[This proposed Tor extension has not been implemented yet. It is currently
in request-for-comments state. -RD]

                  Tor Unreliable Datagram Extension Proposal

                               Marc Liberatore

Abstract

Contents

0. Introduction

  Tor is a distributed overlay network designed to anonymize low-latency
  TCP-based applications.  The current tor specification supports only
  TCP-based traffic.  This limitation prevents the use of tor to anonymize
  other important applications, notably voice over IP software.  This document
  is a proposal to extend the tor specification to support UDP traffic.

  The basic design philosophy of this extension is to add support for
  tunneling unreliable datagrams through tor with as few modifications to the
  protocol as possible.  As currently specified, tor cannot directly support
  such tunneling, as connections between nodes are built using transport layer
  security (TLS) atop TCP.  The latency incurred by TCP is likely unacceptable
  to the operation of most UDP-based application level protocols.

  Thus, we propose the addition of links between nodes using datagram
  transport layer security (DTLS).  These links allow packets to traverse a
  route through tor quickly, but their unreliable nature requires minor
  changes to the tor protocol.  This proposal outlines the necessary
  additions and changes to the tor specification to support UDP traffic.

  We note that a separate set of DTLS links between nodes creates a second
  overlay, distinct from the that composed of TLS links.  This separation and
  resulting decrease in each anonymity set's size will make certain attacks
  easier.  However, it is our belief that VoIP support in tor will
  dramatically increase its appeal, and correspondingly, the size of its user
  base, number of deployed nodes, and total traffic relayed.  These increases
  should help offset the loss of anonymity that two distinct networks imply.

1. Overview of Tor-UDP and its complications

  As described above, this proposal extends the Tor specification to support
  UDP with as few changes as possible.  Tor's overlay network is managed
  through TLS based connections; we will re-use this control plane to set up
  and tear down circuits that relay UDP traffic.  These circuits be built atop
  DTLS, in a fashion analogous to how Tor currently sends TCP traffic over
  TLS.

  The unreliability of DTLS circuits creates problems for Tor at two levels:

      1. Tor's encryption of the relay layer does not allow independent
      decryption of individual records. If record N is not received, then
      record N+1 will not decrypt correctly, as the counter for AES/CTR is
      maintained implicitly.

      2. Tor's end-to-end integrity checking works under the assumption that
      all RELAY cells are delivered.  This assumption is invalid when cells
      are sent over DTLS.

  The fix for the first problem is straightforward: add an explicit sequence
  number to each cell.  To fix the second problem, we introduce a
  system of nonces and hashes to RELAY packets.

  In the following sections, we mirror the layout of the Tor Protocol
  Specification, presenting the necessary modifications to the Tor protocol as
  a series of deltas.

2. Connections

  Tor-UDP uses DTLS for encryption of some links.  All DTLS links must have
  corresponding TLS links, as all control messages are sent over TLS.  All
  implementations MUST support the DTLS ciphersuite "[TODO]".

  DTLS connections are formed using the same protocol as TLS connections.
  This occurs upon request, following a CREATE_UDP or CREATE_FAST_UDP cell,
  as detailed in section 4.6.

  Once a paired TLS/DTLS connection is established, the two sides send cells
  to one another.  All but two types of cells are sent over TLS links.  RELAY
  cells containing the commands RELAY_UDP_DATA and RELAY_UDP_DROP, specified
  below, are sent over DTLS links.  [Should all cells still be 512 bytes long?
  Perhaps upon completion of a preliminary implementation, we should do a
  performance evaluation for some class of UDP traffic, such as VoIP. - ML]
  Cells may be sent embedded in TLS or DTLS records of any size or divided
  across such records.  The framing of these records MUST NOT leak any more
  information than the above differentiation on the basis of cell type.  [I am
  uncomfortable with this leakage, but don't see any simple, elegant way
  around it. -ML]

  As with TLS connections, DTLS connections are not permanent.

3. Cell format

  Each cell contains the following fields:

        CircID                                [2 bytes]
        Command                               [1 byte]
        Sequence Number                       [2 bytes]
        Payload (padded with 0 bytes)         [507 bytes]
                                         [Total size: 512 bytes]

  The 'Command' field holds one of the following values:
       0 -- PADDING         (Padding)                     (See Sec 6.2)
       1 -- CREATE          (Create a circuit)            (See Sec 4)
       2 -- CREATED         (Acknowledge create)          (See Sec 4)
       3 -- RELAY           (End-to-end data)             (See Sec 5)
       4 -- DESTROY         (Stop using a circuit)        (See Sec 4)
       5 -- CREATE_FAST     (Create a circuit, no PK)     (See Sec 4)
       6 -- CREATED_FAST    (Circuit created, no PK)      (See Sec 4)
       7 -- CREATE_UDP      (Create a UDP circuit)        (See Sec 4)
       8 -- CREATED_UDP     (Acknowledge UDP create)      (See Sec 4)
       9 -- CREATE_FAST_UDP (Create a UDP circuit, no PK) (See Sec 4)
      10 -- CREATED_FAST_UDP(UDP circuit created, no PK)  (See Sec 4)

  The sequence number allows for AES/CTR decryption of RELAY cells
  independently of one another; this functionality is required to support
  cells sent over DTLS.  The sequence number is described in more detail in
  section 4.5.

  [Should the sequence number only appear in RELAY packets?  The overhead is
  small, and I'm hesitant to force more code paths on the implementor. -ML]
  [There's already a separate relay header that has other material in it,
  so it wouldn't be the end of the world to move it there if it's
  appropriate. -RD]

  [Having separate commands for UDP circuits seems necessary, unless we can
  assume a flag day event for a large number of tor nodes. -ML]

4. Circuit management

4.2. Setting circuit keys

  Keys are set up for UDP circuits in the same fashion as for TCP circuits.
  Each UDP circuit shares keys with its corresponding TCP circuit.

  [If the keys are used for both TCP and UDP connections, how does it
  work to mix sequence-number-less cells with sequenced-numbered cells --
  how do you know you have the encryption order right? -RD]

4.3. Creating circuits

  UDP circuits are created as TCP circuits, using the *_UDP cells as
  appropriate.

4.4. Tearing down circuits

  UDP circuits are torn down as TCP circuits, using the *_UDP cells as
  appropriate.

4.5. Routing relay cells

  When an OR receives a RELAY cell, it checks the cell's circID and
  determines whether it has a corresponding circuit along that
  connection.  If not, the OR drops the RELAY cell.

  Otherwise, if the OR is not at the OP edge of the circuit (that is,
  either an 'exit node' or a non-edge node), it de/encrypts the payload
  with AES/CTR, as follows:
       'Forward' relay cell (same direction as CREATE):
           Use Kf as key; decrypt, using sequence number to synchronize
           ciphertext and keystream.
       'Back' relay cell (opposite direction from CREATE):
           Use Kb as key; encrypt, using sequence number to synchronize
           ciphertext and keystream.
  Note that in counter mode, decrypt and encrypt are the same operation.
  [Since the sequence number is only 2 bytes, what do you do when it
  rolls over? -RD]

  Each stream encrypted by a Kf or Kb has a corresponding unique state,
  captured by a sequence number; the originator of each such stream chooses
  the initial sequence number randomly, and increments it only with RELAY
  cells.  [This counts cells; unlike, say, TCP, tor uses fixed-size cells, so
  there's no need for counting bytes directly.  Right? - ML]
  [I believe this is true. You'll find out for sure when you try to
  build it. ;) -RD]

  The OR then decides whether it recognizes the relay cell, by
  inspecting the payload as described in section 5.1 below.  If the OR
  recognizes the cell, it processes the contents of the relay cell.
  Otherwise, it passes the decrypted relay cell along the circuit if
  the circuit continues.  If the OR at the end of the circuit
  encounters an unrecognized relay cell, an error has occurred: the OR
  sends a DESTROY cell to tear down the circuit.

  When a relay cell arrives at an OP, the OP decrypts the payload
  with AES/CTR as follows:
        OP receives data cell:
           For I=N...1,
               Decrypt with Kb_I, using the sequence number as above.  If the
               payload is recognized (see section 5.1), then stop and process
               the payload.

  For more information, see section 5 below.

4.6. CREATE_UDP and CREATED_UDP cells

  Users set up UDP circuits incrementally.  The procedure is similar to that
  for TCP circuits, as described in section 4.1.  In addition to the TLS
  connection to the first node, the OP also attempts to open a DTLS
  connection.  If this succeeds, the OP sends a CREATE_UDP cell, with a
  payload in the same format as a CREATE cell.  To extend a UDP circuit past
  the first hop, the OP sends an EXTEND_UDP relay cell (see section 5) which
  instructs the last node in the circuit to send a CREATE_UDP cell to extend
  the circuit.

  The relay payload for an EXTEND_UDP relay cell consists of:
         Address                       [4 bytes]
         TCP port                      [2 bytes]
         UDP port                      [2 bytes]
         Onion skin                    [186 bytes]
         Identity fingerprint          [20 bytes]

  The address field and ports denote the IPV4 address and ports of the next OR
  in the circuit.

  The payload for a CREATED_UDP cell or the relay payload for an
  RELAY_EXTENDED_UDP cell is identical to that of the corresponding CREATED or
  RELAY_EXTENDED cell.  Both circuits are established using the same key.

  Note that the existence of a UDP circuit implies the
  existence of a corresponding TCP circuit, sharing keys, sequence numbers,
  and any other relevant state.

4.6.1 CREATE_FAST_UDP/CREATED_FAST_UDP cells

  As above, the OP must successfully connect using DTLS before attempting to
  send a CREATE_FAST_UDP cell.  Otherwise, the procedure is the same as in
  section 4.1.1.

5. Application connections and stream management

5.1. Relay cells

  Within a circuit, the OP and the exit node use the contents of RELAY cells
  to tunnel end-to-end commands, TCP connections ("Streams"), and UDP packets
  across circuits.  End-to-end commands and UDP packets can be initiated by
  either edge; streams are initiated by the OP.

  The payload of each unencrypted RELAY cell consists of:
        Relay command           [1 byte]
        'Recognized'            [2 bytes]
        StreamID                [2 bytes]
        Digest                  [4 bytes]
        Length                  [2 bytes]
        Data                    [498 bytes]

  The relay commands are:
        1 -- RELAY_BEGIN        [forward]
        2 -- RELAY_DATA         [forward or backward]
        3 -- RELAY_END          [forward or backward]
        4 -- RELAY_CONNECTED    [backward]
        5 -- RELAY_SENDME       [forward or backward]
        6 -- RELAY_EXTEND       [forward]
        7 -- RELAY_EXTENDED     [backward]
        8 -- RELAY_TRUNCATE     [forward]
        9 -- RELAY_TRUNCATED    [backward]
       10 -- RELAY_DROP         [forward or backward]
       11 -- RELAY_RESOLVE      [forward]
       12 -- RELAY_RESOLVED     [backward]
       13 -- RELAY_BEGIN_UDP    [forward]
       14 -- RELAY_DATA_UDP     [forward or backward]
       15 -- RELAY_EXTEND_UDP   [forward]
       16 -- RELAY_EXTENDED_UDP [backward]
       17 -- RELAY_DROP_UDP     [forward or backward]

  Commands labelled as "forward" must only be sent by the originator
  of the circuit. Commands labelled as "backward" must only be sent by
  other nodes in the circuit back to the originator. Commands marked
  as either can be sent either by the originator or other nodes.

  The 'recognized' field in any unencrypted relay payload is always set to
  zero. 

  The 'digest' field can have two meanings.  For all cells sent over TLS
  connections (that is, all commands and all non-UDP RELAY data), it is
  computed as the first four bytes of the running SHA-1 digest of all the
  bytes that have been sent reliably and have been destined for this hop of
  the circuit or originated from this hop of the circuit, seeded from Df or Db
  respectively (obtained in section 4.2 above), and including this RELAY
  cell's entire payload (taken with the digest field set to zero).  Cells sent
  over DTLS connections do not affect this running digest.  Each cell sent
  over DTLS (that is, RELAY_DATA_UDP and RELAY_DROP_UDP) has the digest field
  set to the SHA-1 digest of the current RELAY cells' entire payload, with the
  digest field set to zero.  Coupled with a randomly-chosen streamID, this
  provides per-cell integrity checking on UDP cells.
  [If you drop malformed UDP relay cells but don't close the circuit,
  then this 8 bytes of digest is not as strong as what we get in the
  TCP-circuit side. Is this a problem? -RD]

  When the 'recognized' field of a RELAY cell is zero, and the digest
  is correct, the cell is considered "recognized" for the purposes of
  decryption (see section 4.5 above).

  (The digest does not include any bytes from relay cells that do
  not start or end at this hop of the circuit. That is, it does not
  include forwarded data. Therefore if 'recognized' is zero but the
  digest does not match, the running digest at that node should
  not be updated, and the cell should be forwarded on.)

  All RELAY cells pertaining to the same tunneled TCP stream have the
  same streamID.  Such streamIDs are chosen arbitrarily by the OP.  RELAY
  cells that affect the entire circuit rather than a particular
  stream use a StreamID of zero.

  All RELAY cells pertaining to the same UDP tunnel have the same streamID.
  This streamID is chosen randomly by the OP, but cannot be zero.

  The 'Length' field of a relay cell contains the number of bytes in
  the relay payload which contain real payload data. The remainder of
  the payload is padded with NUL bytes.

  If the RELAY cell is recognized but the relay command is not
  understood, the cell must be dropped and ignored. Its contents
  still count with respect to the digests, though. [Before
  0.1.1.10, Tor closed circuits when it received an unknown relay
  command. Perhaps this will be more forward-compatible. -RD]

5.2.1.  Opening UDP tunnels and transferring data

  To open a new anonymized UDP connection, the OP chooses an open
  circuit to an exit that may be able to connect to the destination
  address, selects a random streamID not yet used on that circuit,
  and constructs a RELAY_BEGIN_UDP cell with a payload encoding the address
  and port of the destination host.  The payload format is:

        ADDRESS | ':' | PORT | [00]

  where  ADDRESS can be a DNS hostname, or an IPv4 address in
  dotted-quad format, or an IPv6 address surrounded by square brackets;
  and where PORT is encoded in decimal.

  [What is the [00] for? -NM]
  [It's so the payload is easy to parse out with string funcs -RD]

  Upon receiving this cell, the exit node resolves the address as necessary.
  If the address cannot be resolved, the exit node replies with a RELAY_END
  cell.  (See 5.4 below.)  Otherwise, the exit node replies with a
  RELAY_CONNECTED cell, whose payload is in one of the following formats:
      The IPv4 address to which the connection was made [4 octets]
      A number of seconds (TTL) for which the address may be cached [4 octets]
   or
      Four zero-valued octets [4 octets]
      An address type (6)     [1 octet]
      The IPv6 address to which the connection was made [16 octets]
      A number of seconds (TTL) for which the address may be cached [4 octets]
  [XXXX Versions of Tor before 0.1.1.6 ignore and do not generate the TTL
  field.  No version of Tor currently generates the IPv6 format.]

  The OP waits for a RELAY_CONNECTED cell before sending any data.
  Once a connection has been established, the OP and exit node
  package UDP data in RELAY_DATA_UDP cells, and upon receiving such
  cells, echo their contents to the corresponding socket.
  RELAY_DATA_UDP cells sent to unrecognized streams are dropped.

  Relay RELAY_DROP_UDP cells are long-range dummies; upon receiving such
  a cell, the OR or OP must drop it.

5.3. Closing streams

  UDP tunnels are closed in a fashion corresponding to TCP connections.

6. Flow Control

  UDP streams are not subject to flow control.

7.2. Router descriptor format.

The items' formats are as follows:
   "router" nickname address ORPort SocksPort DirPort UDPPort

      Indicates the beginning of a router descriptor.  "address" must be
      an IPv4 address in dotted-quad format. The last three numbers
      indicate the TCP ports at which this OR exposes
      functionality. ORPort is a port at which this OR accepts TLS
      connections for the main OR protocol; SocksPort is deprecated and
      should always be 0; DirPort is the port at which this OR accepts
      directory-related HTTP connections; and UDPPort is a port at which
      this OR accepts DTLS connections for UDP data.  If any port is not
      supported, the value 0 is given instead of a port number.

Other sections:

What changes need to happen to each node's exit policy to support this? -RD

Switching to UDP means managing the queues of incoming packets better,
so we don't miss packets. How does this interact with doing large public
key operations (handshakes) in the same thread?

========================================================================
COMMENTS
========================================================================

[16 May 2006]

I don't favor this approach; it makes packet traffic partitioned from
stream traffic end-to-end.  The architecture I'd like to see is:

  A *All* Tor-to-Tor traffic is UDP/DTLS, unless we need to fall back on
    TCP/TLS for firewall penetration or something.  (This also gives us an
    upgrade path for routing through legacy servers.)

  B Stream traffic is handled with end-to-end per-stream acks/naks and
    retries.  On failure, the data is retransmitted in a new RELAY_DATA cell;
    a cell isn't retransmitted.

We'll need to do A anyway, to fix our behavior on packet-loss.  Once we've
done so, B is more or less inevitable, and we can support end-to-end UDP
traffic "for free".

(Also, there are some details that this draft spec doesn't address.  For
example, what happens when a UDP packet doesn't fit in a single cell?)

-NM
