— point to point protocol network
layer for synchronous lines
network layer implements the state machine and the
Link Control Protocol (LCP) of the point to point protocol
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 ifconfig(8)
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 passive
mode. This means, the administrative
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
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. 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.
layer supports the debug
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
. 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 course
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
<proto> illegal <event> in state <statename>
- 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 state automaton.
- The state automaton detected a line loopback (that is, it
was talking with itself). The interface will be temporarily disabled.
- The LCP layer is running again, after a line loopback had
previously been detected.
- The keepalive facility detected the line being
unresponsive. Keepalive must be explicitly requested by the lower layers
in order to take place.
W. Simpson, Editor,
The Point-to-Point Protocol (PPP),
G. McGregor, The PPP
Internet Protocol Control Protocol (IPCP), RFC
B. Lloyd and W.
Simpson, PPP Authentication Protocols,
W. Simpson, PPP
Challenge Handshake Authentication Protocol (CHAP),
The original implementation of sppp
was written in 1994 at
Cronyx Ltd., Moscow by Serge Vakulenko
⟨firstname.lastname@example.org⟩. Jörg Wunsch
⟨email@example.com⟩ 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 Jörg
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
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.