specification - Onion Routing

1. Introduction
This document specifies the requirements for each of the sub-systems of the onion routing network, version number
2.0, and for the common data objects shared by those sub-systems.
2. Common Data Objects
This section describes the common data objects, their content, and requirements for operations related to their use.
2.1 Cells
A cell is a data structure consisting of a cell header section and a payload section. The cell is the minimum message
unit transmitted on an anonymous circuit or a virtual pipe. Cells carry both user application data and network control
and maintenance data.
2.1.1 Cell Structure
Cells consist of a header section and a payload section. Cell header structures are common to all cells. However,
cell payload structures are different for each type of cell. The following diagram outlines the cell structure.
3 2 1
1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
Command
Anonymous Circuit Identifier
Payload (132 bytes)
ÿ
ÿ
ÿ
Figure 2.1.1 - Cell Structure
All of the fields in all of the various cell types, except an ONION_INFO cell, are formatted in network byte order.
Therefore, conversion routines such as htonl(3N), htons(3N), ntohl(3N), and ntohs(3N) must be used when
formatting or retrieving data from cell fields respectively. The ONION_INFO cell is different because the data
fields in the payload of an ONION_INFO cell are specified in host byte order. The fields of the cell will be
explained in more detail in subsequent sections.
2.1.1.1 Cell Header Field Structure
The header consists of two fields, the command field and the anonymous circuit identifier.
The command field is one byte and identifies the cell type. The command field can be set to one of the following
symbolic values:
a) CREATE
b) DATA
c) DESTROY
d) PADDING
e) ONION_INFO
f) DB_UPDATE
g) DB_QUERY
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