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/* -*- c -*- */

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/**@MODULEPAGE "nta" - SIP Transactions Module
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 *
 * @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
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 * intended for both network and user elements. The public interface for @b
 * nta is mostly defined in <sofia-sip/nta.h>, but tag parameters are
 * defined in <sofia-sip/nta_tag.h>.
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 *
 * @CONTACT Pekka Pessi <Pekka.Pessi@nokia.com>
 *
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 * @STATUS @SofiaSIP Core library
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 *
 * @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
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 * messages, as defined in <sofia-sip/msg.h> and <sofia-sip/sip.h>,
 * respectively. The various SIP headers are also defined in
 * <sofia-sip/sip.h>.
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 * 
 * @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,
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 * and pass them to the application. It also takes care of resolving the
 * domain names and sending the messages.
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 * 
 * 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
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 * @From and @To headers, and optionally with other headers like
 * @CallID, @Route, or @CSeq.
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 * 
 * 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
 *
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 * @note In the example above, the @From and @To are reversed. This
 * happens if the headers are taken from an incoming request; the @From
 * and @To headers change direction when an outgoing request is initiated.
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 *
 * @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
 * 
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 * All the SIP UAS elements are required to tag the @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 @To header in the reply messages,
 * and as the @From header in the requests. The function nta_incoming_tag()
 * adds a tag to a incoming transaction. They are usually used in together,
 * using the tag from initial response to the dialog, too:
 * e.g.,
 * @code
 *   if (!nta_leg_tag(leg, nta_incoming_tag(irq, NULL)))
 *    nta_incoming_treply(irq, SIP_500_INTERNAL_SERVER_ERROR, TAG_END());
 * @endcode
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 * 
 * @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
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 * phrase, then @Via, @To, @From, @CallID, @CSeq, and @ContentLength headers.
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 * 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
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 * headers, @RSeq and @RAck, that are used to identify different
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 * response messages. PRACK method is usable on INVITE requests only.
 * 
 * Using reliable responses is negotiated using the "100rel" option tag. The
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 * UAC (party sending the INVITE) can include the option tag to the
 * @Supported or @Require header. In the first case, the UAC just
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 * 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
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 * engine automatically inserts the "100rel" option tag to the @Supported
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 * 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
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 * automatically adds the 100rel option tag to the @Require header.
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 *
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 * Also, if a request with 100rel in @Require header is responded with usual
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 * 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
 *
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 * When a UAC receives a provisional response with a @RSeq header, it is
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 * 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
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 * @CSeq structure as parameter).
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 *
 * 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
 * 
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 * @note The ACK transaction should be sent to the @Contact specified in the 
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 * 2xx reply.  
 * 
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 * <a name="nta_register_f"></a>
 * @section nta_stateless_callback Stateless Processing of SIP Messages
 *
 * When an NTA agent is created, it is possible to provide it with a
 * stateless callback function. The callback function will be called when an
 * incoming SIP request or response message does not match with an existing
 * transaction.
 *
 * Before invoking the stateless callback the agent will try to match the
 * incoming request message with an existing dialog or dialog-less leg (or
 * default leg). So, if you have created a default leg, all request messages
 * are processed statefully by it instead of being passed to the stateless
 * callback function.
 *
 * If you want to process request messages with stateless callback and still
 * use dialog-less legs (for instance, in order to look up domains with
 * nta_leg_by_uri()), you have to switch over to @em stateless @em mode by
 * including NTATAG_STATELESS(1) in nta_agent_create() or
 * nta_agent_set_params() arguments.
 *
 * Also, if a response message does not match with an existing client
 * transaction, the agent will try to use the default outgoing (client)
 * transaction. If you have created an default outgoing transaction, all
 * stray response messages are passed to it instead of the stateless
 * processing function.
 *
 * @section nta_message_f_example Stateless Callback Function
 *
 * In addition to the message (@a msg) and its
 * parsed contents (@a sip) the callback function gets the
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 * 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
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 * messages in stateless manner:
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 * @li nta_msg_discard() ignores and destroys the message,
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 * @li nta_msg_tsend() forwards a request or response message, and
 * @li nta_msg_treply() replies to a request message in a stateless way.
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 * 
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 * Additionally, it is possible to process a request message statefully with
 * nta_incoming_create().
 *
 * The functionality of the stateless callback function can vary greatly,
 * depending the purpose of the application. An user-agent, a proxy or a
 * registrar/redirect server each have very different callback functions.
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 * 
 * A simple redirect server could have a message callback function as
 * follows.
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 *
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 * @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
 *
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 * The incoming response messages are simply ignored. The @b ACK requests can
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 * 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
 *
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 *  All other requests are answered normally with a 302 response. 
 *  The location service is
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 *  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
 * 
 */

/**@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;
  }
  
  ...
}