2. Overview

Packetization Layer Path MTU Discovery (PLPMTUD) is a method for TCP or other Packetization Protocols to dynamically discover the MTU of a path by probing with progressively larger packets. It is most efficient when used in conjunction with the ICMP-based Path MTU Discovery mechanism as specified in RFC 1191 and RFC 1981, but resolves many of the robustness problems of the classical techniques since it does not depend on the delivery of ICMP messages.

This method is applicable to TCP and other transport- or application-level protocols that are responsible for choosing packet boundaries (e.g., segment sizes) and have an acknowledgment structure that delivers to the sender accurate and timely indications of which packets were lost.

General Strategy

The general strategy is for the Packetization Layer to find an appropriate Path MTU by probing the path with progressively larger packets. If a probe packet is successfully delivered, then the effective Path MTU is raised to the probe size.

The isolated loss of a probe packet (with or without an ICMP Packet Too Big message) is treated as an indication of an MTU limit, and not as a congestion indicator. In this case alone, the Packetization Protocol is permitted to retransmit any missing data without adjusting the congestion window.

If there is a timeout or additional packets are lost during the probing process, the probe is considered to be inconclusive (e.g., the lost probe does not necessarily indicate that the probe exceeded the Path MTU). Furthermore, the losses are treated like any other congestion indication: window or rate adjustments are mandatory per the relevant congestion control standards [RFC2914]. Probing can resume after a delay that is determined by the nature of the detected failure.

Searching Technique

PLPMTUD uses a searching technique to find the Path MTU. Each conclusive probe narrows the MTU search range, either by raising the lower limit on a successful probe or lowering the upper limit on a failed probe, converging toward the true Path MTU. For most transport layers, the search should be stopped once the range is narrow enough that the benefit of a larger effective Path MTU is smaller than the search overhead of finding it.

The most likely (and least serious) probe failure is due to the link experiencing congestion-related losses while probing. In this case, it is appropriate to retry a probe of the same size as soon as the Packetization Layer has fully adapted to the congestion and recovered from the losses. In other cases, additional losses or timeouts indicate problems with the link or Packetization Layer. In these situations, it is desirable to use longer delays depending on the severity of the error.

An optional verification process can be used to detect situations where raising the MTU raises the packet loss rate.