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17
doc/FAQ
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@ -38,17 +38,16 @@ for you.]
3. Running tor.
3.1. What's this about roles? What kind of server should I run?
3.1. What kind of server should I run?
The same executable ("or") functions as both client and server, depending
on the value of the config variable named 'Role'. Role represents a
combination of which tasks this particular tor server will do. The default
Role (role 15) is an onion router: it listens for onion routers, listens
for onion proxies, listens for application proxies, and it connects to
all other onion routers it learns about. A directory server (role 63)
does all of the above and also serves directory requests. A simple
onion proxy, on the other hand (role 8), only listens for application
proxies. See part 3.1 of the HACKING document for more technical details.
on which ports are specified in the configuration file. You can specify:
* APPort: client applications (eg privoxy, Mozilla) can speak socks to
this port.
* OPPort: onion proxies (client onion routers) connect to this port.
* ORPort: other onion routers connect to this port
* DirPort: onion proxies and onion routers speak http to this port, to
pull down a directory of which nodes are currently available.
3.2. So I can just run a full onion router and join the network?

58
doc/HACKING
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@ -8,16 +8,20 @@ Read the README file first, so you can get familiar with the basics.
1. The programs.
1.1. "or". This is the main program here. It functions as both a server
and a client, depending on which config file you give it. ...
1.1. "or". This is the main program here. It functions as either a server
or a client, depending on which config file you give it.
1.2. "orkeygen". Use "orkeygen file-for-privkey file-for-pubkey" to
generate key files for an onion router.
2. The pieces.
2.1. Routers. Onion routers, as far as the 'or' program is concerned,
are a bunch of data items that are loaded into the router_array when
the program starts. After it's loaded, the router information is never
changed. When a new OR connection is started (see below), the relevant
information is copied from the router struct to the connection struct.
the program starts. Periodically it downloads a new set of routers
from a directory server, and updates the router_array. When a new OR
connection is started (see below), the relevant information is copied
from the router struct to the connection struct.
2.2. Connections. A connection is a long-standing tcp socket between
nodes. A connection is named based on what it's connected to -- an "OR
@ -26,34 +30,36 @@ an onion proxy on the other end, an "exit connection" has a website or
other server on the other end, and an "AP connection" has an application
proxy (and thus a user) on the other end.
2.3. Circuits. A circuit is a single conversation between two
participants over the onion routing network. One end of the circuit has
an AP connection, and the other end has an exit connection. AP and exit
2.3. Circuits. A circuit is a path over the onion routing
network. Applications can connect to one end of the circuit, and can
create exit connections at the other end of the circuit. AP and exit
connections have only one circuit associated with them (and thus these
connection types are closed when the circuit is closed), whereas OP and
OR connections multiplex many circuits at once, and stay standing even
when there are no circuits running over them.
2.4. Topics. Topics are specific conversations between an AP and an exit.
Topics are multiplexed over circuits.
2.4. Cells. Some connections, specifically OR and OP connections, speak
"cells". This means that data over that connection is bundled into 128
byte packets (8 bytes of header and 120 bytes of payload). Each cell has
"cells". This means that data over that connection is bundled into 256
byte packets (8 bytes of header and 248 bytes of payload). Each cell has
a type, or "command", which indicates what it's for.
3. Important parameters in the code.
3.1. Role.
4. Robustness features.
4.1. Bandwidth throttling. Each cell-speaking connection has a maximum
bandwidth it can use, as specified in the routers.or file. Bandwidth
throttling occurs on both the sender side and the receiving side. The
sending side sends cells at regularly spaced intervals (e.g., a connection
with a bandwidth of 12800B/s would queue a cell every 10ms). The receiving
side protects against misbehaving servers that send cells more frequently,
by using a simple token bucket:
throttling can occur on both the sender side and the receiving side. If
the LinkPadding option is on, the sending side sends cells at regularly
spaced intervals (e.g., a connection with a bandwidth of 25600B/s would
queue a cell every 10ms). The receiving side protects against misbehaving
servers that send cells more frequently, by using a simple token bucket:
Each connection has a token bucket with a specified capacity. Tokens are
added to the bucket each second (when the bucket is full, new tokens
@ -79,22 +85,12 @@ he owns, and then refuse to read any of the bytes at the webserver end
of the circuit. These bottlenecks can propagate back through the entire
network, mucking up everything.
To handle this congestion, each circuit starts out with a receive
window at each node of 100 cells -- it is willing to receive at most 100
cells on that circuit. (It handles each direction separately; so that's
really 100 cells forward and 100 cells back.) The edge of the circuit
is willing to create at most 100 cells from data coming from outside the
onion routing network. Nodes in the middle of the circuit will tear down
the circuit if a data cell arrives when the receive window is 0. When
data has traversed the network, the edge node buffers it on its outbuf,
and evaluates whether to respond with a 'sendme' acknowledgement: if its
outbuf is not too full, and its receive window is less than 90, then it
queues a 'sendme' cell backwards in the circuit. Each node that receives
the sendme increments its window by 10 and passes the cell onward.
(See the tor-spec.txt document for details of how congestion control
works.)
In practice, all the nodes in the circuit maintain a receive window
close to 100 except the exit node, which stays around 0, periodically
receiving a sendme and reading 10 more data cells from the webserver.
close to maximum except the exit node, which stays around 0, periodically
receiving a sendme and reading more data cells from the webserver.
In this way we can use pretty much all of the available bandwidth for
data, but gracefully back off when faced with multiple circuits (a new
sendme arrives only after some cells have traversed the entire network),
@ -108,7 +104,7 @@ congestion control; so far it's enough.
4.3. Router twins. In many cases when we ask for a router with a given
address and port, we really mean a router who knows a given key. Router
twins are two or more routers that all share the same private key. We thus
twins are two or more routers that share the same private key. We thus
give routers extra flexibility in choosing the next hop in the circuit: if
some of the twins are down or slow, it can choose the more available ones.

20
doc/tor-spec.txt
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@ -163,7 +163,7 @@ which reveals the downstream node.
First, the OP generates a pair of 8-byte symmetric keys (one
[K_f] for the 'forward' stream from OP to OR, and one
[K_b] for the 'backward' stream from OR to OP.
[K_b] for the 'backward' stream from OR to OP).
The OP generates a message [M] in the following format:
Maximum bandwidth (bytes/s) [4 bytes]
@ -223,15 +223,15 @@ which reveals the downstream node.
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
proxies is a fixed-width "cell". Each cell contains the following
fields:
ACI (anonymous circuit identifier) [2 bytes]
Command [1 byte]
Length [1 byte]
Sequence number (unused, set to 0) [4 bytes]
Payload (padded with 0 bytes) [120 bytes]
[Total size: 128 bytes]
Payload (padded with 0 bytes) [248 bytes]
[Total size: 256 bytes]
The 'Command' field holds one of the following values:
0 -- PADDING (Padding) (See Sec 6.2)
@ -242,10 +242,10 @@ which reveals the downstream node.
The interpretation of 'Length' and 'Payload' depend on the type of
the cell.
PADDING: Length is 0; Payload is 120 bytes of 0's.
CREATE: Length is a value between 1 and 120; the first 'length'
PADDING: Length is 0; Payload is 248 bytes of 0's.
CREATE: Length is a value between 1 and 248; the first 'length'
bytes of payload contain a portion of an onion.
DATA: Length is a value between 4 and 120; the first 'length'
DATA: Length is a value between 4 and 248; the first 'length'
bytes of payload contain useful data.
DESTROY: Neither field is used.
SENDME: Length encodes a window size, payload is unused.
@ -335,10 +335,10 @@ which reveals the downstream node.
side, then let the high bit of the ACI be 1, else 0.
3. To send M over the wire, prepend a 4-byte integer containing
Len(M). Call the result M'. Let N=ceil(Len(M')/120).
Len(M). Call the result M'. Let N=ceil(Len(M')/248).
Divide M' into N chunks, such that:
Chunk_I = M'[(I-1)*120:I*120] for 1 <= I <= N-1
Chunk_N = M'[(N-1)*120:Len(M')]
Chunk_I = M'[(I-1)*248:I*248] for 1 <= I <= N-1
Chunk_N = M'[(N-1)*248:Len(M')]
4. Send N CREATE cells along the connection, setting the ACI
on each to the selected ACI, setting the payload on each to