[go-nuts] internet checksum calculation

Hi All, i'm try to calculate checksum of ip packet and stuck =(.
My code looks like

func checksum(b []byte) (sum uint16) {
         if len(b)%2 != 0 {
                 b = append(b, byte(0))
         }

         for _, v := range b {
                 sum += uint16(v)
         }

         sum = (sum & 0xffff) + (sum >> 16)
         sum += (sum >> 16)
         return (^sum & 0xffff)
}

That used like this:
func main() {
         hdr := new(bytes.Buffer)
         buf := new(bytes.Buffer)
         var verIhl uint8 = (i.Version << 4) + i.IHL
         binary.Write(hdr, binary.BigEndian, verIhl)
         var dscpEcn uint8 = (i.DSCP << 2) + i.ECN
         binary.Write(hdr, binary.BigEndian, dscpEcn)
         binary.Write(hdr, binary.BigEndian, i.Length)
         binary.Write(hdr, binary.BigEndian, i.ID)
         var flagsFrag uint16 = (i.Flags << 13) + i.FragmentOffset
         binary.Write(hdr, binary.BigEndian, flagsFrag)
         binary.Write(hdr, binary.BigEndian, i.TTL)
         binary.Write(hdr, binary.BigEndian, i.Protocol)
         binary.Write(hdr, binary.BigEndian, i.Checksum)
         binary.Write(hdr, binary.BigEndian, i.NWSrc)
         binary.Write(hdr, binary.BigEndian, i.NWDst)
         binary.Write(hdr, binary.BigEndian, i.Options)
         if i.Checksum == 0 {
                 i.Checksum = checksum(hdr.Bytes())
         }
         io.CopyN(buf, hdr, 10)
         binary.Write(buf, binary.BigEndian, i.Checksum)
         binary.Write(buf, binary.BigEndian, i.NWSrc)
         binary.Write(buf, binary.BigEndian, i.NWDst)
         binary.Write(buf, binary.BigEndian, i.Options)
}

but wireshark says that my checksum not right.
Can somebody helps me?