67 lines
2.9 KiB
Markdown
67 lines
2.9 KiB
Markdown
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<h1>Data Packet</h1>
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<h2>Table of Contents</h2>
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- [Introduction](#introduction)
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- [Packet Header](#packet-header)
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- [Body](#body)
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***
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## Introduction
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A `data packet` contains user specified data. This can be any data, as long as the sender and receiver
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both understand what it is, without further help. Intermediate hops, which route the data have sufficient
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information with the header to know where to forward the packet. In practice, the data will be encrypted
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to avoid eavesdropping by intermediate hops.
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## Packet Header
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The packet header has a fixed size of 36 bytes, with the following layout:
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```
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0 1 2 3
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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|Reserved | Length | Hop Limit |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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+ +
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+ Source IP +
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+ +
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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+ +
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+ Destination IP +
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+ +
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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```
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The first 5 bits are reserved and must be set to 0.
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The next 19 bits are used to specify the length of the body. It is expected that
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the actual length of a packet does not exceed 256K right now, so the 19th bit is
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only needed because we have to account for some overhead related to the encryption.
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The next byte is the hop-limit. Every node decrements this value by 1 before sending
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the packet. If a node decrements this value to 0, the packet is discarded.
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The next 16 bytes contain the sender IP address.
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The final 16 bytes contain the destination IP address.
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## Body
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Following the header is a variable length body. The protocol does not have any requirements for the
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body, and the only requirement imposed is that the body is as long as specified in the header length
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field. It is technically legal according to the protocol to transmit a data packet without a body,
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i.e. a body length of 0. This is useless however, as there will not be any data to interpret.
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