Hi
i would like to know how the network layer does to know what is the upper
layer protocol in order to fill in correctly the protocol field in the
header it adds to the packet before sending it. I'm doing a ethernet device
that doesn't add any header to the packet but change the output device,
then i should say the network device that the packet is like if it has been
sent by the ip protocol.
Thanks,
Romain Giry
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Romain Giry wrote:
Hi,
> Hi
>
> i would like to know how the network layer does to know what is the upper
> layer protocol in order to fill in correctly the protocol field in the
> header it adds to the packet before sending it.
see, their might be either API provided by DLL (ethernet, ATM, FR etc) to
network layer (IP, IPX etc) to sent packets over a physical device. Now, the
Network protocol field can be passed through the API to be filled in by
DLL header by DLL. Another case may be, DLL can easily know what's the ifIndex
the packet is coming from. Network layer might have registration policy by
ifIndex to DLL, which can be used now to infer Network layer protocol id.
> I'm doing a ethernet device
> that doesn't add any header to the packet but change the output device,
> then i should say the network device that the packet is like if it has been
> sent by the ip protocol.
I didn't understand what did you mean by "ethernet device doesn't add any
header but change the output device"? May be after you explain a bit more I
can suggest something more.
Thanks,
Dipak
>
>
> Thanks,
>
> Romain Giry
>
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Hi,
> > I'm doing a ethernet device
> > that doesn't add any header to the packet but change the output device,
> > then i should say the network device that the packet is like if it has been
> > sent by the ip protocol.
>
>I didn't understand what did you mean by "ethernet device doesn't add any
>header but change the output device"? May be after you explain a bit more I
>can suggest something more.
For my thesis, I should do a module for linux that allows the user to
switch the
physical device at run-time. In a first time this should be done by user
commands
and later the module should decide himself to switch the physical device.
That's why it seems to me natural to create a dummy ethernet device driver
which
does nothing else apart from forwarding the packets received from the IP stack
to a real (= physical) network device. Therefore i need to fake that the
packet was
sent by the IP stack so as the physical device fill in correctly the
protocol field in
the header it adds.
Otherwise i thought i can do a transparent firewall that decides to which real
interface to switch the packets after beeing sure that the physical device is
running otherwise it should change.
One thing that may be difficult to implement is that i want to keep a TCP
connection running when i change the physical device. That's why maybe the
firewall solution may be better because when receiving packets i could fake
that they all come from the same physical device and have therefore the same
IP.
Thanks
Romain
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> One thing that may be difficult to implement is that i want to keep a TCP
> connection running when i change the physical device. That's why maybe the
> firewall solution may be better because when receiving packets i could fake
> that they all come from the same physical device and have therefore the same
> IP.
You set up multiple physical devices with the same IP and use the "route"
command. Thats worked with TCP/IP protocols since day 1