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?

1 Answer 1


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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