By default, the MSS value is defined as 1380 bytes in a NetScaler SD-WAN appliance, assuming the WAN infrastructure allows a standard IP packet maximum transmission unit (MTU) size of 1,500 bytes. 

Some WAN infrastructures might not provide enough space for MSS of 1380 bytes because additional protocol headers are added to the IP packets (VPDN, VPNs, IPSec). It is recommended to verify the maximum size of IP Data Payload that can traverse WAN environment without fragmentation. Refer to the following article for more information CTX115434 - How to Find Maximum Size of IP Data Payload that can Traverse WAN Environment Without Fragmentation.

Once the correct value is defined, this value should be configured on all NetScaler SD-WAN. This value is not negotiated but only advertised.For example, let us suppose we have the following scenario: 

All NetScaler SD-WAN appliances in branches A, B, C and the DC need to have their MSS configured as 1310 (which is the lowest value of all 3).