Hi here, please answer to my mail address (ive not
subscribed) :
I've studied the hardware checksum computation. i've
read in include/linux/skbuff.h that when ip->summed is
set to CHECKSUM_HW, skb->csum contains the checksum on
all the packet (as seen by the device).
The fact is, the ip layer does'nt care about skb->csum
(the checksum is verified from the scratch) but
skb->csum ne is used to compute the final checksum of
tcp (completed with the pseudo header checksum).
As mentionned in the skbuff.h file, as the skb->csum
covers all the packet, i think skb->csum can't be used
for tcp computing "as is". (as we cover ip header by
this checksum).
So i imagine that we substract the value of ip header
checksum before that tcp uses skb->csum in :
from tcp_ipv4.c
if ((skb->ip_summed != CHECKSUM_UNNECESSARY &&
tcp_v4_checksum_init(skb) < 0))
goto bad_packet;
with :
static int tcp_v4_checksum_init(struct sk_buff *skb)
{
if (skb->ip_summed == CHECKSUM_HW) {
skb->ip_summed = CHECKSUM_UNNECESSARY;
if
(!tcp_v4_check(skb->h.th,skb->len,skb->nh.iph->saddr,
skb->nh.iph->daddr,skb->csum))
return 0;
[...]
skb->ip_summed = CHECKSUM_NONE;
}
That is to say, as the skb->csum field is filled with
a sum on the entire packets contents, i guess it must
be transformed when used by tcp to finally compute the
checksum (completing with the pseudo header).
I found no trace of this skb->csum transormation
before use in tcp_v4_checksum_init on the receive
path.
Thanks
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