F-SECURE INTERNET GATEKEEPER WINDOWS 2000-2003 SERVER 6.61 Administrator's Manual page 336

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Layer 4 Load Balancing
Layer 7 Load Balancing
Benefits
Supports several load-balancing models (for example
round-robin, least connections, weighted round-robin)
Proxying on the TCP/IP level. Capable of performing OSI Layer 2
to 7 load balancing.
Can be implemented transparently with a non-transparent proxy,
so no configuration changes need to be done at client hosts.
System maintenance can be performed during business hours
without service outages.
Failed host detection.
Drawbacks
Requires additional set up and maintenance.
Load-balancing online traffic at OSI Layer 4 is a very fast solution for
managing servers which offer various services such as e-mail service,
Web service, FTP service and DNS service. Each of these services and
their corresponding servers can be grouped and managed separately.
Layer 4 operates on TCP/IP and UDP protocol levels and enables very
fast load balancing, but it is cannot handle diverse HTTP traffic optimally.
Layer 7 load balancing is a feature rich solution for web traffic distribution
across web servers. In contrast to Layer 4, you can group servers based
on the type of content they serve with load balancing at Layer 7 (HTTP),
which makes it easy to manage and update the served content. Layer 7
load balancing has much more flexible cookie management and more
intelligent persistence implementation.
F5 Labs http://www.f5.com/