Move much of 00-overview.md into doxygen.

2 files changed, 115 insertions, 122 deletions

diff --git a/doc/HACKING/design/00-overview.md b/doc/HACKING/design/00-overview.md
index ff40a566be..1c14dc8c10 100644
--- a/doc/HACKING/design/00-overview.md
+++ b/doc/HACKING/design/00-overview.md
@@ -1,124 +1,6 @@
 
 ## 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. ###
 
diff --git a/src/mainpage.dox b/src/mainpage.dox
index 84eea3c526..7c64e0dffb 100644
--- a/src/mainpage.dox
+++ b/src/mainpage.dox
@@ -3,9 +3,120 @@
 
 @section intro Getting to know Tor
 
-Welcome to the Tor source code documentation!  Here we have documentation for
-nearly every function, type, and module in the Tor source code.  The high-level
-documentation is a work in progress.  For now, have a look at the source code
-overview in doc/HACKING/design.
+Welcome!
+
+This documentation 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.  It also includes a reference for
+nearly every function, type, file, and module in the Tor source code.  The
+high-level documentation is a work in progress.
+
+Tor itself remains a work in progress too: 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 we
+describe a piece of code, we'll sometimes give a note on how it got that way,
+and whether we still think that's a good idea.
+
+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.
+
+@section highlevel 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.
+
+   - `ext` -- Code maintained elsewhere that we include in the Tor
+     source distribution.
+
+   - \refdir{lib} -- Lower-level utility code, not necessarily
+     tor-specific.
+
+   - `trunnel` -- Automatically generated code (from the Trunnel
+     tool): used to parse and encode binary formats.
+
+   - \refdir{core} -- Networking code that is implements the central
+     parts of the Tor protocol and main loop.
+
+   - \refdir{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.
+
+   - \refdir{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.
+
+   - \refdir{tools} -- Binaries other than Tor that we produce.
+      Currently this is tor-resolve, tor-gencert, and the tor_runner.o helper
+      module.
+
+   - `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
+\refdir{lib} are well-factored, but there are many layer violations ("upward
+dependencies") in \refdir{core} and \refdir{feature}.
+We aim to eliminate those over time.
+
+@section keyabstractions Some key high-level abstractions
+
+The most important abstractions at Tor's high-level are Connections,
+Channels, Circuits, and Nodes.
+
+A 'Connection' (connection_t) 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_connection_t (eg a socks connection or
+a connection from an exit relay to a destination), or_connection_t (a TLS
+stream connecting to a relay), dir_connection_t (an HTTP connection to learn
+about the network), and control_connection_t (a connection from a
+controller).
+
+A 'Circuit' (circuit_t) 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 (origin_circuit_t), and the
+cryptography associated with each hop.  Relays, on the other hand, keep track
+only of their hop of each circuit (or_circuit_t).
+
+A 'Channel' (channel_t) is an abstract view of sending cells to and from a
+Tor relay.  Currently, all channels are implemented using OR connections
+(channel_tls_t).  If we switch to other strategies in the future, we'll have
+more connection types.
+
+A 'Node' (node_t) is a view of a Tor instance's current knowledge and opinions
+about a Tor relay or bridge.
 
 **/