Web OS 10.0 Application GuideChapter 15: Content Intelligent Switching n 373212777-A, February 2002Parsing ContentExamining session content places heavier demands upon the Web switch than examiningTCP/IP headers for the following reasons: Content is non-deterministic. Content identifiers such as URLs and cookies can be ofvarying lengths and can appear at unpredictable locations within a request. Scanning ses-sion traffic for a specific string is far more processor-intensive than looking at a knownlocation in a session for a specific number of bytes. To parse a content request, the Web switch must temporarily terminate the TCP connec-tion from the client. This temporary termination is called a delayed binding. While theconnection is suspended, the Web switch acknowledges the client connection on behalf ofthe server, buffers and examines the client request, and finally opens a connection to anappropriate server based on the requested content.For more information on delayed binding, see “Delayed Binding” on page 146. Delayed binding causes two independent TCP connections to span a Web session: onefrom the client to the Web switch and the second from the Web switch to the selectedserver. The Web switch must modify the TCP header, including performing TCP sequencenumber translation and recalculating checksums on every packet that travels between theclient and the server, for the duration of the session. This function, known as TCP connec-tion splicing, heavily tasks a Web switch, particularly when the switch must process thou-sands of these sessions simultaneously. In addition to real-time traffic and connection processing, a content intelligent Web switchmust monitor servers to ensure that requests are forwarded to the best-performing andhealthiest servers. This monitoring involves more than simple ICMP or TCP connectiontests, as servers can continue to process network protocols while failing to retrieve con-tent. If content is segregated in different servers or server farms, the Web switch must provide aflexible, user-customizable mechanism allowing a relevant set of application and contenttests to be applied to each server or server farm.In addition to implementing content intelligent switching, the switch periodically performsbackground functions such as updating network topology, measuring server performance, andhealth checking for servers, applications, and server sites.
