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simple isdn4linux PPP FAQ .. to be continued .. not 'debugged' 
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Q01: what's pppd,ipppd, syncPPP , asyncPPP ??
Q02: error message "this systems lacks PPP support"
Q03: strange information using 'ifconfig'
Q04: MPPP?? What's that and how can I use it ...
Q05: I tried MPPP but it doesn't work 
Q06: can I use asynchronous PPP encapsulation with network devices
Q07: A SunISDN machine can't connect to my i4l system
Q08: I wanna talk to several machines, which need different configs
Q09: Starting the ipppd, I get only error messages from i4l
Q10: I wanna use dynamic IP address assignment 
Q11: I can't connect. How can I check where the problem is.
Q12: How can I reduce login delay? 

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Q01: pppd,ipppd, syncPPP , asyncPPP .. what is that ?
   what should I use?
A: The pppd is for asynchronous PPP .. asynchronous means
   here, the framing is character based. (e.g when
   using ttyI* or tty* devices)

   The ipppd handles PPP packets coming in HDLC
   frames (bit based protocol) ... The PPP driver
   in isdn4linux pushes all IP packets direct
   to the network layer and all PPP protocol
   frames to the /dev/ippp* device. 
   So, the ipppd is a simple external network
   protocol handler.

   If you login into a remote machine using the
   /dev/ttyI* devices and then enable PPP on the
   remote terminal server -> use the 'old' pppd

   If your remote side immediately starts to send
   frames ... you probably connect to a 
   syncPPP machine .. use the network device part
   of isdn4linux with the 'syncppp' encapsulation
   and make sure, that the ipppd is running and 
   connected to at least one /dev/ippp*. Check the 
   isdn4linux manual on how to configure a network device.

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Q02: when I start the ipppd .. I only get the
   error message "this systems lacks PPP support"
A: check that at least the device 'ippp0' exists.
   (you can check this e.g with the program 'ifconfig')
   The ipppd NEEDS this device under THIS name .. 
   If this device doesn't exists, use:
        isdnctrl addif ippp0
        isdnctrl encap ippp0 syncppp
        ... (see isdn4linux doc for more) ...
A: Maybe you have compiled the ipppd with another
   kernel source tree than the kernel you currently
   run ... 

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Q03: when I list the netdevices with ifconfig I see, that
   my ISDN interface has a HWaddr and IRQ=0 and Base 
   address = 0 
A: The device is a fake ethernet device .. ignore IRQ and baseaddr
   You need the HWaddr only for ethernet encapsulation.
   
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Q04: MPPP?? What's that and how can I use it ...

A: MPPP or MP or MPP (Warning: MP is also an 
   acronym for 'Multi Processor') stands for
   Multi Point to Point and means bundling
   of several channels to one logical stream.
   To enable MPPP negotiation you must call the
   ipppd with the '+mp' option. 
   You must also configure a slave device for
   every additional channel. (see the i4l manual
   for more)
   To use channel bundling you must first activate
   the 'master' or initial call. Now you can add 
   the slave channels with the command:
       isdnctrl addlink <device>
   e.g:
       isdnctrl addlink ippp0
   This is different from other encapsulations of
   isdn4linux! With syncPPP, there is no automatic
   activation of slave devices.

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Q05: I tried MPPP but it doesn't work .. the ipppd
   writes in the debug log something like:
   .. rcvd [0][proto=0x3d] c0 00 00 00 80 fd 01 01 00 0a ...
   .. sent [0][LCP ProtRej id=0x2 00 3d c0 00 00 00 80 fd 01 ...

A: you forgot to compile MPPP/RFC1717 support into the
   ISDN Subsystem. Recompile with this option enabled.

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Q06: can I use asynchronous PPP encapsulation
   over the network interface of isdn4linux ..

A: No .. that's not possible .. Use the standard
   PPP package over the /dev/ttyI* devices. You
   must not use the ipppd for this.
   
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Q07: A SunISDN machine tries to connect my i4l system,
   which doesn't work.
   Checking the debug log I just saw garbage like:
!![ ... fill in the line ... ]!!

A: The Sun tries to talk asynchronous PPP ... i4l
   can't understand this ... try to use the ttyI*
   devices with the standard PPP/pppd package

A: (from Alexanter Strauss: )
!![ ... fill in mail ]!!

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Q08: A wanna talk to remote machines, which need
   a different configuration. The only way
   I found to do this is to kill the ipppd and
   start a new one with another config to connect
   to the second machine. 

A: you must bind a network interface explicitly to
   an ippp device, where you can connect a (for this
   interface) individually configured ipppd.

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Q09: When I start the ipppd I only get error messages
   from the i4l driver .. 

A: When starting, the ipppd calls functions which may 
   trigger a network packet. (e.g gethostbyname()).
   Without the ipppd (at this moment, it is not
   fully started) we can't handle this network request.
   Try to configure hostnames necessary for the ipppd
   in your local /etc/hosts file or in a way, that
   your system can resolve it without using an
   isdn/ippp network-interface.

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Q10: I wanna use dynamic IP address assignment ... How 
   must I configure the network device.

A: At least you must have a routing, which forwards
   a packet to the ippp network-interface to trigger
   the dial-on-demand.
   A default routing to the ippp-interface will work.
   Now you must choose a dummy IP address for your
   interface.
   If for some reason you can't set the default
   routing to the ippp interface, you may take any 
   address of the subnet from which you expect your
   dynamic IP number and set a 'network route' for
   this subnet to the ippp interface.
   To allow overriding of the dummy address you
   must call the ipppd with the 'ipcp-accept-local' option.

A: You must know, how the ipppd gets the addresses it wanna
   configure. If you don't give any option, the ipppd
   tries to negotiate the local host address!
   With the option 'noipdefault' it requests an address
   from the remote machine. With 'useifip' it gets the
   addresses from the net interface. Or you set the address
   on the option line with the <a.b.c.d:e.f.g.h> option.
   Note: the IP address of the remote machine must be configured
   locally or the remote machine must send it in an IPCP request.
   If your side doesn't know the IP address after negotiation, it
   closes the connection!
   You must allow overriding of address with the 'ipcp-accept-*'
   options, if you have set your own or the remote address 
   explicitly.

A: Maybe you try these options .. e.g:   

    /sbin/ipppd :$REMOTE noipdefault /dev/ippp0

   where REMOTE must be the address of the remote machine (the
   machine, which gives you your address)

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Q11: I can't connect. How can I check where the problem is.

A: A good help log is the debug output from the ipppd...
   Check whether you can find there:
   - only a few LCP-conf-req SENT messages (less then 10)
     and then a Term-REQ:
     -> check whether your ISDN card is well configured
        it seems, that your machine doesn't dial
        (IRQ,IO,Proto, etc problems)
        Configure your ISDN card to print debug messages and
        check the /dev/isdnctrl output next time. There
        you can see, whether there is activity on the card/line.
   - there are at least a few RECV messages in the log:
     -> fine: your card is dialing and your remote machine
        tries to talk with you. Maybe only a missing 
        authentication. Check your ipppd configuration again.
   - the ipppd exits for some reason:
     -> not good ... check /var/adm/syslog and /var/adm/daemon.
        Could be a bug in the ipppd.

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Q12: How can I reduce login delay?

A: Log a login session ('debug' log) and check which options 
  your remote side rejects. Next time configure your ipppd
  to not negotiate these options. Another 'side effect' is, that
  this increases redundancy. (e.g your remote side is buggy and
  rejects options in a wrong way).