DDSSEC12 — Sectiob 8.2.1 Expand set of submessages that may be sent on TSN streams

Section 8.2.1 Message Module says:

To provide a deterministic behavior and a deterministic message size, DataWriters associated with an TsnTalker in the Deployment configuration (see subclause 7.2.3.1) may need to limit their RTPS Message exchange to RTPS Messages that include the following RTPS Submessages:
• InfoTimestamp
• Data
• DataFrag

This should be expanded to include the RTPS HeaderExtension as well as GAP.

Later in the section there is an explanation of why GAP may not be needed:

RTPS Submessages responsible for achieving reliability or in-order delivery, such as Gap, AckNack, NackFrag; as well as the rest of Interpreter Submessages, may be sent as part of “Best Effort” Streams. However, given the guarantees of the underlying TSN system, this sort of traffic may be unnecessary for TSN-enabled DataReaders and DataWriters. Also, retransmissions and other types of aperiodic traffic may fail to meet the schedule and configuration of the TSN Streams associated with the delivery of time-critical data.

However GAP is not just for reliability or in-order delivery. It is also used in best-efforts in situation where a sample is not relevant to a reader (e.g. because of writer-side content filtering), even if the sample is sent best-efforts.
Gaps are needed to distinguish lost samples from samples that are intentionally-skipped. If the gap is not present, the reader side will interpret non sequential sequence numbers as sample loss, impacting status variables in in the DataReader.