Page "Extensible Messaging and Presence Protocol" Paragraph 34

One of the original design goals of the early Jabber open-source community was enabling users to connect to multiple instant messaging systems ( especially non-XMPP systems ) through a single client application.

This was done though entities called transports or gateways to other instant messaging protocols, but also to protocols such as SMS or email.

Unlike multi-protocol clients, XMPP provides this access at the server level by communicating via special gateway services running alongside an XMPP server.

Any user can " register " with one of these gateways by providing the information needed to log on to that network, and can then communicate with users of that network as though they were XMPP users.

Thus, such gateways function as client proxies ( the gateway authenticates on the user's behalf on the non-XMPP service ).

As a result, any client that fully supports XMPP can access any network with a gateway without extra code in the client, and without the need for the client to have direct access to the Internet.

However, the client proxy model may violate terms of service on the protocol used ( although such terms of service are not legally enforceable in several countries ) and also requires the user to send their IM username and password to the third-party site that operates the transport ( which may raise privacy and security concerns ).