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Version negotiation for the Tor protocol.
1.0. Version numbers
The node-to-node TLS-based "OR connection" protocol and the multi-hop
"circuit" protocol are versioned quasi-independently. (Certain versions
of the circuit protocol may require a minimum version of the connection
protocol to be used.)
Version numbers are incremented for backward-incompatible protocol changes
only. Backward-compatible changes are generally implemented by adding
additional fields to existing structures; implementations MUST ignore
fields they do not expect.
2.1. VERSIONS cells
When a Tor connection is established, both parties normally send a
VERSIONS cell before sending any other cells. (But see below.)
NumVersions [1 byte]
Versions [NumVersions bytes]
"Versions" is a sequence of NumVersions link connection protocol versions,
each one byte long. Parties should list all of the versions which they
are able and willing to support. Parties can only communicate if they
have some connection protocol version in common.
Version 0.1.x.y-alpha and earlier don't understand VERSIONS cells,
and therefore don't support version negotiation. Thus, waiting until
the other side has sent a VERSIONS cell won't work for these servers:
if they send no cells back, it is impossible to tell whether they
have sent a VERSIONS cell that has been stalled, or whether they have
dropped our own VERSIONS cell as unrecognized. Thus, immediately after
a TLS connection has been established, the parties check whether the
other side has an obsolete certificate (organizationName equal to "Tor"
or "TOR"). If the other party presented an obsolete certificate,
we assume a v1 connection. Otherwise, both parties send VERSIONS
cells listing all their supported versions. Upon receiving the
other party's VERSIONS cell, the implementation begins using the
highest-valued version common to both cells. If the first cell from
the other party is _not_ a VERSIONS cell, we assume a v1 protocol.
Implementations MUST discard cells that are not the first cells sent on a
connection.
2.2. MITM-prevention and time checking
If we negotiate a v2 connection or higher, the first cell we send SHOULD
be a NETINFO cell. Implementations SHOULD NOT send NETINFO cells at other
times.
A NETINFO cell contains:
Timestamp [4 bytes]
This OR's address [variable]
Other OR's address [variable]
Timestamp is the OR's current Unix time, in seconds since the epoch. If
an implementation receives time values from many validated ORs that
indicate that its clock is skewed, it SHOULD try to warn the
administrator.
Each address contains Type/Length/Value as used in Section 6.4. The first
address is the address of the interface the party sending the VERSIONS cell
used to connect to or accept connections from the other -- we include it
to block a man-in-the-middle attack on TLS that lets an attacker bounce
traffic through his own computers to enable timing and packet-counting
attacks.
The second address is the one that the party sending the VERSIONS cell
believes the other has -- it can be used to learn what your IP address
is if you have no other hints.
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