05-protocol-docs

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Protocol Documentation

Integration Layer — Detailing the communication contracts between components

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Definition

Protocol documentation describes the rules, formats, and sequences governing communication between system components. This includes message structures, state machines, handshake procedures, and error handling conventions.

Relationship to the Specification

The specification defines what communications must occur and what guarantees they must provide. Protocol documentation elaborates how those communications are structured at the wire level.

Specification Requirement	Protocol Documentation Details
”Nodes shall authenticate before exchanging data”	Handshake sequence, challenge-response format
”Messages shall be ordered within a session”	Sequence numbering scheme, reordering buffer
”Delivery shall be confirmed”	Acknowledgment message format, retry timing
”Communication shall be encrypted”	Cipher suites, key exchange procedure

Core Components

• Message Formats: Byte-level structure of each message type
• State Machines: Valid sequences of messages and state transitions
• Handshakes: Connection establishment and teardown procedures
• Error Handling: Protocol-level error codes and recovery procedures
• Timing: Timeouts, keepalives, and retry intervals
• Versioning: How protocol versions are negotiated and signaled
• Wire Examples: Annotated byte dumps showing real messages

Dependency Chain

Specification
    ↓
Architecture Docs ← component boundaries and interactions
    ↓
Protocol Docs ← wire-level communication details
    ↓
Data Schemas ← structure of protocol payloads
    ↓
Reference Manuals ← complete protocol reference

Why Spec-Grounding Matters

Protocols are contracts. Without clear derivation from specifications:

• Interoperability failures: Different implementations interpret the protocol differently
• Security gaps: Security requirements in the spec may not be reflected in the protocol
• Version confusion: Protocol changes without clear ties to spec versions
• Incomplete implementations: Implementers don’t know which behaviors are mandatory

Specification Traceability Example

Good protocol documentation maintains explicit links:

## Message Authentication Code
 
Per SPEC-6.1.3 (Message Integrity), all messages must include a MAC.
 
### Format
 
    +----------------+----------------+------------------+
    | Payload Length | Payload        | MAC (32 bytes)   |
    | (4 bytes, BE)  | (variable)     | HMAC-SHA256      |
    +----------------+----------------+------------------+
 
### Computation
 
    MAC = HMAC-SHA256(session_key, length || payload)
 
### Verification
 
Per SPEC-6.1.3.2, messages with invalid MACs must be:
1. Discarded without processing
2. Logged for security monitoring (SPEC-8.2.1)
3. NOT acknowledged to sender

State Machine Documentation

Protocols often involve state machines. These should trace to specification:

## Connection State Machine
 
Implements SPEC-4.1 (Connection Lifecycle)
 
    ┌──────────────┐
    │ DISCONNECTED │
    └──────┬───────┘
           │ connect() [SPEC-4.1.1]
           ▼
    ┌──────────────┐
    │ CONNECTING   │──timeout [SPEC-4.1.2]──→ DISCONNECTED
    └──────┬───────┘
           │ handshake complete
           ▼
    ┌──────────────┐
    │ CONNECTED    │──heartbeat timeout [SPEC-4.1.5]──→ RECONNECTING
    └──────┬───────┘
           │ disconnect() [SPEC-4.1.6]
           ▼
    ┌──────────────┐
    │ DISCONNECTING│
    └──────┬───────┘
           │ cleanup complete
           ▼
    ┌──────────────┐
    │ DISCONNECTED │
    └──────────────┘

Best Practices

• Include specification references for every protocol behavior
• Provide both human-readable descriptions and machine-readable schemas
• Document both the happy path and all error/edge cases
• Include wire-level examples with annotated byte sequences
• Maintain a test vector suite that validates protocol implementations against spec
• Version protocol documentation alongside specification versions