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+PCEP Module Internals
+=====================
+
+Introduction
+------------
+
+The PCEP module for the pathd daemon implements the PCEP protocol described in
+:rfc:`5440` to update the policies and candidate paths.
+
+The protocol encoding/decoding and the basic session management is handled by
+the `pceplib external library 1.2 <https://github.com/volta-networks/pceplib/tree/devel-1.2>`_.
+
+Together with pceplib, this module supports at least partially:
+
+  - :rfc:`5440`
+
+        Most of the protocol defined in the RFC is implemented.
+        All the messages can be parsed, but this was only tested in the context
+        of segment routing. Only a very small subset of metric types can be
+        configured, and there is a known issue with some Cisco routers not
+        following the IANA numbers for metrics.
+
+  - :rfc:`8231`
+
+        Support delegation of candidate path after performing the initial
+        computation request. If the PCE does not respond or cannot compute
+        a path, an empty candidate path is delegated to the PCE.
+        Only tested in the context of segment routing.
+
+  - :rfc:`8408`
+
+        Only used to comunicate the support for segment routing to the PCE.
+
+  - :rfc:`8664`
+
+        All the NAI types are implemented, but only the MPLS NAI are supported.
+        If the PCE provide segments that are not MPLS labels, the PCC will
+        return an error.
+
+Note that pceplib supports more RFCs and drafts, see pceplib
+`README <https://github.com/volta-networks/pceplib/blob/master/README.md>`_
+for more details.
+
+
+Architecture
+------------
+
+Overview
+........
+
+The module is separated into multiple layers:
+
+ - pathd interface
+ - command-line console
+ - controller
+ - PCC
+ - pceplib interface
+
+The pathd interface handles all the interactions with the daemon API.
+
+The command-line console handles all the VTYSH configuration commands.
+
+The controller manages the multiple PCC connections and the interaction between
+them and the daemon interface.
+
+The PCC handles a single connection to a PCE through a pceplib session.
+
+The pceplib interface abstracts the API of the pceplib.
+
+.. figure:: ../figures/pcep_module_threading_overview.svg
+
+
+Threading Model
+---------------
+
+The module requires multiple threads to cooperate:
+
+ - The main thread used by the pathd daemon.
+ - The controller pthread used to isolate the PCC from the main thread.
+ - The possible threads started in the pceplib library.
+
+To ensure thread safety, all the controller and PCC state data structures can
+only be read and modified in the controller thread, and all the global data
+structures can only be read and modified in the main thread. Most of the
+interactions between these threads are done through FRR timers and events.
+
+The controller is the bridge between the two threads, all the functions that
+**MUST** be called from the main thread start with the prefix `pcep_ctrl_` and
+all the functions that **MUST** be called from the controller thread start
+with the prefix `pcep_thread_`. When an asynchronous action must be taken in
+a different thread, an FRR event is sent to the thread. If some synchronous
+operation is needed, the calling thread will block and run a callback in the
+other thread, there the result is **COPIED** and returned to the calling thread.
+
+No function other than the controller functions defined for it should be called
+from the main thread. The only exception being some utility functions from
+`path_pcep_lib.[hc]`.
+
+All the calls to pathd API functions **MUST** be performed in the main thread,
+for that, the controller sends FRR events handled in function
+`path_pcep.c:pcep_main_event_handler`.
+
+For the same reason, the console client only runs in the main thread. It can
+freely use the global variable, but **MUST** use controller's `pcep_ctrl_`
+functions to interact with the PCCs.
+
+
+Source Code
+-----------
+
+Generic Data Structures
+.......................
+
+The data structures are defined in multiple places, and where they are defined
+dictates where they can be used.
+
+The data structures defined in `path_pcep.h` can be used anywhere in the module.
+
+Internally, throughout the module, the `struct path` data structure is used
+to describe PCEP messages. It is a simplified flattened structure that can
+represent multiple complex PCEP message types. The conversion from this
+structure to the PCEP data structures used by pceplib is done in the pceplib
+interface layer.
+
+The data structures defined in `path_pcep_controller.h` should only be used
+in `path_pcep_controller.c`. Even if a structure pointer is passed as a parameter
+to functions defined in `path_pcep_pcc.h`, these should consider it as an opaque
+data structure only used to call back controller functions.
+
+The same applies to the structures defined in `path_pcep_pcc.h`, even if the
+controller owns a reference to this data structure, it should never read or
+modify it directly, it should be considered an opaque structure.
+
+The global data structure can be accessed from the pathd interface layer
+`path_pcep.c` and the command line client code `path_pcep_cli.c`.
+
+
+Interface With Pathd
+....................
+
+All the functions calling or called by the pathd daemon are implemented in
+`path_pcep.c`. These functions **MUST** run in the main FRR thread, and
+all the interactions with the controller and the PCCs **MUST** pass through
+the controller's `pcep_ctrl_` prefixed functions.
+
+To handle asynchronous events from the PCCs, a callback is passed to
+`pcep_ctrl_initialize` that is called in the FRR main thread context.
+
+
+Command Line Client
+...................
+
+All the command line configuration commands (VTYSH) are implemented in
+`path_pcep_cli.c`. All the functions there run in the main FRR thread and
+can freely access the global variables. All the interaction with the
+controller's and the PCCs **MUST** pass through the controller `pcep_ctrl_`
+prefixed functions.
+
+
+Debugging Helpers
+.................
+
+All the functions formating data structures for debugging and logging purposes
+are implemented in `path_pcep_debug.[hc]`.
+
+
+Interface with pceplib
+......................
+
+All the functions calling the pceplib external library are defined in
+`path_pcep_lib.[hc]`. Some functions are called from the main FRR thread, like
+`pcep_lib_initialize`, `pcep_lib_finalize`; some can be called from either
+thread, like `pcep_lib_free_counters`; some function must be called from the
+controller thread, like `pcep_lib_connect`. This will probably be formalized
+later on with function prefix like done in the controller.
+
+
+Controller
+..........
+
+The controller is defined and implemented in `path_pcep_controller.[hc]`.
+Part of the controller code runs in FRR main thread and part runs in its own
+FRR pthread started to isolate the main thread from the PCCs' event loop.
+To communicate between the threads it uses FRR events, timers and
+`event_execute` calls.
+
+
+PCC
+...
+
+Each PCC instance owns its state and runs in the controller thread. They are
+defined and implemented in `path_pcep_pcc.[hc]`. All the interactions with
+the daemon must pass through some controller's `pcep_thread_` prefixed function.