On Thu, Dec 18, 2014 at 08:35:32AM -0600, Jeff Webb wrote:
> On 12/17/2014 04:31 AM, Gilles Chanteperdrix wrote:
> > On Tue, Dec 16, 2014 at 03:51:16PM -0600, Jeff Webb wrote:
> >> I am using rtnet without rtmac/tdma. I previously sent a patch
> >> that allows one to use the "rtnet" script and rtnet.conf in this
> >> configuration. I would also like to be able to specify a list of
> >> IP addresses in rtnet.conf that are passed along to "rtroute
> >> solicit". I have attached patches for the rtnet and
> >> xenomai-3.git/next repositories. I think others will find them
> >> useful.
> >
> > Well, you should probably be using "nomac" if you are not interested
> > in tdma.
> >
>
> Thank you for your response. I appreciate your advice, Gilles, and
> even more so, since I am new to RTnet. I am somewhat confused by
> the rtmac/nomac configuration, since I haven't found a lot of
> documentation on it. Something I read indicated that the primary
> purpose was for use as skeleton when writing new MAC
> implementations, but it also seems that it is useful for other
> purposes. I have a couple questions that might clarify things for
> me a bit.
>
> There are a couple of ASCII-art drawings at the beginning of the
> README.rtmac file in the rtnet documentation. The first one seems
> to indicate that rtnet can be used without rtmac, if all
> communication is done from RT applications. I see that with rtmac
> inserted, standard linux programs can also send information over
> the real-time interface. The RTmac.spec document indicates that
> the rtmac layer wraps data sent through rtmac with an rtmac frame
> header, and specifies how the non-real-time packets can be
> tunneled through a real-time network. My first question is: does
> the rtmac/nomac configuration still add the rtmac header? I
> assumed the answer was "yes", but perhaps I am wrong.
The answer is yes, but only for the packets sent from non real-time
applications. The UDP packets sent using RTnet sockets are plain IP
packets compatible with normal IP stacks.
>
> My initial RTnet application is to send UDP packets in real-time
> to a hardware device manufactured by someone else. Since I am not
> in control of the receiving device, I cannot have any additional
> headers added to my packets. Can I do this without the rtmac
> module inserted? Can I do this with rtmac and nomac inserted? If
> so, what would be the advantage in my application? At some point I
> may need to send/receive TCP and UDP packets for non-real-time
> configuration purposes, but I don't see any need to communicate
> from standard linux programs at this time.
>
> Thank you again to everyone for their input. The documentation is
> not very clear on how (or even if) rtnet can be used to do what I
> need.
To be able to send plain IP packets using Linux sockets without the
rtmac tunneling, you can use rtnetproxy. The problem with rtnetproxy
is that it creates only one linux interface, and then routing is
done by rtnet. While it may be sufficient for typical applications,
it is insufficient for network oriented equipments which need
NAT, or per-interface QOS settings.
A solution to this need is to create, like rtmac does, one linux
interface by rtnet interface, but do not do the tunneling that rtmac
does. A long time ago, when I worked for a company which was using
RTnet, I did exactly that, modifying the nomac policy, but the
patches did not went far into rtnet (you can probably find them in
rtnet archives though).
My plan is to do the same thing, but adding a new mac policy which
would be dubbed xmac (like cross-mac) in xenomai 3 tree. I can not
make any promise on that though, since I have plenty to do already
with rtnet, as I have posted. Ideally, what I would like to do, is
to have, for each NIC, one RT interface which gives good latencies,
and a plain linux interface which allows high throughput.
--
Gilles.
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