The path under test exhibits a behavior that violates a documented network standard. A network device is not following the standards outlined in RFC 1191 pertaining to Path Maximum Transmission Unit (PMTU) discovery.

Recommended action

If one or more diagnostic messages has been generated, take the recommended course of action for each message. Start with the diagnostic message with the highest certainty value.

Detailed explanation

This violation of RFC 1191 often results in slow TCP data rates, typically in the order of one sixth of normal capacity. When using protocols other than TCP, connections often fail. Typically this problem is experienced in one direction of data transfer, but not the other. For example, downloads may work but uploads fail.

RFC 1191 refers to a technique for using the "DF" (Don't Fragment) bit in the IP header to dynamically discover the MTU of a path. The basic idea is that a source host initially assumes that the Path MTU (PMTU) is the known MTU of its local interface, and sends all packets on that path with the DF bit set. If any of the packets are too large to be forwarded without fragmentation by some device along the path, that device will discard them and return ICMP "destination unreachable" messages with a code meaning "fragmentation needed and DF set".

Upon receipt of such a message, the source host reduces its assumed PMTU for the path, and stores that value in its dynamic routing table. The PMTU discovery process ends when the host's estimate of the PMTU is low enough that its packets can be delivered without fragmentation or packet loss.

Possible secondary messages

  • "A network device does not specify its MTU - out of compliance with RFC 1191"
  • "A network device incorrectly specifies its MTU - broken compliance with RFC 1191"
  • "A network device permits inappropriate packet fragmentation - Don't Fragment bit in IP header ignored"

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