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1 office 1 Raw TCP/IP interface for lwIP
2  
3 Authors: Adam Dunkels, Leon Woestenberg, Christiaan Simons
4  
5 lwIP provides three Application Program's Interfaces (APIs) for programs
6 to use for communication with the TCP/IP code:
7 * low-level "core" / "callback" or "raw" API.
8 * higher-level "sequential" API.
9 * BSD-style socket API.
10  
11 The raw API (sometimes called native API) is an event-driven API designed
12 to be used without an operating system that implements zero-copy send and
13 receive. This API is also used by the core stack for interaction between
14 the various protocols. It is the only API available when running lwIP
15 without an operating system.
16  
17 The sequential API provides a way for ordinary, sequential, programs
18 to use the lwIP stack. It is quite similar to the BSD socket API. The
19 model of execution is based on the blocking open-read-write-close
20 paradigm. Since the TCP/IP stack is event based by nature, the TCP/IP
21 code and the application program must reside in different execution
22 contexts (threads).
23  
24 The socket API is a compatibility API for existing applications,
25 currently it is built on top of the sequential API. It is meant to
26 provide all functions needed to run socket API applications running
27 on other platforms (e.g. unix / windows etc.). However, due to limitations
28 in the specification of this API, there might be incompatibilities
29 that require small modifications of existing programs.
30  
31 ** Multithreading
32  
33 lwIP started targeting single-threaded environments. When adding multi-
34 threading support, instead of making the core thread-safe, another
35 approach was chosen: there is one main thread running the lwIP core
36 (also known as the "tcpip_thread"). When running in a multithreaded
37 environment, raw API functions MUST only be called from the core thread
38 since raw API functions are not protected from concurrent access (aside
39 from pbuf- and memory management functions). Application threads using
40 the sequential- or socket API communicate with this main thread through
41 message passing.
42  
43 As such, the list of functions that may be called from
44 other threads or an ISR is very limited! Only functions
45 from these API header files are thread-safe:
46 - api.h
47 - netbuf.h
48 - netdb.h
49 - netifapi.h
50 - pppapi.h
51 - sockets.h
52 - sys.h
53  
54 Additionaly, memory (de-)allocation functions may be
55 called from multiple threads (not ISR!) with NO_SYS=0
56 since they are protected by SYS_LIGHTWEIGHT_PROT and/or
57 semaphores.
58  
59 Netconn or Socket API functions are thread safe against the
60 core thread but they are not reentrant at the control block
61 granularity level. That is, a UDP or TCP control block must
62 not be shared among multiple threads without proper locking.
63  
64 If SYS_LIGHTWEIGHT_PROT is set to 1 and
65 LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT is set to 1,
66 pbuf_free() may also be called from another thread or
67 an ISR (since only then, mem_free - for PBUF_RAM - may
68 be called from an ISR: otherwise, the HEAP is only
69 protected by semaphores).
70  
71  
72 ** The remainder of this document discusses the "raw" API. **
73  
74 The raw TCP/IP interface allows the application program to integrate
75 better with the TCP/IP code. Program execution is event based by
76 having callback functions being called from within the TCP/IP
77 code. The TCP/IP code and the application program both run in the same
78 thread. The sequential API has a much higher overhead and is not very
79 well suited for small systems since it forces a multithreaded paradigm
80 on the application.
81  
82 The raw TCP/IP interface is not only faster in terms of code execution
83 time but is also less memory intensive. The drawback is that program
84 development is somewhat harder and application programs written for
85 the raw TCP/IP interface are more difficult to understand. Still, this
86 is the preferred way of writing applications that should be small in
87 code size and memory usage.
88  
89 All APIs can be used simultaneously by different application
90 programs. In fact, the sequential API is implemented as an application
91 program using the raw TCP/IP interface.
92  
93 Do not confuse the lwIP raw API with raw Ethernet or IP sockets.
94 The former is a way of interfacing the lwIP network stack (including
95 TCP and UDP), the later refers to processing raw Ethernet or IP data
96 instead of TCP connections or UDP packets.
97  
98 Raw API applications may never block since all packet processing
99 (input and output) as well as timer processing (TCP mainly) is done
100 in a single execution context.
101  
102 --- Callbacks
103  
104 Program execution is driven by callbacks functions, which are then
105 invoked by the lwIP core when activity related to that application
106 occurs. A particular application may register to be notified via a
107 callback function for events such as incoming data available, outgoing
108 data sent, error notifications, poll timer expiration, connection
109 closed, etc. An application can provide a callback function to perform
110 processing for any or all of these events. Each callback is an ordinary
111 C function that is called from within the TCP/IP code. Every callback
112 function is passed the current TCP or UDP connection state as an
113 argument. Also, in order to be able to keep program specific state,
114 the callback functions are called with a program specified argument
115 that is independent of the TCP/IP state.
116  
117 The function for setting the application connection state is:
118  
119 - void tcp_arg(struct tcp_pcb *pcb, void *arg)
120  
121 Specifies the program specific state that should be passed to all
122 other callback functions. The "pcb" argument is the current TCP
123 connection control block, and the "arg" argument is the argument
124 that will be passed to the callbacks.
125  
126  
127 --- TCP connection setup
128  
129 The functions used for setting up connections is similar to that of
130 the sequential API and of the BSD socket API. A new TCP connection
131 identifier (i.e., a protocol control block - PCB) is created with the
132 tcp_new() function. This PCB can then be either set to listen for new
133 incoming connections or be explicitly connected to another host.
134  
135 - struct tcp_pcb *tcp_new(void)
136  
137 Creates a new connection identifier (PCB). If memory is not
138 available for creating the new pcb, NULL is returned.
139  
140 - err_t tcp_bind(struct tcp_pcb *pcb, ip_addr_t *ipaddr,
141 u16_t port)
142  
143 Binds the pcb to a local IP address and port number. The IP address
144 can be specified as IP_ADDR_ANY in order to bind the connection to
145 all local IP addresses.
146  
147 If another connection is bound to the same port, the function will
148 return ERR_USE, otherwise ERR_OK is returned.
149  
150 - struct tcp_pcb *tcp_listen(struct tcp_pcb *pcb)
151  
152 Commands a pcb to start listening for incoming connections. When an
153 incoming connection is accepted, the function specified with the
154 tcp_accept() function will be called. The pcb will have to be bound
155 to a local port with the tcp_bind() function.
156  
157 The tcp_listen() function returns a new connection identifier, and
158 the one passed as an argument to the function will be
159 deallocated. The reason for this behavior is that less memory is
160 needed for a connection that is listening, so tcp_listen() will
161 reclaim the memory needed for the original connection and allocate a
162 new smaller memory block for the listening connection.
163  
164 tcp_listen() may return NULL if no memory was available for the
165 listening connection. If so, the memory associated with the pcb
166 passed as an argument to tcp_listen() will not be deallocated.
167  
168 - struct tcp_pcb *tcp_listen_with_backlog(struct tcp_pcb *pcb, u8_t backlog)
169  
170 Same as tcp_listen, but limits the number of outstanding connections
171 in the listen queue to the value specified by the backlog argument.
172 To use it, your need to set TCP_LISTEN_BACKLOG=1 in your lwipopts.h.
173  
174 - void tcp_accept(struct tcp_pcb *pcb,
175 err_t (* accept)(void *arg, struct tcp_pcb *newpcb,
176 err_t err))
177  
178 Specified the callback function that should be called when a new
179 connection arrives on a listening connection.
180  
181 - err_t tcp_connect(struct tcp_pcb *pcb, ip_addr_t *ipaddr,
182 u16_t port, err_t (* connected)(void *arg,
183 struct tcp_pcb *tpcb,
184 err_t err));
185  
186 Sets up the pcb to connect to the remote host and sends the
187 initial SYN segment which opens the connection.
188  
189 The tcp_connect() function returns immediately; it does not wait for
190 the connection to be properly setup. Instead, it will call the
191 function specified as the fourth argument (the "connected" argument)
192 when the connection is established. If the connection could not be
193 properly established, either because the other host refused the
194 connection or because the other host didn't answer, the "err"
195 callback function of this pcb (registered with tcp_err, see below)
196 will be called.
197  
198 The tcp_connect() function can return ERR_MEM if no memory is
199 available for enqueueing the SYN segment. If the SYN indeed was
200 enqueued successfully, the tcp_connect() function returns ERR_OK.
201  
202  
203 --- Sending TCP data
204  
205 TCP data is sent by enqueueing the data with a call to
206 tcp_write(). When the data is successfully transmitted to the remote
207 host, the application will be notified with a call to a specified
208 callback function.
209  
210 - err_t tcp_write(struct tcp_pcb *pcb, const void *dataptr, u16_t len,
211 u8_t apiflags)
212  
213 Enqueues the data pointed to by the argument dataptr. The length of
214 the data is passed as the len parameter. The apiflags can be one or more of:
215 - TCP_WRITE_FLAG_COPY: indicates whether the new memory should be allocated
216 for the data to be copied into. If this flag is not given, no new memory
217 should be allocated and the data should only be referenced by pointer. This
218 also means that the memory behind dataptr must not change until the data is
219 ACKed by the remote host
220 - TCP_WRITE_FLAG_MORE: indicates that more data follows. If this is omitted,
221 the PSH flag is set in the last segment created by this call to tcp_write.
222 If this flag is given, the PSH flag is not set.
223  
224 The tcp_write() function will fail and return ERR_MEM if the length
225 of the data exceeds the current send buffer size or if the length of
226 the queue of outgoing segment is larger than the upper limit defined
227 in lwipopts.h. The number of bytes available in the output queue can
228 be retrieved with the tcp_sndbuf() function.
229  
230 The proper way to use this function is to call the function with at
231 most tcp_sndbuf() bytes of data. If the function returns ERR_MEM,
232 the application should wait until some of the currently enqueued
233 data has been successfully received by the other host and try again.
234  
235 - void tcp_sent(struct tcp_pcb *pcb,
236 err_t (* sent)(void *arg, struct tcp_pcb *tpcb,
237 u16_t len))
238  
239 Specifies the callback function that should be called when data has
240 successfully been received (i.e., acknowledged) by the remote
241 host. The len argument passed to the callback function gives the
242 amount bytes that was acknowledged by the last acknowledgment.
243  
244  
245 --- Receiving TCP data
246  
247 TCP data reception is callback based - an application specified
248 callback function is called when new data arrives. When the
249 application has taken the data, it has to call the tcp_recved()
250 function to indicate that TCP can advertise increase the receive
251 window.
252  
253 - void tcp_recv(struct tcp_pcb *pcb,
254 err_t (* recv)(void *arg, struct tcp_pcb *tpcb,
255 struct pbuf *p, err_t err))
256  
257 Sets the callback function that will be called when new data
258 arrives. The callback function will be passed a NULL pbuf to
259 indicate that the remote host has closed the connection. If
260 there are no errors and the callback function is to return
261 ERR_OK, then it must free the pbuf. Otherwise, it must not
262 free the pbuf so that lwIP core code can store it.
263  
264 - void tcp_recved(struct tcp_pcb *pcb, u16_t len)
265  
266 Must be called when the application has received the data. The len
267 argument indicates the length of the received data.
268  
269  
270 --- Application polling
271  
272 When a connection is idle (i.e., no data is either transmitted or
273 received), lwIP will repeatedly poll the application by calling a
274 specified callback function. This can be used either as a watchdog
275 timer for killing connections that have stayed idle for too long, or
276 as a method of waiting for memory to become available. For instance,
277 if a call to tcp_write() has failed because memory wasn't available,
278 the application may use the polling functionality to call tcp_write()
279 again when the connection has been idle for a while.
280  
281 - void tcp_poll(struct tcp_pcb *pcb,
282 err_t (* poll)(void *arg, struct tcp_pcb *tpcb),
283 u8_t interval)
284  
285 Specifies the polling interval and the callback function that should
286 be called to poll the application. The interval is specified in
287 number of TCP coarse grained timer shots, which typically occurs
288 twice a second. An interval of 10 means that the application would
289 be polled every 5 seconds.
290  
291  
292 --- Closing and aborting connections
293  
294 - err_t tcp_close(struct tcp_pcb *pcb)
295  
296 Closes the connection. The function may return ERR_MEM if no memory
297 was available for closing the connection. If so, the application
298 should wait and try again either by using the acknowledgment
299 callback or the polling functionality. If the close succeeds, the
300 function returns ERR_OK.
301  
302 The pcb is deallocated by the TCP code after a call to tcp_close().
303  
304 - void tcp_abort(struct tcp_pcb *pcb)
305  
306 Aborts the connection by sending a RST (reset) segment to the remote
307 host. The pcb is deallocated. This function never fails.
308  
309 ATTENTION: When calling this from one of the TCP callbacks, make
310 sure you always return ERR_ABRT (and never return ERR_ABRT otherwise
311 or you will risk accessing deallocated memory or memory leaks!
312  
313  
314 If a connection is aborted because of an error, the application is
315 alerted of this event by the err callback. Errors that might abort a
316 connection are when there is a shortage of memory. The callback
317 function to be called is set using the tcp_err() function.
318  
319 - void tcp_err(struct tcp_pcb *pcb, void (* err)(void *arg,
320 err_t err))
321  
322 The error callback function does not get the pcb passed to it as a
323 parameter since the pcb may already have been deallocated.
324  
325  
326 --- UDP interface
327  
328 The UDP interface is similar to that of TCP, but due to the lower
329 level of complexity of UDP, the interface is significantly simpler.
330  
331 - struct udp_pcb *udp_new(void)
332  
333 Creates a new UDP pcb which can be used for UDP communication. The
334 pcb is not active until it has either been bound to a local address
335 or connected to a remote address.
336  
337 - void udp_remove(struct udp_pcb *pcb)
338  
339 Removes and deallocates the pcb.
340  
341 - err_t udp_bind(struct udp_pcb *pcb, ip_addr_t *ipaddr,
342 u16_t port)
343  
344 Binds the pcb to a local address. The IP-address argument "ipaddr"
345 can be IP_ADDR_ANY to indicate that it should listen to any local IP
346 address. The function currently always return ERR_OK.
347  
348 - err_t udp_connect(struct udp_pcb *pcb, ip_addr_t *ipaddr,
349 u16_t port)
350  
351 Sets the remote end of the pcb. This function does not generate any
352 network traffic, but only set the remote address of the pcb.
353  
354 - err_t udp_disconnect(struct udp_pcb *pcb)
355  
356 Remove the remote end of the pcb. This function does not generate
357 any network traffic, but only removes the remote address of the pcb.
358  
359 - err_t udp_send(struct udp_pcb *pcb, struct pbuf *p)
360  
361 Sends the pbuf p. The pbuf is not deallocated.
362  
363 - void udp_recv(struct udp_pcb *pcb,
364 void (* recv)(void *arg, struct udp_pcb *upcb,
365 struct pbuf *p,
366 ip_addr_t *addr,
367 u16_t port),
368 void *recv_arg)
369  
370 Specifies a callback function that should be called when a UDP
371 datagram is received.
372  
373  
374 --- System initalization
375  
376 A truly complete and generic sequence for initializing the lwIP stack
377 cannot be given because it depends on additional initializations for
378 your runtime environment (e.g. timers).
379  
380 We can give you some idea on how to proceed when using the raw API.
381 We assume a configuration using a single Ethernet netif and the
382 UDP and TCP transport layers, IPv4 and the DHCP client.
383  
384 Call these functions in the order of appearance:
385  
386 - lwip_init()
387  
388 Initialize the lwIP stack and all of its subsystems.
389  
390 - netif_add(struct netif *netif, const ip4_addr_t *ipaddr,
391 const ip4_addr_t *netmask, const ip4_addr_t *gw,
392 void *state, netif_init_fn init, netif_input_fn input)
393  
394 Adds your network interface to the netif_list. Allocate a struct
395 netif and pass a pointer to this structure as the first argument.
396 Give pointers to cleared ip_addr structures when using DHCP,
397 or fill them with sane numbers otherwise. The state pointer may be NULL.
398  
399 The init function pointer must point to a initialization function for
400 your Ethernet netif interface. The following code illustrates its use.
401  
402 err_t netif_if_init(struct netif *netif)
403 {
404 u8_t i;
405  
406 for (i = 0; i < ETHARP_HWADDR_LEN; i++) {
407 netif->hwaddr[i] = some_eth_addr[i];
408 }
409 init_my_eth_device();
410 return ERR_OK;
411 }
412  
413 For Ethernet drivers, the input function pointer must point to the lwIP
414 function ethernet_input() declared in "netif/etharp.h". Other drivers
415 must use ip_input() declared in "lwip/ip.h".
416  
417 - netif_set_default(struct netif *netif)
418  
419 Registers the default network interface.
420  
421 - netif_set_link_up(struct netif *netif)
422  
423 This is the hardware link state; e.g. whether cable is plugged for wired
424 Ethernet interface. This function must be called even if you don't know
425 the current state. Having link up and link down events is optional but
426 DHCP and IPv6 discover benefit well from those events.
427  
428 - netif_set_up(struct netif *netif)
429  
430 This is the administrative (= software) state of the netif, when the
431 netif is fully configured this function must be called.
432  
433 - dhcp_start(struct netif *netif)
434  
435 Creates a new DHCP client for this interface on the first call.
436  
437 You can peek in the netif->dhcp struct for the actual DHCP status.
438  
439 - sys_check_timeouts()
440  
441 When the system is running, you have to periodically call
442 sys_check_timeouts() which will handle all timers for all protocols in
443 the stack; add this to your main loop or equivalent.
444  
445  
446 --- Optimization hints
447  
448 The first thing you want to optimize is the lwip_standard_checksum()
449 routine from src/core/inet.c. You can override this standard
450 function with the #define LWIP_CHKSUM <your_checksum_routine>.
451  
452 There are C examples given in inet.c or you might want to
453 craft an assembly function for this. RFC1071 is a good
454 introduction to this subject.
455  
456 Other significant improvements can be made by supplying
457 assembly or inline replacements for htons() and htonl()
458 if you're using a little-endian architecture.
459 #define lwip_htons(x) <your_htons>
460 #define lwip_htonl(x) <your_htonl>
461 If you #define them to htons() and htonl(), you should
462 #define LWIP_DONT_PROVIDE_BYTEORDER_FUNCTIONS to prevent lwIP from
463 defining hton*/ntoh* compatibility macros.
464  
465 Check your network interface driver if it reads at
466 a higher speed than the maximum wire-speed. If the
467 hardware isn't serviced frequently and fast enough
468 buffer overflows are likely to occur.
469  
470 E.g. when using the cs8900 driver, call cs8900if_service(ethif)
471 as frequently as possible. When using an RTOS let the cs8900 interrupt
472 wake a high priority task that services your driver using a binary
473 semaphore or event flag. Some drivers might allow additional tuning
474 to match your application and network.
475  
476 For a production release it is recommended to set LWIP_STATS to 0.
477 Note that speed performance isn't influenced much by simply setting
478 high values to the memory options.
479  
480 For more optimization hints take a look at the lwIP wiki.
481  
482 --- Zero-copy MACs
483  
484 To achieve zero-copy on transmit, the data passed to the raw API must
485 remain unchanged until sent. Because the send- (or write-)functions return
486 when the packets have been enqueued for sending, data must be kept stable
487 after that, too.
488  
489 This implies that *ALL* pbufs passed to send functions must *not* be reused by
490 the application unless the send function returns an error indicating the pbuf
491 is not sent/queued for sending.
492  
493  
494 Also, data passed to tcp_write without the copy-flag must not be changed until
495 sent and ACKed (check the amount of bytes marked as 'sent')!
496  
497 Therefore, be careful which type of PBUF you use and if you copy TCP data
498 or not!