TCP接收窗口--确定Window Scaling

1. rcv_scale的确定

    缓存的配置对后面三次握手交互时，设置window scale的值有很大的关系，window scale的值是在三次握手时通过tcp option确定。

1.1 client发送syn

在三次握手的过程中会根据配置的缓存大小，确定scale的大小，在发送syn数据包和syn+ack报文中，在创建syn数据包时，调用tcp_connect_init函数，在该函数中tcp_select_initial_window函数，对rcv_wscale进行赋值。

void tcp_select_initial_window(int __space, __u32 mss,
                   __u32 *rcv_wnd, __u32 *window_clamp,
             int wscale_ok, __u8 *rcv_wscale,
                   __u32 init_rcv_wnd)
{
    unsigned int space = (__space < 0 ? 0 : __space);

    /* If no clamp set the clamp to the max possible scaled window */
    if (*window_clamp == 0)
        (*window_clamp) = (65535 << 14);
    space = min(*window_clamp, space);

    /* Quantize space offering to a multiple of mss if possible. */
    if (space > mss)
        space = (space / mss) * mss;

    if (sysctl_tcp_workaround_signed_windows)
        (*rcv_wnd) = min(space, MAX_TCP_WINDOW);
    else
        (*rcv_wnd) = space;
    (*rcv_wscale) = 0;
    if (wscale_ok) { /*这里的wscale_ok对应于sysctl_tcp_window_scaling参数，该参数内核默认配置为1，表示启用scale*/
        space = max_t(u32, sysctl_tcp_rmem[2], sysctl_rmem_max);
        space = min_t(u32, space, *window_clamp);
        while (space > 65535 && (*rcv_wscale) < 14) { /*如果可用的space大于win能表示的最大值，并且wsacle小于14，则对rcv_wscale进行加1，对space缩小。*/
            space >>= 1;
            (*rcv_wscale)++;
        }
    }

    if (mss > (1 << *rcv_wscale)) {
        if (!init_rcv_wnd) /* Use default unless specified otherwise */
            init_rcv_wnd = tcp_default_init_rwnd(mss);
        *rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
    }

    /* Set the clamp no higher than max representable value */
    (*window_clamp) = min(65535U << (*rcv_wscale), *window_clamp);
}

1.2  syn_ack报文设置rcv_wscale

在接收到syn报文后，发送syn+ack报文下面是设置rcv_scale的流程，也是调用tcp_select_initial_window函数进行初始化。

static unsigned int tcp_synack_options(struct sock *sk,
                   struct request_sock *req,
                   unsigned int mss, struct sk_buff *skb,
                   struct tcp_out_options *opts,
                 const struct tcp_md5sig_key *md5,
                   struct tcp_fastopen_cookie *foc)
{
    struct inet_request_sock *ireq = inet_rsk(req);
    unsigned int remaining = MAX_TCP_OPTION_SPACE;

#ifdef CONFIG_TCP_MD5SIG
    if (md5) {
        opts->options |= OPTION_MD5;
        remaining -= TCPOLEN_MD5SIG_ALIGNED;
        ireq->tstamp_ok &= !ireq->sack_ok;
    }
#endif

    /* We always send an MSS option. */
    opts->mss = mss;
    remaining -= TCPOLEN_MSS_ALIGNED;
    if (likely(ireq->wscale_ok)) { //设置tcp的选项信息wscale
        opts->ws = ireq->rcv_wscale;
        opts->options |= OPTION_WSCALE;
        remaining -= TCPOLEN_WSCALE_ALIGNED;
    }
    if (likely(ireq->tstamp_ok)) {
        opts->options |= OPTION_TS;
        opts->tsval = tcp_skb_timestamp(skb);
        opts->tsecr = req->ts_recent;
        remaining -= TCPOLEN_TSTAMP_ALIGNED;
    }
    if (likely(ireq->sack_ok)) {
        opts->options |= OPTION_SACK_ADVERTISE;
        if (unlikely(!ireq->tstamp_ok))
            remaining -= TCPOLEN_SACKPERM_ALIGNED;
    }
    if (foc != NULL && foc->len >= 0) {
        u32 need = foc->len;

        need += foc->exp ? TCPOLEN_EXP_FASTOPEN_BASE :
                   TCPOLEN_FASTOPEN_BASE;
        need = (need + 3) & ~3U; /* Align to 32 bits */
        if (remaining >= need) {
            opts->options |= OPTION_FAST_OPEN_COOKIE;
            opts->fastopen_cookie = foc;
            remaining -= need;
        }
    }
    return MAX_TCP_OPTION_SPACE - remaining;
}

1.3 解析scale选项

接收到带有tcp选项scale数据报文时，处理流程如下,只针对syn数据包：