Re: advice needed

[Babarovic Ivica <ivica@asist-traffic.com>]

James Carlson wrote:

>Babarovic Ivica writes:
>  
>
>>Since I need to keep vxworks setup and programs untouched  (it needs to
>>work as
>>plug in linux box, plug out linux box) I've come with a following idea.
>>
>>I made a program that connects serial ports on linux board so vxworks
>>box can
>>freely access modem across linux box and initialize it. I'm catching the
>>"CONNECT" string here  and after that, I break the serial port
>>connection/exit
>>program. I start ppp link on serial side towards vxworks and that makes
>>it happy.
>>    
>>
>
>PPP itself (irrespective of the hardware or OS in use) generally can't
>work that way because it's a network protocol that involves shared
>state between the peers.  The local problems of having this somehow
>"work" on Linux pale in comparison to the high-level problems.
>
>I can think of two ways to make the overall model work, and perhaps a
>few variations on them.  The first (and certainly easiest) method
>would be to have two _separate_ PPP links, like this:
>
>	VxWorks <---PPP1---> Linux <---PPP2---> GPRS
>
>In this scenario, the Linux box would see two network interfaces --
>probably named "ppp0" and "ppp1."  To get packets from VxWorks out to
>the GPRS system, any of the following variations would work:
>
>  - Run NAT on the Linux box so that both VxWorks and the Linux
>    applications are behind the same IP address, as far as the GPRS
>    device can tell.  If you have inbound connections over GPRS, this
>    may be harder to use.
>  
>
This is exactly my setup and since I don't have inbound connections
things are simple.
I manage to get things working last night. I'm having minor problems
with establishing
a new GPRS ppp from linux box after I break the connection stream that
vxworks box
created. I need to break that stream with some commands to chat. I'm working
on it now.


>  - Route (forward) between these interfaces.  The VxWorks system and
>    the Linux system will need separate IP addresses, and the GPRS
>    system will need to know to treat your Linux box as the next hop
>    router for the VxWorks system.  If you have cooperation of the
>    GPRS provider (unlikely, I'd imagine), this solution would be
>    simple.
>
>  - Run application layer proxies on the Linux box.  The VxWorks
>    system would connect to SOCKS proxies or other (perhaps custom)
>    applications that in turn open connections over the second PPP
>    link to the rest of the world.
>
>The other solution, which is much closer to what I think you're
>suggesting, but is also much closer to being science fiction, would be
>to have the Linux box behave as a custom L2 bridge for PPP traffic.
>You'd have to modify the kernel so that it knows how to forward
>particular IP packets to the intended receiver, and the pppd daemon so
>that it knows how to forward PPP negotiation parameters (IPCP
>addresses and the like) between two separate links.  The result would
>be something like:
>
>	VxWorks <---PPP1a---> Linux <---PPP1b---> GPRS
>
>Doable, perhaps, but quite an involved project, and something that
>would likely require the help of people who know both the Linux kernel
>and pppd code _very_ well.
>
>  
>
Very interesting ideas indeed. I think I don't have enough time and
knowledge yet to
implement them. It is an interesting solution to the problem though.

Best regards,
I.B.