The SIP applications can behave only as proxies or user agent servers (UAS) in the current version of the Microsoft Lync Server 2013 SIP Application Managed API. As a proxy, the Lync Server 2013 SIP Application API application serves to forward a processed SIP message to the next host or recipient.

The API supports two ways to handle and proxy a SIP message. The simplest way to handle and proxy a message involves using an application manifest and setting the value of the proxyByDefault element element to true . This amounts to creating a script-only Lync Server 2013 SIP Application API application. The more advanced approach involves using the Microsoft.Rtc.Sip namespace to process messages and manage transaction. This corresponds to a managed Lync Server 2013 SIP Application API application.

The Lync Server 2013 SIP Application API documentation discusses application manifests and includes information about script-based and managed code-based application components.

All applications require an application manifest, regardless of whether they implement any managed components.