What is MTU?

Standard Ethernet has an MTU of 1,500 bytes. However, complex paths involving VPNs, MPLS, or PPPoE often add headers that reduce the available space for original data. When a router receives a packet larger than its egress interface's MTU, it has two choices:

1. Fragment: Slice the packet into smaller pieces and forward them.
2. Drop & Notify: Discard the packet and send an ICMP 'Destination Unreachable - Fragmentation Needed' message back to the sender.

The time required to reassemble fragmented packets at the destination increases CPU overhead and dramatically elevates Latency.

Path MTU Discovery (PMTUD)

Modern operating systems attempt to avoid fragmentation by discovering the smallest MTU along the entire path before sending data. This is done by setting the DF (Don't Fragment) bit in the IP header.

Calculating the Overhead

To calculate the payload efficiency, we look at the ratio of data to headers.

By optimizing the Infrastructure standards and ensuring consistent MTU across the backplane, engineers can eliminate the jitter and latency associated with packet slicing.