SIP Media Flow Attribute Support

Overview

The IMG 2020 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 on 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). With the media flow direction attributes, a call can now be put on hold and then the call can be resumed by sending a UPDATE or Re-INVITE message with the flow attributes.

Call Flow Attributes

The following attributes are supported on the IMG 2020. These attributes are coded in the IMG 2020 and do not need to be configured. The attributes were added to support the call on hold/resume feature and are supported in the Re-INVITE message only. Below is some background information on the media call flow attributes added.

sendrecv - Used to establish a 2-way media stream.

recvonly - The SIP endpoint would only receive (listen mode) and not send media.

sendonly - The SIP endpoint would only send and not receive media.

inactive -  The SIP endpoint would neither send nor receive media.

Table 1 below shows the response the IMG 2020 will use when it receives a media flow attribute on the incoming side.

Call Flow Information

Call Flow: Call Hold/Resume using Re-INVITE

Open

Call Flow: Call Hold/Resume using UPDATE

Open

Additional Information

• Absence of a call flow attribute in the UPDATE or Re-INVITE will default to sendrecv.

• When on hold, RTP is stopped but RTCP will still be transmitted.

• SIP UPDATE method for putting the call on hold/resume is also supported in early media.