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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: 
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