[tor-bugs] #25139 [Core Tor/Tor]: Link protocol negotiation without common version

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#25139: Link protocol negotiation without common version
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     Reporter:  atagar        |      Owner:  (none)
         Type:  defect        |     Status:  new
     Priority:  Low           |  Milestone:
    Component:  Core Tor/Tor  |    Version:
     Severity:  Minor         |   Keywords:
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 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.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25139>
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