A Layer 2 Tunneling Protocol (L2TP) performs a sender initiated recovery
algorithm (SIRA) upon receiving, from an L2TP receiver, a predefined
number of packets including the same "next received" (Nr) sequence
number. The L2TP sender transmits a payload message that includes the
"Reset Sr" (R-bit) indicator, which resets the value for Nr (at the
receiver) to either just beyond the first missing packet or to the
current send sequence number of the sender.