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3.1.5.12.2 Receiving an SDP Answer in Provisional Response and Starting Media Streams

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Media streams are started after receiving an SDP answer in the provisional response, depending upon whether the SIP INVITE request was forked to multiple points of presence (MPOP).

A SIP INVITE request was forked if an ms-forking SIP header exists in any provisional response. The ms-forking header is added when the call is forked to MPOP by the SIP proxy or server. For a more detailed specification of this header, see [MS-SIPRE].

Depending on whether the SIP INVITE request was forked, media streams are started as follows:

  • If an SDP answer is received in a provisional response and the SIP INVITE request was forked, the following are applicable:

    • The received streams SHOULD<15> be started, if they are not already started.

    • The send stream SHOULD<16> be started for sending DTMF only after receiving one or more RTP media packets via the corresponding receive stream.

  • If an SDP answer is received in a provisional response and the SIP INVITE request was not forked, both the receive and send streams for sending DTMF only SHOULD<17> be started with the consideration that the send stream is started only after receiving one or more RTP media packets via the corresponding receive stream.

Additionally, speech for the media streams in the forward direction SHOULD NOT<18> be started until the 200 OK is received for the INVITE.