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Comment about GRO and RSC in README 

git-svn-id: file:///Users/braun/svn/vermont/branches/vermont/dtls-merge@2438 aef3b71b-58ee-0310-9ba9-8811b9f0742f
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muenz 2010-06-23 14:40:52 +00:00
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@ -131,3 +131,41 @@ the utilization of pcap library 1.0.0 or higher.
For earlier versions of pcap, the pcap-mmap patch can be applied to
improve the performance: http://public.lanl.gov/cpw/
-------------------------------------
EFFECTS OF RECEIVE OFFLOAD MECHANISMS
-------------------------------------
Several mechanisms have been implemented in modern network interface cards,
drivers, and kernels to offload common functions from the protocol stack and
the application. One particular focus is on TCP segmentation and reassembly.
Receive offload mechanisms aim at reassembling subsequent TCP segments into
a single large packet before passing it to the IP/TCP protocol stack and
finally to the application. In the Linux kernel, this is done by generic
receive offload (GRO) if the network interface card and the driver support
NAPI. Latest Intel 10GE controllers (e.g., 82599) support receive side
coalescing (RSC) which performs TCP reassembly in hardware.
If any receive offload mechanism is enabled, VERMONT (like any other
pcap-based application) does not observe the actually captured TCP packets
but the reassembled ones. One consequence is that packet counts of flows will
be smaller than the true number of packets.
In order to avoid such distortions, all receive offload mechanisms need to
be disabled. In the case of GRO (and the older LRO), this can be done with
ethtool. The following call returns a list of the current status of all
offload mechanisms for interface <dev>:
$ ethtool -k <dev>
If GRO is not shown, you probably need to install a newer version of ethtool.
To disable GRO (and LRO), execute:
$ ethtool -K <dev> gro off
$ ethtool -K <dev> lro off
Hardware-based RSC can be deactivated at compile time of the driver as
explained here:
http://downloadmirror.intel.com/19004/eng/README-2.0.72.4.txt