RTCSdpType "rollback" and why sdp field is needed?

Hi,

When calling setLocalDescription() or setRemoteDescription() with {
type: "rollback" , sdp: ??? }, what is sdp field supposed to have?

According to the doc:

> An RTCSdpType of rollback indicates that a description must be treated as canceling the current SDP negotiation and moving the SDP [SDP] offer and answer back to what it was in the previous stable state. Note the local or remote SDP descriptions in the previous stable state could be null if there has not yet been a successful offer-answer negotiation.

Note that sdp field in RTCSessionDescriptionInit is not
nullable/optional, so what is it supposed to hold when using type
"rollback" and why is it needed at all such a sdp field?

-- 
Iñaki Baz Castillo
<ibc@aliax.net>