18.5. TRANSPORT LAYER Frequently Asked Questions

18.5.1. Transport API

What is the role of the "TransportDescriptorInterface"?


It acts as a builder for a given transport, allowing to configure the transport and building it, using its create_transport factory member function. For further information, refer to TransportDescriptorInterface.

How is the transport instance created?


Applications do not create the Transport instance themselves. Instead, applications use a TransportDescriptor instance to configure the desired transport, and add this configured instance to the list of user-defined transports of the DomainParticipant. The DomainParticipant will use the factory function on the TransportDescriptor to create the Transport when required. For further information, refer to TransportInterface.

Can I modify the "transport_kind_" data member?


No. transport_kind_ is a protected data member for internal use. It cannot be accessed nor modified from the public API. However, users that are implementing a custom Transport need to fill it with a unique constant value in the new implementation. For further information, refer to TransportInterface.

What is the function of a "Locator_t" in a DDS system?


A Locator_t uniquely identifies a communication channel with a remote peer for a particular transport. For example, on UDP transports, the Locator will contain the information of the IP address and port of the remote peer. The listening Locators, such as Multicast locators (listen to multicast communications), Unicast locators (listen to unicast communications), Metatraffic locators (used to receive metatraffic information, usually used by built-in endpoints to perform discovery), User locators (used by the endpoints created by the user to receive user Topic data changes), are used to receive incoming traffic on the DomainParticipant. For further information, refer to Locator.

What is the primary purpose of the "IPLocator" class?


IPLocator is an auxiliary static class that offers methods to manipulate IP-based locators. It is convenient when setting up a new UDP Transport or TCP Transport, as it simplifies setting IPv4 and IPv6 addresses, or manipulating ports. For further information, refer to Configuring IP locators with IPLocator.

18.5.2. UDP Transport

What is the primary characteristic of UDP transport in terms of communication?


UDP is a connectionless transport, where the receiving DomainParticipant must open a UDP port listening for incoming messages, and the sending DomainParticipant sends messages to this port. For further information, refer to UDP Transport.

What is the best configuration for high-frequency best-effort writers?


Setting non_blocking_send to true. It is also applicable for TCP Transport. For further information, refer to UDPTransportDescriptor.

What is required to enable a new UDP transport in a DomainParticipant?


Fast DDS enables a UDPv4 transport by default. Nevertheless, the application can enable other UDP transports if needed. To enable a new UDP transport in a DomainParticipant, first create an instance of UDPv4TransportDescriptor (for UDPv4) or UDPv6TransportDescriptor (for UDPv6), and add it to the user transport list of the DomainParticipant. For further information, refer to Enabling UDP Transport.

18.5.3. TCP Transport

What is the primary requirement for establishing a connection using TCP transport in DDS?


TCP is a connection-oriented transport, so the DomainParticipant must establish a TCP connection to the remote peer before sending data messages. Therefore, one of the communicating DomainParticipants (the one acting as server) must open a TCP port listening for incoming connections, and the other one (the one acting as client) must connect to this port. For further information, refer to TCP Transport.

What happens if there are multiple listening ports in TCP transport?


Only the first listening port will be effectively used. The rest of the ports will be ignored. For further information, refer to TCPTransportDescriptor.

What happens if "listening_ports" is left empty?


If listening_ports is left empty, the participant will not be able to receive incoming connections but will be able to connect to other participants that have configured their listening ports. For further information, refer to TCPTransportDescriptor.

How is TCP transport enabled?


There are several ways of enabling TCP transport in Fast-DDS. The first option is to modify the builtin transports that are responsible for the creation of the DomainParticipant transports. The second option is to set up a Server-Client configuration. You need to create an instance of TCPv4TransportDescriptor (for TCPv4) or TCPv6TransportDescriptor (for TCPv6), and add it to the user transport list of the DomainParticipant. For further information, refer to Enabling TCP Transport.

What configuration is required on the server side to enable communication with the client over TCP?


The router must be configured to forward traffic incoming to port 5100. For further information, refer to TCPTransportDescriptor.

Which configuration is needed to allow incoming connections through a WAN?


The TCPv4TransportDescriptor must indicate its public IP address in the wan_addr data member. On the client side, the DomainParticipant must be configured with the public IP address and listening_ports of the TCP as Initial peers. For further information, refer to WAN or Internet Communication over TCPv4.

Is there any configuration that makes the TCP transport more secure?


Yes, with TLS by including apply_security=true data member and tls_config data member on the TCPTransportDescriptor. For further information, refer to TLS over TCP.

What are the advantages and disadvantages of using TCP transport compared to UDP transport?


TCP offers reliable data transmission with built-in mechanisms for error detection, correction, and ordered delivery of packets, ensuring data integrity and sequencing. It also includes flow control to adjust the transmission rate between sender and receiver. However, this reliability comes with increased overhead, resulting in slower transmission speeds and higher latency compared to UDP. UDP, on the other hand, is faster with lower latency, as it does not ensure packet delivery, order, or perform error correction, making it ideal for real-time applications like video streaming or gaming, but less reliable for applications requiring data accuracy.

18.5.4. Shared Memory Transport

What is the primary advantage of using the Shared Memory Transport (SHM) compared to other network transports like UDP/TCP?


The shared memory (SHM) transport enables fast communications between entities running in the same processing unit/machine, relying on the shared memory mechanisms provided by the host operating system. For further information, refer to Shared Memory Transport.

How are peers running in the same host identified?


Using the DomainParticipant’s GuidPrefix_t. Two participants with identical 4 first bytes on the GuidPrefix_t are considered to be running in the same host. For further information, refer to Shared Memory Transport.

What is a Segment Buffer in the context of the Shared Memory Transport?


A buffer allocated in the shared memory Segment. It works as a container for a DDS message that is placed in the Segment. In other words, each message that the DomainParticipant writes on the Segment will be placed in a different buffer. For further information, refer to Segment Buffer.

What is a Buffer Descriptor?


It acts as a pointer to a specific Segment Buffer in a specific Segment. It contains the segmentId and the offset of the Segment Buffer from the base of the Segment. When communicating a message to other DomainParticipants, Shared Memory Transport only distributes the Buffer Descriptor, avoiding the copy of the message from a DomainParticipant to another. With this descriptor, the receiving DomainParticipant can access the message written in the buffer, as is uniquely identifies the Segment (through the segmentId) and the Segment Buffer (through its offset). For further information, refer to Buffer Descriptor.

What is the primary purpose of creating a listener for a DomainParticipant's receiving port in the context of Shared Memory Transport?


DomainParticipants create a listener to their receiving port, so that they can be notified when a new Buffer Descriptor is pushed to the port. For further information, refer to Port.

What is the purpose of performing a health check when a DomainParticipant opens a Port?


Every time a DomainParticipant opens a Port (for reading or writing), a health check is performed to assess its correctness. The reason is that if one of the processes involved crashes while using a Port, that port can be left inoperative. If the attached listeners do not respond in a given timeout, the Port is considered damaged, and it is destroyed and created again. For further information, refer to Port Health Check.

How is SHM transport enabled?


Fast DDS enables a SHM transport by default. Nevertheless, the application can enable other SHM transports if needed. To enable a new SHM transport in a DomainParticipant, first create an instance of SharedMemTransportDescriptor, and add it to the user transport list of the DomainParticipant. For further information, refer to Enabling Shared Memory Transport.

What happens to discovery traffic when multiple transports are enabled in a DomainParticipant?


The discovery traffic is always performed using the UDP/TCP transport, even if the SHM transport is enabled in both participants running in the same machine. For further information, refer to Enabling Shared Memory Transport.

18.5.5. Data Sharing Delivery

What is the difference between Data Sharing Delivery and Shared Memory Transport?


Although Data-sharing delivery uses shared memory, it differs from Shared Memory Transport in that Shared Memory is a full-compliant transport. That means that with Shared Memory Transport the data being transmitted must be copied from the DataWriter history to the transport and from the transport to the DataReader. With Data-sharing these copies can be avoided. For further information, refer to Data-sharing delivery.

Does the use of Data-sharing avoid data copies between the application and the DataReader and DataWriter?


No, it does not prevent data copies. Data-sharing helps avoid copies between the DataWriter and DataReader by using shared memory for communication, but data still needs to be copied between the application and the DataWriter or DataReader. To avoid these copies, a different mechanism, such as Zero-Copy communication, is required. For further information, refer to Data-sharing delivery.

How are data copies prevented?


Data copies can be prevented in some cases using Zero-Copy communication. For further information, refer to Data-sharing delivery.

What is required for two entities to use data-sharing delivery between them, according to their DataSharingQosPolicy configuration?


Two entities will be able to use data-sharing delivery between them only if both have at least a common domain. For further information, refer to Data-sharing delivery.

What is the default value for the maximum number of Data-sharing domain identifiers, and what does it affect?


By default, there is no limit to the number of identifiers. The default value can be changed using the max_domains() function. For further information, refer to Data-sharing delivery.

What is the consequence of the DataWriter reusing a sample from the pool to publish new data?


The DataReader loses access to the old data sample. For further information, refer to DataReader and DataWriter history coupling.

Can the communications between entities be sped up?


Yes, but only within the same process using intra-process delivery. For further information, refer to Intra-process delivery.

How are peers running in the same process identified?


Fast DDS utilizes the DomainParticipant’s GuidPrefix_t to identify peers running in the same process. Two participants with identical 8 first bytes on the GuidPrefix_t are considered to be running in the same process. For further information, refer to GUID Prefix considerations for intra-process delivery.