PGM_IO_STATUS_TIMER_PENDING

Under ideal conditions there is a constant stream of data on the network and every call to pgm_recv returns data and there is no data loss or dropped packets.  Ideal conditions are rare though, we might see bursty data from senders, senders may close or crash, packets may be lost.
At the most basic level we need to be maintained that the senders exist, senders notify their presence by repeated broadcast of SPM packets.  Packet loss, closed or crashed applications would cause an absence of SPM broadcasts and this situation can be caught by a timer.  If no packets are seen within say 30 seconds we consider the sender to no longer to be operational.
The receive window extends beyond that to monitor every incoming packet, NAK elimination to prevent transmission of duplicate NAKs from the same or different receivers, and retransmission of NAKs for when the retransmit request itself was lost in the network.  Each state is driven by configurable timers or timeouts.

This means that as soon as a single packet from a sender is received the common return values expected are PGM_IO_STATUS_NORMAL for data and PGM_IO_STATUS_TIMER_PENDING for no-data or receive-state transitions.