INTRA Transport

This user-guide explains how the built-in INTRA transport works.

Overview

The built-in intra participant transport (INTRA) is a transport that bypasses RTPS and reduces the number of data-copies from three to one for data published by a data-writer to a data-reader within the same participant. When a sample is published it is copied directly to the data reader’s cache (if there is space). This transport is used for communication between data-readers and data-writers created within the same participant by default.

Please refer to Threading Model for important details regarding application constraints when using this transport.

Registering the INTRA Transport

The built-in INTRA transport is a RTI Connext DDS Micro component that is automatically registered when the DDS_DomainParticipantFactory_get_instance() method is called. By default, data published by a data-writer is sent to all data-readers within the same participant using the INTRA transport.

In order to prevent the INTRA transport from being used it is necessary to remove it as a transport and a user-data transport. The following code shows how to only use the builtin UDP transport for user-data.

    struct DDS_DomainParticipantQos dp_qos =
                                DDS_DomainParticipantQos_INITIALIZER;
    
    REDA_StringSeq_set_maximum(&dp_qos.transports.enabled_transports,1);
    REDA_StringSeq_set_length(&dp_qos.transports.enabled_transports,1);
    *REDA_StringSeq_get_reference(&dp_qos.transports.enabled_transports,0) =
                                        REDA_String_dup(NETIO_DEFAULT_UDP_NAME);

    /* Use only unicast for user-data traffic. */
    REDA_StringSeq_set_maximum(&dp_qos.user_traffic.enabled_transports,1);
    REDA_StringSeq_set_length(&dp_qos.user_traffic.enabled_transports,1);
    *REDA_StringSeq_get_reference(&dp_qos.user_traffic.enabled_transports,0) =
                                        REDA_String_dup("_udp://");

Note that the INTRA transport is never used for discovery traffic internally. It is not possible to disable matching of data-readers and data-writers within the same participant.

Reliability and Durability

The Reliability and Durability Qos policies are not supported by the INTRA transport. However, by creating all the data-readers before the data-writers durability is not required.

Threading Model

The INTRA transport does not create any threads. Instead, a data-reader receives data over the INTRA transport in the context of the data-writer’s send thread.

This model has two important limitations:

• Because a data-readers’s on_data_available() listener is called in the context of the datawriter's send thread, a data-reader may potentially process data at a different priority than intended (the data-writer's). While it is generally not recommended to process data in a data-reader's on_data_available() callback, it is particularly important to not do so when using the INTRA transport. Instead, use a DDS WaitSet or a similar construct to wake up a separate thread to process data.

• Because a data-readers’s on_data_available() listener is called in the context of the data-writer's send thread, any method called in the on_data_available() callback is done in the context of the data-writer's stack. Thus, it is not recommended call any RTI Connext DDS Micro APIs in this listener.