This article describes how to enable Citrix SD-WAN auto MTU detect feature.

Background

• Adding headers to packets affect the maximum transmission (MTU) size
• Downstream devices may do their own encapsulation as well (e.g. VPNs)
• The per packet delivery that the Virtual WAN provides is accomplished by adding headers to packets that affect the maximum transmission (MTU) size for user IP datagrams.  In addition, depending on the environment, it is possible downstream devices may do their own encapsulation as well.  As an example scenario our Virtual Path encapsulated UDP packet sent through a third party VPN tunnel.  The Virtual WAN has auto MTU discovery feature that can be enabled to detect the maximum size packet that can be sent across each WAN Path that make up the Virtual Path.  Once MTU is detected, the Citrix SD-WAN can act on the MTU size in different ways depending on the protocol.  If a TCP session is initiated, the Citrix SD-WAN adjusts the TCP headers (MSS) to reflect the transmission size ability.  This will prevent oversized packets from being sent to the WAN afterward, but this does not handle all situations. 
• It is possible non-TCP traffic, or even TCP traffic, to send oversized packets if the MTU changed after the initial session handshake was performed.  In this case, the Citrix SD-WAN segment packets prior to sending and reassembles them upon reception at the partner device.  Although the Citrix SD-WAN does fragmentation of large packets and reassembles them prior to transmission between nodes, the Citrix SD-WAN does not perform IP fragmentation. 
• Citrix SD-WAN auto MTU discovery feature has the following benefits:
  • Auto detects the maximum size packet that can be sent across each WAN Path
  • Once MTU is detected, the Citrix SD-WAN can act on the MTU size
  • Citrix SD-WAN adjusts the TCP headers (MSS) to reflect the transmission size ability.
  • When oversized packets is still encountered, Citrix SD-WAN segment packets prior to sending and reassembles them upon reception at the partner device.  Although the Citrix SD-WAN does fragmentation of large packets and reassembles them prior to transmission between nodes, the Citrix SD-WAN does not perform IP fragmentation.
  • If the packet is fragmented before Citrix SD-WAN touches it, it will be delivered to the other side of the Citrix SD-WAN tunnel in the same fragmented state.

Instructions

Complete the following steps to enable Citrix SD-WAN auto MTU detect feature:
Note: This setting should be adjusted on both site node’s WAN link

Versions pre-9.3.x and 9.3.x:

Versions 10.x

• CTX115434 - How to Find Maximum Size of IP Data Payload that can Traverse WAN Environment Without Fragmentation