This document defines various limits that may be applied to receiving, sending, and otherwise processing packets that contain IPv6 extension headers. Limits are pragmatic to facilitate interoperability amongst hosts and routers, thereby increasing the deployability of extension headers. The limits described herein establish the minimum baseline of support for use of extension headers on the Internet. If it is known that all communicating parties for a particular communication, including destination hosts and any routers in the path, are capable of supporting more than the baseline then these default limits may be freely exceeded.