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    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: 
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