[Freeswitch-users] Multiple 183 SDP Change Codecs - allowed?

[Anthony Minessale]

This is another case of what works vs what the rules say and how they
should be interpreted.
It for sure is not a bug because there are ways to prove its optional.

I could be wrong, and like kris an operating from the top of my head
but I am pretty sure:

If you have 100rel on (which we don't by default because of issues in
the sofia stack) then once you answer with a sdp, you must not send
anything other than that exact sdp on subsequent responses. The RFC
mandates you ignore any sdp that differes from the last.

if you DONT have 100rel enabled, they you are allowed to completely
ignore the 1xx + sdp messages.

So either way we could claim not to support changing the codec in the
fashion described but on top of any rules we simply do not do it by
default because there are a ton of broken devices who unintentionally
send a series of packets that cause us to break.


NDLB-allow-nondup-sdp will unlock this to the best we support it.
However once we have chosen a codec we will only support that one
codec in this mode iirc.  You need a new offer and
renegotiate-codec-on-reinvite to change the codecs.



On Tue, Jul 3, 2012 at 10:22 AM, Kristian Kielhofner <kris at kriskinc.com> wrote:
> That's pretty nasty of voxbeam to do...  Considering they return the
> first 183 less than one second after the initial INVITE I'd say it's
> very likely they're providing false ringback and/or FAS (false answer
> supervision).  Meanwhile the actual ringback comes over two seconds
> later with PCMA.  That's shady and if I were you I'd switch carriers
> just on principle.
>
> However, as I said before they're not doing anything that's not
> allowed by the various specifications although FAS is considered by
> many as fraud.
>
> What are your codec settings like?  Perhaps FS is rejecting PCMA
> because you're not allowing it?
>
> On Tue, Jul 3, 2012 at 9:38 AM, Avi Marcus <avi at avimarcus.net> wrote:
>> If so.. which I think is not.. is this a bug then in FS?
>>
>> http://pastebin.freeswitch.org/19423
>> You see from the first SDP that PCMU and PCMA were both options.
>> Then for the next SDP FreeSWITCH was only considering PCMU which was the one
>> that got chosen by the first SDP.
>>
>> Voxbeam claims it's not their fault.. is it?
>>
>> -Avi
>>
>>
>>
>> On Tue, Jul 3, 2012 at 4:26 PM, Kristian Kielhofner <kris at kriskinc.com>
>> wrote:
>>>
>>> Avi,
>>>
>>>   Multiple 18x responses that change the SDP are allowed.  I can't
>>> find the specific document text now but as a random guy on the
>>> internet (for whatever that's worth) I'm certain it is allowed.
>>>
>>>   A re-INVITE can't be sent from either side until the dialog has been
>>> established (200+ACK).  In a case where the UAC (caller) would like to
>>> update the session before it is established method UPDATE must be
>>> used.
>>>
>>> On Tue, Jul 3, 2012 at 1:04 AM, Avi Marcus <avi at avimarcus.net> wrote:
>>> > Are multiple 183s from the endpoint that changes the SDP allowed? I'm
>>> > under
>>> > the impression this is broken, similar to
>>> > http://wiki.freeswitch.org/wiki/NDLB#NDLB-allow-nondup-sdp
>>> > ... which is why when the codec changes, FS freaks out and cancels the
>>> > call
>>> > because of codec negotiation error.
>>> > Does that NDLB flag allow this, too?
>>> >
>>> > First has: audio 10116 RTP/AVP 0 101 13
>>> > Second has:  audio 49020 RTP/AVP 8 13 101
>>> > PCAP: http://ge.tt/7MpyBwJ
>>> >
>>> > Can someone point me to the specific RFC so I can tell the supplier to
>>> > fix
>>> > it?
>>> > And just curious.. what would make this allowed? A re-INVITE..?
>>> >
>>> > -Avi
>>> >
>>> >
>>> > _________________________________________________________________________
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>>> > 
>>> > 
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>>>
>>>
>>>
>>> --
>>> Kristian Kielhofner
>>>
>>> _________________________________________________________________________
>>> Professional FreeSWITCH Consulting Services:
>>> consulting at freeswitch.org
>>> http://www.freeswitchsolutions.com
>>>
>>> 
>>> 
>>>
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>>
>>
>>
>> _________________________________________________________________________
>> Professional FreeSWITCH Consulting Services:
>> consulting at freeswitch.org
>> http://www.freeswitchsolutions.com
>>
>> 
>> 
>>
>> Official FreeSWITCH Sites
>> http://www.freeswitch.org
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>>
>> Join Us At ClueCon - Aug 7-9, 2012
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>
>
>
> --
> Kristian Kielhofner
>
> _________________________________________________________________________
> Professional FreeSWITCH Consulting Services:
> consulting at freeswitch.org
> http://www.freeswitchsolutions.com
>
> 
> 
>
> Official FreeSWITCH Sites
> http://www.freeswitch.org
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>
> Join Us At ClueCon - Aug 7-9, 2012
>
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-- 
Anthony Minessale II

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