[Asterisk-Dev] MWI From tag in SIP message

[Olle E. Johansson]

Kevin P. Fleming wrote:
> Chris A. Icide wrote:
> 
>> In the following message, there is a tag after the From: header.  What
>> is the function of this tag?  The MWI indication comes right after the
>> call that left a voicemail, but the tag isn't the same as the tag on
>> the previous call.  As you can see the device (a softswitch) receiving
>> the MWI indication rejects the notify due to the fact that the
>> tag=as1020fb8e exists after the From message.  The softswitch engineer
>> says:
>>
>> "It looks like the SoftSwitch doesn't like the tag on the end of the From
>> header. The reason for this is that tags generally indicate that the
>> message is part of a dialog (such as a subscription or a call), whereas
>> SoftSwitch only supports unsolicited (out-of-dialog) NOTIFYs on the
>> voicemail side."
> 
> 
> I would agree, the tag is unnecessary for an unsolicited NOTIFY message
> (same goes for OPTIONS and all the other non-dialog-invoking SIP
> methods). However, that's not usually an acceptable reason to ignore the
> NOTIFY; the receiving SIP parser can easily ignore the tag if it has no
> purpose.

I would say that the core problem is that we send NOTIFY messages
WITHOUT a preceeding SUBSCRIBE for notifications, so there's no
transaction to match. The NOTIFY has nothing to do with the actuall call
to voicemail, but should be related to a subscription... Well, it works
anyway with a lot of phones and ignoring it is ok and propably a correct
way of handling it.

/O