6.3. TCP Transport

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.

Note

The server and client concepts are independent from the DDS concepts of Publisher, Subscriber, DataWriter, and DataReader. Also, these concepts are independent from the eProsima Discovery Server servers and clients (Discovery Server Settings). Any of them can act as a TCP Server or TCP Client when establishing the connection, and the DDS communication will work over this connection.

Warning

This documentation assumes the reader has basic knowledge of TCP/IP concepts, since terms like Time To Live (TTL), Cyclic Redundancy Check (CRC), Transport Layer Security (TLS), socket buffers, and port numbering are not explained in detail. However, it is possible to configure a basic TCP transport on Fast DDS without this knowledge.

6.3.1. TCPTransportDescriptor

eProsima Fast DDS implements TCP transport for both TCPv4 and TCPv6. Each of these transports is independent from the other, and has its own TransportDescriptorInterface. However, they share many of their features, and most of the TransportDescriptorInterface data members are common.

The following table describes the common data members for both TCPv4 and TCPv6.

Member	Data type	Default	Description
sendBufferSize	uint32_t	0	Size of the sending buffer of the socket (octets).
receiveBufferSize	uint32_t	0	Size of the receiving buffer of the socket (octets).
interfaceWhiteList	vector<string>	Empty vector	List of allowed interfaces See Interface Whitelist.
TTL	uint8_t	1	Time to live, in number of hops.
listening_ports	vector<uint16_t>	Empty vector	List of ports to listen as server. If a port is set to 0, an available port will be automatically assigned.
keep_alive_frequency_ms	uint32_t	5000	Frequency of RTCP keep alive requests (in ms).
keep_alive_timeout_ms	uint32_t	15000	Time since sending the last keep alive request to consider a connection as broken (in ms).
max_logical_port	uint16_t	100	Maximum number of logical ports to try during RTCP negotiation.
logical_port_range	uint16_t	20	Maximum number of logical ports per request to try during RTCP negotiation.
logical_port_increment	uint16_t	2	Increment between logical ports to try during RTCP negotiation.
enable_tcp_nodelay	bool	false	Enables the TCP_NODELAY socket option.
calculate_crc	bool	true	True to calculate and send CRC on message headers.
check_crc	bool	true	True to check the CRC of incoming message headers.
apply_security	bool	false	True to use TLS. See TLS over TCP.
tls_config	TLSConfig		Configuration for TLS. See TLS over TCP.

Note

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.

6.3.1.1. TCPv4TransportDescriptor

The following table describes the data members that are exclusive for TCPv4TransportDescriptor.

Member	Data type	Default	Description
wan_addr	octet[4]	[0, 0, 0, 0]	Configuration for WAN. See WAN or Internet Communication over TCPv4.

Note

The kind value for a TCPv4TransportDescriptor is given by the value LOCATOR_KIND_TCPv4.

6.3.1.2. TCPv6TransportDescriptor

TCPv6TransportDescriptor has no additional data members from the common ones described in TCPTransportDescriptor.

Note

The kind value for a TCPv6TransportDescriptor is given by the value LOCATOR_KIND_TCPv6.

6.3.2. Enabling TCP Transport

There are several ways of enabling TCP transport in eprosima Fast DDS. According to the facet of each scenario, one method might suit better than the others.

6.3.2.1. Configuration of Builtin Transports

The first option is to modify the builtin transports that are responsible of the creation of the DomainParticipant transports. The existing configuration that enables TCP transports is LARGE_DATA. This option instantiates a UDPv4, a TCPv4 and a SHM transport, respectively. UDP protocol will be used for multicast announcements during the participant discovery phase (see Discovery phases) while the participant liveliness and the application data delivery occurs over TCP or SHM. This configuration enables auto discovery and does not require to manually set up each participant IP and listening port. Hence, avoiding the typical Server-Client configuration.

Builtin Transports can be configured via code, XML (see RTPS element type) or using the FASTDDS_BUILTIN_TRANSPORTS environment variable (see FASTDDS_BUILTIN_TRANSPORTS).

C++XML

eprosima::fastdds::dds::DomainParticipantQos qos;
qos.setup_transports(eprosima::fastdds::rtps::BuiltinTransports::LARGE_DATA);

Note

Note that LARGE_DATA configuration of the builtin transports will also create a SHM transport along the UDP and TCP transports. Shared Memory will be used whenever it is possible. Manual configuration will be required if a TCP communication is required when SHM is feasible. (See TCP / SHM Communication with Multicast Discovery).

6.3.2.2. Server-Client Configuration

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.

If you provide listening_ports on the descriptor, the DomainParticipant will act as TCP server, listening for incoming remote connections on the given ports. The examples below show this procedure in both C++ code and XML file.

C++XML

DomainParticipantQos qos;

// Create a descriptor for the new transport.
auto tcp_transport = std::make_shared<TCPv4TransportDescriptor>();
tcp_transport->sendBufferSize = 9216;
tcp_transport->receiveBufferSize = 9216;
tcp_transport->add_listener_port(5100);
tcp_transport->set_WAN_address("80.80.99.45");

// Link the Transport Layer to the Participant.
qos.transport().user_transports.push_back(tcp_transport);

// Avoid using the default transport
qos.transport().use_builtin_transports = false;

If you provide initialPeersList to the DomainParticipant, it will act as TCP client, trying to connect to the remote servers at the given addresses and ports. The examples below show this procedure in both C++ code and XML file. See Initial peers for more information about their configuration.

C++XML

DomainParticipantQos qos;

// Disable the built-in Transport Layer.
qos.transport().use_builtin_transports = false;

// Create a descriptor for the new transport.
// Do not configure any listener port
auto tcp_transport = std::make_shared<TCPv4TransportDescriptor>();
qos.transport().user_transports.push_back(tcp_transport);

// Set initial peers.
Locator_t initial_peer_locator;
initial_peer_locator.kind = LOCATOR_KIND_TCPv4;
IPLocator::setIPv4(initial_peer_locator, "80.80.99.45");
initial_peer_locator.port = 5100;

qos.wire_protocol().builtin.initialPeersList.push_back(initial_peer_locator);

// Avoid using the default transport
qos.transport().use_builtin_transports = false;

HelloWorldExampleTCP shows how to use and configure a TCP transport.

6.3.3. WAN or Internet Communication over TCPv4

Fast DDS is able to connect through the Internet or other WAN networks when configured properly. To achieve this kind of scenarios, the involved network devices such as routers and firewalls must add the rules to allow the communication.

For example, imagine we have the scenario represented on the following figure:

• A DomainParticipant acts as a TCP server listening on port 5100 and is connected to the WAN through a router with public IP 80.80.99.45.

• Another DomainParticipant acts as a TCP client and has configured the server’s IP address and port in its Initial peers list.

On the server side, the router must be configured to forward to the TCP server all traffic incoming to port 5100. Typically, a NAT routing of port 5100 to our machine is enough. Any existing firewall should be configured as well.

In addition, to allow incoming connections through a WAN, the TCPv4TransportDescriptor must indicate its public IP address in the wan_addr data member. The following examples show how to configure the DomainParticipant both in C++ and XML.

C++XML

DomainParticipantQos qos;

// Create a descriptor for the new transport.
auto tcp_transport = std::make_shared<TCPv4TransportDescriptor>();
tcp_transport->sendBufferSize = 9216;
tcp_transport->receiveBufferSize = 9216;
tcp_transport->add_listener_port(5100);
tcp_transport->set_WAN_address("80.80.99.45");

// Link the Transport Layer to the Participant.
qos.transport().user_transports.push_back(tcp_transport);

// Avoid using the default transport
qos.transport().use_builtin_transports = false;

On the client side, the DomainParticipant must be configured with the public IP address and listening_ports of the TCP server as Initial peers.

C++XML

DomainParticipantQos qos;

// Disable the built-in Transport Layer.
qos.transport().use_builtin_transports = false;

// Create a descriptor for the new transport.
// Do not configure any listener port
auto tcp_transport = std::make_shared<TCPv4TransportDescriptor>();
qos.transport().user_transports.push_back(tcp_transport);

// Set initial peers.
Locator_t initial_peer_locator;
initial_peer_locator.kind = LOCATOR_KIND_TCPv4;
IPLocator::setIPv4(initial_peer_locator, "80.80.99.45");
initial_peer_locator.port = 5100;

qos.wire_protocol().builtin.initialPeersList.push_back(initial_peer_locator);

// Avoid using the default transport
qos.transport().use_builtin_transports = false;

6.3.4. HelloWorldExampleTCP

A TCP version of helloworld example can be found in the HelloWorldExampleTCP folder. It shows a publisher and a subscriber that communicate through TCP. The publisher is configured as TCP server while the Subscriber is acting as TCP client.