DDSSEC12 — Provide efficient way to skip encrypted implementation-specific content

The RTPS specification allows implementations to send implementation-specific RTPS sub-messages inside an RTPS message. These submessages are distinguished by having submessageId's within a specific range.

Prior to DDS security skipping implementation-specific RTPS sub-messages was very efficient requiring only the receiver to jump their length indicated in. the header.
However with DDS security the submessages can be encrypted inside a SRTPS_PREFIX/SUFFIX envelope this requires the receiver to always decrypt them even if it will end up skipping them.
An extreme, but likely common case is when the SRTPS_PREFIX/SUFFIX contains exclusively implementation-specific RTPS sub-messages which makes the effort to decrypt the RTPS message fruitless.

A way to resolve this would be to designate a flag in the SRTPA_PREFIX submessage to indicate that the content contains only vendor-specific submessages. A receiver can use this flag to skip the whole RTPS message avoiding the effort to decrypt i n the event that the sender vendorId does not match the receiver's.

The proposal therefore is to designate the leftmost flag of the SRTPS_PREFIX submessage to indicate it contains only "vendor specific content".