It's probably fine but this seemed safer and more consistent as you're not using it (or any other last headers) anywhere else except ack_bye. In my upcoming SIP MESSAGE branch, I have renamed ack_bye (and aliased it) to okay because nothing about it was actually specific to BYE and I therefore was able to use it to ack messages too. In either case, as it is a response rather than a request, using last headers here seems appropriate.