.. _wrtd_message:

-------------------
WRTD Event Messages
-------------------

WRTD Event messages follow the LXI Event Messaging format, as defined in the "LXI Event Messaging
Extended Function" specification, revision 1.0.

To ensure compatibility with LXI devices, WRTD Event messages are transmitted using multicast UDP on
address 224.0.23.159, port 5044 (Rule 3.3.1 of the LXI Event Messaging Extended Function).

Each Event message is transmitted as a single Ethernet frame, with a UDP header and a payload as
shown in :numref:`fig-wrtd-message`.


.. figure:: graphics/wrtd_message.png
   :name: fig-wrtd-message
   :align: center
   :alt: alternate text
   :figclass: align-center

   Contents of a WRTD Event message

The contents of a WRTD Event message are again based on LXI Event messages, with the "Domain" and
Flag bytes (octets 3 and 36 respectively) fixed to zero.

Altough there are currently no "Data Fields" defined (octets 37 and beyond), it should be
highlighted that the LXI Event message format supports an arbitrary number of data fields, in the
form of Type/Length/Value (TLV) triplets, which could be used to provide additional functionality to
WRTD in the future.

Please refer to Section 4.3 of the "LXI Event Messaging Extended Function" specification for more
details.
