The nua module contains the user-agent library taking care of basic SIP User Agent functions. Its functionality includes call management, messaging and event retrieval.
The NUA API gives the high-level application programmer transparent and full control to the SIP protocol engine below it. NUA provides the call semantics on top of existing transaction semantics found in nta module. With NUA it is possible to create different kind of SIP User Agents, like terminals, gateways or MCUs.
The nua engine hides many low-level signaling and media management aspects from the application programmer. It is possible to use different kind of media interfaces - even remote ones - in a fully transparent way.
The application and the protocol engine within User Agent library can be run in separate threads. The protocol engine communicates with the application using events, delivered to the application with a a callback function. The callback function is called within the thread context of the application, represented with a su_root_t object.
The Sofia software suite is based on certain basic ideas and concepts that are used in all levels of Sofia software. Many of those are implemented in Sofia utility library (su) providing unified interface to the most important OS services and utilities .
The following sections contain descriptions of the concepts that a user of NUA library must understand to create a working application. The other utilities (in the SU library and other libraries of Sofia software suite) might also be useful for an application developer but one must be careful when using them because they might change the behavior of the Sofia software suite in a way that causes NUA library to work incorrectly. See [su] for more detailed description of the SU services.
The NUA uses the reactor pattern (also known as dispatcher pattern and notifier pattern) for event driven systems (see [Using Design Patterns to Develop Reusable Object-oriented Communication Software, D.C. Schmidt, CACM October '95, 38(10): 65-74]). Sofia uses a task as basic execution unit for the programming model. According to the model, the program can ask that the event loop invokes a callback function when a certain event occurs. Such events include I/O activity, timers or a asynchronously delivered messages from other task.
The root object is a handle representing the task in the application. Another way of seeing the same thing is that the root object represents the main event loop of the task. Through the root object the task code can access its context information (magic) and thread-synchronization features like wait objects, timers, and messages.
An application using NUA services must create a root object and the callback routine to handle NUA events. The root object is created with su_root_create() function and the callback routine is registered with nua_create() function.
Root object has type su_root_t.
See documentation of <sofia-sip/su_wait.h> and <su_root.c> for more information of root object.
See section nua_event_e for more information of the callback function.
The magic is a term used for the context pointer that can be bound to various objects in Sofia stack (for example root object and operation handle) by the application code. This context pointer is passed back to the application code when a registered callback function is called by the main event loop. The Sofia stack retains the context information between calls to the callback function. An application can use the context information to store any information it needs for processing the events.
The home-based memory management is useful when a lot of memory blocks are allocated for given task. The allocations are done via the memory home, which keeps a reference to each allocated memory block. When the memory home is then freed, it will free all memory blocks to which it has reference. This simplifies application logic because application code does not need to keep track of the allocated memory and free every allocated block separately.
An application using NUA services can use the memory management services provided by the SU library but it is not mandatory.
See documentation of <sofia-sip/su_alloc.h> for more information of memory management services.
Tagging is the mechanism used in Sofia software for packing parameters to functions. It enables passing a variable number of parameters having non-fixed types. For an application programmer the tagging is visible as macros that are used to encapsulate the passed parameters. When evaluated a tagging macro creates a structure that contains a tag (telling what is the type of a parameter) and a value (pointer to opaque data). By checking the tag the layers of Sofia software check whether they can handle the parameter or should it just be passed to lower layers for processing.
There are some tags with special meaning:
The NUA functions can be called with a list of tagged values if they have following parameters at the end of parameter list:
tag_type_t tag, tag_value_t value, ...);
The last tagged value on the parameter list must be TAG_NULL() (or TAG_END(), synonym for TAG_NULL()).
Every tag has two versions:
NUTAG_<tagname>
which takes a value parameter and
NUTAG_<tagname>_REF
which takes a reference parameter. The latter is used with tl_gets() function to retrieve tag values from tag list.
For SIP headers there exists also additional version of tags:
SIPTAG_<tagname>_STR
This tag version takes a C-language character string as parameter. The corresponding tag without _STR suffix takes a parsed value structure as parameter.
The following is an example of call to NUA function containing tagged values:
nua_unregister(op->op_handle, TAG_IF(use_registrar, NUTAG_REGISTRAR(registrar)), SIPTAG_CONTACT_STR("*"), SIPTAG_EXPIRES_STR("0"), TAG_NULL());
An application using NUA services must use tagged arguments for passing the parameters to functions. See nua_invite() for discussion on how a SIP message is constructed from the tags.
See documentation of <sofia-sip/su_tag.h> for more information of tags and the module-specific documentation of each Sofia module for information of tags specific for that module.
The modules of Sofia stack contain configurable debugging and logging functionality based on the services defined in <sofia-sip/su_log.h>. The debugging and logging details (for example level of details on output and output file name) can be configured by environment variables, directives in configuration files and compilation directives in the source files.
Examples of useful directives/ environment variables are:
The defined debug output levels are:
An application using NUA services can also use the debugging and logging services provided by the Sofia stack but it is not mandatory.
See documentation of <sofia-sip/su_log.h> for more information of debugging and logging services.
Stack object represents an instance of SIP stack and media engine. It contains reference to root object of that stack, user-agent-specific settings, and reference to the SIP transaction engine, for example.
A NUA stack object is created by nua_create() function and deleted by nua_destroy() function. The nua_shutdown() function is used to gracefully release active the sessions by nua engine.
NUA stack object has type nua_t.
Operation handle represents an abstract SIP call/session. It contains information of SIP dialog and media session, and state machine that takes care of the call, high-level SDP offer-answer protocol, registration, subscriptions, publications and simple SIP transactions. An operation handle may contain list of tags used when SIP messages are created by NUA (e.g. From and To headers).
An operation handle is created explicitly by the application using NUA for sending messages (function nua_handle()) and by stack for incoming calls/sessions (starting with INVITE or MESSAGE). The handle is destroyed by the application using NUA (function nua_handle_destroy()).
Indication and response events are associated with an operation handle.
NUA operation handle has type nua_handle_t.
The stack thread is a separate thread from application that provides the real-time protocol stack operations so that application thread can for example block or redraw UI as it likes.
The communication between stack thread and application thread is asynchronous. Most of the NUA API functions cause a send of a message to the stack thread for processing and similarly when something happens in the stack thread it sends a message to the application thread. The messages to the application thread are delivered as invokes of the application callback function when the application calls su_root_run() or su_root_step() function.
SIP messages are manipulated with typesafe SIPTAG_ tags. There are three versions of each SIP tag:
For example a header named "Example" would have tags names SIPTAG_EXAMPLE(), SIPTAG_EXAMPLE_STR(), and SIPTAG_EXAMPLE_REF().
When tags are used in NUA calls the corresponding headers are added to the message. In case the header can be present only once in a message and there already exists a value for the header the value given by tag replaces the existing header value. Passing tag value NULL has no effect on headers. Passing tag value (void *)-1 removes corresponding headers from the message.
For example:
nua_subscribe(nh, SIPTAG_EVENT_STR("presence"), SIPTAG_ACCEPT(accept1), SIPTAG_ACCEPT(accept2), TAG_END());
sip_accept_t *ac = NULL; sip_event_t *o = NULL; tl_gets(tl, SIPTAG_EVENT_REF(o), /* _REF takes a reference! */ SIPTAG_ACCEPT_REF(ac), TAG_END());
This section describes basic usage scenarios of NUA/Sofia stack using message sequence charts.
The following sections will present code examples from a simple application that uses services of NUA. The example is not complete but should present all relevant details of the basic use of NUA.
On sourceforge.net there is available an example application sofisip_cli.c that can be studied for more complete example.
An application using services of NUA normally defines data areas that are used to store context information (i.e., "magic"). The types of pointers to these context information areas are passed to NUA by defines.
/* type for application context data */ typedef struct application application; #define NUA_MAGIC_T application /* type for operation context data */ typedef union oper_ctx_u oper_ctx_t; #define NUA_HMAGIC_T oper_ctx_t
The information area contents themselves can be defined as C structures or unions:
/* example of application context information structure */ typedef struct application { su_home_t home[1]; /* memory home */ su_root_t *root; /* root object */ nua_t *nua; /* NUA stack object */ /* other data as needed ... */ } application; /* Example of operation handle context information structure */ typedef union operation { nua_handle_t *handle; /* operation handle / struct { nua_handle_t *handle; /* operation handle / ... /* call-related information */ } call; struct { nua_handle_t *handle; /* operation handle / ... /* subscription-related information */ } subscription; /* other data as needed ... */ } operation;
NUA stack object and handle are opaque to the application programmer. Likewise, the application context is completely opaque to the NUA stack module. NUA functions are passed a pointer, and that pointer is then given back to the application within the callback parameters. In this case the application context information structure is also used to store a root object and memory home for memory handling. The application context information also contains the NUA stack object information.
The following code is an example of application function that initializes the system, enters the main loop for processing the messages, and, after message processing is ended, deinitalizes the system.
If the application is not just responding to incoming SIP messages there must also be means to send messages to NUA. This can be handled for example by having a separate thread that calls NUA functions to send messages or by having a socket connection to the application for sending commands to the application (see documentation of su_wait_create() and su_root_register()).
/* Application context structure */ application appl[1] = {{{{(sizeof appl)}}}}; /* initialize system utilities */ su_init(); /* initialize memory handling */ su_home_init(appl->home); /* initialize root object */ appl->root = su_root_create(appl); if (appl->root != NULL) { /* create NUA stack */ appl->nua = nua_create(appl->root, app_callback, appl, /* tags as necessary ...*/ TAG_NULL()); if (appl->nua != NULL) { /* set necessary parameters */ nua_set_params(appl->nua, /* tags as necessary ... */ TAG_NULL()); /* enter main loop for processing of messages */ su_root_run(appl->root); /* destroy NUA stack */ nua_destroy(appl->nua); } /* deinit root object */ su_root_destroy(appl->root); appl->root = NULL; } /* deinitialize memory handling */ su_home_deinit(appl->home); /* deinitialize system utilities */ su_deinit();
Handling of the events coming from NUA stack is done in the callback function that is registered for NUA stack with the nua_create() function when the application is initialized. The content of callback function is in its simplest form just a switch/case statement that dispatches the incoming events for processing to separate functions.
void app_callback(nua_event_t event, int status, char const *phrase, nua_t *nua, nua_magic_t *magic, nua_handle_t *nh, nua_hmagic_t *hmagic, sip_t const *sip, tagi_t tags[]) { switch (event) { case nua_i_invite: app_i_invite(status, phrase, nua, magic, nh, hmagic, sip, tags); break; case nua_r_invite: app_r_invite(status, phrase, nua, magic, nh, hmagic, sip, tags); break; /* and so on ... */ default: /* unknown event -> print out error message */ if (status > 100) { printf("unknown event %d: %03d %s\n", event, status, phrase); } else { printf("unknown event %d\n", event); } tl_print(stdout, "", tags); break; } } /* app_callback */
The following three functions show an example of how a basic SIP call is created.
The place_a_call() function creates an operation handle and invokes the SIP INVITE method.
operation *place_a_call(char const *name, url_t const *url) { operation *op; sip_to_t *to; /* create operation context information */ op = su_zalloc(appl->home, (sizeof *op)); if (!op) return NULL; /* Destination address */ to = sip_to_create(NULL, url); if (!to) return NULL; to->a_display = name; /* create operation handle */ op->handle = nua_handle(appl->nua, op, SIPTAG_TO(to), TAG_END()); if (op->handle == NULL) { printf("cannot create operation handle\n"); return NULL; } nua_invite(op->handle, /* other tags as needed ... */ TAG_END()); } /* place_a_call */
The app_r_invite() function is called by callback function when response to INVITE message is received. Here it is assumed that automatic acknowledge is not enabled so ACK response must be sent explicitly.
void app_r_invite(int status, char const *phrase, nua_t *nua, nua_magic_t *magic, nua_handle_t *nh, nua_hmagic_t *hmagic, sip_t const *sip, tagi_t tags[]) { if (status == 200) { nua_ack(nh, TAG_END()); } else { printf("response to INVITE: %03d %s\n", status, phrase); } } /* app_r_invite */
The nua_i_state event is sent (and app_i_state() function called by callback function) when the call state changes (see client-side call model).
void app_i_state(int status, char const *phrase, nua_t *nua, nua_magic_t *magic, nua_handle_t *nh, nua_hmagic_t *hmagic, sip_t const *sip, tagi_t tags[]) { nua_callstate_t state = nua_callstate_init; tl_gets(tags, NUTAG_CALLSTATE_REF(state), NUTAG__REF(state), state = (nua_callstate_t)t->t_value; printf("call %s\n", nua_callstate_name(state)); } /* app_i_state */
The app_i_invite() function is called by callback function when incoming INVITE message is received. This example assumes that autoanswer is not enabled so the response must be sent explicitly.
void app_i_invite(int status, char const *phrase, nua_t *nua, nua_magic_t *magic, nua_handle_t *nh, nua_hmagic_t *hmagic, sip_t const *sip, tagi_t tags[]) { printf("incoming call\n"); nua_respond(nh, 200, "OK", SOA_USER_SDP(magic->sdp), TAG_END()); } /* app_i_invite */
The app_i_state() function is called by the callback function when call has been successfully set up and the media has been activated.
void app_i_active(int status, char const *phrase, nua_t *nua, nua_magic_t *magic, nua_handle_t *nh, nua_hmagic_t *hmagic, sip_t const *sip, tagi_t tags[]) { printf("call active\n"); } /* app_i_active */
The following three functions show an example of how a basic SIP call is terminated.
The terminate_call() function sends the SIP BYE message.
The app_r_bye() function is called by the callback function when answer to the BYE message is received. The function destroys the call handle and releases the memory allocated to operation context information.
void app_r_bye(int status, char const *phrase, nua_t *nua, nua_magic_t *magic, nua_handle_t *nh, nua_hmagic_t *hmagic, sip_t const *sip, tagi_t tags[]) { if (status < 200) return; printf("call released\n"); /* release operation handle */ nua_handle_destroy(hmagic->handle); op->handle = NULL; /* release operation context information */ su_free(appl->home, hmagic); } /* app_r_bye */
The app_i_bye() function is called by the callback function when an incoming BYE message is received. The function destroys the call handle and releases the memory allocated to operation context information.
void app_i_bye(int status, char const *phrase, nua_t *nua, nua_magic_t *magic, nua_handle_t *nh, nua_hmagic_t *hmagic, sip_t const *sip, tagi_t tags[]) { printf("call released\n"); /* release operation handle */ nua_handle_destroy(hmagic->handle); op->handle = NULL; /* release operation context information */ su_free(appl->home, hmagic); } /* app_i_bye */
The following functions show an example of how a SIP MESSAGE is sent.
The send_message() function sends the SIP MESSAGE.
void send_message(void) { op_t *op; /* create operation context information */ op = su_zalloc(appl->home, sizeof(op_t)); if (op = NULL) { printf("cannot create operation context information\n"); return; } /* how we create destination_address? */ /* create operation handle */ op->handle = nua_handle(appl->nua, op, NUTAG_URL(destination_address), TAG_END()); if (op->handle == NULL) { printf("cannot create operation handle\n"); return; } /* send MESSAGE */ nua_message(op->handle, SIPTAG_CONTENT_TYPE_STR("text/plain"), SIPTAG_PAYLOAD_STR("Hello, world!"), /* other tags as needed ... */ TAG_END()); } /* send_message */
The app_r_message() function is called by the callback function when answer to the MESSAGE is received.
void app_r_message(int status, char const *phrase, nua_t *nua, nua_magic_t *magic, nua_handle_t *nh, nua_hmagic_t *hmagic, sip_t const *sip, tagi_t tags[]) { printf("response to MESSAGE: %03d %s\n", status, phrase); } /* app_r_message */
The following function shows an example of how a SIP MESSAGE is received.
The app_i_message() function is called by the callback function when a SIP MESSAGE is received.
void app_i_message(int status, char const *phrase, nua_t *nua, nua_magic_t *magic, nua_handle_t *nh, nua_hmagic_t *hmagic, sip_t const *sip, tagi_t tags[]) { printf("received MESSAGE: %03d %s\n", status, phrase); printf("From: %s%s" URL_PRINT_FORMAT "\n", sip->sip_from->a_display ? sip->sip_from->a_display : "", sip->sip_from->a_display ? " " : "", URL_PRINT_ARGS(sip->sip_from->a_url)); if (sip->sip_subject) { printf("Subject: %s\n", sip->sip_subject->g_value); } if (sip->sip_payload) { fwrite(sip->sip_payload->pl_data, sip->sip_payload->pl_len, 1, stdout); fputs("\n", stdout); } } /* app_i_message */
... application_t *app; operation_t *oper; ... oper->app = app; app->nua = nua_create(ssip->s_root, app_callback, app, TAG_NULL()); ... oper->handle = nua_handle(app->nua, app, NUTAG_URL(to->a_url), SIPTAG_TO(to), ta_tags(ta)); ... nua_notifier(oper->handle, SIPTAG_EXPIRES_STR("3600"), SIPTAG_EVENT_STR("presence"), SIPTAG_CONTENT_TYPE_STR("application/pidf-partial+xml"), NUTAG_SUBSTATE(nua_substate_pending), TAG_END());
After the nua_notifier object -- the presence server -- is created, an event nua_r_notifier is returned. Status and phrase values of the app_callback function indicate the success of the creation.
Authorization of an incoming subscription (to the local presence server) can be handled in the callback function.
void app_callback(nua_event_t event, int status, char const *phrase, nua_t *nua, application_t *app, nua_handle_t *nh, oper_t *op, sip_t const *sip, tagi_t tags[]) { nea_sub_t *subscriber = NULL; switch (event) { case nua_i_subscription: tl_gets(tags, NEATAG_SUB_REF(subscriber), TAG_END()); nua_authorize(nua_substate_active); default: break; }
The following functions show an example of how application terminates the NUA stack.
The shutdown() function starts the termination.
void shutdown(void) { nua_shutdown(appl->nua); } /* shutdown */
The app_r_shutdown() function is called by the callback function when NUA stack termination is either finished or failed.
void app_r_shutdown(int status, char const *phrase, nua_t *nua, nua_magic_t *magic, nua_handle_t *nh, nua_hmagic_t *hmagic, sip_t const *sip, tagi_t tags[]) { printf("shutdown: %d %s\n", status, phrase); if (status < 200) { /* shutdown in progress -> return */ return; } /* end the event loop. su_root_run() will return */ su_root_break(magic->root); } /* app_r_shutdown */
