Overview - ISO 14229-2:2021 (UDS Part 2)
ISO 14229-2:2021 is the session-layer part of the Unified Diagnostic Services (UDS) family for road vehicles. It specifies common session layer services and requirements that separate the UDS application (ISO 14229-1) from transport protocols and network layers. The standard defines a consistent service primitive interface between OSI layer 5 (session) and layer 4 (transport) so that diagnostics work independently of the underlying transport (e.g., DoIP, DoCAN, FlexRay, K‑Line, CXPI).
Key topics and technical requirements
- Service interface and primitives: Defines the session service primitives used by UDS - notably S_Data.request, S_Data.indication, and S_Data.confirm - and the rules for their use between application and session layers.
- Service primitive parameters (SPP): Standardizes parameters such as S_Mtype (message type), S_TAtype/S_TA (target address type/address), S_SA (source address), S_AE (address extension), S_Length, S_Data, and S_Result.
- S_PDU / T_PDU mapping: Specifies how session layer PDUs (S_PDU) are mapped onto transport PDUs (T_PDU) and vice versa, enabling consistent message transmission across different transport protocols.
- Timing and timers: Details timing parameter definitions for default and non‑default sessions, including support for enhanced response timing, minimum inter-message times, and client/server timer resource requirements.
- Communication modes: Covers behavior for physical vs. functional communication, start-of-message (SOM) indications, and error handling scenarios.
- Conformance and architecture: Includes normative T_PDU interface (Annex A) and informative vehicle diagnostic OSI architecture examples (Annex B).
Applications and who uses it
ISO 14229-2 is essential for ensuring interoperable vehicle diagnostics across multiple transport technologies. Typical users:
- Automotive OEMs and Tier‑1 suppliers defining diagnostic behavior and ECU requirements
- ECU firmware and gateway developers implementing UDS session services
- Diagnostic tool vendors (scan tools, testers) ensuring compatibility across networks
- System integrators and test labs verifying timing, mapping, and error handling
Practical applications include enabling standardized session management for on‑vehicle diagnostics, supporting secure and remote diagnostics, and simplifying integration across DoIP, CAN, FlexRay, K‑Line, and CXPI networks.
Related standards
This standard is critical for implementing robust, transport‑independent vehicle diagnostics and for achieving consistent behavior across diagnostic tools and ECUs.
Open preview (PDF)Road vehicles — Unified diagnostic services (UDS) — Part 2: Session layer services
