Forum Discussion

brad_11440's avatar
brad_11440
Icon for Nimbostratus rankNimbostratus
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...
config
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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.
JRahm's avatar
JRahm
Icon for Admin rankAdmin
Jan 16, 2009
"Throttling down" is not a shortcoming in the product, it's really the LTM's tcp stack complying with the configuration in the system. I had a table in my tech tip on windows and buffers (http://devcentral.f5.com/Default.aspx?tabid=63&articleType=ArticleView&articleId=287 Click here) that presents a table that shows the practical max throughput numbers at various window sizes and latencies, and it's really shocking how poor throughput is when latency grows even a little bit, and that's not even taking into account loss. A small congestion window can really hamper a tcp sender's ability to deliver data due to the nature of tcp being a reliable transport protocol, which requires feedback from the receiver before pushing too much data at once.