Another significant security issue applies mostly to sites that are protected by Internet firewalls. These sites have implemented security policies in their firewalls that describe the network services they want to permit within their organizations. A feature of URLs is that they support several resource types: file, Ftp, Http, Gopher, Wais, Nntp, Mailto, Prospero, Telnet, and Rlogin (other resources are in the works). It is likely that the firewall implementation permits only a subset of these resources. Therefore, since web servers provide these services independently of their normal mechanisms (independent of the normal Ftp channels, for example), it is possible to bypass a firewall security policy by using web services.