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. 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).
- <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
<
[email protected]>.
Jörg Wunsch
<
[email protected]>
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.
Many.
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.