class apext::tsync::FVM

The class apext::tsync::FVM serves as the module's interface for providing freshness values for secured time synchronization messages [SWS_TS_14199]. Prior to operational use, the instance must be initialized, after which freshness values can be obtained for sending as well as for verifying received messages [SWS_TS_14200]. Upon reception, truncated freshness values are passed together with an attempt counter to reconstruct the full value for authenticator verification. All interface methods are specified as exception-safe and thread-safe.

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Context

The class apext::tsync::FVM represents an interface for freshness value management within module tsync of the AUTOSAR Adaptive Platform. Its primary responsibility is to provide current freshness values (Freshness Values) for the time synchronization service ara::tsync so that messages can be cryptographically secured against replay and spoofing attacks [SWS_TS_14198], [SWS_TS_14199].

The TimeSync functionality acts as the central interaction partner, calling the FVM to retrieve freshness value containers (apext::tsync::FVContainer) for outgoing as well as received secured messages [SWS_TS_14198], [SWS_TS_14199]. The class declaration is included via header file #include "apext/tsync/fvm.h". All specified methods are declared thread-safe and exception-safe (noexcept), allowing calls to take place concurrently from different execution contexts of the time synchronization [SWS_TS_14198], [SWS_TS_14199], [SWS_TS_14200].

Lifecycle

The functional lifecycle of an FVM instance is divided into an explicit initialization phase and the subsequent operational phase:

• Initialization: Prior to using query functions, the method Initialize() must be called [SWS_TS_14200]. Upon successful completion, it returns an empty ara::core::Result<void>. If initialization fails, it returns the error code TsyncFvmErrc::kFVInitializeFailed with rollback semantics [SWS_TS_14200].
• Operational state: Successful initialization is likely the necessary prerequisite for subsequent queries for freshness values to yield valid results. Question Q-df4ee3e749313b94 (open) Once initialized, the instance remains in the operational state and answers queries for configured freshness value IDs.
• Concurrency: Since all methods are marked as thread-safe, access to freshness values for sending and receiving can occur concurrently without external locking mechanisms [SWS_TS_14198], [SWS_TS_14199].

Usage Path

The typical calling sequence by the TimeSync component comprises initialization as well as the continuous determination of freshness values for the transmit and receive paths:

1. Service initialization: TimeSync executes the method Initialize() at startup and checks the returned ara::core::Result for errors [SWS_TS_14200].
2. Sending secured messages: Prior to sending a message, TimeSync calls GetTxFreshness() with the respective FreshnessValueID [SWS_TS_14199]. The obtained FVContainer object provides the freshness value used to calculate the authenticator [SWS_TS_14199].
3. Receiving secured messages: Upon message reception, TimeSync calls GetRxFreshness() and passes the FreshnessValueID, the truncated value TruncatedFreshnessValue contained in the message, and the number of previous authentication attempts AuthVerifyAttempts [SWS_TS_14198]. The FVM determines the complete freshness value from this to verify the authenticator [SWS_TS_14198].

Sequence diagram: Typical sequence of initialization and retrieval of transmit and receive freshness values by TimeSync. Sources: [SWS_TS_14198], [SWS_TS_14199], [SWS_TS_14200]

Error Scenarios

All interface methods signal errors via ara::core::Result and are specified with noexcept [SWS_TS_14198], [SWS_TS_14199], [SWS_TS_14200]. The following error states and preconditions are relevant:

• Failed initialization (TsyncFvmErrc::kFVInitializeFailed): Occurs when the FVM cannot be initialized [SWS_TS_14200]. Thanks to rollback semantics, the system remains in the uninitialized, defined initial state.
• Freshness value not available (TsyncFvmErrc::kFVNotAvailable): Returned by GetTxFreshness() or GetRxFreshness() if no freshness value is available for the requested ID [SWS_TS_14198], [SWS_TS_14199]. In the event of an error, the operations do not alter their state (rollback semantics).
• Repeated verification attempts upon reception: The parameter AuthVerifyAttempts starts at 0 and is incremented with each failed authentication attempt [SWS_TS_14198]. The incrementing likely serves the FVM to adjust an internal acceptance window step by step in order to tolerate slightly time-shifted freshness values. Question Q-8f151c47d2607f90 (open)