Summary

In a network path that has various MTU values along the path, this hop represents the point within the network where the MTU is the smallest.

Recommended action

In cases where you are planning to increase the overall MTU of a network path, concentrate on this point of the network.

Detailed explanation

MTU, or Maximum Transmission Unit, is also known as “link MTU”. It represents the maximum packet size that can be sent in one piece over a link (or hop) without being fragmented. It is measured in bytes.

The “path MTU” is the minimum MTU of the hops in a network path. It defines the end-to-end MTU of the path.

A “path MTU constriction point” is associated with a hop that has a lower MTU value than the hop before it on a network path, measured in the direction of the test, i.e. from Sequencer to target. A test path may contain more than one path MTU constriction point. Delivery monitoring will not flag any constriction points if the measured path MTU remains the same for each hop along the test path.

Similar to capacity, Delivery monitoring cannot measure MTU beyond a given hop any higher than the MTU value that hop has passed. Effectively then, each decrease in MTU represents a constriction on the path MTU to the target host.

Of particular interest are the MTU constrictions from values of MTU greater than 1500 bytes to 1500 bytes (or less). These indicate the boundaries between so-called jumbo frame networks (typically gigE) where MTU can range as high as 16000 bytes and more common 10/100 Ethernet networks where 1500 bytes is the standard MTU.

Possible secondary messages

  • "Path MTU to target host is defined by this hop" This hop is the last constriction point detected and therefore represents the smallest MTU value and defines the end-to-end MTU of the test path, or the "path MTU". It is effectively the MTU bottleneck.
  • "Packets may be lost if path MTU discovery is not correctly used by applications" Interfaces that do not report their proper MTU are detected by means of packet loss.
  • "Interface between jumbo frame and 1500 byte networks present" The MTU of this hop is 1500 bytes, and is lower than the MTU of the previous hop in the test path, measured in the direction of the test, i.e. from Sequencer to target.
  • "Packet loss conditions may have generated an incorrect diagnosis" In cases where there are packet loss, path MTU discovery may underestimate the path MTU. </ul>

    Related topics

    Maximum Transmission Unit (MTU)

    Other resources

    RFC 791 RFC 792 RFC 1191