Hi Shishir,
balance aca appears to be a load balancing algorithm. See these links for more info:
http://www.cisco.com/en/US/products/hw/contnetw/ps792/products_configuration_example09186a008016c8e5.shtml
https://supportforums.cisco.com/thread/129625
ArrowPoint Content Aware (ACA)
The CSS 11500 uses the ArrowPoint Content Aware (ACA) service to gather response time data for every flow for building statistical averages/variances for every service and content rule. The best servers are used, while the slow servers are pruned from the eligible list. This service also can manage persistent connections for e-commerce based on IP addresses (range), Secure Socket Layer (SSL) application IDs, and cookies.
You could use a dynamic LTM algorithm like Observed or Predictive:
sol6406: Overview of Least Connections, Fastest, Observed, and Predictive load balancing modes
http://support.f5.com/kb/en-us/solutions/public/6000/400/sol6406.html
I think string prefix is for parsing a cookie:
http://www.cisco.com/en/US/products/hw/contnetw/ps792/products_configuration_example09186a0080093de2.shtml
Configuring the Content Rule
Note: Cookies require a Layer 5 rule. You can create a Layer 5 rule by adding a URL. For example, . A Layer 4 rule can be promoted to a Layer 5 rule by issuing the advanced-balance cookies command.
...
Specify the prefix located in the string range.
training4(config-owner-content[cookie-layer5])
string prefix "ASPSESSION"
"Quoted textual information"(Len: 0-32)
Regardless, you can use an iRule like this one to persist on a cookie name:
http://devcentral.f5.com/wiki/default.aspx/iRules/ASP_SessionID_Persistence.html
Or on a URI token and/or cookie:
http://devcentral.f5.com/wiki/default.aspx/iRules/Weblogic_JSessionID_Persistence.html
Aaron