Avi Load balancer's TCP 'auto_window_growth' does not work without TCP timestamps option from the client

Products

• VMware Avi Load Balancer

Issue/Introduction

Avi's TCP network profile has an 'Auto learn' option for buffer management where the receive window advertised to the client and server dynamically changes.

• Reference KB - https://docs.vmware.com/en/VMware-NSX-Advanced-Load-Balancer/30.1/Configuration-Guide/GUID-D7FD197F-4E38-4D9E-B797-451B42140F7E.html#auto-learn-1 

There is a limitation on Avi that if the incoming client packets do not contain the TCP timestamp option, this feature does not work and may impact the virtual service performance.

• RFC for TCP timestamp option - https://www.ietf.org/rfc/rfc1323.txt 

Resolution

The workaround is two-part and is as follows -

1. For the 'Receive window' in the TCP profile, select 'Custom' mode and configure a static value between 32KB and 65536 KB as per the application requirements. 
   • This can be done via the UI. Reference KB - https://docs.vmware.com/en/VMware-NSX-Advanced-Load-Balancer/30.1/Configuration-Guide/GUID-35E9B9F0-9647-405E-A6A4-0A3F729EE3A1.html#buffer-management-parameters-2 
2. Disable 'auto_window_growth'. This can only be done via the CLI. Below are the steps for the same -

   > configure networkprofile <tcp-profile-name>
   > profile tcp_proxy_profile 
   > no auto_window_growth
   > save
   > save

 

The fix for this limitation is being worked upon and will be addressed in future Avi releases. There is no timeline or target version known currently.