4 // Fraunhofer Institute for Open Communication Systems (FOKUS)
5 // Competence Center NETwork research (NET), St. Augustin, GERMANY
6 // Stefan Bund <g0dil@berlios.de>
8 // This program is free software; you can redistribute it and/or modify
9 // it under the terms of the GNU General Public License as published by
10 // the Free Software Foundation; either version 2 of the License, or
11 // (at your option) any later version.
13 // This program is distributed in the hope that it will be useful,
14 // but WITHOUT ANY WARRANTY; without even the implied warranty of
15 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
16 // GNU General Public License for more details.
18 // You should have received a copy of the GNU General Public License
19 // along with this program; if not, write to the
20 // Free Software Foundation, Inc.,
21 // 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
25 /** \mainpage The SENF Socket Library
27 The Socket library provides a high level and object oriented abstraction based on the BSD socket
28 API (but not limited to it).
32 \section socket_intro Introduction
33 \seechapter \ref structure \n
34 \seechapter \ref usage
36 The socket library abstraction is based on several concepts:
38 \li The basic visible interface is a \link handle_group handle object\endlink
39 \li The socket interface relies on a \link policy_group policy framework \endlink to configure
41 \li The rest of the socket API is accessible using a classic inheritance hierarchy of \link
42 protocol_group protocol classes \endlink
43 \li There is a family of auxilliary \ref addr_group to supplement the socket library
46 \section socket_handle Socket Handles
47 \seechapter \ref handle_group \n
48 \seechapter \ref concrete_protocol_group
50 The handle/body architecture provides automatic reference counted management of socket
51 instances. This is the visible interface to the socket library.
53 Each specific protocol is used primarily via a protocol specific handle (a typedef
54 symbol). However, more generic kinds of handles can be defined for more generic functionality.
58 \section socket_policy The Policy interface
59 \seechapter \ref policy_group
61 The policy framework configures the exact features, a specific type of socket handle
62 provides. This offers highly efficient access to the most important socket functions (like
63 reading and writing). The policy interface however is a \e static, non-polymorphic interface.
66 \section socket_protocol The Protocol interface
67 \seechapter \ref protocol_group
70 The protocol interface provides further protocol dependent and (possibly) polymorphic access to
71 further socket funcitonality. On the other hand, this type of interface is not as flexible,
72 generic and fast as the policy interface.
74 \section socket_addr Auxilliary Addressing classes
75 \seechapter \ref addr_group
77 To supplement the socket library, there are a multitude of addressing classes. These come in two
79 \li Protocol specific addresses (e.g. INet4Address, MACAddress)
80 \li Socket addresses (\c sockaddr) (e.g. INet4SocketAddress, LLSocketAddress)
82 Whereas the protocol specific addresses are custom value types which represent their
83 corresponding low-level address, the socket addresses are based on the corresponding \c sockaddr
86 \section socket_further Going further
87 \seechapter \ref extend \n
88 \seechapter \ref implementation
90 The socket library is highly flexible and extensible. The implementation is not restricted to
91 plain BSD sockets: Any type of read/write communication can be wrapped into the socket library
92 (one Example is the TapSocketHandle which provides access to a Linux \c tap device).
96 /** \page structure Overview of the Socket Library Structure
98 \image html Handle.png
100 This diagram tries to give a structural overview of the Socket Library, it does \e not directly
101 show, how the library is implemented. This will be explained later.
103 The outside interface to the library is a Handle object. This is the only object, the library
104 user directly interacts with. Every handle references some socket. This is like the ordinary
105 POSIX API: the file descriptor (also called file handle, an integer number) references a socket
106 structure which lives in kernel space. In this library, the Handle object (which is not a simple
107 integer any more but an object) references the Socket (which is part of the
108 implementation). Several handles may reference the same Socket. In contrast to the kernel API,
109 the library employs reference counting to release a socket when the last Handle to it goes out
112 The behavior of a Socket is defined by it's Protocol. It is divided into two parts: the
113 <em>policy interface</em> and the <em>protocol interface</em>. Together they provide the
114 complete API for a specific type of Socket as defined by the Protocol. The <em>policy
115 interface</em> provides highly efficient access to the most frequently used operations whereas
116 the <em>protocol interface</em> completes the interface by providing a complete set of all
117 protocol specific operations not found in the policy interface. This structure allows us to
118 combine the benefits of two design methodologies: The policy interface utilizes a policy based
119 design technique and is highly efficient albeit more complex to implement, whereas the protocol
120 interface is based on a more common inheritance architecture which is not as optimized for
121 performance but much simpler to implement. We reduce the complexity of the implementation by
122 reducing the policy interface to a minimal sensible subset of the complete API.
124 \section over_policy The Policy Interface
126 The policy of a Socket consists of several parts, called <em>policy axis</em>. Each axis
127 corresponds to one specific interface aspect of the Socket. The exact meaning of the policy axis
128 are defined elsewhere (see \ref policy_group). The Protocol will always provide a complete set
129 of <em>policy classes</em>, one for each axis.
131 This <em>complete socket policy</em> defines the policy interface of the protocol. This
132 interface is carried over into the Handle. The socket policy as defined in the Handle however
133 may be <em>incomplete</em>. This mans, that the \e accessible interface of the Socket depends on
134 the type of Handle used. The inherent interface does not change but the view of this interface
135 does if the Handle does not provide the \e complete policy interface. This feature is very
136 important. It allows to define generic Handle types. A generic Handle with an incompletely
137 defined policy can point to an arbitrary Socket as long as all those policy axis which \e are
138 defined match those defined in that Socket's protocol. Using such a generic handle decouples the
139 implementation parts using this handle from the other socket aspects (e.g. you may define a
140 generic socket handle for TCP based communication leaving the addressingPolicy undefined which
141 makes your code independent of the type of addressing, IPv4 or IPv6).
143 This can be described as generalized compile-time polymorphism: A base class reference to some
144 derived class will only give access to a reduced interface (the base class interface) of a
145 class. The class still is of it's derived type (and inherently has the complete interface) but
146 only part of it is accessible via the base class reference. Likewise a generic handle (aka base
147 class reference) will only provide a reduced interface (aka base class interface) to the derived
148 class instance (aka socket).
150 \section over_protocol The Protocol Interface
152 The protocol interface is provided by a set of <em>protocol facets</em>. Each facet provides a
153 part of the interface. Whereas the policy interface is strictly defined (the number and type of
154 policy axis is fixed and also the possible members provided by the policy interface are fixed),
155 the protocol interface is much more flexible. Any member needed to provide a complete API for
156 the specific protocol may be defined, the number and type of facets combined to provide the
157 complete interface is up to the Protocol implementor. This flexibility is necessary to provide a
158 complete API for every possible protocol.
160 However this flexibility comes at a cost: To access the protocol interface the user must know
161 the exact protocol of the socket. With other words, the protocol is only accessible if the
162 handle you use is a <em>protocol specific</em> handle. A protocol specific Handle differs from a
163 generic Handle in two ways: It always has a complete policy and it knows the exact protocol type
164 of the socket (which generic handles don't). This allows to access to the complete protocol
167 \section over_impl Implementation of the Socket Libarary Structure
169 In the Implementation, the socket policy is identified by an instance of the senf::SocketPolicy
170 template. The Socket representation is internally represented in a senf::SocketBody which is not
171 outside visible. The Handle is provided by a hierarchy of handle templates. Each Handle template
172 uses template arguments for the policy and/or protocol as needed (see \ref handle_group).
174 The Handle hierarchy divides the interface into two separate strains: the client interface
175 (senf::ClientSocketHandle and senf::ProtocolClientSocketHandle) provides the interface of a
176 client socket whereas the server interface (senf::ServerSocketHandle and
177 senf::ProtocolServerSocketHandle) provides the interface as used by server sockets.
179 The protocol interface is implemented using inheritance: The Protocol class inherits from each
180 protocol facet using multiple (virtual public) inheritance. The Protocol class therefore
181 provides the complete protocol API in a unified (see \ref protocol_group).
184 /** \page usage Using the Socket Library
186 Whenever you use the socket library, what you will be dealing with are FileHandle derived
187 instances. The socket library relies on reference counting to automatically manage the
188 underlying socket representation. This frees you of having to manage the socket lifetime
191 \section usage_create Creating a Socket Handle
193 To create a new socket handle (opening a socket), you will need to use
194 ProtocolClientSocketHandle or ProtocolServerSocketHandle. You will probably not use these
195 templates as is but use proper typedefs (for example TCPv4ClientSocketHandle or
196 PacketSocketHandle). The documentation for these socket handles are found in the protocol class
197 (for example TCPv4SocketProtocol or PacketSocketProtocol).
199 \section usage_reusable Writing Reusable Components
201 To make your code more flexible, you should not pass around your socket in this form. Most of
202 your code will be using only a small subset of the ProtocolClientSocketHandle or
203 ProtocolServerSocketHandle API.
205 If instead of using the fully specified handle type you use a more incomplete type, you allow
206 your code to be used with all sockets which fulfill the minimal requirements of your code. These
207 types are based on the ClientSocketHandle and ServerSocketHandle templates which implement the
208 policy interface without providing the concrete protocol interface. To use those templates you
209 may define a special reduced policy or handle for your code. By giving only an incomplete policy
210 you thereby reduce the interface to that required by your module:
213 typedef ClientSocketHandle<
217 ConnectedCommunicationPolicy > > MyReadableHandle;
221 This defines \c MyReadableHandle as a ClientSocketHandle which will have only read
222 functionality. Your code expects a stream interface (in contrast to a packet or datagram based
223 interface). You will not have \c write or \c readfrom members. \c write will be disabled since
224 the WritePolicy is unknown, \c readfrom will be disabled since a socket with the
225 ConnectedCommunicationPolicy does not have a \c readfrom member.
233 /** \page extend Extending the Library
235 There are two layers, on which the socket library can be extended: On the protocol layer and on
236 the policy layer. Extending the protocol layer is quite simple and works as long as the desired
237 protocol does use the same BSD API used by the standard internet protocols as implemented in the
238 standard policies (i.e. it uses ordinary read() and write() or rcvfrom() or sendto() calls and
241 If however the implementation of a policy feature needs to be changed, a new policy class has to
242 be written. This also is not very complicated however the integration is more complex.
244 \section extend_protocol Writing a new protocol class
246 Most protocols can be implemented by just implementing a new protocol class. The protocol class
247 must be derived from ConcreteSocketProtocol and takes the socket policy (as created by
248 MakeSocketPolicy) as a template argument. See the documentation of this class for the interface.
250 \attention You may want to use multiple inheritance as it is used in the implementation of the
251 standard protocols (See \ref protocol_group). You must however be extra careful to ensure, that
252 every class ultimately has SocketPolicy as a public \e virtual base.
254 After the protocol class has been defined, you will probably want to provide typedefs for the
255 new protocol sockets. If the new protocol is connection oriented, this will be like
257 typedef ProtocolClientSocketHandle<MySocketProtocolClass> MySocketProtocolClientSocketHandle;
258 typedef ProtocolServerSocketHandle<MySocketProtocolClass> MySocketProtocolServerSocketHandle;
261 \section extend_policy Extending the policy framework
263 If you have to extend the policy framework, you will need to be aware of some important
264 limitations of the socket library:
266 \li When you define a new policy for some axis, this new policy <em>must not</em> be derived
267 from one of the existing concrete policy classes (except of course the respective policy
268 axis base class). This is important since the policy type is \e not polymorphic. The policy
269 to be used is selected by the compiler using the \e static type, which is exactly what is
270 desired, since this allows calls to be efficiently inlined.
272 \li Therefore, extending the policy framework will make the new socket probably \e incompatible
273 with generic code which relies on the policy axis which is extended. Example: If you write a
274 new write policy because your protocol does not use ordinary write() system calls but some
275 protocol specific API, Then any generic function relying on WritablePolicy will \e not work
276 with the new socket, since the socket does \e not have this policy, it has some other kind
279 Therefore you need to be careful of what you are doing. The first step is to find out, which
280 policy you will have to implement. For this, find the ClientSocketHandle and/or
281 ServerSocketHandle members you want to change (see \ref ClientSocketHandle and \ref
282 ServerSocketHandle). Not all policy axis directly contribute to the SocketHandle
283 interface. However, some policy members additionally depend on other policy axis (example:
284 AddressingPolicy::connect is only defined if the communication policy is
285 ConnectedCommunication).
290 /** \page implementation Implementation notes
292 \section class_diagram Class Diagram
294 \diaimage SocketLibrary-classes.dia
296 \section impl_notes Arbitrary Implementation Notes
298 \li The implementation tries to isolate the library user as much as possible from the system
299 header files since those headers define a lot of define symbols and introduce a host of
300 symbols into the global namespace. This is, why some classes define their own \c enum types
301 to replace system defined define constants. This also precludes inlining some functionality.
303 \li To reduce overhead, template functions/members which are more than one-liners are often
304 implemented in terms of a non-template function/member. This is also used to further the
305 isolation from system headers as defined above (template code must always be included into
306 every compilation unit together with all headers need for the implementation).
315 // c-file-style: "senf"
316 // indent-tabs-mode: nil
317 // ispell-local-dictionary: "american"
320 // compile-command: "scons -u doc"