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     sppp - point to point protocol network layer for synchronous lines


     device sppp


     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
     hardware drivers.

     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 (like with an ISDN line).  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.

     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  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  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 〈〉.  Jörg Wunsch
     〈〉 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
     through spppcontrol(8).

     Passive mode has not been tested extensively.

     Link-level compression protocols should be supported.