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brad_11440's avatar
brad_11440
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Jan 15, 2009

FastL4 vs Optimized TCP Profiles

We run active/standby 6400 (platform D63a) LTM's for our production applications. They are accessed via clients on a private network with an average latency around 80ms. Nearly all of our virtual se...
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  • smp_86112's avatar
    smp_86112
    Jan 20, 2009
    Maybe it's about the right time to have you post your VIP config from the bigip.conf. It's possible you have other settings applied to the VIP which conflict with a FastL4 profile??? Also it might be helpful to paste the values of the TCP profile you have applied.

     

     

    Regarding the window size...the behavior you see was bothering me for a long time until it finally clicked in my brain. Yes, the TCP window size always starts at 4380. The trick is to understand whether the LTM is sending or receiving data. When the LTM is sending data, as in this case, there is no need for the LTM to increase its receive window - which is the parameter you refer to in the trace.

     

     

    When the LTM is receiving large amounts of data and you do a trace, you'll see that the LTM receive window starts out at 4380, and quickly increases to the value of the Receive Window as specified in the TCP profile that is applied.

     

     

    The LTM is smart - it starts out with a small window size so it does not need to allocate more memory than necessary for the TCP connection. If it needs to adjust the receive window (when it is receiving data), you'll see it go up.
brad_11440's avatar
brad_11440
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Jan 20, 2009
Well, we made the change this morning. For reasons we are still investigating, after moving to FastL4, the VIP went down and the application stopped working. It did not come back up until we switched over to TCP-LAN-Optimized on both sides (did not try reverting back to the default standard).

 

 

After moving to this optimized profile, we've seen roughly a 100ms decrease in Data Transfer Time (the time between the first reply sent by the server and the last packet sent to fufill the request). The server response time has dropped by about 30ms. I was expecting this to drop further but it's probably due to the application being poorly written.

 

 

One question I do have is in regards to the Server TCP window size. I ran a packet trace after the change and when the server responds, the initial TCP window size is always 4380. The client's initial request is always at 65535. By using the TCP Lan Optimized profile, the F5 is acting as a proxy and negotiating the TCP parameters, correct ? I was under the impression that when others here have made this same change, they noticed the TCP window size opened up on the server side. Any ideas ?