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RTI Connext C API Version 7.4.0
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<<spec>> This QoS policy allows the application to take advantage of transports that are capable of sending messages with different priorities. More...
Data Fields | |
| DDS_Long | value |
| <<spec>> This policy is a hint to the infrastructure as to how to set the priority of the underlying transport used to send the data. More... | |
<<spec>> This QoS policy allows the application to take advantage of transports that are capable of sending messages with different priorities.
The Transport Priority QoS policy is optional and only supported on certain OSs and transports. It allows you to specify on a per-DataWriter or DataReader basis that the data sent by that endpoint is of a different priority.
The DDS specification does not indicate how a DDS implementation should treat data of different priorities. It is often difficult or impossible for DDS implementations to treat data of higher priority differently than data of lower priority, especially when data is being sent (delivered to a physical transport) directly by the thread that called FooDataWriter_write. Also, many physical network transports themselves do not have a end-user controllable level of data packet priority.
In RTI Connext, for the IP-based transports (UDPv4, UDPv6, Real-Time WAN, and TCP), the value set in the Transport Priority QoS policy can be used to set the differentiated services field (DS field) bits of the IPv4 and IPv6 headers for datagrams sent by a DDS_DataWriter or DDS_DataReader. It is platform-dependent on how and whether setting the DS field has an effect. Some platforms may require external permissions in order to set the DS field.
The transport priority value is not provided as is to the transports, but transformed according to the following fields: transport_priority_mask (for example, see NDDS_Transport_UDPv4_Property_t::transport_priority_mask), transport_priority_mapping_low (for example, see NDDS_Transport_UDPv4_Property_t::transport_priority_mapping_low), and transport_priority_mapping_high (for example, see NDDS_Transport_UDPv4_Property_t::transport_priority_mapping_high). If you want the priority value to be exactly equal to the DS value, then the only change you need to make is to set transport_priority_mask to 0xff (and keep the transport_priority_mapping_low and transport_priority_mapping_high defaults).
It is incorrect to assume that using the Transport Priority QoS policy will have any effect at all on the end-to-end delivery of data from a DDS_DataWriter and a DDS_DataReader. All network elements, including switches and routers, must have the capability and be enabled to actually use the DS field to treat higher priority packets differently. Thus the ability to use the Transport Priority QoS policy must be designed and configured at a system level; just turning it on in an application may have no effect at all at a transport level.
For additional details on how to set the DS field in IP-based transports, see the "TRANSPORT_PRIORITY QosPolicy" section of the User's Manual.
| DDS_Long DDS_TransportPriorityQosPolicy::value |
<<spec>> This policy is a hint to the infrastructure as to how to set the priority of the underlying transport used to send the data.
You may choose any value within the range of a 32-bit signed integer; higher values indicate higher priority. However, any further interpretation of this policy is specific to a particular transport and a particular DDS implementation. For example, a particular transport is permitted to treat a range of priority values as equivalent to one another.
[default] 0