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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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