/*!
\page kernelppp The Kernel PPP Kit (libkernelppp.a)

The PPP stack is a kernel module that interfaces with the network stack.
Thanks to its modular nature new protocols and extensions can be added without
having to recompile the PPP stack.

The kernel API is mostly intended for developers who want to write extension modules,
but you can also create and control PPP interfaces from within your kernel module.

Every instance of a PPP interface initially consists of the following classes:

- KPPPInterface (the main interface class)
- KPPPLCP (the Link Control Protocol)
- KPPPStateMachine (the state machine for the Link Control Protocol)
- KPPPProfile (this manages the current interface profile)

KPPPLCP, KPPPStateMachine, and KPPPProfile are integrated into KPPPInterface as
member variables. All these classes have private constructors. Only the PPP
interface manager can create a new instance of KPPPInterface. \n
Such an interface does nothing useful because it needs at least a device to connect
to a PPP server. \n
These are the base classes for extending an interface:

- KPPPDevice (a PPP transport device: PPPoE, modem, cell phone, etc.)
- KPPPLCPExtension (adds new basic codes to the LCP protocol)
- KPPPOptionHandler (adds new configure-packet types to the LCP protocol)
- KPPPProtocol (a base class for protocols; e.g.: LCP and IPCP)

\section kppppackethandling Packet handling

KPPPInterface, KPPPDevice, and KPPPProtocol derive from KPPPLayer. When sending,
packets are passed down the stack to every layer that can handle the packet.
This facilitates creating packet encapsulators like compression protocols. You can
even catch packets before they arrive at the device layer. \n
A different system is used for receiving packets: KPPPDevice passes the received
packet to KPPPInterface which in turn tries to find the best fitting KPPPProtocol
that can handle this kind of packet. If one is found it may pass the packet to the
netstack or, in case of an encapsulator, it can put the packet back into the decode
queue by calling KPPPInterface::Receive().

\section kpppmultilink Multilink interfaces

Interfaces can have one parent and multiple children. The first interface of a
multilink bundle is split into a parent and a child. All other children must be added
to the parent when they are needed. Normally, a bundle (main) interface has no parent
and a sub-interface (child) has no children. \n
A multilink protocol can use any desired technique for creating the additional
children. For instance, it could save the descriptions of all children as private
parameters in the interface description file. Alternatively, it could use internal
bundle names to bundle all interfaces with the equal bundle names. \n
If a received packet could not be handled by a child it is passed up to the parent.
On the other hand, an interface without a device (no next layer after the interface)
will \e not pass the packet to its children.

\section kpppwritingmodules Writing modules

PPP makes use of the kernel module API. Every module must export a \c ppp_module_info
structure. \n
KPPPInterface loads all specified modules and calls their \c ppp_module_info::add_to()
function. Depending on the type of module you should add the corresponding object
to the interface.

\sa ppp_module_info

*/