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3.9.5.1 Call Processing and Routing Elements

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User agents that are publishing can publish any preamble that is in accordance with the preamble XSD. However, the server SHOULD only act on a specific list of elements, and other elements MUST be ignored. The server that is a SIP proxy authorized for the domain of the target user's address-of-record SHOULD apply the routing rules based on the preamble only for INVITEs that meet one of the following criteria:

  • The Content-Type header field has the value "application/SDP" and the Session Description Protocol (SDP) body includes audio.

  • The Content-Type header field contains the string "application/ms-conf-invite" and the request body is an XML document that contains an XML element named "audio". The said element should also contain an XML attribute named "available" with value "true".<33>

  • The content type is "multipart/MIME" and at least one part contains an SDP body that includes audio.<34>

The construction of the INVITE requests with an "application/SDP" content type is described in [RFC3264], the "multipart/MIME" content type is defined [RFC2046].

All other INVITEs SHOULD be routed as specified in [RFC3261]. The routing mechanism specified in this section is applicable only if one of the preceding three conditions is met.

An INVITE whose Content-Type header field contains the string application/ms-conf-invite and the XML body indicates that audio is available is called an audio app-invite.