Nortel Web OS 10.0, Application Manual

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Web OS 10.0 Application Guide
Chapter 15: Content Intelligent Switching
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Parsing Content
Examining session content places heavier demands upon the Web switch than examining
TCP/IP headers for the following reasons:
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Content is non-deterministic. Content identifiers such as URLs and cookies can be of
varying 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 known
location in a session for a specific number of bytes.
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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 the
connection is suspended, the Web switch acknowledges the client connection on behalf of
the server, buffers and examines the client request, and finally opens a connection to an
appropriate server based on the requested content.
For more information on delayed binding, see “Delayed Binding” on page 146 .
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Delayed binding causes two independent TCP connections to span a Web session: one
from the client to the Web switch and the second from the Web switch to the selected
server. The Web switch must modify the TCP header, including performing TCP sequence
number translation and recalculating checksums on every packet that travels between the
client 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.
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In addition to real-time traffic and connection processing, a content intelligent Web switch
must monitor servers to ensure that requests are forwarded to the best-performing and
healthiest servers. This monitoring involves more than simple ICMP or TCP connection
tests, as servers can continue to process network protocols while failing to retrieve con-
tent.
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If content is segregated in different servers or server farms, the Web switch must provide a
flexible, user-customizable mechanism allowing a relevant set of application and content
tests to be applied to each server or server farm.
In addition to implementing content intelligent switching, the switch periodically performs
background functions such as updating network topology, measuring server performance, and
health checking for servers, applications, and server sites.