隨手記錄一下exosip的學習過程：

1. nist 非invite事件服務端處理，nict 非invite事件客戶端處理，ist invite服務器處理，ict invite客戶端處理。

2. 狀態(tài)機處理鏈表，每種狀態(tài)對應一個處理方式，當事務處于trying狀態(tài)時，根據(jù)超時類型處理，這種鏈表結構實屬優(yōu)秀了。。

typedef struct _transition_t transition_t;

struct _transition_t {
  state_t state;
  type_t type;
  void (*method) (void *, void *);
  struct _transition_t *next;
  struct _transition_t *parent;
};

transition_t nict_transition[12] = {
  {
   NICT_PRE_TRYING,
   SND_REQUEST,
   (void (*)(void *, void *)) &nict_snd_request,
   &nict_transition[1], NULL}
  ,
  {
   NICT_TRYING,
   TIMEOUT_F,
   (void (*)(void *, void *)) &osip_nict_timeout_f_event,
   &nict_transition[2], NULL}
  ,
  {
   NICT_TRYING,
   TIMEOUT_E,
   (void (*)(void *, void *)) &osip_nict_timeout_e_event,
   &nict_transition[3], NULL}
  ,
  {
   NICT_TRYING,
   RCV_STATUS_1XX,
   (void (*)(void *, void *)) &nict_rcv_1xx,
   &nict_transition[4], NULL}
  ,
  {
   NICT_TRYING,
   RCV_STATUS_2XX,
   (void (*)(void *, void *)) &nict_rcv_23456xx,
   &nict_transition[5], NULL}
  ,
  {
   NICT_TRYING,
   RCV_STATUS_3456XX,
   (void (*)(void *, void *)) &nict_rcv_23456xx,
   &nict_transition[6], NULL}
  ,
  {
   NICT_PROCEEDING,
   TIMEOUT_F,
   (void (*)(void *, void *)) &osip_nict_timeout_f_event,
   &nict_transition[7], NULL}
  ,
  {
   NICT_PROCEEDING,
   TIMEOUT_E,
   (void (*)(void *, void *)) &osip_nict_timeout_e_event,
   &nict_transition[8], NULL}
  ,
  {
   NICT_PROCEEDING,
   RCV_STATUS_1XX,
   (void (*)(void *, void *)) &nict_rcv_1xx,
   &nict_transition[9], NULL}
  ,
  {
   NICT_PROCEEDING,<