The MASCOTS conference is a well-established forum for state-of-the-art research on the measurement, modeling, and performance analysis of computer systems and networks. The 21st edition of this conference will take place August 14-16, 2013 in San Francisco, CA. The conference will bring together academics and industry practitioners to present and discuss their latest research results. The technical program for the 3-day conference will include keynote talks, refereed full and work-in-progress papers, and posters. We encourage original submissions describing state-of-the-art research in the areas of computer networks and performance evaluation of computer systems. The MASCOTS conference encourages papers describing results of theoretic and/or practical significance. Papers presenting new performance evaluation methods or those applying existing methods to provide new insights to design tradeoffs in computer and networked systems are particularly encouraged.