Networking interfaces

Eclipse Cyclone DDS can use multiple network interfaces simultaneously (the default is a single network interface). The set of enabled interfaces determines the addresses that the host advertises in the discovery information (see Discovery of DDSI participants and endpoints).

Default behaviour

To determine the default network interface, the eligible interfaces are ranked by quality, and the interface with the highest quality is selected. If there are multiple interfaces of the highest quality, it selects the first enumerated one. Eligible interfaces are those that are connected and have the correct type of address family (IPv4 or IPv6). Priority is determined (in decreasing priority) as follows:

  • Interfaces with a non-link-local address are preferred over those with a link-local one.

  • Multicast-capable (see General/Interfaces/NetworkInterface[@multicast]).

  • Non-multicast capable and not point-to-point.

  • Point-to-point.

  • Loopback.

If this selection procedure does not automatically return the desired interface, to override the selection, set: General/Interfaces adding any of the following:

  • Name of the interface (<NetworkInterface name='interface_name' />).

  • IP address of the host on the desired interface (<NetworkInterface address='' />).

  • Network portion of the IP address for the host on the desired interface (<NetworkInterface address='' />).


An exact match on the address is always preferred, and is the only option that allows selecting the desired interface when multiple addresses are tied to a single interface.

The default address family is IPv4. To change the address family to IPv6, set: General/Transport to udp6 or tcp6.


Eclipse Cyclone DDS does not mix IPv4 and IPv6 addressing. Therefore, all DDSI participants in the network must use the same addressing mode. When interoperating, this behaviour is the same. That is, it looks at either IPv4 or IPv6 addresses in the advertised address information in the SPDP and SEDP discovery protocols.

IPv6 link-local addresses are considered undesirable because they must be published and received via the discovery mechanism (see Discovery behaviour). There is no way to determine to which interface a received link-local address is related.

If IPv6 is requested and the selected interface has a non-link-local address, Eclipse Cyclone DDS operates in a global addressing mode and will only consider discovered non-link-local addresses. In this mode, you can select any set of interfaces for listening to multicasts.


This behaviour is identical to that when using IPv4, as IPv4 does not have the formal notion of address scopes that IPv6 has. If only a link-local address is available, Eclipse Cyclone DDS runs in a link-local addressing mode. In this mode, it accepts any address in a discovery packet (assuming that a link-local address is valid on the selected interface). To minimise the risk involved in this assumption, it only allows the selected interface for listening to multicasts.

Multiple network interfaces

Multiple network interfaces can be used simultaneously by listing multiple NetworkInterface elements. The default behaviour still applies, but with extended network interfaces. For example, the SPDP packets advertise multiple addresses and sends these packets out on all interfaces. If link-local addresses are used, the issue with link-local addressing gains importance.

In a configuration with a single network interface, it is obvious which one to use for sending packets to a peer. When there are multiple network interfaces, it is necessary to establish the set of interfaces through which multicasts can be sent (these are sent on a specific interface). This in turn requires determining via which subset of interfaces a peer is reachable.

Cyclone DDS checks which interfaces match the addresses advertised by a peer in its SPDP or SEDP messages, which assumes that:

  • The peer is attached to at least one of the configured networks.

  • That checking the network parts of the addresses results in a subset of the interfaces.

The network interfaces in this subset are the interfaces on which the peer is assumed to be reachable via multicast. This leaves open two classes of addresses:

  • Loopback addresses: these are ignored unless:

    • The configuration has enabled only loopback interfaces.

    • No other addresses are advertised in the discovery message.

    • A non-loopback address matches that of the machine.

  • Routable addresses that do not match an interface: these are ignored if the General/DontRoute option is set, otherwise it is assumed that the network stack knows how to route them, and any of the interfaces may be used.

When a message needs to be sent to a set of peers, Eclipse Cyclone DDS uses the set of addresses spanning the set of intended recipients with the lowest cost. That is, the number of nodes that:

  • Receive it without having a use for it.

  • Unicast vs multicast.

  • Loopback vs real network interface.

  • Configured priority.

Eclipse Cyclone DDS uses some heuristics rather than computing the optimal solution. The address selection can be influenced in two ways:

  • By using the priority attribute, which is used as an offset in the cost calculation. The default configuration gives loopback interfaces a slightly higher priority than other network types.

  • By setting the prefer_multicast attribute, which raises the assumed cost of a unicast message.

The General/RedundantNetworking setting forces the address selection code to consider all interfaces advertised by a peer.