The Benefits of Using a Layered Model

The benefits to using a layered model to describe network protocols and operations include:

• 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.

As shown in the figure, the TCP/IP model and the Open Systems Interconnection (OSI) model are the primary models used when discussing network functionality. They each represent a basic type of layered networking models:

• Protocol 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.

• Reference model - This type of model provides consistency within all types of network protocols and services by describing what has to be done at a particular layer, but not prescribing how it should be accomplished. The OSI model is a widely known internetwork reference model, but is also a protocol model for the OSI protocol suite.

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Electronics and Communications Standard Organizations

Other standard 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.
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. Figure 1 shows several of the standards related to networking.
Electronic Industries Alliance (EIA) - Best known for its standards related to electrical wiring, connectors, and the 19-inch racks used to mount networking equipment.
Telecommunications Industry Association (TIA) - Responsible for developing communication standards in a variety of areas including radio equipment, cellular towers, Voice over IP (VoIP) devices, satellite communications, and more. Figure 2 shows an example of an Ethernet cable meeting TIA/EIA standards.
International Telecommunications Union-Telecommunication Standardization Sector (ITU-T) - One of the largest and oldest communication standard organizations. The ITU-T defines standards for video compression, Internet Protocol Television (IPTV), and broadband communications, such as a digital subscriber line (DSL).

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Open Standards

Open standards encourage interoperability, competition, and innovation. They also guarantee that no single company’s product 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, DHCP, 802.3 (Ethernet), and 802.11 (Wireless LAN). These open standards also allow a client running Apple’s 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 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.
Standards organizations are usually vendor-neutral, non-profit organizations established to develop and promote the concept of open standards.
Click the logo in the figure to visit the website for each standards organization.

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TCP/IP Communication Process

Figures 1 and 2 demonstrate the complete communication process using an example of a web server transmitting data to a client. This process and these protocols will be covered in more detail in later chapters.
Click the Play button to view the animated demonstrations:
1. In Figure 1, the animation begins with the web server preparing the Hypertext Markup Language (HTML) page as data to be sent.
2. The application protocol HTTP header is added to the front of the HTML data. The header contains various information, including the HTTP version the server is using and a status code indicating it has information for the web client.
3. The HTTP application layer protocol delivers the HTML-formatted web page data to the transport layer. The TCP transport layer protocol is used to manage individual conversations, in this example between the web server and web client.
4. Next, the IP information is added to the front of the TCP information. IP assigns the appropriate source and destination IP addresses. This information is known as an IP packet.
5. The Ethernet protocol adds information to both ends of the IP packet, known as a data link frame. This frame is delivered to the nearest router along the path towards the web client. This router removes the Ethernet information, analyzes the IP packet, determines the best path for the packet, inserts the packet into a new frame, and sends it to the next neighboring router towards the destination. Each router removes and adds new data link information before forwarding the packet.
6. This data is now transported through the internetwork, which consists of media and intermediary devices.
7. In Figure 2, the animation begins with the client receiving the data link frames that contain the data. Each protocol header is processed and then removed in the opposite order it was added. The Ethernet information is processed and removed, followed by the IP protocol information, the TCP information, and finally the HTTP information.
8. The web page information is then passed on to the client’s web browser software.

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Protocol Suites and Industry Standards

A protocol suite is a set of protocols that work together to provide comprehensive network communication services. A protocol suite may be specified by a standards organization or developed by a vendor. Protocol suites, like the four shown in the figure, can be a bit overwhelming. However, this course will only cover the protocols of the TCP/IP protocol suite.
The TCP/IP protocol suite is an open standard, meaning these protocols are freely available to the public, and any vendor is able to implement these protocols on their hardware or in their software.
A standards-based protocol is a process that has been endorsed by the networking industry and approved by a standards organization. The use of standards in developing and implementing protocols ensures that products from different manufacturers can interoperate successfully. If a protocol is not rigidly observed by a particular manufacturer, their equipment or software may not be able to successfully communicate with products made by other manufacturers.
Some protocols are proprietary which means one company or vendor controls the definition of the protocol and how it functions. Examples of proprietary protocols are AppleTalk and Novell Netware, which are legacy protocol suites. It is not uncommon for a vendor (or group of vendors) to develop a proprietary protocol to meet the needs of its customers and later assist in making that proprietary protocol an open standard.
For example, click here to view a video presentation by Bob Metcalfe describing the story of how Ethernet was developed.

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Network Protocols

At the human level, some communication rules are formal and others are simply understood based on custom and practice. For devices to successfully communicate, a network protocol suite must describe precise requirements and interactions. Networking protocols define a common format and set of rules for exchanging messages between devices. Some common networking protocols are Hypertext Transfer Protocol (HTTP), Transmission Control Protocol (TCP), and Internet Protocol (IP).
Note: IP in this course refers to both the IPv4 and IPv6 protocols. IPv6 is the most recent version of IP and the replacement for the more common IPv4.
The figures illustrate networking protocols that describe the following processes:

• How the message is formatted or structured, as shown in Figure 1.

• The process by which networking devices share information about pathways with other networks, as shown in Figure 2.

• How and when error and system messages are passed between devices, as shown in Figure 3.

• The setup and termination of data transfer sessions, as shown in Figure 4.

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Message Delivery Options

A message can be delivered in different ways, as shown in Figure 1. Sometimes, a person wants to communicate information to a single individual. At other times, the person may need to send information to a group of people at the same time, or even to all people in the same area.
There are also times when the sender of a message needs to be sure that the message is delivered successfully to the destination. In these cases, it is necessary for the recipient to return an acknowledgment to the sender. If no acknowledgment is required, the delivery option is referred to as unacknowledged.
Hosts on a network use similar delivery options to communicate, as shown in Figure 2.
A one-to-one delivery option is referred to as a unicast, meaning there is only a single destination for the message.
When a host needs to send messages using a one-to-many delivery option, it is referred to as a multicast. Multicasting is the delivery of the same message to a group of host destinations simultaneously.
If all hosts on the network need to receive the message at the same time, a broadcast may be used. Broadcasting represents a one-to-all message delivery option. Some protocols use a special multicast message that is sent to all devices, making it essentially the same as a broadcast. Additionally, hosts may be required to acknowledge the receipt of some messages while not needing to acknowledge others.

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