Link protocol negotiation without common version

Hi lovely core tor folks. I'm presently teaching Stem to communicate over tor's ORPort, and wanted to check about edge case behavior I ran into with the integ tests.

The first step of establishing an ORPort connection is to negotiate the protocol. This is done by...

• Sending a VERSIONS cell with the link protocol versions we support.
• Receive a VERSIONS cell in reply with versions the other side supports.
• All further cells are formatted using the highest common link protocol version.

This is all well and good, but when there isn't a common link protocol version the sender never receives a VERSIONS reply. That is to say, if I send a VERSIONS cell with 3, 4, or 5 things work, but if I send a cell with only other values (1, 2, 6, 20, etc) negotiation terminates right away.

The tor-spec is clear that the connection will be closed, but not if the caller should expect a VERSIONS reply...

If they have no such version in common, they cannot communicate and MUST close the connection.

Personally I have a slight preference for the sender to get a VERSIONS reply, then mutually close the socket. This way the caller will know why the connection was closed...

• "They're a newer tor version than me and only speak higher protocol versions."

... verses...

• "This is a really old relay that doesn't speak modern protocol versions."

Just food for thought. I'm not heartbroken that connections end right away - just makes for a vague error response to the user.