AgentRules.org
RulesStacksClientsAuditorPrunerSelf-HealRFCsBenchmarkCLICI/CDAuthorsSubmit
This specification is maintained by the AgentRules Working Group. Have enhancements or want to claim maintainership?Submit Specification →
devopscritical PriorityID: cilium-wireguard-pmtu-mss-clamping-standard

Cilium WireGuard Cross-Cluster PMTU and TCP MSS Clamping Invariant

Enforces deterministic Maximum Segment Size (MSS) clamping and dynamic Path MTU (PMTU) discovery configuration across multi-cluster Cilium mesh overlays utilizing WireGuard encryption. Prevents silent packet drops, intermittent TCP stalls, and TLS handshake truncation caused by unhandled encapsulation overhead in heterogeneous cross-cloud transit links.

Prohibited Anti-Pattern: Unclamped TCP MSS Over Encrypted Overlays Causing Silent Blackhole Failures During TLS Exchanges

Export Ready-to-Drop Configuration

Select format below to copy code or download file directly.

---
description: Cilium WireGuard Cross-Cluster PMTU and TCP MSS Clamping Invariant - Enforces deterministic Maximum Segment Size (MSS) clamping and dynamic Path MTU (PMTU) discovery configuration across multi-cluster Cilium mesh overlays utilizing WireGuard encryption. Prevents silent packet drops, intermittent TCP stalls, and TLS handshake truncation caused by unhandled encapsulation overhead in heterogeneous cross-cloud transit links.
globs: infrastructure/**/*.yaml, k8s/**/*.yaml, helm/**/*.yaml, src/config/network/**/*.ts
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/cilium-wireguard-pmtu-mss-clamping-standard"
---

# Cilium WireGuard Cross-Cluster PMTU and TCP MSS Clamping Invariant

> **Severity**: CRITICAL | **Category**: devops
> **Anti-Pattern Warning**: Unclamped TCP MSS Over Encrypted Overlays Causing Silent Blackhole Failures During TLS Exchanges

## System Constraints
You are a Principal Engineer enforcing the Cilium Multi-Cluster Service Mesh WireGuard PMTU Clamping Protocol. Mandate invariants that eliminate TCP packet blackholing over WireGuard-encrypted overlays. Ensure all cluster manifests, Helm value definitions, and networking templates explicitly account for outer IP, WireGuard, and Geneve/VXLAN encapsulation overheads by dynamically configuring MSS clamping, ICMP Type 3 Code 4 preservation, and Cilium eBPF host routing constraints.

## Strict Guidelines
- Engineers MUST ALWAYS calculate explicit MTU offsets accounting for outer IP header (20B IPv4 / 40B IPv6), UDP header (8B), WireGuard header (32B), and authentication tag (16B), deducting at least 60 to 80 bytes from base cloud provider MTU (e.g., 1420 MTU on standard 1500 MTU fabrics).
- Engineers MUST NOT rely on unassisted Path MTU Discovery (PMTUD) across public cloud interconnects or transit gateways where ICMP Fragmentation Needed (Type 3, Code 4) packets are commonly filtered.
- Code SHOULD enforce TCP MSS clamping at the ingress/egress boundaries using Cilium eBPF or iptables mangle rules to restrict TCP SYN/SYN-ACK segments to (MTU - 40) bytes for IPv4.
- Services MUST validate that inter-cluster gateway nodes maintain identical MTU baselines across peering topologies to avoid asymmetric MTU blackholes.
- Components NEVER permit Cilium Helm charts to run with implicit default MTU auto-detection in multi-cloud WireGuard mesh architectures.

## Bad vs Good Implementations
### Cilium Helm Values WireGuard MTU Calculation and MSS Clamping

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Omitting explicit MTU and MSS clamping causes Cilium to inherit the host default (typically 1500 bytes). With WireGuard encapsulation adding 60-80 bytes, large packets (like TLS ClientHello/Certificates) exceed underlying cloud interface MTUs and are silently dropped when PMTU ICMP responses are blocked.
```typescript
// Anti-pattern: Defaulting MTU configuration in multi-cluster WireGuard setups
export const ciliumHelmConfig = {
  cluster: { name: 'cluster-us-east-1', id: 1 },
  encryption: {
    enabled: true,
    type: 'wireguard'
  },
  clustermesh: {
    useAPIServer: true
  }
  // Implicit MTU defaults to 1500, leading to packet drops on 1420 WireGuard links
};
```

#### Production Standard (Correct)
> **Standard**: Explicitly reserves 80 bytes of overhead for WireGuard encapsulation, dynamically setting Cilium MTU to 1420 bytes on 1500 MTU fabrics, and activates eBPF-level handling to ensure SYN/SYN-ACK segments are clamped to prevent silent inter-cluster truncation.
```typescript
// Production standard: Explicit WireGuard MTU calculation and MSS clamping
export interface CiliumNetworkConfig {
  baseMtu: number;
  wireguardOverhead: number;
  calculatedMtu: number;
  clampMss: boolean;
}

export function generateCiliumValues(baseHostMtu: number = 1500): Record<string, unknown> {
  // WireGuard IPv4: 20B (IPv4) + 8B (UDP) + 32B (WireGuard) + 16B (Auth Tag) = 76B (rounded to 80B safety margin)
  const wireguardOverhead = 80;
  const targetMtu = baseHostMtu - wireguardOverhead;

  return {
    cluster: { name: 'cluster-us-east-1', id: 1 },
    mtu: targetMtu,
    encryption: {
      enabled: true,
      type: 'wireguard'
    },
    // Enforce dynamic TCP MSS clamping via Cilium eBPF host routing
    bpf: {
      masquerade: true,
      tproxy: true
    },
    autoDirectNodeRoutes: true,
    ipv4: {
      enabled: true
    },
    clustermesh: {
      useAPIServer: true
    }
  };
}
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

Kubernetes Resource Inspector
Inspects live Cilium ConfigMaps, DaemonSets, and Node MTU configurations across active clusters.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

Cilium CLI Multi-Cluster Validator
Executes 'cilium connectivity test' with large frame payload options to detect PMTU truncation across clusters.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Cilium WireGuard Cross-Cluster PMTU and TCP MSS Clamping Invariant

Cilium WireGuard Cross-Cluster PMTU and TCP MSS Clamping Invariant defines an industrial-grade behavioral standard for devops engineering. Enforces deterministic Maximum Segment Size (MSS) clamping and dynamic Path MTU (PMTU) discovery configuration across multi-cluster Cilium mesh overlays utilizing WireGuard encryption. Prevents silent packet drops, intermittent TCP stalls, and TLS handshake truncation caused by unhandled encapsulation overhead in heterogeneous cross-cloud transit links. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Unclamped TCP MSS Over Encrypted Overlays Causing Silent Blackhole Failures During TLS Exchanges Without an explicit system instruction file, developers spend hours debugging silent failures.
Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftUnclamped TCP MSS Over Encrypted Overlays Causing Silent Blackhole Failures During TLS ExchangesCritical
Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production.
Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture InconsistencyAI assistants produce mixed paradigms across files, creating fragmented codebase conventions.Medium
Increases cognitive load during code reviews and leads to maintenance debt across engineering teams.
Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.
Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

Engineers MUST ALWAYS calculate explicit MTU offsets accounting for outer IP header (20B IPv4 / 40B IPv6), UDP header (8B), WireGuard header (32B), and authentication tag (16B), deducting at least 60 to 80 bytes from base cloud provider MTU (e.g., 1420 MTU on standard 1500 MTU fabrics).

Engineers MUST ALWAYS calculate explicit MTU offsets accounting for outer IP header (20B IPv4 / 40B IPv6), UDP header (8B), WireGuard header (32B), and authentication tag (16B), deducting at least 60 to 80 bytes from base cloud provider MTU (e.g., 1420 MTU on standard 1500 MTU fabrics).

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

Engineers MUST NOT rely on unassisted Path MTU Discovery (PMTUD) across public cloud interconnects or transit gateways where ICMP Fragmentation Needed (Type 3, Code 4) packets are commonly filtered.

Engineers MUST NOT rely on unassisted Path MTU Discovery (PMTUD) across public cloud interconnects or transit gateways where ICMP Fragmentation Needed (Type 3, Code 4) packets are commonly filtered.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

Code SHOULD enforce TCP MSS clamping at the ingress/egress boundaries using Cilium eBPF or iptables mangle rules to restrict TCP SYN/SYN-ACK segments to (MTU - 40) bytes for IPv4.

Code SHOULD enforce TCP MSS clamping at the ingress/egress boundaries using Cilium eBPF or iptables mangle rules to restrict TCP SYN/SYN-ACK segments to (MTU - 40) bytes for IPv4.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
4

Services MUST validate that inter-cluster gateway nodes maintain identical MTU baselines across peering topologies to avoid asymmetric MTU blackholes.

Services MUST validate that inter-cluster gateway nodes maintain identical MTU baselines across peering topologies to avoid asymmetric MTU blackholes.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
5

Components NEVER permit Cilium Helm charts to run with implicit default MTU auto-detection in multi-cloud WireGuard mesh architectures.

Components NEVER permit Cilium Helm charts to run with implicit default MTU auto-detection in multi-cloud WireGuard mesh architectures.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is Cilium WireGuard Cross-Cluster PMTU and TCP MSS Clamping Invariant critical for AI coding assistants?

Enforces deterministic Maximum Segment Size (MSS) clamping and dynamic Path MTU (PMTU) discovery configuration across multi-cluster Cilium mesh overlays utilizing WireGuard encryption. Prevents silent packet drops, intermittent TCP stalls, and TLS handshake truncation caused by unhandled encapsulation overhead in heterogeneous cross-cloud transit links. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (Unclamped TCP MSS Over Encrypted Overlays Causing Silent Blackhole Failures During TLS Exchanges) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

Open Specification · CC BY 4.0·Free for Personal, Team & Commercial In-Repo Use

Citation & Open Source Attribution Guidelines

This specification is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0). You are completely free to adapt and share this rule in your workspace. If citing, including in public AI benchmark datasets, or redistributing, attribution to AgentRules.org is required.

Related DEVOPS Rules

View all rules →
devops
86

Kubernetes Gateway API v1 HTTPRoute Canary and ReferenceGrant Invariants

Enforces strict conformance to Kubernetes Gateway API v1 specifications during migration from legacy Ingress controllers. Mandates explicit ReferenceGrant resources for cross-namespace routing, strict relative integer weighting for HTTPRoute canary releases, and explicit backendRef port declarations.

devops
118

ArgoCD ApplicationSet Matrix Generator Invariants & Cascade Deletion Guards

Enforces strict structural constraints on ArgoCD ApplicationSet Matrix generators combining cluster selectors and Git directories. Prevents catastrophic multi-cluster cascade pruning, accidental secret exposure through parameter interpolation, and runaway Application CR explosion via explicit sync policies, preservation finalizers, and parameter validation.

devops
40

Karpenter Resilient Topology, Interruption Queue, and Consolidation Budget Standard

Enforces deterministic disruption budgets, multi-instance-type diversification, and event-driven SQS interruption queue handling across Karpenter NodePools and EC2NodeClasses. Eliminates cascading eviction loops, instance starvation, and availability downtime during AWS Spot rebalances and underutilized node consolidation.

1 Rule in Stack

Cilium