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diff --git a/core/lwip/doc/rawapi.txt b/core/lwip/doc/rawapi.txt new file mode 100644 index 00000000..c727da99 --- /dev/null +++ b/core/lwip/doc/rawapi.txt @@ -0,0 +1,505 @@ +Raw TCP/IP interface for lwIP + +Authors: Adam Dunkels, Leon Woestenberg, Christiaan Simons + +lwIP provides three 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. +* BSD-style socket 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 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 (e.g. unix / windows etc.). However, due to limitations +in the specification of this API, there might be incompatibilities +that require small modifications of existing programs. + +** Threading + +lwIP started targeting single-threaded environments. When adding multi- +threading support, instead of making the core thread-safe, another +approach was chosen: there is one main thread running the lwIP core +(also known as the "tcpip_thread"). The raw API may only be used from +this thread! Application threads using the sequential- or socket API +communicate with this main thread through message passing. + + As such, the list of functions that may be called from + other threads or an ISR is very limited! Only functions + from these API header files are thread-safe: + - api.h + - netbuf.h + - netdb.h + - netifapi.h + - sockets.h + - sys.h + + Additionaly, memory (de-)allocation functions may be + called from multiple threads (not ISR!) with NO_SYS=0 + since they are protected by SYS_LIGHTWEIGHT_PROT and/or + semaphores. + + Only since 1.3.0, if SYS_LIGHTWEIGHT_PROT is set to 1 + and LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT is set to 1, + pbuf_free() may also be called from another thread or + an ISR (since only then, mem_free - for PBUF_RAM - may + be called from an ISR: otherwise, the HEAP is only + protected by semaphores). + + +** 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. + +- struct tcp_pcb *tcp_listen_with_backlog(struct tcp_pcb *pcb, u8_t backlog) + + Same as tcp_listen, but limits the number of outstanding connections + in the listen queue to the value specified by the backlog argument. + To use it, your need to set TCP_LISTEN_BACKLOG=1 in your lwipopts.h. + +- void tcp_accepted(struct tcp_pcb *pcb) + + Inform lwIP that an incoming connection has been accepted. This would + usually be called from the accept callback. This allows lwIP to perform + housekeeping tasks, such as allowing further incoming connections to be + queued in the listen backlog. + +- 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 "err" + callback function of this pcb (registered with tcp_err, see below) + will be called. + + 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. If + there are no errors and the callback function is to return + ERR_OK, then it must free the pbuf. Otherwise, it must not + free the pbuf so that lwIP core code can store it. + +- void tcp_recved(struct tcp_pcb *pcb, u16_t len) + + Must be called when the application has received the data. The len + argument indicates the length of the received data. + + +--- Application polling + +When a connection is idle (i.e., no data is either transmitted or +received), lwIP will repeatedly poll the application by calling a +specified callback function. This can be used either as a watchdog +timer for killing connections that have stayed idle for too long, or +as a method of waiting for memory to become available. For instance, +if a call to tcp_write() has failed because memory wasn't available, +the application may use the polling functionality to call tcp_write() +again when the connection has been idle for a while. + +- void tcp_poll(struct tcp_pcb *pcb, + err_t (* poll)(void *arg, struct tcp_pcb *tpcb), + u8_t interval) + + Specifies the polling interval and the callback function that should + be called to poll the application. The interval is specified in + number of TCP coarse grained timer shots, which typically occurs + twice a second. An interval of 10 means that the application would + be polled every 5 seconds. + + +--- Closing and aborting connections + +- err_t tcp_close(struct tcp_pcb *pcb) + + Closes the connection. The function may return ERR_MEM if no memory + was available for closing the connection. If so, the application + should wait and try again either by using the acknowledgment + callback or the polling functionality. If the close succeeds, the + function returns ERR_OK. + + The pcb is deallocated by the TCP code after a call to tcp_close(). + +- void tcp_abort(struct tcp_pcb *pcb) + + Aborts the connection by sending a RST (reset) segment to the remote + host. The pcb is deallocated. This function never fails. + + ATTENTION: When calling this from one of the TCP callbacks, make + sure you always return ERR_ABRT (and never return ERR_ABRT otherwise + or you will risk accessing deallocated memory or memory leaks! + + +If a connection is aborted because of an error, the application is +alerted of this event by the err callback. Errors that might abort a +connection are when there is a shortage of memory. The callback +function to be called is set using the tcp_err() function. + +- void tcp_err(struct tcp_pcb *pcb, void (* err)(void *arg, + err_t err)) + + The error callback function does not get the pcb passed to it as a + parameter since the pcb may already have been deallocated. + + +--- Lower layer TCP interface + +TCP provides a simple interface to the lower layers of the +system. During system initialization, the function tcp_init() has +to be called before any other TCP function is called. When the system +is running, the two timer functions tcp_fasttmr() and tcp_slowtmr() +must be called with regular intervals. The tcp_fasttmr() should be +called every TCP_FAST_INTERVAL milliseconds (defined in tcp.h) and +tcp_slowtmr() should be called every TCP_SLOW_INTERVAL milliseconds. + + +--- UDP interface + +The UDP interface is similar to that of TCP, but due to the lower +level of complexity of UDP, the interface is significantly simpler. + +- struct udp_pcb *udp_new(void) + + Creates a new UDP pcb which can be used for UDP communication. The + pcb is not active until it has either been bound to a local address + or connected to a remote address. + +- void udp_remove(struct udp_pcb *pcb) + + Removes and deallocates the pcb. + +- err_t udp_bind(struct udp_pcb *pcb, struct ip_addr *ipaddr, + u16_t port) + + Binds the pcb to a local address. The IP-address argument "ipaddr" + can be IP_ADDR_ANY to indicate that it should listen to any local IP + address. The function currently always return ERR_OK. + +- err_t udp_connect(struct udp_pcb *pcb, struct ip_addr *ipaddr, + u16_t port) + + Sets the remote end of the pcb. This function does not generate any + network traffic, but only set the remote address of the pcb. + +- err_t udp_disconnect(struct udp_pcb *pcb) + + Remove the remote end of the pcb. This function does not generate + any network traffic, but only removes the remote address of the pcb. + +- err_t udp_send(struct udp_pcb *pcb, struct pbuf *p) + + Sends the pbuf p. The pbuf is not deallocated. + +- void udp_recv(struct udp_pcb *pcb, + void (* recv)(void *arg, struct udp_pcb *upcb, + struct pbuf *p, + struct ip_addr *addr, + u16_t port), + void *recv_arg) + + Specifies a callback function that should be called when a UDP + datagram is received. + + +--- System initalization + +A truly complete and generic sequence for initializing the lwip stack +cannot be given because it depends on the build configuration (lwipopts.h) +and additional initializations for your runtime environment (e.g. timers). + +We can give you some idea on how to proceed when using the raw API. +We assume a configuration using a single Ethernet netif and the +UDP and TCP transport layers, IPv4 and the DHCP client. + +Call these functions in the order of appearance: + +- stats_init() + + Clears the structure where runtime statistics are gathered. + +- sys_init() + + Not of much use since we set the NO_SYS 1 option in lwipopts.h, + to be called for easy configuration changes. + +- lwip_mem_init() + + Initializes the dynamic memory heap defined by MEM_SIZE. + +- memp_init() + + Initializes the memory pools defined by MEMP_NUM_x. + +- pbuf_init() + + Initializes the pbuf memory pool defined by PBUF_POOL_SIZE. + +- etharp_init() + + Initializes the ARP table and queue. + Note: you must call etharp_tmr at a ARP_TMR_INTERVAL (5 seconds) regular interval + after this initialization. + +- ip_init() + + Doesn't do much, it should be called to handle future changes. + +- udp_init() + + Clears the UDP PCB list. + +- tcp_init() + + Clears the TCP PCB list and clears some internal TCP timers. + Note: you must call tcp_fasttmr() and tcp_slowtmr() at the + predefined regular intervals after this initialization. + +- netif_add(struct netif *netif, struct ip_addr *ipaddr, + struct ip_addr *netmask, struct ip_addr *gw, + void *state, err_t (* init)(struct netif *netif), + err_t (* input)(struct pbuf *p, struct netif *netif)) + + Adds your network interface to the netif_list. Allocate a struct + netif and pass a pointer to this structure as the first argument. + Give pointers to cleared ip_addr structures when using DHCP, + or fill them with sane numbers otherwise. The state pointer may be NULL. + + The init function pointer must point to a initialization function for + your ethernet netif interface. The following code illustrates it's use. + + err_t netif_if_init(struct netif *netif) + { + u8_t i; + + for(i = 0; i < ETHARP_HWADDR_LEN; i++) netif->hwaddr[i] = some_eth_addr[i]; + init_my_eth_device(); + return ERR_OK; + } + + For ethernet drivers, the input function pointer must point to the lwip + function ethernet_input() declared in "netif/etharp.h". Other drivers + must use ip_input() declared in "lwip/ip.h". + +- netif_set_default(struct netif *netif) + + Registers the default network interface. + +- netif_set_up(struct netif *netif) + + When the netif is fully configured this function must be called. + +- dhcp_start(struct netif *netif) + + Creates a new DHCP client for this interface on the first call. + Note: you must call dhcp_fine_tmr() and dhcp_coarse_tmr() at + the predefined regular intervals after starting the client. + + You can peek in the netif->dhcp struct for the actual DHCP status. + + +--- Optimalization hints + +The first thing you want to optimize is the lwip_standard_checksum() +routine from src/core/inet.c. You can override this standard +function with the #define LWIP_CHKSUM <your_checksum_routine>. + +There are C examples given in inet.c or you might want to +craft an assembly function for this. RFC1071 is a good +introduction to this subject. + +Other significant improvements can be made by supplying +assembly or inline replacements for htons() and htonl() +if you're using a little-endian architecture. +#define LWIP_PLATFORM_BYTESWAP 1 +#define LWIP_PLATFORM_HTONS(x) <your_htons> +#define LWIP_PLATFORM_HTONL(x) <your_htonl> + +Check your network interface driver if it reads at +a higher speed than the maximum wire-speed. If the +hardware isn't serviced frequently and fast enough +buffer overflows are likely to occur. + +E.g. when using the cs8900 driver, call cs8900if_service(ethif) +as frequently as possible. When using an RTOS let the cs8900 interrupt +wake a high priority task that services your driver using a binary +semaphore or event flag. Some drivers might allow additional tuning +to match your application and network. + +For a production release it is recommended to set LWIP_STATS to 0. +Note that speed performance isn't influenced much by simply setting +high values to the memory options. + +For more optimization hints take a look at the lwIP wiki. + +--- Zero-copy MACs + +To achieve zero-copy on transmit, the data passed to the raw API must +remain unchanged until sent. Because the send- (or write-)functions return +when the packets have been enqueued for sending, data must be kept stable +after that, too. + +This implies that PBUF_RAM/PBUF_POOL pbufs passed to raw-API send functions +must *not* be reused by the application unless their ref-count is 1. + +For no-copy pbufs (PBUF_ROM/PBUF_REF), data must be kept unchanged, too, +but the stack/driver will/must copy PBUF_REF'ed data when enqueueing, while +PBUF_ROM-pbufs are just enqueued (as ROM-data is expected to never change). + +Also, data passed to tcp_write without the copy-flag must not be changed! + +Therefore, be careful which type of PBUF you use and if you copy TCP data +or not! |