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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

     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.

     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


     <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 disabled.

     <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), 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 1994.


     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.