The OSI and Tcpip Models
OSI is the Open System Interconnection reference model for communications, developed as a standard by the International Organization for Standardization (ISO). Some participants in OSI's creation and development expected OSI to become the networking protocol used by all applications on all computers in the world. The U.S. government went so far as to require OSI support on every computer it purchased, as of a certain date in the early 1990s, which certainly gave vendors some incentive to write OSI code. Some networking vendors even suggested that by the mid-1990s the TCP/IP installed base would begin to decrease, and eventually go away, with OSI as the protocol from which the 21st century Internet would be built.
Today, several of the OSI model's protocols are still used, and ISO is still a very important standards body. However, the OSI model as a whole never gained widespread market acceptance. Today, the OSI model is mainly used as a point of reference for discussing other protocol specifications.
OSI Layers
The OSI reference model consists of seven layers, each defining a set of typical networking functions. Like TCP/IP, the OSI model consists of several standards unique to OSI, as well as references to other well-known standards from other standards bodies. Figure 24-1 shows the OSI layers, as compared with TCP/IP and Novell NetWare.
Figure 24-1 Comparing OSI, TCP/IP, and NetWare
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OSI |
TCP/IP |
NetWare |
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Application |
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Presentation |
Application |
SAP, NCP |
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Session |
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Transport |
Transport |
SPX |
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Network |
Internet |
IPX |
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Data Link |
Network |
Mac |
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Physical |
Interface |
Protocols |
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Because the OSI reference model does have a well-defined set of functions associated with each of its seven layers, you can examine any other networking protocol or specification, and make some determination as to whether it most closely matches OSI Layer 1, 2, or 3, and so on. For instance, the TCP/IP Internetwork layer (IP) closely equates to the OSI network layer, hence the use of the terms network layer protocol and Layer 3 protocol when referring to IP.
Although most discussions of networking focus on the functions in the lower four layers of the OSI model, the functions at all seven layers are important in most networks today. The three upper layers of the OSI reference model define functions focused on the application, whereas the lower four layers define functions focused on end-to-end delivery of the data. With the advent of switching devices that look at layers above the transport layer header—devices often called either Layer 5-7 switches or Layer 7 switches—network engineers need more knowledge and insight into the higher-layer protocols.
Table 24-2 lists the layers of the OSI model and provides some insights into the functions performed by each layer.
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Layer |
Functional Description |
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7 |
The application layer defines the interface between the communications software and any applications that need to communicate outside the computer on which the application resides. For example, a web browser is an application on a computer. The browser needs to get the contents of a web page; OSI Layer 7 defines the protocols used on behalf of the application to get the web page. |
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6 |
The presentation layer's main purpose is to define data formats, such as ASCII text, EBCDIC text, binary, BCD, and JPEG. Encryption is also defined by OSI as a presentation layer service. For example, FTP enables you to choose binary or ASCII transfer. If binary is selected, the sender and receiver do not modify the contents of the file. If ASCII is chosen, the sender translates the text from the sender's character set to a standard ASCII and sends the data. The receiver translates back from the standard ASCII to the character set used on the receiving computer. |
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Layer |
Functional Description |
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5 |
The session layer defines how to start, control, and end conversations (called sessions). This includes the control and management of multiple bidirectional messages so that the application can be notified if only some of a series of messages are completed. This allows the presentation layer to have a seamless view of an incoming stream of data. The presentation layer can be presented with data if all flows occur in some cases. For example, an automated teller machine transaction in which you withdraw cash from your checking account should not debit your account and then fail before handing you the cash, recording the transaction even though you did not receive money. The session layer creates ways to imply which flows are part of the same session and which flows must complete before any are considered complete. |
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4 |
Transport layer protocols provide a large number of services, as discussed in Chapter 5, "IP Services." Whereas Layers 5 to 7 focus on issues related to the application, Layer 4 focuses on issues related to data delivery to the other computer—for instance, error recovery, segmentation of large application data blocks into smaller ones for transmission, and reassembly of those blocks of data on the receiving computer. |
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3 |
The network layer defines end-to-end delivery of packets. To accomplish this, the network layer defines logical addressing so that any endpoint can be identified. It also defines how routing works and how routes are learned so that the packets can be delivered. Chapter 4, "IP Addressing," examines Layer 3 concepts in detail. The network layer defines most of the details that a Cisco router considers when routing. For example, Layer 3 running in a Cisco router is responsible for examining the destination IP address of a packet, comparing that address to the IP routing table, fragmenting the packet if the outgoing interface requires smaller packets, and queuing the packet to be sent out to the interface. |
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2 |
The data link layer specifications deliver data across one particular link or medium. These protocols are necessarily concerned with the type of media in question; for example, 802.3 and 802.2 define Ethernet for the IEEE, which are referenced by OSI as valid data link layer protocols. Other protocols, such as High-Level Data Link Control (HDLC) for a point-to-point WAN link, deal with the different details of a WAN link. |
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1 |
Physical layer specifications, which are also typically standards from other organizations that are referred to by OSI, deal with the physical characteristics of the transmission medium. Connectors, pins, use of pins, electrical currents, encoding, and light modulation are all part of different physical layer specifications. Multiple specifications are sometimes used to complete all details of the physical layer. For example, RJ-45 defines the shape of the connector and the number of wires or pins in the cable. Ethernet and 802.3 define the use of wires or pins 1, 2, 3, and 6. So, to use a Category 5 cable with an RJ-45 connector for an Ethernet connection, Ethernet and RJ-45 physical layer specifications are used. |
Table 24-3 lists the OSI reference model layers and examples of some of the more popular protocols that most closely match them.
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Layer Name |
Examples |
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Application (Layer 7) |
Telnet, HTTP, FTP, NFS, SMTP gateways (Eudora, Exchange), SNMP |
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Presentation (Layer 6) |
JPEG, ASCII, EBCDIC, TIFF, GIF, PICT, encryption, MPEG, MIDI |
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Session (Layer 5) |
RPC, SQL, NFS, NetBIOS, AppleTalk ASP, DECnet SCP, RADIUS, TACACS+, RTP |
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Transport (Layer 4) |
TCP, UDP, SPX |
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Network (Layer 3) |
IP, IPX, AppleTalk DDP |
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Data link (Layer 2) |
IEEE 802.3/802.2, HDLC, Frame Relay, PPP, FDDI, ATM, IEEE 802.5/802.2 |
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Physical (Layer 1) |
EIA/TIA-232, V.35, EIA/TIA-449, RJ45, Ethernet, 802.3, 802.5, B8ZS, T1, E1 |
The most popular internetworking forwarding devices can also be considered to be operating at a particular layer. The following is a list of devices and the highest layer at which they operate.
■ Multilayer switch: Layer 3
■ Content switch: Layers 4-7
OSI Layering Concepts and Benefits
Many benefits can be gained by defining protocol functions in layers, and then defining standard interfaces between those layers. The layers break a large, complex set of concepts and protocols into smaller pieces, making the complex set easier to talk about, easier to implement with hardware and software, and easier to troubleshoot. The following list summarizes the benefits of layered protocol specifications:
■ Easier to learn—Humans can more easily discuss and learn about the many details of a protocol specification.
■ Easier to develop—Reduced complexity allows easier program changes and faster product development.
■ Multivendor interoperability—By creating products to meet the same networking standards, computers and networking gear from multiple vendors can work in the same network.
■ Modular engineering—One vendor can write software that implements higher layers—for example, a web browser—and another vendor can write software that implements the lower layers—for example, Microsoft's built-in TCP/IP software in its operating systems.
OSI Terminology
OSI includes many terms not used by other protocol architectures. Many of you have memorized the names of the layers simply through repetition. However, if you find mnemonic devices useful for memorizing such details, the following list shows four 7-word phrases whose first letters correspond to the first letters of the names of the OSI layers:
■ All People Seem To Need Data Processing (Layer 7 to 1)
■ Please Do Not Take Sausage Pizzas Away (Layer 1 to 7)
■ Pew! Dead Ninja Turtles Smell Particularly Awful (Layer 1 to 7)
■ Please Do Not Tell Sales People Anything (Layer 1 to 7)
Because the OSI layers' names and numbers are used so pervasively throughout networking terminology, it is important to memorize the numbers and names of each layer. Additionally, it is important to know which layers in a non-OSI protocol architecture best match the OSI layers. Figure 24-2 depicts these details for TCP/IP, along with some of the typical TCP/IP protocols at each layer.
Figure 24-2 Using OSI Layers to Reference Other Protocols
OSI Model
KEY POINT
TCP/IP Model
Figure 24-2 Using OSI Layers to Reference Other Protocols
OSI Model
TCP/IP Model
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Application |
Application |
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Presentation |
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Session |
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Transport |
Transport |
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Network |
Internet |
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Data Link |
Network Interface |
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Physical |
TCP/IP Protocols
HTTP, SMTP, POP3
TCP, UDP
Ethernet, Frame Relay, PPP
As you can see, not all TCP/IP layers correspond to a single OSI layer, and vice versa. For instance, the TCP/IP network interface layer defines both the physical network specifications and the protocols used to control the physical network. (Note that the term network access is sometimes used to describe the lowest TCP/IP layer instead of the term network interface.) OSI separates the physical network specifications into the physical layer and the control functions into the data link layer. (In fact, many people consider TCP/IP to actually have a separate physical and data link layer.)
The term protocol data unit (PDU) is an OSI term that makes generic reference to some data structure used when sending data over a network. A PDU represents the bits that include the headers and trailers for some layer, as well as the encapsulated data. For instance, an IP packet is a PDU. In that case, it is called a Layer 3 PDU (L3PDU) by virtue of IP being a Layer 3 protocol.
Figure 24-3 represents the encapsulation process used by OSI on a computer when sending data. The figure points out the names of the PDUs at each layer.
Figure 24-3 OSI Encapsulation and Protocol Data Units
Figure 24-3 OSI Encapsulation and Protocol Data Units
OSI Layer Interactions
Every layered networking architecture, like OSI or TCP/IP, is implemented by a series of protocols. Part of the definition of a protocol defines how that protocol works between multiple devices. Other parts of the protocol define that protocol's interface to another protocol running on the same computer. For example, consider the HTTP protocol. Figure 24-4 shows the simple interaction of HTTP on two computers (Bob and Larry), with Bob retrieving a web page from web server Larry.
Figure 24-4 Basic Application Logic to Get a Web Page
Figure 24-4 Basic Application Logic to Get a Web Page
Figure 24-4 shows an example of same-layer interaction. The HTTP protocol is a TCP/IP application layer protocol. The HTTP software on each computer in the figure sits at the same layer, and the computers interact by exchanging messages and data—hence the term same-layer interaction.
Next, consider how adjacent layers on the same computer work together, with each layer providing a service to the layer above it. For example, in Figure 24-4, Bob and Larry use HTTP to transfer the home page from Larry to Bob. But what would happen if Bob's HTTP Get request was lost in transit through the TCP/IP network? Or what would happen if Larry's response, which includes the contents of the home page, was lost? Well, the page would not show up in Bob's browser, as you might expect. So, HTTP uses a protocol from its lower adjacent layer (TCP at the transport layer) to provide a service of guaranteed delivery across the network, as shown in Figure 24-5.
Figure 24-5 Example of Adjacent-Layer Interaction: TCP and HTTP
Web Server Larry
Figure 24-5 Example of Adjacent-Layer Interaction: TCP and HTTP
Web Server Larry
- Web Browser Bob
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TCP |
HTTP OK |
Web Page |
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This example outlines the concepts of how protocols in adjacent layers, and on the same computer, work together. In this case, the higher-layer protocol (HTTP) needed to do something it could not do (error recovery), so it asked the next-lower-layer protocol (TCP) for help. TCP provided a guaranteed delivery service to HTTP. The interaction between these adjacent layers is aptly called adjacent-layer interaction. Table 24-4 summarizes the key points of same-layer interaction and adjacent-layer interaction. Table 24-4 Summary: Same-Layer and Adjacent-Layer Interactions Table 24-4 Summary: Same-Layer and Adjacent-Layer Interactions KEY POINT
KEY POINT |
Continue reading here: CCIE Routing and Switching Exam Updates Version
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