Add possibility to mark packets(packet marking) for QoS for usage in clustered setups (especially for Galera)

In some scenarios (large and complex network setups) being able to prioritize traffic based on the service type is critical to assure that right services get the right priority(QoS).
Galera nodes for example could properly communicate over a busy network, Replication could also benefit from this feature.
Not always is possible to use IP and PORT for that especially in natted scenarios.
Firewall (mangle) rules on the host with the service can mark packets but apparently has an important impact on performances.
The ideal solution should be that the software providing the service would be able to do Packet Marking.
An evaluation of the above statements from a network/protocol expert is required anyhow.