IEC 62351 - Power Systems Communication Security 5:5.4: Part 5, 5.4: general principles (application layer only, generic definition mapped to protocols, bi-directional, key management, backwards tolerance, upgradeable, multiple connections)

The mechanism lives in the application layer only so it survives any data link, is defined generically and mapped onto each protocol by that protocol's standard, protects both directions, includes management of its cryptographic keys, tolerates devices without security during migration, is upgradeable in algorithms and key sizes, and supports multiple simultaneous connections.