The OSI model is a layered model that has been standardized for defining network communications. The OSI model breaks the complex process of network communications into seven distinct layers, each with it own distinct responsibilities. As shown in Figure 3-1, the seven layers of the OSI model are as follows:
• The application layer (Layer 7) Primarily responsible for interfacing with the end user
• The presentation layer (Layer 6) Primarily responsible for translating the data from something the user understands into something the network understands and vice versa
• The session layer (Layer 5) Primarily responsible for dialog and session control functions between systems
• The transport layer (Layer 4) Primarily responsible for the formatting and handling of the transport of data between systems
• The network layer (Layer 3) Primarily responsible for logical addressing
• The data link layer (Layer 2) Primarily responsible for physical addressing
• The physical layer (Layer 1) Primarily responsible for the physical transport of the data on the network
Figure 3-1. Layers of the OSI Model
Rather than focusing on detailing explicitly how communications occur, either in total or in each layer, the OSI model merely defines what needs to occur, and what each host attempting to communicate should be able to expect in the communications process. After this concept of what needs to occur has been defined, protocols, applications, or services can then be designed and implemented to handle the details of how the process occurs.
The application layer provides the user access to network resources via network-aware applications. The application layer handles identifying and establishing that network resources are available and displays the data that is presented from the network in a format that is understandable to the end user.
Not all applications are defined at the application layer, only network-aware applications. For example, Microsoft Word is not a network-aware application and therefore is not really defined at the application layer. Web browsers, on the other hand, are network aware and therefore are defined at the application layer. Some common application layer protocols, services, and applications are as follows:
• Messaging gateways Post Office Protocol (POP3), Simple Mail Transfer Protocol (SMTP), and x.400 e-mail gateways are used to deliver messaging data between systems.
• Newsgroup, instant messaging and Internet Relay Chat (IRC) protocol applications Applications such as Forte Agent or Microsoft Messenger are used to communicate between systems using protocols such as Network News Transport Protocol (NNTP).
• WWW applications Applications such as Firefox, Microsoft Internet Explorer, Apache Web Server, and Internet Information Services provide web-based access to and from resources.
The presentation layer is responsible for presenting data to/from the application and session layers in a format that is understood by the respective layer. Therefore, the presentation layer is frequently referred to as the "translator" of the network. The presentation layer also handles encryption (not to be confused with network encryption such as IPsec or application encryption such as Pretty Good Privacy [PGP]) and protocol-conversion functionality. Some common protocols at the presentation layer are as follows:
• Graphics formats Formats that handle the display and presentation of graphical data such as Joint Photographic Experts Group (JPEG), Graphics Interface Format (GIF), and Bitmap (BMP)
• Sound and movie formats Formats such as Windows Media File (WMF), Digital Video Express (DiVX), and Moving Pictures Experts Group Layer-3 Audio (MP3) provide a means to translate and present sound and audio files across the network.
• Network redirectors Handles protocol conversion for data from the application to the corresponding network format through the use of protocols such as Server Message Block (SMB) and Netware Core Protocol (NCP).
The session layer is responsible for the establishment, maintenance, and teardown of communications channels that allow systems to differentiate network data that is received. The reason for this is that a network host may be communicating with multiple remote systems using multiple applications. Sessions allow the host to identify the data that belongs to a specific application or host, ensuring that data is not inadvertently delivered to the wrong application or remote host. Some examples of session layer protocols are as follows:
• Remote procedure calls A client/server redirection mechanism for requesting data from and executing procedures on a remote system (the server) from a requesting system (the client).
• NetBIOS An application programming interface (API) typically used on Microsoft systems to provide for remote network access to resources and data.
• Structured Query Language (SQL) SQL provides the mechanisms and methods for connecting to, querying and retrieving remote data, typically from a database.
The transport layer is primarily responsible for the formatting and handling of the transport of data in a transparent manner. The transport layer provides an application independent method of delivering data across the network while doing so in such a manner as to ensure that the data can be properly put back together on the receiving end. This process is known as segmentation and reassembly, and in fact the data that is received from the higher layers are known as segments. Some examples of transport layer protocols are TCP and UDP, both of which are defined in greater detail later in this chapter.
The network layer is responsible for the logical addressing and routing of data, known as packets at this point, across the network. This allows two hosts to communicate with each other regardless of physical location or direct connectivity by using logical addresses that have a global significance. Two common protocols that reside at the network layer are these:
• Internet Protocol (IP) IP uses a hierarchal addressing scheme to identify hosts regardless of physical location. Because IP is hierarchal in nature, using subnets to define hosts that are local to each other, it scales to be able to provide a global addressing scheme and has become the de facto method of logical addressing across the Internet as well as within most organizations.
• Internetwork Packet Exchange (IPX) IPX is used primarily on legacy Novell networks. IPX provides for logical addressing through the use of network and host addresses.
The data link layer is responsible for the physical addressing of data, known as frames, across the network. Whereas logical addresses have a global significance and can be used to identify hosts regardless of physical proximity, physical addresses are used to differentiate between hosts that are able to receive the same electrical signal on the wire.
In addition to physical addressing, the data link layer also ensures the error free delivery of data through the use of a cyclic redundancy check (CRC) to ensure that the data that is received is the same data that was transmitted. Some common protocols that exist at the data link layer are as follows:
• Institute of Electrical and Electronics Engineers (IEEE)802.2 This protocol defines the interface between the network layer and the underlying network architecture. IEEE 802.2 is sometimes referred to as the logical link control (LLC) sublayer of the data link layer.
• IEEE 802.3 This protocol defines how the frames are transmitted and received on the physical media and defines the physical addressing that will be used to identify hosts. IEEE 802.3 is sometimes referred to as the MAC sublayer of the data link layers because it controls how the data will be transmitted on the media.
The physical layer is primarily responsible for the physical transmission of the data, generating the electric signals or pulses of light that contain the bits of data to be transmitted. The physical layer handles things such as the modulation of the data and how the hosts will access the media itself. Some examples of physical layer protocols are as follows:
• 10BASE-T 10BASE-T is a form of Ethernet communications across twisted pair cables at 10 Mbps.
• 100BASE-TX 100BASE-TX is similar to 10BASE-T but defines the communications of Ethernet at 100 Mbps, typically using Category 5 or greater twisted-pair cabling.
Although it is important to understand what processes and functions occur at each layer, the OSI model has no real value without understanding the process of encapsulation. Encapsulation is the process of taking the data received from a higher layer, adding the appropriate data and information for the current layer, and then passing the modified data down to the next layer. This process is repeated as the data passes down the OSI model and is eventually transmitted across the network. For the receiving host to be able to process the data it receives properly, it reverses this process, removing the data specific to each layer and passing the remaining data up to the next layer.
Figure 3-2 illustrates the encapsulation process of the OSI model. As the data from the application on the source host is defined it begins the process of being transmitted across the network. At the application, presentation and session layer, the data is manipulated and formatted in a manner that will be transmitted across the network. At the transport layer, the upper-layer data is encapsulated with the appropriate transport header information, (for example, the TCP header), creating a protocol data unit (PDU) known as a segment. The segment is then passed down to the network layer, where it is encapsulated with the network layer header information, such as the IP header, creating a PDU known as a packet. The packet is passed down to the data link layer, where data link header and footer information (the frame check sequence [FCS]) encapsulates the packet to create a PDU known as a frame. The frame is then passed down to the physical layer, where it is turned into the 1s and 0s that will be electronically transmitted across the network media.
Figure 3-2. Encapsulation Process and OSI
Figure 3-2. Encapsulation Process and OSI
The encapsulation process allows each layer on one host to logically communicate directly with the corresponding layer on the other host, while at the same time providing the means for each host to know what to do next with the data (passing it up or down the communications stack to the next layer as appropriate). So, for all intents and purposes, the transport layer of the transmitting host is directly communicating with the transport layer of the receiving host, because the decapsulation process has removed all the lower-layer data by the time the transport layer sees it. From the perspective of the transport layer on the destination host, it merely has a segment of data that needs to be processed accordingly. Figure 3-3 depicts this process.
Figure 3-3. Logical Communication Between Layers
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