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1. static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
2. gfp_t gfp_mask)
3. {
4. ……………………..
5. if (unlikely(tcb->tcp_flags & TCPHDR_SYN)) //如果是SYN包，则单独处理tcp的选项信息
6. tcp_options_size = tcp_syn_options(sk, skb, &opts, &md5);
7. else
8. tcp_options_size = tcp_established_options(sk, skb, &opts,
9. &md5);
10. tcp_header_size = tcp_options_size + sizeof(struct tcphdr);
11. ………..
12. if (unlikely(tcb->tcp_flags & TCPHDR_SYN)) {
13. /* RFC1323: The window in SYN & SYN/ACK segments
14. * is never scaled.
15. 在三次握手阶段，接收窗口并没有按扩大因子进行缩放*/
16. th->window    = htons(min(tp->rcv_wnd, 65535U));
17. } else {
18. th->window    = htons(tcp_select_window(sk));
19. }
20. …………………………
21. }
22. static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
23. struct tcp_out_options *opts,
24. struct tcp_md5sig_key **md5)
25. {
26. struct tcp_sock *tp = tcp_sk(sk);
27. unsigned int remaining = MAX_TCP_OPTION_SPACE;
28. struct tcp_fastopen_request *fastopen = tp->fastopen_req;
29. #ifdef CONFIG_TCP_MD5SIG
30. *md5 = tp->af_specific->md5_lookup(sk, sk);
31. if (*md5) {
32. opts->options |= OPTION_MD5;
33. remaining -= TCPOLEN_MD5SIG_ALIGNED;
34. }
35. #else
36. *md5 = NULL;
37. #endif
38. opts->mss = tcp_advertise_mss(sk);
39. remaining -= TCPOLEN_MSS_ALIGNED;
40. if (likely(sysctl_tcp_timestamps && !*md5)) {
41. opts->options |= OPTION_TS;
42. opts->tsval = tcp_skb_timestamp(skb) + tp->tsoffset;
43. opts->tsecr = tp->rx_opt.ts_recent;
44. remaining -= TCPOLEN_TSTAMP_ALIGNED;
45. }
46. if (likely(sysctl_tcp_window_scaling)) { /* 如果开启sysctl_tcp_window_scaling 选项则在syn报文中添加scale的选项信息*/
47. opts->ws = tp->rx_opt.rcv_wscale;
48. opts->options |= OPTION_WSCALE;
49. remaining -= TCPOLEN_WSCALE_ALIGNED;
50. }
51. if (likely(sysctl_tcp_sack)) {
52. opts->options |= OPTION_SACK_ADVERTISE;
53. if (unlikely(!(OPTION_TS & opts->options)))
54. remaining -= TCPOLEN_SACKPERM_ALIGNED;
55. }
56. if (fastopen && fastopen->cookie.len >= 0) {
57. u32 need = fastopen->cookie.len;
58. need += fastopen->cookie.exp ? TCPOLEN_EXP_FASTOPEN_BASE :
59. TCPOLEN_FASTOPEN_BASE;
60. need = (need + 3) & ~3U; /* Align to 32 bits */
61. if (remaining >= need) {
62. opts->options |= OPTION_FAST_OPEN_COOKIE;
63. opts->fastopen_cookie = &fastopen->cookie;
64. remaining -= need;
65. tp->syn_fastopen = 1;
66. tp->syn_fastopen_exp = fastopen->cookie.exp ? 1 : 0;
67. }
68. }
69. return MAX_TCP_OPTION_SPACE - remaining;
70. }

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1. static u16 tcp_select_window(struct sock *sk)
2. {
3. struct tcp_sock *tp = tcp_sk(sk);
4. u32 old_win = tp->rcv_wnd;
5. u32 cur_win = tcp_receive_window(tp); /*计算当前剩余接收窗口*/
6. u32 new_win = __tcp_select_window(sk);/*这里根据剩余的接收缓存，计算新的接收窗口*/
7. /* Never shrink the offered window 不允许缩小已经分配的窗口大小*/
8. if (new_win < cur_win) {
9. if (new_win == 0)
10. NET_INC_STATS(sock_net(sk),
11. LINUX_MIB_TCPWANTZEROWINDOWADV);
12. new_win = ALIGN(cur_win, 1 << tp->rx_opt.rcv_wscale);
13. }
14. tp->rcv_wnd = new_win;
15. tp->rcv_wup = tp->rcv_nxt;
16. /* Make sure we do not exceed the maximum possible
17. * scaled window.
18. 确保接收窗口不超过规定的最大值*/
19. if (!tp->rx_opt.rcv_wscale && sysctl_tcp_workaround_signed_windows)
20. new_win = min(new_win, MAX_TCP_WINDOW);
21. else
22. new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));
23. /* RFC1323 scaling applied 根据三次握手时确定的scale，按照scale缩小接收窗口的大小*/
24. new_win >>= tp->rx_opt.rcv_wscale;
25. /* If we advertise zero window, disable fast path. */
26. if (new_win == 0) {
27. tp->pred_flags = 0;
28. if (old_win)
29. NET_INC_STATS(sock_net(sk),
30. LINUX_MIB_TCPTOZEROWINDOWADV);
31. } else if (old_win == 0) {
32. NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPFROMZEROWINDOWADV);
33. }
34. /*返回新的可用的接收窗口，这个值就是在wireshark抓包时看到的值。*/
35. return new_win;
36. }

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1. u32 __tcp_select_window(struct sock *sk)
2. {
3. struct inet_connection_sock *icsk = inet_csk(sk);
4. struct tcp_sock *tp = tcp_sk(sk);
5. /* MSS for the peer's data. Previous versions used mss_clamp
6. * here. I don't know if the value based on our guesses
7. * of peer's MSS is better for the performance. It's more correct
8. * but may be worse for the performance because of rcv_mss
9. * fluctuations. --SAW 1998/11/1
10. */
11. int mss = icsk->icsk_ack.rcv_mss; //接收端mss的大小
12. int free_space = tcp_space(sk); // 可以使用的接收缓存大小，是(sk->sk_rcvbuf-sk->sk_rmem_alloc) /2
13. int allowed_space = tcp_full_space(sk); // 为sk->sk_rcvbuf/2
14. int full_space = min_t(int, tp->window_clamp, allowed_space); //总的接收缓存
15. int window;
16. if (mss > full_space)
17. mss = full_space;
18. /* 如果可用的缓存小于总接收缓存的1/2的话，则说明内存吃紧*/
19. if (free_space < (full_space >> 1)) {
20. icsk->icsk_ack.quick = 0;
21. if (sk_under_memory_pressure(sk)) /*如果处于内存压力的状态，则修改接收窗口的阈值*/
22. tp->rcv_ssthresh = min(tp->rcv_ssthresh,
23. 4U * tp->advmss);
24. /* free_space might become our new window, make sure we don't
25. * increase it due to wscale.
26. */
27. free_space = round_down(free_space, 1 << tp->rx_opt.rcv_wscale);
28. /*剩余接收缓存不足时，则直接返回0*/
29. if (free_space < (allowed_space >> 4) || free_space < mss)
30. return 0;
31. }
32. if (free_space > tp->rcv_ssthresh)
33. free_space = tp->rcv_ssthresh;
34. /* Don't do rounding if we are using window scaling, since the
35. * scaled window will not line up with the MSS boundary anyway.
36. */
37. window = tp->rcv_wnd;
38. if (tp->rx_opt.rcv_wscale) { /*接收扩大因子不为0 的话，则重新设置window*/
39. window = free_space;
40. /* Advertise enough space so that it won't get scaled away.
41. * Import case: prevent zero window announcement if
42. * 1<<rcv_wscale > mss.
43. 这里进行防治四舍五入导致的接收窗口较小*/
44. if (((window >> tp->rx_opt.rcv_wscale) << tp->rx_opt.rcv_wscale) != window)
45. window = (((window >> tp->rx_opt.rcv_wscale) + 1)
46. << tp->rx_opt.rcv_wscale);
47. } else {
48. if (window <= free_space - mss || window > free_space)
49. window = (free_space / mss) * mss;
50. else if (mss == full_space &&
51. free_space > window + (full_space >> 1))
52. window = free_space;
53. }
54. /*返回及时后的window*/
55. return window;
56. }