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Introduction Version 1 2023 Introduction to Networking Table of contents I - Introduction 4 II - Computer Networks...
Introduction Version 1 2023 Introduction to Networking Table of contents I - Introduction 4 II - Computer Networks 5 III - LANs and WANs 6 IV - What is the Internet? 8 1. Internet definition......................................................................................................8 V - Edge Network 13 1. End systems.............................................................................................................13 1.1. End devices......................................................................................................................................14 1.2. Client/Server model........................................................................................................................14 1.3. Peer-To-Peer Model.........................................................................................................................15 2. Access networks.......................................................................................................15 2.1. Home and Small Office Internet Connections...............................................................................16 2.2. Businesses Internet Connections...................................................................................................17 3. Physical media.........................................................................................................17 3.1. Network media................................................................................................................................17 VI - The Network Core 19 1. Intermediary devices...............................................................................................20 2. Forwarding and Routing..........................................................................................20 3. Internet Architecture...............................................................................................21 VII - Activity: Network Representation 22 VIII - Communication Protocols 25 1. What is a Protocol?...................................................................................................25 2. Communication Protocol Types..............................................................................26 3. Network Protocol Functions....................................................................................26 4. Protocol Interaction.................................................................................................26 5. Network Protocol Requirements.............................................................................27 5.1. Message Encoding...........................................................................................................................27 5.2. Message Formatting........................................................................................................................28 5.3. Message Size....................................................................................................................................29 5.4. Message Timing...............................................................................................................................29 5.5. Message Delivery Options...............................................................................................................30 IX - Network Protocols Reference Model 31 1. Layered model.........................................................................................................31 2 Table of contents 2. Two Network Layered Models.................................................................................32 3. Data Encapsulation..................................................................................................34 4. Addressing................................................................................................................36 5. Protocol Suite...........................................................................................................37 X - Activity: Protocols and reference model 40 XI - Small network configuration 43 1. Small network topology..........................................................................................43 2. IP Addressing and connectivity checking for a small Network..............................43 3. Small Network Applications and Protocols............................................................45 XII - Knowledge Check 47 XIII - Conclusion 52 XIV - Exercise solutions 53 3 Introduction Today’s Internet is arguably the largest engineered system ever created by mankind: hundreds of millions of connected computers, communication links, and switches billions of users who connect via laptops, tablets, and smartphones new Internet-connected “things” including game consoles, surveillance systems, watches, eye glasses, thermostats, and cars. This first chapter presents a broad overview of computer networking and the Internet. We’ll cover a lot of ground in this introductory chapter and discuss a lot of the pieces of a computer network, without losing sight of the big picture. 4 Computer Networks I Definition A computer network is a set of computers sharing resources. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are made up of telecommunication network technologies, based on physically wired, optical, and wireless radio-frequency methods that may be arranged in a variety of network topologies. [Wikipedia-2022] Networks of many sizes Networks come in all sizes. They range from simple networks consisting of two computers, to networks connecting millions of devices. Small Home Networks – connect a few computers to each other and the Internet Small Office/Home Office – enables computer within a home or remote office to connect to a corporate network Medium to Large Networks – many locations with hundreds or thousands of interconnected computers World Wide Networks – connects hundreds of millions of computers world-wide – such as the internet 5 LANs and WANs II LANs and WANs Network infrastructures vary greatly in terms of: Size of the area covered Number of users connected Number and types of services available Area of responsibility The two most common types of network infrastructures are Local Area Networks (LANs), and Wide Area Networks (WANs). A LAN is a network infrastructure that provides access to users and end devices in a small geographical area. A LAN is typically used in a department within an enterprise, a home, or a small business network. A WAN is a network infrastructure that provides access to other networks over a wide geographical area, which is typically owned and managed by a larger corporation or a telecommunications service provider. The figure shows LANs connected to a WAN. 6 LANs and WANs LAN Definition A LAN is a network infrastructure that spans a small geographical area. LANs have specific characteristics: LANs interconnect end devices in a limited area such as a home, school, office building, or campus. A LAN is usually administered by a single organization or individual. Administrative control is enforced at the network level and governs the security and access control policies. LANs provide high-speed bandwidth to internal end devices and intermediary devices, as shown in the figure. WAN Definition A WAN is a network infrastructure that spans a wide geographical area. WANs are typically managed by service providers (SPs) or Internet Service Providers (ISPs). WANs have specific characteristics: WANs interconnect LANs over wide geographical areas such as between cities, states, provinces, countries, or continents. WANs are usually administered by multiple service providers. WANs typically provide slower speed links between LANs. The figure shows a WAN which interconnects two LANs. 7 What is the Internet? III 1. Internet definition The Internet Definition The internet is a worldwide collection of interconnected networks (internetworks, or internet for short). The figure shows one way to view the internet as a collection of interconnected LANs and WANs. Internet Architecture The Internet is a computer network that interconnects billions of computing devices throughout the world. The following figure illustrates the components of the Internet. 8 What is the Internet? The Internet Evolution Note Not too long ago, these computing devices were primarily traditional desktop computers, Linux workstations, and so-called servers that store and transmit information such as Web pages and e-mail messages. Increasingly, however, users connect to the Internet with smartphones and tablets—today, close to half of the world’s population are active mobile Internet users with the percentage expected to increase to 75% by 2025 [Statista 2019]. 9 What is the Internet? Internet of Things Furthermore, nontraditional Internet “things” such as TVs, gaming consoles, thermostats, home security systems, home appliances, watches, eye glasses, cars, traffic control systems, and more are being connected to the Internet. In Internet jargon, all of these devices are called hosts or end systems. How do communicate end-systems through the network? Method End systems (devices) are connected together by a network of communication links and packet switches. When one end system has data to send to another end system, the sending end system segments the data and adds header bytes to each segment. The resulting packets are then sent through the network to the destination end system, where they are reassembled into the original data. The Internet Routing function Fundamental A packet switch takes a packet arriving on one of its incoming communication links and forwards that packet on one of its outgoing communication links. Packet switches come in many shapes and flavors, but the two most prominent types in today’s Internet are routers and link- layer switches. Link-layer switches are typically used in access networks, while routers are typically used in the network core. The sequence of communication links and packet switches traversed by a packet from the sending end system to the receiving end system is known as a route or path through the network. The Internet 10 What is the Internet? Internet Service Providers (ISP) Definition End systems access the Internet through Internet Service Providers (ISPs), residential ISPs such as local cable or telephone companies; corporate ISPs; university ISPs; ISPs that provide WiFi access in airports, hotels, coffee shops, and other public places; and cellular data ISPs, providing mobile access to our smartphones and other devices. Each ISP is in itself a network of packet switches and communication links. The Internet ISPs interconnexion ISPs that provide access to end systems must also be interconnected. These lowertier ISPs are thus interconnected through national and international upper-tier ISPs and these upper- tier ISPs are connected directly to each other. An upper-tier ISP consists of high-speed routers interconnected with high-speed fiber-optic links. Each ISP network, whether upper-tier or lower-tier, is managed independently, runs the IP protocol (see below), and conforms to certain naming and address conventions. The Internet 11 What is the Internet? TCP/IP Protocols End systems, packet switches, and other pieces of the Internet run protocols that control the sending and receiving of information within the Internet. The Transmission Control Protocol (TCP) and the Internet Protocol (IP) are two of the most important protocols in the Internet. The IP protocol specifies the format of the packets that are sent and received among routers and end systems. The Internet’s principal protocols are collectively known as TCP/IP. The Internet 12 Edge Network IV We begin in this section at the edge of the network and look at the components with which we are most familiar—namely, the computers, laptops, smartphones and other devices that we use on a daily basis such as access points and modems. At the edge of the network we distinguish three main categories of network components: End systems, also called hosts. These are also called end- devices. Access networks, set of technologies allowing access to the Internet such as routers, switches and access points. The latter are also called intermediary devices. Physical media, that physically connect hosts to the network, such as: copper cables, fiber, wireless connection The Internet 1. End systems End systems also called end devices or hosts can be classified into clients and servers. We distinguish two interaction models of end-systems: Client/Server model Peer-to-Peer model 13 Edge Network 1.1. End devices End devices An end device is either the source or destination of a message transmitted over the network. When an end device initiates communication, it uses the address of the destination end device to specify where to deliver the message. Depending on the used software, and end-device could play either the role of Client, Server or both. 1.2. Client/Server model Clients and Servers All computers that are connected to a network and participate directly in network communication are classified as hosts. Hosts can be called end devices. Servers are computers with software that allow them to provide information, like email or web pages, to other end devices on the network. Each service requires separate server software. For example, a server requires web server software in order to provide web services to the network. A computer with server software can provide services simultaneously to many different clients. Clients have software for requesting and displaying the information obtained from the server, as shown in the figure. Clients and Servers Example An example of client software is a web browser, like Chrome or FireFox. A single computer can also run multiple types of client software. For example, a user can check email and view a web page while instant messaging and listening to an audio stream. The table lists three common types of server software. 14 Edge Network 1.3. Peer-To-Peer Model Peer-To-Peer model Client and server software usually run on separate computers, but it is also possible for one computer to be used for both roles at the same time. In small businesses and homes, many computers function as the servers and clients on the network. This type of network is called a peer-to-peer network. P2P network Example In the figure, the print sharing PC has a Universal Serial Bus (USB) connection to the printer and a network connection, using a network interface card (NIC), to the file sharing PC. Advantages vs. disadvantages of P2P model Warning The advantages of peer-to-peer networking: Easy to set up Less complex Lower cost because network devices and dedicated servers may not be required Can be used for simple tasks such as transferring files and sharing printers The disadvantages of peer-to-peer networking: No centralized administration Not as secure Not scalable All devices may act as both clients and servers which can slow their performance 2. Access networks There are many different ways to actually connect users and organizations to the internet. Home users, remote workers, and small offices typically require a connection to an ISP to access the internet. Connection options vary greatly between ISPs and geographical locations. However, popular choices include 15 Edge Network broadband cable, digital subscriber line (DSL), wireless WANs, and mobile services. Organizations usually need access to other corporate sites as well as the internet. Fast connections are required to support business services including IP phones, video conferencing, and data center storage. ISPs offer business-class interconnections. Popular business-class services include business DSL, leased lines, and Metro Ethernet. 2.1. Home and Small Office Internet Connections Home Internet Access Technologies The figure illustrates common connection options for small office and home office users: Cable - Typically offered by cable television service providers, the internet data signal transmits on the same cable that delivers cable television. It provides a high bandwidth, high availability, and an always-on connection to the internet. DSL - Digital Subscriber Lines also provide high bandwidth, high availability, and an always-on connection to the internet. DSL runs over a telephone line. In general, small office and home office users connect using Asymmetrical DSL (ADSL), which means that the download speed is faster than the upload speed. Cellular - Cellular internet access uses a cell phone network to connect. Wherever you can get a cellular signal, you can get cellular internet access. Performance is limited by the capabilities of the phone and the cell tower to which it is connected. Satellite - The availability of satellite internet access is a benefit in those areas that would otherwise have no internet connectivity at all. Satellite dishes require a clear line of sight to the satellite. Dial-up Telephone - An inexpensive option that uses any phone line and a modem. The low bandwidth provided by a dial-up modem connection is not sufficient for large data transfer, although it is useful for mobile access while traveling. 16 Edge Network 2.2. Businesses Internet Connections Businesses may require higher bandwidth, dedicated bandwidth, and managed services. Connection options that are available differ depending on the type of service providers located nearby. The figure illustrates common connection options for businesses. Dedicated Leased Line - Leased lines are reserved circuits within the service provider’s network that connect geographically separated offices for private voice and/or data networking. The circuits are rented at a monthly or yearly rate. Metro Ethernet - This is sometimes known as Ethernet WAN. In this module, we will refer to it as Metro Ethernet. Metro ethernets extend LAN access technology into the WAN. Ethernet is a LAN technology you will learn about in a later module. Business DSL - Business DSL is available in various formats. A popular choice is Symmetric Digital Subscriber Line (SDSL) which is similar to the consumer version of DSL but provides uploads and downloads at the same high speeds. Satellite - Satellite service can provide a connection when a wired solution is not available. The choice of connection varies depending on geographical location and service provider availability. 3. Physical media 3.1. Network media Different types of media Communication transmits across a network on media. The media provides the channel over which the message travels from source to destination. Modern networks primarily use three types of media to interconnect devices, as shown in the figure: Metal wires within cables - Data is encoded into electrical impulses. Glass or plastic fibers within cables (fiber-optic cable) - Data is encoded into pulses of light. Wireless transmission - Data is encoded via modulation of specific frequencies of electromagnetic waves. 17 Edge Network Criteria for choosing a network media Note The four main criteria for choosing network media are these: What is the maximum distance that the media can successfully carry a signal? What is the environment in which the media will be installed? What is the amount of data and at what speed must it be transmitted? What is the cost of the media and installation? 18 The Network Core V The network core is a mesh of intermediary devices and links that interconnect the Internet’s end systems. This figure highlights the network core with thick, shaded lines. 19 The Network Core 1. Intermediary devices Intermediary devices Intermediary devices connect the individual end devices to the network. They can connect multiple individual networks to form an internetwork. These intermediary devices provide connectivity and ensure that data flows across the network. Intermediary devices use the destination end device address, in conjunction with information about the network interconnections, to determine the path that messages should take through the network. Examples of the more common intermediary devices and a list of functions are shown in the figure. Intermediary network devices perform some or all of these functions: Regenerate and retransmit communication signals Maintain information about what pathways exist through the network and internetwork Notify other devices of errors and communication failures Direct data along alternate pathways when there is a link failure Classify and direct messages according to priorities Permit or deny the flow of data, based on security settings 2. Forwarding and Routing Two Key Network-Core functions Fundamental There are two key Network-Core functions: forwarding: routers/switches move arriving packets from router’s input link to appropriate router output link according to what is indicated in a locally stored forwarding table. This table indicates for each destination what should be the output link to reach that destination. routing: routers run algorithms to compute the best routes that should be taken by packets between source and destination end systems. This function allows building forwarding tables inside routers. Example In the following figure, the router reads the destination address 0111 in the packet header. The local forwarding table of the router indicates that this destination can be reached through link 2. So the router forwards this packet to link 2. 20 The Network Core 3. Internet Architecture A hierarchical structure As illustrated in this figure, the Internet is a complex network of networks where: hosts connect to Internet via access Internet Service Providers (ISPs) access ISPs in turn must be interconnected so that any two hosts (anywhere!) can send packets to each other So, access ISPs connect to regional ISPs with larger coverage Regional ISPs, in their turn, connect to Tier-1 ISPs that ensure national and international coverage. There are a dozen of 1-Tier ISPs IXPs (Internet eXchange Points) allow peer connection between ISPs content provider networks (e.g., Google, Facebook) connect their data centers to Internet, often bypassing tier-1 and regional ISPs 21 VI OK Activity: Network Representation Download then open the below Packet Tracer activity. Answer the following questions: [cf. res.zip] [solution n°1 p. 53] Quiz 1 The icon toolbar at the bottom left hand corner has various categories of networking components. List the intermediary device categories. [solution n°2 p. 53] Quiz 2 Without entering into the internet cloud or intranet cloud, how many icons in the topology represent endpoint devices (only one connection leading to them)? [solution n°3 p. 53] Quiz 3 Without counting the two clouds, how many icons in the topology represent intermediary devices (multiple connections leading to them)? [solution n°4 p. 53] Quiz 4 How many different types of media connections are used in this network topology? [solution n°5 p. 53] Quiz 5 In Packet Tracer, only the Server-PT device can act as a server. Desktop or Laptop PCs cannot act as a server. Based on your studies so far, explain the client-server model. 22 Activity: Network Representation [solution n°6 p. 54] Quiz 6 Select three functions of intermediary devices. Regenerate and retransmit data signals Provide services to end-systems Maintain information about what pathways exist through the network and internetwork Permit or deny the flow of data, based on security settings. Display rendered services by servers [solution n°7 p. 54] Quiz 7 List important criteria for choosing a network media type. [solution n°8 p. 54] Quiz 8 In the Packet Tracer network, how many WANs do you see? [solution n°9 p. 54] Quiz 9 How many LANs do you see? [solution n°10 p. 54] Quiz 10 What are common ways a home user connects to the internet? Cable Dedicated Leased line DSL Dial-up Business DSL Cellular Metro-Ethernet Satellite [solution n°11 p. 55] Quiz 11 What are common methods that businesses use to connect to the internet? 23 Activity: Network Representation Cable Dedicated Leased line DSL Dial-up Business DSL Cellular Metro-Ethernet Satellite 24 Communication Protocols VII 1. What is a Protocol? A communication protocol Definition A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event. Human vs. Computer Protocols Example Consider what you do when you want to ask someone for the time of day. A typical exchange is shown in the Figure beside. Human protocol (or good manners, at least) dictates that one first offer a greeting (the first “Hi” in Figure) to initiate communication with someone else. The typical response to a “Hi” is a returned “Hi” message. Implicitly, one then takes a cordial “Hi” response as an indication that one can proceed and ask for the time of day. As an example of a computer network protocol with which you are probably familiar, consider what happens when you make a request to a Web server, that is, when you type the URL of a Web page into your Web browser. The scenario is illustrated in the right half of the Figure: First, your computer will send a connection request message to the Web server and wait for a reply. The Web server will eventually receive your connection request message and return a connection reply message. Knowing that it is now OK to request the Web document, your computer then sends the name of the Web page it wants to fetch from that Web server in a GET message. Finally, the Web server returns the Web page (file) to your computer. Human vs. Computer Protocol 25 Communication Protocols 2. Communication Protocol Types Protocol Types Network protocols define a common format and set of rules for exchanging messages between devices. Protocols are implemented by end devices and intermediary devices in software, hardware, or both. Each network protocol has its own function, format, and rules for communications. The table lists the various types of protocols that are needed to enable communications across one or more networks. 3. Network Protocol Functions Protocol functions Network communication protocols are responsible for a variety of functions necessary for network communications between end devices. Computers and network devices use agreed-upon protocols to communicate. The table lists the functions of these protocols. 4. Protocol Interaction A message sent over a computer network typically requires the use of several protocols, each one with its own functions and format. The figure shows some common network protocols that are used when a device sends a request to a web server for its web page. 26 Communication Protocols 5. Network Protocol Requirements In addition to identifying the source and destination, computer and network protocols define the details of how a message is transmitted across a network. Common computer protocols include the following requirements: Message encoding Message formatting and encapsulation Message size Message timing Message delivery options 5.1. Message Encoding Encoding Definition One of the first steps to sending a message is encoding. Encoding is the process of converting information into another acceptable form, for transmission. 27 Communication Protocols Decoding Definition Decoding reverses this process to interpret the information. 5.2. Message Formatting When a message is sent from source to destination, it must use a specific format or structure. Message formats depend on the type of message and the channel that is used to deliver the message. IPv6 Formatting Example Internet Protocol (IP) is a protocol with a similar function to the envelope example. In the figure, the fields of the Internet Protocol version 6 (IPv6) packet identify the source of the packet and its destination. IP is responsible for sending a message from the message source to destination over one or more networks. 28 Communication Protocols 5.3. Message Size Size restrictions and segmentation When a long message is sent from one host to another over a network, it is necessary to break the message into smaller pieces, as shown in the figure. The rules that govern the size of the pieces, or frames, communicated across the network are very strict. Frames that are too long or too short are not delivered. The size restrictions of frames require the source host to break a long message into individual pieces that meet both the minimum and maximum size requirements. The long message will be sent in separate frames (see figure below), with each frame containing a piece of the original message. At the receiving host, the individual pieces of the message are reconstructed into the original message. 5.4. Message Timing When to send a message? Message timing includes the following: Flow Control – Manages the rate of data transmission and defines how much information can be sent and the speed at which it can be delivered. Response Timeout – Manages how long a device waits when it does not hear a reply from the destination. Access method - Determines when someone can send a message. There may be various rules governing issues like “collisions”. This is when more than one device sends traffic at the same time and the messages become corrupt. Some protocols are proactive and attempt to prevent collisions; other protocols are reactive and establish a recovery method after the collision occurs. 29 Communication Protocols 5.5. Message Delivery Options Delivery options Message delivery may one of the following methods: Unicast – one to one communication Multicast – one to many, typically not all Broadcast – one to all 30 Network Protocols Reference Model VIII The Internet is an extremely complicated system with numerous applications and protocols, various types of end systems, packet switches, and various types of link-level media. Given this enormous complexity, it is necessary to define a reference model to organize network architecture and ensure its evolution in a comprehensive way. 1. Layered model Layered model Definition The layered model allows describing complex systems where each layer provides a specific service in addition to services provided by lower layers. Airline system Example To describe an airline system, the following figure has divided the airline functionality into layers, providing a framework in which we can discuss airline travel. Note that each layer, combined with the layers below it, implements some functionality, some service: At the ticketing layer and below, airline-counter-to-airline-counter transfer of a person is accomplished. At the baggage layer and below, baggage-check-to-baggage-claim transfer of a person and bags is accomplished. Note that the baggage layer provides this service only to an already-ticketed person. At the gate layer, departure-gate-to-arrival-gate transfer of a person and bags is accomplished. At the takeoff/landing layer, runway-to-runway transfer of people and their bags is accomplished. 31 Network Protocols Reference Model Each layer provides its service by: 1. performing certain actions within that layer (for example, at the gate layer, loading and unloading people from an airplane) and by 2. using the services of the layer directly below it (for example, in the gate layer, using the runway- to-runway passenger transfer service of the takeoff/landing layer). Layered model provides modularity Method A layered architecture allows us to discuss a well-defined, specific part of a large and complex system. This simplification itself is of considerable value by providing modularity, making it much easier to change the implementation of the service provided by the layer. As long as the layer provides the same service to the layer above it, and uses the same services from the layer below it, the remainder of the system remains unchanged when a layer’s implementation is changed. Benefits of Layered Model Note These are the benefits of using a layered model to describe network protocols and operations: Assisting in protocol design because protocols that operate at a specific layer have defined information that they act upon and a defined interface to the layers above and below Fostering competition because products from different vendors can work together Preventing technology or capability changes in one layer from affecting other layers above and below Providing a common language to describe networking functions and capabilities 2. Two Network Layered Models ISO/OSI vs. TCP/IP Layered Models Two layered models describe network operations: Open System Interconnection (OSI) Reference Model TCP/IP Reference Model 32 Network Protocols Reference Model ISO/OSI Model The OSI reference model provides an extensive list of functions and services that can occur at each layer. This type of model provides consistency within all types of network protocols and services by describing what must be done at a particular layer, but not prescribing how it should be accomplished. Note: Whereas the TCP/IP model layers are referred to only by name, the seven OSI model layers are more often referred to by number rather than by name. For instance, the physical layer is referred to as Layer 1 of the OSI model, data link layer is Layer2, and so on. TCP/IP Model The TCP/IP protocol model for internetwork communications was created in the early 1970s and is sometimes referred to as the internet model. This type of model closely matches the structure of a particular protocol suite. The TCP/IP model is a protocol model because it describes the functions that occur at each layer of protocols within the TCP/IP suite. TCP/IP is also used as a reference model. 33 Network Protocols Reference Model The definitions of the standard and the TCP/IP protocols are discussed in a public forum : IETF (Internet Engineering Task Force) They are defined in a publicly available set of IETF RFCs (Request For Comments). An RFC is authored by networking engineers and sent to other IETF members for comments. 3. Data Encapsulation Protocol Data Unit (PDU) Definition The form that a piece of data takes at any layer is called a Protocol Data Unit (PDU). Encapsulation Definition Encapsulation is the process where protocols add their information to the data. PDU Naming Warning At each stage of the encapsulation process, a PDU has a different name to reflect its new functions. There is no universal naming convention for PDUs, in this course, the PDUs are named according to the protocols of the TCP/IP suite. PDUs passing down the stack are as follows: 1. Data (Data Stream) 2. Segment 3. Packet 4. Frame 5. Bits (Bit Stream) 34 Network Protocols Reference Model Data encapsulation through the reference model Method The following figure illustrates the important concept of encapsulation: At the sending host, an application-layer message (M) is passed to the transport layer. The transport layer takes the message and appends additional information (so-called transport- layer header information, Ht that will be used by the receiver-side transport layer. The application-layer message and the transport-layer header information together constitute the transport-layer segment. The transport-layer segment thus encapsulates the application-layer message. The added information might include information allowing the receiver-side transport layer to deliver the message up to the appropriate application, and error-detection bits that allow the receiver to determine whether bits in the message have been changed in route. The transport layer then passes the segment to the network layer, which adds network-layer header information Hn such as source and destination end system addresses, creating a network-layer datagram. The datagram is then passed to the link layer, which will add its own link-layer header information and create a link-layer frame. Thus, we see that at each layer, a packet has two types of fields: header fields and a payload field. The payload is typically a packet from the layer above. Example The following figure shows the physical path that data takes down a sending end system’s protocol stack, up and down the protocol stacks of an intervening link-layer switch and router, and then up the protocol stack at the receiving end system. 35 Network Protocols Reference Model 4. Addressing Addressing To access forward data from source to destination, four levels of addresses are used in an internet employing the TCP/IP protocols: physical, logical, port, and specific. Physical Addresses (MAC addresses) Physical addresses are imprinted on the Network Interface Card (NIC). Ethernet local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon. Example: 07:01:02:01:2C:4B Layer 3 Logical Addresses (IP addresses) An IP address is the network layer, or Layer 3, logical address used to deliver the IP packet from the original source to the final destination, as shown in the figure. 36 Network Protocols Reference Model The IP packet contains two IP addresses: Source IP address - The IP address of the sending device, which is the original source of the packet. Destination IP address - The IP address of the receiving device, which is the final destination of the packet. Port Addresses Port address is a 16-bit address represented by one decimal number ranged from (0-65535) to choose a process among multiple processes on the host to receive and handle the message. Destination port number is needed for delivery. Source port number is needed for receiving a reply such as an acknowledgment. Well-known ports Note Port numbers from 0 to 1023 are reserved for common TCP/IP applications and are called well-known ports. The use of well-known ports allows client applications to easily locate the corresponding server application processes on other hosts. For instance: 80 for HTTP (web) 25 for SMTP (email) 53 for DNS Specific Addresses Specific addresses are generally used in the application layer protocols. For instance: Email addresses: [email protected] Fully Qualified Domain Names: www.udst.edu.qa Address Translation Note The Domain Name System (DNS) translates human-friendly fully qualified domain names into IP addresses. For example, www.example.com is translated to 208.77.188.166 The Address Resolution Protocol (ARP) translates IP addresses into physical MAC addresses. 5. Protocol Suite Protocols interaction to provide services In many cases, protocols must be able to work with other protocols so that your online experience gives you everything you need for network communications. Protocol suites are designed to work with each other seamlessly. A protocol suite is a group of inter-related protocols necessary to perform a communication function. 37 Network Protocols Reference Model TCP/IP protocols interaction Example The figure shows an example of the three TCP/IP protocols used to send packets between the web browser of a host and the web server. HTTP, TCP, and IP are the TCP/IP protocols used. At the network access layer, Ethernet is used in the example. However, this could also be a wireless standard such as WLAN or cellular service. TCP/IP Protocol Suite Today, the TCP/IP protocol suite includes many protocols and continues to evolve to support new services. Some of the more popular ones are shown in the figure. 38 Network Protocols Reference Model TCP/IP is the protocol suite used by the internet and the networks of today. TCP/IP has two important aspects for vendors and manufacturers: Open standard protocol suite - This means it is freely available to the public and can be used by any vendor on their hardware or in their software. Standards-based protocol suite - This means it has been endorsed by the networking industry and approved by a standards organization. This ensures that products from different manufacturers can interoperate successfully. 39 Activity: Protocols and reference IX OK model [solution n°12 p. 55] Quiz 1:Layered Architecture In the scenario below, consider that the source sends an http request to destination Quiz What layer in the IP stack best corresponds to the phrase: 'handles the delivery of segments from the application layer, may be reliable or unreliable' Application Layer Transport Layer Network Layer Link Layer Physical Layer Quiz What layer in the IP stack best corresponds to the phrase: 'bits live on the wire' Application Layer Transport Layer Network Layer Link Layer Physical Layer 40 Activity: Protocols and reference model Quiz What layer in the IP stack best corresponds to the phrase: 'moves packets from the source host to the destination host' Application Layer Transport Layer Network Layer Link Layer Physical Layer Quiz What layer in the IP stack best corresponds to the phrase: 'passes frames from one node to another across some medium' Application Layer Transport Layer Network Layer Link Layer Physical Layer Quiz What layer in the IP stack best corresponds to the phrase: 'handles messages from a variety of network applications' Application Layer Transport Layer Network Layer Link Layer Physical Layer [solution n°13 p. 57] Quiz 7:Matching Layers In the scenario below, consider that the source sends an http request to destination 41 Activity: Protocols and reference model Match the numbered layers to the correct layer on the left 11 10 5 2 8 9 6 7 12 15 3 13 14 4 1 Application Layer Transport Layer Network Layer Link Layer Physical Layer 42 Small network configuration X 1. Small network topology The majority of businesses are small most of the business networks are also small. A small network design is usually simple. Small networks typically have a single WAN connection provided by DSL, cable, or an Ethernet connection. 2. IP Addressing and connectivity checking for a small Network Address configuration All devices within a network must have a unique IP address Devices must be configured with the address of a Default Gateway: the router that connects the network to the Internet You can display host network configuration using an OS command IP Configuration on a Windows Host In Windows 10, you can access the IP address details from the Network and Sharing Center to quickly view the four important settings: address, mask, router, and DNS. Or you can issue the ipconfig command at the command line of a Windows computer. Use the ipconfig /all command to view the MAC address, as well as a number of details regarding the Layer 3 addressing of the device. 43 Small network configuration Network connectivity checking using ping Whether your network is small and new, or you are scaling an existing network, you will always want to be able to verify that your components are properly connected to each other and to the internet. The ping command, available on most operating systems, is the most effective way to quickly test Layer 3 connectivity between a source and destination IP address. The ping command uses the Internet Control Message Protocol (ICMP) echo (ICMP Type 8) and echo reply (ICMP Type 0) messages. On a Windows 10 host, the ping command sends four consecutive ICMP echo messages and expects four consecutive ICMP echo replies from the destination. Verify Connectivity with Traceroute Traceroute can help locate Layer 3 problem areas in a network. A trace returns a list of hops as a packet is routed through a network. The syntax of the trace command varies between operating systems. tracert in Windows 10 Example The following is a sample output of tracert command on a Windows 10 host. Note: Use Ctrl-C to interrupt a tracert in Windows. The only successful response was from the gateway on R1. Trace requests to the next hop timed out as indicated by the asterisk (*), meaning that the next hop router did not respond or there is a failure in the network path. In this example there appears to be a problem between R1 and R2. 44 Small network configuration Do it yourself Advice 1. Display your desktop network configuration: open the command prompt in your workstation display your workstation network configuration ipconfig /all What is the IP address assigned to your machine? What is the IP address of the default gateway (default router through which your machine reaches the Internet) What is the Physical MAC address of the wired NIC of your machine? 2. Test connectivity: Test connectivity between your workstation and the one of your neighbor using ping. Note the average round trip time. Test connectivity between your workstation and www.google.com using ping. Compare the round trip time with the previous one. What do you notice? Explain 3. Trace the route to destination: Trace the route to www.google.com using tracert What is the IP address of the first hit router? How is commonly called this router? 3. Small Network Applications and Protocols After you have set it up, your network still needs certain types of applications and protocols in order to work. The network is only as useful as the applications that are on it. 45 Small network configuration Common applications There are two forms of software programs or processes that provide access to the network: network applications and application layer services. Network Applications Applications are the software programs used to communicate over the network. Some end-user applications are network-aware, meaning that they implement application layer protocols and are able to communicate directly with the lower layers of the protocol stack. Email clients and web browsers are examples of this type of application. Application Layer Services Other programs may need the assistance of application layer services to use network resources like file transfer or network print spooling. Though transparent to an employee, these services are the programs that interface with the network and prepare the data for transfer. Common protocols Network protocols support the applications and services used by employees in a small network. Network administrators commonly require access to network devices and servers. The two most common remote access solution is Secure Shell (SSH). Hypertext Transfer Protocol (HTTP) and Hypertext Transfer Protocol Secure (HTTPS) are used between web clients and web servers. Simple Mail Transfer Protocol (SMTP) is used to send email, Post Office Protocol (POP3) or Internet Mail Access Protocol (IMAP) are used by clients to retrieve email. File Transfer Protocol (FTP) and Security File Transfer Protocol (SFTP) are used to download and upload files between a client and an FTP server. Dynamic Host Configuration Protocol (DHCP) is used by clients to acquire an IP configuration from a DHCP Server. The Domain Name Service (DNS) resolves domain names to IP addresses. Note: A server could provide multiple network services. For instance, a server could be an email, FTP and SSH server. 46 XI OK Knowledge Check [solution n°14 p. 57] Quiz 1 Which of the following is the name for all computers connected to a network that participate directly in network communication? Servers Intermediary devices Hosts Media [solution n°15 p. 57] Quiz 2 Which two devices are intermediary devices? (Choose two) Hosts Switches Servers Routers [solution n°16 p. 58] Quiz 3 Which connection physically connects the end device to the network? Port NIC Interface [solution n°17 p. 58] Quiz 4 Which network infrastructure provides access to users and end devices in a small geographical area, which is typically a network in a department in an enterprise, a home, or small business? Extranet Intranet LAN WAN 47 Knowledge Check [solution n°18 p. 58] Quiz 5 Which network infrastructure provides access to other networks over a large geographical area, which is often owned and managed by a telecommunications service provider? Extranet Intranet LAN WAN [solution n°19 p. 58] Quiz 6 Which device performs the function of determining the path that messages should take through internetworks? a router a firewall DSL modem web server [solution n°20 p. 59] Quiz 7 Match the function of a layer in the Internet protocol stack to its its name Transfer of data between neighboring network devices. Delivery of packets from a source host to a destination host (typically). Transfer of data between one process and another process (typically on different hosts). Protocols that are part of a distributed network application. Transfer of a bit into and out of a transmission media. Application Layer Transport Layer Network Layer Link Layer Physical Layer 48 Knowledge Check [solution n°21 p. 59] Quiz 8 Match the name of an Internet layer with unit of data that is exchanged among protocol entities at that layer Frame Segment Datagram Packet Bit Message Application layer Transport Layer Network Layer Link Layer Physical Layer [solution n°22 p. 59] Quiz 9 Consider the figure below, showing a link-layer frame heading from a host to a router. There are three header fields shown. Match the name of a header with a header label shown in the figure. Physical Layer Network Layer Transport Layer Link Layer Application Layer H1 H2 H3 49 Knowledge Check [solution n°23 p. 60] Quiz 10 Which of the definitions below describe what is meant by the term "encapsulation"? Determining the name of the destination host, translating that name to an IP address and then placing that value in a packet header field. Receiving a “packet” from the layer below, extracting the payload field, and after some internal actions possibly delivering that payload to an upper layer protocol. Starting a transport layer timer for a transmitted segment, and then if an ACK segment isn’t received before the timeout, placing that segment in a retransmission queue Computing the sum of all of the bytes within a packet and placing that value in the packet header field. Taking data from the layer above, adding header fields appropriate for this layer, and then placing the data in the payload field of the “packet” for that layer. [solution n°24 p. 60] Quiz 11 Which of the following descriptions below correspond to a definition of the Internet? Select one or more of the answers below that are correct. A "network of networks". Google and Social media A collection of billions of computing devices, and packet switches interconnected by links. A collection of hardware and software components executing protocols that define the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event. A set of smart connected devices at home and around a person: smart TV, smart watch, connected car,... [solution n°25 p. 60] Quiz 12 Choose one the following two definitions that makes the correct distinction between routing versus forwarding Routing is the local action of moving arriving packets from router’s input link to appropriate router output link, while forwarding is the global action of determining the source-destination paths taken by packets. Forwarding is the local action of moving arriving packets from router’s input link to appropriate router output link, while routing is the global action of determining the source-destination paths taken by packets. 50 Knowledge Check [solution n°26 p. 61] Quiz 13 When we say that the Internet is a “network of networks,” we mean? Check all that apply (hint: check two or more). The Internet is the fastest network ever built. The Internet is made up of a lot of different networks that are interconnected to each other. The Internet is the largest network ever built. The Internet is made up of access networks at the edge, tier-1 networks at the core, and interconnected regional and content provider networks as well. 51 Conclusion This chapter was an overview of computer networks and the Internet. We reviewed the Internet architecture and its components; Edge network, Network-core. We also introduced fundamental concepts of the Internet: encapsulation, protocols, layered reference model. At the end, we introduced elements to build a small network. In the following chapters we'll dive into details of each layer of the TCP/IP reference model. 52 Exercise solutions [exercice p. 22] Solution n°1 The icon toolbar at the bottom left hand corner has various categories of networking components. List the intermediary device categories. Routers, Switches, Hubs, Wireless Devices, WAN Emulation [exercice p. 22] Solution n°2 Without entering into the internet cloud or intranet cloud, how many icons in the topology represent endpoint devices (only one connection leading to them)? 15 [exercice p. 22] Solution n°3 Without counting the two clouds, how many icons in the topology represent intermediary devices (multiple connections leading to them)? 11 [exercice p. 22] Solution n°4 How many different types of media connections are used in this network topology? 4 Copper Straight-Through Serial DTE (WAN) Coaxial Wireless [exercice p. 22] Solution n°5 In Packet Tracer, only the Server-PT device can act as a server. Desktop or Laptop PCs cannot act as a server. Based on your studies so far, explain the client-server model. In modern networks, a host can act as a client, a server, or both. Software installed on the host determines the role it plays on the network. Servers are hosts that have software installed that enables them to provide information and services, like email or web pages, to other hosts on the network. Clients are hosts that have software installed that enables them to request and display the information 53 Exercise solutions obtained from the server. A client could also be configured as a server simply by installing server software. [exercice p. 23] Solution n°6 Select three functions of intermediary devices. Regenerate and retransmit data signals Provide services to end-systems Maintain information about what pathways exist through the network and internetwork Permit or deny the flow of data, based on security settings. Display rendered services by servers [exercice p. 23] Solution n°7 List important criteria for choosing a network media type. The distance the media can successfully carry a signal. The environment in which the media is to be installed. The amount of data and the speed at which it must be transmitted. The cost of the media and installation. [exercice p. 23] Solution n°8 In the Packet Tracer network, how many WANs do you see? 2 The internet and the intranet WANs [exercice p. 23] Solution n°9 How many LANs do you see? 3 [exercice p. 23] Solution n°10 What are common ways a home user connects to the internet? Cable Dedicated Leased line DSL Dial-up Business DSL 54 Exercise solutions Cellular Metro-Ethernet Satellite [exercice p. 23] Solution n°11 What are common methods that businesses use to connect to the internet? Cable Dedicated Leased line DSL Dial-up Business DSL Cellular Metro-Ethernet Satellite [exercice p. 40] Solution n°12 Quiz What layer in the IP stack best corresponds to the phrase: 'handles the delivery of segments from the application layer, may be reliable or unreliable' Application Layer Transport Layer Network Layer Link Layer Physical Layer Quiz What layer in the IP stack best corresponds to the phrase: 'bits live on the wire' Application Layer Transport Layer Network Layer Link Layer Physical Layer 55 Exercise solutions Quiz What layer in the IP stack best corresponds to the phrase: 'moves packets from the source host to the destination host' Application Layer Transport Layer Network Layer Link Layer Physical Layer Quiz What layer in the IP stack best corresponds to the phrase: 'passes frames from one node to another across some medium' Application Layer Transport Layer Network Layer Link Layer Physical Layer Quiz What layer in the IP stack best corresponds to the phrase: 'handles messages from a variety of network applications' Application Layer Transport Layer Network Layer Link Layer Physical Layer 56 Exercise solutions [exercice p. 41] Solution n°13 In the scenario below, consider that the source sends an http request to destination Match the numbered layers to the correct layer on the left Application Layer Transport Layer Network Layer Link Layer Physical Layer 1 2 3 4 5 15 14 13 12 11 10 7 6 9 8 [exercice p. 47] Solution n°14 Which of the following is the name for all computers connected to a network that participate directly in network communication? Servers Intermediary devices Hosts Media [exercice p. 47] Solution n°15 Which two devices are intermediary devices? (Choose two) Hosts Switches 57 Exercise solutions Servers Routers [exercice p. 47] Solution n°16 Which connection physically connects the end device to the network? Port NIC Interface [exercice p. 47] Solution n°17 Which network infrastructure provides access to users and end devices in a small geographical area, which is typically a network in a department in an enterprise, a home, or small business? Extranet Intranet LAN WAN [exercice p. 48] Solution n°18 Which network infrastructure provides access to other networks over a large geographical area, which is often owned and managed by a telecommunications service provider? Extranet Intranet LAN WAN [exercice p. 48] Solution n°19 Which device performs the function of determining the path that messages should take through internetworks? a router a firewall DSL modem web server 58 Exercise solutions [exercice p. 48] Solution n°20 Match the function of a layer in the Internet protocol stack to its its name Application Transport Layer Network Layer Link Layer Physical Layer Layer Transfer of data Delivery of Transfer of Protocols that between one process packets from a data Transfer of a are part of a and another process source host to a between bit into and distributed (typically on different destination host neighboring out of a network hosts). (typically). network transmission application. devices. media. [exercice p. 49] Solution n°21 Match the name of an Internet layer with unit of data that is exchanged among protocol entities at that layer Application layer Transport Layer Network Layer Link Layer Physical Layer Message Segment Packet Frame Bit Datagram [exercice p. 49] Solution n°22 Consider the figure below, showing a link-layer frame heading from a host to a router. There are three header fields shown. Match the name of a header with a header label shown in the figure. 59 Exercise solutions H1 H2 H3 Link Layer Network Layer Transport Layer [exercice p. 50] Solution n°23 Which of the definitions below describe what is meant by the term "encapsulation"? Determining the name of the destination host, translating that name to an IP address and then placing that value in a packet header field. Receiving a “packet” from the layer below, extracting the payload field, and after some internal actions possibly delivering that payload to an upper layer protocol. Starting a transport layer timer for a transmitted segment, and then if an ACK segment isn’t received before the timeout, placing that segment in a retransmission queue Computing the sum of all of the bytes within a packet and placing that value in the packet header field. Taking data from the layer above, adding header fields appropriate for this layer, and then placing the data in the payload field of the “packet” for that layer. [exercice p. 50] Solution n°24 Which of the following descriptions below correspond to a definition of the Internet? Select one or more of the answers below that are correct. A "network of networks". Google and Social media A collection of billions of computing devices, and packet switches interconnected by links. A collection of hardware and software components executing protocols that define the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event. A set of smart connected devices at home and around a person: smart TV, smart watch, connected car,... [exercice p. 50] Solution n°25 Choose one the following two definitions that makes the correct distinction between routing versus forwarding Routing is the local action of moving arriving packets from router’s input link to appropriate router output link, while forwarding is the global action of determining the source-destination paths taken by packets. 60 Exercise solutions Forwarding is the local action of moving arriving packets from router’s input link to appropriate router output link, while routing is the global action of determining the source-destination paths taken by packets. [exercice p. 51] Solution n°26 When we say that the Internet is a “network of networks,” we mean? Check all that apply (hint: check two or more). The Internet is the fastest network ever built. The Internet is made up of a lot of different networks that are interconnected to each other. The Internet is the largest network ever built. The Internet is made up of access networks at the edge, tier-1 networks at the core, and interconnected regional and content provider networks as well. 61
