Hi Bart,
As Paul has indicated, though the t38 media line is actually not useful for
_this_ offer-answer, it can be certainly an indication to the remote side that
it supports such a media type. T.38 fax could encounter a number of
interoperability issues, e.g., a node _not_ supporting this media type, a node
supporting only 1-m line, scenarios involving SBCs, PSTN, etc. Hence this could
be a proactive indication from the offerer in this case.
Regards,
Swami.
-----Original Message-----
From: sip-implementors-boun...@lists.cs.columbia.edu
<sip-implementors-boun...@lists.cs.columbia.edu> On Behalf Of Paul Kyzivat
Sent: Monday, September 17, 2018 3:19 AM
To: sip-implementors@lists.cs.columbia.edu
Subject: Re: [Sip-implementors] t.38 port equal 0 in SDP
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On 9/15/18 5:32 AM, Qter wrote:
> Dear Experts,
>
> I see that one device is sending re-INVITE message with such SDP
> parameters:
>
> v=0
> o=- 1092631119 2 IN IP4 10.10.10.100
> s=-
> c=IN IP4 10.10.10.100
> t=0 0
> m=image 0 udptl t38
> m=audio 29044 RTP/AVP 8
> a=rtpmap:8 PCMA/8000
> a=silenceSupp:off - - - -
>
> The question is if m=image 0 udptl t38 is correct (port number equal 0)?
> Is it according to some RFC?
There is nothing wrong with such an offer. An m= line with a zero port number
is inoperable for this round of offer/answer, because the answer must also have
a zero port number. It can serve as a place-holder where the offerer has plans
to use it in a future offer, or hopes you will use it in a future offer. I
suppose it might be intended as an indicator that the offerer is *capable* of
supporting that, but AFAIK there is no such standardized meaning.
Thanks,
Paul
> BR
>
> Bart
>
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