234Protocols - ch02 Page 13 Monday, June 2, 2003 3:02 PM 2...

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13 2 Understanding Layer 2, 3, and 4 Protocols hile many of the concepts well known to traditional Layer 2 and Layer 3 net- working still hold true in content switching applications, the area introduces new and more complex themes that need to be well understood for any success- ful implementation. Within the discussion of content networking, we will replace terms such as packets and frames with sessions and transactions as we move our attention further up the OSI Seven Layer Model. Before we move into these new terms, however, let’s look at some standard Layer 2, 3, and 4 network- ing concepts. The OSI Seven Layer Model—What Is a Layer? Established in 1947, the International Organization for Standardization (ISO) was formed to bring together the standards bodies from countries around the world. Their deFnition of the model for Open Systems Interconnection, or OSI, is used to deFne modes of interconnection between different components in a networking system. This means that the physical method of transport can be designed independently of the protocols and applications running over it. ±or example, TCP/IP can be run over both Ethernet and ±DDI networks, and Novell’s IPX and Apple’s AppleTalk protocols can both be run over Token Ring networks. These are examples of having independence between the physical net- work type and the upper layer protocols running across them. Consider also, two TCP/IP-enabled end systems communicating across a multitude of different W ch02 Page 13 Monday, June 2, 2003 3:02 PM
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14 Chapter 2 U NDERSTANDING L AYER 2, 3, AND 4 P ROTOCOLS network types, such as Ethernet, Frame Relay, and ATM. Figure 2–1 shows the OSI Seven Layer Model. When we talk about Layer 2 and Layer 3 networking, it is these layers that we’re referring to, and logically the further up the OSI model we move, the greater intelligence we can use in networking decisions. Each layer plays its part in moving data from one device to another across a net- work infrastructure by providing a standard interface to the surrounding layers. The Application Layer (Layer 7) The top layer in the stack, the Application layer is where the end-user applica- tion resides. Think of the Application layer as the browser application or email client for a user sur±ng the Web or sending email. Many protocols are de±ned for use at the Application layer, such as HTTP, FTP, SMTP, and Telnet. In content switching terms, Layer 7 refers to the ability to parse information directly generated by the user or application in decision making, such as the URL typed by the user in the Web browser. For example, http://www.foocorp.com is an example of Application layer data. The Presentation Layer (Layer 6) The Presentation layer is used to provide a common way for applications (resid- ing at the Application layer) to translate between data formats or perform encryption and decryption. Mechanisms to convert between text formats such as ASCII and Unicode may be considered part of the Presentation layer, along with compression techniques for image ±les such as GIF and JPEG.
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234Protocols - ch02 Page 13 Monday, June 2, 2003 3:02 PM 2...

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