PPP程序基本流程.doc

PPPD主函数main.cPPP程序基本流程图PPPD程序用到的几个重要的结构体：typedef struct fsm int unit;/* Interface unit number */ int protocol;/* Data Link Layer Protocol field value */ int state;/* State */ int flags;/* Contains option bits */ u_char id;/* Current id */ u_char reqid;/* Current request id */ u_char seen_ack;/* Have received valid Ack/Nak/Rej to Req */ int timeouttime;/* Timeout time in milliseconds */ int maxconfreqtransmits;/* Maximum Configure-Request transmissions */ int retransmits;/* Number of retransmissions left */ int maxtermtransmits;/* Maximum Terminate-Request transmissions */ int nakloops;/* Number of nak loops since last ack */ int rnakloops;/* Number of naks received */ int maxnakloops;/* Maximum number of nak loops tolerated */ struct fsm_callbacks *callbacks;/* Callback routines */ char *term_reason;/* Reason for closing protocol */ int term_reason_len;/* Length of term_reason */ fsm;typedef struct fsm_callbacks void (*resetci)/* Reset our Configuration Information */_P(fsm *); int (*cilen)/* Length of our Configuration Information */_P(fsm *); void (*addci) /* Add our Configuration Information */_P(fsm *, u_char *, int *); int (*ackci)/* ACK our Configuration Information */_P(fsm *, u_char *, int); int (*nakci)/* NAK our Configuration Information */_P(fsm *, u_char *, int, int); int (*rejci)/* Reject our Configuration Information */_P(fsm *, u_char *, int); int (*reqci)/* Request peers Configuration Information */_P(fsm *, u_char *, int *, int); void (*up)/* Called when fsm reaches OPENED state */_P(fsm *); void (*down)/* Called when fsm leaves OPENED state */_P(fsm *); void (*starting)/* Called when we want the lower layer */_P(fsm *); void (*finished)/* Called when we dont want the lower layer */_P(fsm *); void (*protreject)/* Called when Protocol-Reject received */_P(int); void (*retransmit)/* Retransmission is necessary */_P(fsm *); int (*extcode)/* Called when unknown code received */_P(fsm *, int, int, u_char *, int); char *proto_name;/* String name for protocol (for messages) */ fsm_callbacks;typedef struct char*name;/* name of the option */enum opt_type type;void*addr;char*description;unsigned int flags;void*addr2;intupper_limit;intlower_limit;const char *source;short int priority;short int winner; option_t;struct protent u_short protocol;/* PPP protocol number */ /* Initialization procedure */ void (*init) _P(int unit); /* Process a received packet */ void (*input) _P(int unit, u_char *pkt, int len); /* Process a received protocol-reject */ void (*protrej) _P(int unit); /* Lower layer has come up */ void (*lowerup) _P(int unit); /* Lower layer has gone down */ void (*lowerdown) _P(int unit); /* Open the protocol */ void (*open) _P(int unit); /* Close the protocol */ void (*close) _P(int unit, char *reason); /* Print a packet in readable form */ int (*printpkt) _P(u_char *pkt, int len, void (*printer) _P(void *, char *, .), void *arg); /* Process a received data packet */ void (*datainput) _P(int unit, u_char *pkt, int len); bool enabled_flag;/* 0 iff protocol is disabled */ char *name;/* Text name of protocol */ char *data_name;/* Text name of corresponding data protocol */ option_t *options;/* List of command-line options */ /* Check requested options, assign defaults */ void (*check_options) _P(void); /* Configure interface for demand-dial */ int (*demand_conf) _P(int unit); /* Say whether to bring up link for this pkt */ int (*active_pkt) _P(u_char *pkt, int len);struct channel /* set of options for this channel */option_t *options;/* find and process a per-channel options file */void (*process_extra_options) _P(void);/* check all the options that have been given */void (*check_options) _P(void);/* get the channel ready to do PPP, return a file descriptor */int (*connect) _P(void);/* were finished with the channel */void (*disconnect) _P(void);/* put the channel into PPP mode */int (*establish_ppp) _P(int);/* take the channel out of PPP mode, restore loopback if demand */void (*disestablish_ppp) _P(int);/* set the transmit-side PPP parameters of the channel */void (*send_config) _P(int, u_int32_t, int, int);/* set the receive-side PPP parameters of the channel */void (*recv_config) _P(int, u_int32_t, int, int);/* cleanup on error or normal exit */void (*cleanup) _P(void);/* close the device, called in children after fork */void (*close) _P(void);PPP的状态转换图：对程序流程的基本框架的说明因为程序是利用protent结构指针指向当前所在协议层的方法来实现的因此，每一层都要经历几个阶段，直到本层达到OPENED状态时才可进入下一阶段来实现下一阶段的协议。所以对每一层的协议都有相同的函数指针，只是函数指针指向的协议不同而已。整个程序的主体实现是从主函数的LCP_OPEN()开始的，在这个函数里，调用了有限状态机FSM_OPEN(),而在FSM_OPEN()中，callback指针指向了starting，于是就到了LCP_STARTING()函数来实现一个OPEN事件从而使得PPP状态准备从DEAD到ESTABLISHED的转变。接下来，回到主函数，下面一步是调用START_LINK()，在此函数中会把一个串口设备作为PPP的接口，并把状态转变为ESTABLISHED，然后调用lcp_lowerup()来告诉上层底层已经UP，lcp_lowerup()中调用FSM_LOWERUP()来发送一个configure-request请求，再把当前状态设置为REQSENT状态，至此，第一个LCP协商的报文已经发送出去。while (phase != PHASE_DEAD) handle_events(); get_input(); if (kill_link)lcp_close(0, User request); if (asked_to_quit) bundle_terminating = 1;if (phase = PHASE_MASTER) mp_bundle_terminated(); if (open_ccp_flag) if (phase = PHASE_NETWORK | phase = PHASE_RUNNING) ccp_fsm0.flags = OPT_RESTART; /* clears OPT_SILENT */ (*ccp_protent.open)(0); 接下来的流程实现主要就是在这个while循环中实现了。之前说过了我们已经发送了第一个配置协商报文，所以handle_events()主要就是做等待接收数据包的时间处理了，在handle_events（）里主要调用了两个函数一个是wait_input()，他的任务是等待并判断是否超时。还有一个是calltimeout()他主要是做超时的处理。当等待并未超时而且有数据包过来，则调用整个PPPD中最重要的函数get_input()函数。他主要接收过来的数据包并做出相应的动作。接下来就get_input()函数进行详细的说明。 首先对包进行判断，丢弃所有不在LCP阶段和没有OPENED状态的包，然后protop指针指向当前协议的input函数。于是就进入了LCP_INPUT()，同理LCP_INPUT()调用了FSM_INPUT()对收到的包进行代码域的判断，判断收到的是什么包。假设比较顺利，我们收到的是CONFACK的包，于是调用fsm_rconack()函数，在此函数中根据当前自身的状态来决定下一步的状态如何改变，这里我们假设也很顺利，已经发送完了configure-ack,因此我们把FSM当前状态变成了OPENED状态，并把callback指针指向UP.所以我们马上就调用LCP_UP（）在那里我们又调用了LINK_ESTABLISHED()函数来进入认证的协商，或者如果没有认证则直接进入网络层协议。当然这里我们还是要认证的所有在LINK_ESTABLISHED()里我们选择是利用何种认证方式是PAP,还是EAP，还是CHAP.假设我们这里采用CHAP而且是选择CHAP WITH PEER，意思是等待对端先发送CHALLENGE挑战报文。于是我们又调用了chap_with_peer()函数，并等待接收挑战报文。于是从新又来到handle_events()等待接收。再利用get_input()来接收包，在get_input()里这次调用chap_input()，再调用FSM_INPUT()，在那里我们再对包的代码域进行判断，这次判断出是CHAP_CHALLENGE包，则我们要调用chap_respons（）函数来回应对端，继续等待对方的报文，再次利用CHAP_INPUT()，FSM_INPUT()来判断，如果是SUCCESS，则调用handle_status（），在这个函数里调用success_chap_with_peer函数（），从而进入网络层阶段，调用network_phase()函数。网络层的互动是从st