Ericsson OutSys Cisco Systems Nokia This document describes an optional extension to a Simple Two-way Active Measurement Protocol (STAMP) that enables the use of STAMP test and reflected packets of variable length during a single STAMP test session. In some use cases, the use of asymmetrical test packets allow for the creation of more realistic flows of test packets and, thus, a closer approximation between active performance measurements and conditions experienced by the monitored application. Also, the document includes an analysis of challenges related to performance monitoring in a multicast network. It defines procedures and STAMP extensions to achieve more efficient measurements with a lesser impact on a network.