7.1 A Better Way to Detect PMTU Increases

7.1 A Better Way to Detect PMTU Increases

Section 6.3 suggests detecting increases in the PMTU value by periodically increasing the PTMU estimate to the first-hop MTU. Since it is likely that this process will simply "rediscover" the current PTMU estimate, at the cost of several dropped datagrams, it should not be done often.

A better approach is to periodically increase the PMTU estimate to the next-highest value in the plateau table (or the first-hop MTU, if that is smaller). If the increased estimate is wrong, at most one round-trip time is wasted before the correct value is rediscovered. If the increased estimate is still too low, a higher estimate will be attempted somewhat later.

Because it may take several such periods to discover a significant increase in the PMTU, we recommend that a short timeout period should be used after the estimate is increased, and a longer timeout be used after the estimate is decreased because of a Datagram Too Big message. For example, after the PTMU estimate is decreased, the timeout should be set to 10 minutes; once this timer expires and a larger MTU is attempted, the timeout can be set to a much smaller value (say, 2 minutes). In no case should the timeout be shorter than the estimated round-trip time, if this is known.