The sppp network layer implements the state machine and the Link Control Protocol (LCP) of the point to point protocol (PPP) as described in RFC 1661. Note that this layer does not provide network interfaces of its own, it is rather intended to be layered on top of drivers providing a synchronous point-to-point connection that wish to run a PPP stack over it. The corresponding network interfaces have to be provided by these hardware drivers.
The sppp layer provides three basic modes of operation. The default mode, with no special flags to be set, is to create the PPP connection (administrative Open event to the LCP layer) as soon as the interface is taken up with the ifconfig(8) command. Taking the interface down again will terminate the LCP layer and thus all other layers on top. The link will also terminate itself as soon as no Network Control Protocol (NCP) is open anymore, indicating that the lower layers are no longer needed.
Setting the link-level flag link0 with ifconfig(8) will cause the respective network interface to go into passive mode. This means, the administrative Open event to the LCP layer will be delayed until after the lower layers signals an Up event (rise of "carrier"). This can be used by lower layers to support a dialin connection where the physical layer is not available immediately at startup, but only after some external event arrives. Receipt of a Down event from the lower layer will not take the interface completely down in this case.
Finally, setting the flag link1 will cause the interface to operate in dial-on-demand mode. This is also only useful if the lower layer supports the notion of a carrier (like with an ISDN line). Upon configuring the respective interface, it will delay the administrative Open event to the LCP layer until either an outbound network packet arrives, or until the lower layer signals an Up event, indicating an inbound connection. As with passive mode, receipt of a Down event (loss of carrier) will not automatically take the interface down, thus it remains available for further connections.
The sppp layer supports the debug interface flag that can be set with ifconfig(8). If this flag is set, the various control protocol packets being exchanged as well as the option negotiation between both ends of the link will be logged at level LOG_DEBUG. This can be helpful to examine configuration problems during the first attempts to set up a new configuration. Without this flag being set, only the major phase transitions will be logged at level LOG_INFO.
It is possible to leave the local interface IP address open for negotiation by setting it to 0.0.0.0. This requires that the remote peer can correctly supply a value for it based on the identity of the caller, or on the remote address supplied by this side. Due to the way the IPCP option negotiation works, this address is being supplied late during the negotiation, which might cause the remote peer to make wrong assumptions.
In a similar spirit the remote address can be set to the magical value 0.0.0. * which means that we do not care what address the remote side will use, as long as it is not 0.0.0.0. This is useful if your ISP has several dial-in servers. You can of course route add something_or_other 0.0.0. * and it will do exactly what you would want it to.
The PAP and CHAP authentication protocols as described in RFC 1334, and RFC 1994 resp., are also implemented. Their parameters are being controlled by the spppcontrol(8) utility.
VJ header compression is implemented, and enabled by default. It can be disabled using spppcontrol(8).