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  • 2014-10-30 09:08
Raw TCP/IP interface for lwIP Authors: Adam Dunkels, Leon Woestenberg lwIP provides two Application Program's Interfaces (APIs) for programs to use for communication with the TCP/IP code: * low-level "core" / "callback" or "raw" API. * higher-level "sequential" API. 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). ** The remainder of this document discusses the "raw" API. ** The raw TCP/IP interface allows the application program to integrate better with the TCP/IP code. Program execution is event based by having callback functions being called from within the TCP/IP code. The TCP/IP code and the application program both run in the same thread. The sequential API has a much higher overhead and is not very well suited for small systems since it forces a multithreaded paradigm on the application. The raw TCP/IP interface is not only faster in terms of code execution time but is also less memory intensive. The drawback is that program development is somewhat harder and application programs written for the raw TCP/IP interface are more difficult to understand. Still, this is the preferred way of writing applications that should be small in code size and memory usage. Both APIs can be used simultaneously by different application programs. In fact, the sequential API is implemented as an application program using the raw TCP/IP interface. --- Callbacks Program execution is driven by callbacks. Each callback is an ordinary C function that is called from within the TCP/IP code. Every callback function is passed the current TCP or UDP connection state as an argument. Also, in order to be able to keep program specific state, the callback functions are called with a program specified argument that is independent of the TCP/IP state. The function for setting the application connection state is: - void tcp_arg(struct tcp_pcb *pcb, void *arg) Specifies the program specific state that should be passed to all other callback functions. The "pcb" argument is the current TCP connection control block, and the "arg" argument is the argument that will be passed to the callbacks. --- TCP connection setup The functions used for setting up connections is similar to that of the sequential API and of the BSD socket API. A new TCP connection identifier (i.e., a protocol control block - PCB) is created with the tcp_new() function. This PCB can then be either set to listen for new incoming connections or be explicitly connected to another host. - struct tcp_pcb *tcp_new(void) Creates a new connection identifier (PCB). If memory is not available for creating the new pcb, NULL is returned. - err_t tcp_bind(struct tcp_pcb *pcb, struct ip_addr *ipaddr, u16_t port) Binds the pcb to a local IP address and port number. The IP address can be specified as IP_ADDR_ANY in order to bind the connection to all local IP addresses. If another connection is bound to the same port, the function will return ERR_USE, otherwise ERR_OK is returned. - struct tcp_pcb *tcp_listen(struct tcp_pcb *pcb) Commands a pcb to start listening for incoming connections. When an incoming connection is accepted, the function specified with the tcp_accept() function will be called. The pcb will have to be bound to a local port with the tcp_bind() function. The tcp_listen() function returns a new connection identifier, and the one passed as an argument to the function will be deallocated. The reason for this behavior is that less memory is needed for a connection that is listening, so tcp_listen() will reclaim the memory needed for the original connection and allocate a new smaller memory block for the listening connection. tcp_listen() may return NULL if no memory was available for the listening connection. If so, the memory associated with the pcb passed as an argument to tcp_listen() will not be deallocated. - void tcp_accept(struct tcp_pcb *pcb, err_t (* accept)(void *arg, struct tcp_pcb *newpcb, err_t err)) Specified the callback function that should be called when a new connection arrives on a listening connection. - err_t tcp_connect(struct tcp_pcb *pcb, struct ip_addr *ipaddr, u16_t port, err_t (* connected)(void *arg, struct tcp_pcb *tpcb, err_t err)); Sets up the pcb to connect to the remote host and sends the initial SYN segment which opens the connection. The tcp_connect() function returns immediately; it does not wait for the connection to be properly setup. Instead, it will call the function specified as the fourth argument (the "connected" argument) when the connection is established. If the connection could not be properly established, either because the other host refused the connection or because the other host didn't answer, the "connected" function will be called with an the "err" argument set accordingly. The tcp_connect() function can return ERR_MEM if no memory is available for enqueueing the SYN segment. If the SYN indeed was enqueued successfully, the tcp_connect() function returns ERR_OK. --- Sending TCP data TCP data is sent by enqueueing the data with a call to tcp_write(). When the data is successfully transmitted to the remote host, the application will be notified with a call to a specified callback function. - err_t tcp_write(struct tcp_pcb *pcb, void *dataptr, u16_t len, u8_t copy) Enqueues the data pointed to by the argument dataptr. The length of the data is passed as the len parameter. The copy argument is either 0 or 1 and indicates whether the new memory should be allocated for the data to be copied into. If the argument is 0, no new memory should be allocated and the data should only be referenced by pointer. The tcp_write() function will fail and return ERR_MEM if the length of the data exceeds the current send buffer size or if the length of the queue of outgoing segment is larger than the upper limit defined in lwipopts.h. The number of bytes available in the output queue can be retrieved with the tcp_sndbuf() function. The proper way to use this function is to call the function with at most tcp_sndbuf() bytes of data. If the function returns ERR_MEM, the application should wait until some of the currently enqueued data has been successfully received by the other host and try again. - void tcp_sent(struct tcp_pcb *pcb, err_t (* sent)(void *arg, struct tcp_pcb *tpcb, u16_t len)) Specifies the callback function that should be called when data has successfully been received (i.e., acknowledged) by the remote host. The len argument passed to the callback function gives the amount bytes that was acknowledged by the last acknowledgment. --- Receiving TCP data TCP data reception is callback based - an application specified callback function is called when new data arrives. When the application has taken the data, it has to call the tcp_recved() function to indicate that TCP can advertise increase the receive window. - void tcp_recv(struct tcp_pcb *pcb, err_t (* recv)(void *arg, struct tcp_pcb *tpcb, struct pbuf *p, err_t err)) Sets the callback function that will be called when new data arrives. The callback function will be passed a NULL pbuf to indicate that the remote host has closed the connection. - void tcp_recved(struct tcp_pcb *pcb, u16_t len) Must be called when the application has received the data. Th