Since RFC 2198 does not mandate on sending the "fmtp" attribute line therefore the UAS should negotiate for the "RED" payload format.
as per RFC 2198 - "However to send a redundant stream, the sender needs to know which codecs are recommended for the primary and secondary (and tertiary, etc) encodings" which means that "fmtp" attribute line is required for ordering the encodings. In the absence of it, the encoding ordering can be assumed to be in serail order. ie the default format for sender is redundancy with PCMU (0) as the primary encoding and PCMA (8) as the secondary encoding. rgds Anuradha Aricent ________________________________________ From: [email protected] [[email protected]] On Behalf Of Jagan Mohan [[email protected]] Sent: Friday, February 20, 2009 7:58 PM To: SIP Implementors Subject: [Sip-implementors] Clarification needed related to 'a=fmtp' linefor Redundant Audio in RFC 2198 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 _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors "DISCLAIMER: This message is proprietary to Aricent and is intended solely for the use of the individual to whom it is addressed. It may contain privileged or confidential information and should not be circulated or used for any purpose other than for what it is intended. If you have received this message in error,please notify the originator immediately. If you are not the intended recipient, you are notified that you are strictly prohibited from using, copying, altering, or disclosing the contents of this message. Aricent accepts no responsibility for loss or damage arising from the use of the information transmitted by this email including damage from virus." _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
