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Chapter 6 In RTI Data Distribution Service, for the UDPv4 built-in transport, the value set in the TRANSPORT_PRIORITY QosPolicy is used in a setsockopt call to set the TOS (type of service) bits of the IPv4 header for datagrams sent by a DataWriter. It is platform dependent on how and whether or not the setsockopt has an effect. On some platforms such as Windows and Linux, external permissions must be given to the user application in order to set the TOS bits. It is incorrect to assume that using the TRANSPORT_PRIORITY QosPolicy will have any effect at all on the end-to-end delivery of data from a DataWriter to a DataReader. All network elements such as switches and routers must have the capability and be enabled to actually use the TOS bits to treat higher priority packets differently. Thus the ability to use the TRANSPORT_PRIORITY QosPolicy must be designed and configured at a system level; just turning it on in an application may have no effect at all. It includes the member in Table 6.56. For the default and valid range, please refer to the online documentation. Table 6.56 DDS_TransportPriorityQosPolicy Type DDS_Long Field Name value Description Hint as to how to set the priority. RTI Data Distribution Service will propagate the value set on a per-DataWriter basis to the transport when the DataWriter publishes data. It is up to the implementation of the transport to do something with the value, if anything. Although you can set the TRANSPORT_PRIORITY QosPolicy on Topics, its value can only be used to initialize the TRANSPORT_PRIORITY QosPolicies of a DataWriter. It does not directly affect the operation of RTI Data Distribution Service, see Section 5.1.3. 6.5.20.1 Example Should RTI Data Distribution Service be configured with a transport that can use and will honor the concept of a prioritized message, then you would be able to create a DataWriter of a Topic whose data samples, when published, will be sent at a higher priority than other DataWriters that use the same transport. 6.5.20.2 Properties This QosPolicy may be modified after the entity is created. It does not apply to DataReaders, so there is no requirement that the publishing and subscribing sides use compatible values. 6-156
