Modification of HTTP Requests 
An HTTP request passes several stations on its way from the HTTP client to the application server.
If an SAP Web Dispatcher is configured, HTTP requests first go the Web Dispatcher, which then filters the requests and distributes them to the application servers connected in the SAP system. If no Web Dispatcher is configured, HTTP requests go directly to the Internet Communication Manager (ICM) on an application server.
It is more practical to modify an HTTP request before it is forwarded to the application server. The rewrite handler, which is implemented as part of the HTTP plug-in in Web Dispatcher and in Internet Communication Manager, provides the functions to perform modification actions. You can perform different request modifications in the HTTP server (SAP Web Dispatcher or Internet Communication Manager):
Delete, add, and enhance HTTP header fields:
You can delete or add HTTP header fields, or enhance them with additional values.
Rewrite URLs:
You can rewrite both the URL path and the query string of an incoming HTTP request into another previously-defined URL path or query string. You can link the execution of URL modifications to one or more conditions.
Redirect or filter URLs:
You can redirect the URL of an incoming HTTP request to a different application server or URL. You can also filter incoming HTTP requests by defined patterns.
No modification action / nop action (no operation):
Modifiers (options), such as break, restart, and noescape can occur after a modification action (rule). So that you can use modifiers without definitions too, the nop action is provided.
You can link the execution of request modifications to one or more conditions.
The rewrite handler is configured in profile parameter icm/HTTP/mod_<xx>.
You define modification actions in an action file.