tcp. do not accept packets beyond window

判断tcp seq 是否在正常范围内的时候；

只要报文在 [RCV.WUP, RCV.NXT + RCV.WND] 这个区间内，就认为“sequence 合法”

RCV.WUP：最后一次 window update 使用的接收点

旧内核代码；

static inline bool tcp_sequence(const struct tcp_sock *tp, u32 seq, u32 end_seq)
{
    return    !before(end_seq, tp->rcv_wup) &&
        !after(seq, tp->rcv_nxt + tcp_receive_window(tp));
}

新的代码：

/* Check segment sequence number for validity.
 *
 * Segment controls are considered valid, if the segment
 * fits to the window after truncation to the window. Acceptability
 * of data (and SYN, FIN, of course) is checked separately.
 * See tcp_data_queue(), for example.
 *
 * Also, controls (RST is main one) are accepted using RCV.WUP instead
 * of RCV.NXT. Peer still did not advance his SND.UNA when we
 * delayed ACK, so that hisSND.UNA<=ourRCV.WUP.
 * (borrowed from freebsd)
 */

static enum skb_drop_reason tcp_sequence(const struct sock *sk,
                     u32 seq, u32 end_seq)
{
    const struct tcp_sock *tp = tcp_sk(sk);
/*整个 segment 完全落在窗口左侧
典型场景：重传延迟;ACK 延迟导致对端误判;reordered + delayed ACK*/
    if (before(end_seq, tp->rcv_wup))
        return SKB_DROP_REASON_TCP_OLD_SEQUENCE;


    if (after(end_seq, tp->rcv_nxt + tcp_receive_window(tp))) {
        if (after(seq, tp->rcv_nxt + tcp_receive_window(tp)))
            return SKB_DROP_REASON_TCP_INVALID_SEQUENCE;//整个 segment 完全在窗口右边之外

        /* Only accept this packet if receive queue is empty. 
        seq <= rcv_nxt + rcv_wnd
        end_seq > rcv_nxt + rcv_wnd;
        segment 的头在窗口内，但尾巴越界
        */
        if (skb_queue_len(&sk->sk_receive_queue))
            return SKB_DROP_REASON_TCP_INVALID_END_SEQUENCE; //不为空直接丢弃
            /*为什么要“接收队列为空”才允许？
    这是为了解决一个非常真实的 TCP 边界场景：
    应用层消费慢;RCV.WND 很小
    对端发了一个 稍微大一点的 segment
    segment 头部是我们想要的数据
    但尾部越界;如果：接收队列是空的
说明：没有 out-of-order 数据;没有乱序复杂性
那么：允许这个 segment 进入后续逻辑
在 tcp_data_queue() 中做 truncation*/
    }
/**减少“窗口边界抖动”导致的误丢包
特别是在：小窗口、高 RTT、delayed ACK、应用读慢的情况下。
 避免对“可部分接受的 segment”过早判死刑*/
    return SKB_NOT_DROPPED_YET;
}