Linux 操作系統中基于 TCP keep-alive的超時檢測和保活機制
作者：IT明哥
如果執行 JAVA 程序時，遇到如下錯誤，Exception in thread "main" java.lang.NoSuchFieldError: TCP_KEEPIDLE，通常是因為使用的 JDK 版本不支持 TCP_KEEPIDLE等jdk.net.ExtendedSocketOptions 擴展選項， 這些擴展選項是 Java 9 才正式引入的，只有部分版本的Java 8 支持該選項。
1. OBPROXY 基于TCP keep-alive的超時檢測和保活機制
- 在分析某數據包時，我們發現，針對空閑的TCP連接，obproxy 服務端每隔5分鐘都會發送一些特殊的 tcp數據包，這些數據包在wireshark中顯示為 [TCP KEEP-ALIVE]，其示例如下：：
- 這些數據包其實就是大名鼎鼎的tcp keep-alive 心跳包。而 obproxy 之所以會發送這些心跳包，其實是因為obproxy 開啟了其SO_KEEPALIVE 選項(keep-alive packets are sent only when the SO_KEEPALIVE socket option is enabled)，并使用了 LINUX 操作系統的 socket 套接字級別的基于 tcp keep-alive的超時檢測和保活機制，該機制的詳情見后文。
- obproxy相關參數和配置方式如下：
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;2. LINUX基于TCP keep-alive的超時檢測和保活機制
Linux操作系統中，基于TCP keep-alive的超時檢測和保活機制，分為兩個層面，一個是操作系統級別的，一個是 socket 套接字級別的。
2.1. LINUX中操作系統級別的基于TCP keep-alive的超時檢測和保活機制
操作系統級別的，基于TCP keep-alive的超時檢測和保活機制，主要跟以下幾個內核參數相關，可以在操作系統層面，通過 sysctl 命令查看和更改這些內核參數：
- /proc/sys/net/ipv4/tcp_keepalive_intvl: 默認 75秒，The number of seconds between TCP keep-alive probes；
- /proc/sys/net/ipv4/tcp_keepalive_probes: 默認 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: 默認 7200 秒即2小時，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套接字級別的基于TCP keep-alive的超時檢測和保活機制
socket 套接字級別的，基于TCP keep-alive的超時檢測和保活機制，則需要相關應用在其代碼中，指定如下這些 socket 套接字選項，事實上 obproxy 就是利用了該機制：
- 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端口級別的基于TCP keep-alive的超時檢測和保活機制
JDK11及之后的版本，也支持socket端口級別的，基于TCP keep-alive的超時檢測和保活機制配置，事實上大部分 JDK8 版本，也在代碼層面通過 backport 支持了該機制，相關源碼如下：
- 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.
圖片
圖片
注意：如果執行 JAVA 程序時，遇到如下錯誤，Exception in thread "main" java.lang.NoSuchFieldError: TCP_KEEPIDLE，通常是因為使用的 JDK 版本不支持 TCP_KEEPIDLE等jdk.net.ExtendedSocketOptions 擴展選項， 這些擴展選項是 Java 9 才正式引入的，只有部分版本的Java 8 支持該選項。
4. 參考鏈接
責任編輯：武曉燕
來源：
明哥的IT隨筆
