Layer: L6 Presentation · Build flags: PC_ENABLE_WS_DEFLATE (requires PC_ENABLE_WEBSOCKET)

What this example teaches

This is the WebSocket echo with one flag added: PC_ENABLE_WS_DEFLATE turns on permessage-deflate (RFC 7692), transparent two-way compression. The handler code is unchanged - it still reads plaintext - but frames travel compressed on the wire.

Negotiated in the handshake. With the extension compiled in, the server advertises permessage-deflate; client_no_context_takeover; server_no_context_takeover during the upgrade. Any browser that offered the extension (Chrome/Firefox/Safari do by default) then sends DEFLATE-compressed frames with the RSV1 bit set, and the library inflates them before ws_message() runs:

void ws_message(uint8_t ws_id) {
    // buf is already decompressed plaintext, even when the frame arrived compressed.
    char out[WS_FRAME_SIZE + 8];
    snprintf(out, sizeof(out), "echo: %s", (const char *)ws_pool[ws_id].buf);
    server.ws_send_text(ws_id, out);
}

Replies compress too, opportunistically. ws_send_text() / ws_send_binary() DEFLATE the payload and set RSV1 only when the result is smaller; a frame that would not shrink goes out uncompressed (the per-message flag makes that legal).

Bounded, no per-connection buffers. Both INFLATE and DEFLATE use bounded tables drawn from the shared per-dispatch scratch arena rather than a buffer per connection, so the determinism guarantee holds; a message must both arrive and decompress within WS_FRAME_SIZE.

You can confirm it in browser devtools: the /ws 101 response shows Sec-WebSocket-Extensions: permessage-deflate, and echoed frames show a compressed length.

Build and run

pio ci --board=esp32dev --project-option="framework=arduino" \
  --project-option="build_flags=-DPC_ENABLE_WS_DEFLATE=1" \
  --lib="." examples/L6-Presentation/WebSocketCompression/WebSocketCompression.ino

Flash, browse to http://<ip>/, type, and inspect the /ws frames in devtools.

Annotated source

The complete sketch (WebSocketCompression.ino), reproduced verbatim with added explanatory comments:

// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
// Enable WebSocket permessage-deflate for this sketch (overrides default-off).
#define PC_ENABLE_WS_DEFLATE 1
 
#include "protocore.h"
#include "network_drivers/physical/physical.h"
 
static const char *SSID = "YOUR_SSID";
static const char *PASSWORD = "YOUR_PASSWORD";
 
PC server;
 
static const char PAGE[] = "<!doctype html><meta charset=utf-8><title>WS deflate echo</title>"
                           "<input id=i autofocus><pre id=o></pre><script>"
                           "var w=new WebSocket('ws://'+location.host+'/ws');"
                           "w.onmessage=function(e){o.textContent+=e.data+'\\n'};"
                           "i.onkeydown=function(e){if(e.key=='Enter'){w.send(i.value);i.value=''}}</script>";
 
void ws_connect(uint8_t ws_id)
{
    server.ws_send_text(ws_id, "connected to /ws (permessage-deflate) - type something");
}
 
void ws_message(uint8_t ws_id)
{
    // buf is already decompressed plaintext, even when the frame arrived compressed.
    char out[WS_FRAME_SIZE + 8];
    snprintf(out, sizeof(out), "echo: %s", (const char *)ws_pool[ws_id].buf);
    server.ws_send_text(ws_id, out); // reply is DEFLATE'd only if it ends up smaller
}
 
void ws_close(uint8_t ws_id)
{
    (void)ws_id;
}
 
void setup()
{
    Serial.begin(115200);
    init_wifi_physical(SSID, PASSWORD);
    Serial.print("Connecting to WiFi");
    while (!wifi_ready())
    {
        delay(250);
        Serial.print('.');
    }
    uint32_t ip = pc_net_egress_ip(); // library egress IP (network byte order), no Arduino WiFi
    Serial.printf("IP: %u.%u.%u.%u\n", (unsigned)(ip & 0xFF), (unsigned)((ip >> 8) & 0xFF),
                  (unsigned)((ip >> 16) & 0xFF), (unsigned)((ip >> 24) & 0xFF));
 
    server.on("/", HttpMethod::HTTP_GET, [](uint8_t id, HttpReq *) { server.send(id, 200, "text/html", PAGE); });
    server.on_ws("/ws", ws_connect, ws_message, ws_close);
    server.begin(80);
}
 
void loop()
{
    server.handle();
}