http://tomcat.apache.org/tomcat-6.0-doc/config/http.html says that "The maximum size of the request and response HTTP header, specified in bytes. If not specified, this attribute is set to 8192 (8 KB)." It could be changed to 16Kb, 32Kb, 64Kb depending upon the usage. My app works fine with 64k.
What could be the issues by increasing this value and how is the default value of 8192 bytes benchmarked?