lwip源码解析之TCP协议定时器tcp_slowtmr()；和tcp_fasttmr()；

TCP协议中许多地方是需要使用到定时功能的，如定时重传功能，保活keepalive功能，坚持定时器功能，这些定时功能会在lwip中的两个定时器函数中实现。

一，定时器时钟

二，快速定时任务

void tcp_fasttmr(void)比较简单，它的功能主要是每250ms处理延时发送的ack报文和fin报文，同时通知上层应用处理数据。

void
tcp_fasttmr(void)
{struct tcp_pcb *pcb;++tcp_timer_ctr;tcp_fasttmr_start:pcb = tcp_active_pcbs; //在active中遍历while (pcb != NULL) {if (pcb->last_timer != tcp_timer_ctr) {struct tcp_pcb *next;pcb->last_timer = tcp_timer_ctr;//发送延时的ackif (pcb->flags & TF_ACK_DELAY) {LWIP_DEBUGF(TCP_DEBUG, ("tcp_fasttmr: delayed ACK\n"));tcp_ack_now(pcb);tcp_output(pcb);pcb->flags &= ~(TF_ACK_DELAY | TF_ACK_NOW);}//发送延时的finif (pcb->flags & TF_CLOSEPEND) {LWIP_DEBUGF(TCP_DEBUG, ("tcp_fasttmr: pending FIN\n"));pcb->flags &= ~(TF_CLOSEPEND);tcp_close_shutdown_fin(pcb);}next = pcb->next;//若当前tcp有未被上层应用接收的数据if (pcb->refused_data != NULL) {tcp_active_pcbs_changed = 0;tcp_process_refused_data(pcb); //通过回调函数使上层处理数据if (tcp_active_pcbs_changed) {goto tcp_fasttmr_start;}}pcb = next; //下一个} else {pcb = pcb->next;}}
}


三，低速定时任务

void tcp_slowtmr(void)每500ms调用，该函数完成了超时重传，tcp保活功能，并会遍历active和timewait链表的PCB，删除那些超时或者出错的PCB，同时将PCB中unsent队列中的数据发送出去。一般使用tcp_write();写入数据后，数据不会马上发送，而是在定时任务中发送。

1，超时重传

重点的代码注释如下：

//请求连接次数超出限制if (pcb->state == SYN_SENT && pcb->nrtx >= TCP_SYNMAXRTX) {++pcb_remove; //移除增加LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: max SYN retries reached\n"));}//数据重发次数超出限制else if (pcb->nrtx >= TCP_MAXRTX) {++pcb_remove;LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: max DATA retries reached\n"));} else {//如果坚持定时器已经开启if (pcb->persist_backoff > 0) {//获取坚持定时器触发值u8_t backoff_cnt = tcp_persist_backoff[pcb->persist_backoff-1];//坚持定时器不超过触发值，则加1if (pcb->persist_cnt < backoff_cnt) {pcb->persist_cnt++;}//坚持定时器触发，发送窗口探查if (pcb->persist_cnt >= backoff_cnt) {if (tcp_zero_window_probe(pcb) == ERR_OK) {pcb->persist_cnt = 0; //发送成功，清除计数值if (pcb->persist_backoff < sizeof(tcp_persist_backoff)) {pcb->persist_backoff++; //发送探查次数+1}}}} else { //无开启坚持定时器//如果开启了超时重传定时器，则加1if (pcb->rtime >= 0) {++pcb->rtime;}//有未确认报文且重传时间到，要重传了if (pcb->unacked != NULL && pcb->rtime >= pcb->rto) {LWIP_DEBUGF(TCP_RTO_DEBUG, ("tcp_slowtmr: rtime %"S16_F" pcb->rto %"S16_F"\n",pcb->rtime, pcb->rto));ESP_STATS_TCP_PCB(pcb);if (pcb->state != SYN_SENT) {u8_t backoff_idx = LWIP_MIN(pcb->nrtx, sizeof(tcp_backoff)-1); //获得重传次数，但重传次数不会超过7//动态设置rto，每次超时后，rto时间会增加pcb->rto = ((pcb->sa >> 3) + pcb->sv) << tcp_backoff[backoff_idx]; }pcb->rtime = 0; //重置超时重传定时器//TODO 出现重传，说明报文丢失了，可能是网络出现阻塞，减小拥塞窗口eff_wnd = LWIP_MIN(pcb->cwnd, pcb->snd_wnd);pcb->ssthresh = eff_wnd >> 1; //ssthresh减少到拥塞窗口的一半//若ssthresh比最大报文长度的两倍还小，后者的数值（限制了ssthresh的最小值）if (pcb->ssthresh < (tcpwnd_size_t)(pcb->mss << 1)) {pcb->ssthresh = (pcb->mss << 1);}pcb->cwnd = pcb->mss; //拥塞窗口设置成最大报文长度，一个报文长度LWIP_DEBUGF(TCP_CWND_DEBUG, ("tcp_slowtmr: cwnd %"TCPWNDSIZE_F" ssthresh %"TCPWNDSIZE_F"\n",pcb->cwnd, pcb->ssthresh));//重传报文tcp_rexmit_rto(pcb);}}}if (pcb->state == FIN_WAIT_2) {//TODO 处于FIN_WAIT_2的时间太长（由于对方长时间无反应）则删除if (pcb->flags & TF_RXCLOSED) {/* PCB was fully closed (either through close() or SHUT_RDWR):normal FIN-WAIT timeout handling. */if ((u32_t)(tcp_ticks - pcb->tmr) >TCP_FIN_WAIT_TIMEOUT / TCP_SLOW_INTERVAL) {++pcb_remove;LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: removing pcb stuck in FIN-WAIT-2\n"));}}}


2，保活keepalive

服务端需要检查客户端是否还能通信，若两小时内无通信，客户端发送探查报文，若客户端ack，则更新保活计时器，否则，每隔75s发送一个探查报文，若发送超过9个报文，则认为客户端已挂掉

/* Check if KEEPALIVE should be sent *///服务端需要检查客户端是否还能通信，若两小时内无通信，客户端发送探查报文，若客户端ack，则更新保活计时器//否则，每隔75s发送一个探查报文，若发送超过9个报文，则认为客户端已挂掉if (ip_get_option(pcb, SOF_KEEPALIVE) &&((pcb->state == ESTABLISHED) ||(pcb->state == CLOSE_WAIT))) {//发送9个以上探查报文，对方仍无反应，应该关闭tcpif ((u32_t)(tcp_ticks - pcb->tmr) >(pcb->keep_idle + TCP_KEEP_DUR(pcb)) / TCP_SLOW_INTERVAL){LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: KEEPALIVE timeout. Aborting connection to "));ip_addr_debug_print(TCP_DEBUG, &pcb->remote_ip);LWIP_DEBUGF(TCP_DEBUG, ("\n"));++pcb_remove;++pcb_reset; //复位对方} else if ((u32_t)(tcp_ticks - pcb->tmr) >(pcb->keep_idle + pcb->keep_cnt_sent * TCP_KEEP_INTVL(pcb))/ TCP_SLOW_INTERVAL) //发送探查报文{err = tcp_keepalive(pcb); if (err == ERR_OK) {pcb->keep_cnt_sent++; //探查次数加一}}}


3，删除超时PCB

//若pcb的osseq队列中无序的数据超过一定时长会被丢弃if (pcb->ooseq != NULL &&(u32_t)tcp_ticks - pcb->tmr >= pcb->rto * TCP_OOSEQ_TIMEOUT) {tcp_segs_free(pcb->ooseq);pcb->ooseq = NULL;LWIP_DEBUGF(TCP_CWND_DEBUG, ("tcp_slowtmr: dropping OOSEQ queued data\n"));}//一直等不到服务端回答的tcp要被移除if (pcb->state == SYN_RCVD) {if ((u32_t)(tcp_ticks - pcb->tmr) >TCP_SYN_RCVD_TIMEOUT / TCP_SLOW_INTERVAL) {++pcb_remove;LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: removing pcb stuck in SYN-RCVD\n"));}}//在last_ack超过一定时间也被删除if (pcb->state == LAST_ACK) {if ((u32_t)(tcp_ticks - pcb->tmr) > 2 * TCP_MSL / TCP_SLOW_INTERVAL) {++pcb_remove;LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: removing pcb stuck in LAST-ACK\n"));}}


以上的代码中若当前PCB需要被删除，则pcb_remove不为0，具体的删除代码如下：

if (pcb_remove) { //不为0则需要删除pcbstruct tcp_pcb *pcb2;tcp_err_fn err_fn = pcb->errf; //错误回调函数void *err_arg;enum tcp_state last_state;tcp_pcb_purge(pcb); //释放pcb部分成员，pcb结构体不会被释放if (prev != NULL) {LWIP_ASSERT("tcp_slowtmr: middle tcp != tcp_active_pcbs", pcb != tcp_active_pcbs);prev->next = pcb->next;} else {/* This PCB was the first. */LWIP_ASSERT("tcp_slowtmr: first pcb == tcp_active_pcbs", tcp_active_pcbs == pcb);tcp_active_pcbs = pcb->next;}//需要发送重置报文if (pcb_reset) {tcp_rst(pcb->snd_nxt, pcb->rcv_nxt, &pcb->local_ip, &pcb->remote_ip,pcb->local_port, pcb->remote_port);}err_arg = pcb->callback_arg;last_state = pcb->state;pcb2 = pcb;pcb = pcb->next; //获取下一个pcbmemp_free(MEMP_TCP_PCB, pcb2); //释放pcb结构体tcp_active_pcbs_changed = 0;TCP_EVENT_ERR(last_state, err_fn, err_arg, ERR_ABRT); //调用错误回调函数if (tcp_active_pcbs_changed) {goto tcp_slowtmr_start;}} else {//不需要删除pcbprev = pcb;pcb = pcb->next; //获取下一个pcb//TODO 定时周期到，调用回调函数++prev->polltmr;if (prev->polltmr >= prev->pollinterval) {prev->polltmr = 0;LWIP_DEBUGF(TCP_DEBUG, ("tcp_slowtmr: polling application\n"));tcp_active_pcbs_changed = 0;TCP_EVENT_POLL(prev, err); //!回调周期性函数if (tcp_active_pcbs_changed) {goto tcp_slowtmr_start;}if (err == ERR_OK) {tcp_output(prev); //输出unsent报文}}}}


以上都是处理active链表的pcb，接下来处理timewait的pcb，苍天饶过谁？

/*--------------------------------遍历所有timewait的pcb-----------------------*//* Steps through all of the TIME-WAIT PCBs. */prev = NULL;pcb = tcp_tw_pcbs;while (pcb != NULL) {LWIP_ASSERT("tcp_slowtmr: TIME-WAIT pcb->state == TIME-WAIT", pcb->state == TIME_WAIT);pcb_remove = 0;//是否达到2MSL，是则删除if ((u32_t)(tcp_ticks - pcb->tmr) > 2 * TCP_MSL / TCP_SLOW_INTERVAL) {++pcb_remove;}if (pcb_remove) {struct tcp_pcb *pcb2;tcp_pcb_purge(pcb);/* Remove PCB from tcp_tw_pcbs list. */if (prev != NULL) {LWIP_ASSERT("tcp_slowtmr: middle tcp != tcp_tw_pcbs", pcb != tcp_tw_pcbs);prev->next = pcb->next;} else {/* This PCB was the first. */LWIP_ASSERT("tcp_slowtmr: first pcb == tcp_tw_pcbs", tcp_tw_pcbs == pcb);tcp_tw_pcbs = pcb->next;}pcb2 = pcb;pcb = pcb->next;memp_free(MEMP_TCP_PCB, pcb2);} else {prev = pcb;pcb = pcb->next;}}


四，小结

tcp定时任务是tcp接收发送的动力来源，需要关注。