TCP窗口比例选项非零的原因 [英] The reason of non-zero TCP window scale option

问题描述

我读了一些有关 TCP窗口缩放和

I read some stuff about TCP window scaling and BDP(not quite clear), and I can't figure out what exactly cause sender's TCP realization to set non-zero WS and could the user-mode client program affect it somehow? I think that logically it cannot be based on some data transferring because it happens on SYN-SYN+ACK TCP stage.

有人可以从编程的角度解释一下，用户模式客户端代码如何影响TCP窗口缩放选项(例如，在connect()调用之前)? TCP堆栈如何知道何时将WS设置为非零?

Can someone explain from programming point of view, how can user-mode client code affect on TCP window scale option (e.g. before connect() call)? And how TCP stack knows when to set WS to non-zero?

很抱歉，如果很明显.

推荐答案

TCP窗口缩放索引rcv_wscale(发送 SYN 或 SYN-ACK 时的fe)是在Linux内核中基于套接字 tcp_select_initial_window() :

The TCP window scaling index rcv_wscale (f.e. while sending SYN or SYN-ACK) is calculated inside Linux kernel based on socket's receive buffer in function tcp_select_initial_window():

/* 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;
//...
(*rcv_wscale) = 0;
if (wscale_ok) {
    /* Set window scaling on max possible window
     * See RFC1323 for an explanation of the limit to 14
     */
    space = max_t(u32, space, sysctl_tcp_rmem[2]);
    space = max_t(u32, space, sysctl_rmem_max);
    space = min_t(u32, space, *window_clamp);
    while (space > 65535 && (*rcv_wscale) < 14) {
        space >>= 1;
        (*rcv_wscale)++;
    }
}

此处space是根据sk_rcvbuf从tcp_full_space()中提取的.

Here space is taken from the tcp_full_space() based on sk_rcvbuf.

知道您可以通过更改接收缓冲区的大小来影响此计算:

Knowing that you can affect this calculation via changing size of receive buffer:

int buflen = 12345;
if (setsockopt(sockfd, SOL_SOCKET, SO_RCVBUF, &buflen, sizeof int) < 0)
    perror("setsockopt():");
//...

这可以为您提供零缩放( WS=0 或 wscale 0 ).

This can give you zero scaling (WS=0 or wscale 0).

P.S.请记住，在服务器端，应该在侦听套接字上完成此操作，因为在TCP握手之后您不能影响它.

P.S. keep in mind that at server side it should be done on listening socket, because you can't affect it after TCP-handshake.

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