Imagine an INVITE with an SDP that looks roughly like this that is
received by a UA:

v=0
m=audio 10000 AVP/RTP 0 101
a=rtpmap:0 PCMU/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-11


If that UA can send, but not receive telephone-event, what is the answer
SDP then supposed to look like? More exactly what would the SDP answer
below indicate?

v=0
m=audio 20000 AVP/RTP 0
a=rtpmap:0 PCMU/8000

Of course it means that both sides will use PCMU for both send and
receive with PT=0. The question is if the UA that sent the answer if
allowed to send telephone-event's with PT=101? Should the UA that sent
the offer always be prepared to receive with all the codecs it announced
or should it "cease to listen" to codecs that wasn't mentioned in the
SDP answer?

/ Christian Jansson


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