HAL and PPP interfaces

Andrei Yurkevich urruru at ru.ru
Wed Dec 8 10:31:04 PST 2004

Dan Williams wrote:

>>I haven't used PPP in a while, so I'm not sure how it works at the
>>system level, but I think it makes a lot of sense to merge these
>>properties onto the modem device, similar to how the normal ethernet and
>>wireless properties are merged onto the network card device.
> Same here, but could you explain a bit how the PPP stuff works Andrei?
> For example, you have a PPP interface, but that traffic has to get moved
> over some physical interface like your modem.  How do you tell the PPP
> interface that its supposed to send traffic over a particular modem
> device?  What does the routing table look like when the PPP interface is
> active?

When pppd establishes a PPP connection, it talks over some particular 
device - that could be a serial port, a USB-serial conventor, a RFComm 
device (something like serial-over-bluetooth), a pty or whatever else. 
It is possible to know what device is used, and if it is a hardware 
device then there is a reason to have that interface as a sub-device of 
some hardware device, or at least as a property for it - in case that 
some other hardware gets connected over that PPP interface - I know of 
PDAs and ADSL modems, but there could be something else.

> Since modems would most likely show up as serial devices, (right?) of
> course they won't show in ifconfig, but I assume that the PPP interface
> does.  HAL, however, should only really show _hardware_ network devices
> (since its a Hardware Abstraction Layer), and in this Joe's idea of
> merging the PPP properties to the modem device makes sense.  We
> shouldn't be having the PPP devices be top-level, since they really
> aren't hardware.

Right, ppp's do show under ifconfig and you can learn what hardware 
device a ppp interface is bound to, but there are some exctic cases when 
a ppp interface is connected to a pseudo-tty that is backed up with some 
software (this is the situation with PPTP and 
<imnotsure>PPPoE</imnotsure) - where should the propertis go in this 
case? Here, having a virtual device node makes sense - it could go under 
some modem/usb device if the PPP is spoken over it, or appear as a 
top-level device in case it is backed with software.

> So, if we have stuff like an IP address for the PPP connection, we
> should probably define some property hierarchy for modems and other
> devices over which PPP can run, like:
> Modem 1:
> info.bus	serial
> net.interface	/dev/ttyS0
> net.ppp.interface	/dev/ppp0
> net.ppp.link	1

Sounds reasonable, but only when ppp is run over hardware.

> We already break down network connection stuff by type, so we have for
> example "net.80211.mac_address" and "net.80203.link", so I think it
> would be acceptable to have "net.ppp.link" or whatever.


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