[tor-commits] [tor/master] More module-level documentation.

[nickm at torproject.org]

commit a5f07a6c2a2b937f58548680f9b87b1e75b3c8f8
Author: Nick Mathewson <nickm at torproject.org>
Date:   Wed Oct 26 11:02:57 2016 -0400

    More module-level documentation.
---
 src/or/directory.c     | 35 ++++++++++++++++++++++++++++++++---
 src/or/networkstatus.c | 30 ++++++++++++++++++++++++++++--
 src/or/nodelist.c      | 26 ++++++++++++++++++++++++++
 src/or/relay.c         | 35 +++++++++++++++++++++++++++++++++++
 src/or/torcert.c       | 17 +++++++++++++++++
 5 files changed, 138 insertions(+), 5 deletions(-)

diff --git a/src/or/directory.c b/src/or/directory.c
index a73680a..fdfb339 100644
--- a/src/or/directory.c
+++ b/src/or/directory.c
@@ -40,9 +40,38 @@
 
 /**
  * \file directory.c
- * \brief Code to send and fetch directories and router
- * descriptors via HTTP.  Directories use dirserv.c to generate the
- * results; clients use routers.c to parse them.
+ * \brief Code to send and fetch information from directory authorities and
+ * caches via HTTP.
+ *
+ * Directory caches and authorities use dirserv.c to generate the results of a
+ * query and stream them to the connection; clients use routerparse.c to parse
+ * them.
+ *
+ * Every directory request has a dir_connection_t on the client side and on
+ * the server side.  In most cases, the dir_connection_t object is a linked
+ * connection, tunneled through an edge_connection_t so that it can be a
+ * stream on the Tor network.  The only non-tunneled connections are those
+ * that are used to upload material (descriptors and votes) to authorities.
+ * Among tunneled connections, some use one-hop circuits, and others use
+ * multi-hop circuits for anonymity.
+ *
+ * Directory requests are launched by calling
+ * directory_initiate_command_rend() or one of its numerous variants. This
+ * launch the connection, will construct an HTTP request with
+ * directory_send_command(), send the and wait for a response.  The client
+ * later handles the response with connection_dir_client_reached_eof(),
+ * which passes the information received to another part of Tor.
+ *
+ * On the server side, requests are read in directory_handle_command(),
+ * which dispatches first on the request type (GET or POST), and then on
+ * the URL requested. GET requests are processed with a table-based
+ * dispatcher in url_table[].  The process of handling larger GET requests
+ * is complicated because we need to avoid allocating a copy of all the
+ * data to be sent to the client in one huge buffer.  Instead, we spool the
+ * data into the buffer using logic in connection_dirserv_flushed_some() in
+ * dirserv.c.  (TODO: If we extended buf.c to have a zero-copy
+ * reference-based buffer type, we could remove most of that code, at the
+ * cost of a bit more reference counting.)
  **/
 
 /* In-points to directory.c:
diff --git a/src/or/networkstatus.c b/src/or/networkstatus.c
index f30fe16..85a73c8 100644
--- a/src/or/networkstatus.c
+++ b/src/or/networkstatus.c
@@ -6,8 +6,34 @@
 
 /**
  * \file networkstatus.c
- * \brief Functions and structures for handling network status documents as a
- * client or cache.
+ * \brief Functions and structures for handling networkstatus documents as a
+ * client or as a directory cache.
+ *
+ * A consensus networkstatus object is created by the directory
+ * authorities.  It authenticates a set of network parameters--most
+ * importantly, the list of all the relays in the network.  This list
+ * of relays is represented as an array of routerstatus_t objects.
+ *
+ * There are currently two flavors of consensus.  With the older "NS"
+ * flavor, each relay is associated with a digest of its router
+ * descriptor. Tor instances that use this consensus keep the list of
+ * router descriptors as routerinfo_t objects stored and managed in
+ * routerlist.c.  With the newer "microdesc" flavor, each relay is
+ * associated with a digest of the microdescriptor that the authorities
+ * made for it.  These are stored and managed in microdesc.c. Information
+ * about the router is divided between the the networkstatus and the
+ * microdescriptor according to the general rule that microdescriptors
+ * should hold information that changes much less frequently than the
+ * information in the networkstatus.
+ *
+ * Modern clients use microdescriptor networkstatuses.  Directory caches
+ * need to keep both kinds of networkstatus document, so they can serve them.
+ *
+ * This module manages fetching, holding, storing, updating, and
+ * validating networkstatus objects.  The download-and-validate process
+ * is slightly complicated by the fact that the keys you need to
+ * validate a consensus are stored in the authority certificates, which
+ * you might not have yet when you download the consensus.
  */
 
 #define NETWORKSTATUS_PRIVATE
diff --git a/src/or/nodelist.c b/src/or/nodelist.c
index 070e2e9..2802d5b 100644
--- a/src/or/nodelist.c
+++ b/src/or/nodelist.c
@@ -10,6 +10,32 @@
  * \brief Structures and functions for tracking what we know about the routers
  *   on the Tor network, and correlating information from networkstatus,
  *   routerinfo, and microdescs.
+ *
+ * The key structure here is node_t: that's the canonical way to refer
+ * to a Tor relay that we might want to build a circuit through.  Every
+ * node_t has either a routerinfo_t, or a routerstatus_t from the current
+ * networkstatus consensus.  If it has a routerstatus_t, it will also
+ * need to have a microdesc_t before you can use it for circuits.
+ *
+ * The nodelist_t is a global singleton that maps identities to node_t
+ * objects.  Access them with the node_get_*() functions.  The nodelist_t
+ * is maintained by calls throughout the codebase
+ *
+ * Generally, other code should not have to reach inside a node_t to
+ * see what information it has.  Instead, you should call one of the
+ * many accessor functions that works on a generic node_t.  If there
+ * isn't one that does what you need, it's better to make such a function,
+ * and then use it.
+ *
+ * For historical reasons, some of the functions that select a node_t
+ * from the list of all usable node_t objects are in the routerlist.c
+ * module, since they originally selected a routerinfo_t. (TODO: They
+ * should move!)
+ *
+ * (TODO: Perhaps someday we should abstract the remaining ways of
+ * talking about a relay to also be node_t instances. Those would be
+ * routerstatus_t as used for directory requests, and dir_server_t as
+ * used for authorities and fallback directories.)
  */
 
 #include "or.h"
diff --git a/src/or/relay.c b/src/or/relay.c
index 1794215..823e074 100644
--- a/src/or/relay.c
+++ b/src/or/relay.c
@@ -8,6 +8,41 @@
  * \file relay.c
  * \brief Handle relay cell encryption/decryption, plus packaging and
  *    receiving from circuits, plus queuing on circuits.
+ *
+ * This is a core modules that makes Tor work. It's responsible for
+ * dealing with RELAY cells (the ones that travel more than one hop along a
+ * circuit), by:
+ *  <ul>
+ *   <li>constructing relays cells,
+ *   <li>encrypting relay cells,
+ *   <li>decrypting relay cells,
+ *   <li>demultiplexing relay cells as they arrive on a connection,
+ *   <li>queueing relay cells for retransmission,
+ *   <li>or handling relay cells that are for us to receive (as an exit or a
+ *   client).
+ *  </ul>
+ *
+ * RELAY cells are generated throughout the code at the client or relay side,
+ * using relay_send_command_from_edge() or one of the functions like
+ * connection_edge_send_command() that calls it.  Of particular interest is
+ * connection_edge_package_raw_inbuf(), which takes information that has
+ * arrived on an edge connection socket, and packages it as a RELAY_DATA cell
+ * -- this is how information is actually sent across the Tor network.  The
+ * cryptography for these functions is handled deep in
+ * circuit_package_relay_cell(), which either adds a single layer of
+ * encryption (if we're an exit), or multiple layers (if we're the origin of
+ * the circuit).  After construction and encryption, the RELAY cells are
+ * passed to append_cell_to_circuit_queue(), which queues them for
+ * transmission and tells the circuitmux (see circuitmux.c) that the circuit
+ * is waiting to send something.
+ *
+ * Incoming RELAY cells arrive at circuit_receive_relay_cell(), called from
+ * command.c.  There they are decrypted and, if they are for us, are passed to
+ * connection_edge_process_relay_cell(). If they're not for us, they're
+ * re-queued for retransmission again with append_cell_to_circuit_queue().
+ *
+ * The connection_edge_process_relay_cell() function handles all the different
+ * types of relay cells, launching requests or transmitting data as needed.
  **/
 
 #define RELAY_PRIVATE
diff --git a/src/or/torcert.c b/src/or/torcert.c
index a6a33c6..37a201e 100644
--- a/src/or/torcert.c
+++ b/src/or/torcert.c
@@ -6,6 +6,23 @@
  *
  * \brief Implementation for ed25519-signed certificates as used in the Tor
  * protocol.
+ *
+ * This certificate format is designed to be simple and compact; it's
+ * documented in tor-spec.txt in the torspec.git repository.  All of the
+ * certificates in this format are signed with an Ed25519 key; the
+ * contents themselves may be another Ed25519 key, a digest of a
+ * RSA key, or some other material.
+ *
+ * In this module there is also support for a crooss-certification of
+ * Ed25519 identities using (older) RSA1024 identities.
+ *
+ * Tor uses other types of certificate too, beyond those described in this
+ * module. Notably, our use of TLS requires us to touch X.509 certificates,
+ * even though sensible people would stay away from those. Our X.509
+ * certificates are represented with tor_x509_cert_t, and implemented in
+ * tortls.c.  We also have a separate certificate type that authorities
+ * use to authenticate their RSA signing keys with their RSA identity keys:
+ * that one is authority_cert_t, and it's mostly handled in routerlist.c.
  */
 
 #include "crypto.h"