### Contribution links: an SRT sender probes a stalled tail

- **[live-contribution](extensions/live-contribution.md) §4.1:** a receiver finds a gap only behind a
  later arrival, so a lost newest packet went unrecovered until it expired. When the acknowledged
  position has not moved for 2 × RTT + 20 ms, the sender sends its newest packet again, backing off
  to 16 times that wait. The reference sender counts these as `probes`, apart from
  `retransmitted`, and reports what it holds `unacknowledged`.
- **§6.1:** a packet whose first arrival is a retransmission, with nothing behind it yet to show
  the gap, is **recovered** (found missing as it arrives, a recovery of 0 µs), never delivered at
  the first try: its first send was lost. The SRT and RIST receivers count it so; before, a
  probed tail read as a first try.

