[tor-bugs] #22929 [Core Tor/Tor]: What cells can be sent before a VERSIONS cell, and what is their CIRCID_LEN?

[Tor Bug Tracker & Wiki]

#22929: What cells can be sent before a VERSIONS cell, and what is their
CIRCID_LEN?
--------------------------+------------------------------------
 Reporter:  teor          |          Owner:
     Type:  defect        |         Status:  new
 Priority:  Medium        |      Milestone:  Tor: 0.3.2.x-final
Component:  Core Tor/Tor  |        Version:
 Severity:  Normal        |     Resolution:
 Keywords:  tor-spec      |  Actual Points:
Parent ID:  #18856        |         Points:
 Reviewer:                |        Sponsor:
--------------------------+------------------------------------

Comment (by teor):

 Replying to [ticket:22929 teor]:
 > tor-spec.txt says:
 > {{{
 >    CIRCID_LEN is 2 for link protocol versions 1, 2, and 3.  CIRCID_LEN
 >    is 4 for link protocol version 4 or higher.  The VERSIONS cell itself
 >    always has CIRCID_LEN == 2 for backward compatibility.
 > }}}

 T1. This isn't strictly true: relays parse and drop duplicate VERSIONS
 cells, as long as those versions cells are formatted according to the
 *current* link version. In particular, relays will parse VERSIONS cells
 with CIRCID_LEN == 4 when link protocol 4 has been negotiated.
 See #22931 for details.

 > But what is the CIRCID_LEN for early VPADDING, AUTHORIZE, or PADDING
 cells?

 T2. PADDING cells are not allowed as the first cell, relays say:

 `[info] channel_tls_handle_cell: Received unexpected cell command 0 in
 chan state opening / conn state waiting for renegotiation or V3 handshake;
 closing the connection.`

 VPADDING cells are ignored, and the connection proceeds as normal.
 It's possible to send any number of VPADDING cells.

 > {{{
 >    When this handshake is in use, the first cell must
 >    be VERSIONS, VPADDING or AUTHORIZE, and no other cell type is allowed
 to
 >    intervene besides those specified, except for PADDING and VPADDING
 cells.
 > }}}
 >
 > Is it valid to send VPADDING, then PADDING, then VERSIONS?

 T2. PADDING cells are not allowed before the VERSIONS cell, relays say:

 `[info] channel_tls_handle_cell: Received unexpected cell command 0 in
 chan state opening / conn state waiting for renegotiation or V3 handshake;
 closing the connection.`

 T3. In fact, PADDING cells don't seem to work after the VERSIONS cell,
 either. See #22934.

 > If so, what is their CIRCID_LEN?

 T4. Before the first VERSIONS cell, 2. Afterwards, whatever was
 negotiated.

 > Which sentence prevails, the one above, or the one below?
 >
 > {{{
 >    Parties MUST NOT send any
 >    other cells on a connection until they have received a VERSIONS cell.
 > }}}

 T5. VPADDING works fine.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/22929#comment:2>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online