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	BIRD Programmer's Guide: Protocols
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	(c) 2000 Martin Mares <mj@ucw.cz>
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<sect>Routing protocols
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<sect1>Introduction
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<p>The routing protocols are the BIRD's heart and a fine amount of code
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is dedicated to their management and for providing support functions to them.
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(-: Actually, this is the reason why the directory with sources of the core
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code is called <tt/nest/ :-).
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<p>When talking about protocols, one need to distinguish between <em/protocols/
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and protocol <em/instances/. A protocol exists exactly once, not depending on whether
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it's configured on not and it can have an arbitrary number of instances corresponding
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to its "incarnations" requested by the configuration file. Each instance is completely
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autonomous, has its own configuration, its own status, its own set of routes and its
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own set of interfaces it works on.
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<p>A protocol is represented by a <struct/protocol/ structure containing all the basic
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information (protocol name, default settings and pointers to most of the protocol
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hooks). All these structures are linked in the <param/protocol_list/ list.
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<p>Each instance has its own <struct/proto/ structure describing all its properties: protocol
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type, configuration, a resource pool where all resources belonging to the instance
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live, various protocol attributes (take a look at the declaration of <struct/proto/ in
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<tt/protocol.h/), protocol states (see below for what do they mean), connections
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to routing tables, filters attached to the protocol
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and finally a set of pointers to the rest of protocol hooks (they
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are the same for all instances of the protocol, but in order to avoid extra
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indirections when calling the hooks from the fast path, they are stored directly
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in <struct/proto/). The instance is always linked in both the global instance list
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(<param/proto_list/) and a per-status list (either <param/active_proto_list/ for
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running protocols, <param/initial_proto_list/ for protocols being initialized or
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<param/flush_proto_list/ when the protocol is being shut down).
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<p>The protocol hooks are described in the next chapter, for more information about
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configuration of protocols, please refer to the configuration chapter and also
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to the description of the <func/proto_commit/ function.
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<sect1>Protocol states
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<p>As startup and shutdown of each protocol are complex processes which can be affected
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by lots of external events (user's actions, reconfigurations, behaviour of neighboring routers etc.),
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we have decided to supervise them by a pair of simple state machines -- the protocol
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state machine and a core state machine.
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<p>The <em/protocol state machine/ corresponds to internal state of the protocol
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and the protocol can alter its state whenever it wants to. There exist
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the following states:
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<descrip>
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	<tag/PS_DOWN/ The protocol is down and waits for being woken up by calling its
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	start() hook.
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	<tag/PS_START/ The protocol is waiting for connection with the rest of the
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	network. It's active, it has resources allocated, but it still doesn't want
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	any routes since it doesn't know what to do with them.
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	<tag/PS_UP/ The protocol is up and running. It communicates with the core,
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	delivers routes to tables and wants to hear announcement about route changes.
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	<tag/PS_STOP/ The protocol has been shut down (either by being asked by the
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	core code to do so or due to having encountered a protocol error).
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</descrip>
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<p>Unless the protocol is in the <tt/PS_DOWN/ state, it can decide to change
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its state by calling the <func/proto_notify_state/ function.
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<p>At any time, the core code can ask the protocol to shut down by calling its stop() hook.
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<p>The <em/core state machine/ takes care of the core view of protocol state.
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The states are traversed according to changes of the protocol state machine, but
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sometimes the transitions are delayed if the core needs to finish some actions
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(for example sending of new routes to the protocol) before proceeding to the
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new state. There exist the following core states:
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<descrip>
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	<tag/FS_HUNGRY/ The protocol is down, it doesn't have any routes and
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	doesn't want them.
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	<tag/FS_FEEDING/ The protocol has reached the <tt/PS_UP/ state, but
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	we are still busy sending the initial set of routes to it.
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	<tag/FS_HAPPY/ The protocol is up and has complete routing information.
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	<tag/FS_FLUSHING/ The protocol is shutting down (it's in either <tt/PS_STOP/
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	or <tt/PS_DOWN/ state) and we're flushing all of its routes from the
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	routing tables.
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</descrip>
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<sect1>Functions of the protocol module
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<p>The protocol module provides the following functions: