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SACK是接收方用来向发送方通知已经接收到哪些序列号段的一种机制，这样发送方在重传时就只需要重传接收方真正未收到的部分即可。

初始化

sack提供了proc接口用来控制是否支持sack能力（/proc/sys/net/ipv4/tcp_sack），该选项默认为1，是能sack能力。

1）、发送端发送sync报文时，判断本地是否开启sack选项，如果开启，则options选项置上SACK_ADVERTISE标志

	if (likely(sysctl_tcp_sack)) {
		opts->options |= OPTION_SACK_ADVERTISE;
		if (unlikely(!(OPTION_TS & opts->options)))
			remaining -= TCPOLEN_SACKPERM_ALIGNED;
	}

2）、填充SACK选项内容时，置上TCPOPT_SACK_PERM，用于接收端解析；

	if (unlikely(OPTION_SACK_ADVERTISE & options)) {
		*ptr++ = htonl((TCPOPT_NOP << 24) |
			       (TCPOPT_NOP << 16) |
			       (TCPOPT_SACK_PERM << 8) |
			       TCPOLEN_SACK_PERM);
	}

3）、接收端在解析tcp option选项时，根据TCPOPT_SACK_PERM选项以及本地的sysctl_tcp_sack值判断是否支持sack，如果支持，则opt_rx->sack_ok置上TCP_SACK_SEEN标志，后续协议栈会通过tcp_is_sack函数来判断是否支持sack能力；

        case TCPOPT_SACK_PERM:
            if (opsize == TCPOLEN_SACK_PERM && th->syn &&
                !estab && sysctl_tcp_sack) {
                opt_rx->sack_ok = TCP_SACK_SEEN;
                tcp_sack_reset(opt_rx);
            }
            break;

接收端流程：

tcp_data_queue_ofo

接收端收到消息包后进入tcp_rcv_established，该函数分为快路径和慢路径两种情况，当接收到的数据包序列号不是期望接收的下一个序列号(rcv_next)时，判断为乱序，乱序最终会通过慢路径走到函数tcp_data_queue_ofo将乱序数据包放到队列tp->out_of_order_queue里，放队列前会先进行skb的合并、排序等常规操作。

static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *skb1;
	u32 seq, end_seq;

	TCP_ECN_check_ce(tp, skb);

	if (unlikely(tcp_try_rmem_schedule(sk, skb, skb->truesize))) {
		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPOFODROP);
		__kfree_skb(skb);
		return;
	}

	/* Disable header prediction. */
	tp->pred_flags = 0;
	inet_csk_schedule_ack(sk);

	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPOFOQUEUE);
	SOCK_DEBUG(sk, "out of order segment: rcv_next %X seq %X - %X\n",
		   tp->rcv_nxt, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);

	skb1 = skb_peek_tail(&tp->out_of_order_queue);
	if (!skb1) {
		/* Initial out of order segment, build 1 SACK. */
		if (tcp_is_sack(tp)) {
			tp->rx_opt.num_sacks = 1;
			//乱序的首包，初始化sack信息
			tp->selective_acks[0].start_seq = TCP_SKB_CB(skb)->seq;
			tp->selective_acks[0].end_seq =
						TCP_SKB_CB(skb)->end_seq;
		}
		__skb_queue_head(&tp->out_of_order_queue, skb);
		goto end;
	}

	seq = TCP_SKB_CB(skb)->seq;
	end_seq = TCP_SKB_CB(skb)->end_seq;

	//新到的skb为乱序队列的下一个skb，做合并处理
	if (seq == TCP_SKB_CB(skb1)->end_seq) {
		bool fragstolen;

		//尝试将skb的内容合并到skb1里， 如果合并失败，则将skb1添加到skb的队列后面
		if (!tcp_try_coalesce(sk, skb1, skb, &fragstolen)) {
			__skb_queue_after(&tp->out_of_order_queue, skb1, skb);
		} else {
			tcp_grow_window(sk, skb);
			kfree_skb_partial(skb, fragstolen);
			skb = NULL;
		}

		if (!tp->rx_opt.num_sacks ||
		    tp->selective_acks[0].end_seq != seq)
			goto add_sack;

		/* Common case: data arrive in order after hole. */
		tp->selective_acks[0].end_seq = end_seq;
		goto end;
	}

	/* Find place to insert this segment. */
	//按序列号的顺序，找到需要插入的skb节点
	while (1) {
		if (!after(TCP_SKB_CB(skb1)->seq, seq))
			break;
		if (skb_queue_is_first(&tp->out_of_order_queue, skb1)) {
			skb1 = NULL;
			break;
		}
		skb1 = skb_queue_prev(&tp->out_of_order_queue, skb1);
	}

	/* Do skb overlap to previous one? */
	if (skb1 && before(seq, TCP_SKB_CB(skb1)->end_seq)) {
		if (!after(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
			/* All the bits are present. Drop. */
			NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPOFOMERGE);
			__kfree_skb(skb);
			skb = NULL;
			tcp_dsack_set(sk, seq, end_seq);
			goto add_sack;
		}
		if (after(seq, TCP_SKB_CB(skb1)->seq)) {
			/* Partial overlap. */
			tcp_dsack_set(sk, seq,
				      TCP_SKB_CB(skb1)->end_seq);
		} else {
			if (skb_queue_is_first(&tp->out_of_order_queue,
					       skb1))
				skb1 = NULL;
			else
				skb1 = skb_queue_prev(
					&tp->out_of_order_queue,
					skb1);
		}
	}
	//找不到插入的skb1节点，则将新到的skb插入乱序列表头
	if (!skb1)
		__skb_queue_head(&tp->out_of_order_queue, skb);
	//否则插到找到的skb1的后面
	else
		__skb_queue_after(&tp->out_of_order_queue, skb1, skb);

	/* And clean segments covered by new one as whole. */
	while (!skb_queue_is_last(&tp->out_of_order_queue, skb)) {
		skb1 = skb_queue_next(&tp->out_of_order_queue, skb);

		if (!after(end_seq, TCP_SKB_CB(skb1)->seq))
			break;
		if (before(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
			tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
					 end_seq);
			break;
		}
		__skb_unlink(skb1, &tp->out_of_order_queue);
		tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
				 TCP_SKB_CB(skb1)->end_seq);
		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPOFOMERGE);
		__kfree_skb(skb1);
	}

add_sack:
	//收到乱序包后，构建回复的sack信息
	if (tcp_is_sack(tp))
		tcp_sack_new_ofo_skb(sk, seq, end_seq);
end:
	if (skb) {
		tcp_grow_window(sk, skb);
		skb_set_owner_r(skb, sk);
	}
}

在tcp_data_queue_ofo里最终会调用tcp_sack_new_ofo_skb添加回复发送端的sack信息；

static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct tcp_sack_block *sp = &tp->selective_acks[0];
	int cur_sacks = tp->rx_opt.num_sacks;
	int this_sack;

	if (!cur_sacks)
		goto new_sack;

	//判断跟已有的sack信息是否有可以合并的
	for (this_sack = 0; this_sack < cur_sacks; this_sack++, sp++) {
		if (tcp_sack_extend(sp, seq, end_seq)) {
			/* Rotate this_sack to the first one. */
			for (; this_sack > 0; this_sack--, sp--)
				swap(*sp, *(sp - 1));
			if (cur_sacks > 1)
				tcp_sack_maybe_coalesce(tp);
			return;
		}
	}

	/* Could not find an adjacent existing SACK, build a new one,
	 * put it at the front, and shift everyone else down.  We
	 * always know there is at least one SACK present already here.
	 *
	 * If the sack array is full, forget about the last one.
	 */
	if (this_sack >= TCP_NUM_SACKS) {
		this_sack--;
		tp->rx_opt.num_sacks--;
		sp--;
	}
	for (; this_sack > 0; this_sack--, sp--)
		*sp = *(sp - 1);

new_sack:
	/* Build the new head SACK, and we're done. */
	//新创建一个sack block
	sp->start_seq = seq;
	sp->end_seq = end_seq;
	tp->rx_opt.num_sacks++;
}

__tcp_ack_snd_check

当接收端接收端乱序包时，会立即回复ack，这里的ofo_possible设置为1；

static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
{
	struct tcp_sock *tp = tcp_sk(sk);

	    /* More than one full frame received... */
	//ofo_possible表示出现乱序了，那么马上回复ack
	if (((tp->rcv_nxt - tp->rcv_wup) > inet_csk(sk)->icsk_ack.rcv_mss &&
	     /* ... and right edge of window advances far enough.
	      * (tcp_recvmsg() will send ACK otherwise). Or...
	      */
	     __tcp_select_window(sk) >= tp->rcv_wnd) ||
	    /* We ACK each frame or... */
	    tcp_in_quickack_mode(sk) ||
	    /* We have out of order data. */
	    (ofo_possible && skb_peek(&tp->out_of_order_queue))) {
		/* Then ack it now */
		//马上回复ack，ack的seq号为snd_nxt；
		//ack的ack_seq为rcv_nxt(在tcp_transmit_skb里设置)
		tcp_send_ack(sk);
	} else {
		/* Else, send delayed ack. */
		tcp_send_delayed_ack(sk);
	}
}

ack流程最终通过tcp_send_ack->tcp_transmit_skb走到tcp_options_write函数，在tcp_options_write里，将之前构建的sack信息填充到skb头部里，另外，由于本次收到的数据包没有ack新的包，所以tp->rcv_nxt不会更新，因此本次回复的ack_seq为旧的序列号。

static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
			      struct tcp_out_options *opts)
{
        th = tcp_hdr(skb);
	th->source		= inet->inet_sport;
	th->dest		= inet->inet_dport;
	th->seq			= htonl(tcb->seq);
	th->ack_seq		= htonl(tp->rcv_nxt);

	if (unlikely(opts->num_sack_blocks)) {
		struct tcp_sack_block *sp = tp->rx_opt.dsack ?
			tp->duplicate_sack : tp->selective_acks;
		int this_sack;

		*ptr++ = htonl((TCPOPT_NOP  << 24) |
			       (TCPOPT_NOP  << 16) |
			       (TCPOPT_SACK <<  8) |
			       (TCPOLEN_SACK_BASE + (opts->num_sack_blocks *
						     TCPOLEN_SACK_PERBLOCK)));

		for (this_sack = 0; this_sack < opts->num_sack_blocks;
		     ++this_sack) {
			*ptr++ = htonl(sp[this_sack].start_seq);
			*ptr++ = htonl(sp[this_sack].end_seq);
		}

		tp->rx_opt.dsack = 0;
	}

}