code: function writeTlsa(buff, val) {
    assertUndefined(val.usage, 'TLSA record requires "usage"');
    assertUndefined(val.selector, 'TLSA record requires "selector"');
    assertUndefined(val.matchingtype, 'TLSA record requires "matchingtype"');
    assertUndefined(val.buff, 'TLSA record requires "buff"');
    buff.writeUInt8(val.usage);
    buff.writeUInt8(val.selector);
    buff.writeUInt8(val.matchingtype);
    buff.copy(val.buff);
    return WRITE_RESOURCE_DONE;
  }
docs: https://tools.ietf.org/html/rfc6698