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：