Annotation of embedaddon/bird/doc/prog-intro.sgml, revision 1.1
1.1 ! misho 1: <chapt>BIRD Design
! 2:
! 3: <sect>Introduction
! 4:
! 5: <p>This document describes the internal workings of BIRD, its architecture,
! 6: design decisions and rationale behind them. It also contains documentation on
! 7: all the essential components of the system and their interfaces.
! 8:
! 9: <p>Routing daemons are complicated things which need to act in real time
! 10: to complex sequences of external events, respond correctly even to the most erroneous behavior
! 11: of their environment and still handle enormous amount of data with reasonable
! 12: speed. Due to all of this, their design is very tricky as one needs to carefully
! 13: balance between efficiency, stability and (last, but not least) simplicity of
! 14: the program and it would be possible to write literally hundreds of pages about
! 15: all of these issues. In accordance to the famous quote of Anton Chekhov "Shortness
! 16: is a sister of talent", we've tried to write a much shorter document highlighting
! 17: the most important stuff and leaving the boring technical details better explained
! 18: by the program source itself together with comments contained therein.
! 19:
! 20: <sect>Design goals
! 21:
! 22: <p>When planning the architecture of BIRD, we've taken a close look at the other existing routing
! 23: daemons and also at some of the operating systems used on dedicated routers, gathered all important
! 24: features and added lots of new ones to overcome their shortcomings and to better match the requirements
! 25: of routing in today's Internet: IPv6, policy routing, route filtering and so on. From this
! 26: planning, the following set of design goals has arisen:
! 27:
! 28: <itemize>
! 29:
! 30: <item><it>Support all the standard routing protocols and make it easy to add new ones.</it>
! 31: This leads to modularity and clean separation between the core and the protocols.
! 32:
! 33: <item><it>Support both IPv4 and IPv6 in the same source tree, re-using most of the code.</it>
! 34: This leads to abstraction of IP addresses and operations on them.
! 35:
! 36: <item><it>Minimize OS dependent code to make porting as easy as possible.</it>
! 37: Unfortunately, such code cannot be avoided at all as the details of communication with
! 38: the IP stack differ from OS to OS and they often vary even between different
! 39: versions of the same OS. But we can isolate such code in special modules and
! 40: do the porting by changing or replacing just these modules.
! 41: Also, don't rely on specific features of various operating systems, but be able
! 42: to make use of them if they are available.
! 43:
! 44: <item><it>Allow multiple routing tables.</it>
! 45: Easily solvable by abstracting out routing tables and the corresponding operations.
! 46:
! 47: <item><it>Offer powerful route filtering.</it>
! 48: There already were several attempts to incorporate route filters to a dynamic router,
! 49: but most of them have used simple sequences of filtering rules which were very inflexible
! 50: and hard to use for non-trivial filters. We've decided to employ a simple loop-free
! 51: programming language having access to all the route attributes and being able to
! 52: modify the most of them.
! 53:
! 54: <item><it>Support easy configuration and re-configuration.</it>
! 55: Most routers use a simple configuration language designed ad hoc with no structure at all
! 56: and allow online changes of configuration by using their command-line interface, thus
! 57: any complex re-configurations are hard to achieve without replacing the configuration
! 58: file and restarting the whole router. We've decided to use a more general approach: to
! 59: have a configuration defined in a context-free language with blocks and nesting, to
! 60: perform all configuration changes by editing the configuration file, but to be able
! 61: to read the new configuration and smoothly adapt to it without disturbing parts of
! 62: the routing process which are not affected by the change.
! 63:
! 64: <item><it>Be able to be controlled online.</it>
! 65: In addition to the online reconfiguration, a routing daemon should be able to communicate
! 66: with the user and with many other programs (primarily scripts used for network maintenance)
! 67: in order to make it possible to inspect contents of routing tables, status of all
! 68: routing protocols and also to control their behavior (disable, enable or reset a protocol without restarting all the others). To achieve
! 69: this, we implement a simple command-line protocol based on those used by FTP and SMTP
! 70: (that is textual commands and textual replies accompanied by a numeric code which makes
! 71: them both readable to a human and easy to recognize in software).
! 72:
! 73: <item><it>Respond to all events in real time.</it>
! 74: A typical solution to this problem is to use lots of threads to separate the workings
! 75: of all the routing protocols and also of the user interface parts and to hope that
! 76: the scheduler will assign time to them in a fair enough manner. This is surely a good
! 77: solution, but we have resisted the temptation and preferred to avoid the overhead of threading
! 78: and the large number of locks involved and preferred a event driven architecture with
! 79: our own scheduling of events. An unpleasant consequence of such an approach
! 80: is that long lasting tasks must be split to more parts linked by special
! 81: events or timers to make the CPU available for other tasks as well.
! 82:
! 83: </itemize>
! 84:
! 85: <sect>Architecture
! 86:
! 87: <p>The requirements set above have lead to a simple modular architecture containing
! 88: the following types of modules:
! 89:
! 90: <descrip>
! 91:
! 92: <tagp>Core modules</tagp> implement the core functions of BIRD: taking care
! 93: of routing tables, keeping protocol status, interacting with the user using
! 94: the Command-Line Interface (to be called CLI in the rest of this document)
! 95: etc.
! 96:
! 97: <tagp>Library modules</tagp> form a large set of various library functions
! 98: implementing several data abstractions, utility functions and also functions
! 99: which are a part of the standard libraries on some systems, but missing on other
! 100: ones.
! 101:
! 102: <tagp>Resource management modules</tagp> take care of resources, their allocation
! 103: and automatic freeing when the module having requested shuts itself down.
! 104:
! 105: <tagp>Configuration modules</tagp> are fragments of lexical analyzer,
! 106: grammar rules and the corresponding snippets of C code. For each group
! 107: of code modules (core, each protocol, filters) there exist a configuration
! 108: module taking care of all the related configuration stuff.
! 109:
! 110: <tagp>The filter</tagp> implements the route filtering language.
! 111:
! 112: <tagp>Protocol modules</tagp> implement the individual routing protocols.
! 113:
! 114: <tagp>System-dependent modules</tagp> implement the interface between BIRD
! 115: and specific operating systems.
! 116:
! 117: <tagp>The client</tagp> is a simple program providing an easy, though friendly
! 118: interface to the CLI.
! 119:
! 120: </descrip>
! 121:
! 122: <sect>Implementation
! 123:
! 124: <p>BIRD has been written in GNU C. We've considered using C++, but we've
! 125: preferred the simplicity and straightforward nature of C which gives us fine
! 126: control over all implementation details and on the other hand enough
! 127: instruments to build the abstractions we need.
! 128:
! 129: <p>The modules are statically linked to produce a single executable file
! 130: (except for the client which stands on its own).
! 131:
! 132: <p>The building process is controlled by a set of Makefiles for GNU Make,
! 133: intermixed with several Perl and shell scripts.
! 134:
! 135: <p>The initial configuration of the daemon, detection of system features
! 136: and selection of the right modules to include for the particular OS
! 137: and the set of protocols the user has chosen is performed by a configure
! 138: script generated by GNU Autoconf.
! 139:
! 140: <p>The parser of the configuration is generated by the GNU Bison.
! 141:
! 142: <p>The documentation is generated using <file/SGMLtools/ with our own DTD
! 143: and mapping rules which produce both an online version in HTML and
! 144: a neatly formatted one for printing (first converted
! 145: from SGML to &latex; and then processed by &tex; and <file/dvips/ to
! 146: get a PostScript file).
! 147:
! 148: <p>The comments from C sources which form a part of the programmer's
! 149: documentation are extracted using a modified version of the <file/kernel-doc/
! 150: tool.
! 151:
! 152: <p>If you want to work on BIRD, it's highly recommended to configure it
! 153: with a <tt/--enable-debug/ switch which enables some internal consistency
! 154: checks and it also links BIRD with a memory allocation checking library
! 155: if you have one (either <tt/efence/ or <tt/dmalloc/).
! 156:
! 157: <!--
! 158: LocalWords: IPv IP CLI snippets Perl Autoconf SGMLtools DTD SGML dvips
! 159: LocalWords: PostScript
! 160: -->
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