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Reachability is a percentage which applies to destinations, rather than individual measurement of IP packets. A destination is considered reachable during IPDI scan if it responds to at least one of the measurement packets.
Typically algorithms for reachability that require preprocessing (and their corresponding data structures) are called oracles (similarly there are oracles for distance and approximate distance queries). For the latter case, resolving a single reachability query can be done in linear time using algorithms such as breadth first search or iterative deepening depth-first search.
Packet loss reflects the percentage of all measurement packets that do not result in a response from the destination; those are considered lost. For example, four responses to five packets indicates 20% packet loss (one packet in five is lost). These numbers are summarized by a method similar to that for latency. ISPs are constantly modifying their networks, so destinations sometimes become inaccessible. This causes packet loss values to be slightly inflated. To minimize this effect, periodically CloudCover will remove such dead destinations from it's IPDI ping lists.
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