OSIlag

OSIlag is a hypothetical metric used in computer networking to describe end-to-end latency as a data unit moves through the seven layers of the Open Systems Interconnection (OSI) model. The concept is used to attribute portions of total delay to individual layers and the transitions between them, separating processing time from transmission and propagation delays where possible.

Origin and scope: The term does not refer to a formal standard. It appears in theoretical analyses and educational materials to illustrate how layer boundaries can influence performance and to aid in debugging inter-layer bottlenecks.

Definition and decomposition: In a typical framing, OSIlag represents the sum of latencies incurred at each layer (such as protocol processing, buffering, and segmentation/reassembly) plus the overhead of passing data between adjacent layers and any inter-layer coordination. Propagation and link-transfer delays may be included as layer-crossing costs or treated as separate components depending on the model.

Measurement and challenges: Measuring OSIlag requires instrumentation across multiple layers and careful attribution, since many delays overlap and some layers are not strictly isolated in practice. Encrypted payloads, middleware, and hardware acceleration can obscure layer boundaries, complicating attribution.

Applications and limitations: OSIlag is a useful thought experiment for understanding where latency originates and for guiding optimization priorities, but it is not standardized and results depend on how layers are defined and implemented in a given system.

Related concepts include the OSI model and general network latency.