Mpd consists of several layers, corresponding to the layered nature of the PPP protocol. The two main concepts in mpd are bundles and links. A link is a single physical point-to-point connection between the local machine and a remote peer machine, such as a modem connection or a synchronous serial connection. A bundle is a collection of one or more links, all connecting to the same remote peer, that together form a single multi-link PPP connection whose effective bandwidth is the sum of the bandwidths of the individual links.
The bundle layer in effect lies just above the link layer. The bundle layer handles the task of making multiple physical links appear as a single virtual link. Bundles and layers exist regarless of whether the link(s) are actually connected and transmitting packets.
At the bundle layer you configure multi-link PPP settings and the link management policy. The link management policy determines whether mpd tries to keep all the links connected all the time, or whether it adds and subtracts links depending on demand, and if so, according to what parameters.
At this time, mpd requires the links that constitute each bundle to be declared when the bundle is created; after that, they cannot be changed. A future revision may lift this restriction.
With each bundle is a corresponding
interface layer,
which corresponds directly to a netgraph interface
accessible via ifconfig(8), such as ng0.
The netgraph interface is where IP packets ultimately appear
after travelling across the link.
The interface layer handles configuring the interface,
bringing it up or down as appropriate, assigning
IP addresses, setting up static routes, and configuring proxy-ARP.
The interface layer is also responsible for implementing
dial-on-demand and idle timeout functionality.
Each bundle has a corresponding IP Control Protocol (IPCP) layer, which manages the IP specific configuration of the bundle. This layer handles the negotiation of local and remote IP addresses and Van Jacobson TCP header compression, as well as other optional IP related information such as DNS servers and NBNS servers.
Each bundle also has corresponding compression and encryption layers, which allow you to enable and configure compression and encryption for data sent and received over the bundle.
Individual links in a bundle are configured at the link layer. There are two parts to the link configuration, the device independent part and the device specific part. Device independent parameters include the authentication type (PAP or CHAP), whether authentication is enabled in either direction, keep-alive packets, and various other link specific parameters.
Beneath the link layer is the device layer. Configuration of device specific parameters happens at this layer. Each link has a specific type corresponding to one of the supported device types in mpd. The type dictates how the device dependent part of the link is configured and what its capabilities are.
While the combination of all the various layers presents a large number of configuration options, mpd tries to have reasonable defaults for everything.