RE: [Sip] INFO message belongs only to INVITE dialog usage?

["Christer Holmberg (JO/LMF)" <[e-mail]>]

Hi, 

>>I have read rfc2976 - The SIP INFO Method, but I haven't really 
>>understood that INFO message belongs only to INVITE dialog usage, 
>>which however draft-ietf-sipping-dialogusage-04 claims in its part
>>5.3:
>>
>>
>>      5.3. Matching requests to usages
>>
>>
>>
>>   For many mid-dialog requests, identifying the usage they  belong to

>>   is obvious.  A dialog can have at most one invite usage, so 
>>   any INVITE, UPDATE, PRACK, ACK, CANCEL, BYE, or INFO requests
belong to it.
>>
>> So my, question is:
>>    Can INFO message  belong to other usages, besides INVITE, such are
SUBSCRIBE  or REFER?
> >
> 
> 
>There was IIRC no intent in the writing of RFC 2976 to restrict INFO  
>to occurring within an INVITE dialog usage. So it could technically  
>be used outside of one, including in non-dialog cases.

Chapter 2 of 2976 says:

"The INFO method is used for communicating mid-session signaling
information along the signaling path for the call."

I think "mid-session" rules out the non-dialog cases, doesn't it?


On the other hand, chapter 4 says:

"There is potential that INFO messages could be forked by a SIP Proxy
Server."

That should not happen in a mid-dialo case, should it?

>However, RFC 2976 is considered to be somewhat underspecified, and so  
>far the only place we've further specified its usage is in the  
>context of transporting binary data for telephony protocol tunelling.  
>We have a standing consensus to NOT use it for arbitrary data 
>at this time. If we were to start using it, we'd need to do something
like  
>the event-packages model for INFO. So don't use it, and you won't  
>have to deal with this . . .

Eventhough not specified, in real life it IS used for sending DTMFs
(which is also listed in 2976 as an example usage, eventhough we may
have standing consensus on something else) and "non-telephony protocol"
information. I would even use the word "de facto standard" in some
cases. However, I am not aware of any non-INVITE dialog usage.

Regards,

Christer



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