I found the documentation on setting the QoS as shown below, I can;t seem to figure out what my xml file should look like to set 'max_app_ack_response_length'
These operations can only be used when the DataReader’sRELIABILITY QosPolicy has an acknowledgment_kind set to DDS_APPLICATION_EXPLICIT_ACKNOWLEDGMENT_MODE. You must also set max_app_ack_response_length (in the DATA_READER_RESOURCE_LIMITS QosPolicy (DDS Extension)) to a value greater than zero.
The max_app_ack_response_length is part of the data reader reader_resource_limits. You can set it like this:
<datareader_qos>
<reader_resource_limits>
<max_app_ack_response_length>10</max_app_ack_response_length>
</reader_resource_limits>
</datareader_qos>
(Note I used RTI's System Designer to generate this xml code).
You can also look at the file $NDDSHOME/resource/xml/BuiltinProfiles.documentationONLY.xml to see how RTI sets that field in it's built-in profiles.