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0. Intro.
Onion Routing is still very much in development stages. This document
aims to get you started in the right direction if you want to understand
the code, add features, fix bugs, etc.
Read the README file first, so you can get familiar with the basics.
1. The pieces.
1.1 Connections. A connection is a long-standing tcp socket between
nodes. A connection is named based on what it's connected to -- an "OR
connection" has an onion router on the other end, an "OP connection" has
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.
1.2. 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
connections have only one circuit associated with them, whereas OP and
OR connections multiplex many circuits at once.
1.3. 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
a type, or "command", which indicates what it's for.
2. Other features.
2.1. 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:
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
are discarded.) Each token represents permission to receive one byte
from the network --- to receive a byte, the connection must remove a
token from the bucket. Thus if the bucket is empty, that connection must
wait until more tokens arrive. The number of tokens we add enforces a
longterm average rate of incoming bytes, yet we still permit short-term
bursts above the allowed bandwidth. Currently bucket sizes are set to
ten seconds worth of traffic.
The bandwidth throttling uses TCP to push back when we stop reading.
We extend it with token buckets to allow more flexibility for traffic
bursts.
2.2. Data congestion control.
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