When TCP enters fast recovery it records the highest outstanding unacknowledged packet sequence number.
2.
The HMAC is calculated over the packet payload and material from the packet header, including the packet sequence number.
3.
At the destination, the original message / data is reassembled in the correct order, based on the packet sequence number.
4.
A problem occurs with New Reno when there are no packet losses but instead, packets are reordered by more than 3 packet sequence numbers.
5.
The control word contains a 16-bit packet sequence number ( needed to detect packet re-ordering and packet loss ), payload length, and flags indicating defect conditions.