Path MTU

Problems Related to the MTU

During transmission, IPv6 packets cannot be fragmented on the transit node. The packet length is thus often greater than the path MTU (PMTU). The source node then needs to retransmit the IPv6 packets continuously. This reduces transmission efficiency. If the source node uses the minimum IPv6 MTU of 1280 bytes as the maximum fragment length, in most cases, the PMTU is greater than the minimum IPv6 MTU of the link, and the fragments sent by a node are always smaller than the PMTU. As a result, network resources are wasted. The PMTU discovery protocol is introduced to solve this problem.

Principle of the Path MTU

PMTU is the process of discovering the minimum IPv6 MTU on the path from the source to the destination. PMTU discovery describes a method of dynamically discovering the PMTU for a path. When an IPv6 node sends a great deal of data to another node, data is transmitted through a series of IPv6 fragments. When these fragments are of the maximum length allowed in successful transmission between the source node and destination node, the fragment length is considered optimal and called PMTU.

A source node assumes that a PMTU of a path is the known IPv6 MTU of the first hop on the path. If the packet sent from this path is too large to be forwarded along the path, the transit node discards this packet and returns an ICMPv6 Datagram Too Big message to the source node. The source node then sets the PMTU of the path according to the IPv6 MTU in the message.

When the PMTU learned by the node is smaller than or equal to the actual PMTU, the PMTU discovery process is complete. Before the PMTU discovery process is complete, ICMPv6 Datagram Too Big messages may be repeatedly sent and received because smaller IPv6 MTUs may be found on farther paths.

As shown in Figure 1, the PMTU discovery works as follows:

• A source host first sends a packet to the destination host with its MTU value.
• If an intermediate device returns an error message that contains the network MTU value, the source host resends this packet with this network MTU value.
• The process is repeated until the destination host receives this packet. In this manner, the maximum size of a packet that is processed between two nodes on the network can be determined.

Figure 1: Working procedure of PMTU discovery