LwIP is a small independent implementations of the TCP/IP protocol suite that has been initially developed by Adam Dunkels . The focus of the lwIP TCP/IP implementation is to reduce resource usage while still having a full scale TCP. This makes lwIP suitable for use in embedded systems with tens of kilobytes of free RAM and room for around 40 kilobytes of code ROM.
Currently LwIP 2.1.2 is ported to Kinetis and I.MX RT families in MCUXpresso SDK 2.6 .
Features:
LwIP comes with the following protocols:
IPv4 and IPv6, including packet forwarding over multiple network interfaces.
ICMP for network maintenance and debugging
IGMP for multicast traffic management
MLD (Multicast listener discovery for IPv6).
ND (Neighbor discovery and stateless address autoconfiguration for IPv6)
DHCP and DHCPv6.
UDP (User Datagram Protocol)
TCP (Transmission Control Protocol)
Raw/native API for enhanced performance
Optional Berkeley - like socket API.
TLS: optional layered TCP for nearly transparent TLS for any TCP-based protocol
PPPoS and PPPoE
DNS (Domain name resolver incl. mDNS)
6LoWPAN
Organization of the LwIP stack
Let’s take SDK_2.6.1_EVKB-IMXRT1050 for example,
LwIP code in under SDK_2.6.1_EVKB-IMXRT1050\middleware\lwip
Port: This folder includes the adapter code which can make the LwIP stack run on the bare metal and FreeRTOS.
Enet_ethernetif.c/h: Adapt the LwIP stack to the underlying MCUXpresso SDK ethernet driver, provides ethernet interfaces , like phy init and ethernetif_input.
cc.c/h: provide typedefs, compiler hints for packing and platform specific diagnostic output.
Perf.h: architecture specific performance measurement, current the definitions is NULL, reserve for future use.
Src: This folder includes the latest stable LwIP 2.1.2 source code which can be downloaded from below link download.savannah.gnu.org/releases/lwip/
--api: netconn and socket API files
--core: LwIP core files
--include: LwIP include files
--netif: network interface files
LwIP APIs overview
LwIP provides three application programming interfaces (API) for programmers to use for communication with the TCP/IP code:
Raw APIs: Non thread-safe APIs, callback style for maximum performance and minimum memory footprint. Program execution is driven by callbacks functions, which are then invoked by the lwIP core when activity related to that application occurs. A particular application may register to be notified via a callback function for events such as incoming data available, outgoing data sent, error notifications, poll timer expiration, connection closed, etc. An application can provide a callback function to perform processing for any or all of these events.
Sequential-style APIs: blocking functions. More overhead, but can be called from any thread except TCPIP thread. The sequential API provides a way for ordinary, sequential, programs to use the lwIP stack. It is quite similar to the BSD socket API. The model of execution is based on the blocking open-read-write-close paradigm. Since the TCP/IP stack is event based by nature, the TCP/IP code and the application program must reside in different execution contexts (threads).
Socket API: BSD-style socket API. The socket API is a compatibility API for existing applications, currently it is built on top of the sequential API. It is meant to provide all functions needed to run socket API applications running on other platforms.
TCP Raw API
|
| API function | Description |
| TCP connection setup
| Creates a new TCP PCB | |
| Binds a TCP PCB to a local IP address and port | ||
| Starts the listening process on the TCP PCB. | ||
| Assigns a callback function that will be called when a new TCP connection arrives. | ||
| Used to connect to a remote TCP host. | ||
| Sending TCP data | Queues up data to be sent. | |
| Forces queued data to be sent | ||
| Assigns a callback function that will be called when sent data is acknowledged by the remote host. | ||
| Receiving TCP data | Sets the callback function that will be called when new data arrives. | |
| Must be called when the application has processed the incoming data packet (for TCP window management). | ||
| Application polling | Assigns a callback functions that will be called periodically. It can be used by the application to check if there is remaining application data that needs to be sent or if there are connections that need to be closed. | |
| Closing and aborting connections | Closes a TCP connection with a remote host. | |
| Aborts a TCP connection. | ||
| Assigns a callback function for handling connections aborted by the lwIP due to errors (such as memory shortage errors). |
UDP Raw API
| API | Description |
| udp_bind | Binds a UDP PCB with a local IP address and port. |
| udp_new | Creates a new UDP PCB. |
| udp_send | Sends UDP data. |
| udp_recv | Specifies a callback function which is called when a datagram is received |
| udp_remove | Removes and de-allocates a UDP PCB. |
| udp_connect | Sets up a UDP PCB remote IP address and port |
| udp_disconnect | Removes a UDP PCB remote IP and port. |
Netconn API
| Creates a new connection | |
| Sets a TCP connection into a listening mode | |
| Sends data on a connected TCP netconn | |
| Accepts an incoming connection on a listening TCP connection | |
| Closes a TCP connection without deleting it. | |
| Sends data to the currently connected remote IP/port (not applicable for TCP connections). | |
| Binds a connection to a local IP address and port. | |
| Receives data from a netconn. | |
| Connects to a remote IP address and port. | |
| … |
|
Socket API
| Macro | API | description |
| accept | lwip_accept | Accepts a new connection on a socket |
| lwip_bind | Binds a socket to an IP address and port | |
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| ||
| lwip_getpeername |
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| lwip_getsockname |
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| lwip_setsockopt |
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| lwip_close |
| |
| lwip_connect | Connects a socket to a remote host IP address and port. | |
| Listens for socket connections | ||
| lwip_recv |
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| lwip_recvmsg |
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| lwip_recvfrom |
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| lwip_send |
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| lwip_sendmsg |
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| lwip_sendto |
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| lwip_socket | Creates an new socket. | |
| lwip_poll |
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| lwip_ioctl |
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| lwip_inet_ntop |
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| lwip_inet_pton |
| |
| lwip_read | Reads data from a socket | |
| lwip_readv |
| |
| lwip_write | Writes data on a socket | |
| lwip_writev |
| |
| lwip_close | Closes a socket (socket is deleted). | |
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| ||
| lwip_ioctl |
| |
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LwIP Configure
Configuration file is placed in application source folder: Lwipopts.h:
Lwipopts.h is a user file that you can use to fully configure LwIP and all of it’s modules. You do not need to define every option that LwIP provides; if you do not define an option, a default value will be used. Therefore, your lwipopts.h provides a way to override much of the behavior of LwIP.
LwIP Demos in MCUXpresso SDK
MCUXpresso SDK comes with lots of demos that use different API sets.
| Name | Description |
| Lwip_tcpecho | a TCP echo demo on the lwIP TCP/IP stack with bare metal KSDK or FreeRTOS. The demo uses the TCP protocol and acts as an echo server |
| Lwip_udpecho | a UDP echo demo on the lwIP TCP/IP stack with bare metal KSDK or FreeRTOS, which uses the UDP protocol and acts as an echo server. |
| Lwip_ping | a Ping Demo on the lwIP TCP/IP stack which using the ICMP protocol |
| lwip_nghttp2cli_mbedTLS | an NGHTTP2 client set up on lwIP TCP/IP and the MbedTLS stack with FreeRTOS |
| lwip_mqtt | MQTT client connecting to MQTT broker via unsecured socket |
| lwip_iperf | check your bandwidth using the network performance measurement IPerf application on a PC as a client or a server |
| lwip_httpsrv | an HTTPServer on the lwIP TCP/IP stack with bare metal SDK or FreeRTOS |
| lwip_httpscli_mbedTLS | an HTTPS client set up on lwIP TCP/IP and the MbedTLS stack with FreeRTOS |
| lwip_dhcp_usb | a DHCP and ping demo on the lwIP TCP/IP and USB stack |
| lwip_dhcp | a DHCP client and prints the status as it is progressing |
