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.
layer provides three basic modes of operation.
The default mode,
with no special flags to be set, is to create the PPP connection
event to the LCP layer) as soon as the interface is taken up with the
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
will cause the respective network interface to go into
This means, the administrative
event to the LCP layer will be delayed until after the lower layers
event (rise of
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
event from the lower layer will not take the interface completely down
in this case.
Finally, setting the flag
will cause the interface to operate in
This is also only useful if the lower layer supports the notion
of a carrier.
Upon configuring the
respective interface, it will delay the administrative
event to the LCP layer until either an outbound network packet
arrives, or until the lower layer signals an
event, indicating an inbound connection.
As with passive mode, receipt
event (loss of carrier) will not automatically take the interface down,
thus it remains available for further connections.
layer supports the
interface flag that can be set with
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
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
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
In a similar spirit the remote address can be set to the magical
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
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
VJ header compression is implemented, and enabled by default.
It can be
Currently, only the
control protocol and
network protocol is supported.
More NCPs should be implemented, as well as other control protocols
for authentication and link quality reporting.
Negotiation loop avoidance is not fully implemented.
If the negotiation
does not converge, this can cause an endless loop.
The various parameters that should be adjustable per RFC 1661 are
currently hard-coded into the kernel, and should be made accessible
mode has not been tested extensively.
Link-level compression protocols should be supported.