Default UDP socket for user-traffic of a Subscriber [14236]

[JerrySciangula]

Hi all,

imagine a scenario with a 2 DomainParticipants: 1 Publisher with multiple DataWriters publishing data over different Topics and 1 Subscriber, including multiple DataReaders, each subscribed to the DataWriters in a 1-to-1 fashion.

The best-effort sending of DataWriters data is managed by the Asynchronous thread using the FlowController policy HIGH_PRIORITY. So, each DataWriter is associated to a certain priority and the output flow is ordered according to this priority.

Now, for the Subscriber, if you disable the SHM transport, FastDDS by default creates 3 threads listening on UDP sockets (2 for meta-traffic data and 1 for user-traffic).

Regarding data user-traffic:

1. Is all the traffic (DataWriters' data) handled in FIFO order by the single UDP socket?
2. Is there a way to guarantee that DataWriters' data is received in the same order it has been sent?
3. Can I create multiple user-traffic UDP threads to differentiate and manage the prioritized user-traffic?

Thank you in advance

[JLBuenoLopez]

Hi @JerrySciangula,

1. Is all the traffic (DataWriters' data) handled in FIFO order by the single UDP socket?

Yes, the reception UDP socket is handled in FIFO order.

2. Is there a way to guarantee that DataWriters' data is received in the same order it has been sent?

This is only guaranteed in a per Topic basis. Fast DDS ensures that the samples received in a specific Topic are received in the same order as they have been sent. However, in a multi-topic scenario, this is not guaranteed.

DDS specification includes within the PresentationPolicyQos the ordered_access option which is not yet implemented in Fast DDS.

3. Can I create multiple user-traffic UDP threads to differentiate and manage the prioritized user-traffic?

If you set a custom listening locator to each DataReader, Fast DDS internally will create a single reception thread for each of them. This can be done using the RTPSEndpointQos defining the locator within the unicast_locator_list.

[JerrySciangula]

Thank you very much for the clear and complete reply!