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As I understand it, Tor clients can download "descriptors" from "directory caches" instead of download them directly from the directory authorities. This is to avoid overloading the network.
But I have seen that these three terms are sometimes used identically. In the context of the Tor network, are these three terms synonymous? Are there differences between them?
In version the directory spec v2 directory cache appear to be synonymous:
… Routers may act as directory mirrors (also called "caches"), …
I also took a look at the current spec (v3) which isn't as clear about it but I got the impression that it is used interchangeably.
When it comes to directory servers I found this:
Every Tor that is not a directory server (that is, those that do not have a DirPort set) …
I believe that includes directory caches/mirrors and authorities which would mean the meaning of directory server slightly differs.
Keep in mind that just because there is a formal definition for the terms, it doesn't mean it is used that way in the wild. I'd not be surprised to see all three word used synonymously.
