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Tor Protocol Specification
Roger Dingledine
Nick Mathewson
Note: This document aims to specify Tor as implemented in 0.2.1.x. Future
versions of Tor may implement improved protocols, and compatibility is not
guaranteed. Compatibility notes are given for versions 0.1.1.15-rc and
later; earlier versions are not compatible with the Tor network as of this
writing.
This specification is not a design document; most design criteria
are not examined. For more information on why Tor acts as it does,
see tor-design.pdf.
0. Preliminaries
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
RFC 2119.
0.1. Notation and encoding
PK -- a public key.
SK -- a private key.
K -- a key for a symmetric cipher.
a|b -- concatenation of 'a' and 'b'.
[A0 B1 C2] -- a three-byte sequence, containing the bytes with
hexadecimal values A0, B1, and C2, in that order.
All numeric values are encoded in network (big-endian) order.
H(m) -- a cryptographic hash of m.
0.2. Security parameters
Tor uses a stream cipher, a public-key cipher, the Diffie-Hellman
protocol, and a hash function.
KEY_LEN -- the length of the stream cipher's key, in bytes.
PK_ENC_LEN -- the length of a public-key encrypted message, in bytes.
PK_PAD_LEN -- the number of bytes added in padding for public-key
encryption, in bytes. (The largest number of bytes that can be encrypted
in a single public-key operation is therefore PK_ENC_LEN-PK_PAD_LEN.)
DH_LEN -- the number of bytes used to represent a member of the
Diffie-Hellman group.
DH_SEC_LEN -- the number of bytes used in a Diffie-Hellman private key (x).
HASH_LEN -- the length of the hash function's output, in bytes.
PAYLOAD_LEN -- The longest allowable cell payload, in bytes. (509)
CELL_LEN -- The length of a Tor cell, in bytes.
0.3. Ciphers
For a stream cipher, we use 128-bit AES in counter mode, with an IV of all
0 bytes.
For a public-key cipher, we use RSA with 1024-bit keys and a fixed
exponent of 65537. We use OAEP-MGF1 padding, with SHA-1 as its digest
function. We leave the optional "Label" parameter unset. (For OAEP
padding, see ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-1/pkcs-1v2-1.pdf)
For Diffie-Hellman, we use a generator (g) of 2. For the modulus (p), we
use the 1024-bit safe prime from rfc2409 section 6.2 whose hex
representation is:
"FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E08"
"8A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B"
"302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9"
"A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6"
"49286651ECE65381FFFFFFFFFFFFFFFF"
As an optimization, implementations SHOULD choose DH private keys (x) of
320 bits. Implementations that do this MUST never use any DH key more
than once.
[May other implementations reuse their DH keys?? -RD]
[Probably not. Conceivably, you could get away with changing DH keys once
per second, but there are too many oddball attacks for me to be
comfortable that this is safe. -NM]
For a hash function, we use SHA-1.
KEY_LEN=16.
DH_LEN=128; DH_SEC_LEN=40.
PK_ENC_LEN=128; PK_PAD_LEN=42.
HASH_LEN=20.
When we refer to "the hash of a public key", we mean the SHA-1 hash of the
DER encoding of an ASN.1 RSA public key (as specified in PKCS.1).
All "random" values should be generated with a cryptographically strong
random number generator, unless otherwise noted.
The "hybrid encryption" of a byte sequence M with a public key PK is
computed as follows:
1. If M is less than PK_ENC_LEN-PK_PAD_LEN, pad and encrypt M with PK.
2. Otherwise, generate a KEY_LEN byte random key K.
Let M1 = the first PK_ENC_LEN-PK_PAD_LEN-KEY_LEN bytes of M,
and let M2 = the rest of M.
Pad and encrypt K|M1 with PK. Encrypt M2 with our stream cipher,
using the key K. Concatenate these encrypted values.
[XXX Note that this "hybrid encryption" approach does not prevent
an attacker from adding or removing bytes to the end of M. It also
allows attackers to modify the bytes not covered by the OAEP --
see Goldberg's PET2006 paper for details. We will add a MAC to this
scheme one day. -RD]
0.4. Other parameter values
CELL_LEN=512
1. System overview
Tor is a distributed overlay network designed to anonymize
low-latency TCP-based applications such as web browsing, secure shell,
and instant messaging. Clients choose a path through the network and
build a ``circuit'', in which each node (or ``onion router'' or ``OR'')
in the path knows its predecessor and successor, but no other nodes in
the circuit. Traffic flowing down the circuit is sent in fixed-size
``cells'', which are unwrapped by a symmetric key at each node (like
the layers of an onion) and relayed downstream.
1.1. Keys and names
Every Tor server has multiple public/private keypairs:
- A long-term signing-only "Identity key" used to sign documents and
certificates, and used to establish server identity.
- A medium-term "Onion key" used to decrypt onion skins when accepting
circuit extend attempts. (See 5.1.) Old keys MUST be accepted for at
least one week after they are no longer advertised. Because of this,
servers MUST retain old keys for a while after they're rotated.
- A short-term "Connection key" used to negotiate TLS connections.
Tor implementations MAY rotate this key as often as they like, and
SHOULD rotate this key at least once a day.
Tor servers are also identified by "nicknames"; these are specified in
dir-spec.txt.
2. Connections
Connections between two Tor servers, or between a client and a server,
use TLS/SSLv3 for link authentication and encryption. All
implementations MUST support the SSLv3 ciphersuite
"SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA", and SHOULD support the TLS
ciphersuite "TLS_DHE_RSA_WITH_AES_128_CBC_SHA" if it is available.
There are three acceptable ways to perform a TLS handshake when
connecting to a Tor server: "certificates up-front", "renegotiation", and
"backwards-compatible renegotiation". ("Backwards-compatible
renegotiation" is, as the name implies, compatible with both other
handshake types.)
Before Tor 0.2.0.21, only "certificates up-front" was supported. In Tor
0.2.0.21 or later, "backwards-compatible renegotiation" is used.
In "certificates up-front", the connection initiator always sends a
two-certificate chain, consisting of an X.509 certificate using a
short-term connection public key and a second, self- signed X.509
certificate containing its identity key. The other party sends a similar
certificate chain. The initiator's ClientHello MUST NOT include any
ciphersuites other than:
TLS_DHE_RSA_WITH_AES_256_CBC_SHA
TLS_DHE_RSA_WITH_AES_128_CBC_SHA
SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA
SSL_DHE_DSS_WITH_3DES_EDE_CBC_SHA
In "renegotiation", the connection initiator sends no certificates, and
the responder sends a single connection certificate. Once the TLS
handshake is complete, the initiator renegotiates the handshake, with each
party sending a two-certificate chain as in "certificates up-front".
The initiator's ClientHello MUST include at least one ciphersuite not in
the list above. The responder SHOULD NOT select any ciphersuite besides
those in the list above.
[The above "should not" is because some of the ciphers that
clients list may be fake.]
In "backwards-compatible renegotiation", the connection initiator's
ClientHello MUST include at least one ciphersuite other than those listed
above. The connection responder examines the initiator's ciphersuite list
to see whether it includes any ciphers other than those included in the
list above. If extra ciphers are included, the responder proceeds as in
"renegotiation": it sends a single certificate and does not request
client certificates. Otherwise (in the case that no extra ciphersuites
are included in the ClientHello) the responder proceeds as in
"certificates up-front": it requests client certificates, and sends a
two-certificate chain. In either case, once the responder has sent its
certificate or certificates, the initiator counts them. If two
certificates have been sent, it proceeds as in "certificates up-front";
otherwise, it proceeds as in "renegotiation".
All new implementations of the Tor server protocol MUST support
"backwards-compatible renegotiation"; clients SHOULD do this too. If
this is not possible, new client implementations MUST support both
"renegotiation" and "certificates up-front" and use the router's
published link protocols list (see dir-spec.txt on the "protocols" entry)
to decide which to use.
In all of the above handshake variants, certificates sent in the clear
SHOULD NOT include any strings to identify the host as a Tor server. In
the "renegotiation" and "backwards-compatible renegotiation" steps, the
initiator SHOULD choose a list of ciphersuites and TLS extensions
to mimic one used by a popular web browser.
Responders MUST NOT select any TLS ciphersuite that lacks ephemeral keys,
or whose symmetric keys are less then KEY_LEN bits, or whose digests are
less than HASH_LEN bits. Responders SHOULD NOT select any SSLv3
ciphersuite other than those listed above.
Even though the connection protocol is identical, we will think of the
initiator as either an onion router (OR) if it is willing to relay
traffic for other Tor users, or an onion proxy (OP) if it only handles
local requests. Onion proxies SHOULD NOT provide long-term-trackable
identifiers in their handshakes.
In all handshake variants, once all certificates are exchanged, all
parties receiving certificates must confirm that the identity key is as
expected. (When initiating a connection, the expected identity key is
the one given in the directory; when creating a connection because of an
EXTEND cell, the expected identity key is the one given in the cell.) If
the key is not as expected, the party must close the connection.
When connecting to an OR, all parties SHOULD reject the connection if that
OR has a malformed or missing certificate. When accepting an incoming
connection, an OR SHOULD NOT reject incoming connections from parties with
malformed or missing certificates. (However, an OR should not believe
that an incoming connection is from another OR unless the certificates
are present and well-formed.)
[Before version 0.1.2.8-rc, ORs rejected incoming connections from ORs and
OPs alike if their certificates were missing or malformed.]
Once a TLS connection is established, the two sides send cells
(specified below) to one another. Cells are sent serially. All
cells are CELL_LEN bytes long. Cells may be sent embedded in TLS
records of any size or divided across TLS records, but the framing
of TLS records MUST NOT leak information about the type or contents
of the cells.
TLS connections are not permanent. Either side MAY close a connection
if there are no circuits running over it and an amount of time
(KeepalivePeriod, defaults to 5 minutes) has passed since the last time
any traffic was transmitted over the TLS connection. Clients SHOULD
also hold a TLS connection with no circuits open, if it is likely that a
circuit will be built soon using that connection.
(As an exception, directory servers may try to stay connected to all of
the ORs -- though this will be phased out for the Tor 0.1.2.x release.)
To avoid being trivially distinguished from servers, client-only Tor
instances are encouraged but not required to use a two-certificate chain
as well. Clients SHOULD NOT keep using the same certificates when
their IP address changes. Clients MAY send no certificates at all.
3. Cell Packet format
The basic unit of communication for onion routers and onion
proxies is a fixed-width "cell".
On a version 1 connection, each cell contains the following
fields:
CircID [2 bytes]
Command [1 byte]
Payload (padded with 0 bytes) [PAYLOAD_LEN bytes]
On a version 2 connection, all cells are as in version 1 connections,
except for the initial VERSIONS cell, whose format is:
Circuit [2 octets; set to 0]
Command [1 octet; set to 7 for VERSIONS]
Length [2 octets; big-endian integer]
Payload [Length bytes]
The CircID field determines which circuit, if any, the cell is
associated with.
The 'Command' field holds one of the following values:
0 -- PADDING (Padding) (See Sec 7.2)
1 -- CREATE (Create a circuit) (See Sec 5.1)
2 -- CREATED (Acknowledge create) (See Sec 5.1)
3 -- RELAY (End-to-end data) (See Sec 5.5 and 6)
4 -- DESTROY (Stop using a circuit) (See Sec 5.4)
5 -- CREATE_FAST (Create a circuit, no PK) (See Sec 5.1)
6 -- CREATED_FAST (Circuit created, no PK) (See Sec 5.1)
7 -- VERSIONS (Negotiate proto version) (See Sec 4)
8 -- NETINFO (Time and address info) (See Sec 4)
9 -- RELAY_EARLY (End-to-end data; limited)(See Sec 5.6)
The interpretation of 'Payload' depends on the type of the cell.
PADDING: Payload is unused.
CREATE: Payload contains the handshake challenge.
CREATED: Payload contains the handshake response.
RELAY: Payload contains the relay header and relay body.
DESTROY: Payload contains a reason for closing the circuit.
(see 5.4)
Upon receiving any other value for the command field, an OR must
drop the cell. Since more cell types may be added in the future, ORs
should generally not warn when encountering unrecognized commands.
The payload is padded with 0 bytes.
PADDING cells are currently used to implement connection keepalive.
If there is no other traffic, ORs and OPs send one another a PADDING
cell every few minutes.
CREATE, CREATED, and DESTROY cells are used to manage circuits;
see section 5 below.
RELAY cells are used to send commands and data along a circuit; see
section 6 below.
VERSIONS and NETINFO cells are used to set up connections. See section 4
below.
4. Negotiating and initializing connections
4.1. Negotiating versions with VERSIONS cells
There are multiple instances of the Tor link connection protocol. Any
connection negotiated using the "certificates up front" handshake (see
section 2 above) is "version 1". In any connection where both parties
have behaved as in the "renegotiation" handshake, the link protocol
version is 2 or higher.
To determine the version, in any connection where the "renegotiation"
handshake was used (that is, where the server sent only one certificate
at first and where the client did not send any certificates until
renegotiation), both parties MUST send a VERSIONS cell immediately after
the renegotiation is finished, before any other cells are sent. Parties
MUST NOT send any other cells on a connection until they have received a
VERSIONS cell.
The payload in a VERSIONS cell is a series of big-endian two-byte
integers. Both parties MUST select as the link protocol version the
highest number contained both in the VERSIONS cell they sent and in the
versions cell they received. If they have no such version in common,
they cannot communicate and MUST close the connection.
Since the version 1 link protocol does not use the "renegotiation"
handshake, implementations MUST NOT list version 1 in their VERSIONS
cell.
4.2. NETINFO cells
If version 2 or higher is negotiated, each party sends the other a
NETINFO cell. The cell's payload is:
Timestamp [4 bytes]
Other OR's address [variable]
Number of addresses [1 byte]
This OR's addresses [variable]
The address format is a type/length/value sequence as given in section
6.4 below. The timestamp is a big-endian unsigned integer number of
seconds since the Unix epoch.
Implementations MAY use the timestamp value to help decide if their
clocks are skewed. Initiators MAY use "other OR's address" to help
learn which address their connections are originating from, if they do
not know it. Initiators SHOULD use "this OR's address" to make sure
that they have connected to another OR at its canonical address.
[As of 0.2.0.23-rc, implementations use none of the above values.]
5. Circuit management
5.1. CREATE and CREATED cells
Users set up circuits incrementally, one hop at a time. To create a
new circuit, OPs send a CREATE cell to the first node, with the
first half of the DH handshake; that node responds with a CREATED
cell with the second half of the DH handshake plus the first 20 bytes
of derivative key data (see section 5.2). To extend a circuit past
the first hop, the OP sends an EXTEND relay cell (see section 5)
which instructs the last node in the circuit to send a CREATE cell
to extend the circuit.
The payload for a CREATE cell is an 'onion skin', which consists
of the first step of the DH handshake data (also known as g^x).
This value is hybrid-encrypted (see 0.3) to Bob's onion key, giving
an onion-skin of:
PK-encrypted:
Padding [PK_PAD_LEN bytes]
Symmetric key [KEY_LEN bytes]
First part of g^x [PK_ENC_LEN-PK_PAD_LEN-KEY_LEN bytes]
Symmetrically encrypted:
Second part of g^x [DH_LEN-(PK_ENC_LEN-PK_PAD_LEN-KEY_LEN)
bytes]
The relay payload for an EXTEND relay cell consists of:
Address [4 bytes]
Port [2 bytes]
Onion skin [DH_LEN+KEY_LEN+PK_PAD_LEN bytes]
Identity fingerprint [HASH_LEN bytes]
The port and address field denote the IPv4 address and port of the next
onion router in the circuit; the public key hash is the hash of the PKCS#1
ASN1 encoding of the next onion router's identity (signing) key. (See 0.3
above.) Including this hash allows the extending OR verify that it is
indeed connected to the correct target OR, and prevents certain
man-in-the-middle attacks.
The payload for a CREATED cell, or the relay payload for an
EXTENDED cell, contains:
DH data (g^y) [DH_LEN bytes]
Derivative key data (KH) [HASH_LEN bytes] <see 5.2 below>
The CircID for a CREATE cell is an arbitrarily chosen 2-byte integer,
selected by the node (OP or OR) that sends the CREATE cell. 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: 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.
(An OP with no public key MAY choose any CircID it wishes, since an OP
never needs to process a CREATE cell.)
Public keys are compared numerically by modulus.
As usual with DH, x and y MUST be generated randomly.
5.1.1. CREATE_FAST/CREATED_FAST cells
When initializing the first hop of a circuit, the OP has already
established the OR's identity and negotiated a secret key using TLS.
Because of this, it is not always necessary for the OP to perform the
public key operations to create a circuit. In this case, the
OP MAY send a CREATE_FAST cell instead of a CREATE cell for the first
hop only. The OR responds with a CREATED_FAST cell, and the circuit is
created.
A CREATE_FAST cell contains:
Key material (X) [HASH_LEN bytes]
A CREATED_FAST cell contains:
Key material (Y) [HASH_LEN bytes]
Derivative key data [HASH_LEN bytes] (See 5.2 below)
The values of X and Y must be generated randomly.
If an OR sees a circuit created with CREATE_FAST, the OR is sure to be the
first hop of a circuit. ORs SHOULD reject attempts to create streams with
RELAY_BEGIN exiting the circuit at the first hop: letting Tor be used as a
single hop proxy makes exit nodes a more attractive target for compromise.
5.2. Setting circuit keys
Once the handshake between the OP and an OR is completed, both can
now calculate g^xy with ordinary DH. Before computing g^xy, both client
and server MUST verify that the received g^x or g^y value is not degenerate;
that is, it must be strictly greater than 1 and strictly less than p-1
where p is the DH modulus. Implementations MUST NOT complete a handshake
with degenerate keys. Implementations MUST NOT discard other "weak"
g^x values.
(Discarding degenerate keys is critical for security; if bad keys
are not discarded, an attacker can substitute the server's CREATED
cell's g^y with 0 or 1, thus creating a known g^xy and impersonating
the server. Discarding other keys may allow attacks to learn bits of
the private key.)
If CREATE or EXTEND is used to extend a circuit, the client and server
base their key material on K0=g^xy, represented as a big-endian unsigned
integer.
If CREATE_FAST is used, the client and server base their key material on
K0=X|Y.
From the base key material K0, they compute KEY_LEN*2+HASH_LEN*3 bytes of
derivative key data as
K = H(K0 | [00]) | H(K0 | [01]) | H(K0 | [02]) | ...
The first HASH_LEN bytes of K form KH; the next HASH_LEN form the forward
digest Df; the next HASH_LEN 41-60 form the backward digest Db; the next
KEY_LEN 61-76 form Kf, and the final KEY_LEN form Kb. Excess bytes from K
are discarded.
KH is used in the handshake response to demonstrate knowledge of the
computed shared key. Df is used to seed the integrity-checking hash
for the stream of data going from the OP to the OR, and Db seeds the
integrity-checking hash for the data stream from the OR to the OP. Kf
is used to encrypt the stream of data going from the OP to the OR, and
Kb is used to encrypt the stream of data going from the OR to the OP.
5.3. Creating circuits
When creating a circuit through the network, the circuit creator
(OP) performs the following steps:
1. Choose an onion router as an exit node (R_N), such that the onion
router's exit policy includes at least one pending stream that
needs a circuit (if there are any).
2. Choose a chain of (N-1) onion routers
(R_1...R_N-1) to constitute the path, such that no router
appears in the path twice.
3. If not already connected to the first router in the chain,
open a new connection to that router.
4. Choose a circID not already in use on the connection with the
first router in the chain; send a CREATE cell along the
connection, to be received by the first onion router.
5. Wait until a CREATED cell is received; finish the handshake
and extract the forward key Kf_1 and the backward key Kb_1.
6. For each subsequent onion router R (R_2 through R_N), extend
the circuit to R.
To extend the circuit by a single onion router R_M, the OP performs
these steps:
1. Create an onion skin, encrypted to R_M's public onion key.
2. Send the onion skin in a relay EXTEND cell along
the circuit (see section 5).
3. When a relay EXTENDED cell is received, verify KH, and
calculate the shared keys. The circuit is now extended.
When an onion router receives an EXTEND relay cell, it sends a CREATE
cell to the next onion router, with the enclosed onion skin as its
payload. As special cases, if the extend cell includes a digest of
all zeroes, or asks to extend back to the relay that sent the extend
cell, the circuit will fail and be torn down. The initiating onion
router chooses some circID not yet used on the connection between the
two onion routers. (But see section 5.1. above, concerning choosing
circIDs based on lexicographic order of nicknames.)
When an onion router receives a CREATE cell, if it already has a
circuit on the given connection with the given circID, it drops the
cell. Otherwise, after receiving the CREATE cell, it completes the
DH handshake, and replies with a CREATED cell. Upon receiving a
CREATED cell, an onion router packs it payload into an EXTENDED relay
cell (see section 5), and sends that cell up the circuit. Upon
receiving the EXTENDED relay cell, the OP can retrieve g^y.
(As an optimization, OR implementations may delay processing onions
until a break in traffic allows time to do so without harming
network latency too greatly.)
5.3.1. Canonical connections
It is possible for an attacker to launch a man-in-the-middle attack
against a connection by telling OR Alice to extend to OR Bob at some
address X controlled by the attacker. The attacker cannot read the
encrypted traffic, but the attacker is now in a position to count all
bytes sent between Alice and Bob (assuming Alice was not already
connected to Bob.)
To prevent this, when an OR we gets an extend request, it SHOULD use an
existing OR connection if the ID matches, and ANY of the following
conditions hold:
- The IP matches the requested IP.
- The OR knows that the IP of the connection it's using is canonical
because it was listed in the NETINFO cell.
- The OR knows that the IP of the connection it's using is canonical
because it was listed in the server descriptor.
[This is not implemented in Tor 0.2.0.23-rc.]
5.4. Tearing down circuits
Circuits are torn down when an unrecoverable error occurs along
the circuit, or when all streams on a circuit are closed and the
circuit's intended lifetime is over. Circuits may be torn down
either completely or hop-by-hop.
To tear down a circuit completely, an OR or OP sends a DESTROY
cell to the adjacent nodes on that circuit, using the appropriate
direction's circID.
Upon receiving an outgoing DESTROY cell, an OR frees resources
associated with the corresponding circuit. If it's not the end of
the circuit, it sends a DESTROY cell for that circuit to the next OR
in the circuit. If the node is the end of the circuit, then it tears
down any associated edge connections (see section 6.1).
After a DESTROY cell has been processed, an OR ignores all data or
destroy cells for the corresponding circuit.
To tear down part of a circuit, the OP may send a RELAY_TRUNCATE cell
signaling a given OR (Stream ID zero). That OR sends a DESTROY
cell to the next node in the circuit, and replies to the OP with a
RELAY_TRUNCATED cell.
When an unrecoverable error occurs along one connection in a
circuit, the nodes on either side of the connection should, if they
are able, act as follows: the node closer to the OP should send a
RELAY_TRUNCATED cell towards the OP; the node farther from the OP
should send a DESTROY cell down the circuit.
The payload of a RELAY_TRUNCATED or DESTROY cell contains a single octet,
describing why the circuit is being closed or truncated. When sending a
TRUNCATED or DESTROY cell because of another TRUNCATED or DESTROY cell,
the error code should be propagated. The origin of a circuit always sets
this error code to 0, to avoid leaking its version.
The error codes are:
0 -- NONE (No reason given.)
1 -- PROTOCOL (Tor protocol violation.)
2 -- INTERNAL (Internal error.)
3 -- REQUESTED (A client sent a TRUNCATE command.)
4 -- HIBERNATING (Not currently operating; trying to save bandwidth.)
5 -- RESOURCELIMIT (Out of memory, sockets, or circuit IDs.)
6 -- CONNECTFAILED (Unable to reach server.)
7 -- OR_IDENTITY (Connected to server, but its OR identity was not
as expected.)
8 -- OR_CONN_CLOSED (The OR connection that was carrying this circuit
died.)
9 -- FINISHED (The circuit has expired for being dirty or old.)
10 -- TIMEOUT (Circuit construction took too long)
11 -- DESTROYED (The circuit was destroyed w/o client TRUNCATE)
12 -- NOSUCHSERVICE (Request for unknown hidden service)
5.5. Routing relay cells
When an OR receives a RELAY or RELAY_EARLY cell, it checks the cell's
circID and determines whether it has a corresponding circuit along that
connection. If not, the OR drops the 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 the stream cipher, as follows:
'Forward' relay cell (same direction as CREATE):
Use Kf as key; decrypt.
'Back' relay cell (opposite direction from CREATE):
Use Kb as key; encrypt.
Note that in counter mode, decrypt and encrypt are the same operation.
The OR then decides whether it recognizes the relay cell, by
inspecting the payload as described in section 6.1 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 the stream cipher as follows:
OP receives data cell:
For I=N...1,
Decrypt with Kb_I. If the payload is recognized (see
section 6..1), then stop and process the payload.
For more information, see section 6 below.
5.6. Handling relay_early cells
A RELAY_EARLY cell is designed to limit the length any circuit can reach.
When an OR receives a RELAY_EARLY cell, and the next node in the circuit
is speaking v2 of the link protocol or later, the OR relays the cell as a
RELAY_EARLY cell. Otherwise, it relays it as a RELAY cell.
If a node ever receives more than 8 RELAY_EARLY cells on a given
outbound circuit, it SHOULD close the circuit. (For historical reasons,
we don't limit the number of inbound RELAY_EARLY cells; they should
be harmless anyway because clients won't accept extend requests. See
bug 1038.)
When speaking v2 of the link protocol or later, clients MUST only send
EXTEND cells inside RELAY_EARLY cells. Clients SHOULD send the first ~8
RELAY cells that are not targeted at the first hop of any circuit as
RELAY_EARLY cells too, in order to partially conceal the circuit length.
[In a future version of Tor, servers will reject any EXTEND cell not
received in a RELAY_EARLY cell. See proposal 110.]
6. Application connections and stream management
6.1. Relay cells
Within a circuit, the OP and the exit node use the contents of
RELAY packets to tunnel end-to-end commands and TCP connections
("Streams") across circuits. End-to-end commands 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 [CELL_LEN-14 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] [sometimes control]
6 -- RELAY_EXTEND [forward] [control]
7 -- RELAY_EXTENDED [backward] [control]
8 -- RELAY_TRUNCATE [forward] [control]
9 -- RELAY_TRUNCATED [backward] [control]
10 -- RELAY_DROP [forward or backward] [control]
11 -- RELAY_RESOLVE [forward]
12 -- RELAY_RESOLVED [backward]
13 -- RELAY_BEGIN_DIR [forward]
32..40 -- Used for hidden services; see rend-spec.txt.
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 is computed as the first four bytes of
the running digest of all the bytes that 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 5.2 above),
and including this RELAY cell's entire payload (taken with the digest
field set to zero).
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 5.5 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 stream have the
same stream ID. StreamIDs are chosen arbitrarily by the OP. RELAY
cells that affect the entire circuit rather than a particular
stream use a StreamID of zero -- they are marked in the table above
as "[control]" style cells. (Sendme cells are marked as "sometimes
control" because they can take include a StreamID or not depending
on their purpose -- see Section 7.)
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.
6.2. Opening streams and transferring data
To open a new anonymized TCP connection, the OP chooses an open
circuit to an exit that may be able to connect to the destination
address, selects an arbitrary StreamID not yet used on that circuit,
and constructs a RELAY_BEGIN 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 a decimal integer between 1 and 65535, inclusive.
[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, and opens a new TCP connection to the target port. If the
address cannot be resolved, or a connection can't be established, the
exit node replies with a RELAY_END cell. (See 6.4 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 No version of Tor currently generates the IPv6 format.]
[Tor servers before 0.1.2.0 set the TTL field to a fixed value. Later
versions set the TTL to the last value seen from a DNS server, and expire
their own cached entries after a fixed interval. This prevents certain
attacks.]
The OP waits for a RELAY_CONNECTED cell before sending any data.
Once a connection has been established, the OP and exit node
package stream data in RELAY_DATA cells, and upon receiving such
cells, echo their contents to the corresponding TCP stream.
RELAY_DATA cells sent to unrecognized streams are dropped.
Relay RELAY_DROP cells are long-range dummies; upon receiving such
a cell, the OR or OP must drop it.
6.2.1. Opening a directory stream
If a Tor server is a directory server, it should respond to a
RELAY_BEGIN_DIR cell as if it had received a BEGIN cell requesting a
connection to its directory port. RELAY_BEGIN_DIR cells ignore exit
policy, since the stream is local to the Tor process.
If the Tor server is not running a directory service, it should respond
with a REASON_NOTDIRECTORY RELAY_END cell.
Clients MUST generate an all-zero payload for RELAY_BEGIN_DIR cells,
and servers MUST ignore the payload.
[RELAY_BEGIN_DIR was not supported before Tor 0.1.2.2-alpha; clients
SHOULD NOT send it to routers running earlier versions of Tor.]
6.3. Closing streams
When an anonymized TCP connection is closed, or an edge node
encounters error on any stream, it sends a 'RELAY_END' cell along the
circuit (if possible) and closes the TCP connection immediately. If
an edge node receives a 'RELAY_END' cell for any stream, it closes
the TCP connection completely, and sends nothing more along the
circuit for that stream.
The payload of a RELAY_END cell begins with a single 'reason' byte to
describe why the stream is closing, plus optional data (depending on
the reason.) The values are:
1 -- REASON_MISC (catch-all for unlisted reasons)
2 -- REASON_RESOLVEFAILED (couldn't look up hostname)
3 -- REASON_CONNECTREFUSED (remote host refused connection) [*]
4 -- REASON_EXITPOLICY (OR refuses to connect to host or port)
5 -- REASON_DESTROY (Circuit is being destroyed)
6 -- REASON_DONE (Anonymized TCP connection was closed)
7 -- REASON_TIMEOUT (Connection timed out, or OR timed out
while connecting)
8 -- REASON_NOROUTE (Routing error while attempting to
contact destination)
9 -- REASON_HIBERNATING (OR is temporarily hibernating)
10 -- REASON_INTERNAL (Internal error at the OR)
11 -- REASON_RESOURCELIMIT (OR has no resources to fulfill request)
12 -- REASON_CONNRESET (Connection was unexpectedly reset)
13 -- REASON_TORPROTOCOL (Sent when closing connection because of
Tor protocol violations.)
14 -- REASON_NOTDIRECTORY (Client sent RELAY_BEGIN_DIR to a
non-directory server.)
(With REASON_EXITPOLICY, the 4-byte IPv4 address or 16-byte IPv6 address
forms the optional data, along with a 4-byte TTL; no other reason
currently has extra data.)
OPs and ORs MUST accept reasons not on the above list, since future
versions of Tor may provide more fine-grained reasons.
Tors SHOULD NOT send any reason except REASON_MISC for a stream that they
have originated.
[*] Older versions of Tor also send this reason when connections are
reset.
--- [The rest of this section describes unimplemented functionality.]
Because TCP connections can be half-open, we follow an equivalent
to TCP's FIN/FIN-ACK/ACK protocol to close streams.
An exit connection can have a TCP stream in one of three states:
'OPEN', 'DONE_PACKAGING', and 'DONE_DELIVERING'. For the purposes
of modeling transitions, we treat 'CLOSED' as a fourth state,
although connections in this state are not, in fact, tracked by the
onion router.
A stream begins in the 'OPEN' state. Upon receiving a 'FIN' from
the corresponding TCP connection, the edge node sends a 'RELAY_FIN'
cell along the circuit and changes its state to 'DONE_PACKAGING'.
Upon receiving a 'RELAY_FIN' cell, an edge node sends a 'FIN' to
the corresponding TCP connection (e.g., by calling
shutdown(SHUT_WR)) and changing its state to 'DONE_DELIVERING'.
When a stream in already in 'DONE_DELIVERING' receives a 'FIN', it
also sends a 'RELAY_FIN' along the circuit, and changes its state
to 'CLOSED'. When a stream already in 'DONE_PACKAGING' receives a
'RELAY_FIN' cell, it sends a 'FIN' and changes its state to
'CLOSED'.
If an edge node encounters an error on any stream, it sends a
'RELAY_END' cell (if possible) and closes the stream immediately.
6.4. Remote hostname lookup
To find the address associated with a hostname, the OP sends a
RELAY_RESOLVE cell containing the hostname to be resolved with a NUL
terminating byte. (For a reverse lookup, the OP sends a RELAY_RESOLVE
cell containing an in-addr.arpa address.) The OR replies with a
RELAY_RESOLVED cell containing a status byte, and any number of
answers. Each answer is of the form:
Type (1 octet)
Length (1 octet)
Value (variable-width)
TTL (4 octets)
"Length" is the length of the Value field.
"Type" is one of:
0x00 -- Hostname
0x04 -- IPv4 address
0x06 -- IPv6 address
0xF0 -- Error, transient
0xF1 -- Error, nontransient
If any answer has a type of 'Error', then no other answer may be given.
The RELAY_RESOLVE cell must use a nonzero, distinct streamID; the
corresponding RELAY_RESOLVED cell must use the same streamID. No stream
is actually created by the OR when resolving the name.
7. Flow control
7.1. Link throttling
Each client or relay should do appropriate bandwidth throttling to
keep its user happy.
Communicants rely on TCP's default flow control to push back when they
stop reading.
The mainline Tor implementation uses token buckets (one for reads,
one for writes) for the rate limiting.
Since 0.2.0.x, Tor has let the user specify an additional pair of
token buckets for "relayed" traffic, so people can deploy a Tor relay
with strict rate limiting, but also use the same Tor as a client. To
avoid partitioning concerns we combine both classes of traffic over a
given OR connection, and keep track of the last time we read or wrote
a high-priority (non-relayed) cell. If it's been less than N seconds
(currently N=30), we give the whole connection high priority, else we
give the whole connection low priority. We also give low priority
to reads and writes for connections that are serving directory
information. See proposal 111 for details.
7.2. Link padding
Link padding can be created by sending PADDING cells along the
connection; relay cells of type "DROP" can be used for long-range
padding.
Currently nodes are not required to do any sort of link padding or
dummy traffic. Because strong attacks exist even with link padding,
and because link padding greatly increases the bandwidth requirements
for running a node, we plan to leave out link padding until this
tradeoff is better understood.
7.3. Circuit-level flow control
To control a circuit's bandwidth usage, each OR keeps track of two
'windows', consisting of how many RELAY_DATA cells it is allowed to
originate (package for transmission), and how many RELAY_DATA cells
it is willing to consume (receive for local streams). These limits
do not apply to cells that the OR receives from one host and relays
to another.
Each 'window' value is initially set to 1000 data cells
in each direction (cells that are not data cells do not affect
the window). When an OR is willing to deliver more cells, it sends a
RELAY_SENDME cell towards the OP, with Stream ID zero. When an OR
receives a RELAY_SENDME cell with stream ID zero, it increments its
packaging window.
Each of these cells increments the corresponding window by 100.
The OP behaves identically, except that it must track a packaging
window and a delivery window for every OR in the circuit.
An OR or OP sends cells to increment its delivery window when the
corresponding window value falls under some threshold (900).
If a packaging window reaches 0, the OR or OP stops reading from
TCP connections for all streams on the corresponding circuit, and
sends no more RELAY_DATA cells until receiving a RELAY_SENDME cell.
[this stuff is badly worded; copy in the tor-design section -RD]
7.4. Stream-level flow control
Edge nodes use RELAY_SENDME cells to implement end-to-end flow
control for individual connections across circuits. Similarly to
circuit-level flow control, edge nodes begin with a window of cells
(500) per stream, and increment the window by a fixed value (50)
upon receiving a RELAY_SENDME cell. Edge nodes initiate RELAY_SENDME
cells when both a) the window is <= 450, and b) there are less than
ten cell payloads remaining to be flushed at that edge.
A.1. Differences between spec and implementation
- The current specification requires all ORs to have IPv4 addresses, but
allows servers to exit and resolve to IPv6 addresses, and to declare IPv6
addresses in their exit policies. The current codebase has no IPv6
support at all.
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