Overview
ISO 7478:1987, "Information processing systems - Data communication - Multilink procedures," defines multilink procedures (MLP) at the data link layer for aggregating multiple parallel physical or logical links (Single Link Procedures, SLPs) into a single variable‑bandwidth logical data link. The standard describes frame formats, control bits, sequence numbering and timers needed to provide resequencing, load sharing, graceful degradation and retransmission management across a multilink group. It sits above SLPs and below the network layer in the OSI layering model.
Key topics and technical requirements
- Multilink functional organization: MLP acts as an upper sublayer of the data link layer, interfacing between the network layer and multiple SLPs.
- Multilink frame format: A two‑octet Multilink Control (MLC) field precedes the data unit. MLC contains a 12‑bit multilink send sequence number MN(S) (0–4095), split into MNH and MNL, plus four control bits: V (void sequencing), S (sequence check option), R (MLP reset request) and C (MLP reset confirmation).
- Sequencing and resequencing: Receive and send state variables MV(R) and MV(S) track next in‑sequence frames; receive windows and a receive window guard (MX) are used to detect lost or duplicate multilink frames.
- Flow control and windowing: A multilink window size (MW) controls outstanding unacknowledged frames; MW is a system parameter (bounded by the 12‑bit sequence space).
- Timers and error recovery: Timers such as the lost multilink frame timer (MT1), optional group busy timer (MT2) and MLP reset confirmation timer (MT3) govern loss detection, blocked conditions and reset confirmation.
- Retransmission and link failure handling: SLPs notify the MLP of acknowledgments and link failures; unacknowledged frames are reassigned or retransmitted, and links can be added/removed without interrupting service.
- Compatibility note: The standard does not mandate how an SLP confirms successful transmission of multilink frames to the MLP.
Applications and who uses it
ISO 7478 is intended for implementers and designers of data communication systems that need:
- Bandwidth aggregation / load sharing across multiple parallel links
- Resequencing and duplicate detection for packetized higher‑layer data
- Robust operation under link failure and graceful degradation of service
Typical users:
- Network equipment vendors and protocol stack implementers
- Telecom/network engineers designing link aggregation at the data‑link layer
- Integrators working with legacy X.25 or multi‑link environments (ISO 7776 is referenced for related X.25 DTE/DCE link procedures)
Related standards
- ISO Open Systems Interconnection (OSI) conceptual model (layers referenced)
- ISO 7776 (referenced in ISO 7478 for X.25 DTE/DCE interface guidance)
Keywords: ISO 7478, multilink procedures, MLP, SLP, multilink frame, data link layer, sequence numbers, bandwidth aggregation, load sharing, resequencing, flow control.
