4. Protocol Requirements

As discussed in Section 1, IPv6 nodes are not required to implement Path MTU Discovery. The requirements in this section apply only to those implementations that include Path MTU Discovery.

Nodes should appropriately validate the payload of ICMPv6 PTB messages to ensure these are received in response to transmitted traffic (i.e., a reported error condition that corresponds to an IPv6 packet actually sent by the application) per [ICMPv6].

If a node receives a Packet Too Big message reporting a next-hop MTU that is less than the IPv6 minimum link MTU, it must discard it. A node must not reduce its estimate of the Path MTU below the IPv6 minimum link MTU on receipt of a Packet Too Big message.

When a node receives a Packet Too Big message, it must reduce its estimate of the PMTU for the relevant path, based on the value of the MTU field in the message. The precise behavior of a node in this circumstance is not specified, since different applications may have different requirements, and since different implementation architectures may favor different strategies.

After receiving a Packet Too Big message, a node must attempt to avoid eliciting more such messages in the near future. The node must reduce the size of the packets it is sending along the path. Using a PMTU estimate larger than the IPv6 minimum link MTU may continue to elicit Packet Too Big messages. Because each of these messages (and the dropped packets they respond to) consume network resources, nodes using Path MTU Discovery must detect decreases in PMTU as fast as possible.

Nodes may detect increases in PMTU, but because doing so requires sending packets larger than the current estimated PMTU, and because the likelihood is that the PMTU will not have increased, this must be done at infrequent intervals. An attempt to detect an increase (by sending a packet larger than the current estimate) must not be done less than 5 minutes after a Packet Too Big message has been received for the given path. The recommended setting for this timer is twice its minimum value (10 minutes).

A node must not increase its estimate of the Path MTU in response to the contents of a Packet Too Big message. A message purporting to announce an increase in the Path MTU might be a stale packet that has been floating around in the network, a false packet injected as part of a denial-of-service (DoS) attack, or the result of having multiple paths to the destination, each with a different PMTU.