Boeing Phantom Works The nominal Maximum Transmission Unit (MTU) of the Internet has become 1500 bytes, but existing IP/*/IP tunneling mechanisms impose an encapsulation overhead that can reduce the effective path MTU to smaller values. Additionally, existing tunneling mechanisms are limited in their ability to discover and utilize larger MTUs. This document specifies a trivial mechanism for tunnel MTU determination that addresses these issues.1. Introduction The nominal Maximum Transmission Unit (MTU) of today's Internet has become 1500 bytes due to the preponderance of networking gear that configures an MTU of that size. Since not all links in the Internet configure a 1500 byte MTU, however, packets can be dropped due to an MTU restriction on the path. Upper layers see IP/*/IP tunnels as ordinary links, but even for packets no larger than 1500 bytes these links are susceptible to silent loss (e.g., due to path MTU restrictions, lost error messages, layered encapsulations, reassembly buffer limitations, etc.) resulting in poor performance and/or communications failures [RFC2923][RFC4459][RFC4821][RFC4963]. This document specifies a trivial mechanism for IP/*/IP tunnel MTU determination. It updates the functional specifications for Tunnel Endpoints (TEs) found in existing tunneling mechanisms (see: Section 9).