Since network device can auto calculate and verify the checksum of a
packet, for example: some e1000 interface. Different device will set
different value of skb->ip_summed.
a) If device do nothing to checksum, skb->ip_summed would be set to
CHECKSUM_NONE.
b) If device can only calculate a checksum, and the checksum is
correct, skb->ip_summed would be set to CHECKSUM_HW.
c) If device can verify the checksum, and the checksum is correct,
skb->ip_summed would be set to CHECKSUM_UNNECESSARY.
So if I want to trim a skb, I think I must do a checksum even if the
skb->ip_summed is CHECKSUM_UNNECESSARY.
Following is the comment about CHECKSUM_UNNECESSARY in
include/linux/skbuff.h:
* UNNECESSARY: device parsed packet and wouldbe verified checksum.
* skb->csum is undefined.
* It is bad option, but, unfortunately, many of vendors do this.
* Apparently with secret goal to sell you new device, when you
* will add new protocol to your host. F.e. IPv6. 8)
Signed-off-by: Wei Yongjun <[EMAIL PROTECTED]>
--- a/include/linux/skbuff.h 2006-07-17 10:14:23.175070472 -0400
+++ b/include/linux/skbuff.h 2006-07-17 10:18:31.762279472 -0400
@@ -1208,8 +1208,8 @@ static inline int pskb_trim_rcsum(struct
{
if (likely(len >= skb->len))
return 0;
- if (skb->ip_summed == CHECKSUM_HW)
- skb->ip_summed = CHECKSUM_NONE;
+
+ skb->ip_summed = CHECKSUM_NONE;
return __pskb_trim(skb, len);
}
-
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