Internet on FIRE

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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.

<sect1>Functions of the protocol module

<p>The protocol module provides the following functions: