SIP Messages and Media Session

Hi all,

Suppose we have two UAs that one invites the other, then they start a media session and then the connection is terminated. How exactly the media session is being terminated? I mean, one of the UA terminates the Media Session and then sends a BYE SIP message to the other party; or once the media session is opened and the UA decide to close it will send first a BYE message and then will close the media session?

UA1 UA2 |-----------> INVITE Close Media Session

|-----------> BYE INVITE BYE |-----------> Close Media Session

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RFC3261 section 15.1.1: "The UAC MUST consider the session terminated (and therefore stop sending or listening for media) as soon as the BYE request is passed to the client transaction."

So first send BYE, then close the media session.

BTW, your schema below is perhaps incorrect for the Open Media Session. UA2 can start to send media as soon a SDP is received, generally in the INVITE. UA2 must be ready to receive the media as soon it send a SDP, generally in the 180 or in the 200. So in this case if the media session is opened after receiving the ACK, the beginning of the media can be lost. (This is also known as the "hello issue", because one of the two parties does not hear the other party saying "hello")

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