[Users] add/rewrite a tag/parameter in SDP

[Daniel-Constantin Mierla]

On 02/20/06 12:33, Klaus Darilion wrote:
> 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.
This function exists already. It is used by nathelper to update the 
content length when the spd is changed.

Cheers,
Daniel

>
> 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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