OSI encapsulation (P2)

Status: draft

Notes on the OSI network-layer encapsulation shared by CLNP / ES-IS / IS-IS, and on the NSAP address type (M7). The definitions are in crates/wpg-protocols/src/osi.rs; the sample is templates/protocols/l2/dot3-llc-osi.wpg.yaml.

How it is reached

No “next protocol” number precedes an OSI PDU: its own first octet, the NLPID (ISO/IEC TR 9577), says what it is (0x81 CLNP, 0x82 ES-IS, 0x83 IS-IS). llc and ppp therefore peek at the first 8 payload bits (Binding::peek) to pick the next layer. The dedicated definitions are registered in that peek table: CLNP (0x81) and ES-IS (0x82), see CLNP / ES-IS, and IS-IS (0x83), see IS-IS. Other NLPIDs (for example 0x00, the inactive subset of ISO 8473), and PDUs too short for their definition, decode as the generic osi PDU (the NLPID plus the rest as opaque octets).

When building, stack osi or a dedicated PDU (clnp, esis_ish, isis_l1_lan_hello, …) on top of llc; DSAP / SSAP become 0xfe automatically. The PPP protocol likewise becomes 0x0023 automatically.

The NSAP type

• An nsap value is the address itself (1..=20 octets). On the wire it is preceded by a 1-octet length indicator, as is usual in OSI; the builder writes it and the parser reads it.
• Notation: hex such as 49.0001.1921.6800.1001.00 (. and spaces are ignored, a 0x prefix is allowed).
• The GUI field editor splits the NSAP being typed into AFI / area / system ID (6 octets) / NSEL (addresses shorter than 8 octets are not split).
• A range generator steps the system ID (for example 49.0001.1921.6800.1001.00-49.0001.1921.6800.1004.00).

Limits

• The osi NLPID is not Auto.
• A PDU with NLPID 0x81 / 0x82 / 0x83 always decodes as CLNP / ES-IS / IS-IS (when long enough), so a generic osi layer carrying one of them reads back as the dedicated protocol.

Generated reference

Field reference of the protocols on this page (generated from the definitions):

llc · ppp · osi · clnp · esis_ish · isis_l1_lan_hello