On Mon, Dec 3, 2018 at 11:30 PM Cong Wang <xiyou.wangc...@gmail.com> wrote: > > On Mon, Dec 3, 2018 at 11:08 PM Eric Dumazet <eduma...@google.com> wrote: > > > > The hardware has probably validated the L3 & L4 checksum just fine. > > > > Note that if ip_summed is CHECKSUM_UNNECESSARY, the padding bytes (if any) > > have no impact on the csum that has been verified by the NIC. > > > Why? Why does the hardware validates L3/L4 checksum when it > supplies a full-packet checksum? What's its point here?
The point is that the driver author can decide what is best. For native IP+TCP or IP+UDP, the NIC has the ability to fully understand the packet and fully validate the checksum. > > If it really validates L3/L4 checksum, then a full-packet checksum > is not needed. Yes, this is exactly what CHECKSUM_UNNECESSARY means. linux stack does not have to perform the check another time. For example, no call to csum_partial() is needed, even for IPv6+TCP or IPv6+UDP
