9. Application Probing

All implementations MUST include a mechanism where applications using connectionless protocols can send their own probes. This is necessary to implement PLPMTUD in an application protocol as described in Section 10.4 or to implement diagnostic tools for debugging problems with PMTUD. There MUST be a mechanism that permits an application to send datagrams that are larger than eff_pmtu, the operating systems estimate of the Path MTU, without being fragmented. If these are IPv4 packets, they MUST have the DF bit set.

At this time, most operating systems support two modes for sending datagrams: one that silently fragments packets that are too large, and another that rejects packets that are too large. Neither of these modes is suitable for implementing PLPMTUD in an application or diagnosing problems with Path MTU Discovery. A third mode is REQUIRED where the datagram is sent even if it is larger than the current estimate of the Path MTU.

Implementing PLPMTUD in an application also requires a mechanism where the application can inform the operating system about the outcome of the probe as described in Section 7.6, or directly update search_low, search_high, and eff_pmtu, described in Section 7.1.

Diagnostic applications are useful for finding PMTUD problems, such as those that might be caused by a defective router that returns ICMP PTB messages with incorrect size information. Such problems can be most quickly located with a tool that can send probes of any specified size, and collect and display all returned ICMP PTB messages.