sppp - point to point protocol network layer for synchronous


device sppp


The sppp network layer implements the state machine and the
Link Control
Protocol (LCP) of the point to point protocol (PPP) as de
scribed in RFC
1661. Note that this layer does not provide network inter
faces 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 provid
ed by these
hardware drivers.
The sppp layer provides three basic modes of operation. The
mode, with no special flags to be set, is to create the PPP
(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 physi
cal 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). Up
on configuring
the respective interface, it will delay the administrative
Open event to
the LCP layer until either an outbound network packet ar
rives, or until
the lower layer signals an Up event, indicating an inbound
As with passive mode, receipt of a Down event (loss of car
rier) 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 pro
tocol 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 This requires that the re
mote 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
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 re
mote side will
use, as long as it is not This is useful if your
ISP has several dial-in servers. You can of course route add
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 de
fault. It can be
disabled using spppcontrol(8).


<ifname><ifnum>: <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.
<ifname><ifnum>: loopback The state automaton detected a
line loopback
(that is, it was talking with itself). The interface will
be temporarily
<ifname><ifnum>: up The LCP layer is running again, after a
line loopback had previously been detected.
<ifname><ifnum>: down The keepalive facility detected the
line being
unresponsive. Keepalive must be explicitly requested by the
lower layers
in order to take place.


inet(4), intro(4), ppp(4), ifconfig(8), spppcontrol(8)

W. Simpson, Editor, The Point-to-Point Protocol (PPP), RFC
G. McGregor, The PPP Internet Protocol Control Protocol
(IPCP), RFC 1332.
B. Lloyd and W. Simpson, PPP Authentication Protocols, RFC
W. Simpson, PPP Challenge Handshake Authentication Protocol


The original implementation of sppp was written in 1994 at
Cronyx Ltd.,
Moscow by Serge Vakulenko <vak@cronyx.ru>. Jorg Wunsch
<joerg_wunsch@uriah.heep.sax.de> 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 Jorg Wun



Currently, only the IPCP control protocol and ip(4) network
protocol is
supported. More NCPs should be implemented, as well as oth
er control
protocols for authentication and link quality reporting.
Negotiation loop avoidance is not fully implemented. If the
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 acces
sible through
Passive mode has not been tested extensively.
Link-level compression protocols should be supported.
BSD December 30, 2001
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