mptime Support for Packet Cable
The SDP specification lacks the ability to specify unique packetization times per codec when more than one codec is listed in an m= line. The ptime attribute is not related to a specific codec but to the entire m= line. When multiple codecs appear on a single m= line, the PacketCable mptime attribute can specify different packetization times for each codec.
The Oracle® Enterprise Session Border Controller adheres to PKT-SP-NCS1.5-I01-050128 and PKT-SP-EC-MGCP-I06-021127 for processing and generating mptime. The mptime line uses an integer to indicate the packetization time for each corresponding codec in the m= line. The dash character, "-", on an mptime line is used for non-packetized codecs, such as CN or telephone-event.
If the Oracle® Enterprise Session Border Controller receives an invalid mptime, it is ignored and removed. If a valid mptime is received in the incoming SDP, its values will be used for packetization times of each corresponding codec and a valid mptime line will be sent in the outgoing SDP.
Valid:
m=audio 10000 RTP/AVP 0 96 8 a=mptime:20 - 30 a=rtpmap:96 telephone-event/8000
Valid: 'ptime' attribute is ignored
m=audio 10000 RTP/AVP 0 8 a=mptime:20 30 a=ptime:30
Invalid: dash cannot be first mptime value
m=audio 10000 RTP/AVP 96 0 a=mptime: - 20
When Oracle® Enterprise Session Border Controller includes an mptime in an outgoing SDP, it will also always add a ptime attribute with the value of the most preferred codec. This is done to increase the interoperability with devices that do not support mptime.