8.3.1 Modifying Address, Port or Transport

8.3.1 Modifying Address, Port or Transport

The port number for a stream MAY be changed. To do this, the offerer creates a new media description, with the port number in the m line different from the corresponding stream in the previous SDP. If only the port number is to be changed, the rest of the media stream description SHOULD remain unchanged. The offerer MUST be prepared to receive media on both the old and new ports as soon as the offer is sent. The offerer SHOULD NOT cease listening for media on the old port until the answer is received and media arrives on the new port. Doing so could result in loss of media during the transition.

Received, in this case, means that the media is passed to a media sink. This means that if there is a playout buffer, the agent would continue to listen on the old port until the media on the new port reached the top of the playout buffer. At that time, it MAY cease listening for media on the old port.

The corresponding media stream in the answer MAY be the same as the stream in the previous SDP from the answerer, or it MAY be different. If the updated stream is accepted by the answerer, the answerer SHOULD begin sending traffic for that stream to the new port immediately. If the answerer changes the port from the previous SDP, it MUST be prepared to receive media on both the old and new ports as soon as the answer is sent. The answerer MUST NOT cease listening for media on the old port until media arrives on the new port. At that time, it MAY cease listening for media on the old port. The same is true for an offerer that sends an updated offer with a new port; it MUST NOT cease listening for media on the old port until media arrives on the new port.

Of course, if the offered stream is rejected, the offerer can cease being prepared to receive using the new port as soon as the rejection is received.

To change the IP address where media is sent to, the same procedure is followed for changing the port number. The only difference is that the connection line is updated, not the port number.

The transport for a stream MAY be changed. The process for doing this is identical to changing the port, except the transport is updated, not the port.