3c6db4ed9b
Api for saving heap when Client class is used by a Server (WiFiServer class): Client = Server.available(). Suppose the local end is the server and the remote end is the client, we will deal with heap memory at the local end. When the local application (server) decides to close an active connection with a remote end it issues an Client.stop. The stop() function calls the close() function of ClientContext class which in turn calls tcp_close. The connexion is closed by tcp_close and the protocol control block (pcb) can be put in the following states depending on the requests sent by the remote: CLOSING, FIN_WAIT_1 and FIN_WAIT_2. In theses states pcbs are not freed, then consume some memory heap. If an acknowledgment from the remote end is received, the pcb enter in TIME_WAIT state for some minutes but pcbs in TIME_WAIT state are not freed. Then consume some heap memory. TIME_WAIT pcbs are automatically freed after some minutes or can be freed for instance issuing an tcp_kill_timewait() in the local application which will free the oldest pcb in TIME_WAIT state. If the connection is first closed from the remote end (the client), the local end (server) receive a connection termination request. It then acknowledge it and enter in CLOSE_WAIT state waiting for a connection termination request from the local application. It then send a termination request and enter in LAST_ACK state until it receive an acknowledgment from the remote end. After receiving the acknowledgment it enter in ClOSED state and the local pcb is freed leaving some room in the heap memory. To summarize, when a connexion termination request is send by one end (remote or local), the local pcb is not freed immediatly. This pcb can be in the following states: FIN_WAIT_1, FIN_WAIT_2, CLOSING, TIME_WAIT, CLOSE_WAIT, LAST_ACK. As a consequence, some old pcbs from old closed connections are still consuming heap memory. The local application can call tcp_kill_timewait hoping it will free some TIME_WAIT state pcbs. But if the server receive frequent connections requests and close them after sending whatever it has to send, there may be zero pcbs in TIME_WAIT state among all previously closed connections. In case of insufficient memory to accept a new connection, lwip has developped a strategy: it successively tries to kill the oldest pcb in TIME_WAIT state, or in LAST_ACK state or in CLOSING state or the oldest active connection with lower priority than the new one. As a matter of fact this "urgent" strategy is deployed only when very few heap memory remain available (less than some kb). In case of success, Client.available returns a valid Client but the local application will crash when sending or receiving data from the client (Client.read ou readuntil or available) because this need more heap memory and just some kb were freed in lwip to allocate the new pcb structure ans start the new connection. The propose API is intended to avoid this drawback by calling the abort function of ClientContext which in turn calls tcp_abort which calls tcp_abandon. The connection is aborted and notified to the client with a RESET and the pcb and ressources associated are immediately released increasing the available heap memory.
5.2 KiB
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Client Class
Methods documented for Client in Arduino
Methods and properties described further down are specific to ESP8266. They are not covered in Arduino WiFi library documentation. Before they are fully documented please refer to information below.
flush and stop
flush(timeoutMs) and stop(timeoutMs) both
have now an optional argument: timeout in millisecond, and
both return a boolean.
Default input value 0 means that effective value is left at the discretion of the implementer.
flush() returning true indicates that
output data have effectively been sent, and false that a
timeout has occurred.
stop() returns false in case of an issue
when closing the client (for instance a timed-out flush).
Depending on implementation, its parameter can be passed to
flush().
abort
void abort();Originally proposed in #8738 Unlike
stop(), immediately shuts down internal connection
object.
Under usual circumstances, we either enter CLOSE_WAIT or
TIME_WAIT state. But, the connection object is not freed
right away, and requires us to either * wait until malloc()
returns NULL when our TCP stack tries to allocate memory
for a new connection * manually call tcp_kill_timewait() to
forcibly stop the 'oldest' connection
This API frees up resources used by the connection. Consider using it
instead of stop() if your application handles a lot of
clients and frequently runs out of available heap memory.
Example:
setNoDelay
setNoDelay(nodelay)With nodelay set to true, this function
will to disable Nagle
algorithm.
This algorithm is intended to reduce TCP/IP traffic of small packets sent over the network by combining a number of small outgoing messages, and sending them all at once. The downside of such approach is effectively delaying individual messages until a big enough packet is assembled.
Example:
client.setNoDelay(true);getNoDelay
Returns whether NoDelay is enabled or not for the current connection.
setSync
This is an experimental API that will set the client in synchronized
mode. In this mode, every write() is flushed. It means that
after a call to write(), data are ensured to be received
where they went sent to (that is flush semantic).
When set to true in WiFiClient
implementation,
- It slows down transfers, and implicitly disable the Nagle algorithm.
- It also allows to avoid a temporary copy of data that otherwise
consumes at most
TCP_SND_BUF= (2 *MSS) bytes per connection,
getSync
Returns whether Sync is enabled or not for the current connection.
setDefaultNoDelay and setDefaultSync
These set the default value for both setSync and
setNoDelay for every future instance of
WiFiClient (including those coming from
WiFiServer.available() by default).
Default values are false for both NoDelay and
Sync.
This means that Nagle is enabled by default for all new connections.
getDefaultNoDelay and getDefaultSync
Return the values to be used as default for NoDelay and Sync for all future connections.
Other Function Calls
uint8_t status ()
virtual size_t write (const uint8_t *buf, size_t size)
size_t write_P (PGM_P buf, size_t size)
size_t write (Stream &stream)
size_t write (Stream &stream, size_t unitSize) __attribute__((deprecated))
virtual int read (uint8_t *buf, size_t size)
virtual int peek ()
virtual size_t peekBytes (uint8_t *buffer, size_t length)
size_t peekBytes (char *buffer, size_t length)
virtual operator bool ()
IPAddress remoteIP ()
uint16_t remotePort ()
IPAddress localIP ()
uint16_t localPort () Documentation for the above functions is not yet available.