tcp.c文件的tcp_select_window函数（1）

tcp.c文件的tcp_select_window函数

978计划工作组 2009-7-30

1函数源码

int tcp_select_window(struct sock *sk)

{

       int new_window = sk->prot->rspace(sk);

      

       if(sk->window_clamp)

              new_window=min(sk->window_clamp,new_window);

       /*

        *   Two things are going on here.  First, we don't ever offer a

        *   window less than min(sk->mss, MAX_WINDOW/2).  This is the

        *   receiver side of SWS as specified in RFC1122.

        *   Second, we always give them at least the window they

        *   had before, in order to avoid retracting window.  This

        *   is technically allowed, but RFC1122 advises against it and

        *   in practice it causes trouble.

        *

        *   Fixme: This doesn't correctly handle the case where

        *    new_window > sk->window but not by enough to allow for the

        *    shift in sequence space.

        */

       if (new_window < min(sk->mss, MAX_WINDOW/2) || new_window < sk->window)

              return(sk->window);

       return(new_window);

}

2函数用途

用于填充TCP头的窗口大小，也就是struct tcphdr结构的window域。

3语句注释

3.1 int new_window = sk->prot->rspace(sk);

new_window：新窗口大小，窗口大小即接收数据包的最大字节数。TCP首部的第15和16字节声明了本地窗口大小，远端发送的数据包字节数不能超过此值。

sk->prot->rspace(sk)：取得接收缓冲区空闲区域的大小。

3.2 if(sk->window_clamp)

              new_window=min(sk->window_clamp,new_window);

sk->window_clamp：0表示窗口非节制，非0表示窗口节制，窗口节制的情况下新窗口大小不能超过window_clamp的大小。

new_window=min(sk->window_clamp,new_window)：取节制窗口和接收缓冲区空闲区域的最小值作为新窗口的值。

3.3 if (new_window < min(sk->mss, MAX_WINDOW/2) || new_window < sk->window)

      return(sk->window);

sk->mss：最大报文长度，mss = mtu – ip头 – tcp头。

MAX_WINDOW/2：

sk->window：原窗口尺寸，如果新窗口尺寸小于原窗口尺寸，那么就返回原窗口尺寸；这样做是遵循RFC793文档的不降低窗口大小值原则。