[dpdk-dev] where to find ethernet CRC when stripping is off

Ivan Boule ivan.boule at 6wind.com
Wed Jan 20 16:49:06 CET 2016

On 01/20/2016 04:02 PM, Montorsi, Francesco wrote:
> Hi all,
> I need to get access to the Ethernet CRC of received packets.
> To do this, I'm configuring:
> port_conf.rxmode.hw_strip_crc = 0;
> Now my question is: how am I supposed to access the Ethernet CRC from a DPDK mbuf?
> Is the CRC just the 4 final bytes of the packets?
> Is this correct:
>     uint32_t crc = rte_pktmbuf_mtod_offset (mymbuf, uint32_t*, mymbuf->pkt_len) ;
> ?
> Thanks,
> Francesco Montorsi
Hi Francesco,

You would be right... if the PMDs did not transparently strip the CRC in 
software when hardware CRC stripping is disabled at port configuration 
(as described above).
See for instance how the function ixgbe_recv_pkts_lro() in file 
drivers/net/ixgbe/ixgbe_rxtx.c deals with crc_len.

Considering your need, I think now that PMDs should keep the CRC that 
are stored in received packets when hardware CRC stripping is disabled 
by the application, so that the application can access it as needed.

Note that this would impose that the input packet processing of such 
DPDK applications be aware of the CRC presence (+4 in the packet length 
, for instance).

Let's see what others, if any, that might care think about such a change 
into the CRC stripping semantics.


Ivan Boule
6WIND Development Engineer

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