2. TCP keepalive overview

In order to understand what TCP keepalive (which we will just call keepalive) does, you need do nothing more than read the name: keep TCP alive. This means that you will be able to check your connected socket (also known as TCP sockets), and determine whether the connection is still up and running or if it has broken.

2.1. What is TCP keepalive?

The keepalive concept is very simple: when you set up a TCP connection, you associate a set of timers. Some of these timers deal with the keepalive procedure. When the keepalive timer reaches zero, you send your peer a keepalive probe packet with no data in it and the ACK flag turned on. You can do this because of the TCP/IP specifications, as a sort of duplicate ACK, and the remote endpoint will have no arguments, as TCP is a stream-oriented protocol. On the other hand, you will receive a reply from the remote host (which doesn't need to support keepalive at all, just TCP/IP), with no data and the ACK set.

If you receive a reply to your keepalive probe, you can assert that the connection is still up and running without worrying about the user-level implementation. In fact, TCP permits you to handle a stream, not packets, and so a zero-length data packet is not dangerous for the user program.

This procedure is useful because if the other peers lose their connection (for example by rebooting) you will notice that the connection is broken, even if you don't have traffic on it. If the keepalive probes are not replied to by your peer, you can assert that the connection cannot be considered valid and then take the correct action.

2.2. Why use TCP keepalive?

You can live quite happily without keepalive, so if you're reading this, you may be trying to understand if keepalive is a possible solution for your problems. Either that or you've really got nothing more interesting to do instead, and that's okay too. :)

Keepalive is non-invasive, and in most cases, if you're in doubt, you can turn it on without the risk of doing something wrong. But do remember that it generates extra network traffic, which can have an impact on routers and firewalls.

In short, use your brain and be careful.

In the next section we will distinguish between the two target tasks for keepalive:

  • Checking for dead peers

  • Preventing disconnection due to network inactivity

2.3. Checking for dead peers

Keepalive can be used to advise you when your peer dies before it is able to notify you. This could happen for several reasons, like kernel panic or a brutal termination of the process handling that peer. Another scenario that illustrates when you need keepalive to detect peer death is when the peer is still alive but the network channel between it and you has gone down. In this scenario, if the network doesn't become operational again, you have the equivalent of peer death. This is one of those situations where normal TCP operations aren't useful to check the connection status.

Think of a simple TCP connection between Peer A and Peer B: there is the initial three-way handshake, with one SYN segment from A to B, the SYN/ACK back from B to A, and the final ACK from A to B. At this time, we're in a stable status: connection is established, and now we would normally wait for someone to send data over the channel. And here comes the problem: unplug the power supply from B and instantaneously it will go down, without sending anything over the network to notify A that the connection is going to be broken. A, from its side, is ready to receive data, and has no idea that B has crashed. Now restore the power supply to B and wait for the system to restart. A and B are now back again, but while A knows about a connection still active with B, B has no idea. The situation resolves itself when A tries to send data to B over the dead connection, and B replies with an RST packet, causing A to finally to close the connection.

Keepalive can tell you when another peer becomes unreachable without the risk of false-positives. In fact, if the problem is in the network between two peers, the keepalive action is to wait some time and then retry, sending the keepalive packet before marking the connection as broken.

    _____                                                     _____
   |     |                                                   |     |
   |  A  |                                                   |  B  |
   |_____|                                                   |_____|
      ^                                                         ^
      |--->--->--->-------------- SYN -------------->--->--->---|
      |---<---<---<------------ SYN/ACK ------------<---<---<---|
      |--->--->--->-------------- ACK -------------->--->--->---|
      |                                                         |
      |                                       system crash ---> X
      |                                     system restart ---> ^
      |                                                         |
      |--->--->--->-------------- PSH -------------->--->--->---|
      |---<---<---<-------------- RST --------------<---<---<---|
      |                                                         |

2.4. Preventing disconnection due to network inactivity

The other useful goal of keepalive is to prevent inactivity from disconnecting the channel. It's a very common issue, when you are behind a NAT proxy or a firewall, to be disconnected without a reason. This behavior is caused by the connection tracking procedures implemented in proxies and firewalls, which keep track of all connections that pass through them. Because of the physical limits of these machines, they can only keep a finite number of connections in their memory. The most common and logical policy is to keep newest connections and to discard old and inactive connections first.

Returning to Peers A and B, reconnect them. Once the channel is open, wait until an event occurs and then communicate this to the other peer. What if the event verifies after a long period of time? Our connection has its scope, but it's unknown to the proxy. So when we finally send data, the proxy isn't able to correctly handle it, and the connection breaks up.

Because the normal implementation puts the connection at the top of the list when one of its packets arrives and selects the last connection in the queue when it needs to eliminate an entry, periodically sending packets over the network is a good way to always be in a polar position with a minor risk of deletion.

    _____           _____                                     _____
   |     |         |     |                                   |     |
   |  A  |         | NAT |                                   |  B  |
   |_____|         |_____|                                   |_____|
      ^               ^                                         ^
      |--->--->--->---|----------- SYN ------------->--->--->---|
      |---<---<---<---|--------- SYN/ACK -----------<---<---<---|
      |--->--->--->---|----------- ACK ------------->--->--->---|
      |               |                                         |
      |               | <--- connection deleted from table      |
      |               |                                         |
      |--->- PSH ->---| <--- invalid connection                 |
      |               |                                         |

TCP keepalive overview的更多相关文章

  1. TCP keepalive under Linux

    TCP Keepalive HOWTO Prev   Next 3. Using TCP keepalive under Linux Linux has built-in support for ke ...

  2. TCP keepalive

      2. TCP keepalive overview In order to understand what TCP keepalive (which we will just call keepa ...

  3. TCP Keepalive HOWTO

    TCP Keepalive HOWTO Fabio Busatto <fabio.busatto@sikurezza.org> 2007-05-04 Revision History Re ...

  4. 【转载】TCP保活(TCP keepalive)

    下图是我遇到tcp keepalive的例子: 以下为转载: TCP保活的缘起 双方建立交互的连接,但是并不是一直存在数据交互,有些连接会在数据交互完毕后,主动释放连接,而有些不会,那么在长时间无数据 ...

  5. TCP连接探测中的Keepalive和心跳包. 关键字: tcp keepalive, 心跳, 保活

    1. TCP保活的必要性 1) 很多防火墙等对于空闲socket自动关闭 2) 对于非正常断开, 服务器并不能检测到. 为了回收资源, 必须提供一种检测机制. 2. 导致TCP断连的因素 如果网络正常 ...

  6. 【 总结 】Tcp Keepalive 和 HTTP Keepalive 详解

    TCP Keepalive Tcp keepalive的起源          双方建立交互的连接,但是并不是一直存在数据交互,有些连接会在数据交互完毕后,主动释放连接,而有些不会,那么在长时间无数据 ...

  7. tcp keepalive选项

    之前一直对tcp keepalive选项理解有误, 以为通过setsockopt函数设置SO_KEEPALIVE和相关参数后该socket则使用设置的keepalive相关参数 否则使用系统默认的:k ...

  8. TCP长连接保持连接状态TCP keepalive设置

    如有转载,请注明出处:http://blog.csdn.net/embedded_sky/article/details/42077321 作者:super_bert@csdn 对于TCP长连接保活是 ...

  9. TCP keep-alive翻译

    原文链接: http://www.freesoft.org/CIE/RFC/1122/114.htmhttp://www.freesoft.org/CIE/RFC/1122/index.htm 实现T ...


  1. 【JUC】JDK1.8源码分析之ThreadPoolExecutor(一)

    一.前言 JUC这部分还有线程池这一块没有分析,需要抓紧时间分析,下面开始ThreadPoolExecutor,其是线程池的基础,分析完了这个类会简化之后的分析,线程池可以解决两个不同问题:由于减少了 ...

  2. EF实体框架数据操作基类(转)

    //----------------------------------------------------------------// Copyright (C) 2013 河南禄恒软件科技有限公司 ...

  3. [OC笔记] protocol之我的见解

    OC中的protocol就是和JAVA中interface差不多的东西,但是又不是完全一样的.这个protocol常用来实现委托,也就是自己不实现,当事件产生的时候去回调委托者. 让委托者去执行响应的 ...

  4. Spring - 基于注解的组件扫描

    关于Spring的书籍都会花很大篇幅来讲解Spring如何注入各种bean的问题,非常令人头疼,自己在工作中还从来没有用到过. 所以就要跳过那些篇章,直接学习基于注解的组件扫描. 发现spring2是 ...

  5. JDBCTemplate基础学习

    JDBCTemplate:spring提供的用于操作数据库的模板,类似DbUtils.使用时必须设置数据源(DataSource):数据源如DBCP.C3P0等 一.JDBCAPI简单使用Demo 1 ...

  6. 自己遇到的Android虚拟机出现的错误及解决方法【不断更新】

    2012.11.9 第一个: [2012-11-09 13:15:14 - Tesa] Android Launch! [2012-11-09 13:15:14 - Tesa] The connect ...

  7. link与@import

    导入外部样式的两种写法 <link rel="stylesheet" href="xxxx.css"> <style> @import ...

  8. struts2笔记01-环境搭建

    1.官网下载struts2 struts-2.3.28-all.zip,这个包可谓应有尽有,以后全靠它了! 2.jar包怎么选?       (1)struts-2.3.28-all\struts-2 ...

  9. AngularJs的resource服务与Rest服务交互

    前言以后补: * 在使用resource服务返回的资源对象后具有与后台数据交互的五大接口:save query delete remove get 五种默认行为: { "get": ...

  10. 网站开发进阶(十五)JS基础知识充电站

    JS基础知识充电站 1.javascript alert弹出对话框时确定和取消两个按钮返回值? 用的不是alert对话框,是confirm confirm(str); 参数str:你要说的话或问题: ...