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	BIRD Programmer's Guide: Protocols

	(c) 2000 Martin Mares <mj@ucw.cz>
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<sect>Routing protocols

<sect1>Introduction

<p>The routing protocols are the bird's heart and a fine amount of code
is dedicated to their management and for providing support functions to them.
(-: Actually, this is the reason why the directory with sources of the core
code is called <tt/nest/ :-).

<p>When talking about protocols, one need to distinguish between <em/protocols/
and protocol <em/instances/. A protocol exists exactly once, not depending on whether
it's configured or not and it can have an arbitrary number of instances corresponding
to its "incarnations" requested by the configuration file. Each instance is completely
autonomous, has its own configuration, its own status, its own set of routes and its
own set of interfaces it works on.

<p>A protocol is represented by a <struct/protocol/ structure containing all the basic
information (protocol name, default settings and pointers to most of the protocol
hooks). All these structures are linked in the <param/protocol_list/ list.

<p>Each instance has its own <struct/proto/ structure describing all its properties: protocol
type, configuration, a resource pool where all resources belonging to the instance
live, various protocol attributes (take a look at the declaration of <struct/proto/ in
<tt/protocol.h/), protocol states (see below for what do they mean), connections
to routing tables, filters attached to the protocol
and finally a set of pointers to the rest of protocol hooks (they
are the same for all instances of the protocol, but in order to avoid extra
indirections when calling the hooks from the fast path, they are stored directly
in <struct/proto/). The instance is always linked in both the global instance list
(<param/proto_list/) and a per-status list (either <param/active_proto_list/ for
running protocols, <param/initial_proto_list/ for protocols being initialized or
<param/flush_proto_list/ when the protocol is being shut down).

<p>The protocol hooks are described in the next chapter, for more information about
configuration of protocols, please refer to the configuration chapter and also
to the description of the <func/proto_commit/ function.

<sect1>Protocol states

<p>As startup and shutdown of each protocol are complex processes which can be affected
by lots of external events (user's actions, reconfigurations, behavior of neighboring routers etc.),
we have decided to supervise them by a pair of simple state machines -- the protocol
state machine and a core state machine.

<p>The <em/protocol state machine/ corresponds to internal state of the protocol
and the protocol can alter its state whenever it wants to. There are
the following states:

<descrip>
	<tag/PS_DOWN/ The protocol is down and waits for being woken up by calling its
	start() hook.
	<tag/PS_START/ The protocol is waiting for connection with the rest of the
	network. It's active, it has resources allocated, but it still doesn't want
	any routes since it doesn't know what to do with them.
	<tag/PS_UP/ The protocol is up and running. It communicates with the core,
	delivers routes to tables and wants to hear announcement about route changes.
	<tag/PS_STOP/ The protocol has been shut down (either by being asked by the
	core code to do so or due to having encountered a protocol error).
</descrip>

<p>Unless the protocol is in the <tt/PS_DOWN/ state, it can decide to change
its state by calling the <func/proto_notify_state/ function.

<p>At any time, the core code can ask the protocol to shut itself down by calling its stop() hook.

<p>The <em/core state machine/ takes care of the core view of protocol state.
The states are traversed according to changes of the protocol state machine, but
sometimes the transitions are delayed if the core needs to finish some actions
(for example sending of new routes to the protocol) before proceeding to the
new state. There are the following core states:

<descrip>
	<tag/FS_HUNGRY/ The protocol is down, it doesn't have any routes and
	doesn't want them.
	<tag/FS_FEEDING/ The protocol has reached the <tt/PS_UP/ state, but
	we are still busy sending the initial set of routes to it.
	<tag/FS_HAPPY/ The protocol is up and has complete routing information.
	<tag/FS_FLUSHING/ The protocol is shutting down (it's in either <tt/PS_STOP/
	or <tt/PS_DOWN/ state) and we're flushing all of its routes from the
	routing tables.
</descrip>

<sect1>Functions of the protocol module

<p>The protocol module provides the following functions:
