Hi,
Say, UAC sends an INVITE to UAS in the below format:
Session Initiation Protocol
Request-Line: INVITE sip:[email protected]:5060 SIP/2.0
Message Header
Message body
Session Description Protocol
Session Description Protocol Version (v): 0
Owner/Creator, Session Id (o): user1 53655765 2353687637 IN IP4
10.253.6.208
Session Name (s): -
Connection Information (c): IN IP4 10.253.6.208
Time Description, active time (t): 0 0
Media Description, name and address (m): audio 6001 RTP/AVP 116
0 8
Media Attribute (a): rtpmap:116 red/8000
Media Attribute (a): rtpmap:0 PCMU/8000
Media Attribute (a): rtpmap:8 PCMA/8000
Please note, "a=fmtp:" line for RED payload type, is not present in the
INVITE message sent by UAC.
Now, if UAS supports Redundant Audio (RFC 2198), how should an UAS
interpret the primary codec from the above INVITE message, when "a=fmtp"
line is not present. RFC 2198 only tells that "a=fmtp" line could be used
for mentioning the primary codec and does not mandate that "a=fmtp" line
must be there in SDP.
IMO, UAS could use any of the negotiated codecs (PCMU/PCMA) while
sending Redundant Audio in RTP to UAC.
Or Should UAS not negotiate for "RED" payload type when "a=fmtp" line is
not present in the INVITE from UAC?
Thanks,
Jagan
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