Ubuntu Manpage: sppp — point to point protocol network layer for synchronous lines

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NAME

       sppp — point to point protocol network layer for synchronous lines

SYNOPSIS

       device sppp

DESCRIPTION

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. 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 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).

DIAGNOSTICS

       <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.

AUTHORS

       The  original  implementation  of  sppp  was  written  in  1994 at Cronyx Ltd., Moscow by Serge Vakulenko
       <vak@cronyx.ru>.  Jörg 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 Jörg Wunsch.

BUGS