How can i identify the first Invite received by Kamailio in a dialog. I'm checking for the To-Tag in order to identify the first one. However, if the UA retries the same INVITE (because of a timeout for example), it won't have To Tag and it won't be the first INVITE that i'm receiving in that dialog. 

Is there any flag or variable that i can use to identify the first Invite received?

Thanks in advance