/* 
 * Copyright (C) 2008-2011 Teluu Inc. (http://www.teluu.com)
 * Copyright (C) 2003-2008 Benny Prijono <benny@prijono.org>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA 
 */
#include <pjsip-ua/sip_inv.h>
#include <pjsip-ua/sip_100rel.h>
#include <pjsip-ua/sip_timer.h>
#include <pjsip/sip_module.h>
#include <pjsip/sip_endpoint.h>
#include <pjsip/sip_event.h>
#include <pjsip/sip_multipart.h>
#include <pjsip/sip_transaction.h>
#include <pjmedia/sdp.h>
#include <pjmedia/sdp_neg.h>
#include <pjmedia/errno.h>
#include <pj/array.h>
#include <pj/string.h>
#include <pj/pool.h>
#include <pj/assert.h>
#include <pj/os.h>
#include <pj/log.h>
#include <pj/rand.h>

/* 
 * Note on offer/answer:
 *
 * The offer/answer framework in this implementation assumes the occurence
 * of SDP in a particular request/response according to this table:

                  offer   answer    Note:
    ========================================================================
    INVITE          X               INVITE may contain offer
    18x/INVITE      X       X       Response may contain offer or answer
    2xx/INVITE      X       X       Response may contain offer or answer
    ACK                     X       ACK may contain answer

    PRACK                   X       PRACK can only contain answer
    2xx/PRACK                       Response may not have offer nor answer

    UPDATE          X               UPDATE may only contain offer
    2xx/UPDATE              X       Response may only contain answer
    ========================================================================

  *
  */

#define THIS_FILE       "sip_inv.c"

static const char *inv_state_names[] =
{
    "NULL",
    "CALLING",
    "INCOMING",
    "EARLY",
    "CONNECTING",
    "CONFIRMED",
    "DISCONNCTD",
    "TERMINATED",
};

/* UPDATE method */
static const pjsip_method pjsip_update_method =
{
    PJSIP_OTHER_METHOD,
    { "UPDATE", 6 }
};

#define POOL_INIT_SIZE  256
#define POOL_INC_SIZE   256

/*
 * Static prototypes.
 */
static pj_status_t mod_inv_load(pjsip_endpoint *endpt);
static pj_status_t mod_inv_unload(void);
static pj_bool_t   mod_inv_on_rx_request(pjsip_rx_data *rdata);
static pj_bool_t   mod_inv_on_rx_response(pjsip_rx_data *rdata);
static void        mod_inv_on_tsx_state(pjsip_transaction*, pjsip_event*);

static void inv_on_state_null( pjsip_inv_session *inv, pjsip_event *e);
static void inv_on_state_calling( pjsip_inv_session *inv, pjsip_event *e);
static void inv_on_state_incoming( pjsip_inv_session *inv, pjsip_event *e);
static void inv_on_state_early( pjsip_inv_session *inv, pjsip_event *e);
static void inv_on_state_connecting( pjsip_inv_session *inv, pjsip_event *e);
static void inv_on_state_confirmed( pjsip_inv_session *inv, pjsip_event *e);
static void inv_on_state_disconnected( pjsip_inv_session *inv, pjsip_event *e);

static pj_status_t inv_check_sdp_in_incoming_msg( pjsip_inv_session *inv,
                                                  pjsip_transaction *tsx,
                                                  pjsip_rx_data *rdata);
static pj_status_t inv_negotiate_sdp( pjsip_inv_session *inv );
static pjsip_msg_body *create_sdp_body(pj_pool_t *pool,
                                       const pjmedia_sdp_session *c_sdp);
static pj_status_t process_answer( pjsip_inv_session *inv,
                                   int st_code,
                                   pjsip_tx_data *tdata,
                                   const pjmedia_sdp_session *local_sdp);

static pj_status_t handle_timer_response(pjsip_inv_session *inv,
                                         const pjsip_rx_data *rdata,
                                         pj_bool_t end_sess_on_failure);

static pj_bool_t inv_check_secure_dlg(pjsip_inv_session *inv,
                                      pjsip_event *e);

static int print_sdp(pjsip_msg_body *body, char *buf, pj_size_t len);

static void (*inv_state_handler[])( pjsip_inv_session *inv, pjsip_event *e) = 
{
    &inv_on_state_null,
    &inv_on_state_calling,
    &inv_on_state_incoming,
    &inv_on_state_early,
    &inv_on_state_connecting,
    &inv_on_state_confirmed,
    &inv_on_state_disconnected,
};

static struct mod_inv
{
    pjsip_module         mod;
    pjsip_endpoint      *endpt;
    pjsip_inv_callback   cb;
} mod_inv = 
{
    {
        NULL, NULL,                         /* prev, next.              */
        { "mod-invite", 10 },               /* Name.                    */
        -1,                                 /* Id                       */
        PJSIP_MOD_PRIORITY_DIALOG_USAGE,    /* Priority                 */
        &mod_inv_load,                      /* load()                   */
        NULL,                               /* start()                  */
        NULL,                               /* stop()                   */
        &mod_inv_unload,                    /* unload()                 */
        &mod_inv_on_rx_request,             /* on_rx_request()          */
        &mod_inv_on_rx_response,            /* on_rx_response()         */
        NULL,                               /* on_tx_request.           */
        NULL,                               /* on_tx_response()         */
        &mod_inv_on_tsx_state,              /* on_tsx_state()           */
    }
};


/* Invite session data to be attached to transaction. */
struct tsx_inv_data
{
    pjsip_inv_session   *inv;       /* The invite session                   */
    pj_bool_t            sdp_done;  /* SDP negotiation done for this tsx?   */
    pj_bool_t            retrying;  /* Resend (e.g. due to 401/407)         */
    pj_str_t             done_tag;  /* To tag in RX response with answer    */
    pj_bool_t            done_early;/* Negotiation was done for early med?  */
    pj_bool_t            done_early_rel;/* Early med was realiable?         */
    pj_bool_t            has_sdp;   /* Message with SDP?                    */
};

/*
 * Module load()
 */
static pj_status_t mod_inv_load(pjsip_endpoint *endpt)
{
    pj_str_t allowed[] = {{"INVITE", 6}, {"ACK",3}, {"BYE",3}, {"CANCEL",6},
                            { "UPDATE", 6}};
    pj_str_t accepted = { "application/sdp", 15 };

    /* Register supported methods: INVITE, ACK, BYE, CANCEL, UPDATE */
    pjsip_endpt_add_capability(endpt, &mod_inv.mod, PJSIP_H_ALLOW, NULL,
                               PJ_ARRAY_SIZE(allowed), allowed);

    /* Register "application/sdp" in Accept header */
    pjsip_endpt_add_capability(endpt, &mod_inv.mod, PJSIP_H_ACCEPT, NULL,
                               1, &accepted);

    return PJ_SUCCESS;
}

/*
 * Module unload()
 */
static pj_status_t mod_inv_unload(void)
{
    /* Should remove capability here */
    return PJ_SUCCESS;
}

/*
 * Add reference to INVITE session.
 */
PJ_DEF(pj_status_t) pjsip_inv_add_ref( pjsip_inv_session *inv )
{
    PJ_ASSERT_RETURN(inv && inv->ref_cnt, PJ_EINVAL);

    pj_atomic_inc(inv->ref_cnt);

    return PJ_SUCCESS;
}

static void inv_session_destroy(pjsip_inv_session *inv)
{
    if (inv->last_ack) {
        pjsip_tx_data_dec_ref(inv->last_ack);
        inv->last_ack = NULL;
    }
    if (inv->invite_req) {
        pjsip_tx_data_dec_ref(inv->invite_req);
        inv->invite_req = NULL;
    }
    if (inv->pending_bye) {
        pjsip_tx_data_dec_ref(inv->pending_bye);
        inv->pending_bye = NULL;
    }
    pjsip_100rel_end_session(inv);
    pjsip_timer_end_session(inv);
    pjsip_dlg_dec_session(inv->dlg, &mod_inv.mod);

    /* Release the flip-flop pools */
    pj_pool_release(inv->pool_prov);
    inv->pool_prov = NULL;
    pj_pool_release(inv->pool_active);
    inv->pool_active = NULL;

    pj_atomic_destroy(inv->ref_cnt);
    inv->ref_cnt = NULL;
}

/*
 * Decrease INVITE session reference, destroy it when the reference count
 * reaches zero.
 */
PJ_DEF(pj_status_t) pjsip_inv_dec_ref( pjsip_inv_session *inv )
{
    pj_atomic_value_t ref_cnt;

    PJ_ASSERT_RETURN(inv && inv->ref_cnt, PJ_EINVAL);

    ref_cnt = pj_atomic_dec_and_get(inv->ref_cnt);
    pj_assert( ref_cnt >= 0);
    if (ref_cnt == 0) {
        inv_session_destroy(inv);
        return PJ_EGONE;
    } 
    return PJ_SUCCESS;    
}

/*
 * Set session state.
 */
static void inv_set_state(pjsip_inv_session *inv, pjsip_inv_state state,
                          pjsip_event *e)
{
    pjsip_inv_state prev_state = inv->state;
    pj_bool_t dont_notify = PJ_FALSE;
    pj_status_t status;

    /* Prevent STATE_CALLING from being reported more than once because
     * of authentication
     * https://github.com/pjsip/pjproject/issues/1318
     */
    if (state==PJSIP_INV_STATE_CALLING && 
        (inv->cb_called & (1 << PJSIP_INV_STATE_CALLING)) != 0)
    {
        dont_notify = PJ_TRUE;
    }

    /* If state is confirmed, check that SDP negotiation is done,
     * otherwise disconnect the session.
     */
    if (state == PJSIP_INV_STATE_CONFIRMED) {
        struct tsx_inv_data *tsx_inv_data = NULL;

        if (inv->invite_tsx) {
            tsx_inv_data = (struct tsx_inv_data*)
                           inv->invite_tsx->mod_data[mod_inv.mod.id];
        }

        if ((tsx_inv_data && !tsx_inv_data->sdp_done) &&
            (!inv->neg || pjmedia_sdp_neg_get_state(inv->neg)!=
                                                PJMEDIA_SDP_NEG_STATE_DONE))
        {
            pjsip_tx_data *bye;

            PJ_LOG(4,(inv->obj_name, "SDP offer/answer incomplete, ending the "
                      "session"));

            status = pjsip_inv_end_session(inv, PJSIP_SC_NOT_ACCEPTABLE, 
                                           NULL, &bye);
            if (status == PJ_SUCCESS && bye)
                status = pjsip_inv_send_msg(inv, bye);

            return;
        }
    }

    /* Set state. */
    inv->state = state;

    /* If state is DISCONNECTED, cause code MUST have been set. */
    pj_assert(inv->state != PJSIP_INV_STATE_DISCONNECTED ||
              inv->cause != 0);

    /* Mark the callback as called for this state */
    inv->cb_called |= (1 << state);

    /* Call on_state_changed() callback.
     * While in the callback, can the state shift to DISCONNECTED? Perhaps
     * yes, so better avoid premature destroy of the invite session by
     * temporarily increase its ref counter.
     */
    pjsip_inv_add_ref(inv);
    if (mod_inv.cb.on_state_changed && inv->notify && !dont_notify)
        (*mod_inv.cb.on_state_changed)(inv, e);
    pjsip_inv_dec_ref(inv);

    /* The above callback may change the state, so we need to be careful here
     * and only decrement inv under the following conditions:
     * 1. If the state parameter is DISCONNECTED, and previous state is not
     *    already DISCONNECTED.
     *    This is to make sure that dec_ref() is not called more than once.
     * 2. If current state is PJSIP_INV_STATE_DISCONNECTED.
     *    This is to make sure that dec_ref() is not called if user restarts
     *    inv within the callback. Note that this check must be last since
     *    inv may have already been destroyed.
     */
    if (state == PJSIP_INV_STATE_DISCONNECTED &&
        prev_state != PJSIP_INV_STATE_DISCONNECTED &&
        inv->state == PJSIP_INV_STATE_DISCONNECTED) 
    {
        pjsip_inv_dec_ref(inv);
    }
}


/*
 * Set cause code.
 */
static void inv_set_cause(pjsip_inv_session *inv, int cause_code,
                          const pj_str_t *cause_text)
{
    if ((cause_code > inv->cause) || inv->pending_bye) {
        inv->cause = (pjsip_status_code) cause_code;
        if (cause_text)
            pj_strdup(inv->pool, &inv->cause_text, cause_text);
        else if (cause_code/100 == 2)
            inv->cause_text = pj_str("Normal call clearing");
        else
            inv->cause_text = *pjsip_get_status_text(cause_code);
    }
}


/*
 * Check if outgoing request needs to have SDP answer.
 * This applies for both ACK and PRACK requests.
 */
static const pjmedia_sdp_session *inv_has_pending_answer(pjsip_inv_session *inv,
                                                         pjsip_transaction *tsx)
{
    pjmedia_sdp_neg_state neg_state;
    const pjmedia_sdp_session *sdp = NULL;
    pj_status_t status;

    /* If SDP negotiator is ready, start negotiation. */

    /* Start nego when appropriate. */
    neg_state = inv->neg ? pjmedia_sdp_neg_get_state(inv->neg) :
                PJMEDIA_SDP_NEG_STATE_NULL;

    if (neg_state == PJMEDIA_SDP_NEG_STATE_DONE) {

        /* Nothing to do */

    } else if (neg_state == PJMEDIA_SDP_NEG_STATE_WAIT_NEGO &&
               pjmedia_sdp_neg_has_local_answer(inv->neg) )
    {
        struct tsx_inv_data *tsx_inv_data;
        struct tsx_inv_data dummy;

        /* Get invite session's transaction data.
         * Note that tsx may be NULL, for example when application sends
         * delayed ACK request (at this time, the original INVITE 
         * transaction may have been destroyed.
         */
        if (tsx) {
            tsx_inv_data = (struct tsx_inv_data*)tsx->mod_data[mod_inv.mod.id];
        } else {
            tsx_inv_data = &dummy;
            pj_bzero(&dummy, sizeof(dummy));
            dummy.inv = inv;
        }

        status = inv_negotiate_sdp(inv);
        if (status != PJ_SUCCESS)
            return NULL;
        
        /* Mark this transaction has having SDP offer/answer done. */
        tsx_inv_data->sdp_done = 1;

        status = pjmedia_sdp_neg_get_active_local(inv->neg, &sdp);

    } else {
        /* This remark is only valid for ACK.
        PJ_LOG(4,(inv->dlg->obj_name,
                  "FYI, the SDP negotiator state (%s) is in a mess "
                  "when sending this ACK/PRACK request",
                  pjmedia_sdp_neg_state_str(neg_state)));
         */
    }

    return sdp;
}

/* Process pending disconnection
 *  https://github.com/pjsip/pjproject/issues/1712
 */
static void inv_perform_pending_bye(pjsip_inv_session *inv)
{
    if (inv->pending_bye) {
        pjsip_tx_data *bye = inv->pending_bye;
        pj_status_t status;

        PJ_LOG(4,(inv->dlg->obj_name, "Sending pending BYE"));

        inv->pending_bye = NULL;
        status = pjsip_inv_send_msg(inv, bye);

        if (status != PJ_SUCCESS) {
            PJ_PERROR(1,(inv->dlg->obj_name, status,
                         "Failed sending pending BYE"));
        }
    }
}

/*
 * Send ACK for 2xx response.
 */
static pj_status_t inv_send_ack(pjsip_inv_session *inv, pjsip_event *e)
{
    pjsip_rx_data *rdata;
    pjsip_event ack_e;
    pj_status_t status;

    if (e->type == PJSIP_EVENT_TSX_STATE)
        rdata = e->body.tsx_state.src.rdata;
    else if (e->type == PJSIP_EVENT_RX_MSG)
        rdata = e->body.rx_msg.rdata;
    else {
        pj_assert(!"Unsupported event type");
        return PJ_EBUG;
    }

    /* Note that with https://github.com/pjsip/pjproject/issues/1725, this
     * function can be called to send ACK for previous INVITE 200/OK
     * retransmission
     */

    PJ_LOG(5,(inv->obj_name, "Received %s, sending ACK",
              pjsip_rx_data_get_info(rdata)));

    /* Check if we have cached ACK request and if we have sent it.
     * Must not use the cached ACK if it's still marked as pending
     * by transport (#1011).
     */
    if (inv->last_ack && rdata->msg_info.cseq->cseq == inv->last_ack_cseq &&
        inv->last_ack->tp_info.transport != NULL &&
        !inv->last_ack->is_pending)
    {
        pjsip_tx_data_add_ref(inv->last_ack);

    } else if (mod_inv.cb.on_send_ack) {
        /* If application handles ACK transmission manually, just notify the
         * callback
         */
        PJ_LOG(5,(inv->obj_name, "Received %s, notifying application callback",
                  pjsip_rx_data_get_info(rdata)));

        (*mod_inv.cb.on_send_ack)(inv, rdata);
        return PJ_SUCCESS;

    } else {
        status = pjsip_inv_create_ack(inv, rdata->msg_info.cseq->cseq,
                                      &inv->last_ack);
        if (status != PJ_SUCCESS)
            return status;
    }

    PJSIP_EVENT_INIT_TX_MSG(ack_e, inv->last_ack);

    /* Send ACK */
    status = pjsip_dlg_send_request(inv->dlg, inv->last_ack, -1, NULL);
    if (status != PJ_SUCCESS) {
        /* Better luck next time */
        pj_assert(!"Unable to send ACK!");
        return status;
    }


    /* Set state to CONFIRMED (if we're not in CONFIRMED yet).
     * But don't set it to CONFIRMED if we're already DISCONNECTED
     * (this may have been a late 200/OK response.
     */
    if (inv->state < PJSIP_INV_STATE_CONFIRMED) {
        inv_set_state(inv, PJSIP_INV_STATE_CONFIRMED, &ack_e);
    } else if (inv->state == PJSIP_INV_STATE_DISCONNECTED && inv->last_ack) {
        /* Avoid possible leaked tdata when invite session is already
         * destroyed.
         * https://github.com/pjsip/pjproject/pull/2432
         */
        pjsip_tx_data_dec_ref(inv->last_ack);
        inv->last_ack = NULL;
    }

    return PJ_SUCCESS;
}

/*
 * Module on_rx_request()
 *
 * This callback is called for these events:
 *  - endpoint receives request which was unhandled by higher priority
 *    modules (e.g. transaction layer, dialog layer).
 *  - dialog distributes incoming request to its usages.
 */
static pj_bool_t mod_inv_on_rx_request(pjsip_rx_data *rdata)
{
    pjsip_method *method;
    pjsip_dialog *dlg;
    pjsip_inv_session *inv;

    /* Only wants to receive request from a dialog. */
    dlg = pjsip_rdata_get_dlg(rdata);
    if (dlg == NULL)
        return PJ_FALSE;

    inv = (pjsip_inv_session*) dlg->mod_data[mod_inv.mod.id];

    /* Report to dialog that we handle INVITE, CANCEL, BYE, ACK. 
     * If we need to send response, it will be sent in the state
     * handlers.
     */
    method = &rdata->msg_info.msg->line.req.method;

    if (method->id == PJSIP_INVITE_METHOD) {
        return PJ_TRUE;
    }

    /* BYE and CANCEL must have existing invite session */
    if (method->id == PJSIP_BYE_METHOD ||
        method->id == PJSIP_CANCEL_METHOD)
    {
        if (inv == NULL)
            return PJ_FALSE;

        return PJ_TRUE;
    }

    /* On receipt ACK request, when state is CONNECTING,
     * move state to CONFIRMED.
     */
    if (method->id == PJSIP_ACK_METHOD && inv) {

        /* Ignore if we don't have INVITE in progress */
        if (!inv->invite_tsx) {
            return PJ_TRUE;
        }

        /* Ignore ACK if pending INVITE transaction has not finished. */
        if (inv->invite_tsx->state < PJSIP_TSX_STATE_COMPLETED) {
            return PJ_TRUE;
        }

        /* Ignore ACK with different CSeq
         * https://github.com/pjsip/pjproject/issues/1391
         */
        if (rdata->msg_info.cseq->cseq != inv->invite_tsx->cseq) {
            return PJ_TRUE;
        }

        /* Terminate INVITE transaction, if it's still present. */
        if (inv->invite_tsx->state <= PJSIP_TSX_STATE_COMPLETED) {
            /* Before we terminate INVITE transaction, process the SDP
             * in the ACK request, if any. 
             * Only do this when invite state is not already disconnected
             * (https://github.com/pjsip/pjproject/issues/640).
             */
            if (inv->state < PJSIP_INV_STATE_DISCONNECTED) {
                inv_check_sdp_in_incoming_msg(inv, inv->invite_tsx, rdata);

                /* Check if local offer got no SDP answer and INVITE session
                 * is in CONFIRMED state.
                 */
                if (pjmedia_sdp_neg_get_state(inv->neg)==
                    PJMEDIA_SDP_NEG_STATE_LOCAL_OFFER &&
                    inv->state==PJSIP_INV_STATE_CONFIRMED)
                {
                    pjmedia_sdp_neg_cancel_offer(inv->neg);
                }
            }

            /* Now we can terminate the INVITE transaction */
            pj_assert(inv->invite_tsx->status_code >= 200);
            pjsip_tsx_terminate(inv->invite_tsx, 
                                inv->invite_tsx->status_code);
            inv->invite_tsx = NULL;
            if (inv->last_answer) {
                    pjsip_tx_data_dec_ref(inv->last_answer);
                    inv->last_answer = NULL;
            }
        }

        /* On receipt of ACK, only set state to confirmed when state
         * is CONNECTING (e.g. we don't want to set the state to confirmed
         * when we receive ACK retransmission after sending non-2xx!)
         */
        if (inv->state == PJSIP_INV_STATE_CONNECTING) {
            pjsip_event event;

            PJSIP_EVENT_INIT_RX_MSG(event, rdata);
            inv_set_state(inv, PJSIP_INV_STATE_CONFIRMED, &event);

            /* Send pending BYE if any:
             *   https://github.com/pjsip/pjproject/issues/1712
             * Do this after setting the state to CONFIRMED, so that we
             * have consistent CONFIRMED state between caller and callee.
             */
            if (inv->pending_bye)
                inv_perform_pending_bye(inv);
        }
    }

    return PJ_FALSE;
}

/* This function will process Session Timer headers in received 
 * 2xx or 422 response of INVITE/UPDATE request.
 */
static pj_status_t handle_timer_response(pjsip_inv_session *inv,
                                         const pjsip_rx_data *rdata,
                                         pj_bool_t end_sess_on_failure)
{
    pjsip_status_code st_code;
    pj_status_t status;

    status = pjsip_timer_process_resp(inv, rdata, &st_code);
    if (status != PJ_SUCCESS && end_sess_on_failure) {
        pjsip_tx_data *tdata;
        pj_status_t status2;

        status2 = pjsip_inv_end_session(inv, st_code, NULL, &tdata);
        if (tdata && status2 == PJ_SUCCESS)
            pjsip_inv_send_msg(inv, tdata);
    }

    return status;
}

/*
 * Module on_rx_response().
 *
 * This callback is called for these events:
 *  - dialog distributes incoming 2xx response to INVITE (outside
 *    transaction) to its usages.
 *  - endpoint distributes strayed responses.
 */
static pj_bool_t mod_inv_on_rx_response(pjsip_rx_data *rdata)
{
    pjsip_dialog *dlg;
    pjsip_inv_session *inv;
    pjsip_msg *msg = rdata->msg_info.msg;

    dlg = pjsip_rdata_get_dlg(rdata);

    /* Ignore responses outside dialog */
    if (dlg == NULL)
        return PJ_FALSE;

    /* Ignore responses not belonging to invite session */
    inv = pjsip_dlg_get_inv_session(dlg);
    if (inv == NULL)
        return PJ_FALSE;

    /* This MAY be retransmission of 2xx response to INVITE. 
     * If it is, we need to send ACK.
     */
    if (msg->type == PJSIP_RESPONSE_MSG && msg->line.status.code/100==2 &&
        rdata->msg_info.cseq->method.id == PJSIP_INVITE_METHOD)
    {
        /* The code inside "if" is called the second time 200/OK
         * retransmission is received. Also handle the situation
         * when we have another re-INVITE on going and 200/OK
         * retransmission is received. See:
         * https://github.com/pjsip/pjproject/issues/1725.
         * Also send ACK for 200/OK of pending re-INVITE after call is
         * disconnected (see https://github.com/pjsip/pjproject/issues/1755).
         */
        if (inv->invite_tsx == NULL ||
            inv->state == PJSIP_INV_STATE_DISCONNECTED ||
            (inv->last_ack && inv->last_ack_cseq==rdata->msg_info.cseq->cseq))
        {
            pjsip_event e;

            PJSIP_EVENT_INIT_RX_MSG(e, rdata);
            inv_send_ack(inv, &e);
            return PJ_TRUE;
        }
    }

    /* No other processing needs to be done here. */
    return PJ_FALSE;
}

/*
 * Module on_tsx_state()
 *
 * This callback is called by dialog framework for all transactions
 * inside the dialog for all its dialog usages.
 */
static void mod_inv_on_tsx_state(pjsip_transaction *tsx, pjsip_event *e)
{
    pjsip_dialog *dlg;
    pjsip_inv_session *inv;

    dlg = pjsip_tsx_get_dlg(tsx);
    if (dlg == NULL)
        return;

    inv = pjsip_dlg_get_inv_session(dlg);
    if (inv == NULL)
        return;

    /* Call state handler for the invite session. */
    (*inv_state_handler[inv->state])(inv, e);

    /* Clear invite transaction when tsx is terminated. 
     * Necessary for app that wants to send a new re-INVITE request immediately
     * after the transaction is terminated. 
     */
    if (tsx->state==PJSIP_TSX_STATE_TERMINATED  && tsx == inv->invite_tsx) {
        inv->invite_tsx = NULL;    

        if (inv->last_answer) {
                pjsip_tx_data_dec_ref(inv->last_answer);
                inv->last_answer = NULL;
        }
    }

    /* Call on_tsx_state. CANCEL request is a special case and has been
     * reported earlier in inv_respond_incoming_cancel()
     */
    if (mod_inv.cb.on_tsx_state_changed && inv->notify &&
        !(tsx->method.id==PJSIP_CANCEL_METHOD &&
          e->body.tsx_state.type==PJSIP_EVENT_RX_MSG))
    {
        (*mod_inv.cb.on_tsx_state_changed)(inv, tsx, e);
    }

    /* Clear invite transaction when tsx is confirmed.
     * Previously we set invite_tsx to NULL only when transaction has
     * terminated, but this didn't work when ACK has the same Via branch
     * value as the INVITE (see https://github.com/pjsip/pjproject/issues/113)
     */
    if (tsx->state>=PJSIP_TSX_STATE_CONFIRMED && tsx == inv->invite_tsx) {      
        inv->invite_tsx = NULL;

        if (inv->last_answer) {
                pjsip_tx_data_dec_ref(inv->last_answer);
                inv->last_answer = NULL;
        }
    }
}

/* 
 * Check if tx_data has sdp. 
 */
static pj_bool_t tx_data_has_sdp(const pjsip_tx_data *tdata)
{
    pjsip_msg_body *body = tdata->msg->body;
    pjsip_media_type app_sdp;

    PJ_ASSERT_RETURN(tdata, PJ_FALSE);

    pjsip_media_type_init2(&app_sdp, "application", "sdp");

    if (body &&
        pj_stricmp(&body->content_type.type, &app_sdp.type)==0 && 
        pj_stricmp(&body->content_type.subtype, &app_sdp.subtype)==0) 
    {
        return PJ_TRUE;

    } else if (body && 
               pj_stricmp2(&body->content_type.type, "multipart") && 
               (pj_stricmp2(&body->content_type.subtype, "mixed")==0 ||
                pj_stricmp2(&body->content_type.subtype, "alternative")==0))    
    {
        pjsip_multipart_part *part;

        part = pjsip_multipart_find_part(body, &app_sdp, NULL);
        if (part) {
            return PJ_TRUE;
        }
    }

    return PJ_FALSE;
}


/*
 * Initialize the invite module.
 */
PJ_DEF(pj_status_t) pjsip_inv_usage_init( pjsip_endpoint *endpt,
                                          const pjsip_inv_callback *cb)
{
    pj_status_t status;

    /* Check arguments. */
    PJ_ASSERT_RETURN(endpt && cb, PJ_EINVAL);

    /* Some callbacks are mandatory */
    PJ_ASSERT_RETURN(cb->on_state_changed, PJ_EINVAL);

    /* Check if module already registered. */
    PJ_ASSERT_RETURN(mod_inv.mod.id == -1, PJ_EINVALIDOP);

    /* Copy param. */
    pj_memcpy(&mod_inv.cb, cb, sizeof(pjsip_inv_callback));

    mod_inv.endpt = endpt;

    /* Register the module. */
    status = pjsip_endpt_register_module(endpt, &mod_inv.mod);
    if (status != PJ_SUCCESS)
        return status;

    return PJ_SUCCESS;
}

/*
 * Get the instance of invite module.
 */
PJ_DEF(pjsip_module*) pjsip_inv_usage_instance(void)
{
    return &mod_inv.mod;
}



/*
 * Return the invite session for the specified dialog.
 */
PJ_DEF(pjsip_inv_session*) pjsip_dlg_get_inv_session(pjsip_dialog *dlg)
{
    return (pjsip_inv_session*) dlg->mod_data[mod_inv.mod.id];
}


/*
 * Get INVITE state name.
 */
PJ_DEF(const char *) pjsip_inv_state_name(pjsip_inv_state state)
{
    PJ_ASSERT_RETURN(state >= PJSIP_INV_STATE_NULL && 
                     state <= PJSIP_INV_STATE_DISCONNECTED,
                     "??");

    return inv_state_names[state];
}

/*
 * Create UAC invite session.
 */
PJ_DEF(pj_status_t) pjsip_inv_create_uac( pjsip_dialog *dlg,
                                          const pjmedia_sdp_session *local_sdp,
                                          unsigned options,
                                          pjsip_inv_session **p_inv)
{
    pjsip_inv_session *inv;
    pj_status_t status;

    /* Verify arguments. */
    PJ_ASSERT_RETURN(dlg && p_inv, PJ_EINVAL);

    /* Must lock dialog first */
    pjsip_dlg_inc_lock(dlg);

    /* Normalize options */
    if (options & PJSIP_INV_REQUIRE_100REL)
        options |= PJSIP_INV_SUPPORT_100REL;
    if (options & PJSIP_INV_REQUIRE_TIMER)
        options |= PJSIP_INV_SUPPORT_TIMER;
    if (options & PJSIP_INV_REQUIRE_TRICKLE_ICE)
        options |= PJSIP_INV_SUPPORT_TRICKLE_ICE;

    /* Create the session */
    inv = PJ_POOL_ZALLOC_T(dlg->pool, pjsip_inv_session);
    pj_assert(inv != NULL);

    status = pj_atomic_create(dlg->pool, 0, &inv->ref_cnt);
    if (status != PJ_SUCCESS) {
        pjsip_dlg_dec_lock(dlg);
        return status;
    }

    inv->pool = dlg->pool;
    inv->role = PJSIP_ROLE_UAC;
    inv->state = PJSIP_INV_STATE_NULL;
    inv->dlg = dlg;
    inv->options = options;
    inv->notify = PJ_TRUE;
    inv->cause = (pjsip_status_code) 0;

    /* Create flip-flop pool (see ticket #877) */
    /* (using inv->obj_name as temporary variable for pool names */
    pj_ansi_snprintf(inv->obj_name, PJ_MAX_OBJ_NAME, "inv%p", dlg->pool);
    inv->pool_prov = pjsip_endpt_create_pool(dlg->endpt, inv->obj_name,
                                             POOL_INIT_SIZE, POOL_INC_SIZE);
    inv->pool_active = pjsip_endpt_create_pool(dlg->endpt, inv->obj_name,
                                               POOL_INIT_SIZE, POOL_INC_SIZE);

    /* Object name will use the same dialog pointer. */
    pj_ansi_snprintf(inv->obj_name, PJ_MAX_OBJ_NAME, "inv%p", dlg);

    /* Create negotiator if local_sdp is specified. */
    if (local_sdp) {
        status = pjmedia_sdp_neg_create_w_local_offer(inv->pool, 
                                                      local_sdp, &inv->neg);
        if (status != PJ_SUCCESS) {
            pjsip_dlg_dec_lock(dlg);
            return status;
        }
    }

    /* Register invite as dialog usage. */
    status = pjsip_dlg_add_usage(dlg, &mod_inv.mod, inv);
    if (status != PJ_SUCCESS) {
        pjsip_dlg_dec_lock(dlg);
        return status;
    }

    /* Increment dialog session */
    pjsip_dlg_inc_session(dlg, &mod_inv.mod);

    /* Create 100rel handler */
    pjsip_100rel_attach(inv);

    /* Done */
    pjsip_inv_add_ref(inv);
    *p_inv = inv;

    pjsip_dlg_dec_lock(dlg);

    PJ_LOG(5,(inv->obj_name, "UAC invite session created for dialog %s",
              dlg->obj_name));

    return PJ_SUCCESS;
}

PJ_DEF(pjsip_sdp_info*) pjsip_get_sdp_info(pj_pool_t *pool,
                                           pjsip_msg_body *body,
                                           pjsip_media_type *msg_media_type,
                                           const pjsip_media_type *search_media_type)
{
    pjsip_sdp_info *sdp_info;
    pjsip_media_type search_type;
    pjsip_media_type multipart_mixed;
    pjsip_media_type multipart_alternative;
    pjsip_media_type *msg_type;
    pj_status_t status;

    sdp_info = PJ_POOL_ZALLOC_T(pool,
                                pjsip_sdp_info);

    PJ_ASSERT_RETURN(mod_inv.mod.id >= 0, sdp_info);

    if (!body) {
        return sdp_info;
    }

    if (msg_media_type) {
        msg_type = msg_media_type;
    } else {
        if (body->content_type.type.slen == 0) {
            return sdp_info;
        }
        msg_type = &body->content_type;
    }

    if (!search_media_type) {
        pjsip_media_type_init2(&search_type, "application", "sdp");
    } else {
        pj_memcpy(&search_type, search_media_type, sizeof(search_type));
    }

    pjsip_media_type_init2(&multipart_mixed, "multipart", "mixed");
    pjsip_media_type_init2(&multipart_alternative, "multipart", "alternative");

    if (pjsip_media_type_cmp(msg_type, &search_type, PJ_FALSE) == 0)
    {
        /*
         * If the print_body function is print_sdp, we know that
         * body->data is a pjmedia_sdp_session object and came from
         * a tx_data.  If not, it's the text representation of the
         * sdp from an rx_data.
         */
        if (body->print_body == print_sdp) {
            sdp_info->sdp = body->data;
        } else {
            sdp_info->body.ptr = (char*)body->data;
            sdp_info->body.slen = body->len;
        }
    } else if (pjsip_media_type_cmp(&multipart_mixed, msg_type, PJ_FALSE) == 0 ||
        pjsip_media_type_cmp(&multipart_alternative, msg_type, PJ_FALSE) == 0)
    {
        pjsip_multipart_part *part;
        part = pjsip_multipart_find_part(body, &search_type, NULL);
        if (part) {
            if (part->body->print_body == print_sdp) {
                sdp_info->sdp = part->body->data;
            } else {
                sdp_info->body.ptr = (char*)part->body->data;
                sdp_info->body.slen = part->body->len;
            }
        }
    }

    /*
     * If the body was already a pjmedia_sdp_session, we can just
     * return it.  If not and there wasn't a text representation
     * of the sdp either, we can also just return.
     */
    if (sdp_info->sdp || !sdp_info->body.ptr) {
        return sdp_info;
    }

    /*
     * If the body was the text representation of teh SDP, we need
     * to parse it to create a pjmedia_sdp_session object.
     */
    status = pjmedia_sdp_parse(pool,
                                sdp_info->body.ptr,
                                sdp_info->body.slen,
                                &sdp_info->sdp);
    if (status == PJ_SUCCESS)
        status = pjmedia_sdp_validate2(sdp_info->sdp, PJ_FALSE);

    if (status != PJ_SUCCESS) {
        sdp_info->sdp = NULL;
        PJ_PERROR(1, (THIS_FILE, status,
            "Error parsing/validating SDP body"));
    }

    sdp_info->sdp_err = status;

    return sdp_info;
}

PJ_DEF(pjsip_rdata_sdp_info*) pjsip_rdata_get_sdp_info2(
                                            pjsip_rx_data *rdata,
                                            const pjsip_media_type *search_media_type)
{
    pjsip_media_type *msg_media_type = NULL;
    pjsip_rdata_sdp_info *sdp_info;

    if (rdata->endpt_info.mod_data[mod_inv.mod.id]) {
        return (pjsip_rdata_sdp_info *)rdata->endpt_info.mod_data[mod_inv.mod.id];
    }

    /*
     * rdata should have a Content-Type header at this point but we'll
     * make sure.
     */
    if (rdata->msg_info.ctype) {
        msg_media_type = &rdata->msg_info.ctype->media;
    }
    sdp_info = pjsip_get_sdp_info(rdata->tp_info.pool,
                                   rdata->msg_info.msg->body,
                                   msg_media_type,
                                   search_media_type);
    rdata->endpt_info.mod_data[mod_inv.mod.id] = sdp_info;

    return sdp_info;
}

PJ_DEF(pjsip_rdata_sdp_info*) pjsip_rdata_get_sdp_info(pjsip_rx_data *rdata)
{
    return pjsip_rdata_get_sdp_info2(rdata, NULL);
}

PJ_DEF(pjsip_tdata_sdp_info*) pjsip_tdata_get_sdp_info2(
                                            pjsip_tx_data *tdata,
                                            const pjsip_media_type *search_media_type)
{
    pjsip_ctype_hdr *ctype_hdr = NULL;
    pjsip_media_type *msg_media_type = NULL;
    pjsip_tdata_sdp_info *sdp_info;

    if (tdata->mod_data[mod_inv.mod.id]) {
        return (pjsip_tdata_sdp_info *)tdata->mod_data[mod_inv.mod.id];
    }
    /*
     * tdata won't usually have a Content-Type header at this point
     * but we'll check just the same,
     */
    ctype_hdr = pjsip_msg_find_hdr(tdata->msg, PJSIP_H_CONTENT_TYPE, NULL);
    if (ctype_hdr) {
        msg_media_type = &ctype_hdr->media;
    }

    sdp_info = pjsip_get_sdp_info(tdata->pool,
                                   tdata->msg->body,
                                   msg_media_type,
                                   search_media_type);
    tdata->mod_data[mod_inv.mod.id] = sdp_info;

    return sdp_info;
}

PJ_DEF(pjsip_tdata_sdp_info*) pjsip_tdata_get_sdp_info(pjsip_tx_data *tdata)
{
    return pjsip_tdata_get_sdp_info2(tdata, NULL);
}

/*
 * Verify incoming INVITE request.
 */
PJ_DEF(pj_status_t) pjsip_inv_verify_request3(pjsip_rx_data *rdata,
                                              pj_pool_t *tmp_pool,
                                              unsigned *options,
                                              const pjmedia_sdp_session *r_sdp,
                                              const pjmedia_sdp_session *l_sdp,
                                              pjsip_dialog *dlg,
                                              pjsip_endpoint *endpt,
                                              pjsip_tx_data **p_tdata)
{
    pjsip_msg *msg = NULL;
    pjsip_allow_hdr *allow = NULL;
    pjsip_supported_hdr *sup_hdr = NULL;
    pjsip_require_hdr *req_hdr = NULL;
    pjsip_contact_hdr *c_hdr = NULL;
    int code = 200;
    unsigned rem_option = 0;
    pj_status_t status = PJ_SUCCESS;
    pjsip_hdr res_hdr_list;
    pjsip_rdata_sdp_info *sdp_info;

    /* Init return arguments. */
    if (p_tdata) *p_tdata = NULL;

    /* Verify arguments. */
    PJ_ASSERT_RETURN(tmp_pool != NULL && options != NULL, PJ_EINVAL);

    /* Normalize options */
    if (*options & PJSIP_INV_REQUIRE_100REL)
        *options |= PJSIP_INV_SUPPORT_100REL;
    if (*options & PJSIP_INV_REQUIRE_TIMER)
        *options |= PJSIP_INV_SUPPORT_TIMER;
    if (*options & PJSIP_INV_REQUIRE_ICE)
        *options |= PJSIP_INV_SUPPORT_ICE;
    if (*options & PJSIP_INV_REQUIRE_TRICKLE_ICE)
        *options |= PJSIP_INV_SUPPORT_TRICKLE_ICE;

    if (rdata) {
        /* Get the message in rdata */
        msg = rdata->msg_info.msg;
    
        /* Must be INVITE request. */
        PJ_ASSERT_RETURN(msg && msg->type == PJSIP_REQUEST_MSG &&
                         msg->line.req.method.id == PJSIP_INVITE_METHOD,
                         PJ_EINVAL);
    }

    /* If tdata is specified, then either dlg or endpt must be specified */
    PJ_ASSERT_RETURN((!p_tdata) || (endpt || dlg), PJ_EINVAL);

    /* Get the endpoint */
    endpt = endpt ? endpt : dlg->endpt;

    /* Init response header list */
    pj_list_init(&res_hdr_list);

    /* Check the Contact header */
    if (msg) {
        c_hdr = (pjsip_contact_hdr*)
                pjsip_msg_find_hdr(msg, PJSIP_H_CONTACT, NULL);
    }
    if (msg && (!c_hdr || !c_hdr->uri)) {
        /* Missing Contact header or Contact contains "*" */
        pjsip_warning_hdr *w;
        pj_str_t warn_text;

        warn_text = pj_str("Bad/missing Contact header");
        w = pjsip_warning_hdr_create(tmp_pool, 399,
                                     pjsip_endpt_name(endpt),
                                     &warn_text);
        if (w) {
            pj_list_push_back(&res_hdr_list, w);
        }

        code = PJSIP_SC_BAD_REQUEST;
        status = PJSIP_ERRNO_FROM_SIP_STATUS(code);
        goto on_return;
    }

    /* Ticket #1735: Check Contact/Record-Route header in a secure dialog. */
    if (pjsip_cfg()->endpt.disable_secure_dlg_check == PJ_FALSE &&
        msg && PJSIP_URI_SCHEME_IS_SIPS(msg->line.req.uri))
    {
        /* Check Contact header */
        if (!PJSIP_URI_SCHEME_IS_SIPS(c_hdr->uri))
            status = PJSIP_ESESSIONINSECURE;

        /* Check top Record-Route header */
        if (status == PJ_SUCCESS) {
            pjsip_rr_hdr *r = (pjsip_rr_hdr*)
                              pjsip_msg_find_hdr(msg, PJSIP_H_RECORD_ROUTE,
                                                 NULL);
            if (r && !PJSIP_URI_SCHEME_IS_SIPS(&r->name_addr)) {
                /* Not "sips", check if it is "sip" and has param
                 * "transport=tls".
                 */
                if (PJSIP_URI_SCHEME_IS_SIP(&r->name_addr)) {
                    pjsip_sip_uri *sip_uri = (pjsip_sip_uri*)
                                     pjsip_uri_get_uri(r->name_addr.uri);
                    if (pj_stricmp2(&sip_uri->transport_param, "tls")!=0)
                        status = PJSIP_ESESSIONINSECURE;
                } else {
                    /* Not "sips" nor "sip", treat it as insecure? */
                    status = PJSIP_ESESSIONINSECURE;
                }
            }
        }

        if (status != PJ_SUCCESS) {
            pjsip_warning_hdr *w;
            pj_str_t warn_text = pj_str("SIPS Required");
            w = pjsip_warning_hdr_create(tmp_pool, 381,
                                         pjsip_endpt_name(endpt),
                                         &warn_text);
            if (w) {
                pj_list_push_back(&res_hdr_list, w);
            }
            code = PJSIP_SC_TEMPORARILY_UNAVAILABLE;
            goto on_return;
        }
    }

    /* Check the request body, see if it's something that we support,
     * only when the body hasn't been parsed before.
     */
    if (r_sdp == NULL && rdata) {
        sdp_info = pjsip_rdata_get_sdp_info(rdata);
    } else {
        sdp_info = NULL;
    }

    if (r_sdp==NULL && msg && msg->body) {

        /* Check if body really contains SDP. */
        if (sdp_info->body.ptr == NULL && !PJSIP_INV_ACCEPT_UNKNOWN_BODY) {
            /* Couldn't find "application/sdp" */
            code = PJSIP_SC_UNSUPPORTED_MEDIA_TYPE;
            status = PJSIP_ERRNO_FROM_SIP_STATUS(code);

            if (p_tdata) {
                /* Add Accept header to response */
                pjsip_accept_hdr *acc;

                acc = pjsip_accept_hdr_create(tmp_pool);
                PJ_ASSERT_RETURN(acc, PJ_ENOMEM);
                acc->values[acc->count++] = pj_str("application/sdp");
                pj_list_push_back(&res_hdr_list, acc);
            }

            goto on_return;
        }

        if (sdp_info->sdp_err != PJ_SUCCESS) {
            /* Unparseable or invalid SDP */
            code = PJSIP_SC_BAD_REQUEST;

            if (p_tdata) {
                /* Add Warning header. */
                pjsip_warning_hdr *w;

                w = pjsip_warning_hdr_create_from_status(tmp_pool,
                                                         pjsip_endpt_name(endpt),
                                                         sdp_info->sdp_err);
                PJ_ASSERT_RETURN(w, PJ_ENOMEM);

                pj_list_push_back(&res_hdr_list, w);
            }

            goto on_return;
        }

        r_sdp = sdp_info->sdp;
    }

    if (r_sdp) {
        /* Negotiate with local SDP */
        if (l_sdp) {
            pjmedia_sdp_neg *neg;

            /* Local SDP must be valid! */
            status = pjmedia_sdp_validate(l_sdp);
            if (status != PJ_SUCCESS) {
                pj_assert(!"Invalid local SDP");
                code = PJSIP_SC_INTERNAL_SERVER_ERROR;
                goto on_return;
            }

            /* Create SDP negotiator */
            status = pjmedia_sdp_neg_create_w_remote_offer(
                            tmp_pool, l_sdp, r_sdp, &neg);
            if (status != PJ_SUCCESS) {
                pj_assert(!"Failed creating SDP negotiator");
                code = PJSIP_SC_INTERNAL_SERVER_ERROR;
                goto on_return;
            }

            /* Negotiate SDP */
            status = pjmedia_sdp_neg_negotiate(tmp_pool, neg, 0);
            if (status != PJ_SUCCESS) {

                /* Incompatible media */
                code = PJSIP_SC_NOT_ACCEPTABLE_HERE;

                if (p_tdata) {
                    pjsip_accept_hdr *acc;
                    pjsip_warning_hdr *w;

                    /* Add Warning header. */
                    w = pjsip_warning_hdr_create_from_status(
                                            tmp_pool, 
                                            pjsip_endpt_name(endpt), status);
                    PJ_ASSERT_RETURN(w, PJ_ENOMEM);

                    pj_list_push_back(&res_hdr_list, w);

                    /* Add Accept header to response */
                    acc = pjsip_accept_hdr_create(tmp_pool);
                    PJ_ASSERT_RETURN(acc, PJ_ENOMEM);
                    acc->values[acc->count++] = pj_str("application/sdp");
                    pj_list_push_back(&res_hdr_list, acc);

                }

                goto on_return;
            }
        }
    }

    /* Check supported methods, see if peer supports UPDATE.
     * We just assume that peer supports standard INVITE, ACK, CANCEL, and BYE
     * implicitly by sending this INVITE.
     */
    if (msg) {
        allow = (pjsip_allow_hdr*) pjsip_msg_find_hdr(msg, PJSIP_H_ALLOW,
                                                      NULL);
    }
    if (allow) {
        unsigned i;
        const pj_str_t STR_UPDATE = { "UPDATE", 6 };

        for (i=0; i<allow->count; ++i) {
            if (pj_stricmp(&allow->values[i], &STR_UPDATE)==0)
                break;
        }

        if (i != allow->count) {
            /* UPDATE is present in Allow */
            *options |= PJSIP_INV_SUPPORT_UPDATE;
        }

    }

    /* Check Supported header */
    if (msg) {
        sup_hdr = (pjsip_supported_hdr*)
                  pjsip_msg_find_hdr(msg, PJSIP_H_SUPPORTED, NULL);
    }
    if (sup_hdr) {
        unsigned i;
        const pj_str_t STR_100REL = { "100rel", 6};
        const pj_str_t STR_TIMER = { "timer", 5};
        const pj_str_t STR_ICE = { "ice", 3 };
        const pj_str_t STR_TRICKLE_ICE = { "trickle-ice", 11 };

        for (i=0; i<sup_hdr->count; ++i) {
            if (pj_stricmp(&sup_hdr->values[i], &STR_100REL)==0)
                rem_option |= PJSIP_INV_SUPPORT_100REL;
            else if (pj_stricmp(&sup_hdr->values[i], &STR_TIMER)==0)
                rem_option |= PJSIP_INV_SUPPORT_TIMER;
            else if (pj_stricmp(&sup_hdr->values[i], &STR_ICE)==0)
                rem_option |= PJSIP_INV_SUPPORT_ICE;
            else if (pj_stricmp(&sup_hdr->values[i], &STR_TRICKLE_ICE)==0)
                rem_option |= PJSIP_INV_SUPPORT_TRICKLE_ICE;
        }
    }

    /* Check Require header */
    if (msg) {
        req_hdr = (pjsip_require_hdr*)
                  pjsip_msg_find_hdr(msg, PJSIP_H_REQUIRE, NULL);
    }
    if (req_hdr) {
        unsigned i;
        const pj_str_t STR_100REL = { "100rel", 6};
        const pj_str_t STR_REPLACES = { "replaces", 8 };
        const pj_str_t STR_TIMER = { "timer", 5 };
        const pj_str_t STR_ICE = { "ice", 3 };
        const pj_str_t STR_TRICKLE_ICE = { "trickle-ice", 11 };
        unsigned unsupp_cnt = 0;
        pj_str_t unsupp_tags[PJSIP_GENERIC_ARRAY_MAX_COUNT];
        
        for (i=0; i<req_hdr->count; ++i) {
            if ((*options & PJSIP_INV_SUPPORT_100REL) && 
                pj_stricmp(&req_hdr->values[i], &STR_100REL)==0)
            {
                rem_option |= PJSIP_INV_REQUIRE_100REL;

            } else if ((*options & PJSIP_INV_SUPPORT_TIMER) && 
                pj_stricmp(&req_hdr->values[i], &STR_TIMER)==0)
            {
                rem_option |= PJSIP_INV_REQUIRE_TIMER;

            } else if (pj_stricmp(&req_hdr->values[i], &STR_REPLACES)==0) {
                pj_bool_t supp;
                
                supp = pjsip_endpt_has_capability(endpt, PJSIP_H_SUPPORTED, 
                                                  NULL, &STR_REPLACES);
                if (!supp)
                    unsupp_tags[unsupp_cnt++] = req_hdr->values[i];
            } else if ((*options & PJSIP_INV_SUPPORT_ICE) &&
                pj_stricmp(&req_hdr->values[i], &STR_ICE)==0)
            {
                rem_option |= PJSIP_INV_REQUIRE_ICE;

            } else if ((*options & PJSIP_INV_SUPPORT_TRICKLE_ICE) &&
                pj_stricmp(&req_hdr->values[i], &STR_TRICKLE_ICE)==0)
            {
                rem_option |= PJSIP_INV_REQUIRE_TRICKLE_ICE;

            } else if (!pjsip_endpt_has_capability(endpt, PJSIP_H_SUPPORTED,
                                                   NULL, &req_hdr->values[i]))
            {
                /* Unknown/unsupported extension tag!  */
                unsupp_tags[unsupp_cnt++] = req_hdr->values[i];
            }
        }

        /* Check if there are required tags that we don't support */
        if (unsupp_cnt) {

            code = PJSIP_SC_BAD_EXTENSION;
            status = PJSIP_ERRNO_FROM_SIP_STATUS(code);

            if (p_tdata) {
                pjsip_unsupported_hdr *unsupp_hdr;
                const pjsip_hdr *h;

                /* Add Unsupported header. */
                unsupp_hdr = pjsip_unsupported_hdr_create(tmp_pool);
                PJ_ASSERT_RETURN(unsupp_hdr != NULL, PJ_ENOMEM);

                unsupp_hdr->count = unsupp_cnt;
                for (i=0; i<unsupp_cnt; ++i)
                    unsupp_hdr->values[i] = unsupp_tags[i];

                pj_list_push_back(&res_hdr_list, unsupp_hdr);

                /* Add Supported header. */
                h = pjsip_endpt_get_capability(endpt, PJSIP_H_SUPPORTED, 
                                               NULL);
                pj_assert(h);
                if (h) {
                    sup_hdr = (pjsip_supported_hdr*)
                              pjsip_hdr_clone(tmp_pool, h);
                    pj_list_push_back(&res_hdr_list, sup_hdr);
                }
            }

            goto on_return;
        }
    }

    /* Check if there are local requirements that are not supported
     * by peer.
     */
    if ( msg && (((*options & PJSIP_INV_REQUIRE_100REL)!=0 && 
                  (rem_option & PJSIP_INV_SUPPORT_100REL)==0) ||
                 ((*options & PJSIP_INV_REQUIRE_TIMER)!=0 && 
                  (rem_option & PJSIP_INV_SUPPORT_TIMER)==0) ||
                 ((*options & PJSIP_INV_REQUIRE_TRICKLE_ICE)!=0 && 
                  (rem_option & PJSIP_INV_SUPPORT_TRICKLE_ICE)==0)))
    {
        code = PJSIP_SC_EXTENSION_REQUIRED;
        status = PJSIP_ERRNO_FROM_SIP_STATUS(code);

        if (p_tdata) {
            const pjsip_hdr *h;

            /* Add Require header. */
            req_hdr = pjsip_require_hdr_create(tmp_pool);
            PJ_ASSERT_RETURN(req_hdr != NULL, PJ_ENOMEM);

            if (*options & PJSIP_INV_REQUIRE_100REL)
                req_hdr->values[req_hdr->count++] = pj_str("100rel");
            if (*options & PJSIP_INV_REQUIRE_TIMER)
                req_hdr->values[req_hdr->count++] = pj_str("timer");
            if (*options & PJSIP_INV_REQUIRE_TRICKLE_ICE)
                req_hdr->values[req_hdr->count++] = pj_str("trickle-ice");

            pj_list_push_back(&res_hdr_list, req_hdr);

            /* Add Supported header. */
            h = pjsip_endpt_get_capability(endpt, PJSIP_H_SUPPORTED, 
                                           NULL);
            pj_assert(h);
            if (h) {
                sup_hdr = (pjsip_supported_hdr*)
                          pjsip_hdr_clone(tmp_pool, h);
                pj_list_push_back(&res_hdr_list, sup_hdr);
            }

        }

        goto on_return;
    }

    /* If remote Require something that we support, make us Require
     * that feature too.
     */
    if (rem_option & PJSIP_INV_REQUIRE_100REL) {
            pj_assert(*options & PJSIP_INV_SUPPORT_100REL);
            *options |= PJSIP_INV_REQUIRE_100REL;
    }
    if (rem_option & PJSIP_INV_REQUIRE_TIMER) {
            pj_assert(*options & PJSIP_INV_SUPPORT_TIMER);
            *options |= PJSIP_INV_REQUIRE_TIMER;
    }
    if (rem_option & PJSIP_INV_REQUIRE_TRICKLE_ICE) {
            pj_assert(*options & PJSIP_INV_SUPPORT_TRICKLE_ICE);
            *options |= PJSIP_INV_REQUIRE_TRICKLE_ICE;
    }

on_return:

    /* Create response if necessary */
    if (code != 200 && p_tdata) {
        pjsip_tx_data *tdata;
        const pjsip_hdr *h;

        if (!rdata) {
            return PJSIP_ERRNO_FROM_SIP_STATUS(code);
        }

        if (dlg) {
            status = pjsip_dlg_create_response(dlg, rdata, code, NULL, 
                                               &tdata);
        } else {
            status = pjsip_endpt_create_response(endpt, rdata, code, NULL, 
                                                 &tdata);
        }

        if (status != PJ_SUCCESS)
            return status;

        /* Add response headers. */
        h = res_hdr_list.next;
        while (h != &res_hdr_list) {
            pjsip_hdr *cloned;

            cloned = (pjsip_hdr*) pjsip_hdr_clone(tdata->pool, h);
            PJ_ASSERT_RETURN(cloned, PJ_ENOMEM);

            pjsip_msg_add_hdr(tdata->msg, cloned);

            h = h->next;
        }

        *p_tdata = tdata;

        /* Can not return PJ_SUCCESS when response message is produced.
         * Ref: PROTOS test ~#2490
         */
        if (status == PJ_SUCCESS)
            status = PJSIP_ERRNO_FROM_SIP_STATUS(code);

    }

    return status;
}


/*
 * Verify incoming INVITE request.
 */
PJ_DEF(pj_status_t) pjsip_inv_verify_request2(pjsip_rx_data *rdata,
                                              unsigned *options,
                                              const pjmedia_sdp_session *r_sdp,
                                              const pjmedia_sdp_session *l_sdp,
                                              pjsip_dialog *dlg,
                                              pjsip_endpoint *endpt,
                                              pjsip_tx_data **p_tdata)
{
    return pjsip_inv_verify_request3(rdata, rdata->tp_info.pool,
                                     options, r_sdp, l_sdp, dlg, 
                                     endpt, p_tdata);
}


/*
 * Verify incoming INVITE request.
 */
PJ_DEF(pj_status_t) pjsip_inv_verify_request( pjsip_rx_data *rdata,
                                              unsigned *options,
                                              const pjmedia_sdp_session *l_sdp,
                                              pjsip_dialog *dlg,
                                              pjsip_endpoint *endpt,
                                              pjsip_tx_data **p_tdata)
{
    return pjsip_inv_verify_request3(rdata, rdata->tp_info.pool,
                                     options, NULL, l_sdp, dlg, 
                                     endpt, p_tdata);
}

/*
 * Create UAS invite session.
 */
PJ_DEF(pj_status_t) pjsip_inv_create_uas( pjsip_dialog *dlg,
                                          pjsip_rx_data *rdata,
                                          const pjmedia_sdp_session *local_sdp,
                                          unsigned options,
                                          pjsip_inv_session **p_inv)
{
    pjsip_inv_session *inv;
    struct tsx_inv_data *tsx_inv_data;
    pjsip_msg *msg;
    pjsip_rdata_sdp_info *sdp_info;
    pj_status_t status;

    /* Verify arguments. */
    PJ_ASSERT_RETURN(dlg && rdata && p_inv, PJ_EINVAL);

    /* Dialog MUST have been initialised. */
    PJ_ASSERT_RETURN(pjsip_rdata_get_tsx(rdata) != NULL, PJ_EINVALIDOP);

    msg = rdata->msg_info.msg;

    /* rdata MUST contain INVITE request */
    PJ_ASSERT_RETURN(msg->type == PJSIP_REQUEST_MSG &&
                     msg->line.req.method.id == PJSIP_INVITE_METHOD,
                     PJ_EINVALIDOP);

    /* Lock dialog */
    pjsip_dlg_inc_lock(dlg);

    /* Normalize options */
    if (options & PJSIP_INV_REQUIRE_100REL)
        options |= PJSIP_INV_SUPPORT_100REL;
    if (options & PJSIP_INV_REQUIRE_TIMER)
        options |= PJSIP_INV_SUPPORT_TIMER;

    /* Create the session */
    inv = PJ_POOL_ZALLOC_T(dlg->pool, pjsip_inv_session);
    pj_assert(inv != NULL);

    status = pj_atomic_create(dlg->pool, 0, &inv->ref_cnt);
    if (status != PJ_SUCCESS) {
        pjsip_dlg_dec_lock(dlg);
        return status;
    }

    inv->pool = dlg->pool;
    inv->role = PJSIP_ROLE_UAS;
    inv->state = PJSIP_INV_STATE_NULL;
    inv->dlg = dlg;
    inv->options = options;
    inv->notify = PJ_TRUE;
    inv->cause = (pjsip_status_code) 0;

    /* Create flip-flop pool (see ticket #877) */
    /* (using inv->obj_name as temporary variable for pool names */
    pj_ansi_snprintf(inv->obj_name, PJ_MAX_OBJ_NAME, "inv%p", dlg->pool);
    inv->pool_prov = pjsip_endpt_create_pool(dlg->endpt, inv->obj_name,
                                             POOL_INIT_SIZE, POOL_INC_SIZE);
    inv->pool_active = pjsip_endpt_create_pool(dlg->endpt, inv->obj_name,
                                               POOL_INIT_SIZE, POOL_INC_SIZE);

    /* Object name will use the same dialog pointer. */
    pj_ansi_snprintf(inv->obj_name, PJ_MAX_OBJ_NAME, "inv%p", dlg);

    /* Process SDP in message body, if present. */
    sdp_info = pjsip_rdata_get_sdp_info(rdata);
    if (sdp_info->sdp_err) {
        pjsip_dlg_dec_lock(dlg);
        return sdp_info->sdp_err;
    }

    /* Create negotiator. */
    if (sdp_info->sdp) {
        status = pjmedia_sdp_neg_create_w_remote_offer(inv->pool, local_sdp,
                                                       sdp_info->sdp,
                                                       &inv->neg);
                                                
    } else if (local_sdp) {
        status = pjmedia_sdp_neg_create_w_local_offer(inv->pool, 
                                                      local_sdp, &inv->neg);
    } else {
        status = PJ_SUCCESS;
    }

    if (status != PJ_SUCCESS) {
        pjsip_dlg_dec_lock(dlg);
        return status;
    }

    /* Register invite as dialog usage. */
    status = pjsip_dlg_add_usage(dlg, &mod_inv.mod, inv);
    if (status != PJ_SUCCESS) {
        pjsip_dlg_dec_lock(dlg);
        return status;
    }

    /* Increment session in the dialog. */
    pjsip_dlg_inc_session(dlg, &mod_inv.mod);

    /* Save the invite transaction. */
    inv->invite_tsx = pjsip_rdata_get_tsx(rdata);

    /* Attach our data to the transaction. */
    tsx_inv_data = PJ_POOL_ZALLOC_T(inv->invite_tsx->pool, struct tsx_inv_data);
    tsx_inv_data->inv = inv;
    tsx_inv_data->has_sdp = (sdp_info->sdp!=NULL);
    inv->invite_tsx->mod_data[mod_inv.mod.id] = tsx_inv_data;

    /* Create 100rel handler */
    if (inv->options & PJSIP_INV_REQUIRE_100REL) {
        pjsip_100rel_attach(inv);
    }

    /* Done */
    pjsip_inv_add_ref(inv);
    pjsip_dlg_dec_lock(dlg);
    *p_inv = inv;

    PJ_LOG(5,(inv->obj_name, "UAS invite session created for dialog %s",
              dlg->obj_name));

    return PJ_SUCCESS;
}

/*
 * Forcefully terminate the session.
 */
PJ_DEF(pj_status_t) pjsip_inv_terminate( pjsip_inv_session *inv,
                                         int st_code,
                                         pj_bool_t notify)
{
    PJ_ASSERT_RETURN(inv, PJ_EINVAL);

    /* Lock dialog. */
    pjsip_dlg_inc_lock(inv->dlg);

    /* Set callback notify flag. */
    inv->notify = notify;

    /* If there's pending transaction, terminate the transaction. 
     * This may subsequently set the INVITE session state to
     * disconnected.
     */
    if (inv->invite_tsx && 
        inv->invite_tsx->state <= PJSIP_TSX_STATE_COMPLETED)
    {
        pjsip_tsx_terminate(inv->invite_tsx, st_code);

    }

    /* Set cause. */
    inv_set_cause(inv, st_code, NULL);

    /* Forcefully terminate the session if state is not DISCONNECTED */
    if (inv->state != PJSIP_INV_STATE_DISCONNECTED) {
        pjsip_event usr_event;

        PJSIP_EVENT_INIT_USER(usr_event, NULL, NULL, NULL, NULL);
        inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, &usr_event);
    }

    /* Done.
     * The dec_lock() below will actually destroys the dialog if it
     * has no other session.
     */
    pjsip_dlg_dec_lock(inv->dlg);

    return PJ_SUCCESS;
}


/*
 * Restart UAC session, possibly because app or us wants to re-send the 
 * INVITE request due to 401/407 challenge or 3xx response.
 */
PJ_DEF(pj_status_t) pjsip_inv_uac_restart(pjsip_inv_session *inv,
                                          pj_bool_t new_offer)
{
    PJ_ASSERT_RETURN(inv, PJ_EINVAL);

    inv->state = PJSIP_INV_STATE_NULL;
    inv->invite_tsx = NULL;
    if (inv->last_answer) {
        pjsip_tx_data_dec_ref(inv->last_answer);
        inv->last_answer = NULL;
    }

    if (new_offer && inv->neg) {
        pjmedia_sdp_neg_state neg_state;

        neg_state = pjmedia_sdp_neg_get_state(inv->neg);
        if (neg_state == PJMEDIA_SDP_NEG_STATE_LOCAL_OFFER) {
            pjmedia_sdp_neg_cancel_offer(inv->neg);
        }
    }

    return PJ_SUCCESS;
}


static void *clone_sdp(pj_pool_t *pool, const void *data, unsigned len)
{
    PJ_UNUSED_ARG(len);
    return pjmedia_sdp_session_clone(pool, (const pjmedia_sdp_session*)data);
}

static int print_sdp(pjsip_msg_body *body, char *buf, pj_size_t len)
{
    return pjmedia_sdp_print((const pjmedia_sdp_session*)body->data, buf, len);
}


PJ_DEF(pj_status_t) pjsip_create_sdp_body( pj_pool_t *pool,
                                           pjmedia_sdp_session *sdp,
                                           pjsip_msg_body **p_body)
{
    const pj_str_t STR_APPLICATION = { "application", 11};
    const pj_str_t STR_SDP = { "sdp", 3 };
    pjsip_msg_body *body;

    body = PJ_POOL_ZALLOC_T(pool, pjsip_msg_body);
    PJ_ASSERT_RETURN(body != NULL, PJ_ENOMEM);

    pjsip_media_type_init(&body->content_type, (pj_str_t*)&STR_APPLICATION,
                          (pj_str_t*)&STR_SDP);
    body->data = sdp;
    body->len = 0;
    body->clone_data = &clone_sdp;
    body->print_body = &print_sdp;

    *p_body = body;

    return PJ_SUCCESS;
}

static pjsip_multipart_part* create_sdp_part(pj_pool_t *pool, pjmedia_sdp_session *sdp)
{
    pjsip_multipart_part *sdp_part;
    pjsip_media_type media_type;

    pjsip_media_type_init2(&media_type, "application", "sdp");

    sdp_part = pjsip_multipart_create_part(pool);
    PJ_ASSERT_RETURN(sdp_part != NULL, NULL);

    sdp_part->body = PJ_POOL_ZALLOC_T(pool, pjsip_msg_body);
    PJ_ASSERT_RETURN(sdp_part->body != NULL, NULL);

    pjsip_media_type_cp(pool, &sdp_part->body->content_type, &media_type);

    sdp_part->body->data = sdp;
    sdp_part->body->clone_data = clone_sdp;
    sdp_part->body->print_body = print_sdp;

    return sdp_part;
}

PJ_DEF(pj_status_t) pjsip_create_multipart_sdp_body(pj_pool_t *pool,
                                                     pjmedia_sdp_session *sdp,
                                                     pjsip_msg_body **p_body)
{
    pjsip_media_type media_type;
    pjsip_msg_body *multipart;
    pjsip_multipart_part *sdp_part;

    pjsip_media_type_init2(&media_type, "multipart", "mixed");
    multipart = pjsip_multipart_create(pool, &media_type, NULL);
    PJ_ASSERT_RETURN(multipart != NULL, PJ_ENOMEM);

    sdp_part = create_sdp_part(pool, sdp);
    PJ_ASSERT_RETURN(sdp_part != NULL, PJ_ENOMEM);
    pjsip_multipart_add_part(pool, multipart, sdp_part);
    *p_body = multipart;

    return PJ_SUCCESS;
}

static pjsip_msg_body *create_sdp_body(pj_pool_t *pool,
                                       const pjmedia_sdp_session *c_sdp)
{
    pjsip_msg_body *body;
    pj_status_t status;

    status = pjsip_create_sdp_body(pool,
                                   pjmedia_sdp_session_clone(pool, c_sdp),
                                   &body);

    if (status != PJ_SUCCESS)
        return NULL;

    return body;
}

/* Utility to remove a string value from generic array header */
static void remove_val_from_array_hdr(pjsip_generic_array_hdr *arr_hdr,
                                      const pj_str_t *val)
{
    unsigned i;
    for (i=0; i<arr_hdr->count; ++i) {
        if (pj_stricmp(&arr_hdr->values[i], val)==0) {
            pj_array_erase(arr_hdr->values, sizeof(arr_hdr->values[0]),
                           arr_hdr->count, i);
            --arr_hdr->count;
            break;
        }
    }
}


/* Remove disabled extensions, e.g: timer & 100rel, from Allow/Supported
 * headers (see ticket #1858).
 */
static void cleanup_allow_sup_hdr(unsigned inv_option,
                                  pjsip_tx_data *tdata,
                                  pjsip_allow_hdr *allow_hdr,
                                  pjsip_supported_hdr *sup_hdr)
{
    /* If all extensions are enabled, nothing to do */
    if ((inv_option & PJSIP_INV_SUPPORT_100REL) &&
        (inv_option & PJSIP_INV_SUPPORT_TIMER) &&
        (inv_option & PJSIP_INV_SUPPORT_TRICKLE_ICE))
    {
        return;
    }

    if (!allow_hdr && tdata) {
        allow_hdr = (pjsip_allow_hdr*) pjsip_msg_find_hdr(tdata->msg,
                                                          PJSIP_H_ALLOW,
                                                          NULL);
    }
    if (!sup_hdr && tdata) {
        sup_hdr = (pjsip_supported_hdr*) pjsip_msg_find_hdr(tdata->msg,
                                                            PJSIP_H_SUPPORTED,
                                                            NULL);
    }

    /* Remove "timer" from Supported header if Session-Timers is
     * disabled (https://github.com/pjsip/pjproject/issues/1761)
     */
    if ((inv_option & PJSIP_INV_SUPPORT_TIMER) == 0 && sup_hdr) {
        const pj_str_t STR_TIMER = { "timer", 5 };
        remove_val_from_array_hdr(sup_hdr, &STR_TIMER);
    }

    if ((inv_option & PJSIP_INV_SUPPORT_TRICKLE_ICE) == 0 && sup_hdr) {
        const pj_str_t STR_TRICKLE_ICE = { "trickle-ice", 11 };
        remove_val_from_array_hdr(sup_hdr, &STR_TRICKLE_ICE);
    }

    if ((inv_option & PJSIP_INV_SUPPORT_100REL) == 0) {
        const pj_str_t STR_PRACK  = { "PRACK", 5 };
        const pj_str_t STR_100REL = { "100rel", 6 };

        if (allow_hdr)
            remove_val_from_array_hdr(allow_hdr, &STR_PRACK);
        if (sup_hdr)
            remove_val_from_array_hdr(sup_hdr, &STR_100REL);
    }
}

/*
 * Create initial INVITE request.
 */
PJ_DEF(pj_status_t) pjsip_inv_invite( pjsip_inv_session *inv,
                                      pjsip_tx_data **p_tdata )
{
    pjsip_tx_data *tdata;
    const pjsip_hdr *hdr;
    pjsip_allow_hdr *allow_hdr = NULL;
    pjsip_supported_hdr *sup_hdr = NULL;
    pj_bool_t has_sdp;
    pj_status_t status;

    /* Verify arguments. */
    PJ_ASSERT_RETURN(inv && p_tdata, PJ_EINVAL);

    /* State MUST be NULL or CONFIRMED. */
    PJ_ASSERT_RETURN(inv->state == PJSIP_INV_STATE_NULL ||
                     inv->state == PJSIP_INV_STATE_CONFIRMED, 
                     PJ_EINVALIDOP);

    /* Lock dialog. */
    pjsip_dlg_inc_lock(inv->dlg);

    /* Create the INVITE request. */
    status = pjsip_dlg_create_request(inv->dlg, pjsip_get_invite_method(), -1,
                                      &tdata);
    if (status != PJ_SUCCESS)
        goto on_return;


    /* If this is the first INVITE, then copy the headers from inv_hdr.
     * These are the headers parsed from the request URI when the
     * dialog was created.
     */
    if (inv->state == PJSIP_INV_STATE_NULL) {
        hdr = inv->dlg->inv_hdr.next;

        while (hdr != &inv->dlg->inv_hdr) {
            pjsip_msg_add_hdr(tdata->msg, (pjsip_hdr*)
                              pjsip_hdr_shallow_clone(tdata->pool, hdr));
            hdr = hdr->next;
        }
    }

    /* See if we have SDP to send. */
    if (inv->neg) {
        pjmedia_sdp_neg_state neg_state;

        neg_state = pjmedia_sdp_neg_get_state(inv->neg);

        has_sdp = (neg_state == PJMEDIA_SDP_NEG_STATE_LOCAL_OFFER ||
                   (neg_state == PJMEDIA_SDP_NEG_STATE_WAIT_NEGO &&
                    pjmedia_sdp_neg_has_local_answer(inv->neg)));


    } else {
        has_sdp = PJ_FALSE;
    }

    /* Add SDP, if any. */
    if (has_sdp) {
        const pjmedia_sdp_session *offer;

        status = pjmedia_sdp_neg_get_neg_local(inv->neg, &offer);
        if (status != PJ_SUCCESS) {
            pjsip_tx_data_dec_ref(tdata);
            goto on_return;
        }

        tdata->msg->body = create_sdp_body(tdata->pool, offer);
    }

    /* Add Allow header. */
    if (inv->dlg->add_allow) {
        hdr = pjsip_endpt_get_capability(inv->dlg->endpt, PJSIP_H_ALLOW, NULL);
        if (hdr) {
            allow_hdr = (pjsip_allow_hdr*)
                        pjsip_hdr_shallow_clone(tdata->pool, hdr);
            pjsip_msg_add_hdr(tdata->msg, (pjsip_hdr*)allow_hdr);
        }
    }

    /* Add Supported header */
    hdr = pjsip_endpt_get_capability(inv->dlg->endpt, PJSIP_H_SUPPORTED, NULL);
    if (hdr) {
        sup_hdr = (pjsip_supported_hdr*)
                   pjsip_hdr_shallow_clone(tdata->pool, hdr);
        pjsip_msg_add_hdr(tdata->msg, (pjsip_hdr*)sup_hdr);
    }

    /* Cleanup Allow & Supported headers from disabled extensions */
    cleanup_allow_sup_hdr(inv->options, NULL, allow_hdr, sup_hdr);

    /* Add Require header. */
    if ((inv->options & PJSIP_INV_REQUIRE_100REL) ||
        (inv->options & PJSIP_INV_REQUIRE_TIMER) ||
        (inv->options & PJSIP_INV_REQUIRE_TRICKLE_ICE))
    {
        pjsip_require_hdr *hreq;

        hreq = pjsip_require_hdr_create(tdata->pool);

        if (inv->options & PJSIP_INV_REQUIRE_100REL)
            hreq->values[hreq->count++] = pj_str("100rel");
        if (inv->options & PJSIP_INV_REQUIRE_TIMER)
            hreq->values[hreq->count++] = pj_str("timer");
        if (inv->options & PJSIP_INV_REQUIRE_TRICKLE_ICE)
            hreq->values[hreq->count++] = pj_str("trickle-ice");

        pjsip_msg_add_hdr(tdata->msg, (pjsip_hdr*) hreq);
    }

    status = pjsip_timer_update_req(inv, tdata);
    if (status != PJ_SUCCESS)
        goto on_return;

    /* Done. */
    *p_tdata = tdata;


on_return:
    pjsip_dlg_dec_lock(inv->dlg);
    return status;
}


/* Util: swap pool */
static void swap_pool(pj_pool_t **p1, pj_pool_t **p2)
{
    pj_pool_t *tmp = *p1;
    *p1 = *p2;
    *p2 = tmp;
}


/*
 * Initiate SDP negotiation in the SDP negotiator.
 */
static pj_status_t inv_negotiate_sdp( pjsip_inv_session *inv )
{
    pj_status_t status;

    PJ_ASSERT_RETURN(pjmedia_sdp_neg_get_state(inv->neg) ==
                     PJMEDIA_SDP_NEG_STATE_WAIT_NEGO, 
                     PJMEDIA_SDPNEG_EINSTATE);

    status = pjmedia_sdp_neg_negotiate(inv->pool_prov, inv->neg, 0);

    PJ_PERROR(4,(inv->obj_name, status, "SDP negotiation done"));

    if (mod_inv.cb.on_media_update && inv->notify)
        (*mod_inv.cb.on_media_update)(inv, status);

    /* Invite session may have been terminated by the application even 
     * after a successful SDP negotiation, for example when no audio 
     * codec is present in the offer (see ticket #1034).
     */
    if (inv->state != PJSIP_INV_STATE_DISCONNECTED) {

        /* Swap the flip-flop pool when SDP negotiation success. */
        if (status == PJ_SUCCESS) {
            swap_pool(&inv->pool_prov, &inv->pool_active);
        }

        /* Reset the provisional pool regardless SDP negotiation result. */
        pj_pool_reset(inv->pool_prov);

    } else {

        status = PJSIP_ERRNO_FROM_SIP_STATUS(inv->cause);
    }

    return status;
}

/*
 * Check in incoming message for SDP offer/answer.
 */
static pj_status_t inv_check_sdp_in_incoming_msg( pjsip_inv_session *inv,
                                                  pjsip_transaction *tsx,
                                                  pjsip_rx_data *rdata)
{
    struct tsx_inv_data *tsx_inv_data;
    pj_status_t status;
    pjsip_msg *msg;
    pjsip_rdata_sdp_info *sdp_info;

    /* Check if SDP is present in the message. */

    msg = rdata->msg_info.msg;
    if (msg->body == NULL) {
        /* Message doesn't have body. */
        return PJ_SUCCESS;
    }

    sdp_info = pjsip_rdata_get_sdp_info(rdata);
    if (sdp_info->body.ptr == NULL) {
        /* Message body is not "application/sdp" */
        return PJMEDIA_SDP_EINSDP;
    }

    /* Process the SDP body. */
    if (sdp_info->sdp_err) {
        PJ_PERROR(4,(THIS_FILE, sdp_info->sdp_err,
             "Error parsing SDP in %s",
             pjsip_rx_data_get_info(rdata)));
        return PJMEDIA_SDP_EINSDP;
    }

    /* Get/attach invite session's transaction data */
    tsx_inv_data = (struct tsx_inv_data*) tsx->mod_data[mod_inv.mod.id];
    if (tsx_inv_data == NULL) {
        tsx_inv_data = PJ_POOL_ZALLOC_T(tsx->pool, struct tsx_inv_data);
        tsx_inv_data->inv = inv;
        tsx_inv_data->has_sdp = (sdp_info->sdp!=NULL);
        tsx->mod_data[mod_inv.mod.id] = tsx_inv_data;
    }

    /* Initialize info that we are following forked media */
    inv->following_fork = PJ_FALSE;
    inv->updated_sdp_answer = PJ_FALSE;

    /* MUST NOT do multiple SDP offer/answer in a single transaction,
     * EXCEPT previous nego was in 18x (early media) and any of the following
     * condition is met:
     *  - Non-forking scenario:
     *    - 'accept_multiple_sdp_answers' is set, and
     *    - previous early response was not reliable (rfc6337 section 3.1.1).
     *  - Forking scenario:
     *    - This response has different To tag than the previous response, and
     *    - This response is 18x/2xx (early or final). If this is 18x,
     *      only do multiple SDP nego if 'follow_early_media_fork' is set.
     *
     * See also tickets #657, #1644, #1764, and #2123 for more info.
     */
    if (tsx_inv_data->sdp_done) {
        pj_str_t res_tag;
        int st_code;

        res_tag = rdata->msg_info.to->tag;
        st_code = rdata->msg_info.msg->line.status.code;

        if (tsx->role == PJSIP_ROLE_UAC && tsx_inv_data->done_early &&
               (
                   /* Non-forking scenario */
                   (
                       !tsx_inv_data->done_early_rel &&
                       (st_code/100 == 2 || st_code/10 == 18) &&
                       pjsip_cfg()->endpt.accept_multiple_sdp_answers &&
                       !pj_stricmp(&tsx_inv_data->done_tag, &res_tag)
                   )
                   ||
                   /* Forking scenario */
                   (
                       (st_code/100 == 2 ||
                           (st_code/10 == 18 &&
                               pjsip_cfg()->endpt.follow_early_media_fork)) &&
                       pj_stricmp(&tsx_inv_data->done_tag, &res_tag)
                   )
               )
           )
        {
            pjsip_sdp_info *tdata_sdp_info;
            const pjmedia_sdp_session *reoffer_sdp = NULL;

            if (pjmedia_sdp_neg_get_state(inv->neg) !=
                PJMEDIA_SDP_NEG_STATE_DONE)
            {
                PJ_LOG(4,(inv->obj_name, "SDP negotiation in progress, "
                          "message body in %s response is ignored",
                          (st_code/10==18? "early" : "final" )));
                return PJ_SUCCESS;
            }

            PJ_LOG(4,(inv->obj_name, "Received %s response "
                      "after SDP negotiation has been done in early "
                      "media. Renegotiating SDP..",
                      (st_code/10==18? "early" : "final" )));

            /* Retrieve original SDP offer from INVITE request */
            tdata_sdp_info = pjsip_tdata_get_sdp_info(tsx->last_tx);
            reoffer_sdp = tdata_sdp_info->sdp;

            /* Feed the original offer to negotiator */
            status = pjmedia_sdp_neg_modify_local_offer2(inv->pool_prov, 
                                                         inv->neg,
                                                         inv->sdp_neg_flags,
                                                         reoffer_sdp);

            if (status != PJ_SUCCESS) {
                PJ_LOG(1,(inv->obj_name, "Error updating local offer for "
                          "forked 2xx/18x response (err=%d)", status));
                return status;
            }

            inv->following_fork = !!pj_stricmp(&tsx_inv_data->done_tag,
                                               &res_tag);
            inv->updated_sdp_answer = PJ_TRUE;

        } else {

            if (rdata->msg_info.msg->body) {
                PJ_LOG(4,(inv->obj_name, "SDP negotiation done, message "
                          "body is ignored"));
            }
            return PJ_SUCCESS;
        }
    }

    pj_assert(sdp_info->sdp != NULL);

    /* The SDP can be an offer or answer, depending on negotiator's state */

    if (inv->neg == NULL ||
        pjmedia_sdp_neg_get_state(inv->neg) == PJMEDIA_SDP_NEG_STATE_DONE) 
    {

        /* This is an offer. */

        PJ_LOG(5,(inv->obj_name, "Got SDP offer in %s", 
                  pjsip_rx_data_get_info(rdata)));

        if (inv->neg == NULL) {
            status=pjmedia_sdp_neg_create_w_remote_offer(inv->pool, NULL,
                                                         sdp_info->sdp,
                                                         &inv->neg);
        } else {
            status=pjmedia_sdp_neg_set_remote_offer(inv->pool_prov, inv->neg, 
                                                    sdp_info->sdp);
        }

        if (status != PJ_SUCCESS) {
            PJ_PERROR(4,(THIS_FILE, status, "Error processing SDP offer in %",
                      pjsip_rx_data_get_info(rdata)));
            return PJMEDIA_SDP_EINSDP;
        }

        /* Inform application about remote offer. */
        if (mod_inv.cb.on_rx_offer2 && inv->notify) {
            struct pjsip_inv_on_rx_offer_cb_param param;

            param.offer = sdp_info->sdp;
            param.rdata = rdata;
            (*mod_inv.cb.on_rx_offer2)(inv, &param);
        } else if (mod_inv.cb.on_rx_offer && inv->notify) {
            (*mod_inv.cb.on_rx_offer)(inv, sdp_info->sdp);
        }

        /* application must have supplied an answer at this point. */
        if (pjmedia_sdp_neg_get_state(inv->neg) !=
                PJMEDIA_SDP_NEG_STATE_WAIT_NEGO)
        {
            if (mod_inv.cb.on_rx_reinvite && inv->notify &&
                msg->type == PJSIP_REQUEST_MSG &&
                msg->line.req.method.id == PJSIP_INVITE_METHOD)
            {
                /* Do not return failure first, allow the application
                 * to set the answer in the on_rx_reinvite() callback.
                 */
                PJ_LOG(5,(inv->obj_name, "Ignoring on_rx_offer() status "
                          "because on_rx_reinvite() is implemented"));
                return PJ_SUCCESS;
            }
            return PJ_EINVALIDOP;
        }

    } else if (pjmedia_sdp_neg_get_state(inv->neg) == 
                PJMEDIA_SDP_NEG_STATE_LOCAL_OFFER) 
    {
        int status_code;

        /* This is an answer. 
         * Process and negotiate remote answer.
         */

        PJ_LOG(5,(inv->obj_name, "Got SDP answer in %s", 
                  pjsip_rx_data_get_info(rdata)));

        status = pjmedia_sdp_neg_set_remote_answer(inv->pool_prov, inv->neg,
                                                   sdp_info->sdp);

        if (status != PJ_SUCCESS) {
            PJ_PERROR(4,(THIS_FILE, status, "Error processing SDP answer in %s",
                      pjsip_rx_data_get_info(rdata)));
            return PJMEDIA_SDP_EINSDP;
        }

        /* Negotiate SDP */

        inv_negotiate_sdp(inv);

        /* Mark this transaction has having SDP offer/answer done, and
         * save the reference to the To tag
         */

        tsx_inv_data->sdp_done = 1;
        status_code = rdata->msg_info.msg->line.status.code;
        tsx_inv_data->done_early = (status_code/100==1);
        tsx_inv_data->done_early_rel = tsx_inv_data->done_early &&
                                       pjsip_100rel_is_reliable(rdata);
        if (!inv->sdp_done_early_rel)
           inv->sdp_done_early_rel = tsx_inv_data->done_early_rel;
        pj_strdup(tsx->pool, &tsx_inv_data->done_tag, 
                  &rdata->msg_info.to->tag);

    } else {
        
        PJ_LOG(5,(THIS_FILE, "Ignored SDP in %s: negotiator state is %s",
              pjsip_rx_data_get_info(rdata), 
              pjmedia_sdp_neg_state_str(pjmedia_sdp_neg_get_state(inv->neg))));
    }

    return PJ_SUCCESS;
}


/*
 * Process INVITE answer, for both initial and subsequent re-INVITE
 */
static pj_status_t process_answer( pjsip_inv_session *inv,
                                   int st_code,
                                   pjsip_tx_data *tdata,
                                   const pjmedia_sdp_session *local_sdp)
{
    pj_status_t status;
    const pjmedia_sdp_session *sdp = NULL;

    /* If local_sdp is specified, then we MUST NOT have answered the
     * offer before. 
     */
    if (local_sdp && (st_code/100==1 || st_code/100==2)) {

        if (inv->neg == NULL) {
            status = pjmedia_sdp_neg_create_w_local_offer(inv->pool, 
                                                          local_sdp,
                                                          &inv->neg);
        } else if (pjmedia_sdp_neg_get_state(inv->neg)==
                   PJMEDIA_SDP_NEG_STATE_REMOTE_OFFER)
        {
            status = pjmedia_sdp_neg_set_local_answer(inv->pool_prov, inv->neg,
                                                      local_sdp);
        } else {

            /* Can not specify local SDP at this state. */
            pj_assert(0);
            status = PJMEDIA_SDPNEG_EINSTATE;
        }

        if (status != PJ_SUCCESS)
            return status;

    }


     /* If SDP negotiator is ready, start negotiation. */
    if (st_code/100==2 || (st_code/10==18 && st_code!=180 && st_code!=181)) {

        pjmedia_sdp_neg_state neg_state;

        /* Start nego when appropriate. */
        neg_state = inv->neg ? pjmedia_sdp_neg_get_state(inv->neg) :
                    PJMEDIA_SDP_NEG_STATE_NULL;

        if (neg_state == PJMEDIA_SDP_NEG_STATE_LOCAL_OFFER) {

            status = pjmedia_sdp_neg_get_neg_local(inv->neg, &sdp);

        } else if (neg_state == PJMEDIA_SDP_NEG_STATE_WAIT_NEGO &&
                   pjmedia_sdp_neg_has_local_answer(inv->neg) )
        {
            struct tsx_inv_data *tsx_inv_data;

            /* Get invite session's transaction data */
            tsx_inv_data = (struct tsx_inv_data*) 
                           inv->invite_tsx->mod_data[mod_inv.mod.id];

            status = inv_negotiate_sdp(inv);
            if (status != PJ_SUCCESS)
                return status;
            
            /* Mark this transaction has having SDP offer/answer done. */
            tsx_inv_data->sdp_done = 1;

            status = pjmedia_sdp_neg_get_active_local(inv->neg, &sdp);
        }
    }

    /* Include SDP when it's available for 2xx and 18x (but not 180 and 181)
     * response. Subsequent response will include this SDP.
     *
     * Note note:
     *  - When offer/answer has been completed in reliable 183, we MUST NOT
     *    send SDP in 2xx response. So if we don't have SDP to send, clear
     *    the SDP in the message body ONLY if 100rel is active in this 
     *    session.
     */
    if (sdp) {
        tdata->msg->body = create_sdp_body(tdata->pool, sdp);
    } else {
        if (inv->options & PJSIP_INV_REQUIRE_100REL) {
            tdata->msg->body = NULL;
        }
    }

    /* Cancel SDP negotiation if this is a negative reply to a re-INVITE */
    if (st_code >= 300 && inv->neg != NULL &&
        inv->state == PJSIP_INV_STATE_CONFIRMED)
    {
        pjmedia_sdp_neg_state neg_state;
        neg_state = pjmedia_sdp_neg_get_state(inv->neg);
        if (neg_state == PJMEDIA_SDP_NEG_STATE_REMOTE_OFFER) {
            pjmedia_sdp_neg_cancel_offer(inv->neg);
        }
    }

    return PJ_SUCCESS;
}


/*
 * Create first response to INVITE
 */
PJ_DEF(pj_status_t) pjsip_inv_initial_answer(   pjsip_inv_session *inv,
                                                pjsip_rx_data *rdata,
                                                int st_code,
                                                const pj_str_t *st_text,
                                                const pjmedia_sdp_session *sdp,
                                                pjsip_tx_data **p_tdata)
{
    pjsip_tx_data *tdata;
    pj_status_t status;
    pjsip_status_code st_code2;

    /* Verify arguments. */
    PJ_ASSERT_RETURN(inv && p_tdata, PJ_EINVAL);

    /* Must have INVITE transaction. */
    PJ_ASSERT_RETURN(inv->invite_tsx, PJ_EBUG);

    pj_log_push_indent();

    pjsip_dlg_inc_lock(inv->dlg);

    /* Create response */
    status = pjsip_dlg_create_response(inv->dlg, rdata, st_code, st_text,
                                       &tdata);
    if (status != PJ_SUCCESS)
        goto on_return;

    /* Invoke Session Timers module */
    status = pjsip_timer_process_req(inv, rdata, &st_code2);
    if (status != PJ_SUCCESS) {
        pj_status_t status2;

        status2 = pjsip_dlg_modify_response(inv->dlg, tdata, st_code2, NULL);
        if (status2 != PJ_SUCCESS) {
            pjsip_tx_data_dec_ref(tdata);
            goto on_return;
        }
        status2 = pjsip_timer_update_resp(inv, tdata);
        if (status2 == PJ_SUCCESS)
            *p_tdata = tdata;
        else
            pjsip_tx_data_dec_ref(tdata);

        goto on_return;
    }

    /* Process SDP in answer */
    status = process_answer(inv, st_code, tdata, sdp);
    if (status != PJ_SUCCESS) {
        pjsip_tx_data_dec_ref(tdata);
        goto on_return;
    }

    /* Cleanup Allow & Supported headers from disabled extensions */
    cleanup_allow_sup_hdr(inv->options, tdata, NULL, NULL);

    /* Save this answer */
    inv->last_answer = tdata;
    pjsip_tx_data_add_ref(inv->last_answer);
    PJ_LOG(5,(inv->dlg->obj_name, "Initial answer %s",
              pjsip_tx_data_get_info(inv->last_answer)));

    /* Invoke Session Timers */
    pjsip_timer_update_resp(inv, tdata);

    *p_tdata = tdata;

on_return:
    pjsip_dlg_dec_lock(inv->dlg);
    pj_log_pop_indent();
    return status;
}


/*
 * Answer INVITE request.
 */ 
PJ_DEF(pj_status_t) pjsip_inv_answer(   pjsip_inv_session *inv,
                                        int st_code,
                                        const pj_str_t *st_text,
                                        const pjmedia_sdp_session *local_sdp,
                                        pjsip_tx_data **p_tdata )
{
    pjsip_tx_data *last_res;
    pjsip_tx_data *old_res;
    pj_status_t status;

    /* Verify arguments. */
    PJ_ASSERT_RETURN(inv && p_tdata, PJ_EINVAL);

    /* Must have INVITE transaction. */
    PJ_ASSERT_RETURN(inv->invite_tsx, PJ_EBUG);

    /* Must have created an answer before */
    PJ_ASSERT_RETURN(inv->last_answer, PJ_EINVALIDOP);

    pj_log_push_indent();

    pjsip_dlg_inc_lock(inv->dlg);

    /* Clone last response.
     * The tdata (last_answer) is a shared object used by the transaction.
     * Modifying a shared object might lead to a deadlock.
     * Refer to ticket #2137 for more detail.
     */
    status = pjsip_tx_data_clone(inv->last_answer, 0, &last_res);
    if (status != PJ_SUCCESS)
        goto on_return;

    /* Modify last response. */
    status = pjsip_dlg_modify_response(inv->dlg, last_res, st_code, st_text);
    if (status != PJ_SUCCESS) {
        pjsip_tx_data_dec_ref(last_res);
        goto on_return;
    }

    /* For non-2xx final response, strip message body */
    if (st_code >= 300) {
        last_res->msg->body = NULL;
    }

    /* Process SDP in answer */
    status = process_answer(inv, st_code, last_res, local_sdp);
    if (status != PJ_SUCCESS) {
        pjsip_tx_data_dec_ref(last_res);
        goto on_return;
    }

    /* Invoke Session Timers */
    pjsip_timer_update_resp(inv, last_res);

    /* Cleanup Allow & Supported headers from disabled extensions */
    cleanup_allow_sup_hdr(inv->options, last_res, NULL, NULL);

    /* Update last_answer */
    old_res = inv->last_answer;
    inv->last_answer = last_res;

    /* Release old answer */
    pjsip_tx_data_dec_ref(old_res);

    *p_tdata = last_res;

on_return:
    pjsip_dlg_dec_lock(inv->dlg);
    pj_log_pop_indent();
    return status;
}


/*
 * Set local SDP offer/answer.
 */
PJ_DEF(pj_status_t) pjsip_inv_set_local_sdp(pjsip_inv_session *inv,
                                            const pjmedia_sdp_session *sdp )
{
    const pjmedia_sdp_session *offer;
    pj_status_t status;

    PJ_ASSERT_RETURN(inv && sdp, PJ_EINVAL);

    /* If we have remote SDP offer, set local answer to respond to the offer,
     * otherwise we set/modify our local offer (and create an SDP negotiator
     * if we don't have one yet).
     */
    if (inv->neg) {
        pjmedia_sdp_neg_state neg_state = pjmedia_sdp_neg_get_state(inv->neg);

        if ((neg_state == PJMEDIA_SDP_NEG_STATE_REMOTE_OFFER ||
             neg_state == PJMEDIA_SDP_NEG_STATE_WAIT_NEGO) &&
            pjmedia_sdp_neg_get_neg_remote(inv->neg, &offer) == PJ_SUCCESS)
        {
            status = pjsip_inv_set_sdp_answer(inv, sdp);
        }  else if (neg_state == PJMEDIA_SDP_NEG_STATE_DONE) {
            status = pjmedia_sdp_neg_modify_local_offer2(inv->pool, inv->neg,
                                                         inv->sdp_neg_flags,
                                                         sdp);
        } else
            return PJMEDIA_SDPNEG_EINSTATE;
    } else {
        status = pjmedia_sdp_neg_create_w_local_offer(inv->pool, 
                                                      sdp, &inv->neg);
    }

    return status;
}


/*
 * Set SDP answer.
 */
PJ_DEF(pj_status_t) pjsip_inv_set_sdp_answer( pjsip_inv_session *inv,
                                              const pjmedia_sdp_session *sdp )
{
    pj_status_t status;

    PJ_ASSERT_RETURN(inv && sdp, PJ_EINVAL);

    pjsip_dlg_inc_lock(inv->dlg);
    status = pjmedia_sdp_neg_set_local_answer( inv->pool_prov, inv->neg, sdp);
    pjsip_dlg_dec_lock(inv->dlg);

    return status;
}


/*
 * End session.
 */
PJ_DEF(pj_status_t) pjsip_inv_end_session(  pjsip_inv_session *inv,
                                            int st_code,
                                            const pj_str_t *st_text,
                                            pjsip_tx_data **p_tdata )
{
    pjsip_tx_data *tdata;
    pj_status_t status;

    /* Verify arguments. */
    PJ_ASSERT_RETURN(inv && p_tdata, PJ_EINVAL);

    pj_log_push_indent();

    /* Set cause code. */
    inv_set_cause(inv, st_code, st_text);

    /* Create appropriate message. */
    switch (inv->state) {
    case PJSIP_INV_STATE_NULL:
    case PJSIP_INV_STATE_CALLING:
    case PJSIP_INV_STATE_EARLY:
    case PJSIP_INV_STATE_INCOMING:

        if (inv->role == PJSIP_ROLE_UAC) {

            /* For UAC when session has not been confirmed, create CANCEL. */

            /* MUST have the original UAC INVITE transaction. */
            PJ_ASSERT_RETURN(inv->invite_tsx != NULL, PJ_EBUG);

            /* But CANCEL should only be called when we have received a
             * provisional response. If we haven't received any responses,
             * just destroy the transaction.
             */
            if (inv->invite_tsx->status_code < 100) {

                /* Do not stop INVITE retransmission, see ticket #506 */
                //pjsip_tsx_stop_retransmit(inv->invite_tsx);
                inv->cancelling = PJ_TRUE;
                inv->pending_cancel = PJ_TRUE;
                *p_tdata = NULL;
                PJ_LOG(4, (inv->obj_name, "Delaying CANCEL since no "
                           "provisional response is received yet"));
                pj_log_pop_indent();
                return PJ_SUCCESS;
            }

            /* The CSeq here assumes that the dialog is started with an
             * INVITE session. This may not be correct; dialog can be 
             * started as SUBSCRIBE session.
             * So fix this!
             */
            status = pjsip_endpt_create_cancel(inv->dlg->endpt, 
                                               inv->invite_tsx->last_tx,
                                               &tdata);
            if (status != PJ_SUCCESS) {
                pj_log_pop_indent();
                return status;
            }

            /* Set timeout for the INVITE transaction, in case UAS is not
             * able to respond the INVITE with 487 final response. The 
             * timeout value is 64*T1.
             */
            pjsip_tsx_set_timeout(inv->invite_tsx, 64 * pjsip_cfg()->tsx.t1);

        } else {

            /* For UAS, send a final response. */
            tdata = inv->invite_tsx->last_tx;
            if (tdata == NULL)
                tdata = inv->last_answer;

            PJ_ASSERT_RETURN(tdata != NULL, PJ_EINVALIDOP);

            //status = pjsip_dlg_modify_response(inv->dlg, tdata, st_code,
            //                                 st_text);
            status = pjsip_inv_answer(inv, st_code, st_text, NULL, &tdata);
        }
        break;

    case PJSIP_INV_STATE_CONNECTING:
    case PJSIP_INV_STATE_CONFIRMED:
        /* End Session Timer */
        pjsip_timer_end_session(inv);

        /* For established dialog, send BYE */
        status = pjsip_dlg_create_request(inv->dlg, pjsip_get_bye_method(), 
                                          -1, &tdata);
        break;

    case PJSIP_INV_STATE_DISCONNECTED:
        /* No need to do anything. */
        pj_log_pop_indent();
        return PJSIP_ESESSIONTERMINATED;

    default:
        pj_assert(!"Invalid operation!");
        pj_log_pop_indent();
        return PJ_EINVALIDOP;
    }

    if (status != PJ_SUCCESS) {
        pj_log_pop_indent();
        return status;
    }


    /* Done */

    inv->cancelling = PJ_TRUE;
    *p_tdata = tdata;

    pj_log_pop_indent();
    return PJ_SUCCESS;
}

/*
 * Cancel re-INVITE transaction.
 */
PJ_DEF(pj_status_t) pjsip_inv_cancel_reinvite( pjsip_inv_session *inv,
                                               pjsip_tx_data **p_tdata )
{
    pjsip_tx_data *tdata;
    pj_status_t status;

    /* Verify arguments. */
    PJ_ASSERT_RETURN(inv && p_tdata, PJ_EINVAL);
    
    pj_log_push_indent();

    /* Create appropriate message. */
    switch (inv->state) {
    case PJSIP_INV_STATE_CONFIRMED:
        /* MUST have the original UAC INVITE transaction  */
        PJ_ASSERT_RETURN(inv->invite_tsx != NULL, PJ_EBUG);

        /* CANCEL should only be called when we have received a
         * provisional response.
         */
        if (inv->invite_tsx->status_code < 100) {
            inv->cancelling = PJ_TRUE;
            inv->pending_cancel = PJ_TRUE;
            *p_tdata = NULL;
            PJ_LOG(4, (inv->obj_name, "Delaying CANCEL since no "
                       "provisional response is received yet"));
            pj_log_pop_indent();
            return PJ_SUCCESS;
        }

        status = pjsip_endpt_create_cancel(inv->dlg->endpt, 
                                           inv->invite_tsx->last_tx,
                                           &tdata);
        if (status != PJ_SUCCESS) {
            pj_log_pop_indent();
            return status;
        }
        break;

    default:
        /* We cannot send CANCEL to a re-INVITE if the INVITE session is
         * not confirmed.
         */
        pj_log_pop_indent();
        return PJ_EINVALIDOP;
    }

    pj_log_pop_indent();

    *p_tdata = tdata;
    return PJ_SUCCESS;
}

/* Following redirection recursion, get next target from the target set and
 * notify user.
 *
 * Returns PJ_FALSE if recursion fails (either because there's no more target
 * or user rejects the recursion). If we return PJ_FALSE, caller should
 * disconnect the session.
 *
 * Note:
 *   the event 'e' argument may be NULL.
 */
static pj_bool_t inv_uac_recurse(pjsip_inv_session *inv, int code,
                                 const pj_str_t *reason, pjsip_event *e)
{
    pjsip_redirect_op op;
    pjsip_target *target;

    /* Won't redirect if the callback is not implemented. */
    if (mod_inv.cb.on_redirected == NULL)
        return PJ_FALSE;

    if (reason == NULL)
        reason = pjsip_get_status_text(code);

    /* Set status of current target */
    pjsip_target_assign_status(inv->dlg->target_set.current, inv->dlg->pool,
                               code, reason);

    /* Fetch next target from the target set. We only want to
     * process SIP/SIPS URI for now.
     */
    for (;;) {
        target = pjsip_target_set_get_next(&inv->dlg->target_set);
        if (target == NULL) {
            /* No more target. */
            return PJ_FALSE;
        }

        if (!PJSIP_URI_SCHEME_IS_SIP(target->uri) &&
            !PJSIP_URI_SCHEME_IS_SIPS(target->uri))
        {
            code = PJSIP_SC_UNSUPPORTED_URI_SCHEME;
            reason = pjsip_get_status_text(code);

            /* Mark this target as unusable and fetch next target. */
            pjsip_target_assign_status(target, inv->dlg->pool, code, reason);
        } else {
            /* Found a target */
            break;
        }
    }

    /* We have target in 'target'. Set this target as current target
     * and notify callback. 
     */
    pjsip_target_set_set_current(&inv->dlg->target_set, target);

    op = (*mod_inv.cb.on_redirected)(inv, target->uri, e);


    /* Check what the application wants to do now */
    switch (op) {
    case PJSIP_REDIRECT_ACCEPT:
    case PJSIP_REDIRECT_ACCEPT_REPLACE:
    case PJSIP_REDIRECT_STOP:
        /* Must increment session counter, that's the convention of the 
         * pjsip_inv_process_redirect().
         */
        pjsip_dlg_inc_session(inv->dlg, &mod_inv.mod);

        /* Act on the recursion */
        pjsip_inv_process_redirect(inv, op, e);
        return PJ_TRUE;

    case PJSIP_REDIRECT_PENDING:
        /* Increment session so that the dialog/session is not destroyed 
         * while we're waiting for user confirmation.
         */
        pjsip_dlg_inc_session(inv->dlg, &mod_inv.mod);

        /* Also clear the invite_tsx variable, otherwise when this tsx is
         * terminated, it will also terminate the session.
         */
        inv->invite_tsx = NULL;

        /* Done. The processing will continue once the application calls
         * pjsip_inv_process_redirect().
         */
        return PJ_TRUE;

    case PJSIP_REDIRECT_REJECT:
        /* Recursively call  this function again to fetch next target, if any.
         */
        return inv_uac_recurse(inv, PJSIP_SC_REQUEST_TERMINATED, NULL, e);

    }

    pj_assert(!"Should not reach here");
    return PJ_FALSE;
}


/* Process redirection/recursion */
PJ_DEF(pj_status_t) pjsip_inv_process_redirect( pjsip_inv_session *inv,
                                                pjsip_redirect_op op,
                                                pjsip_event *e)
{
    const pjsip_status_code cancel_code = PJSIP_SC_REQUEST_TERMINATED;
    pjsip_event usr_event;
    pj_status_t status = PJ_SUCCESS;

    PJ_ASSERT_RETURN(inv && op != PJSIP_REDIRECT_PENDING, PJ_EINVAL);

    if (e == NULL) {
        PJSIP_EVENT_INIT_USER(usr_event, NULL, NULL, NULL, NULL);
        e = &usr_event;
    }

    pjsip_dlg_inc_lock(inv->dlg);

    /* Decrement session. That's the convention here to prevent the dialog 
     * or session from being destroyed while we're waiting for user
     * confirmation.
     */
    pjsip_dlg_dec_session(inv->dlg, &mod_inv.mod);

    /* See what the application wants to do now */
    switch (op) {
    case PJSIP_REDIRECT_ACCEPT:
    case PJSIP_REDIRECT_ACCEPT_REPLACE:
        /* User accept the redirection. Reset the session and resend the 
         * INVITE request.
         */
        {
            pjsip_tx_data *tdata;
            pjsip_via_hdr *via;

            /* Get the original INVITE request. */
            tdata = inv->invite_req;
            pjsip_tx_data_add_ref(tdata);

            /* Restore strict route set.
             * See https://github.com/pjsip/pjproject/issues/492
             */
            pjsip_restore_strict_route_set(tdata);

            /* Set target */
            tdata->msg->line.req.uri = (pjsip_uri*)
               pjsip_uri_clone(tdata->pool, inv->dlg->target_set.current->uri);

            /* Remove branch param in Via header. */
            via = (pjsip_via_hdr*) 
                  pjsip_msg_find_hdr(tdata->msg, PJSIP_H_VIA, NULL);
            via->branch_param.slen = 0;

            /* Process PJSIP_REDIRECT_ACCEPT_REPLACE */
            if (op == PJSIP_REDIRECT_ACCEPT_REPLACE) {
                pjsip_to_hdr *to;
                pjsip_dialog *dlg = inv->dlg;
                enum { TMP_LEN = PJSIP_MAX_URL_SIZE };
                char tmp[TMP_LEN];
                int len;

                /* Replace To header */
                to = PJSIP_MSG_TO_HDR(tdata->msg);
                to->uri = (pjsip_uri*)
                          pjsip_uri_clone(tdata->pool,
                                          dlg->target_set.current->uri);
                to->tag.slen = 0;
                pj_list_init(&to->other_param);
                
                /* Re-init dialog remote info */
                dlg->remote.info = (pjsip_to_hdr*)
                                   pjsip_hdr_clone(dlg->pool, to);

                /* Remove header param from remote info */
                if (PJSIP_URI_SCHEME_IS_SIP(dlg->remote.info->uri) ||
                    PJSIP_URI_SCHEME_IS_SIPS(dlg->remote.info->uri))
                {
                    pjsip_sip_uri *sip_uri = (pjsip_sip_uri *) 
                                   pjsip_uri_get_uri(dlg->remote.info->uri);
                    if (!pj_list_empty(&sip_uri->header_param)) {
                        /* Remove all header param */
                        pj_list_init(&sip_uri->header_param);
                    }
                }

                /* Print the remote info. */
                len = pjsip_uri_print(PJSIP_URI_IN_FROMTO_HDR,
                                      dlg->remote.info->uri, tmp, TMP_LEN);
                if (len < 1) {
                    pj_ansi_strcpy(tmp, "<-error: uri too long->");
                    len = (int)pj_ansi_strlen(tmp);
                }
                pj_strdup2_with_null(dlg->pool, &dlg->remote.info_str, tmp);

                /* Secure? */
                dlg->secure = PJSIP_URI_SCHEME_IS_SIPS(to->uri);
            }

            /* Reset message destination info (see #1248). */
            pj_bzero(&tdata->dest_info, sizeof(tdata->dest_info));

            /* Must invalidate the message! */
            pjsip_tx_data_invalidate_msg(tdata);

            /* Reset the session */
            pjsip_inv_uac_restart(inv, PJ_FALSE);

            /* (re)Send the INVITE request */
            status = pjsip_inv_send_msg(inv, tdata);
        }
        break;

    case PJSIP_REDIRECT_STOP:
        /* User doesn't want the redirection. Disconnect the session now. */
        inv_set_cause(inv, cancel_code, pjsip_get_status_text(cancel_code));
        inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);

        /* Caller should expect that the invite session is gone now, so
         * we don't need to set status to PJSIP_ESESSIONTERMINATED here.
         */
        break;

    case PJSIP_REDIRECT_REJECT:
        /* Current target is rejected. Fetch next target if any. */
        if (inv_uac_recurse(inv, cancel_code, NULL, NULL) == PJ_FALSE) {
            inv_set_cause(inv, cancel_code, 
                          pjsip_get_status_text(cancel_code));
            inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);

            /* Tell caller that the invite session is gone now */
            status = PJSIP_ESESSIONTERMINATED;
        }
        break;


    case PJSIP_REDIRECT_PENDING:
        pj_assert(!"Should not happen");
        break;
    }


    pjsip_dlg_dec_lock(inv->dlg);

    return status;
}


/*
 * Create re-INVITE.
 */
PJ_DEF(pj_status_t) pjsip_inv_reinvite( pjsip_inv_session *inv,
                                        const pj_str_t *new_contact,
                                        const pjmedia_sdp_session *new_offer,
                                        pjsip_tx_data **p_tdata )
{
    pj_status_t status;
    pjsip_contact_hdr *contact_hdr = NULL;

    /* Check arguments. */
    PJ_ASSERT_RETURN(inv && p_tdata, PJ_EINVAL);

    /* Must NOT have a pending INVITE transaction */
    if (inv->invite_tsx!=NULL)
        return PJ_EINVALIDOP;

    pj_log_push_indent();

    pjsip_dlg_inc_lock(inv->dlg);

    if (new_contact) {
        pj_str_t tmp;
        const pj_str_t STR_CONTACT = { "Contact", 7 };

        pj_strdup_with_null(inv->dlg->pool, &tmp, new_contact);
        contact_hdr = (pjsip_contact_hdr*)
                      pjsip_parse_hdr(inv->dlg->pool, &STR_CONTACT, 
                                      tmp.ptr, tmp.slen, NULL);
        if (!contact_hdr) {
            status = PJSIP_EINVALIDURI;
            goto on_return;
        }
    }


    if (new_offer) {
        if (!inv->neg) {
            status = pjmedia_sdp_neg_create_w_local_offer(inv->pool, 
                                                          new_offer,
                                                          &inv->neg);
            if (status != PJ_SUCCESS)
                goto on_return;

        } else switch (pjmedia_sdp_neg_get_state(inv->neg)) {

            case PJMEDIA_SDP_NEG_STATE_NULL:
                pj_assert(!"Unexpected SDP neg state NULL");
                status = PJ_EBUG;
                goto on_return;

            case PJMEDIA_SDP_NEG_STATE_LOCAL_OFFER:
                PJ_LOG(4,(inv->obj_name, 
                          "pjsip_inv_reinvite: already have an offer, new "
                          "offer is ignored"));
                break;

            case PJMEDIA_SDP_NEG_STATE_REMOTE_OFFER:
                status = pjmedia_sdp_neg_set_local_answer(inv->pool_prov, 
                                                          inv->neg,
                                                          new_offer);
                if (status != PJ_SUCCESS)
                    goto on_return;
                break;

            case PJMEDIA_SDP_NEG_STATE_WAIT_NEGO:
                PJ_LOG(4,(inv->obj_name, 
                          "pjsip_inv_reinvite: SDP in WAIT_NEGO state, new "
                          "offer is ignored"));
                break;

            case PJMEDIA_SDP_NEG_STATE_DONE:
                status = pjmedia_sdp_neg_modify_local_offer2(
                             inv->pool_prov, inv->neg,
                             inv->sdp_neg_flags, new_offer);
                if (status != PJ_SUCCESS)
                    goto on_return;
                break;
        }
    }

    if (contact_hdr)
        inv->dlg->local.contact = contact_hdr;

    status = pjsip_inv_invite(inv, p_tdata);

on_return:
    pjsip_dlg_dec_lock(inv->dlg);
    pj_log_pop_indent();
    return status;
}

/*
 * Create UPDATE.
 */
PJ_DEF(pj_status_t) pjsip_inv_update (  pjsip_inv_session *inv,
                                        const pj_str_t *new_contact,
                                        const pjmedia_sdp_session *offer,
                                        pjsip_tx_data **p_tdata )
{
    pjsip_contact_hdr *contact_hdr = NULL;
    pjsip_tx_data *tdata = NULL;
    pjmedia_sdp_session *sdp_copy;
    const pjsip_hdr *hdr;
    pjsip_supported_hdr *sup_hdr = NULL;
    pj_status_t status = PJ_SUCCESS;

    /* Verify arguments. */
    PJ_ASSERT_RETURN(inv && p_tdata, PJ_EINVAL);

    /* Dialog must have been established */
    PJ_ASSERT_RETURN(inv->dlg->state == PJSIP_DIALOG_STATE_ESTABLISHED,
                     PJ_EINVALIDOP);

    /* Invite session must not have been disconnected */
    PJ_ASSERT_RETURN(inv->state < PJSIP_INV_STATE_DISCONNECTED,
                     PJ_EINVALIDOP);

    pj_log_push_indent();

    /* Lock dialog. */
    pjsip_dlg_inc_lock(inv->dlg);

    /* Process offer, if any */
    if (offer) {
        if (inv->state == PJSIP_INV_STATE_EARLY && !inv->sdp_done_early_rel) {
            PJ_LOG(4,(inv->dlg->obj_name,
                      "RFC 3311 section 5.1 recommends against sending UPDATE"
                      " without reliable prov response"));
#if PJSIP_INV_UPDATE_EARLY_CHECK_RELIABLE
            status = PJ_EINVALIDOP;
            goto on_error;
#endif
        }

        if (pjmedia_sdp_neg_get_state(inv->neg)!=PJMEDIA_SDP_NEG_STATE_DONE) {
            PJ_LOG(4,(inv->dlg->obj_name,
                      "Invalid SDP offer/answer state for UPDATE"));
            status = PJ_EINVALIDOP;
            goto on_error;
        }

        /* Notify negotiator about the new offer. This will fix the offer
         * with correct SDP origin.
         */
        status = pjmedia_sdp_neg_modify_local_offer2(inv->pool_prov, inv->neg,
                                                     inv->sdp_neg_flags, offer);
        if (status != PJ_SUCCESS)
            goto on_error;

        /* Retrieve the "fixed" offer from negotiator */
        pjmedia_sdp_neg_get_neg_local(inv->neg, &offer);
    }

    /* Update Contact if required */
    if (new_contact) {
        pj_str_t tmp;
        const pj_str_t STR_CONTACT = { "Contact", 7 };

        pj_strdup_with_null(inv->dlg->pool, &tmp, new_contact);
        contact_hdr = (pjsip_contact_hdr*)
                      pjsip_parse_hdr(inv->dlg->pool, &STR_CONTACT, 
                                      tmp.ptr, tmp.slen, NULL);
        if (!contact_hdr) {
            status = PJSIP_EINVALIDURI;
            goto on_error;
        }

        inv->dlg->local.contact = contact_hdr;
    }

    /* Create request */
    status = pjsip_dlg_create_request(inv->dlg, &pjsip_update_method,
                                      -1, &tdata);
    if (status != PJ_SUCCESS)
            goto on_error;

    /* Attach SDP body */
    if (offer) {
        sdp_copy = pjmedia_sdp_session_clone(tdata->pool, offer);
        pjsip_create_sdp_body(tdata->pool, sdp_copy, &tdata->msg->body);
    }

    /* Session Timers spec (RFC 4028) says that Supported header MUST be put
     * in refresh requests. So here we'll just put the Supported header in
     * all cases regardless of whether session timers is used or not, just
     * in case this is a common behavior.
     */
    hdr = pjsip_endpt_get_capability(inv->dlg->endpt, PJSIP_H_SUPPORTED, NULL);
    if (hdr) {
        sup_hdr = (pjsip_supported_hdr*)
                   pjsip_hdr_shallow_clone(tdata->pool, hdr);
        pjsip_msg_add_hdr(tdata->msg, (pjsip_hdr*)sup_hdr);
    }

    status = pjsip_timer_update_req(inv, tdata);
    if (status != PJ_SUCCESS)
        goto on_error;

    /* Cleanup Allow & Supported headers from disabled extensions */
    cleanup_allow_sup_hdr(inv->options, NULL, NULL, sup_hdr);

    /* Unlock dialog. */
    pjsip_dlg_dec_lock(inv->dlg);

    *p_tdata = tdata;

    pj_log_pop_indent();
    return PJ_SUCCESS;

on_error:
    if (tdata)
        pjsip_tx_data_dec_ref(tdata);

    /* Unlock dialog. */
    pjsip_dlg_dec_lock(inv->dlg);

    pj_log_pop_indent();
    return status;
}

/*
 * Create an ACK request.
 */
PJ_DEF(pj_status_t) pjsip_inv_create_ack(pjsip_inv_session *inv,
                                         int cseq,
                                         pjsip_tx_data **p_tdata)
{
    const pjmedia_sdp_session *sdp = NULL;
    pj_status_t status;

    PJ_ASSERT_RETURN(inv && p_tdata, PJ_EINVAL);

    /* Lock dialog. */
    pjsip_dlg_inc_lock(inv->dlg);

    /* Destroy last_ack */
    if (inv->last_ack) {
        pjsip_tx_data_dec_ref(inv->last_ack);
        inv->last_ack = NULL;
    }

    /* Create new ACK request */
    status = pjsip_dlg_create_request(inv->dlg, pjsip_get_ack_method(), 
                                      cseq, &inv->last_ack);
    if (status != PJ_SUCCESS) {
        pjsip_dlg_dec_lock(inv->dlg);
        return status;
    }

    /* See if we have pending SDP answer to send */
    sdp = inv_has_pending_answer(inv, inv->invite_tsx);
    if (sdp) {
        inv->last_ack->msg->body = create_sdp_body(inv->last_ack->pool, sdp);
    }

    /* Keep this for subsequent response retransmission */
    inv->last_ack_cseq = cseq;
    pjsip_tx_data_add_ref(inv->last_ack);

    /* Done */
    *p_tdata = inv->last_ack;

    /* Unlock dialog. */
    pjsip_dlg_dec_lock(inv->dlg);

    return PJ_SUCCESS;
}

/*
 * Send a request or response message.
 */
PJ_DEF(pj_status_t) pjsip_inv_send_msg( pjsip_inv_session *inv,
                                        pjsip_tx_data *tdata)
{
    pj_status_t status;

    /* Verify arguments. */
    PJ_ASSERT_RETURN(inv && tdata, PJ_EINVAL);

    pj_log_push_indent();

    PJ_LOG(5,(inv->obj_name, "Sending %s", 
              pjsip_tx_data_get_info(tdata)));

    if (tdata->msg->type == PJSIP_REQUEST_MSG) {
        struct tsx_inv_data *tsx_inv_data;

        pjsip_dlg_inc_lock(inv->dlg);

        /* Check again that we didn't receive incoming re-INVITE */
        if (tdata->msg->line.req.method.id==PJSIP_INVITE_METHOD && 
            inv->invite_tsx) 
        {
            pjsip_tx_data_dec_ref(tdata);
            pjsip_dlg_dec_lock(inv->dlg);
            status = PJ_EINVALIDOP;
            goto on_error;
        }

        /* Don't send BYE before ACK is received
         * https://github.com/pjsip/pjproject/issues/1712
         */
        if (tdata->msg->line.req.method.id == PJSIP_BYE_METHOD &&
            inv->role == PJSIP_ROLE_UAS &&
            inv->state == PJSIP_INV_STATE_CONNECTING &&
            inv->cause != PJSIP_SC_REQUEST_TIMEOUT &&
            inv->cause != PJSIP_SC_TSX_TRANSPORT_ERROR)
        {
            if (inv->pending_bye)
                pjsip_tx_data_dec_ref(inv->pending_bye);

            inv->pending_bye = tdata;
            PJ_LOG(4, (inv->obj_name, "Delaying BYE request until "
                       "ACK is received"));
            pjsip_dlg_dec_lock(inv->dlg);
            goto on_return;
        }

        /* Associate our data in outgoing invite transaction */
        tsx_inv_data = PJ_POOL_ZALLOC_T(inv->pool, struct tsx_inv_data);
        tsx_inv_data->inv = inv;
        tsx_inv_data->has_sdp = tx_data_has_sdp(tdata);

        pjsip_dlg_dec_lock(inv->dlg);

        status = pjsip_dlg_send_request(inv->dlg, tdata, mod_inv.mod.id, 
                                        tsx_inv_data);
        if (status != PJ_SUCCESS) {
            goto on_error;
        }

        /* Check if this is delayed manual ACK (see #416) */
        if (mod_inv.cb.on_send_ack &&
            tdata->msg->line.req.method.id == PJSIP_ACK_METHOD &&
            tdata == inv->last_ack)
        {
            pjsip_dlg_inc_lock(inv->dlg);

            /* Set state to CONFIRMED (if we're not in CONFIRMED yet).
             * But don't set it to CONFIRMED if we're already DISCONNECTED
             * (this may have been a late 200/OK response.
             */
            if (inv->state < PJSIP_INV_STATE_CONFIRMED) {
                pjsip_event ack_e;
                PJSIP_EVENT_INIT_TX_MSG(ack_e, inv->last_ack);
                inv_set_state(inv, PJSIP_INV_STATE_CONFIRMED, &ack_e);
            } else if (inv->state == PJSIP_INV_STATE_DISCONNECTED) {
                /* Avoid possible leaked tdata when invite session is
                 * already destroyed.
                 * https://github.com/pjsip/pjproject/pull/2432
                 */
                pjsip_tx_data_dec_ref(inv->last_ack);
                inv->last_ack = NULL;
            }

            pjsip_dlg_dec_lock(inv->dlg);
        }

    } else {
        pjsip_cseq_hdr *cseq;

        /* Can only do this to send response to original INVITE
         * request.
         */
        PJ_ASSERT_RETURN((cseq=(pjsip_cseq_hdr*)pjsip_msg_find_hdr(tdata->msg, PJSIP_H_CSEQ, NULL))!=NULL
                          && (cseq->cseq == inv->invite_tsx->cseq),
                         PJ_EINVALIDOP);

        if (inv->options & PJSIP_INV_REQUIRE_100REL) {
            status = pjsip_100rel_tx_response(inv, tdata);
        } else 
        {
            status = pjsip_dlg_send_response(inv->dlg, inv->invite_tsx, tdata);
        }

        if (status != PJ_SUCCESS) {
            goto on_error;
        }
    }

    /* Done */
on_return:
    pj_log_pop_indent();
    return PJ_SUCCESS;

on_error:
    pj_log_pop_indent();
    return status;
}


/*
 * Respond to incoming CANCEL request.
 */
static void inv_respond_incoming_cancel(pjsip_inv_session *inv,
                                        pjsip_transaction *cancel_tsx,
                                        pjsip_event *e)
{
    pjsip_tx_data *tdata;
    pjsip_transaction *invite_tsx;
    pjsip_rx_data *rdata;
    pj_str_t key;
    pj_status_t status;

    pj_assert(e->body.tsx_state.type == PJSIP_EVENT_RX_MSG);
    rdata = e->body.tsx_state.src.rdata;

    /* https://github.com/pjsip/pjproject/issues/1651
     * Special treatment for CANCEL. Since here we're responding to CANCEL
     * automatically (including 487 to INVITE), application will see the
     * 200/OK response to CANCEL first in the callback, and then 487 to
     * INVITE, before the CANCEL request itself. And worse, pjsua application
     * may not see the CANCEL request at all because by the time the CANCEL
     * request is reported, call has been disconnected and further events
     * from the INVITE session has been suppressed.
     */
    if (mod_inv.cb.on_tsx_state_changed && inv->notify)
        (*mod_inv.cb.on_tsx_state_changed)(inv, cancel_tsx, e);

    /* See if we have matching INVITE server transaction: */

    pjsip_tsx_create_key(rdata->tp_info.pool, &key, PJSIP_ROLE_UAS,
                         pjsip_get_invite_method(), rdata);
    invite_tsx = pjsip_tsx_layer_find_tsx2(&key, PJ_TRUE);

    if (invite_tsx == NULL) {

        /* Invite transaction not found! 
         * Respond CANCEL with 481 (RFC 3261 Section 9.2 page 55)
         */
        status = pjsip_dlg_create_response( inv->dlg, rdata, 481, NULL, 
                                            &tdata);

    } else {
        /* Always answer CANCEL will 200 (OK) regardless of
         * the state of the INVITE transaction.
         */
        status = pjsip_dlg_create_response( inv->dlg, rdata, 200, NULL, 
                                            &tdata);
    }

    /* See if we have created the response successfully. */
    if (status != PJ_SUCCESS) return;

    /* Send the CANCEL response */
    status = pjsip_dlg_send_response(inv->dlg, cancel_tsx, tdata);
    if (status != PJ_SUCCESS) return;


    /* See if we need to terminate the UAS INVITE transaction
     * with 487 (Request Terminated) response. 
     */
    if (invite_tsx && invite_tsx->status_code < 200) {

        pj_assert(invite_tsx->last_tx != NULL);

        tdata = invite_tsx->last_tx;

        status = pjsip_dlg_modify_response(inv->dlg, tdata, 487, NULL);
        if (status == PJ_SUCCESS) {
            /* Remove the message body */
            tdata->msg->body = NULL;
            if (inv->options & PJSIP_INV_REQUIRE_100REL) {
                status = pjsip_100rel_tx_response(inv, tdata);
            } else {
                status = pjsip_dlg_send_response(inv->dlg, invite_tsx, 
                                                 tdata);
            }
        }
    }

    if (invite_tsx)
        pj_grp_lock_dec_ref(invite_tsx->grp_lock);
}


/*
 * Respond to incoming BYE request.
 */
static void inv_respond_incoming_bye( pjsip_inv_session *inv,
                                      pjsip_transaction *bye_tsx,
                                      pjsip_rx_data *rdata,
                                      pjsip_event *e )
{
    pj_status_t status;
    pjsip_tx_data *tdata;

    /* Respond BYE with 200: */

    status = pjsip_dlg_create_response(inv->dlg, rdata, 200, NULL, &tdata);
    if (status != PJ_SUCCESS) return;

    status = pjsip_dlg_send_response(inv->dlg, bye_tsx, tdata);
    if (status != PJ_SUCCESS) return;

    /* Terminate session: */

    if (inv->state != PJSIP_INV_STATE_DISCONNECTED) {
        inv_set_cause(inv, PJSIP_SC_OK, NULL);
        inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);
    }
}

/*
 * Respond to BYE request.
 */
static void inv_handle_bye_response( pjsip_inv_session *inv,
                                     pjsip_transaction *tsx,
                                     pjsip_rx_data *rdata,
                                     pjsip_event *e )
{
    pj_status_t status;
    
    if (e->body.tsx_state.type != PJSIP_EVENT_RX_MSG) {
        inv_set_cause(inv, PJSIP_SC_OK, NULL);
        inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);
        return;
    }

    /* Handle 401/407 challenge. */
    if (tsx->status_code == 401 || tsx->status_code == 407) {

        pjsip_tx_data *tdata;
        
        status = pjsip_auth_clt_reinit_req( &inv->dlg->auth_sess, 
                                            rdata,
                                            tsx->last_tx,
                                            &tdata);
        
        if (status != PJ_SUCCESS) {
            
            /* Does not have proper credentials. 
             * End the session anyway.
             */
            inv_set_cause(inv, PJSIP_SC_OK, NULL);
            inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);
            
        } else {
            struct tsx_inv_data *tsx_inv_data;

            tsx_inv_data = (struct tsx_inv_data*)tsx->mod_data[mod_inv.mod.id];
            if (tsx_inv_data)
                tsx_inv_data->retrying = PJ_TRUE;

            /* Re-send BYE. */
            status = pjsip_inv_send_msg(inv, tdata);
        }

    } else {

        /* End the session. */
        inv_set_cause(inv, PJSIP_SC_OK, NULL);
        inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);
    }

}

/*
 * Respond to incoming UPDATE request.
 */
static void inv_respond_incoming_update(pjsip_inv_session *inv,
                                        pjsip_event *e)
{
    pjmedia_sdp_neg_state neg_state;
    pj_status_t status;
    pjsip_tx_data *tdata = NULL;
    pjsip_rx_data *rdata;
    pjsip_status_code st_code;

    pj_assert(e->type == PJSIP_EVENT_TSX_STATE &&
              e->body.tsx_state.type == PJSIP_EVENT_RX_MSG);
    rdata = e->body.tsx_state.src.rdata;

    /* Check routing URI scheme for secure dialog */
    if (!inv_check_secure_dlg(inv, e))
        return;

    /* Invoke Session Timers module */
    status = pjsip_timer_process_req(inv, rdata, &st_code);
    if (status != PJ_SUCCESS) {
        status = pjsip_dlg_create_response(inv->dlg, rdata, st_code,
                                           NULL, &tdata);
        goto on_return;
    }

    neg_state = pjmedia_sdp_neg_get_state(inv->neg);

    /* If UPDATE doesn't contain SDP, just respond with 200/OK.
     * This is a valid scenario according to session-timer draft.
     */
    if (rdata->msg_info.msg->body == NULL) {

        status = pjsip_dlg_create_response(inv->dlg, rdata, 
                                           200, NULL, &tdata);
    }
    /* Send 491 if we receive UPDATE while we're waiting for an answer */
    else if (neg_state == PJMEDIA_SDP_NEG_STATE_LOCAL_OFFER) {
        status = pjsip_dlg_create_response(inv->dlg, rdata, 
                                           PJSIP_SC_REQUEST_PENDING, NULL,
                                           &tdata);
    }
    /* Send 500 with Retry-After header set randomly between 0 and 10 if we 
     * receive UPDATE while we haven't sent answer.
     */
    else if (neg_state == PJMEDIA_SDP_NEG_STATE_REMOTE_OFFER ||
             neg_state == PJMEDIA_SDP_NEG_STATE_WAIT_NEGO)
    {
        pjsip_retry_after_hdr *ra_hdr;
        int val;

        status = pjsip_dlg_create_response(inv->dlg, rdata, 
                                           PJSIP_SC_INTERNAL_SERVER_ERROR,
                                           NULL, &tdata);

        val = (pj_rand() % 10);
        ra_hdr = pjsip_retry_after_hdr_create(tdata->pool, val);
        pjsip_msg_add_hdr(tdata->msg, (pjsip_hdr*)ra_hdr);

    } else {
        /* We receive new offer from remote */
        inv_check_sdp_in_incoming_msg(inv, pjsip_rdata_get_tsx(rdata), rdata);

        /* Application MUST have supplied the answer by now.
         * If so, negotiate the SDP.
         */
        neg_state = pjmedia_sdp_neg_get_state(inv->neg);
        if (neg_state != PJMEDIA_SDP_NEG_STATE_WAIT_NEGO ||
            (status=inv_negotiate_sdp(inv)) != PJ_SUCCESS)
        {
            /* Negotiation has failed. If negotiator is still
             * stuck at non-DONE state, cancel any ongoing offer.
             */
            neg_state = pjmedia_sdp_neg_get_state(inv->neg);
            if (neg_state != PJMEDIA_SDP_NEG_STATE_DONE) {
                pjmedia_sdp_neg_cancel_offer(inv->neg);
            }

            status = pjsip_dlg_create_response(inv->dlg, rdata, 
                                               PJSIP_SC_NOT_ACCEPTABLE_HERE,
                                               NULL, &tdata);
        } else {
            /* New media has been negotiated successfully, send 200/OK */
            status = pjsip_dlg_create_response(inv->dlg, rdata, 
                                               PJSIP_SC_OK, NULL, &tdata);
            if (status == PJ_SUCCESS) {
                const pjmedia_sdp_session *sdp;
                status = pjmedia_sdp_neg_get_active_local(inv->neg, &sdp);
                if (status == PJ_SUCCESS)
                    tdata->msg->body = create_sdp_body(tdata->pool, sdp);
            }
        }
    }

on_return:
    /* Invoke Session Timers */
    if (status == PJ_SUCCESS)
        status = pjsip_timer_update_resp(inv, tdata);

    if (status != PJ_SUCCESS) {
        if (tdata != NULL) {
            pjsip_tx_data_dec_ref(tdata);
            tdata = NULL;
        }
        return;
    }

    pjsip_dlg_send_response(inv->dlg, pjsip_rdata_get_tsx(rdata), tdata);
}


/*
 * Handle incoming response to UAC UPDATE request.
 */
static pj_bool_t inv_handle_update_response( pjsip_inv_session *inv,
                                             pjsip_event *e)
{
    pjsip_transaction *tsx = e->body.tsx_state.tsx;
    struct tsx_inv_data *tsx_inv_data;
    pj_bool_t handled = PJ_FALSE;
    pj_status_t status = -1;

    tsx_inv_data = (struct tsx_inv_data*)tsx->mod_data[mod_inv.mod.id];
    pj_assert(tsx_inv_data);

    /* Handle 401/407 challenge. */
    if (tsx->state == PJSIP_TSX_STATE_COMPLETED &&
        (tsx->status_code == 401 || tsx->status_code == 407))
    {
        pjsip_tx_data *tdata;

        status = pjsip_auth_clt_reinit_req( &inv->dlg->auth_sess, 
                                            e->body.tsx_state.src.rdata,
                                            tsx->last_tx,
                                            &tdata);
        
        if (status != PJ_SUCCESS) {
            
            /* Somehow failed. Probably it's not a good idea to terminate
             * the session since this is just a request within dialog. And
             * even if we terminate we should send BYE.
             */
            /*
            inv_set_cause(inv, PJSIP_SC_OK, NULL);
            inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);
            */
            
        } else {
            if (tsx_inv_data)
                tsx_inv_data->retrying = PJ_TRUE;

            /* Re-send request. */
            status = pjsip_inv_send_msg(inv, tdata);
        }

        handled = PJ_TRUE;
    }

    /* Process 422 response */
    else if (tsx->state == PJSIP_TSX_STATE_COMPLETED &&
             tsx->status_code == 422)
    {
        status = handle_timer_response(inv, e->body.tsx_state.src.rdata,
                                       PJ_FALSE);
        handled = PJ_TRUE;
    }

    /* Process 2xx response */
    else if (tsx->state == PJSIP_TSX_STATE_COMPLETED &&
        tsx->status_code/100 == 2)
    {
        pjsip_rx_data *rdata = e->body.tsx_state.src.rdata;

        /* Check routing URI scheme for secure dialog */
        if (inv_check_secure_dlg(inv, e)) {

            status = handle_timer_response(inv, rdata, PJ_FALSE);

            if (rdata->msg_info.msg->body) {
                /* Only process remote SDP if we have sent local offer */
                if (inv->neg && pjmedia_sdp_neg_get_state(inv->neg) == 
                                            PJMEDIA_SDP_NEG_STATE_LOCAL_OFFER)
                {
                    status = inv_check_sdp_in_incoming_msg(inv, tsx, rdata);
                } else {
                    PJ_LOG(5,(THIS_FILE, "Ignored message body in %s as no "
                                         "local offer was sent",
                                         pjsip_rx_data_get_info(rdata)));
                }
            }
        }

        handled = PJ_TRUE;
    }

    /* Process 502/503 error */
    else if ((tsx->state == PJSIP_TSX_STATE_TERMINATED) &&
             (tsx->status_code == 503 || tsx->status_code == 502))
    {
        status = pjsip_timer_handle_refresh_error(inv, e);

        handled = PJ_TRUE;
    }
    
    /* Get/attach invite session's transaction data */
    else 
    {
        /* Session-Timer needs to see any error responses, to determine
         * whether peer supports UPDATE with empty body.
         */
        if (tsx->state == PJSIP_TSX_STATE_COMPLETED &&
            tsx->role == PJSIP_ROLE_UAC)
        {
            status = handle_timer_response(inv, e->body.tsx_state.src.rdata,
                                           PJ_FALSE);
            handled = PJ_TRUE;
        }
    }

    /* Cancel the negotiation if we don't get successful negotiation by now,
     * unless it's authentication challenge and the request is being retried.
     */
    if (pjmedia_sdp_neg_get_state(inv->neg) ==
                PJMEDIA_SDP_NEG_STATE_LOCAL_OFFER &&
        tsx_inv_data && tsx_inv_data->sdp_done == PJ_FALSE &&
        !tsx_inv_data->retrying && tsx_inv_data->has_sdp)
    {
        pjmedia_sdp_neg_cancel_offer(inv->neg);

        /* Prevent from us cancelling different offer! */
        tsx_inv_data->sdp_done = PJ_TRUE;
    }

    return handled;
}


/*
 * Handle incoming reliable response.
 */
static void inv_handle_incoming_reliable_response(pjsip_inv_session *inv,
                                                  pjsip_rx_data *rdata)
{
    pjsip_tx_data *tdata;
    const pjmedia_sdp_session *sdp;
    pj_status_t status;

    /* Create PRACK */
    status = pjsip_100rel_create_prack(inv, rdata, &tdata);
    if (status != PJ_SUCCESS)
        return;

    /* See if we need to attach SDP answer on the PRACK request */
    sdp = inv_has_pending_answer(inv, pjsip_rdata_get_tsx(rdata));
    if (sdp) {
        tdata->msg->body = create_sdp_body(tdata->pool, sdp);
    }

    /* Send PRACK (must be using 100rel module!) */
    pjsip_100rel_send_prack(inv, tdata);
}


/*
 * Handle incoming PRACK.
 */
static void inv_respond_incoming_prack(pjsip_inv_session *inv,
                                       pjsip_rx_data *rdata)
{
    pj_status_t status;

    /* Run through 100rel module to see if we can accept this
     * PRACK request. The 100rel will send 200/OK to PRACK request.
     */
    status = pjsip_100rel_on_rx_prack(inv, rdata);
    if (status != PJ_SUCCESS)
        return;

    /* Now check for SDP answer in the PRACK request */
    if (rdata->msg_info.msg->body) {
        status = inv_check_sdp_in_incoming_msg(inv, 
                                        pjsip_rdata_get_tsx(rdata), rdata);
    } else {
        /* No SDP body */
        status = -1;
    }

    /* If SDP negotiation has been successful, also mark the
     * SDP negotiation flag in the invite transaction to be
     * done too.
     */
    if (status == PJ_SUCCESS && inv->invite_tsx) {
        struct tsx_inv_data *tsx_inv_data;

        /* Get/attach invite session's transaction data */
        tsx_inv_data = (struct tsx_inv_data*) 
                       inv->invite_tsx->mod_data[mod_inv.mod.id];
        if (tsx_inv_data == NULL) {
            tsx_inv_data = PJ_POOL_ZALLOC_T(inv->invite_tsx->pool, 
                                            struct tsx_inv_data);
            tsx_inv_data->inv = inv;
            tsx_inv_data->has_sdp = PJ_TRUE;
            inv->invite_tsx->mod_data[mod_inv.mod.id] = tsx_inv_data;
        }
        
        tsx_inv_data->sdp_done = PJ_TRUE;
    }

    if (pjmedia_sdp_neg_get_state(inv->neg) == PJMEDIA_SDP_NEG_STATE_DONE) {
        inv->sdp_done_early_rel = PJ_TRUE;
    }
}


/* Ticket #1735: If this is a secure dialog, make sure that any incoming
 * initial/subsequent INVITE/UPDATE request or the 2xx response to INVITE/
 * UPDATE specifies secure Contact and Record-Route headers.
 */
static pj_bool_t inv_check_secure_dlg(pjsip_inv_session *inv,
                                      pjsip_event *e)
{
    pjsip_transaction *tsx = e->body.tsx_state.tsx;
    pjsip_dialog *dlg = pjsip_tsx_get_dlg(tsx);

    if (pjsip_cfg()->endpt.disable_secure_dlg_check == PJ_FALSE &&
        dlg->secure && e->body.tsx_state.type==PJSIP_EVENT_RX_MSG &&
        ((tsx->role==PJSIP_ROLE_UAC && tsx->status_code/100 == 2) ||
         (tsx->role==PJSIP_ROLE_UAS && tsx->state == PJSIP_TSX_STATE_TRYING))
        &&
        (tsx->method.id==PJSIP_INVITE_METHOD || 
         pjsip_method_cmp(&tsx->method, &pjsip_update_method)==0))
    {
        const pjsip_msg *msg = e->body.tsx_state.src.rdata->msg_info.msg;
        pj_status_t status = PJ_SUCCESS;
        pjsip_contact_hdr *c;

        /* Check Contact header */
        c = (pjsip_contact_hdr*)
            pjsip_msg_find_hdr(msg,PJSIP_H_CONTACT, NULL);
        if (!(c && c->uri && PJSIP_URI_SCHEME_IS_SIPS(c->uri)))
            status = PJSIP_ESESSIONINSECURE;

        /* Check top Record-Route header */
        if (status == PJ_SUCCESS) {
            pjsip_rr_hdr *r = (pjsip_rr_hdr*)
                              pjsip_msg_find_hdr(msg, PJSIP_H_RECORD_ROUTE,
                                                 NULL);
            if (r && !PJSIP_URI_SCHEME_IS_SIPS(&r->name_addr)) {
                /* Not "sips", check if it is "sip" and has param
                 * "transport=tls".
                 */
                if (PJSIP_URI_SCHEME_IS_SIP(&r->name_addr)) {
                    pjsip_sip_uri *sip_uri = (pjsip_sip_uri*)
                                     pjsip_uri_get_uri(r->name_addr.uri);
                    if (pj_stricmp2(&sip_uri->transport_param, "tls")!=0)
                        status = PJSIP_ESESSIONINSECURE;
                } else {
                    /* Not "sips" nor "sip", treat it as insecure? */
                    status = PJSIP_ESESSIONINSECURE;
                }
            }
        }

        if (status == PJSIP_ESESSIONINSECURE) {
            /* Found non-SIPS scheme in Contact/Record-Route header */

            pj_str_t warn_text = pj_str("SIPS Required");

            if (tsx->role == PJSIP_ROLE_UAC) {

                /* If we are UAC, terminate the session */
                pjsip_tx_data *bye;

                PJ_LOG(4,(inv->obj_name,
                          "Secure dialog requires SIPS scheme in Contact and "
                          "Record-Route headers, ending the session"));

                status = pjsip_inv_end_session(inv, 480, NULL, &bye);
                if (status == PJ_SUCCESS && bye) {
                    pjsip_warning_hdr *w;
                    w = pjsip_warning_hdr_create(bye->pool, 381,
                                                 pjsip_endpt_name(dlg->endpt),
                                                 &warn_text);
                    if (w)
                        pjsip_msg_add_hdr(bye->msg, (pjsip_hdr*)w);

                    status = pjsip_inv_send_msg(inv, bye);
                }

            } else {

                /* If we are UAS, reject the request */
                pjsip_rx_data *rdata = e->body.tsx_state.src.rdata;
                pjsip_tx_data *tdata;

                PJ_LOG(4,(inv->obj_name,
                          "Secure dialog requires SIPS scheme in Contact and "
                          "Route headers, rejecting the request"));

                status = pjsip_dlg_create_response(inv->dlg, rdata, 480,
                                                   NULL, &tdata);
                if (status == PJ_SUCCESS) {
                    pjsip_warning_hdr *w;
                    w = pjsip_warning_hdr_create(tdata->pool, 381,
                                                 pjsip_endpt_name(dlg->endpt),
                                                 &warn_text);
                    if (w)
                        pjsip_msg_add_hdr(tdata->msg, (pjsip_hdr*)w);

                    pjsip_dlg_send_response(dlg, tsx, tdata);
                }

            }

            return PJ_FALSE;
        }
    }

    return PJ_TRUE;
}



/*
 * State NULL is before anything is sent/received.
 */
static void inv_on_state_null( pjsip_inv_session *inv, pjsip_event *e)
{
    pjsip_transaction *tsx = e->body.tsx_state.tsx;
    pjsip_dialog *dlg = pjsip_tsx_get_dlg(tsx);

    PJ_ASSERT_ON_FAIL(tsx && dlg, return);

    if (tsx->method.id == PJSIP_INVITE_METHOD) {

        /* Keep the initial INVITE transaction. */
        if (inv->invite_tsx == NULL)
            inv->invite_tsx = tsx;

        if (dlg->role == PJSIP_ROLE_UAC) {

            /* Save the original INVITE request.
             * We may need to resend the INVITE if we receive redirection
             * or session timer too small response.
             */
            if (1) {
                if (inv->invite_req) {
                    pjsip_tx_data_dec_ref(inv->invite_req);
                    inv->invite_req = NULL;
                }
                inv->invite_req = tsx->last_tx;
                pjsip_tx_data_add_ref(inv->invite_req);
            }

            switch (tsx->state) {
            case PJSIP_TSX_STATE_CALLING:
                inv_set_state(inv, PJSIP_INV_STATE_CALLING, e);
                break;
            default:
                inv_on_state_calling(inv, e);
                break;
            }

        } else {
            switch (tsx->state) {
            case PJSIP_TSX_STATE_TRYING:
                inv_set_state(inv, PJSIP_INV_STATE_INCOMING, e);
                break;
            case PJSIP_TSX_STATE_PROCEEDING:
                inv_set_state(inv, PJSIP_INV_STATE_INCOMING, e);
                if (tsx->status_code > 100)
                    inv_set_state(inv, PJSIP_INV_STATE_EARLY, e);
                break;
            case PJSIP_TSX_STATE_TERMINATED:
                /* there is a failure in sending response. */
                inv_set_cause(inv, tsx->status_code, &tsx->status_text);
                inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);
                break;
            default:
                inv_on_state_incoming(inv, e);
                break;
            }
        }

    } else {
        pj_assert(!"Unexpected transaction type");
    }
}

/*
 * Generic UAC transaction handler:
 *  - resend request on 401 or 407 response.
 *  - terminate dialog on 408 and 481 response.
 *  - resend request on 422 response.
 */
static pj_bool_t handle_uac_tsx_response(pjsip_inv_session *inv, 
                                         pjsip_event *e)
{
    /* RFC 3261 Section 12.2.1.2:
     *  If the response for a request within a dialog is a 481
     *  (Call/Transaction Does Not Exist) or a 408 (Request Timeout), the UAC
     *  SHOULD terminate the dialog.  A UAC SHOULD also terminate a dialog if
     *  no response at all is received for the request (the client
     *  transaction would inform the TU about the timeout.)
     * 
     *  For INVITE initiated dialogs, terminating the dialog consists of
     *  sending a BYE.
     *
     * Note:
     *  according to X, this should terminate dialog usage only, not the 
     *  dialog.
     */
    pjsip_transaction *tsx = e->body.tsx_state.tsx;

    pj_assert(tsx->role == PJSIP_UAC_ROLE);

    /* Note that 481 response to CANCEL does not terminate dialog usage,
     * but only the transaction.
     */
    if (inv->state != PJSIP_INV_STATE_DISCONNECTED &&
        ((tsx->status_code == PJSIP_SC_CALL_TSX_DOES_NOT_EXIST &&
            tsx->method.id != PJSIP_CANCEL_METHOD) ||
         (tsx->status_code == PJSIP_SC_REQUEST_TIMEOUT &&
          !pjsip_cfg()->endpt.keep_inv_after_tsx_timeout)))
    {
        pjsip_tx_data *bye;
        pj_status_t status;

        inv_set_cause(inv, tsx->status_code, &tsx->status_text);
        inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);

        /* Send BYE */
        status = pjsip_dlg_create_request(inv->dlg, pjsip_get_bye_method(), 
                                          -1, &bye);
        if (status == PJ_SUCCESS) {
            pjsip_inv_send_msg(inv, bye);
        }

        return PJ_TRUE; /* Handled */

    } 
    /* Handle 401/407 challenge. */
    else if (tsx->state == PJSIP_TSX_STATE_COMPLETED &&
             (tsx->status_code == PJSIP_SC_UNAUTHORIZED ||
              tsx->status_code == PJSIP_SC_PROXY_AUTHENTICATION_REQUIRED)) 
    {
        pjsip_tx_data *tdata;
        pj_status_t status;

        if (tsx->method.id == PJSIP_INVITE_METHOD)
            inv->invite_tsx = NULL;

        status = pjsip_auth_clt_reinit_req( &inv->dlg->auth_sess, 
                                            e->body.tsx_state.src.rdata,
                                            tsx->last_tx, &tdata);
    
        if (status != PJ_SUCCESS) {
            /* Somehow failed. Probably it's not a good idea to terminate
             * the session since this is just a request within dialog. And
             * even if we terminate we should send BYE.
             */
            /*
            inv_set_cause(inv, PJSIP_SC_OK, NULL);
            inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);
            */
            
        } else {
            struct tsx_inv_data *tsx_inv_data;

            tsx_inv_data = (struct tsx_inv_data*)tsx->mod_data[mod_inv.mod.id];
            if (tsx_inv_data)
                tsx_inv_data->retrying = PJ_TRUE;

            /* Re-send request. */
            status = pjsip_inv_send_msg(inv, tdata);
        }

        return PJ_TRUE; /* Handled */
    }

    /* Handle session timer 422 response. */
    else if (tsx->state == PJSIP_TSX_STATE_COMPLETED &&
             tsx->status_code == PJSIP_SC_SESSION_TIMER_TOO_SMALL) 
    {
        handle_timer_response(inv, e->body.tsx_state.src.rdata, 
                              PJ_FALSE);

        return PJ_TRUE; /* Handled */

    } 

    else {
        return PJ_FALSE; /* Unhandled */
    }
}


/* Handle call rejection, especially with regard to processing call
 * redirection. We need to handle the following scenarios:
 *  - 3xx response is received -- see if on_redirected() callback is
 *    implemented. If so, add the Contact URIs in the response to the
 *    target set and notify user.
 *  - 4xx - 6xx resposne is received -- see if we're currently recursing,
 *    if so fetch the next target if any and notify the on_redirected()
 *    callback.
 *  - for other cases -- disconnect the session.
 */
static void handle_uac_call_rejection(pjsip_inv_session *inv, pjsip_event *e)
{
    pjsip_transaction *tsx = e->body.tsx_state.tsx;
    pj_status_t status;
    
    if (PJSIP_IS_STATUS_IN_CLASS(tsx->status_code, 300)) {

        if (mod_inv.cb.on_redirected == NULL) {

            /* Redirection callback is not implemented, disconnect the
             * call.
             */
            goto terminate_session;

        } else {
            const pjsip_msg *res_msg;

            res_msg = e->body.tsx_state.src.rdata->msg_info.msg;

            /* Gather all Contact URI's in the response and add them
             * to target set. The function will take care of removing
             * duplicate URI's.
             */
            pjsip_target_set_add_from_msg(&inv->dlg->target_set, 
                                          inv->dlg->pool, res_msg);

            /* Recurse to alternate targets if application allows us */
            if (!inv_uac_recurse(inv, tsx->status_code, &tsx->status_text, e))
            {
                /* Recursion fails, terminate session now */
                goto terminate_session;
            }

            /* Done */
        }

    } else if ((tsx->status_code==401 || tsx->status_code==407) &&
                !inv->cancelling) 
    {

        /* Handle authentication failure:
         * Resend the request with Authorization header.
         */
        pjsip_tx_data *tdata;

        status = pjsip_auth_clt_reinit_req(&inv->dlg->auth_sess, 
                                           e->body.tsx_state.src.rdata,
                                           tsx->last_tx,
                                           &tdata);

        if (status != PJ_SUCCESS) {

            /* Does not have proper credentials. If we are currently 
             * recursing, try the next target. Otherwise end the session.
             */
            if (!inv_uac_recurse(inv, tsx->status_code, &tsx->status_text, e))
            {
                /* Recursion fails, terminate session now */
                goto terminate_session;
            }

        } else {

            /* Restart session. */
            pjsip_inv_uac_restart(inv, PJ_FALSE);

            /* Send the request. */
            status = pjsip_inv_send_msg(inv, tdata);
        }

    } else if (tsx->state == PJSIP_TSX_STATE_COMPLETED &&
               tsx->status_code == PJSIP_SC_SESSION_TIMER_TOO_SMALL) 
    {
        /* Handle session timer 422 response:
         * Resend the request with requested session timer setting.
         */
        status = handle_timer_response(inv, e->body.tsx_state.src.rdata,
                                       PJ_FALSE);
        if (status != PJ_SUCCESS)
            goto terminate_session;

    } else if (PJSIP_IS_STATUS_IN_CLASS(tsx->status_code, 600)) {
        /* Global error */
        goto terminate_session;

    } else {
        /* See if we have alternate target to try */
        if (!inv_uac_recurse(inv, tsx->status_code, &tsx->status_text, e)) {
            /* Recursion fails, terminate session now */
            goto terminate_session;
        }
    }
    return;

terminate_session:
    inv_set_cause(inv, tsx->status_code, &tsx->status_text);
    inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);
}


/*
 * State CALLING is after sending initial INVITE request but before
 * any response (with tag) is received.
 */
static void inv_on_state_calling( pjsip_inv_session *inv, pjsip_event *e)
{
    pjsip_transaction *tsx = e->body.tsx_state.tsx;
    pjsip_dialog *dlg = pjsip_tsx_get_dlg(tsx);
    pj_status_t status;

    PJ_ASSERT_ON_FAIL(tsx && dlg, return);
    
    if (tsx == inv->invite_tsx) {

        switch (tsx->state) {

        case PJSIP_TSX_STATE_CALLING:
            inv_set_state(inv, PJSIP_INV_STATE_CALLING, e);
            break;

        case PJSIP_TSX_STATE_PROCEEDING:
            if (inv->pending_cancel) {
                pjsip_tx_data *cancel;

                inv->pending_cancel = PJ_FALSE;

                status = pjsip_inv_end_session(inv, 487, NULL, &cancel);
                if (status == PJ_SUCCESS && cancel)
                    status = pjsip_inv_send_msg(inv, cancel);
            }

            if (tsx->status_code != 100) {

                if (inv->role == PJSIP_ROLE_UAC) {
                    pjsip_rx_data *rdata = e->body.tsx_state.src.rdata;
                    pjsip_allow_hdr *allow = NULL;
                    pjsip_msg *msg = rdata->msg_info.msg;

                    if (msg) {
                        allow = (pjsip_allow_hdr*) pjsip_msg_find_hdr(msg, 
                                 PJSIP_H_ALLOW, NULL);
                    }
                    if (allow) {
                        unsigned i;
                        const pj_str_t STR_UPDATE = { "UPDATE", 6 };

                        for (i=0; i<allow->count; ++i) {
                            if (pj_stricmp(&allow->values[i], &STR_UPDATE)==0) {
                                /* UPDATE is present in Allow */
                                inv->options |= PJSIP_INV_SUPPORT_UPDATE;
                                break;
                            }
                        }
                    }
                }

                if (dlg->remote.info->tag.slen)
                    inv_set_state(inv, PJSIP_INV_STATE_EARLY, e);

                inv_check_sdp_in_incoming_msg(inv, tsx, 
                                              e->body.tsx_state.src.rdata);

                if (pjsip_100rel_is_reliable(e->body.tsx_state.src.rdata)) {
                    inv_handle_incoming_reliable_response(
                        inv, e->body.tsx_state.src.rdata);
                }

            } else {
                /* Ignore 100 (Trying) response, as it doesn't change
                 * session state. It only ceases retransmissions.
                 */
            }
            break;

        case PJSIP_TSX_STATE_COMPLETED:
            if (tsx->status_code/100 == 2) {

                /* This should not happen.
                 * When transaction receives 2xx, it should be terminated
                 */
                pj_assert(0);

                inv_set_state(inv, PJSIP_INV_STATE_CONNECTING, e);

                /* Check routing URI scheme for secure dialog */
                if (!inv_check_secure_dlg(inv, e))
                    break;

                /* Process session timer response. */
                status = handle_timer_response(inv,
                                               e->body.tsx_state.src.rdata,
                                               PJ_TRUE);
                if (status != PJ_SUCCESS)
                    break;

                inv_check_sdp_in_incoming_msg(inv, tsx, 
                                              e->body.tsx_state.src.rdata);

            } else {
                handle_uac_call_rejection(inv, e);
            }
            break;

        case PJSIP_TSX_STATE_TERMINATED:
            /* INVITE transaction can be terminated either because UAC
             * transaction received 2xx response or because of transport
             * error.
             */
            if (tsx->status_code/100 == 2) {
                /* This must be receipt of 2xx response */
                pj_assert(e->body.tsx_state.type == PJSIP_EVENT_RX_MSG);

                /* Set state to CONNECTING */
                inv_set_state(inv, PJSIP_INV_STATE_CONNECTING, e);

                /* Check routing URI scheme for secure dialog */
                if (!inv_check_secure_dlg(inv, e))
                    break;

                /* Process session timer response. */
                status = handle_timer_response(inv,
                                               e->body.tsx_state.src.rdata,
                                               PJ_TRUE);
                if (status != PJ_SUCCESS)
                    break;

                inv_check_sdp_in_incoming_msg(inv, tsx, 
                                              e->body.tsx_state.src.rdata);
                /* Send ACK */
                inv_send_ack(inv, e);

            } else  {
                inv_set_cause(inv, tsx->status_code, &tsx->status_text);
                inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);
            }
            break;

        default:
            break;
        }

    } else if (tsx->role == PJSIP_ROLE_UAC) {
        /*
         * Handle case when outgoing request is answered with 481 (Call/
         * Transaction Does Not Exist), 408, or when it's timed out. In these
         * cases, disconnect session (i.e. dialog usage only).
         * Note that 481 response to CANCEL does not terminate dialog usage,
         * but only the transaction.
         */
        if ((tsx->status_code == PJSIP_SC_CALL_TSX_DOES_NOT_EXIST &&
                tsx->method.id != PJSIP_CANCEL_METHOD) ||
            tsx->status_code == PJSIP_SC_REQUEST_TIMEOUT ||
            tsx->status_code == PJSIP_SC_TSX_TIMEOUT)
        {
            inv_set_cause(inv, tsx->status_code, &tsx->status_text);
            inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);
        }
    } else if (tsx->role == PJSIP_ROLE_UAS &&
               tsx->state == PJSIP_TSX_STATE_TRYING &&
               pjsip_method_cmp(&tsx->method, &pjsip_update_method)==0)
    {
        /*
         * Handle a very early UPDATE
         */
        inv_respond_incoming_update(inv, e);


    }
}

/*
 * State INCOMING is after we received the request, but before
 * responses with tag are sent.
 */
static void inv_on_state_incoming( pjsip_inv_session *inv, pjsip_event *e)
{
    pjsip_transaction *tsx = e->body.tsx_state.tsx;
    pjsip_dialog *dlg = pjsip_tsx_get_dlg(tsx);

    PJ_ASSERT_ON_FAIL(tsx && dlg, return);

    if (tsx == inv->invite_tsx) {

        /*
         * Handle the INVITE state transition.
         */

        switch (tsx->state) {

        case PJSIP_TSX_STATE_TRYING:
            inv_set_state(inv, PJSIP_INV_STATE_INCOMING, e);
            break;

        case PJSIP_TSX_STATE_PROCEEDING:
            /*
             * Transaction sent provisional response.
             */
            if (tsx->status_code > 100)
                inv_set_state(inv, PJSIP_INV_STATE_EARLY, e);
            break;

        case PJSIP_TSX_STATE_COMPLETED:
            /*
             * Transaction sent final response.
             */
            if (tsx->status_code/100 == 2) {
                inv_set_state(inv, PJSIP_INV_STATE_CONNECTING, e);
            } else {
                inv_set_cause(inv, tsx->status_code, &tsx->status_text);
                inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);
            }
            break;

        case PJSIP_TSX_STATE_TERMINATED:
            /* 
             * This happens on transport error (e.g. failed to send
             * response)
             */
            inv_set_cause(inv, tsx->status_code, &tsx->status_text);
            inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);
            break;

        default:
            pj_assert(!"Unexpected INVITE state");
            break;
        }

    } else if (tsx->method.id == PJSIP_CANCEL_METHOD &&
               tsx->role == PJSIP_ROLE_UAS &&
               tsx->state < PJSIP_TSX_STATE_COMPLETED &&
               e->body.tsx_state.type == PJSIP_EVENT_RX_MSG )
    {

        /*
         * Handle incoming CANCEL request.
         */

        inv_respond_incoming_cancel(inv, tsx, e);

    }
}

/*
 * State EARLY is for both UAS and UAC, after response with To tag
 * is sent/received.
 */
static void inv_on_state_early( pjsip_inv_session *inv, pjsip_event *e)
{
    pjsip_transaction *tsx = e->body.tsx_state.tsx;
    pjsip_dialog *dlg = pjsip_tsx_get_dlg(tsx);

    PJ_ASSERT_ON_FAIL(tsx && dlg, return);

    if (tsx == inv->invite_tsx) {

        /*
         * Handle the INVITE state progress.
         */

        switch (tsx->state) {

        case PJSIP_TSX_STATE_PROCEEDING:
            /* Send/received another provisional response. */
            inv_set_state(inv, PJSIP_INV_STATE_EARLY, e);

            if (e->body.tsx_state.type == PJSIP_EVENT_RX_MSG) {
                inv_check_sdp_in_incoming_msg(inv, tsx, 
                                              e->body.tsx_state.src.rdata);

                if (pjsip_100rel_is_reliable(e->body.tsx_state.src.rdata)) {
                    inv_handle_incoming_reliable_response(
                        inv, e->body.tsx_state.src.rdata);
                }
            }
            break;

        case PJSIP_TSX_STATE_COMPLETED:
            if (tsx->status_code/100 == 2) {
                inv_set_state(inv, PJSIP_INV_STATE_CONNECTING, e);
                if (e->body.tsx_state.type == PJSIP_EVENT_RX_MSG) {
                    pj_status_t status;

                    /* Check routing URI scheme for secure dialog */
                    if (!inv_check_secure_dlg(inv, e))
                        break;

                    /* Process session timer response. */
                    status = handle_timer_response(inv, 
                                                   e->body.tsx_state.src.rdata,
                                                   PJ_TRUE);
                    if (status != PJ_SUCCESS)
                        break;

                    inv_check_sdp_in_incoming_msg(inv, tsx, 
                                                  e->body.tsx_state.src.rdata);
                }

            } else if (tsx->role == PJSIP_ROLE_UAC) {

                handle_uac_call_rejection(inv, e);

            } else {
                inv_set_cause(inv, tsx->status_code, &tsx->status_text);
                inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);
            }
            break;

        case PJSIP_TSX_STATE_CONFIRMED:
            /* For some reason can go here (maybe when ACK for 2xx has
             * the same branch value as the INVITE transaction) */

        case PJSIP_TSX_STATE_TERMINATED:
            /* INVITE transaction can be terminated either because UAC
             * transaction received 2xx response or because of transport
             * error.
             */
            if (tsx->status_code/100 == 2) {

                /* This must be receipt of 2xx response */

                /* Set state to CONNECTING */
                inv_set_state(inv, PJSIP_INV_STATE_CONNECTING, e);

                if (e->body.tsx_state.type == PJSIP_EVENT_RX_MSG) {
                    pj_status_t status;
                    
                    /* Check routing URI scheme for secure dialog */
                    if (!inv_check_secure_dlg(inv, e))
                        break;

                    /* Process session timer response. */
                    status = handle_timer_response(inv, 
                                                   e->body.tsx_state.src.rdata,
                                                   PJ_TRUE);
                    if (status != PJ_SUCCESS)
                        break;

                    inv_check_sdp_in_incoming_msg(inv, tsx, 
                                                  e->body.tsx_state.src.rdata);
                }

                /* if UAC, send ACK and move state to confirmed. */
                if (tsx->role == PJSIP_ROLE_UAC) {
                    pj_assert(e->body.tsx_state.type == PJSIP_EVENT_RX_MSG);

                    inv_send_ack(inv, e);
                }

            } else  {
                inv_set_cause(inv, tsx->status_code, &tsx->status_text);
                inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);
            }
            break;

        default:
            pj_assert(!"Unexpected INVITE tsx state");
            break;
        }

    } else if (inv->role == PJSIP_ROLE_UAS &&
               tsx->role == PJSIP_ROLE_UAS &&
               tsx->method.id == PJSIP_CANCEL_METHOD &&
               tsx->state < PJSIP_TSX_STATE_COMPLETED &&
               e->body.tsx_state.type == PJSIP_EVENT_RX_MSG )
    {

        /*
         * Handle incoming CANCEL request.
         */

        inv_respond_incoming_cancel(inv, tsx, e);

    } else if (tsx->role == PJSIP_ROLE_UAS &&
               tsx->state == PJSIP_TSX_STATE_TRYING &&
               pjsip_method_cmp(&tsx->method, &pjsip_update_method)==0)
    {
        /*
         * Handle incoming UPDATE
         */
        inv_respond_incoming_update(inv, e);


    } else if (tsx->role == PJSIP_ROLE_UAC &&
               (tsx->state == PJSIP_TSX_STATE_COMPLETED ||
                tsx->state == PJSIP_TSX_STATE_TERMINATED) &&
               pjsip_method_cmp(&tsx->method, &pjsip_update_method)==0)
    {
        /*
         * Handle response to outgoing UPDATE request.
         */
        inv_handle_update_response(inv, e);

    } else if (tsx->role == PJSIP_ROLE_UAS &&
               tsx->state == PJSIP_TSX_STATE_TRYING &&
               pjsip_method_cmp(&tsx->method, &pjsip_prack_method)==0)
    {
        /*
         * Handle incoming PRACK
         */
        inv_respond_incoming_prack(inv, e->body.tsx_state.src.rdata);

    } else if (tsx->role == PJSIP_ROLE_UAC) {
        
        /* Generic handling for UAC tsx completion */
        handle_uac_tsx_response(inv, e);

    } else if (tsx->role == PJSIP_ROLE_UAS &&
               tsx->method.id == PJSIP_BYE_METHOD &&
               tsx->status_code < 200 &&
               e->body.tsx_state.type == PJSIP_EVENT_RX_MSG)
    {
        /* Received BYE before the 2xx/OK response to INVITE.
         * Assume that the 2xx/OK response is lost and the BYE
         * arrives earlier.
         */
        inv_respond_incoming_bye(inv, tsx, e->body.tsx_state.src.rdata, e);

        if (inv->invite_tsx->role == PJSIP_ROLE_UAC) {
            /* Set timer just in case we will never get the final response
             * for INVITE.
             */
            pjsip_tsx_set_timeout(inv->invite_tsx, 64*pjsip_cfg()->tsx.t1);
        } else if (inv->invite_tsx->status_code < 200) {
            pjsip_tx_data *tdata;
            pjsip_msg *msg;

            /* For UAS, send a final response. */
            tdata = inv->invite_tsx->last_tx;
            PJ_ASSERT_ON_FAIL(tdata != NULL, return);

            msg = tdata->msg;
            msg->line.status.code = PJSIP_SC_REQUEST_TERMINATED;
            msg->line.status.reason =
                    *pjsip_get_status_text(PJSIP_SC_REQUEST_TERMINATED);
            msg->body = NULL;

            pjsip_tx_data_invalidate_msg(tdata);
            pjsip_tx_data_add_ref(tdata);

            pjsip_dlg_send_response(inv->dlg, inv->invite_tsx, tdata);
        }
    }
}

/*
 * State CONNECTING is after 2xx response to INVITE is sent/received.
 */
static void inv_on_state_connecting( pjsip_inv_session *inv, pjsip_event *e)
{
    pjsip_transaction *tsx = e->body.tsx_state.tsx;
    pjsip_dialog *dlg = pjsip_tsx_get_dlg(tsx);

    PJ_ASSERT_ON_FAIL(tsx && dlg, return);

    if (tsx == inv->invite_tsx) {

        /*
         * Handle INVITE state progression.
         */
        switch (tsx->state) {

        case PJSIP_TSX_STATE_CONFIRMED:
            /* It can only go here if incoming ACK request has the same Via
             * branch parameter as the INVITE transaction.
             */
            if (tsx->status_code/100 == 2) {
                if (e->body.tsx_state.type == PJSIP_EVENT_RX_MSG) {
                    inv_check_sdp_in_incoming_msg(inv, tsx,
                                                  e->body.tsx_state.src.rdata);
                }

                inv_set_state(inv, PJSIP_INV_STATE_CONFIRMED, e);

                /* Send pending BYE if any:
                 *   https://github.com/pjsip/pjproject/issues/1712
                 * Do this after setting the state to CONFIRMED, so that we
                 * have consistent CONFIRMED state between caller and callee.
                 */
                if (inv->pending_bye)
                    inv_perform_pending_bye(inv);

            }
            break;

        case PJSIP_TSX_STATE_TERMINATED:
            /* INVITE transaction can be terminated either because UAC
             * transaction received 2xx response or because of transport
             * error.
             */
            if (tsx->status_code/100 != 2) {
                if (tsx->role == PJSIP_ROLE_UAC) {
                    inv_set_cause(inv, tsx->status_code, &tsx->status_text);
                    inv_set_state(inv, PJSIP_INV_STATE_DISCONNECTED, e);
                } else {
                    pjsip_tx_data *bye;
                    pj_status_t status;

                    inv_set_cause(inv, tsx->status_code, &tsx->status_text);

                    /* Send BYE */
                    status = pjsip_dlg_create_request(inv->dlg,
                                                      pjsip_get_bye_method(),
                                                      -1, &bye);
                    if (status == PJ_SUCCESS) {
                        pjsip_inv_send_msg(inv, bye);

                        if (inv->pending_bye) {
                            pjsip_tx_data_dec_ref(inv->pending_bye);
                            inv->pending_bye = NULL;
                        }
                    }
                }
            }
            break;

        case PJSIP_TSX_STATE_DESTROYED:
            /* Do nothing. */
            break;

        default:
            pj_assert(!"Unexpected state");
            break;
        }

    } else if (tsx->role == PJSIP_ROLE_UAS &&
               tsx->method.id == PJSIP_BYE_METHOD &&
               tsx->status_code < 200 &&
               e->body.tsx_state.type == PJSIP_EVENT_RX_MSG) 
    {

        /*
         * Handle incoming BYE.
         */

        inv_respond_incoming_bye( inv, tsx, e->body.tsx_state.src.rdata, e );

    } else if (tsx->method.id == PJSIP_BYE_METHOD &&
               tsx->role == PJSIP_ROLE_UAC &&
               (tsx->state == PJSIP_TSX_STATE_COMPLETED ||
                tsx->state == PJSIP_TSX_STATE_TERMINATED))
    {

        /*
         * Outgoing BYE
         */
        inv_handle_bye_response( inv, tsx, e->body.tsx_state.src.rdata, e);

    }
    else if (tsx->method.id == PJSIP_CANCEL_METHOD &&
             tsx->role == PJSIP_ROLE_UAS &&
             tsx->status_code < 200 &&
             e->body.tsx_state.type == PJSIP_EVENT_RX_MSG) 
    {

        /*
         * Handle strandled incoming CANCEL or CANCEL for re-INVITE
         */
        inv_respond_incoming_cancel(inv, tsx, e);

    } else if (tsx->role == PJSIP_ROLE_UAS &&
               tsx->state == PJSIP_TSX_STATE_TRYING &&
               pjsip_method_cmp(&tsx->method, pjsip_get_invite_method())==0)
    {
        pjsip_rx_data *rdata = e->body.tsx_state.src.rdata;
        pjsip_tx_data *tdata;
        pj_status_t status;

        /* See https://github.com/pjsip/pjproject/issues/1455
         * Handle incoming re-INVITE before current INVITE is confirmed.
         * According to RFC 5407:
         *  - answer with 200 if we don't have pending offer-answer
         *  - answer with 491 if we *have* pending offer-answer
         *
         *  But unfortunately accepting the re-INVITE would mean we have
         *  two outstanding INVITEs, and we don't support that because
         *  we will get confused when we handle the ACK.
         */
        status = pjsip_dlg_create_response(inv->dlg, rdata,
                                           PJSIP_SC_REQUEST_PENDING,
                                           NULL, &tdata);
        if (status != PJ_SUCCESS)
            return;
        pjsip_timer_update_resp(inv, tdata);
        status = pjsip_dlg_send_response(dlg, tsx, tdata);

    } else if (tsx->role == PJSIP_ROLE_UAS &&
               tsx->state == PJSIP_TSX_STATE_TRYING &&
               pjsip_method_cmp(&tsx->method, &pjsip_update_method)==0)
    {
        /*
         * Handle incoming UPDATE
         */
        inv_respond_incoming_update(inv, e);


    } else if (tsx->role == PJSIP_ROLE_UAC &&
               (tsx->state == PJSIP_TSX_STATE_COMPLETED ||
                tsx->state == PJSIP_TSX_STATE_TERMINATED) &&
               pjsip_method_cmp(&tsx->method, &pjsip_update_method)==0)
    {
        /*
         * Handle response to outgoing UPDATE request.
         */
        if (inv_handle_update_response(inv, e) == PJ_FALSE)
            handle_uac_tsx_response(inv, e);

    } else if (tsx->role == PJSIP_ROLE_UAS &&
               tsx->state == PJSIP_TSX_STATE_TRYING &&
               pjsip_method_cmp(&tsx->method, &pjsip_prack_method)==0)
    {
        /*
         * Handle incoming PRACK
         */
        inv_respond_incoming_prack(inv, e->body.tsx_state.src.rdata);

    } else if (tsx->role == PJSIP_ROLE_UAC) {
        
        /* Generic handling for UAC tsx completion */
        handle_uac_tsx_response(inv, e);

    }

}

/*
 * State CONFIRMED is after ACK is sent/received.
 */
static void inv_on_state_confirmed( pjsip_inv_session *inv, pjsip_event *e)
{
    pjsip_transaction *tsx = e->body.tsx_state.tsx;
    pjsip_dialog *dlg = pjsip_tsx_get_dlg(tsx);

    PJ_ASSERT_ON_FAIL(tsx && dlg, return);


    if (tsx->method.id == PJSIP_BYE_METHOD &&
        tsx->role == PJSIP_ROLE_UAC &&
        (tsx->state == PJSIP_TSX_STATE_COMPLETED ||
         tsx->state == PJSIP_TSX_STATE_TERMINATED))
    {

        /*
         * Outgoing BYE
         */

        inv_handle_bye_response( inv, tsx, e->body.tsx_state.src.rdata, e);

    }
    else if (tsx->method.id == PJSIP_BYE_METHOD &&
             tsx->role == PJSIP_ROLE_UAS &&
             tsx->status_code < 200 &&
             e->body.tsx_state.type == PJSIP_EVENT_RX_MSG) 
    {

        /*
         * Handle incoming BYE.
         */

        inv_respond_incoming_bye( inv, tsx, e->body.tsx_state.src.rdata, e );

    }
    else if (tsx->method.id == PJSIP_CANCEL_METHOD &&
             tsx->role == PJSIP_ROLE_UAS &&
             tsx->status_code < 200 &&
             e->body.tsx_state.type == PJSIP_EVENT_RX_MSG) 
    {

        /*
         * Handle strandled incoming CANCEL or CANCEL for re-INVITE
         */
        inv_respond_incoming_cancel(inv, tsx, e);
    }
    else if (tsx->method.id == PJSIP_INVITE_METHOD &&
             tsx->role == PJSIP_ROLE_UAS)
    {

        /*
         * Handle incoming re-INVITE
         */
        if (tsx->state == PJSIP_TSX_STATE_TRYING) {
            
            pjsip_rx_data *rdata = e->body.tsx_state.src.rdata;
            pjsip_tx_data *tdata;
            pj_status_t status;
            pjsip_rdata_sdp_info *sdp_info = NULL;
            pjsip_status_code st_code;

            /* Check if we have INVITE pending. */
            if (inv->invite_tsx && inv->invite_tsx!=tsx) {
                int code;
                pj_str_t reason;

                reason = pj_str("Another INVITE transaction in progress");

                if (inv->invite_tsx->role == PJSIP_ROLE_UAC)
                    code = 491;
                else
                    code = 500;

                /* Can not receive re-INVITE while another one is pending. */
                status = pjsip_dlg_create_response( inv->dlg, rdata, code,
                                                    &reason, &tdata);
                if (status != PJ_SUCCESS)
                    return;

                if (code == 500) {
                    /* MUST include Retry-After header with random value
                     * between 0-10.
                     */
                    pjsip_retry_after_hdr *ra_hdr;
                    int val = (pj_rand() % 10);

                    ra_hdr = pjsip_retry_after_hdr_create(tdata->pool, val);
                    pjsip_msg_add_hdr(tdata->msg, (pjsip_hdr*)ra_hdr);
                }

                status = pjsip_dlg_send_response( inv->dlg, tsx, tdata);
                

                return;
            }

            /* Check routing URI scheme for secure dialog */
            if (!inv_check_secure_dlg(inv, e))
                return;

            /* Save the invite transaction. */
            inv->invite_tsx = tsx;

            /* Process session timers headers in the re-INVITE */
            status = pjsip_timer_process_req(inv, rdata, &st_code);
            if (status != PJ_SUCCESS) {
                status = pjsip_dlg_create_response(inv->dlg, rdata, st_code,
                                                   NULL, &tdata);
                if (status != PJ_SUCCESS)
                    return;

                pjsip_timer_update_resp(inv, tdata);
                status = pjsip_dlg_send_response(dlg, tsx, tdata);
                return;
            }

            /* Send 491 if we receive re-INVITE while another offer/answer
             * negotiation is in progress
             */
            if (pjmedia_sdp_neg_get_state(inv->neg) !=
                    PJMEDIA_SDP_NEG_STATE_DONE)
            {
                status = pjsip_dlg_create_response(inv->dlg, rdata,
                                                   PJSIP_SC_REQUEST_PENDING,
                                                   NULL, &tdata);
                if (status != PJ_SUCCESS)
                    return;
                pjsip_timer_update_resp(inv, tdata);
                status = pjsip_dlg_send_response(dlg, tsx, tdata);
                return;
            }

            /* Process SDP in incoming message. */
            status = inv_check_sdp_in_incoming_msg(inv, tsx, rdata);

            if (status == PJ_SUCCESS && mod_inv.cb.on_rx_reinvite &&
                inv->notify)
            {
                pj_status_t rc;

                sdp_info = pjsip_rdata_get_sdp_info(rdata);
                rc = (*mod_inv.cb.on_rx_reinvite)(inv, sdp_info->sdp,
                                                  rdata);
                if (rc == PJ_SUCCESS) {
                    /* Application will send its own response.
                     * Our job is done. */
                    PJ_LOG(5,(inv->obj_name, "on_rx_reinvite() returns %d",
                              rc));
                    return;
                }

                /* If application lets us answer the re-INVITE which contains 
                 * SDP, application must set the SDP answer with
                 * #pjsip_inv_set_sdp_answer().
                 */
                if (sdp_info->sdp && 
                    (pjmedia_sdp_neg_get_state(inv->neg) !=
                        PJMEDIA_SDP_NEG_STATE_WAIT_NEGO))
                {
                    status = PJ_EINVALIDOP;
                }
            }

            if (status != PJ_SUCCESS) {
                pj_bool_t reject_message = PJ_TRUE;

                if (status == PJMEDIA_SDP_EINSDP)
                {
                    sdp_info = pjsip_rdata_get_sdp_info(rdata);
                    if (sdp_info->body.ptr == NULL && 
                        PJSIP_INV_ACCEPT_UNKNOWN_BODY) 
                    {
                        /* Message body is not "application/sdp" */
                        reject_message = PJ_FALSE;
                    }               
                }

                if (reject_message) {
                    /* Not Acceptable */
                    const pjsip_hdr *accept;

                    /* The incoming SDP is unacceptable. If the SDP negotiator
                     * state has just been changed, i.e: DONE -> REMOTE_OFFER,
                     * revert it back.
                     */
                    if (pjmedia_sdp_neg_get_state(inv->neg) ==
                        PJMEDIA_SDP_NEG_STATE_REMOTE_OFFER)
                    {
                        pjmedia_sdp_neg_cancel_offer(inv->neg);
                    }

                    status = pjsip_dlg_create_response(inv->dlg, rdata, 
                                         (status == PJMEDIA_SDP_EINSDP)?415:488,
                                          NULL, &tdata);

                    if (status != PJ_SUCCESS)
                        return;


                    accept = pjsip_endpt_get_capability(dlg->endpt, 
                                                        PJSIP_H_ACCEPT,
                                                        NULL);
                    if (accept) {
                        pjsip_msg_add_hdr(tdata->msg, (pjsip_hdr*)
                                          pjsip_hdr_clone(tdata->pool, accept));
                    }

                    status = pjsip_dlg_send_response(dlg, tsx, tdata);

                    return;
                }
            }

            /* Create 2xx ANSWER */
            status = pjsip_dlg_create_response(dlg, rdata, 200, NULL, &tdata);
            if (status != PJ_SUCCESS)
                return;

            /* If the INVITE request has SDP body, send answer.
             * Otherwise generate offer from local active SDP.
             */
            if (!sdp_info)
                sdp_info = pjsip_rdata_get_sdp_info(rdata);
            if (sdp_info->sdp != NULL) {
                status = process_answer(inv, 200, tdata, NULL);
            } else {
                /* INVITE does not have SDP. 
                 * If on_create_offer() callback is implemented, ask app.
                 * to generate an offer, otherwise just send active local
                 * SDP to signal that nothing gets modified.
                 */
                pjmedia_sdp_session *sdp = NULL;

                if (mod_inv.cb.on_create_offer)  {
                    (*mod_inv.cb.on_create_offer)(inv, &sdp);
                    if (sdp) {
                        /* Notify negotiator about the new offer. This will
                         * fix the offer with correct SDP origin.
                         */
                        status = 
                            pjmedia_sdp_neg_modify_local_offer2(
                                inv->pool_prov, inv->neg,
                                inv->sdp_neg_flags, sdp);

                        /* Retrieve the "fixed" offer from negotiator */
                        if (status==PJ_SUCCESS) {
                            const pjmedia_sdp_session *lsdp = NULL;
                            pjmedia_sdp_neg_get_neg_local(inv->neg, &lsdp);
                            sdp = (pjmedia_sdp_session*)lsdp;
                        }
                    }
                } 
                
                if (sdp == NULL) {
                    const pjmedia_sdp_session *active_sdp = NULL;
                    status = pjmedia_sdp_neg_send_local_offer(inv->pool_prov,
                                                              inv->neg, 
                                                              &active_sdp);
                    if (status == PJ_SUCCESS)
                        sdp = (pjmedia_sdp_session*) active_sdp;
                }

                if (sdp) {
                    tdata->msg->body = create_sdp_body(tdata->pool, sdp);
                }
            }

            if (status != PJ_SUCCESS) {
                /*
                 * SDP negotiation has failed.
                 */
                pj_status_t rc;
                pj_str_t reason;

                /* Delete the 2xx answer */
                pjsip_tx_data_dec_ref(tdata);
                
                /* Create 500 response */
                reason = pj_str("SDP negotiation failed");
                rc = pjsip_dlg_create_response(dlg, rdata, 500, &reason, 
                                               &tdata);
                if (rc == PJ_SUCCESS) {
                    pjsip_warning_hdr *w;
                    const pj_str_t *endpt_name;

                    endpt_name = pjsip_endpt_name(dlg->endpt);
                    w = pjsip_warning_hdr_create_from_status(tdata->pool, 
                                                             endpt_name,
                                                             status);
                    if (w)
                        pjsip_msg_add_hdr(tdata->msg, (pjsip_hdr*)w);

                    pjsip_inv_send_msg(inv, tdata);
                }
                return;
            }

            /* Invoke Session Timers */
            pjsip_timer_update_resp(inv, tdata);

            /* Send 2xx regardless of the status of negotiation */
            status = pjsip_inv_send_msg(inv, tdata);

        } else if (tsx->state == PJSIP_TSX_STATE_CONFIRMED) {
            /* This is the case where ACK has the same branch as
             * the INVITE request.
             */
            if (tsx->status_code/100 == 2 &&
                e->body.tsx_state.type == PJSIP_EVENT_RX_MSG) 
            {
                inv_check_sdp_in_incoming_msg(inv, tsx,
                                              e->body.tsx_state.src.rdata);

                /* Check if local offer got no SDP answer */
                if (pjmedia_sdp_neg_get_state(inv->neg)==
                    PJMEDIA_SDP_NEG_STATE_LOCAL_OFFER)
                {
                    pjmedia_sdp_neg_cancel_offer(inv->neg);
                }
            }

        }

    }
    else if (tsx->method.id == PJSIP_INVITE_METHOD &&
             tsx->role == PJSIP_ROLE_UAC)
    {

        /*
         * Handle outgoing re-INVITE
         */
        if (tsx->state == PJSIP_TSX_STATE_CALLING) {

            /* Must not have other pending INVITE transaction */
            pj_assert(inv->invite_tsx==NULL || tsx==inv->invite_tsx);

            /* Save pending invite transaction */
            inv->invite_tsx = tsx;

        } else if (tsx->state == PJSIP_TSX_STATE_PROCEEDING) {
            
            /* CANCEL the re-INVITE if necessary */
            if (inv->pending_cancel) {
                pj_status_t status;
                pjsip_tx_data *cancel;

                inv->pending_cancel = PJ_FALSE;

                status = pjsip_inv_cancel_reinvite(inv, &cancel);
                if (status == PJ_SUCCESS && cancel)
                    status = pjsip_inv_send_msg(inv, cancel);
            }

        } else if (tsx->state == PJSIP_TSX_STATE_TERMINATED &&
                   tsx->status_code/100 == 2) 
        {
            pj_status_t status;

            /* Re-INVITE was accepted. */

            /* Check routing URI scheme for secure dialog */
            if (!inv_check_secure_dlg(inv, e))
                return;

            /* Process session timer response. */
            status = handle_timer_response(inv, 
                                           e->body.tsx_state.src.rdata,
                                           PJ_TRUE);
            if (status != PJ_SUCCESS)
                return;

            /* Process SDP */
            inv_check_sdp_in_incoming_msg(inv, tsx, 
                                          e->body.tsx_state.src.rdata);

            /* Check if local offer got no SDP answer */
            if (pjmedia_sdp_neg_get_state(inv->neg)==
                PJMEDIA_SDP_NEG_STATE_LOCAL_OFFER)
            {
                pjmedia_sdp_neg_cancel_offer(inv->neg);
            }

            /* Send ACK */
            inv_send_ack(inv, e);

        } else if (handle_uac_tsx_response(inv, e)) {

            /* Handle response that terminates dialog */
            /* Nothing to do (already handled) */

        } else if (tsx->status_code >= 300 && tsx->status_code < 700 &&
                   e->body.tsx_state.prev_state != PJSIP_TSX_STATE_COMPLETED)
        {
            /* Ticket #1654: do not cancel SDP offer when tsx state changing
             * from 'completed' to 'terminated', as it should have already
             * been cancelled when tsx state is 'completed'.
             */

            pjmedia_sdp_neg_state neg_state;
            struct tsx_inv_data *tsx_inv_data;

            tsx_inv_data = (struct tsx_inv_data*)tsx->mod_data[mod_inv.mod.id];

            /* Outgoing INVITE transaction has failed, cancel SDP nego */
            neg_state = pjmedia_sdp_neg_get_state(inv->neg);
            if (neg_state == PJMEDIA_SDP_NEG_STATE_LOCAL_OFFER &&
                tsx_inv_data->retrying == PJ_FALSE)
            {
                pjmedia_sdp_neg_cancel_offer(inv->neg);
            }

            if (tsx == inv->invite_tsx)
                inv->invite_tsx = NULL;

            /* Process 502/503 error for session timer refresh */
            if (tsx->status_code == 503 || tsx->status_code == 502) {
                pjsip_timer_handle_refresh_error(inv, e);
            }
        }

    } else if (tsx->role == PJSIP_ROLE_UAS &&
               tsx->state == PJSIP_TSX_STATE_TRYING &&
               pjsip_method_cmp(&tsx->method, &pjsip_update_method)==0)
    {
        /*
         * Handle incoming UPDATE
         */
        inv_respond_incoming_update(inv, e);

    } else if (tsx->role == PJSIP_ROLE_UAC &&
               (tsx->state == PJSIP_TSX_STATE_COMPLETED ||
                tsx->state == PJSIP_TSX_STATE_TERMINATED) &&
               pjsip_method_cmp(&tsx->method, &pjsip_update_method)==0)
    {
        /*
         * Handle response to outgoing UPDATE request.
         */
        if (inv_handle_update_response(inv, e) == PJ_FALSE)
            handle_uac_tsx_response(inv, e);

    } else if (tsx->role == PJSIP_ROLE_UAS &&
               tsx->state == PJSIP_TSX_STATE_TRYING &&
               pjsip_method_cmp(&tsx->method, &pjsip_prack_method)==0)
    {
        /*
         * Handle strandled incoming PRACK
         */
        inv_respond_incoming_prack(inv, e->body.tsx_state.src.rdata);

    } else if (tsx->role == PJSIP_ROLE_UAC) {
        /*
         * Handle 401/407/408/481/422 response
         */
        handle_uac_tsx_response(inv, e);
    }

}

/*
 * After session has been terminated, but before dialog is destroyed
 * (because dialog has other usages, or because dialog is waiting for
 * the last transaction to terminate).
 */
static void inv_on_state_disconnected( pjsip_inv_session *inv, pjsip_event *e)
{
    pjsip_transaction *tsx = e->body.tsx_state.tsx;
    pjsip_dialog *dlg = pjsip_tsx_get_dlg(tsx);

    PJ_ASSERT_ON_FAIL(tsx && dlg, return);

    if (tsx->role == PJSIP_ROLE_UAS &&
        tsx->status_code < 200 &&
        e->body.tsx_state.type == PJSIP_EVENT_RX_MSG) 
    {
        pjsip_rx_data *rdata = e->body.tsx_state.src.rdata;

        /*
         * Respond BYE with 200/OK
         */
        if (tsx->method.id == PJSIP_BYE_METHOD) {
            inv_respond_incoming_bye( inv, tsx, rdata, e );
        } else if (tsx->method.id == PJSIP_CANCEL_METHOD) {
            /*
             * Respond CANCEL with 200/OK too.
             */
            pjsip_tx_data *tdata;
            pj_status_t status;

            status = pjsip_dlg_create_response(dlg, rdata, 200, NULL, &tdata);
            if (status != PJ_SUCCESS) return;

            status = pjsip_dlg_send_response(dlg, tsx, tdata);
            if (status != PJ_SUCCESS) return;

        }

    } else if (tsx->role == PJSIP_ROLE_UAC) {
        /*
         * Handle 401/407/408/481/422 response
         */
        handle_uac_tsx_response(inv, e);
    }
}

