IPv6 path MTU discovery
The links that a packet passes from a source to a destination can have different MTUs, among which the minimum MTU is the path MTU. If a packet exceeds the path MTU, the source end fragments the packet to reduce the processing pressure on intermediate devices and to use network resources effectively.
A source end uses path MTU discovery to find the path MTU to a destination, as shown in Figure 81.
Figure 81: Path MTU discovery process
The source host sends a packet no larger than its MTU to the destination host.
If the MTU of a device's output interface is smaller than the packet, the device performs the following operations:
Discards the packet.
Returns an ICMPv6 error message containing the interface MTU to the source host.
Upon receiving the ICMPv6 error message, the source host performs the following operations:
Uses the returned MTU to limit the packet size.
Performs fragmentation.
Sends the fragments to the destination host.
Step 2 and step 3 are repeated until the destination host receives the packet. In this way, the source host finds the minimum MTU of all links in the path to the destination host.