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 55.
The source host sends a packet no larger than its MTU to the destination host.
If the MTU of an intermediate 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.
Figure 55: Path MTU discovery process
