Datapath Processing Architecture for In-Band Congestion Signaling (IBCS)

In-band congestion signaling protocols, such as Congestion Signaling (CSIG) and High Precision Congestion Control (HPCC++), require intermediate Network Elements (NEs) to actively parse scalar congestion metrics from packet headers, evaluate them against local link states, and conditionally rewrite these fields before transmission. To ensure end-to-end algorithmic consistency and avoid unintended interactions with routing topologies (e.g., packet reordering), the datapath of these NEs must adhere to a standardized logical processing model. This document defines the normative datapath processing architecture for Network Elements participating in In-Band Congestion Signaling (IBCS). By establishing abstract topological roles (Edge vs. Transit NEs) and standardizing the "Compare-and-Replace" operational paradigm, this specification abstracts the signal update logic from hardware-specific pipelines. It guarantees strict orthogonality between congestion signaling and Equal-Cost Multi-Path (ECMP) routing invariants, supporting diverse congestion metrics across multi-vendor deployments.