问题：

[多选] Regarding PAR (Positive Acknowledgement and Re-transmission), which of the answer choices below are correct?（）

A . The source device will only retransmit lost packets on the request of the destination device.
B . The source device starts a timer when it sends a segment and retransmits if an acknowledgment is not received before the timer expires.
C . The destination device acknowledges receipt of a segment by sending a packet with a new sequence number and the ACK bit sent.
D . The destination device acknowledges receipt of a segment by sending a packet that indicates the next sequence number it expects.
E . If the destination device does not receive a segment, all segments are retransmitted.
F . The source device keeps a record of all segments sent and expects and acknowledgment of each.