Can the MTU size change on a local network if non of my software attempt to do that?

Question

From the ip manual page (man 7 ip), we read that the MTU may change over time.

When PMTU discovery is enabled, the kernel automatically keeps track of the path MTU per destination host. When it is connected to a specific peer with connect(2), the currently known path MTU can be retrieved conveniently using the IP_MTU socket option (e.g., after an EMSGSIZE error occurred). The path MTU may change over time. For connectionless sockets with many destinations, the new MTU for a given destination can also be accessed using the error queue (see IP_RECVERR). A new error will be queued for every incoming MTU update.

I'm thinking this is a rare event and can happen on devices I have no control over.

My environment is a local network of a few computers connected via a switch. I would imagine that the MTU is never going to change "by itself" in such an environment. Is that a correct assumption?

Answer 1

The MTU on any given link is the MTU on the link. What PMTUD does is to discover the smallest MTU of any link in the path from one host to another host. Because network routing is dynamic, the path may change, so the smallest MTU in the path may change.

If you are concerned about the MTU in your business network, that will not change, unless you make some type of change in your business network (things like link failures or bringing up new links can change the path). Packets are routed one packet at a time, without regard to any other packets, so a change to a network can change the path taken by packets, even within a single packet flow. You should have some type of monitoring on your network to inform you in the case of failures or other changes.

The real concern is the MTU between two hosts across the public Internet, where paths (so possibly the smallest MTU in the path) do change all the time.