对于急躁的人：

如何改变的价值/proc/sys/net/ipv4/tcp_retries2在Linux中单个连接，使用setsockopt()，ioctl()或者这样，或者是这可能吗？

较长的脱毛：

我正在开发一个使用长时间轮询HTTP请求的应用程序。在服务器端，需要知道客户端何时关闭连接。精度并不严格，但肯定不能为15分钟。接近一分钟就可以了。

对于不熟悉此概念的用户，长轮询HTTP请求的工作方式如下：

• 客户端发送请求
• 服务器使用HTTP标头进行响应，但响应保持打开状态。使用分块传输编码，允许服务器在数据可用时发送数据。
• 发送所有数据后，服务器将发送一个“关闭块”以表示响应已完成。

在我的应用程序中，服务器每隔一秒（默认为30秒）向客户端发送“心跳”。心跳只是作为响应块发送的换行符。这是为了保持线路繁忙，以便我们通知连接丢失。

客户端正确关闭时没有问题。但是当它被强制关闭时（例如，客户端计算机断电），则不会发送TCP重置。在这种情况下，服务器发送心跳，客户端不会对其进行确认。此后，在放弃并向应用程序层（我们的HTTP服务器）报告故障之后，服务器将继续重传数据包大约15分钟。在我的情况下，等待15分钟太长了。

我可以通过写入以下文件来控制重传时间/proc/sys/net/ipv4/：

tcp_retries1 - INTEGER
    This value influences the time, after which TCP decides, that
    something is wrong due to unacknowledged RTO retransmissions,
    and reports this suspicion to the network layer.
    See tcp_retries2 for more details.

    RFC 1122 recommends at least 3 retransmissions, which is the
    default.

tcp_retries2 - INTEGER
    This value influences the timeout of an alive TCP connection,
    when RTO retransmissions remain unacknowledged.
    Given a value of N, a hypothetical TCP connection following
    exponential backoff with an initial RTO of TCP_RTO_MIN would
    retransmit N times before killing the connection at the (N+1)th RTO.

    The default value of 15 yields a hypothetical timeout of 924.6
    seconds and is a lower bound for the effective timeout.
    TCP will effectively time out at the first RTO which exceeds the
    hypothetical timeout.

    RFC 1122 recommends at least 100 seconds for the timeout,
    which corresponds to a value of at least 8.

默认值tcp_retries2的确是8，我15分钟（900秒）的重传时间与上面引用的内核文档一致。

tcp_retries2例如，如果我将值更改为5，则连接死亡的速度更快。但是像这样设置它会影响系统中的所有连接，我真的很想只为这个长轮询连接设置它。

引用RFC 1122：

4.2.3.5  TCP Connection Failures

   Excessive retransmission of the same segment by TCP
   indicates some failure of the remote host or the Internet
   path.  This failure may be of short or long duration.  The
   following procedure MUST be used to handle excessive
   retransmissions of data segments [IP:11]:

   (a)  There are two thresholds R1 and R2 measuring the amount
        of retransmission that has occurred for the same
        segment.  R1 and R2 might be measured in time units or
        as a count of retransmissions.

   (b)  When the number of transmissions of the same segment
        reaches or exceeds threshold R1, pass negative advice
        (see Section 3.3.1.4) to the IP layer, to trigger
        dead-gateway diagnosis.

   (c)  When the number of transmissions of the same segment
        reaches a threshold R2 greater than R1, close the
        connection.

   (d)  An application MUST be able to set the value for R2 for
        a particular connection.  For example, an interactive
        application might set R2 to "infinity," giving the user
        control over when to disconnect.

   (e)  TCP SHOULD inform the application of the delivery
        problem (unless such information has been disabled by
        the application; see Section 4.2.4.1), when R1 is
        reached and before R2.  This will allow a remote login
        (User Telnet) application program to inform the user,
        for example.

在我看来，这tcp_retries1和tcp_retries2在Linux中对应于R1与R2在RFC。该RFC明确指出（在d）项，一个符合标准的实现必须允许设置的值R2，但我一直在使用发现没有办法做到这一点setsockopt()，ioctl()还是这样。

另一种选择是在R1超出时获得通知（项目e）。但是，这不如设置好R2，因为我认为R1很快（在几秒钟内）就会被击中，并且R1无法为每个连接设置的值，或者至少RFC不需要它。