Re: [dpdk-dev] [dpdk-users] Invalid TCP/UDP checksum for IPV4 packets

[Thomas Monjalon <thomas@monjalon.net>]

Hi,

31/05/2017 10:16, Julien Castets:
> I had a running application on DPDK 2.2.0 which offloads IP/TCP/UDP
> checksums to the NIC. A few days ago, I started the migration to 17.02
> and started to see invalid TCP/UDP checksum.
> 
> I called the new API rte_eth_tx_prepare() but checksums were still invalid.
> 
> After digging for a few hours in DPDK source code, I discovered the
> flag PKT_TX_IPV4 needs to be set in ol_flags.
> 
> Basically, the function rte_net_intel_cksum_flags_prepare of rte_net.h
> (http://dpdk.org/doc/api/rte__net_8h_source.html) does the following:
> 
> if (offload TCP checksum flag is set) {
>   if (PKT_TX_IPV4 in ol_flags) {
>      // compute ipv4 checksum
>    }
>    else {
>      // compute ipv6 checksum
>    }
> }
> 
> Because I didn't set PKT_TX_IPV4 in ol_flags, I was going into the
> "else" clause and the checksum was computed as if my packet was an
> IPV6 packet.
> 
> Don't you think it would be preferable to assert instead of
> considering a non-explicitely-declared-IPV4-packet as being an IPV6
> packet?

Yes, you're right.
I think we were avoiding wasting 1 cycle here.
Usability was the cost for best performance.
A good trade-off may be to use RTE_ASSERT which will be triggered only
in debug mode.