ISO/IEC 8473-4:1995

Overview

ISO/IEC 8473-4:1995 (also published as ITU‑T Recommendation X.623) specifies how the connectionless‑mode network protocol (CLNP) - defined in ISO/IEC 8473‑1 - can operate when the underlying service is provided by a subnetwork that implements the OSI Data Link service (ISO/IEC 8886 / CCITT X.212). The standard defines the Subnetwork Dependent Convergence Function (SNDCF) behaviors and includes a PICS proforma (Protocol Implementation Conformance Statement) for conformance testing.

Key topics and requirements

• SNDCF model: Defines how SN‑UNITDATA primitives from CLNP map to subnetwork primitives depending on whether the Data Link service is connectionless or connection‑mode.
  • For connectionless Data Link service: SN‑UNITDATA → DL‑UNITDATA; DL‑UNITDATA indications map back to SN‑UNITDATA indications. No explicit SNDCF control information is exchanged.
  • For connection‑mode Data Link service: SNDCF must establish a DL‑CONNECT (if no existing data link) and then use DL‑DATA to carry SN‑Userdata; DL‑DISCONNECT is used to release links.
• SN‑Userdata handling: SN‑Userdata is treated as an ordered octet sequence, transferred transparently between subnetwork attachment points.
• Minimum SDU size: The underlying service must support at least a 512‑octet service data unit; segmentation rules apply if SDU limits are smaller.
• Quality of Service (QoS): SN‑QoS parameters are conveyed to the Data Link layer; available QoS is known before issuing DL primitives.
• PICS proforma (Annex A): Provides the template and instructions required for producing implementation conformance statements in line with ISO/IEC 9646‑1 (X.290).

Applications and who uses it

• Network protocol implementers building CLNP stacks that must interwork with OSI Data Link subnetworks.
• Device and equipment vendors (routers, gateways, link-layer devices) implementing SNDCF mappings to ensure correct delivery semantics.
• Conformance testers and test laboratories using the PICS proforma to validate protocol options and capabilities.
• Systems integrators and standards architects designing interoperable OSI-based networks or migrating legacy X.25/OSI deployments.
• Standards bodies and technical committees referencing OSI layering, SNDCF behavior, and conformance procedures.

Related standards

• ISO/IEC 8473‑1 (CLNP protocol specification)
• ISO/IEC 8886 / CCITT X.212 (OSI Data Link service)
• ISO/IEC 8648 (Network layer internal organization)
• ISO/IEC 9646‑1 / CCITT X.290 (Conformance testing / PICS guidance)
• ISO/IEC 7498‑1 / ITU‑T X.200 (OSI Basic Reference Model)

Keywords: ISO/IEC 8473-4, SNDCF, CLNP, OSI data link service, PICS proforma, connectionless-mode network service, DL-UNITDATA, DL-CONNECT.