Development of an application running Internet communication based on a STM32F4 Discovery kit.
The exact processor version mounted on the kit is a STM32F407VG. This version comes packed in a 100 pins LQFP. It holds an ARM Cortex-M4F core with 1 MB of flash and 192 KB of RAM. My current kit's are marked with a sticker that says C-01 and I assume that this indicates revision C.1.
The kit described above does not have all the hardware needed for Ethernet communication. An additional expansion board called DM-STF4BB (sometimes called STM32F4DIS-BB) is used to add missing functionality.
It is possible to develop C and C++ code for STM32F4 by the use of GNU ARM Eclipse. This environment is used for this project.
Toolchain version used is gcc-arm-none-eabi-4_9-2015q3. It was chosen to be installed at the location:
- C:\Program Files (x86)\GNU Tools ARM Embedded\4.9 2015q3
The company that develops the processor on the Discovery kit provides a software platform with HAL drivers, middle-wares and examples. This platform goes under the name STM32CubeF4. Version 1.10.0 was downloaded for this project and it was then unpacked to my local drive at:
- D:\Program\STMicroelectronics\STM32Cube\STM32CubeF4
A network socket is an endpoint of an inter-process communication across a computer network. Today, most communication between computers is based on the Internet Protocol; therefore most network sockets are Internet sockets.
A socket API is an application programming interface (API), usually provided by the operating system, that allows application programs to control and use network sockets. Internet socket API's are usually based on the Berkeley sockets standard.
A socket address is the combination of an IP address and a port number, much like one end of a telephone connection is the combination of a phone number and a particular extension. Based on this address, internet sockets deliver incoming data packets to the appropriate application process or thread.
An example of how to setup a socket with LwIP:
/* Allocate a new socket setup to run TCP
* - AF_INET: address format is host and port number
* - SOCK_STREAM: connection-based protocol
* - IPPROTO_IP: will combined with previous settings result in a TCP socket */
g_socketHandle = socket(AF_INET, SOCK_STREAM, IPPROTO_IP);A socket can be setup to be either blocking or non-blocking.
A socket set to blocking will not take control of the system and not release it until some event happens. For example when you call receive to read from a stream, control isn't returned to your program until at least one byte of data is read from the remote site. This process of waiting for data to appear is referred to as "blocking".
A socket placed in non-blocking mode, means that we are free to do other things without having to stop and wait for an operation to complete.
Zero-copy describes computer operations in which the CPU does not perform the task of copying data from one memory area to another. This is frequently used to save CPU cycles and memory bandwidth when transmitting a file over a network.
The architecture of LwIP is based on the architecture of the TCP/IP model which specifies format, transmitting and routing of data between two end points.
The model is based on four abstraction layers. From lowest to highest the layers are:
- Link layer
- Internet layer (IP)
- Transport layer
- Application layer
The link layer is the group of methods and communications protocols that only operate on the link that a host is physically connected to. The link is the physical and logical network component used to interconnect hosts or nodes in the network and a link protocol is a suite of methods and standards that operate only between adjacent network nodes of a local area network segment or a wide area network connection.
The link layer is not really part of the LwIP but there is layer called Network interface layer in the file netif.c. This layer functions as an interface to the link layer. The actual link layer is provided by external drivers that are hardware specific.
The internet layer is a group of internetworking methods, protocols, and specifications in the Internet protocol suite that are used to transport datagrams (packets) from the originating host across network boundaries, if necessary, to the destination host specified by a network address (IP address) which is defined for this purpose by the Internet Protocol. The internet layer derives its name from its function of forming an internet (uncapitalized), or facilitating internetworking, which is the concept of connecting multiple networks with each other through gateways.
Internet-layer protocols use IP-based packets. The internet layer does not include the protocols that define communication between local (on-link) network nodes which fulfill the purpose of maintaining link states between the local nodes, such as the local network topology, and that usually use protocols that are based on the framing of packets specific to the link types. Such protocols belong to the link layer.
The internet layer is in LwIP implemented in the file named ip.c.
The transport layer implements the protocols that provide host-to-host communication services for applications. It provides services such as connection-oriented data stream support, reliability, flow control, and multiplexing.
The transport layer is in LwIP implemented in the files named udp.c and tcp.c.
The application layer contains all protocols for specific data communications services on a process-to-process level.
The application layer is in LwIP implemented in a number of different files, for example dhcp.c, dns.c.
LwIP offers three different API's designed for different purposes:
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Raw API is the core API of LwIP. This API aims at providing the best performances while using a minimal code size. Events are handled asynchronously by the use of callbacks. When using the Raw API in a mufti-threaded environment, beware that there is no protection against concurrent access. Consequently, all the network part of the application must remain in a single thread context.
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Netconn API is a sequential API built on top of the Raw API. It allows mufti-threaded operation and therefore requires an operating system. It is easier to use than the Raw API at the expense of lower performances and increased memory footprint.
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BSD Socket API is a Berkeley like Socket implementation (Posix/BSD) built on top of the Netconn API. Its interest is portability. It shares the same drawback than the Netconn API.
All LwIP based projects contain a configuration file named lwipopts.h. Any missing option from this configuration file can be imported (copy/paste) from a file called opt.h.