Open Settlement Protocol (OSP) is a client-server protocol that manages access control, accounting, usage data and inter-domain routing to make it easier for Internet service providers (ISPs) to support IP telephony.
OSP facilitates information exchange through the Extensible Markup Language (EML). The protocol sends message content over HTTP, using a MIME header along with an XML document. Systems that rely on Simple Gateway Control Protocol (SGCP), Media Gateway Control Protocol (MGCP), Mecago and other IP telephony protocols integrate with OSP through gateway controllers or agents. These gateways include H.323 gateways and Session Initiation Protocol (SIP) proxies. Through OSP, Voice over IP (VoIP) service providers can use services from OSP clearinghouses, support secure VoIP peering and arrange end-to-end telephony services across multiple voice and/or data networks.
The OSP format was defined by the European Telecommunications Standards Institute (ETSI) in ETSI Technical Specification 101 321.