TCP will keep you alive

Recently I took some time looking into TCP keep-alive and how it works in a Windows environment. Seeing that I already had a summary written on the subject, I’ve decided this is an opportunity for a blog post.

So, here we go…

Applications are often client-server based. In some cases you’ll want the client to keep the connection to the server open, even if it’s been idle for a while. Usually, you’ll want that if making a new connection is too “costly”, being it time or performance or any other consideration.

Connections are usually interrupted by-design, considering servers usually have a timeout period set, after which they close an idle connection. This mechanism is in place to make sure connections are closed when there is no longer a need for them and the client didn’t take the initiative to close the connection.

There are also cases, when connections are interrupted by a third-party for other reasons. One example of this is firewalls. Firewalls have a security mechanism in place, which closes stale connections to make sure they will not be exploited for some sort of an attack.

Whatever the reason might be, if you find that you have a good enough reason to keep a connection alive, TCP keep-alive is one way to go at it.

A good explanation of TCP keep-alive can be found here: http://msdn.microsoft.com/en-us/library/aa925764.aspx

How TCP Keep-alive works –

First and foremost, TCP Keep-alive is not enabled by default. In order for it to be enabled, you have to enable it in the application layer, meaning you need an application to access this feature.

You can do this by one of the following:

·        setsockopt() with SO_KEEPALIVE option

·        WSAIoctl() with SIO_KEEPALIVE_VALS option  

Or, if you’re using a .NET application:

·        SetSocketOption method from Socket Class in System.Net.Sockets namespace

·        GetSocketOption method from Socket Class in System.Net.Sockets namespace

Otherwise, TCP Keep-alive will not be used.

In windows (2008/Vista and above) TCP Keep alive has 2 registry keys that influence its behavior:

1.      HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\KeepAliveTime

This key is a DWORD measured in milliseconds. It controls how often a TCP connections attempts to verify that an idle connection is still intact. It does so by sending a keep-alive packet.

2.      HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\KeepAliveInterval

This key is also a DWORD measured in milliseconds. It controls how long to wait for a response for the keep-alive packet before sending a retransmission. Once a response is received, keep-alive intervals go back to what was defined in the KeepAliveTime value. The connection will be aborted after 10 failed retransmissions (this number is hard-coded and can’t be modified).

To add to all of this, it’s important to underline that TCP keep-alive is a packet that contains null data, so its impact on the network traffic is minimal. Therefore, its bandwidth usage can and should be neglected.

If your application supports TCP keep-alive it will use it if the values above are configured. To make sure that this is the case, contact the vendor/owner of the application in question. Otherwise, you’ll need to modify to source code in order to accomplish the use of TCP keep-alive.

Until next time,

Dani .H