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。

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1. void tcp_select_initial_window(int __space, __u32 mss,
   
2.              __u32 *rcv_wnd, __u32 *window_clamp,
   
3.              int wscale_ok, __u8 *rcv_wscale,
   
4.              __u32 init_rcv_wnd)
   
5. {
   
6.     unsigned int space = (__space < 0 ? 0 : __space);
   
7. 
   
8.     /* If no clamp set the clamp to the max possible scaled window */
   
9.     if (*window_clamp == 0)
   
10.         (*window_clamp) = (65535 << 14);
    
11.     space = min(*window_clamp, space);
    
12. 
    
13.     /* Quantize space offering to a multiple of mss if possible. */
    
14.     if (space > mss)
    
15.         space = (space / mss) * mss;
    
16. 
    
17.     if (sysctl_tcp_workaround_signed_windows)
    
18.         (*rcv_wnd) = min(space, MAX_TCP_WINDOW);
    
19.     else
    
20.         (*rcv_wnd) = space;
    
21.     (*rcv_wscale) = 0;
    
22.     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*/
    
23.         space = max_t(u32, sysctl_tcp_rmem[2], sysctl_rmem_max);
    
24.         space = min_t(u32, space, *window_clamp);
    
25.         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。*/
    
26.             space >>= 1;
    
27.             (*rcv_wscale)++;
    
28.         }
    
29.     }
    
30. 
    
31.     if (mss > (1 << *rcv_wscale)) {
    
32.         if (!init_rcv_wnd) /* Use default unless specified otherwise */
    
33.             init_rcv_wnd = tcp_default_init_rwnd(mss);
    
34.         *rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
    
35.     }
    
36. 
    
37.     /* Set the clamp no higher than max representable value */
    
38.     (*window_clamp) = min(65535U << (*rcv_wscale), *window_clamp);
    
39. }

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。

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1. static unsigned int tcp_synack_options(struct sock *sk,
   
2.                  struct request_sock *req,
   
3.                  unsigned int mss, struct sk_buff *skb,
   
4.                  struct tcp_out_options *opts,
   
5.                  const struct tcp_md5sig_key *md5,
   
6.                  struct tcp_fastopen_cookie *foc)
   
7. {
   
8.     struct inet_request_sock *ireq = inet_rsk(req);
   
9.     unsigned int remaining = MAX_TCP_OPTION_SPACE;
   
10. 
    
11. #ifdef CONFIG_TCP_MD5SIG
    
12.     if (md5) {
    
13.         opts->options |= OPTION_MD5;
    
14.         remaining -= TCPOLEN_MD5SIG_ALIGNED;
    
15.         ireq->tstamp_ok &= !ireq->sack_ok;
    
16.     }
    
17. #endif
    
18. 
    
19.     /* We always send an MSS option. */
    
20.     opts->mss = mss;
    
21.     remaining -= TCPOLEN_MSS_ALIGNED;
    
22.     if (likely(ireq->wscale_ok)) { //设置tcp的选项信息wscale
    
23.         opts->ws = ireq->rcv_wscale;
    
24.         opts->options |= OPTION_WSCALE;
    
25.         remaining -= TCPOLEN_WSCALE_ALIGNED;
    
26.     }
    
27.     if (likely(ireq->tstamp_ok)) {
    
28.         opts->options |= OPTION_TS;
    
29.         opts->tsval = tcp_skb_timestamp(skb);
    
30.         opts->tsecr = req->ts_recent;
    
31.         remaining -= TCPOLEN_TSTAMP_ALIGNED;
    
32.     }
    
33.     if (likely(ireq->sack_ok)) {
    
34.         opts->options |= OPTION_SACK_ADVERTISE;
    
35.         if (unlikely(!ireq->tstamp_ok))
    
36.             remaining -= TCPOLEN_SACKPERM_ALIGNED;
    
37.     }
    
38.     if (foc != NULL && foc->len >= 0) {
    
39.         u32 need = foc->len;
    
40. 
    
41.         need += foc->exp ? TCPOLEN_EXP_FASTOPEN_BASE :
    
42.                  TCPOLEN_FASTOPEN_BASE;
    
43.         need = (need + 3) & ~3U; /* Align to 32 bits */
    
44.         if (remaining >= need) {
    
45.             opts->options |= OPTION_FAST_OPEN_COOKIE;
    
46.             opts->fastopen_cookie = foc;
    
47.             remaining -= need;
    
48.         }
    
49.     }
    
50.     return MAX_TCP_OPTION_SPACE - remaining;
    
51. }

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：