Mandatory build dependencies:

• glib-2.0 homepage: http://www.gtk.org/download.html debian: libglib2.0-dev
• libevent homepage: http://www.monkey.org/~provos/libevent/ debian: libevent-dev
• json-c homepage: http://oss.metaparadigm.com/json-c/ debian: libjson0-dev

Optional build dependencies:

• python for commands wrappers generator
• doxygen for documentation
• check for unit tests
• valgrind for unit tests analysis

Build steps:

• ./configure
• make
• make install
• fun
• make uninstall

Other make targets:

• make doxygen build documentation
• make check run unit tests
• make check-valgrind run unit tests under valgrind

VDE 3 framework is an event-based library which provides an easy way to build highly customized virtual network tools. These tools can be built by connecting basic components within a context. Such components are provided by modules dynamically loaded at run-time and can be easily developed by others.

As an example let's describe the steps needed to build an ethernet hub capable of accepting connections from VDE 2 (the code can be found in src/vde_hub.c).

The context is the entity responsible for the creation of new components and the environment in which these will run. There are two important configurable properties for a context which can be set through vde_context_init(): the modules path and the event handler. Each module found in the modules path provides an implementation of a particular component. The event handler provides to the library an interface where to register callbacks for read/write events on file descriptors or timeouts.

In this example we can use the default search path of the library and an event handler based on libevent.

Once the context has been successfully initialized it is possible to create new components inside it by calling vde_context_new_component(). Within a context each component is identified by its name, passed to vde_context_new_component() at creation time, thus component names must be unique in a context. There are three kinds of components:

transport

responsible for the creation of communication channels (connections) between the context and the outside world;

engine

processes data to/from various connections;

connection manager

ties a transport to an engine so that new connections originating from the transport will be associated to the engine.

Different implementations of the same component kind are identified by different families.

To create a VDE 2 compatible ethernet hub three components are needed: an engine of the hub family, a transport of the vde2 family and a connection manager of the default family which will tie the two previous components.

After the components are created it is possible to invoke operations on them. Two of the most common operations are listen and connect issued to the connection manager, however each component implementation can provide its own operations.

In this example, to accept connections from VDE 2, we tell the connection manager to listen using vde_conn_manager_listen().

A connection is an abstract object used by an engine to receive and send packets. The engine interacts with the connection by writing packets and registering callbacks to be informed whenever new packets are available to be read.

Each connection has an associated backend which implements the mechanisms to handle packets. When a transport creates a new connection the transport itself implements the backend, but it is also possible to have local connections between two engines in the same context. In this case the backend will call the read callback of an engine as soon as it receives a packet from the other one.

Following the above example, on vde_conn_manager_listen() the connection manager will put the transport in listen state. When the transport has a new connection ready it will call the accept_cb() callback of connection manager, which will in turn will register the new connection to the engine associated with the connection manager by calling vde_engine_new_conn().

Optionally the connection manager before calling vde_engine_new_conn() can negotiate authorization data with the remote connection manager, based on this data and the local engine settings the connection will be either accepted or rejected.

A connection can be closed either explicitly by the engine or as a consequence of a fatal error in the backend. In the former case vde_conn_fini() and vde_conn_delete() are called to shutdown the connection, in the latter case the error_cb() callback of the engine is called to free resources associated with the connection.

A packet is a structured container of data exchanged by engines.

Inside each packet there's a payload plus a VDE-specific header used to identify the payload type and size. The engine expects a full vde packet to be received from a connection, however a specific backend might add or remove the header as needed. For instance connections generated by transport vde2 or tap will add/remove the header on receive/send because the underlying protocol expects standard ethernet frames.

When a connection has a vde packet ready to be processed it calls the read_cb() callback of the engine passing a pointer to the packet. When an engine wants to send a packet on a connection it calls vde_connection_write() passing a pointer to the packet as well.

In both cases it is responsability of the caller to free the packet after the function call returns, thus if the callee wants to preserve the packet it must perform a copy.

An engine may require additional space when processing a packet, for instance to tag/untag an ethernet frame with 802.1Q informations or to build a layer 2 packet from a layer 3 packet. In these cases, instead of copying the packet to a large memory area, the engine can ask the connection to preallocate head and tail space around the payload.

In addition to library and modules API it's possible to remotely control a vde context by setting up a control engine which acts as an RPC interface for the components. This engine exchanges vde packets using vde connections as any other engine does, however the content of these packets is not network data, it is a serialized method call which the control engine is able to decode and dispatch to the right component. Method calls are serialized into JSON strings and follow JSON-RPC 1.0 specifications. The method name format of the remote call is: component_name.method_name.

A component, to remotely expose its functionalities, must dynamically register them at runtime. These functionalities are divided in two categories:

commands

synchronous method calls from the remote user to a component. From the component point of view commands are regular functions which can populate a serializable object that will be used as the remote call reply or error.

signals

asynchronous method calls to the remote user done by the component to notify changes in its properties. To raise a signal a component calls the function vde_component_signal_raise() passing a signal name and a serializable object representing the property changes. The signal will be then propagated to all the registered listeners.

Commands and signals can be registered/unregistered at any time via vde_component_command_[add|del]() and vde_component_signal_[add|del]() respectively. A listener can be registered/unregistered to a signal via vde_component_signal_[attach|detach]().

Components implementations must provide a description for their commands and signals that will be used to generate static checks for the incoming method calls and at the same time to build an online help that can be requested by the remote caller.

The following is an example of a command execution on the hub engine running in the example application src/vde_hub.c:

--> { "method": "e1.printport", "params": [1], "id": 0 }
<-- { "id": 0, "result": "please print something useful for port 1", "error": null }

And this is an example of signal registration and signal delivery on the same engine:

--> { "method": "e2.notify_add", "params": ["e1.port_new"], "id": 0 }
<-- { "id": 0, "result": "Signal attached", "error": null }
... a new connection is added to the hub ...
<-- { "id": null, "method": "e1.port_new", "params": [ 1 ] }