[tor-bugs] #22882 [Core Tor/Tor]: The v4 link protocol requires the initiator to set the most significant bit

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#22882: The v4 link protocol requires the initiator to set the most significant bit
------------------------------+--------------------------------
     Reporter:  teor          |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.3.2.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-spec easy doc
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 tor-spec.txt says:
 {{{
    To prevent CircID collisions, when one node sends a CREATE cell to
    another, it chooses from only one half of the possible values based
    on the ORs' public identity keys.  In link protocol version 3 or
    lower, if the sending node has a lower key, it chooses a CircID with
    an MSB of 0; otherwise, it chooses a CircID with an MSB of 1. (Public
    keys are compared numerically by modulus.)

    In link protocol version 4 or higher, whichever node initiated the
    connection sets its MSB to 1, and whichever node didn't initiate the
    connection sets its MSB to 0.

    (An OP with no public key MAY choose any CircID it wishes, since an OP
    never needs to process a CREATE cell.)
 }}}

 But this last sentence is only true for v3. For v4, if the most
 significant bit is not set by a client, relays close the connection with a
 message like:

 {{{
 Jul 11 20:20:18.000 [info] command_process_create_cell: Received create
 cell with unexpected circ_id 2147483647. Closing.
 }}}

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/22882>
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