1 Network Architectures and Services NET Dissertation Resilient Application Layer Signaling based on Supervised Peer-to-Peer (P2P) Networks Ali Fessi Network Architectures and Services Department of Computer Science Technische Universität München
3 Technische Universität München Lehrstuhl für Netzarchitekturen und Netzdienste Resilient Application Layer Signaling based on Supervised Peer-to-Peer (P2P) Networks Ali Fessi Vollständiger Abdruck der von der Fakultät für Informatik der Technischen Universität München zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) genehmigten Dissertation. Vorsitzender: Univ.-Prof. Dr. Thomas Neumann Prüfer der Dissertation: 1. Univ.-Prof. Dr.-Ing. Georg Carle 2. Prof. Dr. Radu State (Université de Nancy 1, Frankreich) Die Dissertation wurde am bei der Technischen Universität München eingereicht und durch die Fakultät für Informatik am angenommen.
4 Cataloging-in-Publication Data Ali Fessi Resilient Application Layer Signaling based on Supervised Peer-to-Peer (P2P) Networks Dissertation, December 2010 Network Architectures and Services, Department of Computer Science Technischen Universität München ISBN ISSN (print) ISSN (electronic) Network Architectures and Services NET Series Editor: Georg Carle, Technischen Universität München, Germany c 2011, Technischen Universität München, Germany
5 3 Abstract: The Internet has become a critical infrastructure which requires adequate protection from errors and attacks. It plays a critical role in delivering services, e.g., human and device communication, disseminating news and information, power grid control etc. Voice over IP (VoIP) is one of the critical services which require increased resilience on the Internet. It has been widely adopted by telephony providers to reduce costs. However, many incidents have demonstrated the lack of reliability inherent in VoIP. This thesis investigates the benefits of using supervised Peer-to-Peer (P2P) networks for resilient application layer signaling, specifically for VoIP session setup. The supervisor provides the peers with verifiable identities, thus mitigating critical security issues in P2P networks, e.g., Sybil attacks. The supervisor is involved in the signaling for VoIP session setup under normal operation. However, in case the supervisor becomes unreachable, the VoIP service can still be provided by the P2P network. A supervised P2P network can be considered as a solution between server-based and pure P2Pbased signaling solutions. The goal is the combination of the advantages of both architectures leading to improved reliability and security. A reliability analysis of the supervised approach is provided based on reliability theory and traces from the Skype network. Furthermore, different mechanisms are discussed how the approach can optimize the operation of the P2P network, notably load balancing and routing. A security threat analysis evaluates to what extent the supervised P2P approach is able to provide the same security level as in server-based signaling solutions. Finally, storing user contact data in a P2P network raises also privacy issues. Thus, concepts and experiences learned from anonymization networks such as Tor and I2P are adjusted to build a privacy preserving P2P signaling protocol for VoIP. The theoretical analysis in this thesis has been accompanied by implementation work and experimentation on PlanetLab.
7 5 Publications: Parts of this thesis have been pre-published in earlier versions: A. Fessi, H. Niedermayer, H. Kinkelin, and G. Carle. A Cooperative SIP Infrastructure for Highly Reliable Telecommunication Services. In IPTComm 07: Proceedings of the 1st international conference on Principles, Systems and Applications of IP Telecommunications, New York, NY, USA, ACM. A. Fessi, H. Niedermayer, H. Kinkelin, and G. Carle. CoSIP a Hybrid Architecture for Reliable and Secure SIP Services. PIK Praxis der Informationsverarbeitung und Kommunikation, 30(4), N. Kammenhuber, A. Fessi, and G. Carle. Resilience: Widerstandsfähigkeit des Internets gegen Störungen - Stand der Forschung und Entwicklung. Informatik-Spektrum (Special Issue on Future Internet), 33, Number 2: , April A. Fessi, N. Evans, H. Niedermayer, and R. Holz. Pr 2 -P2PSIP: Privacy Preserving P2P Signaling for VoIP and IM. In IPTComm 10: Proceedings of the 1st international conference on Principles, Systems and Applications of IP Telecommunications, Munich, Germany, ACM.
9 CONTENTS Part I Introduction and Background Introduction Outline and Scientific Contributions Resilience of Internet Services Terminology The fault error failure chain Fault Tolerance Resilience Dependability Survivability Security Relationship between Security, Dependability and Resilience Conclusions Challenges Topological Failures Overload Lack of Integrity Software Faults Correlated and Cascaded Failures Mechanisms Protection at the Application Layer Protection at the Lower Layers Reading Recommendations Conclusions Overlay and P2P Networks Terminology P2P networks Overlay Networks Unstructured Overlays Structured Overlays Distributed Hash Tables (DHTs)
10 8 Contents 3.2 Algorithmic Aspects ID Space Distance Metric Consistent Hashing Geometry Routing Operational Aspects Bootstrapping Maintenance Parallel Lookups Recursive vs. Iterative Routing Middlebox Traversal Underlay Proximity Resource awareness Security Conclusions Part II P2P Networks and Applications for Resilient Internet Services Coooperative SIP (CoSIP) Background Session Initiation Protocol (SIP) Challenges in VoIP/SIP Networks P2P-based SIP (P2PSIP) CoSIP Overview Concept Application Scenarios Prototype Implementation Evaluation Reliability Analysis Security Evaluation on PlanetLab Related Work Conclusions Supervised Node IDs Background Near-Optimal Node IDs Load Distribution Simulations Mathematical Model
11 Contents Maximum Load Distribution Simulation Setup Interpretation Related Work Conclusions Low-Diameter Overlays Background Optimal-Diameter Graphs Hypercubes Generalized Hypercubes Overlay Algorithm Network Parameters and Notation Overlay Routing Tables Routing Algorithm Evaluation Mathematical Model Simulations Supervised Low-Diameter Overlays Approach Evaluation Related Work Conclusions Part III Security and Privacy CoSIP Security Security Requirements Threat Model Mechanisms Secure Node ID Assignment Resilient Routing Data Replication Evaluation Related Work Conclusions Privacy-Preserving P2PSIP Design of Pr 2 -P2PSIP Model and Notation Protocol Overview Protocol Operations
12 10 Contents Cryptographic Primitives Evaluating Pr 2 -P2PSIP Threat Analysis Reliability Cost Analysis Cryptographic Overhead End-to-end Signaling Latency Related Work Conclusions Part IV Conclusions and Outlook Conclusions and Outlook Summary Future Directions Appendix 157 A. CoSIP Prototype Implementation Details A.1 Design Decisions A.1.1 Support of Different SIP Clients Software A.1.2 Choice of a DHT A.1.3 A Supernode Approach A.1.4 Putting Everything Together A.1.5 Support of P2P-based SIP A.2 Message Processing A.2.1 Session Establishment with CoSIP A.3 High-Level State Machines A.3.1 REGISTER Session State Machine A.3.2 INVITE Session State Machine B. SIP-Based X.509 Certificate Enrollment C. Taxonomy of Attacks on P2P Networks C.1 Attacks on the Overlay C.2 Attacks on the DHT C.3 Attacks on the Application Bibliography 181
13 Part I INTRODUCTION AND BACKGROUND
15 1. INTRODUCTION Communication has always been a fundamental need for human beings to exchange knowledge, experience and emotions. The Internet is a communication medium which has changed the way we live. From a technical point of view, there are some good reasons for the success of the Internet. Leonard Kleinrock worked in the 1960s on the mathematical model behind packet switching . He demonstrated that packet switching provides better bandwidth utilization and response time than the traditional circuit switching technology used for telephony. Meanwhile, the Internet has dominated other communication technologies such as Asynchronous Transfer Mode (ATM), Public Switched Telephone Network (PSTN) and Integrated Services Digital Network (ISDN). The television network is one of the next candidates to be replaced by IPbased TV. The usage of IP also is being investigated within airplanes . The Internet has increasingly been used for the control of the power grid network . These examples demonstrate how the Internet has become a critical infrastructure on which our life and prosperity depends. The success of another communication technology, GSM (Global System for Mobile Communications: originally from Groupe Spécial Mobile), lead to the extensive exploration of Internet support in 3 rd generation (3G) mobile networks. 3G networks consist of both circuit-switched and packet-switched domains. However, the trend is to discard the circuit-switched domain for the benefit of the packet-switched domain in the successors, the 3GPP Long Term Evolution (LTE) networks . The circuit-switched domain is not required for telephony anymore given that voice data will be transported exclusively using Voice over IP (VoIP). The Lack of Resilience in the Current Internet The pervasive use of the Internet today notably for critical applications increases the resilience requirements on the net. At the early time of the Internet, hosts were stationary, users had a strong technical background and were trustworthy. Applications were limited to web browsing and s. Today, the Internet user base has grown to nearly two billion users  with more than 35,000 registered autonomous systems (ASs) . Even if equipment or protocols are extensively tested during the development phase, it can not be guaranteed that they can provide the required functionality
16 14 1. Introduction under real world conditions, particularly since the deployment conditions evolve continuously over time, e.g., traffic properties, user base, applications and physical or environmental conditions. Examples of physical or environmental conditions are cable damages, power failures or interactions with other protocols and equipments. Jean-Claude Laprie defines resilience in computer systems as the persistence of the avoidance of failures that are unacceptably frequent or severe, when facing changes . Given that resilience includes the aspect of changes, guaranteeing continuous resilience is a very challenging task, most notably because changes are not always predictable. Changes can be triggered by non-malicious or malicious actions. Thus, resilience includes security aspects as well. Peer-to-Peer (P2P) Networks as Resilience Enablers A networking paradigm which garnered much attention since the end of the 1990 s offer attractive properties from a resilience perspective: P2P networking. P2P networks are generally overlays on top of existing networks, which allow for functionality not provided by the underlay, e.g., multicast or distributed lookup. In contrast to client/server systems with asymmetric roles, each peer in a P2P network implements the functions of both client and server. P2P networks are decentralized by design. They are built to cope with churn, i.e., with peers joining and leaving the network regularly. Also, data stored in a P2P network is replicated at multiple nodes in the network. Nodes can detect that their neighboring peers have left the network and thus, can autonomously recover from stale routing table entries. Moreover, P2P networks offer geographic diversity. Inherent decentralization, data replication, autonomous recovery from stale routing table entries, and geographic diversity, these are properties which make P2P networks attractive for building resilient network services. The Lack of Security in Pure P2P Networks On the other hand, building services using P2P networks is not straightforward. Given that the service is provided by the peers, it depends on the trustworthiness of the peers whether they perform their tasks, notably storage and routing, correctly and cooperate to provide a resilient service. If a P2P network is open to the public, which is the case for many file sharing networks, e.g., KAD , then a number of attacks are possible, particularly Sybil  attacks where an attacker emulates a large number of peers in the network, and eclipse  attacks where attackers hide content or nodes in the network and prevent them from being reachable. These attacks can render the aforementioned resilience benefits of P2P networks invalid.
17 1.1. Outline and Scientific Contributions 15 Supervised P2P Networks To address the security issues in P2P networks and benefit appropriately from their resilience advantages, we follow an approach with a supervised P2P network in this thesis. Definition A Supervised P2P network is a P2P network with a node with a special role: the P2P network supervisor. The supervisor is involved in building the P2P network but is not required during the normal network operation. Thus, the failure of the supervisor does not lead to the failure of the service offered by the P2P network. Supervised P2P networks have been under-explored, especially from a security perspective. The P2P research community has invested considerable effort into addressing P2P security issues in a distributed manner [11, 31, 33, 34, 83, 150]. However, these solutions can not provide resilience guarantees. One of the main problems is that they do not provide protection against the above mentioned Sybil attacks. Just as an example,  provides a mechanism to route DHT lookups beyond malicious nodes and show that if 20% of the nodes in the network are malicious, the lookup success rate is at 99.0%. These results are by far not sufficient for critical applications such as telephony. First, an attacker with sufficient resources could easily emulate more than 20% of the nodes in the network. Second, 99.0% lookup success rate is not sufficient since one aims at 5 nines availability for telephony, i.e, an availability of %. P2P systems deployed in practice today are actually rarely pure P2P networks. They generally do require some servers, even though most functionality can be performed among the peers. Even trackerless torrent functions with a server for the initial bootstrapping. However, in contrast to previous work on hybrid (or supervised) P2P networks, in this thesis a supervisor carries out a critical security function. It is considered as a trusted third party. The intention is to provide the same security guarantees as with server-based architectures. This is a requirement for critical applications such as telephony. On the other hand, one benefits from the resilience aspects of P2P networks and enhances the resilience of critical applications. Such a supervised P2P network can be considered as a solution between server-based and pure P2P-based signaling solutions leading to the combination of the advantages of both architectures. 1.1 Outline and Scientific Contributions To guide the reader through this thesis, we provide a short summary of each part and highlight the contributions. Figure 1.1 provides an overview. Part I of this thesis includes this introduction chapter followed by two background chapters. The first background chapter (Chapter 2) is
18 16 1. Introduction Part I: Introduction and Background Chapter 1: Introduction Chapter 2: Resilience of Internet Services Chapter 3: Overlay and P2P Networks Part II: P2P Networks and Applications for Resilient Services Chapter 4: Cooperative SIP (CoSIP) Chapter 5: Supervised Node IDs Chapter 6: Low-Diameter Overlays Part III: Security and Privacy Chapter 7: CoSIP Security Chapter 8: Privacy-Preserving P2PSIP (Pr 2 -P2PSIP) Part IV: Conclusions and Outlook Fig. 1.1: Outline of this thesis. about the state of the art of the resilience of Internet services. We shed light on the notion of resilience and related terms such as reliability, availability, dependability and survivability, which are used throughout this thesis. We provide an overview of challenges in the Internet as well as resilience mechanisms. The second background chapter is about overlay and P2P networks. It introduces the appropriate terminology and describes P2P networks from algorithmic as well as operational perspectives. Part II of this thesis provides contributions in the area of P2P networks, notably supervised P2P networks, and their applications for building resilient services.
19 1.1. Outline and Scientific Contributions 17 In Chapter 4, we investigate the application of supervised P2P networks in the context of the Session Initiation Protocol (SIP). We present Cooperative SIP (CoSIP), a hybrid architecture for SIP signaling which fills the gap between i) server-based telecommunication infrastructures (which are vulnerable to network and service faults, Denial-of-Service (DoS) attacks, etc.) and ii) pure P2P-based telecommunication networks (which are vulnerable to Sybil attacks, eclipse attacks, etc.). Chapters 5 and 6 focus on the design flexibilities of P2P networks. In Chapter 5, we analyze how the P2P network can benefit from supervised node ID assignment. Given n peers in the network, a random node ID distribution used in pure P2P DHTs results into some peers being responsible for a higher load than the average load by a factor of O(log n). A supervised P2P network approach like CoSIP guarantees a near-optimal node ID distribution where each node is responsible for a load in the order of the average load reducing the expected maximum load from a logarithmic to a constant factor. In Chapter 6, we explore a further design flexibility in structured overlay networks. We present an algorithm for structured overlays where lookups can be performed within O(1). More precisely, given random node IDs, routing can be achieved within two hops with a probability (1 ɛ) for an arbitrarily small ɛ. The routing table size is within O( n). Then, the approach with supervised node IDs from Chapter 5 is applied here as well to increase the probability that routing within two hops succeeds to one. Part III addresses the security issues which are raised by adding a P2P network in the CoSIP signaling procedure. Notably, we discuss whether the P2P network introduces new attack vectors and whether attackers are able to invalidate the resilience benefits of the P2P network. For example, storing an object at k peers provides redundancy, but becomes useless if all k peers are controlled by a single malicious node. We evaluate to what extent the supervised approach can solve the security issues inherent in P2P networks and discuss additional security mechanisms, e.g., resilient routing, to achieve the same security level as in server-based signaling solutions. Finally, in CoSIP, storing user contact data in a P2P network raises also severe privacy issues. Thus, we learn from concepts and experiences from anonymization networks such as Tor  and I2P  to build Privacy-Preserving P2PSIP (Pr 2 -P2PSIP) networks. The solution is valid for CoSIP as well as P2PSIP as being standardized in the IETF . We provide a threat analysis as well as an analysis of the performance of Pr 2 -P2PSIP Particularly we analyze cryptographic
20 18 1. Introduction overhead, signaling latency and reliability costs. Table 1.1 summarizes the contributions of this thesis. Tab. 1.1: Overview of contributions of this thesis. Chapter Previous Work Contributions Evaluation Methodology Part II 3 Server-based SIP with lack of reliability. P2PSIP with lack of security. Cooperative SIP (CoSIP) combining advantages of both server and P2P-based architectures. Reliability Analysis based reliability theory and Skype traces. Implementation. Functional and performance evaluation on PlanetLab. 4 DHTs with random node IDs. Skewed load. Supervised node IDs. Provably improved load balancing. Mathematical model. Simulations. 5 DHTs with routing in O(log n) DHTs with routing in O(1) Mathematical model. Simulations. Part III 6 P2P networks with security issues. Secure P2P networks with Verifiable IDs and Resilient routing. Security threat analysis. Implementation of a selected security mechanism: SIP-based certificate enrollement. 7 CoSIP and P2PSIP networks with privacy issues: Location privacy and Social interaction privacy. Pr 2 -P2PSIP: A protocol which addresses privacy issues in CoSIP and P2PSIP. Threat analysis. Analysis of cryptographic overhead, signaling latency and reliability costs.
21 2. RESILIENCE OF INTERNET SERVICES Before we explore the potential of (supervised) P2P networks for building resilient network services, we first provide an overview of the state-of-theart in building resilience Internet resiliences. In this chapter, we first shed light on the notion of resilience and related terms which are used throughout this thesis, for example, reliability and availability. Then, we provide an overview of different challenges in the Internet and appropriate resilience mechanisms. Given that the field of resilient Internet services is an emerging discipline with a large amount of effort, we do not claim to provide a complete list of appropriate resilience mechanisms. However, this chapter will help to position the resilience approach proposed in this thesis and compare it with other approaches. Furthermore, since this thesis is about application layer signaling, a special focus in this chapter is put on the resilience mechanisms at the application layer. Other mechanisms in underlying layers, e.g., IEEE link aggregation, IP Fast ReRoute (IPFRR), etc. are summarized very briefly. 2.1 Terminology The term resilience in computer systems and networks is often used in relationship with other terms, e.g., fault tolerance, dependability, availability, reliability. These terms (including resilience) may sound synonymous or similar at a first glance. Nevertheless, there has been quite some effort by engineers and scientists to provide an appropriate taxonomy and clarify the differences. The IFIP working group (WG) Dependable Computing and Fault Tolerance 1 has been working on fault tolerance and dependability since One of the first results of this working group was a taxonomy document which has been updated iteratively [8, 73, 75]. Starting from the version of 1992 , the authors added security related terms and discussed the relationship between security and dependability. A significant effort dedicated to definitions and taxonomies in the context of network resilience is lead by the Resilinets initiative 2. Some of the results have been recently published by Sterbenz et al. . We summarize the definitions of the relevant terms used in the literature 1 2
22 20 2. Resilience of Internet Services in the context of resilience, notably network resilience. The effort by the IFIP WG and Sterbenz et al. will guide our terminology discussion The fault error failure chain Some of the basic terms that have been agreed on early in the disciplines of fault tolerance and dependability are faults, errors and failures: Correct service is delivered when the service implements the system function . A service failure, or simply failure, is an event that occurs when the delivered service deviates from correct service . A service is a sequence of the system s external states. A service failure means that at least one external state of the system deviates from the correct service state. The deviation of an external state of the system from the correct service state is called an error. Thus, an error is the part of the total state of the system that may lead to its subsequent service failure . The adjudged or hypothesized cause of an error is called a fault. A fault is active when it causes an error, otherwise it is dormant . These definitions lead to the causal relationship: fault error failure Fault Tolerance Fault tolerance means to avoid service failures in the presence of faults . Fault tolerance is one of oldest disciplines in computer science. For example, in 1956 Moore and Shannon provided an approach to build a reliable switching system out of relatively unreliable mechanical relays . Jalote  binds fault tolerance to redundancy: A system is fault tolerant if it can mask the presence of faults in the system by using redundancy. However, redundancy is not necessarily replication of the same object (software or hardware). For example, Laprie et al.  point that fault tolerance can be achieved via separate designs or implementations, i.e., design or implementation diversity. Further, one can differentiate between hardware diversity and software diversity Resilience The origin of the term Resilience can be traced back to the Latin verb resilire, which roughly means jump back. In physics, resilience is a material s property of being able to recover to a normal state after a deformation resulting from external forces. Laprie  provides different examples how the term resilience is used in the context of psychology, ecology, business
23 2.1. Terminology 21 and industrial safety. For example, in ecology, resilience has been used to refer to moving from a stability domain to another one under the influence of disturbances ; in industrial safety, it has been used to refer to anticipating risk changes before damage occurance. The common denominator in the different usages of the term resilience is the ability to successfully accommodate changes. Then, Laprie  defines resilience in computing systems as: i) the persistence of the avoidance of failures that are unacceptably frequent or severe, when facing changes. Sterbenz et al. , define resilience in computer networks as: ii) the ability of the network to provide and maintain an acceptable level of service in the face of various faults and challenges to normal operation. By various faults, Sterbenz et al.  indicate that the faults may vary by type, scope, and number. The definitions of resilience in computer systems by i) Laprie and ii) resilience in computer networks by Sterbenz et al. are not contradictory 3. Both definitions accomodate the dynamic aspect of resilience; Laprie by the term change, Sterbenz by various faults 4 and challenges Dependability The dependability of a system is the ability to avoid service failures that are more frequent and more severe than is acceptable . The concept of dependability has been developed over the last decades and encompasses: availability: readiness for correct service reliability: continuity of correct service safety: absence of catastrophic consequences on the user(s) and the environment integrity: absence of improper system alterations maintainability: ability to be restored after failure. 3 An discussion with the authors Sterbenz et al. lead also to this conclusion. 4 Under the condition that these faults become active. Faults do not cause any change to the system as long as they are dormant.
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