2101-Ch03.docx

Networks vary in size, shape, and function. They can be as complex as devices connected across the internet, or as simple as two computers directly connected to one another with a single cable, and anything in-between. However, simply having a wired or wireless physical connection between end device...

Networks vary in size, shape, and function. They can be as complex as devices connected across the internet, or as simple as two computers directly connected to one another with a single cable, and anything in-between. However, simply having a wired or wireless physical connection between end devices is not enough to enable communication. For communication to occur, devices must know "how" to communicate. People exchange ideas using many different communication methods. However, all communication methods have the following three elements in common: - **Message source (sender)** - Message sources are people, or electronic devices, that need to send a message to other individuals or devices. - **Message Destination (receiver)** - The destination receives the message and interprets it. - **Channel** - This consists of the media that provides the pathway over which the message travels from source to destination. Sending a message, whether by face-to-face communication or over a network, is governed by rules called protocols. These protocols are specific to the type of communication method being used. In our day-to-day personal communication, the rules we use to communicate over one medium, like a telephone call, are not necessarily the same as the rules for using another medium, such as sending a letter. The process of sending a letter is similar to communication that occurs in computer networks. Protocols must account for the following requirements to successfully deliver a message that is understood by the receiver: - An identified sender and receiver - Common language and grammar - Speed and timing of delivery - Confirmation or acknowledgment requirements The protocols that are used in network communications share many of these fundamental traits. 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 One of the first steps to sending a message is encoding. Encoding is the process of converting information into another acceptable form, for transmission. Decoding reverses this process to interpret the information. 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. Message timing is also very important in network communications. Message timing includes the following: - **Flow Control** - This is the process of managing the rate of data transmission. Flow control defines how much information can be sent and the speed at which it can be delivered. For example, if one person speaks too quickly, it may be difficult for the receiver to hear and understand the message. In network communication, there are network protocols used by the source and destination devices to negotiate and manage the flow of information. - **Response Timeout** - If a person asks a question and does not hear a response within an acceptable amount of time, the person assumes that no answer is coming and reacts accordingly. The person may repeat the question or instead, may go on with the conversation. Hosts on the network use network protocols that specify how long to wait for responses and what action to take if a response timeout occurs. - **Access method** - This determines when someone can send a message. Click Play in the figure to see an animation of two people talking at the same time, then a \"collision of information\" occurs, and it is necessary for the two to back off and start again. Likewise, when a device wants to transmit on a wireless LAN, it is necessary for the WLAN network interface card (NIC) to determine whether the wireless medium is available. You know that for end devices to be able to communicate over a network, each device must abide by the same set of rules. These rules are called protocols and they have many functions in a network. This topic gives you a overview of network protocols. 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. **Network Communication Protocols** These protocols govern how data is transmitted and received across a network. Common examples include: - **TCP (Transmission Control Protocol)**: Ensures reliable, ordered, and error-checked delivery of data between applications. - [**UDP (User Datagram Protocol)**: Provides a faster, but less reliable, transmission method without error-checking^1^](https://learningnetwork.cisco.com/s/blogs/a0D3i000002SKPyEAO/communication-protocols). - HTTP **Network Security Protocols** These protocols protect data integrity, confidentiality, and availability across the network. Examples include: - [**SSL/TLS (Secure Sockets Layer/Transport Layer Security)**: Encrypts data transmitted over the internet to ensure privacy and data integrity^2^](https://www.cisco.com/c/en/us/products/security/what-is-network-security.html). - SSH **Routing Protocols** These protocols determine the best path for data to travel across a network. Examples include: - **OSPF (Open Shortest Path First)**: A link-state routing protocol that uses the shortest path first algorithm. - [**BGP (Border Gateway Protocol)**: Manages how packets are routed across the internet through the exchange of routing and reachability information between edge routers^3^](https://community.cisco.com/t5/networking-knowledge-base/dynamic-routing-protocols-ospf-eigrp-ripv2-is-is-bgp/ta-p/4511577)[^4^](https://www.cisco.com/c/en/us/td/docs/net_mgmt/prime/network/3-8/reference/guide/routpro.html). **Service Discovery Protocols** These protocols help network devices discover each other and the services they offer. Examples include: - [**CDP (Cisco Discovery Protocol)**:](https://www.cisco.com/c/en/us/td/docs/net_mgmt/prime/network/3-8/reference/guide/discover.html) - DHCP - DNS 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. One of the best ways to visualize how the protocols within a suite interact is to view the interaction as a stack. A protocol stack shows how the individual protocols within a suite are implemented. The protocols are viewed in terms of layers, with each higher-level service depending on the functionality defined by the protocols shown in the lower levels. The lower layers of the stack are concerned with moving data over the network and providing services to the upper layers, which are focused on the content of the message being sent. As illustrated in the figure, we can use layers to describe the activity occurring in face-to-face communication. At the bottom is the physical layer where we have two people with voices saying words out loud. In the middle is the rules layer that stipulates the requirements of communication including that a common language must be chosen. At the top is the content layer and this is where the content of the communication is actually spoken. A protocol suite is a set of protocols that work together to provide comprehensive network communication services. Since the 1970s there have been several different protocol suites, some developed by a standards organization and others developed by various vendors. During the evolution of network communications and the internet there were several competing protocol suites - ***Internet Protocol Suite or TCP/IP** - This is the most common and relevant protocol suite used today. The TCP/IP protocol suite is an open standard protocol suite maintained by the Internet Engineering Task Force (IETF).* - ***Open Systems Interconnection (OSI) protocols** - This is a family of protocols developed jointly in 1977 by the International Organization for Standardization (ISO) and the International Telecommunications Union (ITU). The OSI protocol also included a seven-layer model called the OSI reference model. The OSI reference model categorizes the functions of its protocols. Today OSI is mainly known for its layered model. The OSI protocols have largely been replaced by TCP/IP.* - ***AppleTalk** - A short-lived proprietary protocol suite released by Apple Inc. in 1985 for Apple devices. In 1995, Apple adopted TCP/IP to replace AppleTalk.* - ***Novell NetWare** - A short-lived proprietary protocol suite and network operating system developed by Novell Inc. in 1983 using the IPX network protocol. In 1995, Novell adopted TCP/IP to replace IPX.* TCP/IP protocols are available for the application, transport, and internet layers. There are no TCP/IP protocols in the network access layer. The most common network access layer LAN protocols are Ethernet and WLAN (wireless LAN) protocols. Network access layer protocols are responsible for delivering the IP packet over the physical medium. 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 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.* Open standards encourage interoperability, competition, and innovation. They also guarantee that the product of no single company can monopolize the market or have an unfair advantage over its competition. A good example of this is when purchasing a wireless router for the home. There are many different choices available from a variety of vendors, all of which incorporate standard protocols such as IPv4, IPv6, DHCP, SLAAC, Ethernet, and 802.11 Wireless LAN. These open standards also allow a client running the Apple OS X operating system to download a web page from a web server running the Linux operating system. This is because both operating systems implement the open standard protocols, such as those in the TCP/IP protocol suite. Standards organizations are usually vendor-neutral, non-profit organizations established to develop and promote the concept of open standards. These organizations are important in maintaining an open internet with freely accessible specifications and protocols that can be implemented by any vendor. A standards organization may draft a set of rules entirely on its own or, in other cases, may select a proprietary protocol as the basis for the standard. If a proprietary protocol is used, it usually involves the vendor who created the protocol. - ***Internet Society (ISOC)** - Responsible for promoting the open development and evolution of internet use throughout the world.* - ***Internet Architecture Board (IAB)** - Responsible for the overall management and development of internet standards.* - ***Internet Engineering Task Force (IETF)**- Develops, updates, and maintains internet and TCP/IP technologies. This includes the process and documents for developing new protocols and updating existing protocols, which are known as Request for Comments (RFC) documents.* - ***Internet Research Task Force (IRTF)**- Focused on long-term research related to internet and TCP/IP protocols such as Anti-Spam Research Group (ASRG), Crypto Forum Research Group (CFRG), and Peer-to-Peer Research Group (P2PRG).* - ***Internet Corporation for Assigned Names and Numbers (ICANN)**- Based in the United States, ICANN coordinates IP address allocation, the management of domain names, and assignment of other information used in TCP/IP protocols.* - ***Internet Assigned Numbers Authority (IANA)**- Responsible for overseeing and managing IP address allocation, domain name management, and protocol identifiers for ICANN.* Other standards organizations have responsibilities for promoting and creating the electronic and communication standards used to deliver the IP packets as electronic signals over a wired or wireless medium. These standard organizations include the following: - **Institute of Electrical and Electronics Engineers**(**IEEE**, pronounced "I-triple-E") - Organization of electrical engineering and electronics dedicated to advancing technological innovation and creating standards in a wide area of industries including power and energy, healthcare, telecommunications, and networking. Important IEEE networking standards include 802.3 Ethernet and 802.11 WLAN standard. Search the internet for other IEEE network standards. - **Electronic Industries Alliance (EIA)** - Organization is best known for its standards relating to electrical wiring, connectors, and the 19-inch racks used to mount networking equipment. - **Telecommunications Industry Association (TIA)** - Organization responsible for developing communication standards in a variety of areas including radio equipment, cellular towers, Voice over IP (VoIP) devices, satellite communications, and more. - **International Telecommunications Union-Telecommunication Standardization Sector (ITU-T)** - One of the largest and oldest communication standards organizations. The ITU-T defines standards for video compression, Internet Protocol Television (IPTV), and broadband communications, such as a digital subscriber line (DSL). You cannot actually watch real packets travel across a real network, the way you can watch the components of a car being put together on an assembly line. so, it helps to have a way of thinking about a network so that you can imagine what is happening. A model is useful in these situations. Complex concepts such as how a network operates can be difficult to explain and understand. For this reason, a layered model is used to modularize the operations of a network into manageable layers. 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 there are two layered models that are used to describe network operations: - Open System Interconnection (OSI) Reference Model - TCP/IP Reference 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. It also describes the interaction of each layer with the layers directly above and below. The TCP/IP protocols discussed in this course are structured around both the OSI and TCP/IP models. The table shows details about each layer of the OSI model. The functionality of each layer and the relationship between layers will become more evident throughout this course as the protocols are discussed in more detail. **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. 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. The table shows details about each layer of the TCP/IP model. 1. **Application Layer**: This is the topmost layer where high-level protocols operate. It provides services for network applications, such as HTTP for web browsing, FTP for file transfer, and SMTP for email. 2. **Transport Layer**: This layer is responsible for host-to-host communication and data transfer. It ensures reliable data transmission with error checking and flow control. Key protocols include TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). 3. **Internet Layer**: This layer handles the routing of data packets across networks. It provides logical addressing and determines the best path for data to travel. The main protocol here is IP (Internet Protocol). 4. **Network Access Layer**: deals with the physical transmission of data over a network. It includes protocols for communication within a single network segment, such as Ethernet and Wi-Fi. OSI TCP/IP comparison *The key similarities are in the transport and network layers; however, the two models differ in how they relate to the layers above and below each layer:* - *OSI Layer 3, the network layer, maps directly to the TCP/IP internet layer. This layer is used to describe protocols that address and route messages through an internetwork.* - *OSI Layer 4, the transport layer, maps directly to the TCP/IP transport layer. This layer describes general services and functions that provide ordered and reliable delivery of data between source and destination hosts.* - *The TCP/IP application layer includes several protocols that provide specific functionality to a variety of end user applications. The OSI model Layers 5, 6, and 7 are used as references for application software developers and vendors to produce applications that operate on networks.* - *Both the TCP/IP and OSI models are commonly used when referring to protocols at various layers. Because the OSI model separates the data link layer from the physical layer, it is commonly used when referring to these lower layers.* Knowing the OSI reference model and the TCP/IP protocol model will come in handy when you learn about how data is encapsulated as it moves across a network. It is not as simple as a physical letter being sent through the mail system. In theory, a single communication, such as a video or an email message with many large attachments, could be sent across a network from a source to a destination as one massive, uninterrupted stream of bits. However, this would create problems for other devices needing to use the same communication channels or links. These large streams of data would result in significant delays. Further, if any link in the interconnected network infrastructure failed during the transmission, the complete message would be lost and would have to be retransmitted in full. A better approach is to divide the data into smaller, more manageable pieces to send over the network. Segmentation is the process of dividing a stream of data into smaller units for transmissions over the network. Segmentation is necessary because data networks use the TCP/IP protocol suite send data in individual IP packets. Each packet is sent separately, similar to sending a long letter as a series of individual postcards. Packets containing segments for the same destination can be sent over different paths. This leads to segmenting messages having two primary benefits: - **Increases speed** - Because a large data stream is segmented into packets, large amounts of data can be sent over the network without tying up a communications link. This allows many different conversations to be interleaved on the network called multiplexing. - **Increases efficiency** -If a single segment is fails to reach its destination due to a failure in the network or network congestion, only that segment needs to be retransmitted instead of resending the entire data stream. The challenge to using segmentation and multiplexing to transmit messages across a network is the level of complexity that is added to the process. Imagine if you had to send a 100-page letter, but each envelope could only hold one page. Therefore, 100 envelopes would be required and each envelope would need to be addressed individually. It is possible that the 100-page letter in 100 different envelopes arrives out-of-order. Consequently, the information in the envelope would need to include a sequence number to ensure that the receiver could reassemble the pages in the proper order. In network communications, each segment of the message must go through a similar process to ensure that it gets to the correct destination and can be reassembled into the content of the original message, as shown in the figure. TCP is responsible for sequencing the individual segments. As application data is passed down the protocol stack on its way to be transmitted across the network media, various protocol information is added at each level. This is known as the encapsulation process. **Note:** Although the UDP PDU is called datagram, IP packets are sometimes also referred to as IP datagrams. The form that a piece of data takes at any layer is called a protocol data unit (PDU). During encapsulation, each succeeding layer encapsulates the PDU that it receives from the layer above in accordance with the protocol being used. At each stage of the process, a PDU has a different name to reflect its new functions. Although there is no universal naming convention for PDUs, in this course, the PDUs are named according to the protocols of the TCP/IP suite. - *Data - The general term for the PDU used at the application layer* - *Segment - Transport layer PDU* - *Packet - Network layer PDU* - *Frame - Data Link layer PDU* - *Bits - Physical layer PDU used when physically transmitting data over the medium* ***Note:** If the Transport header is TCP, then it is a segment. If the Transport header is UDP then it is a datagram.* When messages are being sent on a network, the encapsulation process works from top to bottom. At each layer, the upper layer information is considered data within the encapsulated protocol. For example, the TCP segment is considered data within the IP packet. This process is reversed at the receiving host and is known as de-encapsulation. De-encapsulation is the process used by a receiving device to remove one or more of the protocol headers. The data is de-encapsulated as it moves up the stack toward the end-user application. As you just learned, it is necessary to segment messages in a network. But those segmented messages will not go anywhere if they are not addressed properly. This topic gives an overview of network addresses. You will also get the chance to use the Wireshark tool, which will help you to 'view' network traffic. The network and data link layers are responsible for delivering the data from the source device to the destination device. As shown in the figure, protocols at both layers contain a source and destination address, but their addresses have different purposes: - **Network layer source and destination addresses** - Responsible for delivering the IP packet from the original source to the final destination, which may be on the same network or a remote network. - **Data link layer source and destination addresses** - Responsible for delivering the data link frame from one network interface card (NIC) to another NIC on the same network. 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 he 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. The IP addresses indicate the original source IP address and final destination IP address. This is true whether the source and destination are on the same IP network or different IP networks. An IP address contains two parts: - **Network portion (IPv4) or Prefix (IPv6)** - The left-most part of the address that indicates the network in which the IP address is a member. All devices on the same network will have the same network portion of the address. - **Host portion (IPv4) or Interface ID (IPv6)** - The remaining part of the address that identifies a specific device on the network. This portion is unique for each device or interface on the network. **Note:** The subnet mask (IPv4) or prefix-length (IPv6) is used to identify the network portion of an IP address from the host portion. When the sender of the packet is on a different network from the receiver, the source and destination IP addresses will represent hosts on different networks. This will be indicated by the network portion of the IP address of the destination host. - **Source IPv4 address** - The IPv4 address of the sending device, the client computer - **Destination IPv4 address** - The IPv4 address of the receiving device, the server, Web Server: Notice in the figure that the network portion of the source IPv4 address and destination IPv4 address are on different networks. the data link Layer 2 physical address has a different role. The purpose of the data link address is to deliver the data link frame from one network interface to another network interface on the same network. Before an IP packet can be sent over a wired or wireless network, it must be encapsulated in a data link frame, so it can be transmitted over the physical medium. As the IP packet travels from host-to-router, router-to-router, and finally router-to-host, at each point along the way the IP packet is encapsulated in a new data link frame. Each data link frame contains the source data link address of the NIC card sending the frame, and the destination data link address of the NIC card receiving the frame. The Layer 2, data link protocol is only used to deliver the packet from NIC-to-NIC on the same network. The router removes the Layer 2 information as it is received on one NIC and adds new data link information before forwarding out the exit NIC on its way towards the final destination. The IP packet is encapsulated in a data link frame that contains the following data link information: - **Source data link address** - The physical address of the NIC that is sending the data link frame. - **Destination data link address** - The physical address of the NIC that is receiving the data link frame. This address is either the next hop router or the address of the final destination device.