Real-Time Ethernet Capturing (RTcap)
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RTnet can capture incoming and outgoing Ethernet packets with a very low time
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stamp jitter, typically below 10 us (depends on the hardware).
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When it is configured and compiled with --enable-rtcap, some extensions will be
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added to the RTnet stack and an additional module rtcap.o will be created. This
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module has to be loaded *after* all NIC drivers are inserted and *before* any
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device is started or a RTmac discipline is attached to it. It will create two
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read-only Linux shadow network devices for every NIC:
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<rtdevX> (e.g. rteth0) and
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<rtdevX>-mac (exception: loopback device will only be mirrored to "rtlo").
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The first capturing device mirrors any incoming packet the hardware reports to
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the stack and any outgoing packet sent on the local station using RTnet. The
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second one captures only packets which have be delayed by an active RTmac
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discipline. As the capturing time is dictated by the parent shadow device,
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packet lists can be unchronologic, but it provides a deeper look on the
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influence of RTmac on the packet transmission process.
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After these shadow devices are started up using ifconfig, any capturing tool
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like tcpdump or Ethereal can be used for the actual analysis work. In order to
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get hold of any packet on the network, the real-time NIC should be
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furthermore switched to promiscuous mode when it is configured:
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rtifconfig <rtdevX> up <IP> promisc
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If you notice any potential packet losses while capturing, you can try to
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increase the number of real-time buffer used for storing packets before they
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can be processed by Linux. The module parameter rtcap_rtskb controls this
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parameter. It is set to 128 by default. Generally you should also tell RTcap to
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switch on the RTAI timer (module parameter: start_timer=1) and prevent any
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other module or program to do so as well.
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The capturing support adds a slight overhead to both paths of packets,
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therefore the compilation parameter should only be switched on when the service
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is actually required.
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