DX - Load Balancing & Application Acceleration
DX - Load Balancing & Application Acceleration

SLB active session timeout interval

‎09-02-2008 01:48 PM

We setup Load Balancing for Microsoft Office Communicator.

Client software messaging keeps on disconnecting every 90 seconds and reconnects back.


What I found that seems to explain the disconnect is the Session timers under Default SLB settings.

Change the value from 90 to 600.

Seems to fix it so far.


What issues will expect increasing the default value?

There are other SLB running and this change is global.


DX - Load Balancing & Application Acceleration

Re: SLB active session timeout interval

‎09-03-2008 07:49 AM


The main reason for the session timeout is to be able to clear the inactive sessions from clients that improperly disconnected (people that unplugged their PC before logging out, people that lost their network connection, …). Without it, we'll keep these sessions for ever and our SLB table sessions will grow infinitely.   By setting the session timers higher the sessions will take longer to time out, so the DX will keep track of them which requires memory to be allocated.   You can use 'netstat' on the command line to view the state of connections to the DX.


As well as the global setting the SLB active session timeout can be changed per SLB group.

For the specific applications that require long active timeout (like FTP for instance), you can change the SLB active session timeout from 1 to 604800 (7 days) per SLB group:


In CLI: "set slb group <name> session timeout active [1-604800]"


Or in WebUI: In the SLB Group – "Override Default Settings" section.

DX - Load Balancing & Application Acceleration

Re: SLB active session timeout interval

‎09-03-2008 10:23 AM

Thanks for your response


I don't experience the disconnect from the client messaging anymore since I adjusted from 90 seconds to 600 seconds.

But wouldn't I experience disconnect after 600 seconds?


What about "close wait session timeout interval", is it necessary to adjust this to work hand in hand with Active session timeout?

DX - Load Balancing & Application Acceleration

Re: SLB active session timeout interval

‎09-04-2008 04:01 AM

Yes, with that setting the DX will terminate the session if it is still connected after 600s.  You can set the expiry time for the SLB group up to 604800s (7 days), so depending on the application you can tweak the value to what you expect as the maximum allowed session length.   When the session is terminated a TCP reset is sent by defauly, governed by the DX setting:


% show slb advanced

SLB Advanced Settings
Reset to Client: enabled
Reset to Server: enabled


The other timeout values can also be set per SLB group or globally are:


% show slb session timeout

SLB Session Timeouts (sec)
Active: 150
Close: 12
Ack wait (syn flood): 6


The Close timer is for when a client initiates the session termination, in case the TCP close does not complete gracefully then the DX removes the session after this timeout value, so this has no correlation with the Active session timer and does not ned to be modified if you change the Active timer.

The Ack wait timer is similar but relates to the client initiating a session.  If the client does not send the expected Ack to complete the three-way TCP handshake, the connection is then not in an established state and is taking up resources.   After waiting for the Ack Wait period the embryonic connection is then removed.


Generally these other values can be left as default, if you find that stale connections are building up and causing issues then lowering these values would cause the DX to reap those connections quicker.  Note that lowering the Ack Wait too low could cause 'slow' clients to fail to establish connections.  


As you found out, the main timer is the Active session one which determines the maximum length of time a connection can be in an established state and can need altering if you have long-lived sessions through the SLB group.





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