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Overview:

The IMG supports the media flow direction attributes sendrecv, recvonly, sendonly, and inactive. These attributes, which are interpreted from the senders perspective, set the direction of the media stream to be established in the SIP protocol. The flow attributes present in the SDP portion of the SIP messaging will be used to support the call hold/resume scenario which is explained in RFC 3264 section 6.1 and still be backwards compatible with the call hold feature of RFC 2543 section B5. Up until software version 10.5.2 the IMG supported the Call Hold scenario using the "c=0.0.0.0" line  in the SDP message to put a call on hold. With the addition of the Media Flow direction Attributes a call can now be put on hold and then the call can be resumed by sending a Re-INVITE message with both the c=0.0.0.0 and one of the Flow attributes displayed above.

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