.. include:: /../getting_started/vars.rst

.. _section-General-740:

General Compatibility 7.4.0
***************************

This section describes compatibility issues that affect an upgrade to 7.4.0.
It describes general, wire, and other compatibility issues between *Connext*
7.4.0 and previous releases.

Unless noted otherwise, the issues in this section do not
apply to *RTI* |CONNEXT| *Micro*.

.. attention::

    For important information on new and removed platforms and features in 
    release 7.4.0, see :link_whats_new_740:`What's New in 7.4.0 <>`. See also 
    individual products' :link_rti_community_doc_s:`Release Notes <740>`.

See :ref:`section-Product-Index-740` for specific issues.

See `RTI Connext Performance Benchmarks <https://community.rti.com/static/documentation/performance/benchmarks/index.html>`__
for performance details in this release.

Wire Compatibility
==================

RTPS Versions
-------------
See :link_release_notes_comp_740:`Compatibility, in the RTI Connext DDS Core Libraries Release Notes <>` 
for information about the RTPS versions supported for each |CONNEXT| release.


Best-effort |DR| from before 6.1.0 cannot receive MultiChannel RTPS fragmented data from synchronous |DW| in 7.4.0
------------------------------------------------------------------------------------------------------------------
You cannot have a best-effort |DR| from a release below 6.1.0 receiving 
MultiChannel RTPS fragmented data from a 7.4.0 |DW| publishing synchronously. 
This backward compatibility had to be broken because of the fix for RTI Issue 
ID `CORE-14668 <https://community.rti.com/static/documentation/connext-dds/7.4.0/doc/manuals/connext_dds_professional/release_notes/whats_fixed/740/fixes_usability.html>`__.

.. CORE-14668


Transport Compatibility
=======================

Real-Time WAN Transport updated message_size_max value may cause messages to be dropped from previous versions
--------------------------------------------------------------------------------------------------------------
The ``message_size_max`` for the *RTI Real-Time WAN Transport* has been
decreased from 65535 to 1400 to avoid issues related to IP fragmentation that
are common in WAN environments. (See :link_rwt_release_notes_wn_740:`What's New in 7.4.0 <>`
in the *RTI Real-Time WAN Transport Release Notes*.) Because of this change,
RTPS messages with serialized sizes larger than 1400 bytes may be dropped by
the current release and the following error message will appear when two
|DPs| discover each other: 

::

   ERROR [PARSE MESSAGE|0x01015772,0xEFEA70AC,0xB9D66813:0x000100C7{Entity=DR,MessageKind=DATA}|
   RECEIVE FROM 0x0101C23E,0x7B2BF1E0,0xA1CDCA5F:0x000100C2|:0x000001C1{Domain=0}|ASSERT REMOTE DP|
   LC:Discovery] PRESParticipant_checkTransportInfoMatching:INVALID CONFIGURATION | 
   the message_size_max, 65507, for udpv4_wan, of the remote participant (guid: 0x0101C23E,
   0x7B2BF1E0,0xA1CDCA5F, name: '<UNKNOWN>'), does not match the message_size_max, 1400, of the local 
   participant (guid: 0x01015772,0xEFEA70AC,0xB9D66813, name: '<UNKNOWN>'). Will lose RTPS packets 
   with size > 1400.

The messages are dropped because the ``DDS_ReceiverPoolQosPolicy::buffer_size``
default value is set to an AUTO value that takes the maximum value of the
``message_size_max`` across all installed transports in a |DP|. 
Because it is a maximum across all installed transports, if you have another
transport installed (such as the builtin shared memory or UDPv4 transports)
and have not changed the ``message_size_max`` for those transports, then you
will not have a problem with dropped messages, even if you see the error
message about dropped packets for the UDPv4_WAN transport.

You can avoid dropped messages by doing one of the following:

- Update the ``message_size_max`` in versions before 7.4.0 to 1400. You can use the
  ``BuiltinQosSnippetLib::Transport.UDP.WAN`` snippet. This QoS snippet enables the
  *Real-Time WAN Transport*, sets the ``message_size_max`` to 1400, and enables
  asynchronous publishing, which was required in previous releases to support
  data fragmentation when using reliable communication (that requirement has
  been removed in this release). For example: 

  ::

    <qos_profile name=”MyProfile”>
        <base_name>
            <element>BuiltinQosSnippetLib::Transport.UDP.WAN</element>
        </base_name>
    ....
    </qos_profile>

- Change the value of the ``DDS_ReceiverPoolQosPolicy::buffer_size`` QoS to be
  as large as the ``message_size_max`` of the application running a previous
  version of |CONNEXT| (65535 if it was left to the default). For example:

  ::

    <qos_profile name=”MyProfile”>
        <domain_participant_qos>
            <receiver_pool>
                <buffer_size>65535</buffer_size>
            </receiver_pool>
    ....
        </domain_participant_qos>
    ....
    </qos_profile>

- Update the ``message_size_max`` in 7.4.0 to be the same as the previous
  version (65535 if it was previously left untouched). For example: 

  ::

    <qos_profile name=”MyProfile”>
        <domain_participant_qos>
            <transport_builtin>
                <udpv4_wan>
                <message_size_max>65535</message_size_max>
                </udpv4_wan>
            </transport_builtin>
    ....
        </domain_participant_qos>
    ....
    </qos_profile>

  This solution has the disadvantage that you are likely to run into issues
  related to IP fragmentation. For more information about IP fragmentation and
  DDS fragmentation, see
  :link_fragrwt_usersman_740:`Disabling IP Fragmentation for Real-Time WAN Transport <>`
  in the *RTI Connext Core Libraries User's Manual*. 


.. CORE-14698 or DOC-569