This counts packets flagged as invalid retransmissions, meaning retransmitted TCP segments that don't actually match previously sent data for that connection. This differs from normal TCP retransmission, which is benign and common on lossy links; an invalid retransmit specifically suggests someone is replaying or forging packets into a live session, so admins treat sustained counts here as more suspicious than typical retransmit-rate counters elsewhere on the box. For example, a handful of invalid retransmits per day might be background noise, but hundreds within one minute against a single host would warrant a closer look at that session.