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?