There are 4 media flow direction attributes
1.sendrecv
2.recvonly
3.sendonly
4.inactive
these attributes are interpreted from senders perspective. 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.
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.
Below is a call flow for the hold resume example that shows use of these attributes.
1.sendrecv
2.recvonly
3.sendonly
4.inactive
these attributes are interpreted from senders perspective. 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.
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.
Below is a call flow for the hold resume example that shows use of these attributes.
Below are few pointes to note in this:
1. 183 Session in progress is given by the Media Gateway
2. ACK is transferred between IMG and GW-B and GW-A
3. the 200 OK with a=recvonly in response to the reINVITE with a=sendonly is sent by the IMG.
4. with the above, one way media flows only till IMG.
5. when reINVITE is sent with c=0.0.0.0. and a=inactive 200 OK contains a=inactive.
6. The support for Media flow attribute is limited to reINVITE Method only. If a gateway sends a different method such as the UPDATE method with Media Call Flow attributes set for call hold or resume,IMG will honor the UPDATE method but not put call on hold or off hold/resume.=> this sentence is as is from dialogic
7. Absence of a Call Flow Attribute in the Re-INVITE shall default to sendrecv.
6. Call Flow attributes within an INVITE message will be ignored.
7. RTP as well as RTCP will be stopped when media is put "On Hold".
references:
https://www.dialogic.com/webhelp/IMG1010/10.5.3/WebHelp/media_flow_attribute.htm
1. 183 Session in progress is given by the Media Gateway
2. ACK is transferred between IMG and GW-B and GW-A
3. the 200 OK with a=recvonly in response to the reINVITE with a=sendonly is sent by the IMG.
4. with the above, one way media flows only till IMG.
5. when reINVITE is sent with c=0.0.0.0. and a=inactive 200 OK contains a=inactive.
6. The support for Media flow attribute is limited to reINVITE Method only. If a gateway sends a different method such as the UPDATE method with Media Call Flow attributes set for call hold or resume,IMG will honor the UPDATE method but not put call on hold or off hold/resume.=> this sentence is as is from dialogic
7. Absence of a Call Flow Attribute in the Re-INVITE shall default to sendrecv.
6. Call Flow attributes within an INVITE message will be ignored.
7. RTP as well as RTCP will be stopped when media is put "On Hold".
references:
https://www.dialogic.com/webhelp/IMG1010/10.5.3/WebHelp/media_flow_attribute.htm
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