[Freeswitch-users] SDP with 2 m=audio lines

[Anthony Minessale]

if PCMU at 30 is available it will be selected since its offered the
highest in the list.


On Tue, Feb 14, 2012 at 1:16 PM, Sergey Okhapkin
<sos at sokhapkin.dyndns.org> wrote:
> Which ptime will be selected for codec 0? It is in both m=audio lines.
>
> m=audio 24598 RTP/AVP 0 101 13
> a=rtpmap:101 telephone-event/8000
> a=fmtp:101 0-16
> a=ptime:30
> m=audio 24598 RTP/AVP 98 0 8 3 101 13
> a=rtpmap:98 SILK/8000
> a=fmtp:98 useinbandfec=1; usedtx=0
> a=rtpmap:101 telephone-event/8000
> a=fmtp:101 0-16
> a=ptime:20
>
>
> On Tuesday 14 February 2012 13:10:56 Anthony Minessale wrote:
>> In your case FS to FS, your FS will parse both m= lines in order they
>> are put in the sdp and choose the first stream it prefers based on
>> your codec prefs.  Its functionally identical to putting them all in
>> one m= line only they are grouped by ptime.
>>
>>
>>
>>
>>
>> On Tue, Feb 14, 2012 at 12:40 PM, Sergey Okhapkin
>>
>> <sos at sokhapkin.dyndns.org> wrote:
>> > But RFC 3388 says (in 1. Introduction):
>> >
>> >   An SDP session description typically contains one or more media lines
>> >   - they are commonly known as "m" lines.  When a session description
>> >   contains more than one "m" line, SDP does not provide any means to
>> >   express a particular relationship between two or more of them.  When
>> >   an application receives an SDP session description with more than one
>> >   "m" line, it is up to the application what to do with them.  SDP does
>> >   not carry any information about grouping media streams.
>> >
>> > On Tuesday 14 February 2012 11:40:40 Anthony Minessale wrote:
>> >> The word undefined does not even appear in that RFC.
>> >>
>> >> RFC 2327 (the one that actually describes SDP) is the relevant one:
>> >>
>> >> " A session description may contain a number of media descriptions.
>> >>    Each media description starts with an "m=" field, and is terminated
>> >>    by either the next "m=" field or by the end of the session
>> >>    description.  A media field also has several sub-fields: "
>> >>
>> >>
>> >> The ptime attribute cannot be applied to a single codec mentioned in a
>> >> m= line it must apply to all of them.
>> >> Once you have 2 different ptimes in the same description you have no
>> >> choice but to split it into different m= lines for each sep ptime.
>> >>
>> >> If you dislike this behavior then set:
>> >>
>> >> sdp_m_per_ptime=false
>> >>
>> >> either globally in vars.xml or in your dialplan on a per leg basis.
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >> On Tue, Feb 14, 2012 at 7:39 AM, Sergey Okhapkin
>> >>
>> >> <sos at sokhapkin.dyndns.org> wrote:
>> >> > I noticed that recent freeswitch versions could send SDP with 2
>> >> > m=audio lines. Per RFC 3388, the behavior is undefined this case.
>> >> > I don't know how to reproduce the problem, I see such packets
>> >> > coming from my customers. Here is an example:
>> >> >
>> >> > INVITE sip:xxxxxxxxxxxx at sip.callwithus.com SIP/2.0
>> >> > ...
>> >> > User-Agent: FreeSWITCH-mod_sofia/1.0.head-git-eeeaf41 2012-02-10
>> >> > 17-27-50 -0600 ....
>> >> >
>> >> > v=0
>> >> > o=FreeSWITCH 1329167961 1329167962 IN IP4 xx.xx.xx.xx
>> >> > s=FreeSWITCH
>> >> > c=IN IP4 xx.xx.xx.xx
>> >> > t=0 0
>> >> > m=audio 24598 RTP/AVP 0 101 13
>> >> > a=rtpmap:101 telephone-event/8000
>> >> > a=fmtp:101 0-16
>> >> > a=ptime:30
>> >> > m=audio 24598 RTP/AVP 98 0 8 3 101 13
>> >> > a=rtpmap:98 SILK/8000
>> >> > a=fmtp:98 useinbandfec=1; usedtx=0
>> >> > a=rtpmap:101 telephone-event/8000
>> >> > a=fmtp:101 0-16
>> >> > a=ptime:20
>> >> >
>> >> >
>> >> > Hmm, I just realized that this SDP is buggy - 2 audio streams on
>> >> > the
>> >> > same
>> >> > port, with common codecs, but with different ptime :-)
>> >> >
>> >> > __________________________________________________________________
>> >> > ______ _
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>> >> >
>> >> > 
>> >> > 
>> >> >
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>> > _
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>> >
>> > 
>> > 
>> >
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>
> _________________________________________________________________________
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> http://www.freeswitchsolutions.com
>
> 
> 
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-- 
Anthony Minessale II

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