Re: [dpdk-dev] Regarding UDP checksum offload

[Olivier MATZ <olivier.matz@6wind.com>]

Hi Prashant,

On 01/28/2015 03:57 PM, Prashant Upadhyaya wrote:
>>>         I am using dpdk 1.6r1, intel 82599 NIC.
>>>         I have an mbuf, I have hand-constructed a UDP packet (IPv4) in
>>>         the data
>>>         portion, filled the relevant fields of the headers and I do a tx
>>>         burst. No
>>>         problems, the destination gets the packet. I filled UDP checksum
>>>         as zero
>>>         and there was no checksum offloaded in ol_flags.
>>>
>>>         Now in the same usecase, I want to offload UDP checksum.
>>>         I am aware that the checksum field in UDP header has to be
>>>         filled with the
>>>         pseudo header checksum, I did that, duly added the
>>>         PKT_TX_UDP_CKSUM flag in
>>>         ol_flags, did a tx_burst and the packet does not reach the
>>>         destination.
>>>
>>>         I realized that I have to fill the following fields as well (my
>>>         packet does
>>>         not have vlan tag)
>>>         mbuf->pkt.vlan_macip.f.l2_len
>>>         mbuf->pkt.vlan_macip.f.l3_len
>>>
>>>         so I filled the l2_len as 14 and l3_len as 20 (IP header with no
>>>         options)
>>>         Yet the packet did not reach the destination.
>>>
>>>         So my question is -- am I filling the l2_len and l3_len properly ?
>>>         Is there anything else to be done before I can get this UDP checksum
>>>         offload to work properly for me.
>>
>>
>>
>>     As far as I remember, this should be working on 1.6r1.
>>     When you say "did not reach the destination", do you mean that the
>>     packet is not transmitted at all? Or is it transmitted with a wrong
>>     checksum?
>
>
> The packet is not transmitted to destination. I cannot see it in tcpdump
> at wireshark.
> If I don't do the offload and fill UDP checksum as zero, then
> destination shows the packet in tcpdump
> If I don't do the offload and just fill the pseudo header checksum in
> UDP header (clearly the wrong checksum), then the destination shows the
> packet in tcpdump and wireshark decodes it to complain of wrong UDP
> checksum as expected.

This is strange. I don't see anything obvious in what you are
describing. It looks like the packet is dropped in the driver
or in the hardware. You can check the device statistics.

Another thing you can do is to retry on the latest stable dpdk which
is known to work (see csumonly.c in test-pmd).

> Let me add further, I am _just_ doing the UDP checksum offload and not
> the IP hdr checksum offload. I calculate and set IP header checksum by
> my own code. I hope that this is acceptable and does not interfere with
> UDP checksum offload

This should not be a problem.

Regards,
Olivier