code: function Checksum(type, val) {
      this.type = type;
      this.val = val || 0;
      switch (this.type) {
          case 0x00:
              this._compute = this._computeNone;
              break;
          case 0x01:
              this._compute = this._computeCrc32;
              break;
          case 0x02:
              this._compute = this._computeFarm32;
              break;
          case 0x03:
              this._compute = this._computeCrc32C;
              break;
          default:
              assert(false, 'invalid checksum type ' + this.type);
      }
  }
docs: csumtype:1 (csum:4){0,1}