[reSIProcate] dum: ACK retransmission issues and sending reINVITE with no offer

[Jason Fischl]

We have been implementing the new InviteSession. The following issue 
came up:

In the previous implementation the InviteSession stored all ACK messages 
that were sent in order to retransmit the appropriate one when a 200 was 
received in the Connected state. This was necessary since a second 
reINVITE could occur while the first 200 was still being retransmitted.

In the new implementation, we decided that storing the ACK messages was 
unnecessary. We can build the ACK from the 200/INVITE using the dialog 
and fill in the CSeq sequence number from the 200. This works because 
dum doesn't allow a reINVITE with no offer. We couldn't think of a good 
use case for this. However, we do allow a dum UAS to receive a reINVITE 
with no offer.

In the b2bua case, if the b2bua receives an INVITE with no offer from 
some confused gateway, it can respond with the current session 
description and then deal with the answer when it arrives.

It is worth discussing further whether there are any valid use cases for 
dum sending a reINVITE with no offer - b2bua or otherwise.

Jason