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‎Communication_Protocols/Multi_Protocol_Systems.md‎

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# Multi-Protocol Systems
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Embedded products often bridge, translate, or coordinate multiple protocols (e.g., UART↔CAN, CAN↔Ethernet, I2C↔SPI). This document provides patterns to design reliable, maintainable multi-protocol systems with predictable behavior under load.
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---
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## Design Goals
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- Lossless or controlled-loss routing under bursty traffic
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- Bounded latency with prioritization
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- Backpressure and flow control across protocol boundaries
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- Extensibility for new endpoints and message formats
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---
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## Reference Architecture
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- Hardware interfaces: UART, SPI, I2C, CAN/CAN-FD, Ethernet/Wi‑Fi, USB
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- Software layers:
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- Drivers (ISR/DMA)
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- I/O queues (RX/TX ring buffers)
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- Protocol adapters (frame parse/compose)
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- Router (message classification and forwarding)
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- Application services (control, management, persistence)
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- RTOS building blocks: tasks per interface, central router task, event groups, message queues, memory pools
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```text
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ISR/DMA → RX Queue → Adapter → Router → Adapter → TX Queue → Driver/DMA → Wire
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```
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---
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## Message Model
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- Canonical message:
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- Header: source, destination, protocol, priority, length, timestamp
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- Payload: opaque bytes for adapter
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- Routing table:
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- Maps (source, type) → output(s) with optional transforms
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- Priority classes:
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- Critical, High, Normal, Low mapped to scheduler priorities and queue depths
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---
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## Flow Control and Backpressure
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- UART: software/hardware flow control (XON/XOFF, RTS/CTS)
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- CAN: natural arbitration; still implement queue depth limits and drop policy
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- Ethernet/TCP: TCP flow control assists, but preserve internal backpressure to prevent memory exhaustion
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- Strategies:
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- Watermarks and hysteresis on queues
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- Headroom reservations for critical traffic
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- Leaky-bucket or token-bucket rate limiting between domains
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---
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## Scheduling and Latency
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- Assign priorities: ISR top, then router, then interface tasks by criticality
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- Use DMA for bulk transfers to free CPU cycles
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- Avoid lengthy work in ISR; push to tasks via queues
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- Pre-allocate buffers (no heap on hot path)
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---
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## Error Handling
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- Per-interface error counters and status snapshots
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- End-to-end retries with capped backoff and jitter
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- Drop policies: tail drop for low priority, or RED-like probabilistic drop
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- Persistent fault isolation: disable flapping outputs temporarily
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---
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## Example Router Loop (pseudo C)
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```c
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for (;;) {
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message_t *msg = queue_receive(router_q, ROUTER_WAIT_MS);
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if (!msg) continue;
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route_t *r = route_lookup(msg);
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if (!r) { stats.unknown++; buffer_free(msg); continue; }
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for (int i = 0; i < r->num_outputs; i++) {
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adapter_t *ad = r->outputs[i];
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if (!adapter_try_send(ad, msg)) {
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// Apply backpressure policy
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if (msg->priority <= PRIORITY_NORMAL) { stats.drop++; }
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else { adapter_blocking_send(ad, msg, TIMEOUT_MS); }
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}
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}
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buffer_free(msg);
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}
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```
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---
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## Bridging Examples
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### UART ↔ CAN
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- Packetize UART stream into frames with length + CRC
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- Map UART message types to CAN IDs; enforce CAN payload ≤ 8/64 bytes (Classic/FD)
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- Apply rate limiting from UART→CAN to respect bus utilization targets
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### CAN ↔ Ethernet (UDP)
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- UDP multicast for broadcast-like semantics; include CAN-ID, timestamp, and flags in UDP payload
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- Ensure loss tolerance or implement app-level ACKs if needed
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- Consider DSCP marking for priority traffic
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---
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## Time Synchronization
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- For cross-domain correlation: PTP over Ethernet; propagate time to MCU domain
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- For CAN-only networks: periodic time-sync frames or gateway time stamping
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- Store timestamps in canonical message headers
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---
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## Security Considerations
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- Authenticate management/control messages crossing domains
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- Validate lengths and IDs; drop malformed or unauthorized frames
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- Rate-limit untrusted inputs; partition critical vs non-critical traffic
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---
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## Test Strategy
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- Soak tests with mixed traffic profiles and bursts
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- Fault injection: drop, duplicate, reorder frames; bit errors
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- Latency/jitter measurements per path; 99th/99.9th percentile targets
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- Capacity tests: fill queues to watermarks; verify drop policy and recovery
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---
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## Deployment Checklist
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- Routing table versioned and remotely updatable
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- Queue depths sized from measured traffic distributions
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- Backpressure verified end-to-end
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- Logs include per-path counters and timestamps
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- Watchdog coverage for router and interface tasks
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‎Communication_Protocols/Network_Protocols.md‎

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# Network Protocols for Embedded Systems
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Designing networked embedded devices requires balancing deterministic behavior, small memory footprints, and constrained CPU cycles while speaking interoperable protocols. This guide focuses on practical, production-oriented aspects for IPv4/IPv6, ICMP/ARP/ND, UDP/TCP, and application-layer IoT protocols.
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---
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## Goals
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- Understand the TCP/IP stack in embedded contexts
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- Choose and configure lightweight stacks (e.g., lwIP)
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- Implement robust UDP/TCP clients/servers
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- Integrate IoT app protocols (MQTT/CoAP)
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- Diagnose problems with a structured checklist
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---
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## TCP/IP Stack Overview
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- Network stack models:
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- OSI (7 layers) vs TCP/IP (4 layers). In practice, embedded stacks follow TCP/IP: Link → Internet → Transport → Application.
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- Common link layers: Ethernet, Wi‑Fi, PPP, LTE Cat-x, 802.15.4.
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- Internet layer:
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- IPv4 header (20 bytes min), IPv6 (40 bytes). IPv6 removes header checksum and uses Neighbor Discovery (ND) instead of ARP.
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- ICMPv4/ICMPv6 for diagnostics and error reporting.
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- Transport layer:
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- UDP: message boundaries preserved, lower overhead, no built-in retransmission.
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- TCP: connection-oriented, ordered, reliable stream, congestion and flow control.
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- Application layer:
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- IoT protocols: MQTT (pub/sub over TCP), CoAP (REST over UDP), HTTP/HTTPS, custom binary.
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---
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## Addressing, Name Resolution, and Configuration
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- Address assignment:
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- Static IP, DHCPv4, SLAAC/DHCPv6 for IPv6.
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- Name resolution:
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- DNS (A/AAAA), mDNS for local discovery.
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- ARP (IPv4) and ND (IPv6) for L2 neighbor resolution.
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- NAT traversal considerations for outbound vs inbound connectivity (prefer client-initiated sessions for simplicity).
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---
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## Embedded Network Stacks
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- Common choices: lwIP, uIP (legacy/very small), vendor stacks (e.g., STM32Cube, NXP SDK).
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- Key configuration levers (lwIP examples):
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- Memory pools: `MEM_SIZE`, `PBUF_POOL_SIZE`, `TCP_MSS`, `TCP_SND_BUF`, `TCP_WND`.
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- Features: enable only what you need (e.g., disable `LWIP_IPV6` if not used; disable `LWIP_DNS` if static names).
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- Threading: `tcpip_thread` priority vs application tasks; ensure input path is high priority and bounded work in callbacks.
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- Zero-copy path from driver to stack when possible (DMA-safe RX/TX buffers, proper cache maintenance on MCUs with cache).
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---
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## UDP in Embedded Systems
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Use UDP when latency and simplicity outweigh guaranteed delivery.
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- Pros: small footprint, no connection state, multicast support.
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- Cons: no retransmission, ordering, or congestion control.
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- Design patterns:
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- App-level sequence numbers and ACK/NACK where needed.
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- Timeouts and retry policy with capped backoff.
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- Message authentication (HMAC) if security layer is not provided elsewhere.
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### Minimal UDP Echo (POSIX-like pseudo C)
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```c
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int sock = socket(AF_INET, SOCK_DGRAM, 0);
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struct sockaddr_in addr = { .sin_family = AF_INET, .sin_port = htons(9000), .sin_addr.s_addr = INADDR_ANY };
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bind(sock, (struct sockaddr*)&addr, sizeof(addr));
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for (;;) {
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uint8_t buf[1500];
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struct sockaddr_in peer; socklen_t len = sizeof(peer);
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int n = recvfrom(sock, buf, sizeof(buf), 0, (struct sockaddr*)&peer, &len);
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if (n > 0) {
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sendto(sock, buf, n, 0, (struct sockaddr*)&peer, len);
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}
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}
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```
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---
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## TCP in Embedded Systems
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Use TCP for reliability and compatibility with cloud services.
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- Memory planning:
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- Each TCP PCB and each connection consumes buffers; set limits (`MEMP_NUM_TCP_PCB`, backlog).
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- Choose `TCP_MSS` appropriate to link MTU (Ethernet MTU 1500 → MSS 1460 for IPv4, 1440 for IPv6).
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- Nagle vs latency:
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- Disable Nagle (`TCP_NODELAY`) for request/response with small payloads to reduce latency.
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- Keep Nagle for bulk transfers; or coalesce at application layer.
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- Keepalive:
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- TCP keepalive timers detect dead peers; configure periods conservatively to avoid unnecessary network chatter on cellular.
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### Minimal TCP Client (POSIX-like pseudo C)
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```c
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int sock = socket(AF_INET, SOCK_STREAM, 0);
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struct sockaddr_in srv = { .sin_family = AF_INET, .sin_port = htons(1883), .sin_addr.s_addr = inet_addr("192.0.2.10") };
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connect(sock, (struct sockaddr*)&srv, sizeof(srv));
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const char hello[] = "ping";
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send(sock, hello, sizeof(hello)-1, 0);
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uint8_t buf[1024];
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int n = recv(sock, buf, sizeof(buf), 0);
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// handle response
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```
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---
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## IoT Application Protocols
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### MQTT (over TCP)
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- Pub/Sub with broker; topics, QoS 0/1/2; retain and last will.
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- Embedded notes: persistent sessions reduce handshake cost; limit topic and payload size; batch publishes.
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### CoAP (over UDP)
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- REST-like with confirmable/non-confirmable messages, observe, block-wise transfer.
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- Embedded notes: implement retransmission and token matching carefully; DTLS for security if required.
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### HTTP/HTTPS
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- Ubiquitous, verbose; consider short timeouts and connection pooling if client.
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- Prefer HTTP/1.1 keep-alive; HTTP/2 is heavier for MCUs without proper libraries.
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---
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## Performance Tuning Checklist
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- Link MTU and MSS aligned; avoid IP fragmentation.
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- Pre-allocate RX/TX buffers; avoid heap in hot path.
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- Use DMA and zero-copy pbufs where supported.
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- Configure interrupt coalescing (if NIC supports) vs latency goals.
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- Pin high-priority threads; bound ISR work; defer to RTOS tasks.
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- Use DSCP/ToS to mark latency-sensitive traffic where network honors QoS.
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---
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## Reliability and Robustness
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- Exponential backoff with jitter for reconnects.
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- Dead-peer detection (keepalive, application heartbeats).
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- Validate all lengths and parse defensively (avoid buffer overruns).
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- Implement watchdog resets around networking stalls with recovery paths.
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- Persist credentials and broker endpoints in redundant storage.
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---
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## Diagnostics
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- Packet capture:
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- Mirror/SPAN port or inline hub; or software capture from the driver if feasible.
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- Use filters (examples): `arp`, `icmp`, `tcp.port == 1883`, `udp.port == 5683`.
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- Health metrics:
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- RX/TX drops, retransmits, RTT, DNS latency, reconnect counts.
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- Common issues:
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- ARP/ND resolution failures → check VLAN, gateway, and subnet masks
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- MSS/MTU mismatch → excessive fragmentation, PMTU black holes
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- Head-of-line blocking in single-threaded MQTT clients → worker thread separation
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---
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## Security Notes
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- Prefer TLS/DTLS with modern ciphersuites; validate certificates and time.
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- Use hardware RNG and secure key storage if available.
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- Restrict inbound listeners; favor outbound client connections.
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- Rate-limit and authenticate management endpoints.
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---
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## Production Readiness Checklist
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- IP configuration: static/DHCPv4/IPv6 SLAAC documented
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- DNS fallback and caching behavior defined
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- Robust reconnect and backoff strategy
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- Heartbeats and liveness probes implemented
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- TLS/DTLS configuration with strong RNG and key storage
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- Watchdog and brown-out recovery paths tested
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- Packet captures for success and failure cases recorded
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