doc/websocket: updates the API reference for ESP WebSocket Client

Author: ESP-Marius

docs/en/api-reference/protocols/esp_websocket_client.rst

@@ -3,11 +3,11 @@ ESP WebSocket Client
 
 Overview
 --------
-The ESP WebSocket client is an implementation of `WebSocket protocol client <https://tools.ietf.org/html/rfc6455>`_ for ESP32
+The ESP WebSocket client is an implementation of `WebSocket protocol client <https://tools.ietf.org/html/rfc6455>`_ for {IDF_TARGET_NAME}
 
 Features
 --------
-   * supports WebSocket over TCP, SSL with mbedtls
+   * Supports WebSocket over TCP, TLS with mbedtls
    * Easy to setup with URI
    * Multiple instances (Multiple clients in one application)
 
@@ -19,48 +19,105 @@ URI
 -  Supports ``ws``, ``wss`` schemes
 -  WebSocket samples:
 
-   -  ``ws://websocket.org``: WebSocket over TCP, default port 80
-   -  ``wss://websocket.org``: WebSocket over SSL, default port 443
-  
--  Minimal configurations:
+   -  ``ws://echo.websocket.org``: WebSocket over TCP, default port 80
+   -  ``wss://echo.websocket.org``: WebSocket over SSL, default port 443
+
+Minimal configurations:
 
 .. code:: c
 
     const esp_websocket_client_config_t ws_cfg = {
-        .uri = "ws://websocket.org",
+        .uri = "ws://echo.websocket.org",
     };
 
--  If there are any options related to the URI in
-   ``esp_websocket_client_config_t``, the option defined by the URI will be
-   overridden. Sample:
+The WebSocket client supports the use of both path and query in the URI. Sample:
+
+.. code:: c
+
+    const esp_websocket_client_config_t ws_cfg = {
+        .uri = "ws://echo.websocket.org/connectionhandler?id=104",
+    };
+
+
+If there are any options related to the URI in
+:cpp:type:`esp_websocket_client_config_t`, the option defined by the URI will be
+overridden. Sample:
 
 .. code:: c
 
     const esp_websocket_client_config_t ws_cfg = {
-        .uri = "ws://websocket.org:123",
+        .uri = "ws://echo.websocket.org:123",
         .port = 4567,
     };
     //WebSocket client will connect to websocket.org using port 4567
 
-SSL
+TLS
 ^^^
 
--  Get certificate from server, example: ``websocket.org``
-   ``openssl s_client -showcerts -connect websocket.org:443 </dev/null 2>/dev/null|openssl x509 -outform PEM >websocket_org.pem``
--  Configuration:
+Configuration:
 
-.. code:: cpp
+.. code:: c
 
     const esp_websocket_client_config_t ws_cfg = {
-        .uri = "wss://websocket.org",
+        .uri = "wss://echo.websocket.org",
         .cert_pem = (const char *)websocket_org_pem_start,
     };
 
-For more options on ``esp_websocket_client_config_t``, please refer to API reference below
+.. note:: If you want to verify the server, then you need to provide a certificate in PEM format, and provide to ``cert_pem`` in :cpp:type:`websocket_client_config_t`. If no certficate is provided then the TLS connection will to default not requiring verification.
+
+PEM certificate for this example could be extracted from an openssl `s_client` command connecting to websocket.org.
+In case a host operating system has `openssl` and `sed` packages installed, one could execute the following command to download and save the root or intermediate root certificate to a file (Note for Windows users: Both Linux like environment or Windows native packages may be used).
+```
+echo "" | openssl s_client -showcerts -connect websocket.org:443 | sed -n "1,/Root/d; /BEGIN/,/END/p" | openssl x509 -outform PEM >websocket_org.pem
+```
+
+This command will extract the second certificate in the chain and save it as a pem-file.
+
+Subprotocol
+^^^^^^^^^^^
+
+The subprotocol field in the config struct can be used to request a subprotocol
+
+.. code:: c
+
+    const esp_websocket_client_config_t ws_cfg = {
+        .uri = "ws://websocket.org",
+        .subprotocol = "soap",
+    };
+
+.. note:: The client is indifferent to the subprotocol field in the server response and will accept the connection no matter what the server replies.
+
+For more options on :cpp:type:`esp_websocket_client_config_t`, please refer to API reference below
+
+Events
+------
+* `WEBSOCKET_EVENT_CONNECTED`: The client has successfully established a connection to the server. The client is now ready to send and receive data. Contains no event data.
+* `WEBSOCKET_EVENT_DISCONNECTED`: The client has aborted the connection due to the transport layer failing to read data, e.g. because the server is unavailable. Contains no event data.
+* `WEBSOCKET_EVENT_DATA`: The client has successfully received and parsed a WebSocket frame. The event data contains a pointer to the payload data, the length of the payload data as well as the opcode of the received frame. A message may be fragmented into multiple events if the length exceeds the buffer size. This event will also be posted for non-payload frames, e.g. pong or connection close frames.
+* `WEBSOCKET_EVENT_ERROR`: Not used in the current implementation of the client.
+
+If the client handle is needed in the event handler it can be accessed through the pointer passed to the event handler:
+
+.. code:: c
+
+    esp_websocket_client_handle_t client = (esp_websocket_client_handle_t)handler_args;
+
+
+Limitations and Known Issues
+----------------------------
+* The client is able to request the use of a subprotocol from the server during the handshake, but does not do any subprotocol related checks on the response from the server.
 
 Application Example
 -------------------
-Simple WebSocket example that uses esp_websocket_client to establish a websocket connection and send/receive data with the `websocket.org <https://websocket.org>`_ Server: :example:`protocols/websocket`.
+A simple WebSocket example that uses esp_websocket_client to establish a websocket connection and send/receive data with the `websocket.org <https://websocket.org>`_ server can be found here: :example:`protocols/websocket`.
+
+Sending Text Data
+^^^^^^^^^^^^^^^^^
+The WebSocket client supports sending data as a text data frame, which informs the application layer that the payload data is text data encoded as UTF-8. Example:
+
+.. code:: cpp
+
+    esp_websocket_client_send_text(client, data, len, portMAX_DELAY);
 
 
 API Reference