[SIPForum-discussion] 100 Tring for Re-invite

[Rajangam.Subramanian at alcatel.com]

Hello all,
    I a query related to 100 Trying response for RE-Invite message.

The following is the scenario. A and B are 2 SIP UA's. A successful SIP 
call is established between A and B.

Now A wants to invoke some service and sends a Re-INVITE to B.
This Re-Invite to B will have a valid"To-tag" in the To header (as this 
is a re-invite)

B sends a 100 Trying for this RE-Invite.

My Question is: Is it necessary for  this 100 Trying to Re-Invite to 
have To-tag. Or can UA B send a 100 trying with out a to-tag.
        
Regards,
Rajangam