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    1: <!--
    2: 	BIRD Programmer's Guide: Protocols
    3: 
    4: 	(c) 2000 Martin Mares <mj@ucw.cz>
    5: -->
    6: 
    7: <sect>Routing protocols
    8: 
    9: <sect1>Introduction
   10: 
   11: <p>The routing protocols are the bird's heart and a fine amount of code
   12: is dedicated to their management and for providing support functions to them.
   13: (-: Actually, this is the reason why the directory with sources of the core
   14: code is called <tt/nest/ :-).
   15: 
   16: <p>When talking about protocols, one need to distinguish between <em/protocols/
   17: and protocol <em/instances/. A protocol exists exactly once, not depending on whether
   18: it's configured or not and it can have an arbitrary number of instances corresponding
   19: to its "incarnations" requested by the configuration file. Each instance is completely
   20: autonomous, has its own configuration, its own status, its own set of routes and its
   21: own set of interfaces it works on.
   22: 
   23: <p>A protocol is represented by a <struct/protocol/ structure containing all the basic
   24: information (protocol name, default settings and pointers to most of the protocol
   25: hooks). All these structures are linked in the <param/protocol_list/ list.
   26: 
   27: <p>Each instance has its own <struct/proto/ structure describing all its properties: protocol
   28: type, configuration, a resource pool where all resources belonging to the instance
   29: live, various protocol attributes (take a look at the declaration of <struct/proto/ in
   30: <tt/protocol.h/), protocol states (see below for what do they mean), connections
   31: to routing tables, filters attached to the protocol
   32: and finally a set of pointers to the rest of protocol hooks (they
   33: are the same for all instances of the protocol, but in order to avoid extra
   34: indirections when calling the hooks from the fast path, they are stored directly
   35: in <struct/proto/). The instance is always linked in both the global instance list
   36: (<param/proto_list/) and a per-status list (either <param/active_proto_list/ for
   37: running protocols, <param/initial_proto_list/ for protocols being initialized or
   38: <param/flush_proto_list/ when the protocol is being shut down).
   39: 
   40: <p>The protocol hooks are described in the next chapter, for more information about
   41: configuration of protocols, please refer to the configuration chapter and also
   42: to the description of the <func/proto_commit/ function.
   43: 
   44: <sect1>Protocol states
   45: 
   46: <p>As startup and shutdown of each protocol are complex processes which can be affected
   47: by lots of external events (user's actions, reconfigurations, behavior of neighboring routers etc.),
   48: we have decided to supervise them by a pair of simple state machines -- the protocol
   49: state machine and a core state machine.
   50: 
   51: <p>The <em/protocol state machine/ corresponds to internal state of the protocol
   52: and the protocol can alter its state whenever it wants to. There are
   53: the following states:
   54: 
   55: <descrip>
   56: 	<tag/PS_DOWN/ The protocol is down and waits for being woken up by calling its
   57: 	start() hook.
   58: 	<tag/PS_START/ The protocol is waiting for connection with the rest of the
   59: 	network. It's active, it has resources allocated, but it still doesn't want
   60: 	any routes since it doesn't know what to do with them.
   61: 	<tag/PS_UP/ The protocol is up and running. It communicates with the core,
   62: 	delivers routes to tables and wants to hear announcement about route changes.
   63: 	<tag/PS_STOP/ The protocol has been shut down (either by being asked by the
   64: 	core code to do so or due to having encountered a protocol error).
   65: </descrip>
   66: 
   67: <p>Unless the protocol is in the <tt/PS_DOWN/ state, it can decide to change
   68: its state by calling the <func/proto_notify_state/ function.
   69: 
   70: <p>At any time, the core code can ask the protocol to shut itself down by calling its stop() hook.
   71: 
   72: <p>The <em/core state machine/ takes care of the core view of protocol state.
   73: The states are traversed according to changes of the protocol state machine, but
   74: sometimes the transitions are delayed if the core needs to finish some actions
   75: (for example sending of new routes to the protocol) before proceeding to the
   76: new state. There are the following core states:
   77: 
   78: <descrip>
   79: 	<tag/FS_HUNGRY/ The protocol is down, it doesn't have any routes and
   80: 	doesn't want them.
   81: 	<tag/FS_FEEDING/ The protocol has reached the <tt/PS_UP/ state, but
   82: 	we are still busy sending the initial set of routes to it.
   83: 	<tag/FS_HAPPY/ The protocol is up and has complete routing information.
   84: 	<tag/FS_FLUSHING/ The protocol is shutting down (it's in either <tt/PS_STOP/
   85: 	or <tt/PS_DOWN/ state) and we're flushing all of its routes from the
   86: 	routing tables.
   87: </descrip>
   88: 
   89: <sect1>Functions of the protocol module
   90: 
   91: <p>The protocol module provides the following functions: