Implementation of Go-Back-N ARQ Flow Control Protocol

The sender's window in the sliding window protocol can expand when:
In which protocol can the sender receive an acknowledgement for a frame that falls outside its current window?
The receiver window will ________ in size when the frames are received in the sliding window method of flow control.
For a sliding window protocol with a window size of n-1, what is the maximum number of frames that can be sent but unacknowledged?
In the Go-Back-N ARQ protocol, the size of the receiver window is: