
## Overview ##

This document describes the general structure of the Tor codebase, how
it fits together, what functionality is available for extending Tor,
and gives some notes on how Tor got that way.

Tor remains a work in progress: We've been working on it for nearly two
decades, and we've learned a lot about good coding since we first
started.  This means, however, that some of the older pieces of Tor will
have some "code smell" in them that could stand a brisk
refactoring.  So when I describe a piece of code, I'll sometimes give a
note on how it got that way, and whether I still think that's a good
idea.

The first drafts of this document were written in the Summer and Fall of
2015, when Tor 0.2.6 was the most recent stable version, and Tor 0.2.7
was under development.  There is a revision in progress (as of late
2019), to bring it up to pace with Tor as of version 0.4.2.  If you're
reading this far in the future, some things may have changed.  Caveat
haxxor!

This document is not an overview of the Tor protocol.  For that, see the
design paper and the specifications at https://spec.torproject.org/ .

For more information about Tor's coding standards and some helpful
development tools, see doc/HACKING in the Tor repository.


### The very high level ###

Ultimately, Tor runs as an event-driven network daemon: it responds to
network events, signals, and timers by sending and receiving things over
the network.  Clients, relays, and directory authorities all use the
same codebase: the Tor process will run as a client, relay, or authority
depending on its configuration.

Tor has a few major dependencies, including Libevent (used to tell which
sockets are readable and writable), OpenSSL or NSS (used for many encryption
functions, and to implement the TLS protocol), and zlib (used to
compress and uncompress directory information).

Most of Tor's work today is done in a single event-driven main thread.
Tor also spawns one or more worker threads to handle CPU-intensive
tasks.  (Right now, this only includes circuit encryption and the more
expensive compression algorithms.)

On startup, Tor initializes its libraries, reads and responds to its
configuration files, and launches a main event loop.  At first, the only
events that Tor listens for are a few signals (like TERM and HUP), and
one or more listener sockets (for different kinds of incoming
connections).  Tor also configures several timers to handle periodic
events.  As Tor runs over time, other events will open, and new events
will be scheduled.

The codebase is divided into a few top-level subdirectories, each of
which contains several sub-modules.

   * `src/ext` -- Code maintained elsewhere that we include in the Tor
     source distribution.

   * src/lib` -- Lower-level utility code, not necessarily tor-specific.

   * `src/trunnel` -- Automatically generated code (from the Trunnel
     tool): used to parse and encode binary formats.

   * `src/core` -- Networking code that is implements the central parts of
   the Tor protocol and main loop.

   * `src/feature` -- Aspects of Tor (like directory management, running a
   relay, running a directory authorities, managing a list of nodes,
   running and using onion services) that are built on top of the
   mainloop code.

   * `src/app` -- Highest-level functionality; responsible for setting up
   and configuring the Tor daemon, making sure all the lower-level
   modules start up when required, and so on.

   * `src/tools` -- Binaries other than Tor that we produce.  Currently this
   is tor-resolve, tor-gencert, and the tor_runner.o helper module.

   * `src/test` -- unit tests, regression tests, and a few integration
   tests.

In theory, the above parts of the codebase are sorted from highest-level to
lowest-level, where high-level code is only allowed to invoke lower-level
code, and lower-level code never includes or depends on code of a higher
level.  In practice, this refactoring is incomplete: The modules in `src/lib`
are well-factored, but there are many layer violations ("upward
dependencies") in `src/core` and `src/feature`.  We aim to eliminate those
over time.

### Some key high-level abstractions ###

The most important abstractions at Tor's high-level are Connections,
Channels, Circuits, and Nodes.

A 'Connection' represents a stream-based information flow.  Most
connections are TCP connections to remote Tor servers and clients. (But
as a shortcut, a relay will sometimes make a connection to itself
without actually using a TCP connection.  More details later on.)
Connections exist in different varieties, depending on what
functionality they provide.  The principle types of connection are
"edge" (eg a socks connection or a connection from an exit relay to a
destination), "OR" (a TLS stream connecting to a relay), "Directory" (an
HTTP connection to learn about the network), and "Control" (a connection
from a controller).

A 'Circuit' is persistent tunnel through the Tor network, established
with public-key cryptography, and used to send cells one or more hops.
Clients keep track of multi-hop circuits, and the cryptography
associated with each hop.  Relays, on the other hand, keep track only of
their hop of each circuit.

A 'Channel' is an abstract view of sending cells to and from a Tor
relay.  Currently, all channels are implemented using OR connections.
If we switch to other strategies in the future, we'll have more
connection types.

A 'Node' is a view of a Tor instance's current knowledge and opinions
about a Tor relay or bridge.

### The rest of this document. ###

> **Note**: This section describes the eventual organization of this
> document, which is not yet complete.

We'll begin with an overview of the facilities provided by the modules
in src/lib.  Knowing about these is key to writing portable, simple code
in Tor.

Then we'll move on to a discussion of how parts of the Tor codebase are
initialized, finalized, configured, and managed.

Then we'll go on and talk about the main data-flow of the Tor network:
how Tor generates and responds to network traffic.  This will occupy a
chapter for the main overview, with other chapters for special topics.

After that, we'll mention the main modules in src/features and describe the
functions of each.

We'll close with a meandering overview of important pending issues in
the Tor codebase, and how they affect the future of the Tor software.
