1. TCP接收窗口的调整
在上面配置的rcv_buf都是配置的接收缓存,在tcp层中接收窗口不能完全占满接收缓存,因为TCP层描述的接收窗口,仅仅是tcp层的data,不包含整个数据包的header部分,也就是不包含tcp header IP Header和一些选项信息。
在tcp三次握手的时候会通告各自的接收窗口,包括窗口中的scale因子,在使用wireshark抓包的时候,如果没有抓到三次握手的数据报文,在后续的报文中会有下面的提示信息,wireshark无法准确获取窗口的滑动因子。
在tcp的发送函数中tcp_transmit_skb中调用tcp_select_window函数,选择通告的win大小,接收窗口只有16位,所以最大能表示的65535字节,64KB的大小,可以通过添加的scale参数扩大接收缓存的大小。
1.1 tcp_transmit_skb
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static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
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gfp_t gfp_mask)
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{
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……………………..
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if (unlikely(tcb->tcp_flags & TCPHDR_SYN)) //如果是SYN包,则单独处理tcp的选项信息
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tcp_options_size = tcp_syn_options(sk, skb, &opts, &md5);
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else
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tcp_options_size = tcp_established_options(sk, skb, &opts,
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&md5);
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tcp_header_size = tcp_options_size + sizeof(struct tcphdr);
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………..
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if (unlikely(tcb->tcp_flags & TCPHDR_SYN)) {
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/* RFC1323: The window in SYN & SYN/ACK segments
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* is never scaled.
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在三次握手阶段,接收窗口并没有按扩大因子进行缩放*/
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th->window = htons(min(tp->rcv_wnd, 65535U));
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} else {
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th->window = htons(tcp_select_window(sk));
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}
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…………………………
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}
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static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
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struct tcp_out_options *opts,
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struct tcp_md5sig_key **md5)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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unsigned int remaining = MAX_TCP_OPTION_SPACE;
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struct tcp_fastopen_request *fastopen = tp->fastopen_req;
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#ifdef CONFIG_TCP_MD5SIG
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*md5 = tp->af_specific->md5_lookup(sk, sk);
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if (*md5) {
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opts->options |= OPTION_MD5;
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remaining -= TCPOLEN_MD5SIG_ALIGNED;
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}
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#else
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*md5 = NULL;
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#endif
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opts->mss = tcp_advertise_mss(sk);
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remaining -= TCPOLEN_MSS_ALIGNED;
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if (likely(sysctl_tcp_timestamps && !*md5)) {
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opts->options |= OPTION_TS;
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opts->tsval = tcp_skb_timestamp(skb) + tp->tsoffset;
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opts->tsecr = tp->rx_opt.ts_recent;
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remaining -= TCPOLEN_TSTAMP_ALIGNED;
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}
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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*/
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opts->ws = tp->rx_opt.rcv_wscale;
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opts->options |= OPTION_WSCALE;
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remaining -= TCPOLEN_WSCALE_ALIGNED;
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}
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if (likely(sysctl_tcp_sack)) {
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opts->options |= OPTION_SACK_ADVERTISE;
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if (unlikely(!(OPTION_TS & opts->options)))
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remaining -= TCPOLEN_SACKPERM_ALIGNED;
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}
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if (fastopen && fastopen->cookie.len >= 0) {
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u32 need = fastopen->cookie.len;
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need += fastopen->cookie.exp ? TCPOLEN_EXP_FASTOPEN_BASE :
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TCPOLEN_FASTOPEN_BASE;
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need = (need + 3) & ~3U; /* Align to 32 bits */
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if (remaining >= need) {
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opts->options |= OPTION_FAST_OPEN_COOKIE;
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opts->fastopen_cookie = &fastopen->cookie;
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remaining -= need;
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tp->syn_fastopen = 1;
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tp->syn_fastopen_exp = fastopen->cookie.exp ? 1 : 0;
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}
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}
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return MAX_TCP_OPTION_SPACE - remaining;
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}
1.2 tcp_select_window
tcp_select_window函数更加当前的缓存,确定了最终window的大小。
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static u16 tcp_select_window(struct sock *sk)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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u32 old_win = tp->rcv_wnd;
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u32 cur_win = tcp_receive_window(tp); /*计算当前剩余接收窗口*/
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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*/
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/* Never shrink the offered window 不允许缩小已经分配的窗口大小*/
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if (new_win < cur_win) {
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if (new_win == 0)
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NET_INC_STATS(sock_net(sk),
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LINUX_MIB_TCPWANTZEROWINDOWADV);
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new_win = ALIGN(cur_win, 1 << tp->rx_opt.rcv_wscale);
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}
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tp->rcv_wnd = new_win;
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tp->rcv_wup = tp->rcv_nxt;
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/* Make sure we do not exceed the maximum possible
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* scaled window.
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确保接收窗口不超过规定的最大值*/
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if (!tp->rx_opt.rcv_wscale && sysctl_tcp_workaround_signed_windows)
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new_win = min(new_win, MAX_TCP_WINDOW);
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else
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new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));
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/* 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*/
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new_win >>= tp->rx_opt.rcv_wscale;
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/* If we advertise zero window, disable fast path. */
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if (new_win == 0) {
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tp->pred_flags = 0;
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if (old_win)
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NET_INC_STATS(sock_net(sk),
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LINUX_MIB_TCPTOZEROWINDOWADV);
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} else if (old_win == 0) {
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NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPFROMZEROWINDOWADV);
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}
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/*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。*/
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return new_win;
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}
1.3 __tcp_select_window
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u32 __tcp_select_window(struct sock *sk)
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{
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struct inet_connection_sock *icsk = inet_csk(sk);
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struct tcp_sock *tp = tcp_sk(sk);
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/* MSS for the peer's data. Previous versions used mss_clamp
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* here. I don't know if the value based on our guesses
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* of peer's MSS is better for the performance. It's more correct
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* but may be worse for the performance because of rcv_mss
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* fluctuations. --SAW 1998/11/1
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*/
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int mss = icsk->icsk_ack.rcv_mss; //接收端mss的大小
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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
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int allowed_space = tcp_full_space(sk); // 为sk->sk_rcvbuf/2
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int full_space = min_t(int, tp->window_clamp, allowed_space); //总的接收缓存
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int window;
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if (mss > full_space)
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mss = full_space;
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/* 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*/
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if (free_space < (full_space >> 1)) {
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icsk->icsk_ack.quick = 0;
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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*/
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tp->rcv_ssthresh = min(tp->rcv_ssthresh,
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4U * tp->advmss);
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/* free_space might become our new window, make sure we don't
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* increase it due to wscale.
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*/
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free_space = round_down(free_space, 1 << tp->rx_opt.rcv_wscale);
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/*剩余接收缓存不足时,则直接返回0*/
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if (free_space < (allowed_space >> 4) || free_space < mss)
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return 0;
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}
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if (free_space > tp->rcv_ssthresh)
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free_space = tp->rcv_ssthresh;
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/* Don't do rounding if we are using window scaling, since the
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* scaled window will not line up with the MSS boundary anyway.
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*/
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window = tp->rcv_wnd;
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if (tp->rx_opt.rcv_wscale) { /*接收扩大因子不为0 的话,则重新设置window*/
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window = free_space;
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/* Advertise enough space so that it won't get scaled away.
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* Import case: prevent zero window announcement if
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* 1<<rcv_wscale > mss.
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这里进行防治四舍五入导致的接收窗口较小*/
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if (((window >> tp->rx_opt.rcv_wscale) << tp->rx_opt.rcv_wscale) != window)
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window = (((window >> tp->rx_opt.rcv_wscale) + 1)
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<< tp->rx_opt.rcv_wscale);
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} else {
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if (window <= free_space - mss || window > free_space)
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window = (free_space / mss) * mss;
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else if (mss == full_space &&
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free_space > window + (full_space >> 1))
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window = free_space;
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}
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/*返回及时后的window*/
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return window;
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}
1.4 tcp_space
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/* Note: caller must be prepared to deal with negative returns */
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static inline int tcp_space(const struct sock *sk)
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{
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/*这里的sk_rmem_alloc表示的是该socket已经被接收到的数据包占用的空间大小
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http://vger.kernel.org/~davem/skb_sk.html */
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return tcp_win_from_space(sk->sk_rcvbuf -
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atomic_read(&sk->sk_rmem_alloc));
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}
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static inline int tcp_full_space(const struct sock *sk)
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{
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return tcp_win_from_space(sk->sk_rcvbuf);
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}
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static inline int tcp_win_from_space(int space)
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{
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/*这里sysctl_tcp_adv_win_scale 在较新的内核为1,原先为2,Upstream commit b49960a05e32121d29316cfdf653894b88ac9190 是修改的patch,里面说明了修改的原因。 如果sysctl_tcp_adv_win_scale为1的话,这里的tcp可以使用的空间有原先的3/4修改为1/2,说明tcp数据包中的其他字段开销变大了*/
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return sysctl_tcp_adv_win_scale<=0 ?
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(space>>(-sysctl_tcp_adv_win_scale)) :
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space - (space>>sysctl_tcp_adv_win_scale);
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}
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