[VoiceOps] T.38 re-invites

[Ryan Delgrosso]

Best practice and real-world are often very different things.

I have seen numerous scenarios where a softswitch acting as a forking 
proxy will use a dummy SDP in its outgoing invite (Metaswitch and 
Sylantro both come to mind here), and immediately will re-invite when 
the far end has answered.

I have also seen numerous scenarios where fax servers will send the 
initial invite with G711 and then instantly re-invite to T.38 upon answer.

Dont stand on convention or BCP here, the re-invite often comes from the 
calling party, and sometimes a few times.



On 01/04/2012 06:46 AM, Mark R Lindsey wrote:
> The IETF informational draft: "SIP Support for Real-time Fax: Call Flow Examples And Best Current Practices" shows the re-INVITEs from the called party. This is still conventional best practice.
>
> 	http://tools.ietf.org/html/draft-ietf-sipping-realtimefax-01
>
>
> Mike Coffee's talk at SIPNOC 2011 discussed the expectations, showed how things can fall apart:
>
> 	http://www.sipforum.org/component/option,com_docman/task,cat_view/gid,115/Itemid,261/
>
>
> -- Mark R Lindsey mark at ecg.co +1-229-316-0013  http://ecg.co/lindsey
>
>
>
>
> On Jan 4, 2012, at 04:35 , Nathan Anderson wrote:
>
>> This might not be the most appropriate forum for such a question, but given that implementation bugs in either NOC gear or CPE gear can cause headaches for network operators, I offer this here in the hope that perhaps a fellow VoiceOp here may have run into this before and knows the answer...
>>
>> It is my understanding that with SIP, *either* party to a call can send another INVITE (a so-called "re-invite") at any time during an active session; this could be used to change the endpoint of a call or redirect it elsewhere, and is also commonly used to renegotiate the media in use (audio, video, image, and associated codecs) without dropping the call even while the endpoints remain the same.
>>
>> Does this hold true, though, for re-invites whose purpose is to switch from an RTP audio stream to a T.38 UDPTL stream?  Can *either* peer send that INVITE?  Or is there a rule -- spoken or unspoken -- that states that a T.38 re-invite can only be initiated by the *receiver* of the fax, and not the transmitter?
>>
>> I have some ATAs that appear to send out a T.38 re-invite upon detection of a fax tone regardless of whether the ATA placed the call or answered the call, and if I am understanding what I'm seeing in my packet captures, I think the fact that these ATAs do this even when they are going to play the role of the T.38 transmitter is confusing the heck out of my (Asterisk) SBC and the termination provider.  But I can't find any concrete evidence to support the idea that T.38 re-invites should only be initiated by the receiver.
>>
>> -- 
>> Nathan Anderson
>> First Step Internet, LLC
>> nathana at fsr.com
>> _______________________________________________
>> VoiceOps mailing list
>> VoiceOps at voiceops.org
>> https://puck.nether.net/mailman/listinfo/voiceops
>
> _______________________________________________
> VoiceOps mailing list
> VoiceOps at voiceops.org
> https://puck.nether.net/mailman/listinfo/voiceops