Re: [Xenomai-help] Xenomai and Libpcap

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Fillod Stephane wrote:
> Hej!
> 
> Benjamin Biegel wrote:
>> I have a question regarding the use of Libpcap together with Xenomai. 
>> The Libpcap library provides the ability to capture Ethernet frames
> with 
>> my NIC in promiscuous mode + time stamp the received frames on
> Ethernet 
>> level (through the clock of the NIC).
>>
>> My question is this: How do i set up an Xenomai real-time environment 
>> that makes the time-stamping very accurate. How do I have influence on
> 
>> when pcap is scheduled? How do I make sure pcap is not preempted by
> the OS?
> 
> You have to know that the Linux network stack and libpcap on behalf of
> your process won't be able to run under hard real-time even with
> Xenomai.
> 
> What you're looking for is RTnet: http://www.rtnet.org
> So far, performance for high speed networking is rather poor IMHO,
> esp. regarding the API. Please follow this proposal[1] (sorry for 
> formatting), and feel free to revive the thread and the discussion.
> 
> [1]
> http://thread.gmane.org/gmane.linux.real-time.rtnet.devel/262/focus=263

To be a bit more precise. Rtnet has a capture interface (rtcap), which
sends a copy of each packet received by the rtnet driver, to a Linux
driver, which you can capture with libpcap.

As far as I remember, however, the timestamping of each packet is done
in the Linux domain, so, if you want to get the real timestamp, you have
to modify rtnet to get the timestamp done in the Xenomai domain.

Unfortunately, that is not all, because if you get Xenomai's timestamp,
they drift when compared to Linux timestamps. But if you look only at
relative timestamps over short period of times, that is Ok.

-- 
                                          Gilles