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<chapt>BIRD Design
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<sect>Introduction
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<p>This document describes the internal workings of BIRD, its architecture,
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design decisions and rationale behind them. It also contains documentation on
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all the essential components of the system and their interfaces.
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<p>Routing daemons are complicated things which need to act in real time
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to complex sequences of external events, respond correctly even to the most erroneous behavior
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of their environment and still handle enormous amount of data with reasonable
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speed. Due to all of this, their design is very tricky as one needs to carefully
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balance between efficiency, stability and (last, but not least) simplicity of
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the program and it would be possible to write literally hundreds of pages about
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all of these issues. In accordance to the famous quote of Anton Chekhov "Shortness
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is a sister of talent", we've tried to write a much shorter document highlighting
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the most important stuff and leaving the boring technical details better explained
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by the program source itself together with comments contained therein.
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<sect>Design goals
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<p>When planning the architecture of BIRD, we've taken a close look at the other existing routing
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daemons and also at some of the operating systems used on dedicated routers, gathered all important
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features and added lots of new ones to overcome their shortcomings and to better match the requirements
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of routing in today's Internet: IPv6, policy routing, route filtering and so on. From this
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planning, the following set of design goals has arisen:
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<itemize>
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<item><it>Support all the standard routing protocols and make it easy to add new ones.</it>
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This leads to modularity and clean separation between the core and the protocols.
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<item><it>Support both IPv4 and IPv6 in the same source tree, re-using most of the code.</it>
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This leads to abstraction of IP addresses and operations on them.
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<item><it>Minimize OS dependent code to make porting as easy as possible.</it>
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Unfortunately, such code cannot be avoided at all as the details of communication with
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the IP stack differ from OS to OS and they often vary even between different
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versions of the same OS. But we can isolate such code in special modules and
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do the porting by changing or replacing just these modules.
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Also, don't rely on specific features of various operating systems, but be able
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to make use of them if they are available.
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<item><it>Allow multiple routing tables.</it>
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Easily solvable by abstracting out routing tables and the corresponding operations.
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<item><it>Offer powerful route filtering.</it>
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There already were several attempts to incorporate route filters to a dynamic router,
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but most of them have used simple sequences of filtering rules which were very inflexible
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and hard to use for non-trivial filters. We've decided to employ a simple loop-free
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programming language having access to all the route attributes and being able to
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modify the most of them.
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<item><it>Support easy configuration and re-configuration.</it>
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Most routers use a simple configuration language designed ad hoc with no structure at all
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and allow online changes of configuration by using their command-line interface, thus
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any complex re-configurations are hard to achieve without replacing the configuration
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file and restarting the whole router. We've decided to use a more general approach: to
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have a configuration defined in a context-free language with blocks and nesting, to
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perform all configuration changes by editing the configuration file, but to be able
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to read the new configuration and smoothly adapt to it without disturbing parts of
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the routing process which are not affected by the change.
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<item><it>Be able to be controlled online.</it>
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In addition to the online reconfiguration, a routing daemon should be able to communicate
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with the user and with many other programs (primarily scripts used for network maintenance)
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in order to make it possible to inspect contents of routing tables, status of all
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routing protocols and also to control their behavior (disable, enable or reset a protocol without restarting all the others). To achieve
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this, we implement a simple command-line protocol based on those used by FTP and SMTP
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(that is textual commands and textual replies accompanied by a numeric code which makes
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them both readable to a human and easy to recognize in software).
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<item><it>Respond to all events in real time.</it>
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A typical solution to this problem is to use lots of threads to separate the workings
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of all the routing protocols and also of the user interface parts and to hope that
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the scheduler will assign time to them in a fair enough manner. This is surely a good
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solution, but we have resisted the temptation and preferred to avoid the overhead of threading
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and the large number of locks involved and preferred a event driven architecture with
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our own scheduling of events. An unpleasant consequence of such an approach
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is that long lasting tasks must be split to more parts linked by special
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events or timers to make the CPU available for other tasks as well.
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</itemize>
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<sect>Architecture
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<p>The requirements set above have lead to a simple modular architecture containing
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the following types of modules:
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<descrip>
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<tagp>Core modules</tagp> implement the core functions of BIRD: taking care
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of routing tables, keeping protocol status, interacting with the user using
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the Command-Line Interface (to be called CLI in the rest of this document)
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etc.
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<tagp>Library modules</tagp> form a large set of various library functions
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implementing several data abstractions, utility functions and also functions
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which are a part of the standard libraries on some systems, but missing on other
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ones.
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<tagp>Resource management modules</tagp> take care of resources, their allocation
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and automatic freeing when the module having requested shuts itself down.
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<tagp>Configuration modules</tagp> are fragments of lexical analyzer,
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grammar rules and the corresponding snippets of C code. For each group
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of code modules (core, each protocol, filters) there exist a configuration
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module taking care of all the related configuration stuff.
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<tagp>The filter</tagp> implements the route filtering language.
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<tagp>Protocol modules</tagp> implement the individual routing protocols.
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<tagp>System-dependent modules</tagp> implement the interface between BIRD
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and specific operating systems.
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<tagp>The client</tagp> is a simple program providing an easy, though friendly
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interface to the CLI.
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</descrip>
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<sect>Implementation
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<p>BIRD has been written in GNU C. We've considered using C++, but we've
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preferred the simplicity and straightforward nature of C which gives us fine
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control over all implementation details and on the other hand enough
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instruments to build the abstractions we need.
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<p>The modules are statically linked to produce a single executable file
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(except for the client which stands on its own).
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<p>The building process is controlled by a set of Makefiles for GNU Make,
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intermixed with several Perl and shell scripts.
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<p>The initial configuration of the daemon, detection of system features
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and selection of the right modules to include for the particular OS
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and the set of protocols the user has chosen is performed by a configure
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script generated by GNU Autoconf.
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<p>The parser of the configuration is generated by the GNU Bison.
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<p>The documentation is generated using <file/SGMLtools/ with our own DTD
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and mapping rules which produce both an online version in HTML and
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a neatly formatted one for printing (first converted
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from SGML to &latex; and then processed by &tex; and <file/dvips/ to
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get a PostScript file).
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<p>The comments from C sources which form a part of the programmer's
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documentation are extracted using a modified version of the <file/kernel-doc/
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tool.
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<p>If you want to work on BIRD, it's highly recommended to configure it
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with a <tt/--enable-debug/ switch which enables some internal consistency
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checks and it also links BIRD with a memory allocation checking library
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if you have one (either <tt/efence/ or <tt/dmalloc/).
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