Overview
ISO/IEC 8825-6:2021 (Identical to ITU‑T X.695) defines the registration, specification and application of PER encoding instructions (PER EIs) for ASN.1 Packed Encoding Rules. The standard:
- Specifies the information and format required to describe a PER encoding instruction.
- Defines the approval process and operation of the Registration Authority for PER EIs.
- Describes mechanisms to associate PER EIs with ASN.1 types using type prefixes and an encoding control section.
Note: PER encoding instructions affect the UNALIGNED variant of PER (used to match legacy bit‑patterns); they have no effect on the ALIGNED variant.
Key topics and requirements
- Specification content - A PER EI specification must include: a title, unique identifying keyword, EI syntax (in square brackets), descriptive purpose, full applicability to ASN.1 types, a complete EIDetail production, and a precise statement of effects (e.g., amendment to PER rules, ECN specification, or other implementable description). It must also state disjointness with previously approved EIs.
- Registration Authority operation (clauses 6–9) - Procedures for receiving, recording and publishing proposals. The Registration Authority publishes an Implementers’ Guide (maintained by ITU‑T).
- Approval lifecycle - PER EI proposals move through status categories (REJECTED, POSSIBLE, NEEDED, READY, etc.) and may be progressed to APPROVED or REJECTED via Study Group / Sub‑Committee resolution.
- Application mechanisms (clauses 10–13) - Two ways to attach EIs to types:
- Type prefixes (prefixed encoding instructions) applied directly to ASN.1 type definitions.
- Encoding control section (targeted encoding instructions) that use target lists to identify types or built‑in names.
- Multiple and conflicting assignments - Rules govern multiple assignments, order of application, and negating instructions.
- Documentation and syntax - Uses ASN.1/ECN notation defined by ISO/IEC 8824-1 and cross‑references ISO/IEC 8825-2 for PER semantics.
Applications and users
ISO/IEC 8825-6:2021 is relevant for:
- Protocol designers and implementers who need ASN.1 encodings that precisely match legacy bit‑level formats.
- ASN.1 tool vendors (compilers, encoders/decoders) implementing PER and supporting PER EIs.
- Standards bodies and registries that manage interoperable encoding instructions.
- System architects working on telecommunications, embedded systems, and constrained protocols where bit-efficient canonical encodings are critical.
Practical uses include porting legacy protocols to ASN.1, producing interoperable UNALIGNED‑PER encoders/decoders, and formally registering encoding instruction extensions.
Related standards
- ISO/IEC 8824-1 / ITU‑T X.680 - ASN.1 basic notation (required for EI syntax).
- ISO/IEC 8825-2 / ITU‑T X.691 - Specification of Packed Encoding Rules (PER) whose clauses are amended by PER EIs.
- ISO/IEC 8825 series - Other ASN.1 encoding rules and guidance.
Keywords: ASN.1, PER, Packed Encoding Rules, PER encoding instructions, PER EI, encoding control section, type prefixes, Registration Authority, ISO/IEC 8825-6:2021, ITU‑T X.695.