ethernet-arp-ipv4-icmp

name	ethernet-arp-ipv4-icmp
title	Ethernet, ARP, IPv4, and ICMP
version	1
status	active
owners	[]
triggers	["Ethernet","ARP","IPv4","ICMP","ping"]
prerequisites	["network-stack-architecture","networking-drivers","reliability-fault-injection"]
related_docs	["ROADMAP.md","SECURITY.md"]
category	Networking
conditional_skills	[]
implementation_gates	["Requires packet buffers, timers, and VirtIO net."]
related_milestones	["M4 Devices and Storage"]
last_verified	{"base_commit":"d21a477","date":"2026-07-16","worktree_dirty":true,"context":"base commit plus uncommitted audit and agent-system worktree"}
description	Use when working on Ethernet, ARP, IPv4; provides the FinnOS-specific ethernet, arp, ipv4, and icmp workflow and evidence gates.

Ethernet, ARP, IPv4, and ICMP

1. Name

ethernet-arp-ipv4-icmp (Networking; skill maturity: planning-gated).

2. Purpose

Implement the first bounded L2/L3 path with strict packet/state validation.

3. When to use this skill

Use for requests mentioning Ethernet, ARP, IPv4, ICMP, ping, or when a dependency points to this skill. Load only after repository entry and before design or implementation.

4. When not to use this skill

Requires packet buffers, timers, and VirtIO net. Do not load it only because a future FinnOS document mentions the subsystem.

5. Prerequisite skills

• network-stack-architecture
• networking-drivers
• reliability-fault-injection

Read the full prerequisite closure in topological dependency order. If one cannot be satisfied, move the task to Blocked or Deferred; do not omit the dependency.

Conditional skills:

• None.

Implementation gates are roadmap/runtime conditions, not additional documents to load automatically:

• Requires packet buffers, timers, and VirtIO net.

6. Authoritative repository references

• ROADMAP.md
• SECURITY.md

Re-read implementation and tests referenced by those documents. The documents establish intent/status boundaries, not runtime proof.

7. Current FinnOS context

No packet parser or network service exists.

Registry verification used base commit d21a477 plus the dirty worktree context "base commit plus uncommitted audit and agent-system worktree" on 2026-07-16. This is not an integrated-revision claim. Reverify after HEAD, active PRs, or relevant source changes.

8. Required inputs

• User request or issue with desired outcome and architecture/profile scope.
• Current git status, recent history, active related issue/PR, and selected roadmap item.
• Relevant implementation, tests, invariants, ADRs, and exact baseline output.
• Toolchain/firmware/hardware versions when behavior crosses those boundaries.

9. Expected outputs

An evidence-backed ethernet, arp, ipv4, and icmp result with scoped artifacts, tests, documentation, and handoff. Include acceptance evidence, residual limitations, and next dependency rather than only code.

10. Step-by-step workflow

1. Parse Ethernet with length/type bounds
2. Implement capped ARP cache with expiry
3. Validate IPv4 header/options/fragment policy/checksum
4. Implement bounded ICMP echo/error handling
5. Interoperate through QEMU and fuzz malformed packets
6. Run the narrow regression, then all required subsystem/repository checks.
7. Update canonical docs/status and finish the agent-handoff template.

11. Repository-specific commands

./tools/finn check

Run commands from the repository root. A command listed here is a baseline/gate, not evidence that absent future functionality has a runnable target.

12. Architecture considerations

State shared semantics explicitly; isolate x86-64 and ARM64 mechanisms.

State guest architecture separately from host architecture and emulator model. Maintain a parity row for changed semantics and document intentional differences.

13. Safety constraints

Preserve the boundary described by the current state: No packet parser or network service exists. Apply .agents/checklists/pre-change.md; for kernel, driver, security, architecture, or UI work also apply the matching checklist.

14. Testing requirements

Make Interoperate through QEMU and fuzz malformed packets observable with a negative case, then run the narrow and aggregate gates. Do not permanently hard-code test counts; counts belong to dated evidence reports.

15. Documentation requirements

Update the canonical behavior/status document, relevant architecture/reference material, test/build instructions, limitations, and this skill registry if any command, gate, or current-context statement changes.

16. Review checklist

• The workflow proves Parse Ethernet with length/type bounds before relying on downstream assumptions.
• The observed baseline and first failing/divergent point are recorded.
• Every prerequisite and roadmap dependency is satisfied or explicitly marked blocked.
• Bounds, ownership, rollback, error paths, and resource limits were reviewed.
• x86-64 and ARM64 semantics are mapped without unsupported parity claims.
• Negative/failure tests exist at the layer that owns the behavior.
• Commands, logs, artifact/profile/architecture, and results are preserved.
• Canonical documentation and affected agent skills agree with behavior.

17. Completion criteria

• All stated outputs exist and remain within the task's scope/non-goals.
• The narrow regression and required repository/subsystem gates pass.
• Acceptance criteria are tied to source, test, runtime, or hardware evidence as appropriate.
• Unknowns and unsupported environments remain explicit; no planned feature is promoted.
• The final diff is reviewed and a structured handoff is complete.

18. Common failure modes

• Starting from an audit statement instead of re-reading changed source and tests.
• Using a successful compile or marker as evidence for a broader subsystem claim.
• Ignoring this skill's current constraint: No packet parser or network service exists.
• Changing shared policy while testing only one architecture or one happy path.
• Losing the exact failing artifact/log by rebuilding before capture.

19. Forbidden shortcuts

• Do not weaken, delete, reorder, or broaden validators merely to make a test pass.
• Do not claim ARM64, physical hardware, userspace, Peony, security, or release support without its own acceptance evidence.
• Do not bypass a named prerequisite by embedding a temporary incompatible abstraction.
• Do not commit target/, build/out/, firmware, credentials, local paths, or unreviewed generated output.
• Do not disregard the roadmap/status gate: Requires packet buffers, timers, and VirtIO net.

20. Handoff requirements

Use .agents/templates/handoff-template.md. Include objective, starting/final Git state, task state, skills used, files, exact commands/results, evidence classification, docs/status changes, unknowns, blockers, risks, next action, and next skills. Distinguish locally verified worktree changes from integrated behavior.

21. Examples

Request: "Work on Ethernet." Correct response: begin by parse ethernet with length/type bounds, then implement capped arp cache with expiry, and require evidence for interoperate through qemu and fuzz malformed packets before changing status. Incorrect response: create a plausible subsystem scaffold and mark the roadmap item complete because it compiles.

22. Skill maintenance notes

Canonical source: .agents/scripts/skill_registry.py. Increment version for material policy/workflow/gate changes, update last_verified only after reinspection, run python3 .agents/scripts/render_skills.py, review generated diffs, then run python3 .agents/scripts/validate.py --all. Follow .agents/GOVERNANCE.md; never hand-edit generated skills.