[Asterisk-Dev] MWI From tag in SIP message

[Kevin P. Fleming]

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.