Ericsson OutSys Cisco Systems Nokia University of Cincinnati This document defines an optional STAMP extension that allows a Session-Reflector to send packets whose length or quantity differs from those sent by the Session-Sender. While standard STAMP exchanges packets symmetrically, some measurement scenarios benefit from asymmetric response packets to better reflect real application conditions. This extension enables the Session-Reflector to send packets of different sizes and/or additional packets not tied one- for-one to incoming test packets. The document also analyzes challenges in multicast performance monitoring and specifies STAMP procedures to improve measurement efficiency and reduce network impact.