From Header Parameter Tag in SipSessionKey is `null`

Question

I'm working on an application, where the SIPSession generated by Mobicents has one of it's attribute coming as null. After digging through the source code, I found out that, the value returned from the SipSession#getId() method is nothing but the SessionKey.

The SessionKey internally uses 4 different parameters to generate the String representation of the key. Out of that, one of the attribute is: fromTag. You can look at the source code of SipSessionKey here. Now, I'm unable to understand, what exactly is that fromTag. When I saw the request which is being sent, there is definitely a vlaue in the From header of the SipRequest. The From header is in the form:

From: <tel:+xxxxxxxxx>

That's it. What is fromTag in there? Why I'm getting it as null?

Answer 1

According to section 8.1.1.3 of RFC 3261, the From header MUST have a tag parameter. This is one of the pieces of data used to identify the dialog. (The others are Call-ID and the tag on the To header, generated by the UAS.) One the examples shown in the RFC is:

From: sip:[email protected];tag=887s

When looking at the SIP message received by the Mobicents container, is there a tag parameter on the From header?