[Devel] Re: [Users] add/rewrite a tag/parameter in SDP

[Klaus Darilion]

Daniel-Constantin Mierla wrote:
> Hello,
> 
> On 02/18/06 05:07, Lenir wrote:
>>
>> Hey guys,
>>
>> What is the best way to modify the SDP message to always include (if 
>> not replace) the a= tags in a SDP message?
>>
>> My problem is specifically with g729, according to the RFC, if the 
>> parameter: “a=fmtp:18 annexb=yes” is present or missing then it is 
>> assumed that g729b (g729 with VAD) and if “a=fmtp:18 annexb=no” is 
>> present then the devices will use g729a (g729 with VAD Disabled). My 
>> problem is that some cisco gateways and ATAs behave differently (of 
>> course Cisco always doing it their way), if “a=fmtp:18 annexb=no” is 
>> missing then it assumes that g729a will be used, instead of following 
>> the RFC where it should negotiate g729b. Also, asterisk doesn’t 
>> support g729b so I want to avoid using g729b everywhere in my network.
>>
>> So what is the best way or is there a way, to either:
>> a) insert “a=fmtp:18 annexb=no” if is not present
>>
>> b) replace “a=fmtp:18 annexb=yes” for “a=fmtp:18 annexb=no” if it is 
>> present
>>
> you can use textops module. You have functions to search a string and 
> replace it with another value (replace()). If the line does not exist 
> then you have to use search_append(). It is in roadmap to add specific 
> functions to search/replace only withing the body of the message...
> 
> http://openser.org/docs/modules/1.0.x/textops.html

We would also need a function which counts the new body size and updates 
the Content-Length header.

regards
klaus

> 
> Cheers,
> Daniel
> 
>> Thanks in advance!!
>>
>> Lenir
>>
>> p.s. Please see the below excerpt from RFC 3555 regarding how to 
>> specify G729.
>>
>> RFC 3555 MIME Type Registration of RTP Payload Formats July 2003
>>
>> 4.1.9. Registration of MIME media type audio/G729
>>
>> MIME media type name: audio
>>
>> MIME subtype name: G729
>>
>> Required parameters: None
>>
>> Optional parameters:
>>
>> ptime, maxptime
>>
>> annexb: indicates that Annex B, voice activity detection, is used or 
>> preferred. Permissible values are "yes" and "no" (without the quotes); 
>> "yes" is implied if this parameter is omitted
>>
>> ------------------------------------------------------------------------
>>
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> 
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