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$Id$
Tor Protocol Specification
Roger Dingledine
Nick Mathewson
Note: This document specifies Tor as currently implemented in versions
0.1.2.1-alpha and earlier. Current protocol designs are described in
tor-spec.txt.
0. Preliminaries
0.1. Notation and encoding
PK -- a public key.
SK -- a private key.
K -- a key for a symmetric cypher.
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.
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 padding, with SHA-1 as its digest
function. (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.
For a hash function, we use SHA-1.
KEY_LEN=16.
DH_LEN=128; DH_GROUP_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.
2. Connections
There are two ways to connect to an onion router (OR). The first is
as an onion proxy (OP), which allows the OP to authenticate the OR
without authenticating itself. The second is as another OR, which
allows mutual authentication.
Tor uses TLS for link encryption. All implementations MUST support
the TLS ciphersuite "TLS_EDH_RSA_WITH_DES_192_CBC3_SHA", and SHOULD
support "TLS_DHE_RSA_WITH_AES_128_CBC_SHA" if it is available.
Implementations MAY support other ciphersuites, but MUST NOT
support any suite without ephemeral keys, symmetric keys of at
least KEY_LEN bits, and digests of at least HASH_LEN bits.
An OP or OR always sends a two-certificate chain, consisting of a
certificate using a short-term connection key and a second, self-
signed certificate containing the OR's identity key. The commonName of the
first certificate is the OR's nickname, and the commonName of the second
certificate is the OR's nickname, followed by a space and the string
"<identity>".
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.
All parties SHOULD reject connections to or from ORs that have malformed
or missing certificates. ORs MAY accept or reject connections from OPs
with malformed or missing certificates.
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. An OP or an OR may close a
connection to an OR if there are no circuits running over the
connection, and an amount of time (KeepalivePeriod, defaults to 5
minutes) has passed.
(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.)
3. Cell Packet format
The basic unit of communication for onion routers and onion
proxies is a fixed-width "cell". Each cell contains the following
fields:
CircID [2 bytes]
Command [1 byte]
Payload (padded with 0 bytes) [CELL_LEN-3 bytes]
[Total size: CELL_LEN 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 6.2)
1 -- CREATE (Create a circuit) (See Sec 4.1)
2 -- CREATED (Acknowledge create) (See Sec 4.1)
3 -- RELAY (End-to-end data) (See Sec 4.5 and 5)
4 -- DESTROY (Stop using a circuit) (See Sec 4.4)
5 -- CREATE_FAST (Create a circuit, no PK) (See Sec 4.1)
6 -- CREATED_FAST (Circuit created, no PK) (See Sec 4.1)
7 -- HELLO (Introduce the OR) (See Sec 7.1)
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 4.4)
Upon receiving any other value for the command field, an OR must
drop the cell.
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 4 below.
RELAY cells are used to send commands and data along a circuit; see
section 5 below.
HELLO cells are used to introduce parameters and characteristics of
Tor clients and servers when connections are established.
4. Circuit management
4.1. 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 4.2). 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 public key, giving
an onion-skin of:
PK-encrypted:
Padding 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 4.2 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 OR 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 OR has a lower key, it chooses a CircID with
an MSB of 0; otherwise, it chooses a CircID with an MSB of 1.
Public keys are compared numerically by modulus.
As usual with DH, x and y MUST be generated randomly.
4.1.1. 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 4.2 below)
The values of X and Y must be generated randomly.
[Versions of Tor before 0.1.0.6-rc did not support these cell types;
clients should not send CREATE_FAST cells to older Tor servers.]
4.2. 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.)
(The mainline Tor implementation, in the 0.1.1.x-alpha series, discarded
all g^x values less than 2^24, greater than p-2^24, or having more than
1024-16 identical bits. This served no useful purpose, and we stopped.)
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.
4.3. 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 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. The initiating onion router chooses some circID not yet
used on the connection between the two onion routers. (But see
section 4.1. 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.)
4.4. 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 5.1).
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.)
[Versions of Tor prior to 0.1.0.11 didn't send reasons; implementations
MUST accept empty TRUNCATED and DESTROY cells.]
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 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 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 the stream cipher as follows:
OP receives data cell:
For I=N...1,
Decrypt with Kb_I. If the payload is recognized (see
section 5.1), then stop and process the payload.
For more information, see section 5 below.
5. Application connections and stream management
5.1. 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]
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]
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 4.2 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 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 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.
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. 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 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, 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 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.
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.
5.3. 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 -- (unallocated) [**]
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.)
(With REASON_EXITPOLICY, the 4-byte IPv4 address or 16-byte IPv6 address
forms the optional data; no other reason currently has extra data.
As of 0.1.1.6, the body also contains a 4-byte TTL.)
OPs and ORs MUST accept reasons not on the above list, since future
versions of Tor may provide more fine-grained reasons.
[*] Older versions of Tor also send this reason when connections are
reset.
[**] Due to a bug in versions of Tor through 0095, error reason 8 must
remain allocated until that version is obsolete.
--- [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.
5.4. Remote hostname lookup
To find the address associated with a hostname, the OP sends a
RELAY_RESOLVE cell containing the hostname to be resolved. (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.
6. Flow control
6.1. Link throttling
Each node 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.
6.2. Link 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.
6.3. 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 package for transmission, and how many RELAY_DATA cells
it is willing to deliver to streams outside the network.
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]
6.4. 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.
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