Appendix C

OSI Model Relationship with Switches

About this appendix

This appendix introduces the Open Systems Interconnection (OSI) Reference architectural model to briefly provide background information about the two types of switches that are discussed in this guide--the Layer 4 (L4) and the Layer 7 (L7) switches. These two switches function at different layers of the OSI model and, as a result, have some different capabilities.

What is the OSI model?

The OSI model was developed by the International Standards Organization. It is used to describe the structure and function of data communications protocols. The OSI model contains seven layers that define the functions of communications protocols, as shown in the following illustration.

TCP/IP does not implement Layer 5 or Layer 6 of the OSI model. It is implemented on top of Layer 4.

Layer 4 switch

An L4 switch operates at Layer 4, the Transport layer of the OSI model. An L4 switch looks at port numbers in a TCP/IP packet and, based only on the port number, passes port 80 (HTTP), port 119 (NNTP), port 1755 (MMS), port 554 (RTSP), port 21 (FTP), and port 53 (DNS) traffic to one of the NetCache appliances that it has been configured to be aware of. For Web caching and streaming media caching, an L4 switch can be configured to pass non-HTTP traffic directly to the Web.

These switches are called Layer 4 devices because they base their switching decisions on information in the TCP header, and TCP is a protocol that resides at Layer 4 in the OSI seven-layer model.

Layer 7 switch

An L7 switch operates at Layer 7, the Application layer, of the OSI model. The Application layer includes all processes that use the Transport layer (L4) protocols to deliver data. The most widely known and implemented protocols at the Application layer are Telnet, FTP, SMTP, and HTTP. Because these switches operate at Layer 7, they can understand URLs and can understand much more about the traffic than an L4 switch can.

An L7 switch provides the same features that an L4 switch provides, plus additional, more sophisticated features. Whereas an L4 switch can only examine port numbers in a TCP/IP packet, an L7 switch examines the port number in a TCP/IP packet to determine if it needs to examine the packet further.

For HTTP requests (typically port 80), the L7 switch examines the request and determines whether the object is cacheable. This capability allows you to set up your switch so that requests for obviously uncacheable objects, such as CGI and URLs with cookies, can bypass the cache completely. Uncacheable objects are obtained directly from a Web server.