void checksumadjust(unsigned char *chksum, unsigned char *optr,
   int olen, unsigned char *nptr, int nlen)
   /* assuming: unsigned char is 8 bits, long is 32 bits.
     - chksum points to the chksum in the packet
     - optr points to the old data in the packet
     - nptr points to the new data in the packet
   */
   {
     long x, old, new;
     x=chksum[0]*256+chksum[1];
     x=~x;
     while (olen) {
       if (olen==1) {
         old=optr[0]*256+optr[1];
         x-=old & 0xff00;
         if (x<=0) { x--; x&=0xffff; }
         break;
       }
       else {
         old=optr[0]*256+optr[1]; optr+=2;
         x-=old & 0xffff;
         if (x<=0) { x--; x&=0xffff; }
         olen-=2;
       }
     }
     while (nlen) {
       if (nlen==1) {
         new=nptr[0]*256+nptr[1];
         x+=new & 0xff00;
         if (x & 0x10000) { x++; x&=0xffff; }
         break;
       }
       else {
         new=nptr[0]*256+nptr[1]; nptr+=2;
         x+=new & 0xffff;
         if (x & 0x10000) { x++; x&=0xffff; }
         nlen-=2;
       }
     }
     x=~x;
     chksum[0]=x/256; chksum[1]=x & 0xff;
   }