organization or administration are called Interior Gateway Protocols or IGP. RIP, OSPF are examples of IGP. Routing between different organizations or administrations may have Exterior Gateway Protocol, and there is only one EGP i.e. Border Gateway Protocol. ▪ Tunneling: If they are two geographically separate networks, which want to communicate with each other, they may deploy a dedicated line between or they have to pass their data through intermediate networks. Tunneling is a mechanism by which two or more same networks communicate with each other, by passing intermediate networking complexities. Tunneling is configured at both ends.  When the data enters from one end of Tunnel, it is tagged. This tagged data is then routed inside the intermediate or transit network to reach the other end of Tunnel. When data exists the Tunnel its tag is removed and delivered to the other part of the network. Both ends seem as if they are directly connected and tagging makes data travel through transit network without any modifications. ▪ Packet Fragmentation: Most Ethernet segments have their maximum transmission unit (MTU) fixed to 1500 bytes. A data packet can have more or less packet length depending upon the application. Devices in the transit path also have their hardware and software capabilities which tell what amount of data that device can handle and what size of packet it can process. If the data packet size is less than or equal to the size of packet the transit network can handle, it is processed neutrally. If the packet is larger, it is broken into smaller pieces and then forwarded. This is called packet fragmentation. Each fragment contains the same destination and source address and routed through transit path easily. At the receiving end it is assembled again. If a packet with DF (don’t fragment) bit set to 1 comes to a router which cannot handle the packet because of its length, the packet is dropped. When a packet is received by a router has its MF (more fragments) bit set to 1, the router then knows that it is a fragmented packet and parts of the original packet is on the way. If packet is fragmented too small, the overhead is increases. If the packet is fragmented too large, intermediate router may not be able to process it and it might get dropped. ▪ What is Software-Defined Networking (SDN)? Software-Defined Networking (SDN) is an approach to networking that uses software- based controllers or application programming interfaces (APIs) to communicate with underlying hardware infrastructure and direct traffic on a network. This model differs from that of traditional networks, which use dedicated hardware devices (i.e., routers and switches) to control network traffic. SDN can create and control a virtual network – or control a traditional hardware – via software. While network virtualization allows organizations to segment different virtual networks within a single physical network, or to connect devices on different physical networks to create a single virtual network, software-defined networking enables a new way of controlling the routing of data packets through a centralized server. ▪ Why Software-Defined Networking is important? SDN represents a substantial step forward from traditional networking, in that it enables the following: Increased control with greater speed and flexibility: Instead of manually programming multiple vendor-specific hardware devices, developers can control the flow of traffic over a network simply by programming an open standard software-based controller. Networking administrators also have more flexibility in choosing networking equipment, since they can choose a single protocol to communicate with any number of hardware devices through a central controller. Customizable network infrastructure: With a software-defined network, administrators can configure network services and allocate virtual resources to change the network infrastructure in real time through one centralized location. This allows  network administrators to optimize the flow of data through the network and prioritize applications that require more availability. Robust security: A software-defined network delivers visibility into the entire network, providing a more holistic view of security threats. With the proliferation of smart devices that connect to the internet, SDN offers clear advantages over traditional networking. Operators can create separate zones for devices that require different levels of security, or immediately quarantine compromised devices so that they cannot infect the rest of the network. The key difference between SDN and traditional networking is infrastructure: SDN is software-based, while traditional networking is hardware-based. Because the control plane is software-based, SDN is much more flexible than traditional networking. It allows administrators to control the network, change configuration settings, provision resources, and increase network capacity-all from a centralized user interface, without the need for more hardware. There are also security differences between SDN and traditional networking. Thanks to greater visibility and the ability to define secure pathways, SDN offers better security in many ways. However, because software-defined networks use a centralized controller, securing the controller is crucial to maintaining a secure network. ▪ How does Software-Defined Networking (SDN) work? Here are the SDN basics: In SDN (like anything virtualized), the software is decoupled from the hardware. SDN moves the control plane that determines where to send traffic to software, and leaves the data plane that actually forwards the traffic in the hardware. This allows network administrators who use software-defined networking to program and control the entire network via a single pane of glass instead of on a device by device basis. There are three parts to a typical SDN architecture, which may be located in different physical locations: Applications, which communicate resource requests or information about the network as a whole Controllers, which use the information from applications to decide how to route a data packet Networking devices, which receive information from the controller about where to move the data Physical or virtual networking devices actually move the data through the network. In some cases, virtual switches, which may be embedded in either the software or the hardware, take over the responsibilities of physical switches and consolidate their functions into a single, intelligent switch. The switch checks the integrity of both the data packets and their virtual machine destinations and moves the packets along. ▪ Benefits of Software-Defined Networking (SDN): Many of today’s services and applications, especially when they involve the cloud, could not function without SDN. SDN allows data to move easily between distributed locations, which is critical for cloud applications. Additionally, SDN supports moving workloads around a network quickly. For instance, dividing a virtual network into sections, using a technique called network functions virtualization (NFV), allows telecommunications providers to move customer services to less expensive servers or even to the customer’s own servers. Service providers can use a virtual network infrastructure to shift workloads from private to public cloud infrastructures as necessary, and to make new customer services available instantly. SDN also makes it easier for any network to flex and scale as network administrators add or remove virtual machines, whether those machines are on-premises or in the cloud. Finally, because of the speed and flexibility offered by SDN, it is able to support emerging trends and technologies such as edge computing and the Internet of Things, which require transferring data quickly and easily between remote sites. ▪ How is SDN different from Traditional Networking? The key difference between SDN and traditional networking is infrastructure: SDN is software-based, while traditional networking is hardware-based. Because the control plane is software-based, SDN is much more flexible than traditional networking. It allows administrators to control the network, change configuration settings, provision resources, and increase network capacity-all from a centralized user interface, without adding more hardware. There are also security differences between SDN and traditional networking. Thanks to greater visibility and the ability to define secure pathways, SDN offers better security in many ways. However, because software-defined networks use a centralized controller, securing the controller is crucial to maintaining a secure network, and this single point of failure represents a potential vulnerability of SDN. ▪ What are the different models of SDN? While the premise of centralized software controlling the flow of data in switches and routers applies to all software-defined networking, there are different models of SDN. Open SDN: Network administrators use a protocol like Open Flow to control the b