ISO/IEC 8825-2:2021

Overview - ISO/IEC 8825-2:2021 (Packed Encoding Rules, PER)

ISO/IEC 8825-2:2021 specifies the Packed Encoding Rules (PER) for ASN.1 (Abstract Syntax Notation One). PER defines a compact transfer syntax and the corresponding decoding rules for values defined by ASN.1 (Rec. ITU‑T X.680 | ISO/IEC 8824‑1). The standard is tailored for use at communication time where minimizing the size of serialized data is the primary concern, while supporting forward-compatible extensibility of abstract syntaxes.

Key topics and technical requirements

• Packed Encoding Rules (PER) - formal specification of how ASN.1 types are encoded and decoded to produce a compact binary transfer syntax.
• Size optimization - encodings are designed to minimize representation size (suitable for bandwidth- or storage-constrained systems).
• Extensibility support - rules preserve existing encodings when abstract syntaxes are extended (per X.680 / ISO/IEC 8824‑1 forms of extension).
• PER-visible constraints - treatment of constraints (range, size, enumerations) that affect encoding decisions.
• Type coverage - encoding instructions for core ASN.1 types: BOOLEAN, INTEGER, ENUMERATED, REAL, BIT STRING, OCTET STRING, NULL, SEQUENCE, SET, CHOICE, object identifiers, character strings, times, and structured types (SEQUENCE OF, SET OF).
• Length determinants and constrained numbers - general rules for encoding lengths, constrained/semi-constrained/unconstrained integers and normally-small numbers.
• Aligned and unaligned PER representations - examples and guidance for ALIGNED PER and UNALIGNED PER encodings.
• Conformance and interoperability - conformance clauses and object identifiers for transfer syntaxes; identical text published as ITU‑T X.691 (02/2021).
• Relationship to other ASN.1 rules - can be adapted according to Rec. ITU‑T X.695 | ISO/IEC 8825‑6.

Practical applications and who uses it

• Protocol designers and standards bodies creating compact, interoperable binary protocols.
• Telecommunications and network engineers working with ASN.1-based signaling and messaging.
• Embedded, IoT, mobile and satellite systems where bandwidth and memory are constrained.
• ASN.1 tool vendors and implementers (encoders/decoders) who need to produce ALIGNED or UNALIGNED PER.
• Security and cryptography systems that require standardized binary encodings for certificate or key representations.

Benefits include smaller message sizes, predictable interoperability, and preserved encodings when extending message formats.

Related standards

• ISO/IEC 8824-1 / Rec. ITU‑T X.680 - ASN.1 notation (type definitions)
• ISO/IEC 8825-6 / Rec. ITU‑T X.695 - PER modifications and mapping guidance
• ITU‑T X.691 (02/2021) - identical publication of the same PER specification

Keywords: ISO/IEC 8825-2:2021, Packed Encoding Rules, PER, ASN.1, transfer syntax, ALIGNED PER, UNALIGNED PER, encoding rules, compact binary encoding, ASN.1 constraints.