PMTU Discovery

PMTU Discovery is a diagnostic tool that determines the maximum transmission unit (MTU) on the network path between the SonicWall security appliance and a remote host. It is used to avoid IP fragmentation of traffic between the two hosts.

For IPv4 packets, Path MTU Discovery works by setting the "Don't Fragment" (DF) option bit in the IP headers of outgoing packets. When the DF option bit is set for a packet, and the packet traverses a device with an MTU smaller than the packet size, the device drops the packet and sends back an ICMP Fragmentation Needed message containing its MTU, allowing the source host to reduce its Path MTU appropriately. The process repeats until the MTU is small enough to traverse the entire path without fragmentation. IPv6 functions similarly, but the DF option bit is not required. IPv6 devices automatically send an ICMPv6 Packet Too Big message if the packet exceeds the devices MTU size.

By determining the MTU size on a network path and configuring the MTU for your SonicWall security appliance below the path MTU size, you avoid the potential delay caused by negotiation of the MTU size and other MTU-related network issues.

To configure Path MTU Discovery:
1
On the System > Diagnostics page, select PMTU Discovery for the Diagnostic Tool.

2
In the Path MTU Discovery to this host or IP address, enter the IP address or host name that you want to measure the Path MTU for. This can be either an IPv4 or IPv6 address.
3
Optionally, in the Interface drop-down menu, you can select one of the configured WAN interfaces on the appliance to check the Path MTU for that interface. When the Interface drop-down menu is set to ANY, the appliance chooses among all of its interfaces.
4
Click Go. The Path MTU Discovery results are displayed in a pop-up window.
 
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NOTE: If you do not see this window, ensure your browser allows pop-ups for the SonicWall GUI.

The following example shows the Path MTU Discovery for the route between 192.168.168.168 and 58.63.236.236. The smallest MTU is 1492 bytes between 9.9.9.8 and 0.103.48.1.