Boeing Phantom Works IPv6-in-(foo)*-in-IPv4 tunnels must support a minimum Maximum Transmission Unit (MTU) of 1280 bytes for IPv6 via static prearrangements and/or dynamic MTU determination based on ICMPv4 messages, but these methods have known operational limitations. This document specifies a link adaptation mechanism for IPv6-in-(foo)*-in- IPv4 tunnels that presents an assured MTU to the IPv6 layer using tunnel endpoint-based segmentation/reassembly and dynamic segment size probing.