Is it still true that each onion relay keeps an open TLS connection to every other relay? Why?

Question

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?

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.