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authorJosh Blum <josh@joshknows.com>2010-12-22 19:19:14 -0800
committerJosh Blum <josh@joshknows.com>2010-12-22 19:19:14 -0800
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-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, u8_t interval,
- err_t (* poll)(void *arg, struct tcp_pcb *tpcb))
-
- 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.
-
-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.
-
-- 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.