[reSIProcate] SDP in ACK after intial offer/answer exchange causes BYE from resip/dum

["Justin Matthews" <jmatthewsr@xxxxxxxxx>]

A certain UA, with "bridges in the logo" (:-) I love that comment),  is
sending SDP in the ACK during the initial call setup, but after the
offer/answer has been completed in the invite/200.  This causes DUM in
ServerInviteSession::dispatchAccepted() to handle the event as OnAckAnswer
and send a BYE, terminating the session.

Section 13.2.1 in 3261 states some restrictions here, but it doesn't appear
that this case is spelled out.  To me I can understand how this is
considered bad behavior, with no way to answer this new offer in the ACK.
Question is, is sending a BYE too harsh here?  Could this decision be left
up to the APP?  For example, if the SDP is the same as the initial INVITE,
just ignore this behavior?

UAC                     UAS

INVITE w/offer ->
200 OK w/answer <-
ACK w/SDP ->
BYE <-
ACK ->


Thanks,

-Justin