tcp拥塞算法分析二（bic）

本文分析linux-4.14.69代码的bic拥塞算法

首先回顾下基础的慢启动和拥塞避免函数，慢启动阶段（tcp_slow_start）更新窗口的速度是加acked，acked就是这个ack包对应确认的包个数；拥塞避免阶段(tcp_cong_avoid_ai)更新窗口的速度是加acked/w, w一般情况下就是当前的窗口大小．（在bic中，相当与一个权重，bic会根据当前窗口到最大窗口的差距动态设置这个w．）

u32 tcp_slow_start(struct tcp_sock *tp, u32 acked)
{
	u32 cwnd = min(tp->snd_cwnd + acked, tp->snd_ssthresh);

	acked -= cwnd - tp->snd_cwnd;
	tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp);

	return acked;
}
EXPORT_SYMBOL_GPL(tcp_slow_start);

void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w, u32 acked)
{
	/* If credits accumulated at a higher w, apply them gently now. */
	if (tp->snd_cwnd_cnt >= w) {
		tp->snd_cwnd_cnt = 0;
		tp->snd_cwnd++;
	}

	tp->snd_cwnd_cnt += acked;
	if (tp->snd_cwnd_cnt >= w) {
		u32 delta = tp->snd_cwnd_cnt / w;

		tp->snd_cwnd_cnt -= delta * w;
		tp->snd_cwnd += delta;
	}
	tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_cwnd_clamp);
}
EXPORT_SYMBOL_GPL(tcp_cong_avoid_ai);

再看看bic的拥塞避免函数．

static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bictcp *ca = inet_csk_ca(sk);

	if (!tcp_is_cwnd_limited(sk))
		return;

	if (tcp_in_slow_start(tp))
		tcp_slow_start(tp, acked);
	else {
		bictcp_update(ca, tp->snd_cwnd);
		tcp_cong_avoid_ai(tp, ca->cnt, 1);
	}
}

慢启动就是调用默认的慢启动函数，　拥塞避免阶段则多了一个更新窗口的处理，并将tcp_cong_avoid_ai的第三个参数acked设置固定值１．更新函数就是在更新ca->cnt, 从而控制了拥塞避免阶段增长速度。其中涉及到很多变量，下面一一列出

bictcp结构体
tcp_jiffies32
low_window
BICTCP_B
static int max_increment
smooth_part

/* BIC TCP Parameters */
struct bictcp {
	u32	cnt;		/* increase cwnd by 1 after ACKs */
	u32	last_max_cwnd;	/* last maximum snd_cwnd */
	u32	last_cwnd;	/* the last snd_cwnd */
	u32	last_time;	/* time when updated last_cwnd */
	u32	epoch_start;	/* beginning of an epoch */
#define ACK_RATIO_SHIFT	4
	u32	delayed_ack;	/* estimate the ratio of Packets/ACKs << 4 */
};

/* TCP uses 32bit jiffies to save some space.
 * Note that this is different from tcp_time_stamp, which
 * historically has been the same until linux-4.13.
 */
#define tcp_jiffies32 ((u32)jiffies)static int low_window = 14;

#define BICTCP_B		4	 /*
					  * In binary search,
					  * go to point (max+min)/N
					  */
static int max_increment = 16; /* Limit on increment allowed during binary search */
static int smooth_part = 20; /* log(B/(B*Smin))/log(B/(B-1))+B, # of RTT from Wmax-B to Wmax */

再来看bictcp_update的流程．

１．判断是否要更新（如果窗口没变，而且与上次更新的时间小于HZ/32,即31.25ms就不用更新，直接跳出）.

	if (ca->last_cwnd == cwnd &&
	    (s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32)
		return;

２．更新last_cwnd和last_time，（如果是初次，epoch_start为０，需要设置下epoch_start为当前时间tcp_jiffies32），如果当前发送窗口小于low_window，ca->cnt设置为当前发送窗口，即普通拥塞避免模式．

    ca->last_cwnd = cwnd;
	ca->last_time = tcp_jiffies32;

	if (ca->epoch_start == 0) /* record the beginning of an epoch */
		ca->epoch_start = tcp_jiffies32;

３．二分查找.
3.1 cwnd < last_max_cwnd
3.1.1当max_cwnd - cwnd大于BICTCP_B*max_increment, cnt =cwnd/max_increment。离最大窗口很远的时候，快速增长，一个rtt增max_increment个窗口，经过BICTCP_B个rtt达到最大窗口
3.1.2当max_cwnd - cwnd小于BICTCP_B, cnt = cwnd *smooth_part / BICTCP_B，一个rtt增(BICTCP_B/smooth_part )个窗口. 接近最大窗口时候非常缓慢的增长，大约经过smooth_part个rtt才增大到最大窗口。
3.1.3当max_cwnd - cwnd在[BICTCP_B, BICTCP_B*max_increment], cnt = cwnd / (max_cwnd - cwnd)，一个rtt增max_cwnd-cwnd个窗口，也就是一个rtt达到最大窗口．
3.2 cwnd >= last_max_cwnd
3.2.1当cwnd - max_cwnd小于BICTCP_B, 超过的不多，cnt = cwnd *smooth_part / BICTCP_B，缓慢长，同3.1.2,一个rtt增(BICTCP_B/smooth_part )个窗口
3.2.2当cwnd - max_cwnd在[BICTCP_B, max_increment*(BICTCP_B-1)], cnt = (cwnd * (BICTCP_B-1)) / (cwnd-last_max_cwnd); 一个rtt增大(cwnd-last_max_cwnd)/(BICTCP_B-1)个窗口．速度会越来越快，因为cwnd不断增大．根据if条件范围可以算出1个rtt增加的窗口范围在[BICTCP_B/(BICTCP_B-1), max_increment]
3.2.3当cwnd - max_cwnd大于max_increment，cnt = cwnd / max_increment; ,同3.1.1,一个rtt增max_increment个窗口

/* binary increase */
	if (cwnd < ca->last_max_cwnd) {
		__u32	dist = (ca->last_max_cwnd - cwnd)
			/ BICTCP_B;

		if (dist > max_increment)
			/* linear increase */
			ca->cnt = cwnd / max_increment;
		else if (dist <= 1U)
			/* binary search increase */
			ca->cnt = (cwnd * smooth_part) / BICTCP_B;
		else
			/* binary search increase */
			ca->cnt = cwnd / dist;
	} else {
		/* slow start AMD linear increase */
		if (cwnd < ca->last_max_cwnd + BICTCP_B)
			/* slow start */
			ca->cnt = (cwnd * smooth_part) / BICTCP_B;
		else if (cwnd < ca->last_max_cwnd + max_increment*(BICTCP_B-1))
			/* slow start */
			ca->cnt = (cwnd * (BICTCP_B-1))
				/ (cwnd - ca->last_max_cwnd);
		else
			/* linear increase */
			ca->cnt = cwnd / max_increment;
	}

4. 当last_max_cwnd == 0。即在慢启动开始或出现拥塞的时候，控制cnt不超过20.

	/* if in slow start or link utilization is very low */
	if (ca->last_max_cwnd == 0) {
		if (ca->cnt > 20) /* increase cwnd 5% per RTT */
			ca->cnt = 20;
	}

5.对延迟确认的处理。延迟确认的时候，一个ack不止是确认一个报文，作者的意思是，根据延迟ack的比例1<<ACK_RATIO_SHIFT/delayed_ack，增大窗口的时候需要扩大delayed_ack/2^ACK_RATIO_SHIFT倍。默认延迟比例为50%（ACK_RATIO_SHIFT为常数4, delayed_ack默认值为2<<ACK_RATIO_SHIFT）

	ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
	if (ca->cnt == 0)			/* cannot be zero */
		ca->cnt = 1;

ACK_RATIO_SHIFT为常数4, delayed_ack的设置涉及到bictcp_reset函数和bictcp_acked函数.

bictcp_reset对bictcp结构体做初始化，可以看到初始化的时候delayed_ack设置为32，即默认延迟比例为50%。 bictcp_reset在两种情况下被调用: 初始化时（bictcp_init ）、进入拥塞处理时（bictcp_state 状态为TCP_CA_Loss）

static inline void bictcp_reset(struct bictcp *ca)
{
	ca->cnt = 0;
	ca->last_max_cwnd = 0;
	ca->last_cwnd = 0;
	ca->last_time = 0;
	ca->epoch_start = 0;
	ca->delayed_ack = 2 << ACK_RATIO_SHIFT;
}

bictcp_acked计算延迟ack的比例delayed_ack ,公式没有看懂，sample是什么东西？

/* Track delayed acknowledgment ratio using sliding window
 * ratio = (15*ratio + sample) / 16
 */
static void bictcp_acked(struct sock *sk, const struct ack_sample *sample)
{
	const struct inet_connection_sock *icsk = inet_csk(sk);

	if (icsk->icsk_ca_state == TCP_CA_Open) {
		struct bictcp *ca = inet_csk_ca(sk);

		ca->delayed_ack += sample->pkts_acked -
			(ca->delayed_ack >> ACK_RATIO_SHIFT);
	}
}

最后要关注的就是bic算法对ssthresh的计算了。
每收到一个ack，就会调用tcp_ack(这个函数有点复杂后面慢慢看)。tcp_ack会调用.pkts_acked和.cong_avoid，pkts_acked对应bictcp_acked, cong_avoid对应bictcp_cong_avoid。 tcp_ack中如果检测到丢包，则进入拥塞阶段，调用.ssthresh，对应bic的bictcp_recalc_ssthresh函数，tcp_ack完成重传后，退回到拥塞阶段，调用.undo_cwnd函数，即tcp_reno_undo_cwnd。

bictcp_recalc_ssthresh用于拥塞后计算慢启动阈值ssthresh。里面有几个参数要注意下
#define BICTCP_BETA_SCALE 1024   /* Scale factor beta calculation
static int beta = 819;       /* = 819/1024 (BICTCP_BETA_SCALE) */
1.设置最大窗口
当 snd_cwnd < last_max_cwnd && fast_convergence==1. last_max_cwnd=snd_cwnd*(819+1024)/(2*1024)=0.9*snd_cwnd
当 snd_cwnd >= last_max_cwnd . last_max_cwnd=snd_cwnd
2.设置发送窗口
   当snd_cwnd <= low_window。snd_cwnd = max(snd_cwnd/2, 2)
当snd_cwnd > low_window. snd_cwnd= max(snd_cwnd*beta/BICTCP_BETA_SCALE, 2)=max(snd_cwnd*0.8, 2)

/*
 *	behave like Reno until low_window is reached,
 *	then increase congestion window slowly
 */
static u32 bictcp_recalc_ssthresh(struct sock *sk)
{
	const struct tcp_sock *tp = tcp_sk(sk);
	struct bictcp *ca = inet_csk_ca(sk);

	ca->epoch_start = 0;	/* end of epoch */

	/* Wmax and fast convergence */
	if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
		ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
			/ (2 * BICTCP_BETA_SCALE);
	else
		ca->last_max_cwnd = tp->snd_cwnd;

	if (tp->snd_cwnd <= low_window)
		return max(tp->snd_cwnd >> 1U, 2U);
	else
		return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
}

要深入了解需要了解收到ack后的相关处理细节

tcp拥塞算法分析二（bic）

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