Re: [Sip] Warning header

["Worley, Dale R (Dale)" <[e-mail]>]

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If an initial INVITE from an endpoint offer contains the sdp as follows.

m=audio 15190 RTP/AVP 100 101\r\n
a=fmtp:18 annexb=yes\r\n
a=fmtp:101 0-15\r\n
a=rtpmap:100 UNACCEPTABLECODEC/8000\r\n
a=sendrecv

the terminating endpoint returns an error response 488 with a warning header as follows.

Warning: 306 132.177.120.67:5060 "Attribute not understood"

Is 306 is correct response?
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The response is 488, which is correct if the recipient cannot find suitable codecs in the SDP offer.
This makes sense, as the SDP contains no audio coded.

The warning code 306 indicates that the recipient cannot understand one of the a= lines.  Unfortunately,
the message does not indicate which a= line it does not understand.  My understanding is that
if the interpreter of SDP does not understand an a= line, it should ignore it, which is aligned with
the fact that the response contains a warning (as opposed to an error) about not understanding an a= line.

I also see that the m= line mentions codec 101, but there is no a=rtpmap:101 line, which is I think technically
an error, but in practice means that the recipient must ignore codec 101.

Dale