URL Generation in an AS-ABAP - Web Dispatcher
Configuration 
This section describes a standard configuration for generating URLs.
In a simple system landscape the AS-ABAP and the browsers are usually located in the same network. In this case the browser can access the AS-ABAP server directly using its configured name. Conversely, if the AS-ABAP has to generate an absolute URL for the browser, it can use its configured name to generate the URL.
In more complex system landscapes a reverse proxy server, for instance, the SAP Web dispatcher, is used in the network. This happens, for example, if the reverse proxy server is visible in the Internet and the AS-ABAP is located behind a firewall. In this case the browser uses the name of the reverse proxy server when it is communicating with the server. Conversely, it is not possible for the AS-ABAP to use its own configured name to generate absolute URLs. The URL must contain the name and port of the reverse proxy server (that is, the name and port of the unit with which the browser communicates).
For more information, refer to:
Configuration Table HTTPURLLOC