RFC 3588:Diameter Base Protocol
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client and server


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... service specific authorization information, between client and servers, allowing the peers to decide whether a user's access request should be granted. ...
... forwarding path, they do not alter any AVPs transiting between client and server. Redirect agents do not originate messages and are capable of handling any message type ...


... In addition to client and servers, the Diameter protocol introduces relay, proxy ...


... Session-Id AVP is a means for the client and servers to correlate a Diameter message with a user session ...



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