protocol-design

name: protocol-design department: "sentinel" description: "Use when selecting and designing communication protocol stacks for IoT or embedded systems. Covers physical layer selection, transport and application protocols, security layers, message format design, and error resilience. Do not use for firmware architecture (use embedded-architecture) or fleet-scale operations (use fleet-management)." version: 1 triggers: - "BLE" - "MQTT" - "Matter" - "Thread" - "Zigbee" - "LoRa" - "CoAP" - "protocol" - "wireless" - "radio"

Protocol Design

Purpose

Select and design the communication protocol stack for an IoT/embedded system, including physical layer selection, transport protocol, application protocol, and security layer.

Scope Constraints

Evaluates protocol options against device requirements and produces stack recommendations. Does not implement protocol drivers or radio firmware. Does not perform RF testing or spectrum analysis.

Inputs

• Device hardware capabilities (radio modules available)
• Communication requirements (range, bandwidth, latency, power)
• Network topology (star, mesh, point-to-point)
• Security requirements (encryption, authentication, integrity)
• Ecosystem requirements (Matter certification, HomeKit, Alexa)

Input Sanitization

No user-provided values are used in commands or file paths. All inputs are treated as read-only analysis targets.

Procedure

Step 1: Define Communication Requirements

Quantify what the protocol must deliver:

• Range: Indoor (10m), building (50m), campus (500m), wide-area (km+)
• Bandwidth: Bytes/second needed for typical and peak traffic
• Latency: Maximum acceptable end-to-end delay
• Power budget: mA available for radio operations, duty cycle
• Topology: Star (hub-spoke), mesh (multi-hop), point-to-point
• Device density: How many devices in one radio neighborhood

Step 2: Select Physical/Link Layer

Choose the radio technology:

Step 3: Select Application Protocol

Choose how data is structured and exchanged:

• MQTT: Pub/sub, lightweight, great for telemetry. Needs TCP (Wi-Fi/cellular).
• CoAP: REST-like over UDP, ideal for constrained devices. Built-in discovery.
• HTTP/REST: Universal but heavy. Only for powered devices with good connectivity.
• Custom binary: Maximum efficiency but maintenance burden. Use Protobuf or CBOR instead of raw binary.

Step 4: Design Security Layer

Layer security into the stack:

• Transport encryption: TLS 1.3 (TCP), DTLS (UDP), or link-layer encryption (BLE bonding)
• Authentication: Device certificates, pre-shared keys, or OAuth 2.0 device flow
• Integrity: Message signing, sequence numbers to prevent replay
• Key management: How are keys provisioned, rotated, and revoked?

Step 5: Design Message Format

Define the wire format:

• Serialization: JSON (readable, large), CBOR (compact, schemaless), Protobuf (compact, schema-required)
• Envelope: Header (device ID, message type, sequence, timestamp) + payload
• Size budget: Maximum message size for the chosen transport
• Fragmentation: How to handle messages larger than MTU

Step 6: Design Error Handling and Resilience

Plan for communication failures:

• Retry strategy: Exponential backoff with jitter
• QoS levels: At-most-once, at-least-once, exactly-once — which is needed?
• Offline queuing: Buffer messages during disconnection, sync on reconnect
• Connection management: Keepalive intervals, reconnection strategy

Progress Checklist

• Step 1: Communication requirements quantified
• Step 2: Physical/link layer selected with rationale
• Step 3: Application protocol chosen
• Step 4: Security layer designed with key management
• Step 5: Message format defined within MTU budget
• Step 6: Error handling and resilience strategy complete

Compaction resilience: If context was lost during a long session, re-read the Inputs section to reconstruct what system is being analyzed, check the Progress Checklist for completed steps, then resume from the earliest incomplete step.

Output Format

# Protocol Architecture

## Requirements Summary
| Requirement | Value |
|-------------|-------|
| Range | [X meters/km] |
| Bandwidth | [X bytes/sec] |
| Latency | [X ms max] |
| Power budget | [X mA avg] |
| Topology | [Star/Mesh/P2P] |

## Protocol Stack
| Layer | Choice | Rationale |
|-------|--------|-----------|
| Physical/Link | [BLE/Thread/Wi-Fi/etc.] | [Why] |
| Transport | [TCP/UDP/none] | [Why] |
| Security | [TLS/DTLS/link-layer] | [Why] |
| Application | [MQTT/CoAP/custom] | [Why] |
| Serialization | [JSON/CBOR/Protobuf] | [Why] |

## Message Format

[Header: device_id (4B) | msg_type (1B) | seq (2B) | timestamp (4B)] [Payload: CBOR-encoded application data] [Total: ~128 bytes typical, 512 bytes max]

## Security Design
| Aspect | Approach |
|--------|----------|
| Encryption | [Method] |
| Authentication | [Method] |
| Key management | [Provisioning and rotation strategy] |

## Error Handling
| Scenario | Strategy |
|----------|----------|
| Message lost | [Retry/QoS approach] |
| Disconnection | [Offline queue + sync] |
| Server unreachable | [Backoff + local operation] |

Handoff

• Hand off to embedded-architecture if firmware task decomposition or memory layout decisions arise from protocol stack requirements.
• Hand off to cipher/crypto-review if cryptographic implementation concerns are identified during security layer design.

Quality Checks

• Protocol selection is justified against requirements (not just "we always use MQTT")
• Power consumption of radio operations is calculated against battery budget
• Security covers encryption, authentication, and key management
• Message format fits within the chosen transport's MTU
• Error handling addresses disconnection, retry, and offline operation
• Ecosystem requirements (Matter, HomeKit) are satisfied by protocol choice

Evolution Notes