Project 14: Formally Verified IPC Protocol
Specify and verify a synchronous IPC protocol using TLA+ or Isabelle.
Quick Reference
| Attribute | Value |
|---|---|
| Difficulty | Master |
| Time Estimate | 1-2 months |
| Language | C (implementation) + TLA+/Isabelle (specification) |
| Prerequisites | IPC theory, discrete math, modeling basics |
| Key Topics | Formal specs, invariants, liveness |
1. Learning Objectives
By completing this project, you will:
- Write a formal specification of a synchronous IPC protocol.
- Prove safety properties (no deadlock, no message loss).
- Prove liveness properties (progress, eventual delivery).
- Implement a conforming IPC protocol and map spec to code.
2. Theoretical Foundation
2.1 Core Concepts
- Safety vs Liveness: Safety = nothing bad happens, liveness = something good eventually happens.
- Invariants: Properties that must always hold.
- Model Checking: Exhaustively exploring state space.
- Refinement: Mapping spec to implementation.
2.2 Why This Matters
Formal verification turns microkernel IPC from “probably correct” into “provably correct.” seL4 achieved this with rigorous proofs.
2.3 Historical Context / Background
TLA+ was created by Leslie Lamport and used for distributed systems. seL4 proofs used Isabelle/HOL to verify kernel correctness.
2.4 Common Misconceptions
- “Formal methods are academic only.” They are used in safety-critical systems.
- “Model checking proves the code.” It proves the model, not the implementation.
3. Project Specification
3.1 What You Will Build
A formal spec of synchronous IPC, proofs of key properties, and a reference implementation that aligns with the spec.
3.2 Functional Requirements
- Specification: Model sender, receiver, endpoint state.
- Safety proofs: No deadlock, no lost messages.
- Liveness proofs: Progress when both sides ready.
- Implementation: C or Rust IPC library.
- Refinement mapping: Explain how code matches spec.
3.3 Non-Functional Requirements
- Completeness: All variables and actions defined.
- Clarity: Spec is readable and commented.
- Repeatability: Model checker runs with zero errors.
3.4 Example Usage / Output
NoDeadlock ==
~(sender = "WAITING" /\ receiver = "WAITING")
3.5 Real World Outcome
$ tlc SyncIPC.tla
Model checking completed. No error has been found.
76 states generated, 0 states left on queue.
$ ./ipc_demo
[ipc] ping -> pong
[ipc] no deadlocks observed
4. Solution Architecture
4.1 High-Level Design
┌──────────────┐ Spec ┌──────────────┐ Impl ┌──────────────┐
│ TLA+/Isabelle│ ───────▶ │ Properties │ ───────▶ │ C IPC Code │
└──────────────┘ └──────────────┘ └──────────────┘
4.2 Key Components
| Component | Responsibility | Key Decisions |
|---|---|---|
| Spec model | State + actions | synchronous vs async |
| Property set | Safety/liveness | minimal vs complete |
| Model checker | Explore states | TLC config |
| Implementation | IPC code | alignment with spec |
4.3 Data Structures
VARIABLES sender, receiver, channel, msg
4.4 Algorithm Overview
Key Algorithm: Rendezvous Action
- Sender ready and receiver ready.
- Transfer message and update states.
- Ensure invariant holds.
Complexity Analysis:
- Time: Model checking exponential in state space
- Space: State graph size grows quickly
5. Implementation Guide
5.1 Development Environment Setup
# Install TLA+ toolbox or tlc
# Build C implementation
cc -O2 -g -o ipc_demo *.c
5.2 Project Structure
verified_ipc/
├── spec/
│ └── SyncIPC.tla
├── src/
│ └── ipc.c
├── proofs/
│ └── notes.md
└── tests/
└── test_ipc.c
5.3 The Core Question You’re Answering
“Can I prove my IPC protocol cannot deadlock or lose messages?”
5.4 Concepts You Must Understand First
Stop and research these before coding:
- Safety and liveness properties
- State machines
- Model checking limits
- Refinement mapping
5.5 Questions to Guide Your Design
- What is the minimal state to model?
- Which actions must be atomic in the spec?
- How will you align code with the model?
5.6 Thinking Exercise
Model a Deadlock
Create a state where sender waits and receiver waits. What action breaks the deadlock?
5.7 The Interview Questions They’ll Ask
- “What is the difference between safety and liveness?”
- “How do you validate a formal model against code?”
5.8 Hints in Layers
Hint 1: Start with a 2-state model Keep states minimal to avoid explosion.
Hint 2: Prove safety first Add liveness after invariants hold.
Hint 3: Map code to actions Comment code with spec action names.
5.9 Books That Will Help
| Topic | Book | Chapter |
|---|---|---|
| Formal methods | Lamport TLA+ | Intro chapters |
| seL4 proofs | seL4 papers | Verification sections |
5.10 Implementation Phases
Phase 1: Foundation (2 weeks)
Goals:
- Write minimal spec
Tasks:
- Define variables and actions.
- Run TLC with small bounds.
Checkpoint: TLC runs with no errors.
Phase 2: Core Functionality (2-3 weeks)
Goals:
- Add properties
Tasks:
- Add invariants and temporal properties.
- Fix counterexamples.
Checkpoint: Safety + liveness pass.
Phase 3: Polish & Edge Cases (2-3 weeks)
Goals:
- Implement and map to spec
Tasks:
- Write IPC code.
- Document refinement mapping.
Checkpoint: Implementation behaves as spec.
5.11 Key Implementation Decisions
| Decision | Options | Recommendation | Rationale |
|---|---|---|---|
| Spec language | TLA+, Isabelle | TLA+ | Faster iteration |
| Model size | small, large | small | Avoid state explosion |
| Property focus | safety only, safety+liveness | both | Complete verification |
6. Testing Strategy
6.1 Test Categories
| Category | Purpose | Examples |
|---|---|---|
| Model Tests | TLC runs | safety/liveness |
| Implementation Tests | IPC demo | ping/pong |
| Refinement Tests | Spec vs code | action mapping |
6.2 Critical Test Cases
- Deadlock state is unreachable.
- Message delivery always happens.
- Code behavior matches spec trace.
6.3 Test Data
States: 2 senders, 2 receivers
Messages: simple integers
7. Common Pitfalls & Debugging
7.1 Frequent Mistakes
| Pitfall | Symptom | Solution |
|---|---|---|
| State explosion | TLC runs forever | Reduce state space |
| Weak invariants | False positives | Strengthen invariants |
| Spec/impl mismatch | Unexpected behavior | Align actions to code |
7.2 Debugging Strategies
- Use TLC counterexample traces.
- Add comments in code referencing spec actions.
7.3 Performance Traps
Large models are expensive. Keep state space minimal and increase only when needed.
8. Extensions & Challenges
8.1 Beginner Extensions
- Add timeout semantics to the model.
- Add multiple senders/receivers.
8.2 Intermediate Extensions
- Model capability-based access.
- Add asynchronous IPC variant.
8.3 Advanced Extensions
- Prove refinement in Isabelle.
- Verify a real microkernel IPC path.
9. Real-World Connections
9.1 Industry Applications
- seL4: Formal proofs of kernel correctness.
- Safety-critical systems: Aviation, automotive, medical.
9.2 Related Open Source Projects
- seL4: https://sel4.systems/
- TLA+ tools: https://lamport.azurewebsites.net/tla/tla.html
9.3 Interview Relevance
Formal methods knowledge is rare and valuable in systems roles.
10. Resources
10.1 Essential Reading
- TLA+ Specifying Systems by Lamport.
- seL4 verification papers.
10.2 Video Resources
- Lamport’s TLA+ lectures.
10.3 Tools & Documentation
- TLC model checker
- Isabelle/HOL (optional)
10.4 Related Projects in This Series
- Project 1: IPC implementation.
- Project 8: seL4 verification concepts.
11. Self-Assessment Checklist
11.1 Understanding
- I can explain safety vs liveness.
- I can interpret TLC counterexamples.
11.2 Implementation
- Spec passes TLC.
- IPC implementation matches spec.
11.3 Growth
- I can describe refinement mapping clearly.
12. Submission / Completion Criteria
Minimum Viable Completion:
- Minimal spec written and verified.
- IPC demo runs.
Full Completion:
- Safety and liveness proofs pass.
- Refinement mapping documented.
Excellence (Going Above & Beyond):
- Isabelle proof of refinement.
- Publish spec and explanation.
This guide was generated from LEARN_MICROKERNELS.md. For the complete learning path, see the parent directory.