Jan,
thanks a lot for your quick reply! I have some additional questions below:
On 11/11/06, Jan Kiszka <[EMAIL PROTECTED]> wrote:
> Likely, those packets were received by the same IRQ event. The typical
> RTnet driver looks like this:
>
> irq_handler()
> {
> timestamp = read_timestamp();
> ...
> while ((packet = incoming_packet())) {
> packet.timestamp = timestamp;
> enqueue_received_packet(packet);
> }
> ...
> }
>
I am not fully aware of the details of the mode of operation of the
NIC, but do you suggest that the NIC stores/buffers multiple packets
before raising an interrupt to the CPU? If this is the case, I
understand that the timestamp could be the same (quantized to
microsecond level at least).
Is is possible to make the NIC to raise an IRQ for every single packet
or would this have severe impact of performance?
I really would like to examine the *actual* receive time into the NIC
as close to the real time as possible.
> That's due to the fact that Linux "only" handles timestamps via struct
> timeval, i.e. with microsecond resolution. But that's ok as the error of
> your timestamp is in microsecond range anyway, even on unloaded passive
> sniffers.
Thanks for informing me about the struct timeval, that explains the
behaviour. What kind of timestamp error would I expect on an unloaded
vs loaded system? Do you have any rough figures of the error or could
you refer to any other sources of information?
/ Mats
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