Linux上TCP重传的应用控制 [英] Application control of TCP retransmission on Linux

问题描述

对于不耐烦的人:

如何在Linux中为单个连接更改/proc/sys/net/ipv4/tcp_retries2的值，使用setsockopt()，ioctl() 之类的，或者有可能吗?

How to change the value of /proc/sys/net/ipv4/tcp_retries2 for a single connection in Linux, using setsockopt(), ioctl() or such, or is it possible?

更长的描述:

我正在开发一个使用长轮询 HTTP 请求的应用程序.在服务器端，需要知道客户端何时关闭了连接.准确度并不重要，但肯定不能是 15 分钟.接近一分钟就可以了.

I'm developing an application that uses long polling HTTP requests. On the server side, it needs to be known when the client has closed the connection. The accuracy is not critical, but it certainly cannot be 15 minutes. Closer to a minute would do fine.

对于那些不熟悉这个概念的人，长轮询 HTTP 请求的工作方式如下:

For those not familiar with the concept, a long polling HTTP request works like this:

• 客户端发送请求
• 服务器使用 HTTP 标头进行响应，但让响应保持打开状态.使用分块传输编码，允许服务器在数据可用时发送位数据.
• 发送完所有数据后，服务器发送一个关闭块"；表示响应已完成.

在我的应用程序中，服务器发送心跳"每隔一次发送给客户端(默认为 30 秒).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.

In my application, the server sends "heartbeats" to the client every now an then (30 seconds by default). A heartbeat is just a newline character that is sent as a response chunk. This is meant to keep the line busy so that we notify the connection loss.

客户端正常关闭就没有问题.但是当它被强制关闭时(例如，客户端机器断电)，不会发送 TCP 重置.在这种情况下，服务器发送一个心跳，客户端不会确认.在此之后，服务器在放弃并向应用层(我们的 HTTP 服务器)报告失败后继续重新传输数据包大约 15 分钟.对我来说，15 分钟的等待时间太长了.

There's no problem when the client shuts down correctly. But when it's shut down with force (the client machine loses power, for example), a TCP reset is not sent. In this case, the server sends a heartbeat, which the client doesn't ACK. After this, the server keeps retransmitting the packet for roughly 15 minutes after giving up and reporting the failure to the application layer (our HTTP server). And 15 minutes is too long a wait in my case.

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

I can control the retransmission time by writing to the following files in /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.

tcp_retries2的默认值确实是8，我重传15分钟(900秒)的经验和上面引用的内核文档是一致的.

The default value of tcp_retries2 is indeed 8, and my experience of 15 minutes (900 seconds) of retransmission is in line with the kernel documentation quoted above.

例如，如果我将 tcp_retries2 的值更改为 5，连接会更快地终止.但是这样设置会影响系统中的所有连接，我真的很想为这个长轮询连接设置它.

If I change the value of tcp_retries2 to 5 for example, the connection dies much more quicker. But setting it like this affects all the connections in the system, and I'd really like to set it for this one long polling connection only.

来自 RFC 1122 的引用:

A quote from 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，Linux 中的 tcp_retries1 和 tcp_retries2 对应于 RFC 中的 R1 和 R2.RFC 清楚地指出(在 d 项中)符合要求的实现必须允许设置 R2 的值，但我发现无法使用 setsockopt()、<代码>ioctl() 之类的.

It seems to me that tcp_retries1 and tcp_retries2 in Linux correspond to R1 and R2 in the RFC. The RFC clearly states (in item d) that a conforming implementation MUST allow setting the value of R2, but I have found no way to do it using setsockopt(), ioctl() or such.

另一种选择是在超过 R1 时收到通知(项目 e).但是，这不如设置 R2 好，因为我认为 R1 很快(几秒钟内)就会命中，并且 R1 的值code> 不能为每个连接设置，或者至少 RFC 不需要它.

Another option would be to get a notification when R1 is exceeded (item e). This is not as good as setting R2, though, as I think R1 is hit pretty soon (in a few seconds), and the value of R1 cannot be set per connection, or at least the RFC doesn't require it.

推荐答案

貌似是在 Kernel 2.6.37 中添加的.从内核 Git 提交差异和摘自下面的更改日志；

Looks like this was added in Kernel 2.6.37. Commit diff from kernel Git and Excerpt from change log below;

提交dca43c75e7e545694a9dd6288553f55c53e2a3a3作者:Jerry Chu日期:2010 年 8 月 27 日星期五 19:13:28 +0000

commit dca43c75e7e545694a9dd6288553f55c53e2a3a3 Author: Jerry Chu Date: Fri Aug 27 19:13:28 2010 +0000

tcp: Add TCP_USER_TIMEOUT socket option.

This patch provides a "user timeout" support as described in RFC793. The
socket option is also needed for the the local half of RFC5482 "TCP User
Timeout Option".

TCP_USER_TIMEOUT is a TCP level socket option that takes an unsigned int,
when > 0, to specify the maximum amount of time in ms that transmitted
data may remain unacknowledged before TCP will forcefully close the
corresponding connection and return ETIMEDOUT to the application. If 
0 is given, TCP will continue to use the system default.

Increasing the user timeouts allows a TCP connection to survive extended
periods without end-to-end connectivity. Decreasing the user timeouts
allows applications to "fail fast" if so desired. Otherwise it may take
upto 20 minutes with the current system defaults in a normal WAN
environment.

The socket option can be made during any state of a TCP connection, but
is only effective during the synchronized states of a connection
(ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, or LAST-ACK).
Moreover, when used with the TCP keepalive (SO_KEEPALIVE) option,
TCP_USER_TIMEOUT will overtake keepalive to determine when to close a
connection due to keepalive failure.

The option does not change in anyway when TCP retransmits a packet, nor
when a keepalive probe will be sent.

This option, like many others, will be inherited by an acceptor from its
listener.

Signed-off-by: H.K. Jerry Chu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

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