ESP-IDF uses the open source `lwIP lightweight TCP/IP stack`_. The ESP-IDF version of lwIP (`esp-lwip`_) has some modifications and additions compared to the upstream project.
When using any lwIP API other than the `BSD Sockets API`_, please make sure that the API is thread-safe. To check if a given API call is thread-safe, enable the :ref:`CONFIG_LWIP_CHECK_THREAD_SAFETY` configuration option and run the application. This enables lwIP to assert the correct access of the TCP/IP core functionality. If the API is not accessed or locked properly from the appropriate `lwIP FreeRTOS Task`_, the execution will be aborted. The general recommendation is to use the :doc:`/api-reference/network/esp_netif` component to interact with lwIP.
- Dynamic Host Configuration Protocol (DHCP) Server & Client are supported indirectly via the :doc:`/api-reference/network/esp_netif` functionality.
- Simple Network Time Protocol (SNTP) is also supported via the :doc:`/api-reference/network/esp_netif`, or directly via the :component_file:`lwip/include/apps/esp_sntp.h` functions, which also provide thread-safe API to :component_file:`lwip/lwip/src/include/lwip/apps/sntp.h` functions, see also :ref:`system-time-sntp-sync`.
- ICMP Ping is supported using a variation on the lwIP ping API, see :doc:`/api-reference/protocols/icmp_echo`.
- ICMPv6 Ping, supported by lwIP's ICMPv6 Echo API, is used to test IPv6 network connectivity. For more information, see :example:`protocols/sockets/icmpv6_ping`.
- NetBIOS lookup is available using the standard lwIP API. :example:`protocols/http_server/restful_server` has the option to demonstrate using NetBIOS to look up a host on the LAN.
- mDNS uses a different implementation to the lwIP default mDNS, see :doc:`/api-reference/protocols/mdns`. But lwIP can look up mDNS hosts using standard APIs such as ``gethostbyname()`` and the convention ``hostname.local``, provided the :ref:`CONFIG_LWIP_DNS_SUPPORT_MDNS_QUERIES` setting is enabled.
- The PPP implementation in lwIP can be used to create PPPoS (PPP over serial) interface in ESP-IDF. Please refer to the documentation of the :doc:`/api-reference/network/esp_netif` component to create and configure a PPP network interface, by means of the ``ESP_NETIF_DEFAULT_PPP()`` macro defined in :component_file:`esp_netif/include/esp_netif_defaults.h`. Additional runtime settings are provided via :component_file:`esp_netif/include/esp_netif_ppp.h`. PPPoS interfaces are typically used to interact with NBIoT/GSM/LTE modems. More application-level friendly API is supported by the `esp_modem <https://components.espressif.com/component/espressif/esp_modem>`_ library, which uses this PPP lwIP module behind the scenes.
The BSD Sockets API is a common cross-platform TCP/IP sockets API that originated in the Berkeley Standard Distribution of UNIX but is now standardized in a section of the POSIX specification. BSD Sockets are sometimes called POSIX Sockets or Berkeley Sockets.
As implemented in ESP-IDF, lwIP supports all of the common usages of the BSD Sockets API.
-:example:`protocols/http_request`: this simplified example uses a TCP socket to send an HTTP request, but :doc:`/api-reference/protocols/esp_http_client` is a much better option for sending HTTP requests
-``select()``: via :doc:`/api-reference/storage/vfs`
-``poll()`` : on ESP-IDF, ``poll()`` is implemented by calling ``select()`` internally, so using ``select()`` directly is recommended, if a choice of methods is available
Some lwIP application sample code uses prefixed versions of BSD APIs, e.g., ``lwip_socket()``, instead of the standard ``socket()``. Both forms can be used with ESP-IDF, but using standard names is recommended.
-``select(int maxfdp1, fd_set *readset, fd_set *writeset, fd_set *exceptset, struct timeval *timeout)`` has an exception descriptor indicating that the socket has an error. For more information, see `select() Errors`_.
- When socket API fails, the return value does not contain the failure reason and the application can get the error reason code by accessing ``errno``. Different values indicate different meanings. For more information, see `Socket Error Reason Code`_.
- Socket error when ``select()`` has exception descriptor.
**Get the error reason code**
- If the ``select()`` indicates that the socket fails, we can not get the error reason code by accessing ``errno``, instead we should call ``getsockopt()`` to get the failure reason code. Since ``select()`` has exception descriptor, the error code is not given to ``errno``.
The ``getsockopt()`` function has the following prototype: ``int getsockopt(int s, int level, int optname, void *optval, socklen_t *optlen)``. Its purpose is to get the current value of the option of any type, any state socket, and store the result in ``optval``. For example, when you get the error code on a socket, you can get it by ``getsockopt(sockfd, SOL_SOCKET, SO_ERROR, &err, &optlen)``.
Below is a list of common error codes. For a more detailed list of standard POSIX/C error codes, please see `newlib errno.h <https://github.com/espressif/newlib-esp32/blob/master/newlib/libc/include/sys/errno.h>`_ and the platform-specific extensions :component_file:`newlib/platform_include/errno.h`.
The ``fcntl()`` function is a standard API for manipulating options related to a file descriptor. In ESP-IDF, the :doc:`/api-reference/storage/vfs` layer is used to implement this function.
When the file descriptor is a socket, only the following ``fcntl()`` values are supported:
-``O_NONBLOCK`` to set or clear non-blocking I/O mode. Also supports ``O_NDELAY``, which is identical to ``O_NONBLOCK``.
-``O_RDONLY``, ``O_WRONLY``, ``O_RDWR`` flags for different read or write modes. These flags can only be read using ``F_GETFL``, and cannot be set using ``F_SETFL``. A TCP socket returns a different mode depending on whether the connection has been closed at either end or is still open at both ends. UDP sockets always return ``O_RDWR``.
The ``ioctl()`` function provides a semi-standard way to access some internal features of the TCP/IP stack. In ESP-IDF, the :doc:`/api-reference/storage/vfs` layer is used to implement this function.
When the file descriptor is a socket, only the following ``ioctl()`` values are supported:
The Netconn API is used to implement the BSD Sockets API inside lwIP, and it can also be called directly from ESP-IDF apps. This API has lower resource usage than the BSD Sockets API. In particular, it can send and receive data without firstly copying it into internal lwIP buffers.
Espressif does not test the Netconn API in ESP-IDF. As such, this functionality is **enabled but not supported**. Some functionality may only work correctly when used from the BSD Sockets API.
For more information about the Netconn API, consult `lwip/lwip/src/include/lwip/api.h <http://www.nongnu.org/lwip/2_0_x/api_8h.html>`_ and `part of the **unofficial** lwIP Application Developers Manual <https://lwip.fandom.com/wiki/Netconn_API>`_.
Both IPv4 and IPv6 are supported in a dual-stack configuration and are enabled by default. Both IPv6 and IPv4 may be disabled separately if they are not needed, see :ref:`lwip-ram-usage`.
IPv6 support is limited to **Stateless Autoconfiguration** only. **Stateful configuration** is not supported in ESP-IDF, nor in upstream lwIP.
This configuration option enables IPv6 autoconfiguration for all network interfaces, which differs from the upstream lwIP behavior, where the autoconfiguration needs to be explicitly enabled for each ``netif`` with ``netif->ip6_autoconfig_enabled=1``.
Since the DHCPv6 works only in its stateless configuration, the :ref:`lwip-ivp6-autoconfig` has to be enabled as well via :ref:`CONFIG_LWIP_IPV6_AUTOCONFIG`.
Moreover, the DHCPv6 needs to be explicitly enabled from the application code using:
- DHCPv6 stateless configuration, uses :ref:`lwip-ivp6-dhcp6` to configure DNS servers. Note that this configuration assumes IPv6 Router Advertisement Flags (RFC-5175) to be set to
It is possible to ``close()`` a socket from a different thread than the one that created it. The ``close()`` call blocks, until any function calls currently using that socket from other tasks have returned.
It is, however, not possible to delete a task while it is actively waiting on ``select()`` or ``poll()`` APIs. It is always necessary that these APIs exit before destroying the task, as this might corrupt internal structures and cause subsequent crashes of the lwIP. These APIs allocate globally referenced callback pointers on the stack so that when the task gets destroyed before unrolling the stack, the lwIP could still hold pointers to the deleted stack.
The default lwIP implementation is to have these timers enabled all the time, even if no timeout events are active. This increases CPU usage and power consumption when using automatic Light-sleep mode. ``ESP-lwIP`` default behavior is to set each timer ``on demand``, so it is only enabled when an event is pending.
The original lwIP supports implementing custom compile-time modifications via ``LWIP_HOOK_FILENAME``. This file is already used by the ESP-IDF port layer, but ESP-IDF users could still include and implement any custom additions via a header file defined by the macro ``ESP_IDF_LWIP_HOOK_FILENAME``. Here is an example of adding a custom hook file to the build process, and the hook is called ``my_hook.h``, located in the project's ``main`` folder:
Calling ``send()`` or ``sendto()`` repeatedly on a UDP socket may eventually fail with ``errno`` equal to ``ENOMEM``. This failure occurs due to the limitations of buffer sizes in the lower-layer network interface drivers. If all driver transmit buffers are full, the UDP transmission will fail. For applications that transmit a high volume of UDP datagrams and aim to avoid any dropped datagrams by the sender, it is advisable to implement error code checking and employ a retransmission mechanism with a short delay.
Increasing the number of TX buffers in the :ref:`Wi-Fi <CONFIG_ESP_WIFI_TX_BUFFER>` or :ref:`Ethernet <CONFIG_ETH_DMA_TX_BUFFER_NUM>` project configuration as applicable may also help.
TCP/IP performance is a complex subject, and performance can be optimized toward multiple goals. The default settings of ESP-IDF are tuned for a compromise between throughput, latency, and moderate memory usage.
Espressif tests ESP-IDF TCP/IP throughput using the :example:`wifi/iperf` example in an RF-sealed enclosure.
The :example_file:`wifi/iperf/sdkconfig.defaults` file for the iperf example contains settings known to maximize TCP/IP throughput, usually at the expense of higher RAM usage. To get maximum TCP/IP throughput in an application at the expense of other factors, it is suggested to apply settings from this file into the project sdkconfig.
- If a lot of tasks are competing for CPU time on the system, consider that the lwIP task has configurable CPU affinity (:ref:`CONFIG_LWIP_TCPIP_TASK_AFFINITY`) and runs at fixed priority (18, ``ESP_TASK_TCPIP_PRIO``). To optimize CPU utilization, consider assigning competing tasks to different cores or adjusting their priorities to lower values. For additional details on built-in task priorities, please refer to :ref:`built-in-task-priorities`.
- If there is enough free IRAM, select :ref:`CONFIG_LWIP_IRAM_OPTIMIZATION` and :ref:`CONFIG_LWIP_EXTRA_IRAM_OPTIMIZATION` to improve TX/RX throughput.
Except for increasing buffer sizes, most changes that increase throughput also decrease latency by reducing the amount of CPU time spent in lwIP functions.
Most lwIP RAM usage is on-demand, as RAM is allocated from the heap as needed. Therefore, changing lwIP settings to reduce RAM usage may not change RAM usage at idle, but can change it at peak.
- Reducing :ref:`CONFIG_LWIP_MAX_SOCKETS` reduces the maximum number of sockets in the system. This also causes TCP sockets in the ``WAIT_CLOSE`` state to be closed and recycled more rapidly when needed to open a new socket, further reducing peak RAM usage.
- Reducing :ref:`CONFIG_LWIP_TCPIP_RECVMBOX_SIZE`, :ref:`CONFIG_LWIP_TCP_RECVMBOX_SIZE` and :ref:`CONFIG_LWIP_UDP_RECVMBOX_SIZE` reduce RAM usage at the expense of throughput, depending on usage.
- Reducing :ref:`CONFIG_LWIP_TCP_MSL` and :ref:`CONFIG_LWIP_TCP_FIN_WAIT_TIMEOUT` reduces the maximum segment lifetime in the system. This also causes TCP sockets in the ``TIME_WAIT`` and ``FIN_WAIT_2`` states to be closed and recycled more rapidly.
- Disabling :ref:`CONFIG_LWIP_IPV6` can save about 39 KB for firmware size and 2 KB RAM when the system is powered up and 7 KB RAM when the TCP/IP stack is running. If there is no requirement for supporting IPV6, it can be disabled to save flash and RAM footprint.
- Disabling :ref:`CONFIG_LWIP_IPV4` can save about 26 KB of firmware size and 600 B RAM on power up and 6 KB RAM when the TCP/IP stack is running. If the local network supports IPv6-only configuration, IPv4 can be disabled to save flash and RAM footprint.
The peak heap memory that lwIP consumes is the **theoretically-maximum memory** that the lwIP driver consumes. Generally, the peak heap memory that lwIP consumes depends on:
Some TCP-based applications need only one TCP connection. However, they may choose to close this TCP connection and create a new one when an error occurs (e.g., a sending failure). This may result in multiple TCP connections existing in the system simultaneously, because it may take a long time for a TCP connection to close, according to the TCP state machine, refer to RFC793.
