4. Requirements

All links MUST enforce their MTU: links that might non-deterministically deliver packets that are larger than their rated MTU MUST consistently discard such packets.

All hosts SHOULD use IPv4 fragmentation in a mode that mimics IPv6 functionality. All fragmentation SHOULD be done on the host, and all IPv4 packets, including fragments, SHOULD have the DF bit set such that they will not be fragmented (again) in the network.

The requirements below only apply to those implementations that include PLPMTUD.

To use PLPMTUD, a Packetization Layer MUST have a loss reporting mechanism that provides the sender with timely and accurate indications of which packets were lost in the network.

Normal congestion control algorithms MUST remain in effect under all conditions except when only an isolated probe packet is detected as lost. In this case alone, the normal congestion (window or data rate) reduction SHOULD be suppressed. If any other data loss is detected, standard congestion control MUST take place.

Suppressed congestion control MUST be rate limited such that it occurs less frequently than the worst-case loss rate for TCP congestion control at a comparable data rate over the same path.

Whenever the MTU is raised, the congestion state variables MUST be rescaled so as not to raise the window size in bytes (or data rate in bytes per seconds).

Whenever the MTU is reduced (e.g., when processing ICMP PTB messages), the congestion state variable SHOULD be rescaled so as not to raise the window size in packets.

If PLPMTUD updates the MTU for a particular path, all Packetization Layer sessions that share the path representation SHOULD be notified to make use of the new MTU and make the required congestion control adjustments.

All implementations MUST include mechanisms for applications to selectively transmit packets larger than the current effective Path MTU, but smaller than the first-hop link MTU.

Implementations MAY use different heuristics to select the initial effective Path MTU for each protocol.