09 by Taylor & Francis Group, LLC Contributors Jie Zhang Chuan Zhu Department of Electrical and School of Information Science and Computer Engineering Engineering The University of British Columbia Northeastern University Vancouver, British Columbia, Shenyang, People's Republic of Canada China C 2009 by Taylor & Francis Group, LLC Part I CONCEPTS & SERVICES © 2009 by Taylor & Francis Group, LLC SIP in Peer-to-Peer Networks Holger Schmidt and Franz J. Hauck Ulm University CONTENTS 1.1 Introduction 1.2 Background 1.3 Integration of P2P Technologies into SIP Issues in P2P SIP Field of Application of P2P SIP 1.6 Standardization 1.7 Conclusion References 1.1 Introduction Session Initiation Protocol (SIP [1]) has become the quasi-standard for Voice- over-Internet Protocol (VoIP) communications. As an IETF protocol for ses- sion signalling in general, SIP is, for instance, adopted for session manage- ment in the Third Generation Partnership Project (3GPP) [2], which aims at providing a next generation infrastructure for mobile phone systems. SIP is based on a client-server infrastructure in which user agents represent the end-terminals as clients, proxy servers handle SIP message routing between the user agents, and registrar servers store the client's contact information into a location service. As a result of the client-server infrastructure, SIP networks are in general characterized by high administrative efforts. Therefore, Skype [3] started to integrate peer-to-peer (P2P) mechanisms into VoIP communications, which led to an infrastructure without expensive high-end server machines and with almost no need for client-side configuration. However, as Skype builds on pro- prietary, obfuscated protocols [4] and telecom companies require standards for interoperability, there are efforts to integrate P2P techniques into SIP (P2P SIP). There are several approaches to integrate P2P mechanisms into SIP: an extension of the SIP protocol itself (e.g., Bryan et al. [5]), an integration of P2P mechanisms into the SIP message forw