Layer 4 (Transport) — TCP connection pool, ring buffers, and lwIP integration. More...

#include "DetWebServerConfig.h"
#include "freertos/FreeRTOS.h"
#include "freertos/queue.h"
#include "lwip/tcp.h"
#include <Arduino.h>

Classes
struct	TcpConn
	A single TCP connection context. More...

struct	TcpEvt
	Event record posted from lwIP callbacks to the main-loop task. More...

class	DeterministicAsyncTCP
	Static-only facade over the raw lwIP TCP API. More...

Enumerations
enum	ConnState { CONN_FREE , CONN_ACTIVE , CONN_CLOSING }
	Lifecycle state of a connection pool slot. More...

enum	EvtType { EVT_CONNECT , EVT_DATA , EVT_DISCONNECT , EVT_ERROR }
	Type of connection event posted to the FreeRTOS event queue. More...

Variables
TcpConn	conn_pool [MAX_CONNS]
	Static pool of connection contexts. Defined in transport.cpp.

Detailed Description

Layer 4 (Transport) — TCP connection pool, ring buffers, and lwIP integration.

Defines the static connection pool and the FreeRTOS event queue used to pass connection events from lwIP callbacks (running in tcpip_thread) to the Arduino main-loop task.

Concurrency model

Context	Reads	Writes
lwIP callbacks	rx_head (to check full)	rx_buffer[], rx_head
Arduino loop	rx_buffer[], rx_tail	rx_tail

rx_head and rx_tail are volatile to prevent the compiler from caching them across the inter-task boundary. The single-producer / single-consumer ring buffer design guarantees correctness without a mutex as long as writes to each index are atomic (true for 32-bit Xtensa).

Backpressure When the ring buffer is full, tcp_recved() is called with only the bytes actually copied — not p->tot_len. This shrinks the TCP receive window and applies backpressure to the sender rather than silently dropping data.

Author: Douglas Quigg (dstroy0)

Date: 2026

Definition in file transport.h.

Enumeration Type Documentation

◆ ConnState

enum ConnState

Lifecycle state of a connection pool slot.

Transitions:

CONN_FREE → CONN_ACTIVE : accept callback fires.
CONN_ACTIVE → CONN_FREE : graceful close, error, or timeout.
CONN_ACTIVE → CONN_CLOSING : (reserved for future half-close support).

Enumerator
CONN_FREE	Slot is available; no PCB is attached.
CONN_ACTIVE	Live connection; PCB is valid.
CONN_CLOSING	FIN sent; waiting for final ACK (reserved).

Definition at line 54 of file transport.h.

◆ EvtType

enum EvtType

Type of connection event posted to the FreeRTOS event queue.

Enumerator
EVT_CONNECT	New connection accepted.
EVT_DATA	Data received; bytes are already in the ring buffer.
EVT_DISCONNECT	Remote peer closed the connection gracefully.
EVT_ERROR	lwIP reported an error (PCB may already be freed).

Definition at line 90 of file transport.h.

Variable Documentation

◆ conn_pool

TcpConn conn_pool[MAX_CONNS]

extern

Static pool of connection contexts. Defined in transport.cpp.

Definition at line 25 of file transport.cpp.

Referenced by DeterministicAsyncTCP::check_timeouts(), http_parse(), DeterministicAsyncTCP::init(), DetWebServer::send(), DetWebServer::send_empty(), sse_write(), DeterministicAsyncTCP::stop(), ws_close(), ws_parse(), and ws_send_frame().

Classes