[dpdk-dev] [PATCH] lib/librte_net: fix bug for ipv4 checksumcalculating

[guohongzhi (A)]

Ok, later I will write a patch to solve the problem of tcpdump checksum

-----Original Message-----
From: Morten Brørup [mailto:mb at smartsharesystems.com] 
Sent: Thursday,May 14,2020 20:57
To: guohongzhi (A) <guohongzhi1 at huawei.com>; dev at dpdk.org
Cc: olivier.matz at 6wind.com; konstantin.ananyev at intel.com; jiayu.hu at intel.com; ferruh.yigit at intel.com; nicolas.chautru at intel.com; cristian.dumitrescu at intel.com; Zhoujingbin (Robin, Russell Lab) <zhoujingbin at huawei.com>; chenchanghu <chenchanghu at huawei.com>; Lilijun (Jerry) <jerry.lilijun at huawei.com>; Linhaifeng <haifeng.lin at huawei.com>
Subject: RE: [dpdk-dev] [PATCH] lib/librte_net: fix bug for ipv4 checksumcalculating

> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of guohongzhi
> Sent: Thursday, May 14, 2020 3:27 AM
> 
> The function of rte_ipv4_cksum for calculating the checksum of IPv4 
> header is incorrect.
> This function will return checksum value like 0xffff.
> This value, however, is considered an illegal checksum on some 
> switches(like Trident3).
> 
> RFC 1624 specifies the IPv4 checksum as follows:
> https://tools.ietf.org/rfc/rfc1624
> Since there is guaranteed to be at least one
>    non-zero field in the IP header, and the checksum field in the
>    protocol header is the complement of the sum, the checksum field can
>    never contain ~(+0), which is -0 (0xFFFF).  It can, however, contain
>    ~(-0), which is +0 (0x0000).
> 
> ---
>  lib/librte_net/rte_ip.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/librte_net/rte_ip.h b/lib/librte_net/rte_ip.h index 
> 1ceb7b7..ece2e43 100644
> --- a/lib/librte_net/rte_ip.h
> +++ b/lib/librte_net/rte_ip.h
> @@ -267,7 +267,7 @@ rte_ipv4_cksum(const struct rte_ipv4_hdr 
> *ipv4_hdr)  {
>  	uint16_t cksum;
>  	cksum = rte_raw_cksum(ipv4_hdr, sizeof(struct rte_ipv4_hdr));
> -	return (cksum == 0xffff) ? cksum : (uint16_t)~cksum;
> +	return (uint16_t)~cksum;
>  }
> 
>  /**
> --
> 2.21.0.windows.1
> 
> 

Well spotted!

Reviewed-By: Morten Brørup <mb at smartsharesystems.com>


Would you consider writing another patch splitting rte_ipv4_udptcp_cksum() up into rte_ipv4_udp_cksum() and rte_ipv4_tcp_cksum(), so the TCP checksum will be calculated correctly?

RFC 768 for UDP specifies:

If the computed  checksum  is zero,  it is transmitted  as all ones (the equivalent  in one's complement  arithmetic).   An all zero  transmitted checksum  value means that the transmitter  generated  no checksum  (for debugging or for higher level protocols that don't care).

RFC 793 for TCP has no such special treatment for the checksum of zero, but rte_ipv4_udptcp_cksum() implements the UDP special treatment anyway.