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libuv 与 TCP Keepalive

关于 keepalive

这里的keepalive与HTTP的keepalive不同，这里的keepalive是TCP层的keepalive，用处是当两台机器之间通信时，中间网络出现故障，这时，两端并无法感知网络故障这个事件，无法及时发现网络故障。

HTTP的keepalive是指，一个请求在请求头部增加一个keep alive的行，这时，服务端传输完成后，不会关闭这个TCP连接，还可以继续下次HTTP请求，提高了效率。

Linux内核关于TCP keepalive的说明在这里： http://tldp.org/HOWTO/TCP-Keepalive-HOWTO/usingkeepalive.html

其中：

tcp_keepalive_time
 the interval between the last data packet sent (simple ACKs are not considered data) and the first keepalive probe; after the connection is marked to need keepalive, this counter is not used any further
tcp_keepalive_intvl
 the interval between subsequential keepalive probes, regardless of what the connection has exchanged in the meantime
tcp_keepalive_probes
 the number of unacknowledged probes to send before considering the connection dead and notifying the application layer

大致翻译一下就是

tcp_keepalive_time
当一台机器在 N 秒内，还没有收到对方的任何数据时，开始 keepalive 探测对方是否正常
tcp_keepalive_intvl
keepalive报文发送的间隔，单位秒
tcp_keepalive_probes
keepalive报文探测次数

也就是说，在 tcp_keepalive_time 秒内仍未收到对端数据时，开始发起 keepalive 探测，每隔 tcp_keepalive_intvl 发送一个探测报文，当 发送 tcp_keepalive_probes 探测报文，对方仍未响应时，关闭连接。

在Linux下，可以通过以下方式查看系统的keepalive配置：

[root@localhost ~]# cat /proc/sys/net/ipv4/ 
1800
[root@localhost ~]# cat /proc/sys/net/ipv4/tcp_keepalive_probes 
9
[root@localhost ~]# cat /proc/sys/net/ipv4/tcp_keepalive_intvl  
75

keepalive 接口

设置一个套接字的keepalive的方法如下：

1. enable keepalive

int on = 1;
if (setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE, &on, sizeof(on))) {
	// log
    return -1;
}

2. 设置 tcp_keepalive_time

int idle = 10;
if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPIDLE, &idle, sizeof(int)) < 0) {
  // log
  return -1;
}

3. 设置 tcp_keepalive_probes

int probes = 4;
if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPIDLE, &probes, sizeof(int)) < 0) {
  // log
  return -1;
}

4. 设置 tcp_keepalive_intvl

int intvl = 1;
if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPIDLE, &intvl, sizeof(int)) < 0) {
  // log
  return -1;
}

libuv 的 keepalive

libuv提供的接口只能设置上面的两个：

  1. enable keepalive
  2. 设置 tcp_keepalive_time

libuv提供的接口为 uv_tcp_keepalive, 函数原型如下：

int uv_tcp_keepalive(uv_tcp_t* handle, int enable, unsigned int delay)
   Enable / disable TCP keep-alive. delay is the initial delay in seconds, ignored when enable is zero.

该函数的实现代码如下：

int uv__tcp_keepalive(int fd, int on, unsigned int delay) {
  if (setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE, &on, sizeof(on)))
    return -errno;
#ifdef TCP_KEEPIDLE
  if (on && setsockopt(fd, IPPROTO_TCP, TCP_KEEPIDLE, &delay, sizeof(delay)))
    return -errno;
#endif
  /* Solaris/SmartOS, if you don't support keep-alive,
   * then don't advertise it in your system headers...
   */
  /* FIXME(bnoordhuis) That's possibly because sizeof(delay) should be 1. */
#if defined(TCP_KEEPALIVE) && !defined(__sun)
  if (on && setsockopt(fd, IPPROTO_TCP, TCP_KEEPALIVE, &delay, sizeof(delay)))
    return -errno;
#endif
  return 0;
}
int uv_tcp_keepalive(uv_tcp_t* handle, int on, unsigned int delay) {
  int err;
  if (uv__stream_fd(handle) != -1) {
    err =uv__tcp_keepalive(uv__stream_fd(handle), on, delay);
    if (err)
      return err;
  }
  if (on)
    handle->flags |= UV_TCP_KEEPALIVE;
  else
    handle->flags &= ~UV_TCP_KEEPALIVE;
  /* TODO Store delay if uv__stream_fd(handle) == -1 but don't want to enlarge
   *      uv_tcp_t with an int that's almost never used...
   */
  return 0;
}

从上面的代码可以看出，当 uv__stream_fd(handle) 不成功时，仅仅设置该连接的flags位，实际上并没有用到delay这个参数。也就是说，只有当连接已经成功建立时，才能设置 tcp_keepalive_time，如果连接还没有建立成功，则这个值根本没有设置。

如果要设置后面两个数值的话，需要自己实现, 示例代码如下：

//
// 设置 keepalive 相关的2个参数
// probes: 对应内核 tcp_keepalive_probes, 发送多少次keepalive报文还未收到回应时, close该连接
// intvl:  对应内核 tcp_keepalive_intvl, 发送keepalive报文的间隔时间
// idle:   对应内核 tcp_keepalive_time
int set_keep_alive(const uv_handle_t* handle, int probes, int intvl, int idle) {
    int ret;
    uv_os_fd_t fd;
    ret = uv_fileno(handle, &fd);
    if (ret < 0) {
        return ret;
    }
    if (idle > 0) {
        if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPIDLE, &idle, sizeof(idle)))
            return -1;
    }
    // 设置 tcp_keepalive_intvl
    if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPINTVL,
                (const void *) &intvl, sizeof(int)) < 0 ) {
        return -1;
    }
    // 设置 tcp_keepalive_probes
    if ( setsockopt(fd, IPPROTO_TCP, TCP_KEEPCNT,
                (const void *) &probes, sizeof(int)) < 0) {
        return -1;
    }
    return 0;
}

这个函数只能在连接建立成功后调用，因为只有连接建立成功才有fd，即uv_fileno才会返回成功。

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