The original paper describing Tor says this on section 4:

Each onion router maintains a TLS connection to every other onion router.

That paper is from 2004, when there was a much smaller number of relays on the network. Now there are more than 5000.

  • Is it still 100% true that relays form a complete graph at all times?
  • Wouldn't that number of simultaneous connections exceed some OS limit on number of open sockets or something like that?
  • Does it impose a performance penalty on relays?
  • Is there any privacy consideration that requires those connections to exist at all times?
  • What kind of messages do those relays exchange when they're not part of the same circuit?

Tor relays do not maintain open connections to every other relay. Relays create connections to other relays when asked to by a client, or when they need to connect to retrieve some information (like getting new Consensus documents from a Directory Cache.

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