On 06/03/2015 03:02 PM, April Jones wrote:
SDP on 183 is always missing the a=sendrecv attrib. RFC is kind of unclear of whether sendrecv is necessary, but it feels like sendrecv is assumed. Is that correct?
Correct. Per RFC 3264 Section 5.1 ("Unicast Streams"): If the offerer wishes to both send and receive media with its peer, it MAY include an "a=sendrecv" attribute, or it MAY omit it, since sendrecv is the default. But note, as I said in my previous reply, that any SDP answer present in the 183 is merely a declaration of how the terminating gateway would like to receive media, and says nothing about sending it. In the same section: For recvonly and sendrecv streams, the port number and address in the offer indicate where the offerer would like to receive the media stream. Moreover, RFC 3960, which deals with early media in SIP explicitly, states: Therefore, by only looking at the SIP signalling, a UAC cannot be sure whether or not there will be early media for a particular session. The UAC needs to check if media packets are arriving at a given moment. -- Alex -- Alex Balashov | Principal | Evariste Systems LLC 303 Perimeter Center North, Suite 300 Atlanta, GA 30346 United States Tel: +1-800-250-5920 (toll-free) / +1-678-954-0671 (direct) Web: http://www.evaristesys.com/, http://www.csrpswitch.com/