# Harp 拥塞控制

在input函数中

```c
//1:收到了能让 snd_una 移动的 ACK，或者其他包中的head->una 能让snd_una 移动
if (_itimediff(this->snd_una, prev_una) > 0) { 
	//2：cwnd 无论怎么调整，也不能超过远端窗口rmt_wnd
	if (this->cwnd < this->rmt_wnd) {
		fr_uint32_t mss = this->mss;
		if (this->cwnd < this->ssthresh) {
			/*
             * a:慢启动阶段：cwnd 小于 ssthresh,  this->cwnd 自增1 指数增长
             */
			this->cwnd++;
			this->incr += mss;
		}else {
			/*
             * b:拥塞避免阶段：this->cwnd 自增零点几
             */
			// this->cwnd = cwnd + 1/cwnd + 1/16
			if (this->incr < mss) {
                this->incr = mss;
            }
			this->incr += (mss * mss) / this->incr + (mss / 16);
			if ((this->cwnd + 1) * mss <= this->incr) {
				//当 incr 累计增加的值超过一个 mss 时， cwnd 增加 1
			#if 1
				this->cwnd = (this->incr + mss - 1) / ((mss > 0)? mss : 1);
			#else
				this->cwnd++;
			#endif
			}
		}
		if (this->cwnd > this->rmt_wnd) {
			// 保持 cwnd 不超过 rmt_wnd
			this->cwnd = this->rmt_wnd;
			this->incr = this->rmt_wnd * mss;
		}
	}
}
```

flush函数

```c
	resent = (this->fastresend > 0)? (fr_uint32_t)this->fastresend : 0xffffffff;
	if (change) {
		/*
         * 快速重传,会直接进入b阶段
         */
		//  inflight=发送窗口中报文的数量， 这个值小于等于 cwnd 把 inflight设置为inflight的一半
		// 但 this->cwnd 置为比 this->ssthresh 稍高的值就进入快速恢复阶段，
		//  cwnd 会以与拥塞避免相同的方式,线性增长
		fr_uint32_t inflight = this->snd_nxt - this->snd_una;
		this->ssthresh = inflight / 2;
		if (this->ssthresh < FR_HARP_THRESH_MIN)
			this->ssthresh = FR_HARP_THRESH_MIN;
		this->cwnd = this->ssthresh + resent;
		this->incr = this->cwnd * this->mss; //incr 为 cwnd 的 mss 倍
		this->cwd_status = FR_HARP_STATUS_CONGESTION_FAST_RECOVERY;
	}

	if (lost) {
		/*
        * 超时重传,进入a阶段
        */
		// 修改慢启动阈值this->ssthresh 为窗口的一半 然后 this->cwnd =1 进入慢启动
		this->ssthresh = this->cwd_min / 2;
		if (this->ssthresh < FR_HARP_THRESH_MIN)
			this->ssthresh = FR_HARP_THRESH_MIN;
		this->cwnd = 1;   // cwnd 置为 1, 进入慢启动
		this->incr = this->mss;
		this->cwd_status = FR_HARP_STATUS_SLOW_START_E;
	}

	if (this->cwnd < 1) {
		this->cwnd = 1;
		this->incr = this->mss;
	}
```

策略为：只要发送丢包就进入慢启动阶段-》大于阈值时候-》进入拥塞避免阶段

只要发生快速重传-》就进入拥塞避免阶段，这个时候，更贴切的说法是快速恢复阶段