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tor/doc/tor-spec-udp.txt
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[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
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