DDSIRTP25 — Add an INFO_AGE submessage

The INFO_AGE submessage is semantically similar to INFO_TS, but it contains a Duration_t instead of a Time_t. It assigns a time to subsequent submessages that the service may use for functionality like lifespan and sample ordering.

Transmitting a sample age gives the receiver an inexact timestamp. There will be an unknown time span between the time the sender calculates the age and the time the receiver transforms it back to a local time. If the receiver does no adjustment based on estimated latency, the calculated time will always be too high. However, the error will in practice be bounded and in many scenarios the receiver will have a time that is within one second of the actual sample time.

Kongsberg uses a submessage like this today for deployments where no correct time source is available. This might be because the system has to be fully operational before a good time source is ready, or because some systems have requirements that mandate that the system clock must be adjustable. We have found that this is a better solution than attempting to have some form of synchronized "DDS time" on the network, because coming up with a network time that works very quickly at startup and is robust to network fragmentation is challenging.

The use of the INFO_AGE message is enabled by a vendor specific configuration option to the participant containing the publisher. It may either be sent as an alternative to INFO_TS or in combination with INFO_TS to allow the subscriber to determine which time to use an also for compatibility with systems without support for INFO_AGE.

INFO_AGE allows two features on systems with no time synchronization:

• Full support for sample lifespan QoS, as long as lifespan is an order of magnitude larger than network latency
• Partial support for by source timestamp QoS. It may be used to determine that some samples are clearly older than others, in practical use when the time difference is on the order of seconds. When estimated times are within the uncertainty margin, the subscriber needs to treat them as by destination order QoS to avoid surprising behavior when e.g. a writer publishes several samples in quick succession.

It should be noted that a system based on INFO_AGE with multiple readers and writers on the same topic will have a non-deterministic sample order on the readers.

Merge with DDSIRTP25-12

This can be merged with DDSIRTP25-12