Linux 操作系统中基于 TCP keep-alive的超时检测和保活机制-51CTO.COM

1. OBPROXY 基于TCP keep-alive的超时检测和保活机制

在分析某数据包时，我们发现，针对空闲的TCP连接，obproxy 服务端每隔5分钟都会发送一些特殊的 tcp数据包，这些数据包在wireshark中显示为 [TCP KEEP-ALIVE]，其示例如下：：
这些数据包其实就是大名鼎鼎的tcp keep-alive 心跳包。而 obproxy 之所以会发送这些心跳包，其实是因为obproxy 开启了其SO_KEEPALIVE 选项(keep-alive packets are sent only when the SO_KEEPALIVE socket option is enabled)，并使用了 LINUX 操作系统的 socket 套接字级别的基于 tcp keep-alive的超时检测和保活机制，该机制的详情见后文。
obproxy相关参数和配置方式如下：

alter proxyconfig set server_tcp_user_timeout = 0;
alter proxyconfig set sock_option_flag_out = 3;
alter proxyconfig set server_tcp_keepidle = 5;
alter proxyconfig set server_tcp_keepintvl = 5;
alter proxyconfig set server_tcp_keepcnt = 2;
alter proxyconfig set client_sock_option_flag_out = 3;
alter proxyconfig set client_tcp_keepidle = 5;
alter proxyconfig set client_tcp_keepintvl = 5;
alter proxyconfig set client_tcp_keepcnt = 2;
alter proxyconfig set client_tcp_user_timeout = 0;
alter proxyconfig set skip_proxyro_check=true;1.
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2. LINUX基于TCP keep-alive的超时检测和保活机制

Linux操作系统中，基于TCP keep-alive的超时检测和保活机制，分为两个层面，一个是操作系统级别的，一个是 socket 套接字级别的。

2.1. LINUX中操作系统级别的基于TCP keep-alive的超时检测和保活机制

操作系统级别的，基于TCP keep-alive的超时检测和保活机制，主要跟以下几个内核参数相关，可以在操作系统层面，通过 sysctl 命令查看和更改这些内核参数：

/proc/sys/net/ipv4/tcp_keepalive_intvl: 默认 75秒，The number of seconds between TCP keep-alive probes；
/proc/sys/net/ipv4/tcp_keepalive_probes: 默认 9 次，The maximum number of TCP keep-alive probes to send before giving up and killing the connection if no response is obtained from the other end；
/proc/sys/net/ipv4/tcp_keepalive_time: 默认 7200 秒即2小时，The number of seconds a connection needs to be idle before TCP begins sending out keep-alive probes. Keep-alives are sent only when the SO_KEEPALIVE socket option is enabled. An idle connection is terminated after approximately an additional 11 minutes (9 probes an interval of 75 seconds apart) when keep-alive is enabled；
sysctl net.ipv4.tcp_keepalive_time
sysctl net.ipv4.tcp_keepalive_intvl
sysctl net.ipv4.tcp_keepalive_probes

2.2. LINUX中socket套接字级别的基于TCP keep-alive的超时检测和保活机制

socket 套接字级别的，基于TCP keep-alive的超时检测和保活机制，则需要相关应用在其代码中，指定如下这些 socket 套接字选项，事实上 obproxy 就是利用了该机制：

TCP_KEEPIDLE：the amount of time until the first keepalive packet is sent；
TCP_KEEPCNT：the number of probes to send；
TCP_KEEPINTVL：the interval between keepalive packets；

3. JAVA中如何指定 socket端口级别的基于TCP keep-alive的超时检测和保活机制

JDK11及之后的版本，也支持socket端口级别的，基于TCP keep-alive的超时检测和保活机制配置，事实上大部分 JDK8 版本，也在代码层面通过 backport 支持了该机制，相关源码如下：

java.net.StandardSocketOptions
java.net.StandardSocketOptions#SO_KEEPALIVE
jdk.net.ExtendedSocketOptions
When the SO_KEEPALIVE option is enabled, TCP probes a connection that has been idle for some amount of time. The default value for this idle period is 2 hours which is too long for most applications. The TCP_KEEPIDLE, TCP_KEEPCOUNT, TCP_KEEPINTERVAL option can be used to affect this value for a given socket.
The default idle time for SO_KEEPALIVE is 2 hours, too long for most applications. Some operation systems have support to configure the idle time on a per connection basis (Linux has TCP_KEEPIDLE, Windows has SIO_KEEPALIVE_VALS). We should consider exposing an extended socket option to configure this.
TCP_KEEPIDLE, TCP_KEEPCOUNT, and TCP_KEEPINTERVAL are non-standard socket options supported on several platforms to provide fine control over the TCP/IP keep alive mechanism. It should be possible to set these socket options via the setOption method defined by java.net.Socket and java.nio.channels.SocketChannel.
Add a JDK-specific socket option that supports setting TCP_KEEPIDLE, TCP_KEEPCOUNT, TCP_KEEPINTERVAL, on platforms that support it. The option can be set/get through the existing set/getOption methods on Socket and NetworkChannel.

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注意：如果执行 JAVA 程序时，遇到如下错误，Exception in thread "main" java.lang.NoSuchFieldError: TCP_KEEPIDLE，通常是因为使用的 JDK 版本不支持 TCP_KEEPIDLE等jdk.net.ExtendedSocketOptions 扩展选项，这些扩展选项是 Java 9 才正式引入的，只有部分版本的Java 8 支持该选项。

Linux 操作系统中基于 TCP keep-alive的超时检测和保活机制

1. OBPROXY 基于TCP keep-alive的超时检测和保活机制

2. LINUX基于TCP keep-alive的超时检测和保活机制

2.1. LINUX中操作系统级别的基于TCP keep-alive的超时检测和保活机制

2.2. LINUX中socket套接字级别的基于TCP keep-alive的超时检测和保活机制

3. JAVA中如何指定 socket端口级别的基于TCP keep-alive的超时检测和保活机制

4. 参考链接