nta.docs 20.4 KB
Newer Older
Pekka Pessi's avatar
Pekka Pessi committed
1 2
/* -*- c -*- */

Pekka Pessi's avatar
Pekka Pessi committed
3
/* REMOVED_RCS_ID */
Pekka Pessi's avatar
Pekka Pessi committed
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543

/**@mainpage Sofia Transaction API for SIP - "nta"
 *
 * @section nta_meta Module Meta Information
 *
 * Sofia SIP Transaction API (nta) provides simple interface to the SIP
 * transaction, transport and message handling. The @b nta interface is
 * intended for both network and user elements. The public interface for 
 * @b nta is mostly defined in <nta.h>, but tag parameters are defined in
 * <nta_tag.h>.
 *
 * Functions and datatypes providing compatibility for old sources are in
 * <nta_compat.h>
 * 
 * @CONTACT Pekka Pessi <Pekka.Pessi@nokia.com>
 *
 * @STATUS Core library
 *
 * @LICENSE LGPL
 *
 * @section nta_objects NTA Objects
 * 
 * The NTA deals with a few kinds of objects: @e agent (#nta_agent_t), @e
 * call @e legs (#nta_leg_t), @e outgoing @e client @e requests
 * (#nta_outgoing_t), and @e incoming @e server @e requests
 * (#nta_incoming_t).
 * 
 * NTA also uses SIP message objects #msg_t and #sip_t for handling
 * messages, as defined in <msg.h> and <sip.h>, respectively. The various
 * SIP headers are also defined in <sip.h>.
 * 
 * @section nta_agent_t Creating an NTA Agent
 * 
 * Most of the SIP entities, like @e user @e agent or @e proxy, consist of a
 * SIP server and a SIP client working together.  The NTA provides a simple
 * interface to SIP server and client with the #nta_agent_t objects.
 * 
 * The #nta_agent_t object is created by calling nta_agent_create(). The
 * object listens for incoming connections, receives messages, parses them,
 * and pass them to the application. It also takes care of sending the
 * messages.
 * 
 * The agent needs a #su_root_t object to schedule its execution. A root
 * object is used to wait for the network events, schedule the timer
 * routines, and pass messages asyncronously. A root object can be created
 * by, e.g., the function su_root_create(). The root object can be have its
 * own thread, or its main loop can be executed by an application thread by
 * calling the function su_root_run(). The main loop can be terminated by
 * calling the function su_root_break().
 * 
 * A simple agent could be created as follows:
 * @code
 *  registrar->reg_root = su_root_create(NULL);
 * 
 *  if (registrar->reg_root) {
 *    registrar->reg_agent = nta_agent_create(registrar->reg_root,
 *                                            (url_string_t*)argv[1],
 *                                            NULL,
 *                                            NULL,
 *                                            NULL);
 * 
 *    if (registrar->reg_agent) {
 *      su_root_run(registrar->reg_root);
 *      nta_agent_destroy(registrar->reg_agent);
 *    }
 * 
 *    su_root_destroy(registrar->reg_root);
 *  }
 * @endcode
 * 
 * @section nta_server SIP Server Action
 * 
 * A SIP server responds to the transactions sent by a client. The SIP
 * server can operate in two modes; it can be stateless or stateful. This
 * section describes how a stateful SIP server uses NTA.
 * 
 * @subsection nta_leg_t The NTA Legs
 * 
 * A leg is required for stateful transaction processing.  A default
 * leg is created like this:
 * @code
 * default_leg = nta_leg_tcreate(agent, process_requests, context,
 *                               URLTAG_URL(url), 
 *                               NTATAG_NO_DIALOG(1),
 *                               TAG_END());
 * @endcode			       
 * 
 * The @a url parameter is used to specify which URLs match to the leg. If
 * it is given, only requests with requestURI matching are processed by the
 * leg. The nta_leg_tcreate() is a @ref tagarg "tagarg" function, taking a
 * tagged argument list as its arguments.
 * 
 * Other, ordinary legs can be used to match incoming requests with existing
 * dialogs, calls or transaction contexts, or to provide outgoing requests
 * with consistent headers. When a call leg is created, it is provided with
 * @b From and @b To headers, and optionally with other headers like @b
 * Call-ID, @b Route, or @b CSeq.
 * 
 * A new call leg can be created as follows:
 * @code
 * call_leg = nta_leg_tcreate(agent,
 *                            process_call_requests, call_context,
 *                            SIPTAG_CALL_ID(sip->sip_call_id),
 *                            SIPTAG_TO(sip->sip_from),   
 *                            SIPTAG_FROM(sip->sip_to),
 *                            TAG_END());
 * @endcode
 *
 * @note In the example above, the @b From and @b To are reversed. This
 * happens if the headers are taken from an incoming request; the @b From
 * and @b To headers change direction when an outgoing request is initiated.
 *
 * @note An existing leg can be used in any direction, however.  If the leg
 * was created for an incoming INVITE transaction, it is also possible to
 * use the leg for an outgoing BYE transaction.
 * 
 * @subsection nta_leg_tag Tagging the Call Leg
 * 
 * All the SIP UAS elements (however, not proxy servers) are required to tag
 * the @b To header in their final responses. The function nta_leg_tag()
 * adds a tag to the leg's local address. Local address is used as the @b To
 * header in the reply messages, and as the @b From header in the requests.
 * 
 * @subsection nta_incoming_t Incoming Transactions
 * 
 * An incoming transaction object (nta_incoming_t) is created by NTA for
 * each unique incoming request message. When NTA has created the incoming
 * transaction object, it invokes the callback function provided with 
 * nta_leg_tcreate().  
 * 
 * The simplest way to reply to the request is to return a valid status code
 * from the callback function. Valid status codes are in range of 100 to
 * 699, inclusive.  If no automatic response is desired, the callback
 * function should return 0.
 * 
 * @note If the status code is final, the incoming transaction object will
 * be destroyed immediately after the callback function returns. It can not
 * be used afterwards.
 * 
 * @note It is not possible to respond with a 2xx status code to an incoming
 * INVITE transaction by returning the status code from the callback.
 * 
 * Valid return values for callback function are as follows:
 * @li 0, 100 .. 699 for requests other than INVITE, and
 * @li 0, 100 .. 199, 300..699 for INVITE requests.
 * 
 * All other return codes are interpreted as 500, that is, a @e 500 @e
 * Internal @e Server @e Error reply message is sent back to the client and
 * the request is immediately destroyed.
 * 
 * The simple registrar/redirect server may have a incoming request callback
 * as follows:
 * @code
 * int process_request(server_t *server,
 * 	               nta_leg_t *leg,
 * 	     	       nta_incoming_t *irq,
 * 	     	       sip_t const *sip)
 * {
 *   sip_contact_t *m;
 * 
 *   switch (sip->sip_request->rq_method) {
 *   case sip_method_register:
 *     return registrar_add(server, leg, reply, sip);
 * 
 *   case sip_method_ack:
 *     return 500;
 * 
 *   case sip_method_cancel:
 *     return 200;
 * 
 *   default:
 *     if (registrar_find(server, sip->sip_request->rq_url, &m) {
 *       nta_incoming_treply(irq, SIP_302_MOVED_TEMPORARILY, 
 *                           SIPTAG_CONTACT(m), TAG_END());
 *       return 302;
 *     }
 *     else {
 *       nta_incoming_treply(irq, SIP_404_NOT_FOUND, TAG_END());
 *       return 404;
 *     }
 *   }
 * }
 * @endcode
 * 
 * The default reply message will contain the status line with default
 * phrase, then Via, To, From, Call-ID, CSeq, and Content-Length headers.
 * If a more complex response message is required, the application should
 * respond using the function nta_incoming_treply(): 
 * @code
 * nta_incoming_treply(reply, SIP_200_OK, 
 *                     SIPTAG_CONTACT(contact),
 *                     SIPTAG_CONTENT_TYPE_STR("application/sdp"),
 *                     SIPTAG_PAYLOAD(sdp),
 *                     TAG_END());
 * @endcode
 * 
 * The nta_incoming_treply() is a @ref tagarg "tagarg" function, taking a
 * tagged argument list as its argument.
 * 
 * @note It is possible to send provisional replies (containing 1xx status
 * codes) several times with nta_incoming_treply(), but only one final
 * reply (containing status codes 2xx..6xx) can be sent. However, with
 * INVITE requests, a proxy can send a final 2xx reply even after an error
 * reply (3xx..6xx).
 * 
 * @section nta_100rel Reliable Provisional Responses - "100rel"
 * 
 * The <A href="../specs/rfc3262.txt"><B>100rel</B></A> SIP extension
 * provides reliable provisional responses, provisional responses that are
 * retransmitted until a special acknowledgement request, PRACK, is
 * received. In addition the PRACK method, the extension defines two
 * headers, @b RSeq and @b RAck, that are used to identify different
 * response messages. PRACK method is usable on INVITE requests only.
 * 
 * Using reliable responses is negotiated using the "100rel" option tag. The
 * UAC (party sending the INVITE) can include the option tag to the @b
 * Supported or @b Required header. In the first case, the UAC just
 * announces support for reliable responses, in the second case, the UAC
 * requires that the UAS (party responding to the call) always sends
 * provisional responses in reliable manner.
 *
 * When reliable responses are enabled with NTATAG_REL100() tag, the @b nta
 * engine automatically inserts the "100rel" option tag to the @b Supported
 * header in the INVITE requests.
 *
 * @subsection nta_reliable_t Responding Reliably
 *
 * When a UAS wants to respond reliably to a INVITE request, instead of
 * familiar nta_incoming_treply() or nta_incoming_mreply() it uses the
 * functions nta_reliable_treply() or nta_reliable_mreply(). These functions
 * return a pointer to a special object, nta_reliable_t, that is used to
 * keep track of unacknowledged responses and respond to the the PRACK
 * acknowledgement request. 
 * 
 * Both the functions nta_reliable_treply () and nta_reliable_mreply() take
 * a callback funtion pointer and an application context pointer as their
 * arguments. The callback function is similar to the leg callback function. 
 * The callback is invoked when a corresponding PRACK request is received,
 * or when there is a timeout.
 *
 * The @b nta takes care of assigning a serial number to each reliable
 * response and resending them if no PRACK request is received. It also
 * automatically adds the 100rel option tag to the @b Require header.
 *
 * Also, if a request with 100rel in Require header is responded with usual
 * nta_incoming_treply()/nta_incoming_mreply() functions, the @b nta creates
 * a reliable response object for each provisional response in behalf of
 * application. As the application can not provide a PRACK callback function
 * to @b nta, the PRACK requests are not delivered to the application. 
 *
 * @subsection early_dialog UAC Receives a Reliable Response
 *
 * When a UAC receives a provisional response with a @b RSeq header, it is
 * required to acknowledge it. In order to do that, it must establish an @e
 * early @e dialog with the UAS. In another view, a reliable response is
 * used to establish the early dialog. UAC establishes a leg object for the
 * early dialog by calling nta_leg_tcreate() with the parameters derived
 * from the response message. 
 *
 * @code
 * int invite_callback(call_t *call,
 * 		    nta_outgoing_t *orq,
 * 		    sip_t const *sip)
 * {
 *   int status = sip->sip_status->st_status;
 * 
 *   if (!call->has_dialog && 
 *       (status >= 200 || (status > 100 && sip->sip_rseq))) {
 *     nta_leg_t *early = 
 *       nta_leg_tcreate(call->nta_agent, mid_dialog_request, call,
 * 		      SIPTAG_TO(sip->sip_to),
 * 		      SIPTAG_FROM(sip->sip_from),
 * 		      SIPTAG_CALL_ID(sip->sip_call_id),
 * 		      SIPTAG_CSEQ(sip->sip_cseq),
 * 		      TAG_END());
 * 
 *     nta_leg_client_route(early, 
 * 			 sip->sip_record_route,
 * 			 sip->sip_contact);
 * 
 *     fork = call_fork(call, leg = early);
 * 
 *     if (!fork) {
 *       handle error;
 *     }
 *     call = fork;
 *   }				       
 * @endcode
 * 
 * The original dialog object and client transaction object are used to
 * process other call forks. For instance, if the early dialog is
 * established with an announcement server it will never lead to an fully
 * established call, but an another dialog will be used when the call is
 * completed.
 *
 * @subsection nta_prack Acknowledging Reliable Response
 *
 * After an early dialog has been established, acknowledging the reliable
 * response is trivial. The application can create a PRACK client
 * transaction object by calling nta_outgoing_prack()
 *
 * @section nta_client SIP Client Action
 * 
 * A SIP client initiates the transactions. In some cases, a SIP client is
 * also required to invoke additional transactions, like @b ACK or @b
 * CANCEL, to finalize the original transaction. This section describes how
 * a SIP client uses NTA to make transactions.
 * 
 * @subsection client_nta_leg_t Creating the Call Leg
 * 
 * If the client does not have a suitable call leg, it must create it by
 * calling the function nta_leg_tcreate():
 * @code
 * context->leg = nta_leg_tcreate(agent,
 *                                callback, context,
 *                                SIPTAG_CALL_ID(call_id),
 *                                SIPTAG_FROM(from),
 *                                SIPTAG_TO(to),
 *                                TAG_END());
 * @endcode
 * 
 * The @p callback function and @p context pointer are used for incoming
 * transactions, and they may be @c NULL if no such transactions are
 * expected.  If the callback is @c NULL, NTA responds to incoming
 * transactions with status @e 403 @e Forbidden.
 * 
 * The @a call_id may be @c NULL or left out. In that case, NTA generates a
 * new call ID.
 * 
 * The @a from and @a to are used in outgoing transactions. They are also
 * used to select which incoming messages belong to this leg.
 *
 * The initial sequence number can be supplied with SIPTAG_CSEQ() (taking a
 * CSeq structure as parameter).
 *
 * The additional parameters (after @a to) are included in outgoing messages
 * using this leg. Currently, only @c SIPTAG_ROUTE() is supported.
 * 
 * @note Additional tagged parameters are ignored.
 * 
 * @subsection nta_outgoing_t Outgoing requests
 * 
 * The outgoing request is created and sent by nta_outgoing_tcreate(). It
 * can be used as follows:
 * @code
 * oreq = nta_outgoing_tcreate(leg, response_to_register, reg,
 *                             proxy_url,
 *                             SIP_METHOD_REGISTER,
 *                             registrar_url,
 *                             SIPTAG_CONTACT(my_contact),
 *                             TAG_END());
 * @endcode
 * 
 * NTA invokes the callback function response_to_register() each time a
 * provisional answer is received, and when a final answer is received.
 * 
 * @note There may be multiple final answers to the INVITE request.
 * 
 * If NTA does not receive answer in timely manner, it will generate a
 * @e 408 @e Timeout response and hand that back to the application. 
 * 
 * @note After a provisional answer to the INVITE request, no timeout will
 * occur inside NTA.  Application must itself timeout the INVITE
 * transactions if any answer has been received.
 * 
 * The request can be destroyed with NTA function nta_outgoing_destroy().
 * If no final answer has been received, the request is cancelled when it is
 * destroyed, too.  The application can also cancel the outgoing request by
 * calling nta_outgoing_cancel().
 * 
 * @subsection nta_ack Acknowledging Answers to INVITE
 * 
 * The final answers to the INVITE request must be acknowledged. NTA takes
 * care of acknowledging automatically the 3xx..6xx answers; the appliction
 * must explicitly create a separate acknowledge transaction to final 2xx
 * answers. 
 * 
 * The final answer can be acknowledged like this:
 * @code
 *  url = sip->sip_contact ? sip->sip_contact->m_url : original_url;
 *  ack = nta_outgoing_tcreate(leg, NULL, NULL,
 *                             SIP_METHOD_ACK,
 *                             (url_string_t*)url,
 *                             SIPTAG_CSEQ(sip->sip_cseq),
 *                             SIPTAG_PAYLOAD(sdp),
 *                             TAG_END());
 * @endcode
 * 
 * @note The ACK transaction should be sent to the Contact specified in the 
 * 2xx reply.  
 * 
 * @section nta_register_f Stateless Processing of SIP Messages
 * 
 * The callback function statelessly processing incoming messages is
 * provided to the NTA agent when it is created. The callback function will
 * be called to process incoming SIP messages that do not match with any
 * existing transaction or dialog. In addition to the message (@a msg) and
 * its parsed contents (@a sip) the callback function gets the
 * application-specific context pointer (in this case, @a registrar) and a
 * pointer to the NTA agent (@a agent) as its arguments:
 * 
 * @code 
 * int process_message(nta_agent_context_t *registrar,
 * 		       nta_agent_t *agent,
 * 		       msg_t *msg,
 * 		       sip_t *sip);
 * @endcode
 * 
 * The application has three functions that can be used to process the
 * messages:
 * @li nta_msg_discard() ignores and destroys the message,
 * @li nta_msg_tsend() forwards the message, and
 * @li nta_msg_treply() replies to the message in a stateless way.
 * 
 * The functionality of the callback function can vary greatly, depending
 * the purpose of the application. A proxy or registrar/redirect server 
 * have very different callback functions.
 * 
 * A simple redirect server could have a message callback function as
 * follows.
 * @code
 * int process_message(redirect_t *r,
 * 		       nta_agent_t *agent,
 * 		       msg_t *msg,
 * 		       sip_t *sip)
 * {
 *   sip_contact_t *m;
 *   sip_unsupported_t *u;
 * 
 * @endcode
 *
 * The incoming status messages are simply ignored. The @b ACK requests can
 * safely be discarded, too, because the redirect server keeps no state.
 * 
 * @code
 *   if (!sip->sip_request || sip->sip_request->rq_method == sip_method_ack) {
 *     nta_msg_discard(agent, msg); 
 *     return 0;
 *   }
 * @endcode
 *
 * Next, the redirect server first checks if processing the request requires
 * a feature that is not supported by it:
 * @code
 *   u = sip_unsupported(msg_home(msg), sip->sip_require, r->r_supported);
 *   if (u) {
 *     nta_msg_treply(agent, msg, SIP_420_BAD_EXTENSION, 
 *                    SIPTAG_SUPPORTED(r->r_supported),
 *                    SIPTAG_UNSUPPORTED(u),
 *                    TAG_END());
 *     return 0;
 *   }
 * @endcode
 *
 * The @b CANCEL requests terminate a transacton. A stateless redirect
 * server does not have transactions, so it redirect replies with a @e 481
 * @e Call @e Leg/Transaction @e Does @e Not @e Exist message:
 * @code
 *   if (sip->sip_request->rq_method == sip_method_cancel) {
 *     nta_msg_treply(agent, msg, SIP_481_NO_TRANSACTION, TAG_END());
 *     return 0;
 *   }
 * @endcode
 *
 *  All other requests are answered normally. The location service is
 *  searched for the request uri, and if a matching address was found, a
 *  list of active bindings is returned to the client.
 * @code
 *   m = location_find(redirect, sip->sip_request->rq_url);
 *   if (m) {
 *     nta_msg_treply(agent, msg, SIP_302_MOVED_TEMPORARILY, 
 *                    SIPTAG_CONTACT(m),
 *                    TAG_END());
 *   } 
 * @endcode
 *
 *   Otherwise, @e 404 @e Not @e Found is sent:
 * @code
 *   else {
 *     nta_msg_treply(agent, msg, SIP_404_NOT_FOUND, TAG_END());
 *   }
 * 
 *   return 0;
 * }
 * @endcode
 * 
 * @todo Add an example.
 */

/**@page internal NTA Semantics and Internal Data Flows
 * 
 * NTA implements simple state machines at transaction level. The figure
 * below illustrates how a message is processed by NTA.
 * 
 * @image html nta-receiving-message.gif "NTA processing incoming messages." 
 * @image latex nta-receiving-message.eps "NTA processing incoming messages." 
 * 
 * 
 */

int invite_callback(call_t *call,
		    nta_outgoing_t *orq,
		    sip_t const *sip)
{
  int status = sip->sip_status->st_status;
  nta_leg_t *leg = call->leg;

  if (!call->has_dialog && 
      (status >= 200 || (status > 100 && sip->sip_rseq))) {
    nta_leg_t *early = 
      nta_leg_tcreate(call->nta_agent, mid_dialog_request, call,
		      SIPTAG_TO(sip->sip_to),
		      SIPTAG_FROM(sip->sip_from),
		      SIPTAG_CALL_ID(sip->sip_call_id),
		      SIPTAG_CSEQ(sip->sip_cseq),
		      TAG_END());

    nta_leg_client_route(early, 
			 sip->sip_record_route,
			 sip->sip_contact);

    fork = call_fork(call, leg = early);

    if (!fork) {
      handle error;
    }
    call = fork;
  }				       
  
  if (status > 100 && status < 200 && sip->sip_rseq) {
    nta_outgoing_t *prack = 
      nta_outgoing_prack(leg, orq, NULL, NULL, NULL,
			 sip, 
			 TAG_END());
    nta_outgoing_destroy(prack);
    return 0;
  }
  
  ...
}