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+# ADR-0002: Routing Distance, Ordering & Bypass Rules
+
+## Status
+Accepted
+
+## Date
+2026-02-27
+
+## Context
+The KernBench Graph Latency Simulator must compare kernel execution time
+across different architectures and topologies by computing end-to-end
+latency from graph traversal.
+
+To support meaningful comparison:
+- routing must be deterministic
+- latency must reflect actual interconnect structure
+- local vs remote traffic must be distinguishable
+- “bypass” optimizations must not undermine debuggability or correctness
+
+The simulator also aims to avoid software-managed metadata and hidden
+shortcuts that obscure control paths.
+
+## Decision
+
+### D1. Distance is accumulated latency, not hop count
+- Routing “distance” is defined as the **sum of per-node and per-link latency**.
+- Hop count alone must not be used for ordering or path selection.
+- Size-aware serialization latency (bytes / BW) contributes to distance.
+
+### D2. Routing order is derived from graph traversal
+- The chosen route is the path with minimum accumulated latency
+  given the constructed graph and routing policy.
+- Deterministic ordering must be guaranteed for identical inputs
+  (topology + policy + request).
+
+### D3. Bypass is explicit and graph-represented
+- Any bypass (e.g., local cube HBM access via XBAR instead of NOC) must be:
+  - explicitly represented as a graph path, and
+  - subject to latency accumulation like any other path.
+- Example: PE_DMA has dual egress — one to XBAR (HBM path) and one to NOC (non-HBM path).
+  Both are explicit graph edges; neither is a “bypass” — they are distinct data paths
+  serving different memory domains.
+- Implicit or “magic” bypass paths are disallowed.
+
+### D4. No zero-latency end-to-end paths
+
+- Every routed request must incur **end-to-end** latency > 0.
+- Individual fabric segments (e.g., NOC hops) MAY have distance_mm = 0
+  when the fabric is distributed and distance is not meaningful at that granularity.
+  This is allowed because other components on the same path (e.g., PE_DMA, SRAM,
+  UCIe endpoints) contribute non-zero latency, ensuring the end-to-end invariant holds.
+- Fully zero-latency end-to-end paths are disallowed, except for explicit
+  test-only stubs clearly marked as such.
+
+### D5. Policy vs topology responsibility split
+- Topology builder:
+  - defines nodes and links and their latency/BW parameters
+- Routing policy:
+  - selects among available graph paths based on decoded domains
+- Routing policy must not assume missing links; missing connectivity
+  is a topology construction error.
+
+### D6. No software-managed routing metadata
+- Routing decisions must not rely on per-request software-managed metadata
+  that tracks distance, hop count, or ordering outside the graph model.
+- All distance/order computation is derived from traversal itself.
+
+## Alternatives Considered
+
+1) **Hop-count based routing**
+- Rejected: ignores heterogeneous latency/BW and misrepresents
+  architectural differences.
+
+2) **Implicit local shortcuts**
+- Rejected: breaks debuggability and violates traversal-based latency.
+
+3) **Software-managed distance metadata**
+- Rejected: increases control overhead and obscures routing semantics.
+
+## Consequences
+
+### Positive
+- Clear, debuggable hop-by-hop traces (SPEC R2, R4).
+- Architecture comparisons reflect real interconnect structure.
+- Routing behavior is reproducible and deterministic.
+
+### Tradeoffs / Costs
+- Graph construction must be correct and complete.
+- Bypass modeling requires explicit graph representation,
+  which slightly increases topology description complexity.
+
+## Implementation Notes (Non-normative)
+- Recommended responsibilities:
+  - Graph builder: ensure all required paths exist.
+  - Router: select next hop based on decoded domains and policy.
+- Tests should assert:
+  - non-zero end-to-end latency
+  - deterministic routing for identical inputs
+  - bypass paths appear explicitly in emitted traces
+
+## Links
+- SPEC.md: R1 (routing), R2 (latency), R3 (topology), R5 (multi-domain comm)
+- ADR-0001: PhysAddr layout & decoding contract