Extend Readme.md

Author: ruediste

README.md

@@ -1,5 +1,6 @@
-# AsyncTCP 
-[![Build Status](https://travis-ci.org/me-no-dev/AsyncTCP.svg?branch=master)](https://travis-ci.org/me-no-dev/AsyncTCP) ![](https://github.com/me-no-dev/AsyncTCP/workflows/Async%20TCP%20CI/badge.svg) [![Codacy Badge](https://api.codacy.com/project/badge/Grade/2f7e4d1df8b446d192cbfec6dc174d2d)](https://www.codacy.com/manual/me-no-dev/AsyncTCP?utm_source=github.com&utm_medium=referral&utm_content=me-no-dev/AsyncTCP&utm_campaign=Badge_Grade)
+# AsyncTCP
+
+[![Build Status](https://travis-ci.org/me-no-dev/AsyncTCP.svg?branch=master)](https://travis-ci.org/me-no-dev/AsyncTCP) ![](https://github.com/me-no-dev/AsyncTCP/workflows/Async%20TCP%20CI/badge.svg) [![Codacy Badge](https://api.codacy.com/project/badge/Grade/2f7e4d1df8b446d192cbfec6dc174d2d)](https://www.codacy.com/manual/me-no-dev/AsyncTCP?utm_source=github.com&utm_medium=referral&utm_content=me-no-dev/AsyncTCP&utm_campaign=Badge_Grade)
 
 ### Async TCP Library for ESP32 Arduino
 
@@ -10,4 +11,38 @@ This is a fully asynchronous TCP library, aimed at enabling trouble-free, multi-
 This library is the base for [ESPAsyncWebServer](https://github.com/me-no-dev/ESPAsyncWebServer)
 
 ## AsyncClient and AsyncServer
+
 The base classes on which everything else is built. They expose all possible scenarios, but are really raw and require more skills to use.
+
+### AsyncServer
+
+To setup a server, create an `AsyncServer` instance, specifying either a port or a port and an IP address. Then register a callback using the `onClient()` method, which will be called whenever a new TCP client connects to the server.
+
+By default, when sending the response in small chunks in rapid succession, the server will buffer the chunks until an ack for a previous packet has been received. By using ` setNoDelay(true);` you can disable this buffering and send the data as soon as possible. ([see Nagle's algorithm](https://en.wikipedia.org/wiki/Nagle%27s_algorithm]))
+
+Now you can use `begin()` to start listening for incoming connections. The server can be stopped by calling `end()`.
+
+```cpp
+AsyncServer server(80);
+server.onClient(...);
+server.begin();
+...
+server.end();
+```
+
+Internally, after you call `begin()`, it is first ensured that an event queue ([RTOS queue](https://www.freertos.org/Documentation/02-Kernel/02-Kernel-features/02-Queues-mutexes-and-semaphores/01-Queues)) is created and a service Task ([RTOS task](https://www.freertos.org/Documentation/01-FreeRTOS-quick-start/01-Beginners-guide/01-RTOS-fundamentals)) is created. Both the queue and task are created only once, even if there are multiple servers.
+
+Then [lwip](https://savannah.nongnu.org/projects/lwip/) is configured to accept tcp connections on the specified port and invoke the `_s_accept` function when a new connection is made, which forwards to the `AsyncServer._accept()` function. The `_accept()` function creates a new `AsyncClient` and pushes it to the event queue. The service task will then pick up the event and call the `AsyncServer._accepted()` which in turn calls the callback registered with `onClient()`. Thus the callback is ultimately called in the service task context.
+
+### AsyncClient
+
+A client can either be created by the server when a new connection is made or by the user.
+
+In both cases, the connection registers various callbacks with lwip to push the events to the event queue and thus to the service task. There are the following callbacks:
+
+- **recv:** invoke the `onPacket()` callback if registered, or the `onData()` callback followed by marking the data as received.
+- **sent:** invoke the `onAck()` callback
+- **error:** de-register the callbacks from lwip and call the user's `onError` and `onDisconnect` callbacks.
+- **poll:** used for ack and rx timeouts and to call the user's `onPoll` callback.
+
+To send data, either use `write()` to send data immediately, or a combination of `add()` and `send()` to queue data and send it all together.