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sppp — point to point protocol network layer for synchronous lines
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
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. 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
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).
<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
<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 1661.
G. McGregor, The PPP Internet Protocol Control Protocol (IPCP), RFC 1332.
B. Lloyd and W. Simpson, PPP Authentication Protocols, RFC 1334.
W. Simpson, PPP Challenge Handshake Authentication Protocol (CHAP), RFC
The original implementation of sppp was written in 1994 at Cronyx Ltd.,
Moscow by Serge Vakulenko ⟨email@example.com⟩. Jörg Wunsch
⟨firstname.lastname@example.org⟩ 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 Wunsch.
Currently, only the IPCP control protocol and ip(4) 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
Passive mode has not been tested extensively.
Link-level compression protocols should be supported.