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61 lines
2.5 KiB
Plaintext
61 lines
2.5 KiB
Plaintext
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0. Intro.
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Onion Routing is still very much in development stages. This document
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aims to get you started in the right direction if you want to understand
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the code, add features, fix bugs, etc.
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Read the README file first, so you can get familiar with the basics.
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1. The pieces.
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1.1 Connections. A connection is a long-standing tcp socket between
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nodes. A connection is named based on what it's connected to -- an "OR
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connection" has an onion router on the other end, an "OP connection" has
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an onion proxy on the other end, an "exit connection" has a website or
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other server on the other end, and an "AP connection" has an application
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proxy (and thus a user) on the other end.
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1.2. Circuits. A circuit is a single conversation between two
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participants over the onion routing network. One end of the circuit has
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an AP connection, and the other end has an exit connection. AP and exit
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connections have only one circuit associated with them, whereas OP and
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OR connections multiplex many circuits at once.
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1.3. Cells. Some connections, specifically OR and OP connections, speak
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"cells". This means that data over that connection is bundled into 128
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byte packets (8 bytes of header and 120 bytes of payload). Each cell has
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a type, or "command", which indicates what it's for.
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2. Other features.
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2.1. Bandwidth throttling. Each cell-speaking connection has a maximum
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bandwidth it can use, as specified in the routers.or file. Bandwidth
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throttling occurs on both the sender side and the receiving side. The
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sending side sends cells at regularly spaced intervals (e.g., a connection
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with a bandwidth of 12800B/s would queue a cell every 10ms). The receiving
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side protects against misbehaving servers that send cells more frequently,
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by using a simple token bucket:
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Each connection has a token bucket with a specified capacity. Tokens are
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added to the bucket each second (when the bucket is full, new tokens
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are discarded.) Each token represents permission to receive one byte
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from the network --- to receive a byte, the connection must remove a
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token from the bucket. Thus if the bucket is empty, that connection must
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wait until more tokens arrive. The number of tokens we add enforces a
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longterm average rate of incoming bytes, yet we still permit short-term
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bursts above the allowed bandwidth. Currently bucket sizes are set to
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ten seconds worth of traffic.
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The bandwidth throttling uses TCP to push back when we stop reading.
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We extend it with token buckets to allow more flexibility for traffic
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bursts.
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2.2. Data congestion control.
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