point to point protocol network layer for synchronous
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
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
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
will cause the respective network interface to go into
mode. This means, the administrative
event to the LCP layer will be delayed until
after the lower layers signals an Up
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
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
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 of a Down
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
. 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 wrong assumptions.
In a similar spirit the remote address can be set to the magical value
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
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 disabled
- <ifname><ifnum>: <proto> illegal <event> in state
- An event happened that should not happen for the current state the
respective control protocol is in. See RFC 1661 for a description of the
- <ifname><ifnum>: loopback
- The state automaton detected a line loopback (that is, it was talking with
itself). The interface will be temporarily disabled.
- <ifname><ifnum>: up
- The LCP layer is running again, after a line loopback had previously been
- <ifname><ifnum>: down
- The keepalive facility detected the line being unresponsive. Keepalive
must be explicitly requested by the lower layers in order to take
W. Simpson, Editor,
The Point-to-Point Protocol (PPP),
The PPP Internet Protocol Control Protocol (IPCP),
B. Lloyd and
W. Simpson, PPP Authentication
Protocols, RFC 1334.
PPP Challenge Handshake Authentication Protocol
(CHAP), RFC 1994.
The original implementation of
written in 1994 at Cronyx Ltd., Moscow by Serge
rewrote a large part in 1997 in order to fully implement the state machine as
described in RFC 1661, so it could also be used for dialup lines. He also
wrote this man page. Serge later on wrote a basic implementation for PAP and
CHAP, which served as the base for the current implementation, done again by
Currently, only the IPCP
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 through
mode has not been tested extensively.
Link-level compression protocols should be supported.