Quantitative Analysis of 2-tier P2P- SIP Architecture with ID-based Signature

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1 A Thesis for the degree of Master Quantitative Analysis of 2-tier P2P- SIP Architecture with ID-based Signature Chang-Hwan Lee School of Engineering Information and Communications University 2008 i

2 Quantitative Analysis of 2-tier P2P- SIP Architecture with ID-based Signature ii

3 Quantitative Analysis of 2-tier P2P- SIP Architecture with ID-based Signature Advisor : Professor Young-Hee Lee by Chang-Hwan Lee School of Engineering Information and Communications University A thesis submitted to the faculty of Information and Communications University in partial fulfillment of the requirements for the degree of Master of Science in the School of Engineering Daejeon, Korea January 7, Approved by (signed) Professor Young-Hee Lee Major Advisor iii

4 Quantitative Analysis of 2-tier P2P-SIP Architecture with ID-based Signature Chang-Hwan Lee We certify that this work has passed the scholastic standards requested by the Information and Communications University as a thesis for the degree of Master January 7, 2008 Approved: Chairman of the Committee Young-Hee Lee, Professor School of Engineering Committee Member Kwang-Jo Kim, Professor School of Engineering Committee Member Myung-Chul Kim, Professor School of Engineering iv

5 M.S Chang-Hwan Lee Quantitative Analysis of 2-tier P2P-SIP Architecture with ID-based Signature School of Engineering. 2008, p.40 Major Advisor: Professor. Young-Hee Lee. Text in English Abstract SIP (Session Initiation Protocol) is IETF standard signaling protocol for VoIP (Voice over IP) and other multimedia services. Recently, there have been some efforts to propose P2P-SIP architecture, which makes use of a peer-to-peer network instead of registrar and location SIP servers in order to facilitate SIP user registration and location. However, existing P2P-SIP systems still have some problems. Current P2P-SIP lookup does not meet the requirement for low time delay in multimedia services. Furthermore, security procedure based on PKI for SIP message authentication requires verification of central authorities (CA) during message routing and certificates maintenance of intermediate nodes. It puts more delay in signaling as users increase in the P2P-SIP network. Therefore, we propose 2-tier P2P-SIP architecture, using ID-based signature which has more light-weight authentication procedure and decreases the session setup latency to some extent, and perform quantitative analysis of this architecture. ID-based signature allows users to verify digital signatures using only public information such as the user's identity, so that it reduces the complexity and costs for establishing and managing the public key of nodes. Based on the performance analysis and simulation results, our proposed 2-tier P2P-SIP architecture achieves better scalability as well as shorter session setup i

6 latency than PKI-based P2P-SIP architecture. ii

7 Contents A Thesis for the degree of Master... i Abstract... i Contents...iii List of Tables... v List of Figures... vi List of Abbreviations...vii I Introduction Peer-to-Peer SIP architecture Main Issues in P2P-SIP Session Setup Latency Message Authentication Overhead Problem Statement Thesis Organization... 5 II Related Work Works on P2P-SIP Skype Security Extensions for P2P-SIP ID-based Cryptography... 9 III 2-tier P2P-SIP Architecture with ID-based signature Building 2-tier Hierarchy and Utilizing Localized Cache Adopting the ID-based Signature to P2P-SIP system Key generation Resource registration...16 iii

8 3.2.3 Resource location and verification...18 IV Performance Evaluation Implementation of 2-tier P2P-SIP model, using ID-based signature Simulation environment Metrics for evaluation and Simulation result V Conclusions and Future-work References Acknowledgements Curriculum Vitae iv

9 List of Tables Table 1. Simulation parameters v

10 List of Figures Figure 1. 2-tier P2P-SIP with localized cache Figure 2. Registration with ID-based signature Figure 3. Resource location and verification of reource integrity Figure 4. P2P-SIP node architecture Figure 5. Session setup over P2P-SIP Figure 6. Effect of 2-tier hierarchy & localized cache 10% Figure 7. Effect of 2-tier hierarchy & localized cache 20% Figure 8. Effect of ID-based signature I Figure 9. Effect of ID-based signature II Figure 10. Effect of ID-based signature III Figure 11. Node overall bandwidth vi

11 List of Abbreviations ICU Information and Communications University vii

12 I Introduction SIP (Session Initiation Protocol) [1] is the most popular protocol for various multimedia services based on the Internet, including VoIP, conferencing application and IPTV. It is widely used by enterprise, consumers, and even carriers in the core of their networks. SIP is designed for establishing media sessions of any kind and it is a text-based client-server protocol. But the centralized SIP server becomes the single point of failure. So, Peer-to-Peer SIP has been recently focused on as an approach for handling media sessions with robustness and system scalability [2], [3], [4], [5]. There has been much speculation that this interest can be attributed Skype [6], the most popular P2P communication service provider. Researchers thus try to combine P2P and SIP functionality on the application layer, where participating nodes build an overlay network and locate the resources among these nodes. 1.1 Peer-to-Peer SIP architecture As the alternative to Client-Server SIP, P2P-SIP has been recently studied by many researchers due to its easy deployment and robustness against failure as benefits compared to using centralized servers. P2P inherently has high scalability because the capacity scales with user population, and robustness and fault tolerance because there is no centralized server and the network selforganizes itself. This is achieved at the cost of higher signaling latency for locating the resources of interest in the P2P overlay network. And the selforganizing aspects of P2P lend themselves to systems that are easier to configure and manage than centralized systems. Therefore, P2P-SIP removes or reduces 1

13 the number of centralized SIP servers needed in a SIP deployment and provides robustness because of the lack of a single point of failure. The centralized aspect of SIP architecture is the registrar and location server. In P2P-SIP, the peers store the resources of interest, rather than registrar and location server. In particular, P2P-SIP builds on a DHT (Distributed Hash Table). A peer is assigned its own ID by an authentication server, the hash of an IP address. Once the server assigns peer-id, the server does not need to be asked again during operation. And the user generates resource-id in the same space by hashing the SIP-URI of the user. As with any DHT, each peer knows about some fixed number of peers distributed across the overlay. To route a message, the peer with the closest peer- ID to the requested resource-id is selected from the list of known peers. The message is then sent to this closest known peer. The process repeats, converging on the peer with the nearest value. Usually, there are two approaches on P2P-SIP. The first approach is SIP over P2P, which replaces SIP location service by a P2P protocol and the second approach is P2P over SIP, which uses SIP messages as the syntax to encode the P2P protocol. Some researches [7] have combined two approaches into on hybrid architecture. 1.2 Main Issues in P2P-SIP The integration of SIP and P2P brings many benefits. Some of centralized SIP servers can be reduced and the robustness and scalability of SIP is improved. There has been many works on P2P-SIP now. However, some issues still need to be addressed. 2

14 1.2.1 Session Setup Latency Session setup latency can be intolerable when too many peer nodes join in a P2P-SIP network. Existing P2P-SIP systems store and locate resources in P2P- SIP networks based on DHT algorithms, such as Chord [8], CAN [9] and Pastry [10]. Basically, system scalability and robustness of P2P-SIP are achieved at the cost of higher signaling latency for locating the resources of interest in the P2P overlay network. Session setup latency increase when the number of peer nodes in a DHT based P2P-SIP increases. For example, session setup latency in P2P- SIP with N-nodes based on Chord is O(log N), which will increase when N increases Message Authentication Overhead Without an authentication server, authentication of the resource stored in a P2P-SIP network is difficult task. In order to authenticate the resources in P2P- SIP, the signature can be attached to the end of message under PKI (Public Key Infrastructure) framework. However, the PKI-based signature model requires verification of central authorities (CA) during SIP message routing and certificates maintenance of intermediate nodes for authentication procedure. It puts more delay in signaling as users increase in P2P-SIP. Especially, much delay in signaling of real-time multimedia communication can be a significant problem. 3

15 1.3 Problem Statement The P2P-SIP architecture improves the performance of client-server SIP system in aspects of robustness and system scalability. However, existing P2P- SIP system still needs to address some problems. Basically, session setup in P2P- SIP takes about O(log N) times more than client-server session setup O(1) in aspects of latency. Furthermore, Using PKI-based signature for SIP message authentication burdens session setup with high latency. Thus, the design goal of secure P2P-SIP should achieve low latency for real-time multimedia services In this thesis, we propose 2-tier P2P-SIP architecture, using ID-based signature which has more light-weight authentication procedure and decreases the session setup latency to some extent, perform quantitative analysis about the proposed architecture through the simulation tool and present proper network scalability for this architecture. ID-based signature is firstly proposed in [11], which is signature scheme, based on the bilinear pairings on elliptic curves. IDbased signature allows users to verify digital signatures using only public information such as the user's identity, while PKI-based signature requires verification of each user s public key from CAs. It can utilize public identifier such as ID or SIP-URI as a public key, so that it reduces the complexity and costs for establishing and managing the public key of nodes. Additionally, we build 2-tier hierarchy and utilize localized cache for fast session setup. According to the storage capacity and network connectivity, the P2P-SIP node is classified into two types, which are super node and ordinary node. Once a node locates the resource of interest, the resource will be stored at the localized cache of its super node for certain time of period. 4

16 1.4 Thesis Organization The remaining sections are organized as follows. Section 2 gives brief overview about some draft documents proposed in IETF P2P-SIP Working Group and explains about the ID-based cryptography. In Section 3, the proposed P2P-SIP architecture with ID-based signature is described. And Section 4, we evaluate the performance of our P2P-SIP architecture with ID-based signature in aspects of session setup latency and node bandwidth. Finally, Section 5 concludes the thesis. 5

17 II Related Work 2.1 Works on P2P-SIP The integration of SIP and P2P is interesting work in multimedia domain. In this chapter, we look through the existing works on P2P-SIP architectures. Firstly, Kundan et al [2], present P2P Internet telephony architecture using SIP. They use Chord as the underlying distributed hash table (DHT) and extend SIP protocol to support P2P. In this architecture, P2P messages are sent over the SIP infrastructure. Secondly, David et al [3], combine the SIP/SIMPLE family of IETF standards for VoIP and IM with Gnutella protocol. SIP/SIMPLE is one P2P over SIP protocol. Besides in [4], they use an External DHT as a SIP location service. In this situation, applications use SIP to establish conversation and use P2P as route protocol. The P2P arithmetic is based on Chord. Additionally, there exist already P2P-SIP working groups patronized by IETF [5]. Many people propose the draft documents on critical issues of P2P-SIP and try to solve the issues and standardize the P2P-SIP architecture. 6

18 2.2 Skype A Skype [6] is a free P2P application based on Kazaa [12] architecture that allows making calls over the Internet to any other Skype user. Skype has the following problems: The protocol is proprietary unlike open standards such as SIP. Most importantly, it has centralized elements for login authentication which means that if this element fails, the system may not work. The Skype s architecture is no different from the classical SIP architecture, except that Skype s Global Index Server assigns a super-node for a new joining node. The super-node, similar to the SIP registrar, proxy and presence server, maintains the presence information for this node, and locates other users by communicating with other super-nodes. A node that has enough capacity and availability can become a super-node. We believe that the lookup is based on some variation of flooding, similar to Kazaa. The main advantage of Skype is that it implements the equivalent of STUN [13] and TURN [14] servers in the node itself to handle NAT [15], unlike explicit server configuration in existing SIP applications. 7

19 2.3 Security Extensions for P2P-SIP P2P-SIP provides peer-to-peer registration and resource location that can be used for SIP applications. A secure P2P-SIP network requires protection from a variety of security threats. Joining peers should only be admitted into an overlay if they are authorized members of that overlay. Resources (users) should be authenticated before communication begins. The overlay's links and the data registered on the peers should be protected from attackers. Without such security measures in place, attackers can generate false identities and become peers in the system, where they can interfere with message routing and maintenance of the overlay structure. They can also masquerade as other, valid users or resources in the overlay, possibly generating false responses to resource requests. The goal of P2P-SIP is to scale gracefully from ad hoc groups of a few people to overlays of millions of peers across the globe. As such, there is no one security model that fits the needs of all envisioned environments: for the small network establishing a certificate chain is difficult, while for a global network the unrestricted ability to insert resources and devise useful Peer IDs is a clear invitation to insecurity. To address this issue, some people propose the draft document about the P2P-SIP authentication. [16] This work offers one security model. It is a public key certificate system applicable to larger deployments in which the administrative costs of public key management is preferable to the scalability issues of shared secret keys. 8

20 2.4 ID-based Cryptography ID-based cryptography [11] is an asymmetric key authentication system in which the public key of a user is some unique information about the identity of the user (e.g. a user's address or telephone number). Firstly, the identitybased cryptosystem was a signature scheme developed by Adi Shamir in 1984, which allowed users to verify digital signatures using only public information such as the user's identity. Current ID-based schemes include Boneh/Franklin's pairing-based encryption scheme, and Cocks's encryption scheme based on quadratic residues. Identity-based systems allow any users to generate a public key from a known identity value such as ID or telephone number. A trusted third party, called the Private Key Generator (PKG), constructs the corresponding private keys. To work, the PKG first publishes a "master" public key, and retains the corresponding master private key. Given the master public key, any users can generate a public key corresponding to the identity i by combining the master public key with the identity value. To obtain a corresponding private key, the user is authorized to use the identity i, contacting the PKG, which uses the master private key to generate the private key for identity i. As a result, users may encrypt messages (or verify signatures) with no prior distribution of keys between individual participants. This is remarkably efficient in cases where pre-distribution of authenticated keys is inconvenient or infeasible due to technical restrictions. However, to decrypt or sign messages, 9

21 the authorized user must obtain the appropriate private key from the PKG. A prior settlement of the ID-based schemes is that the PKG must be highly trusted, as it is capable of generating any user's private key and may therefore decrypt (or sign) messages without authentication. Because any user's private key can be computed through the use of the third party's secret, this system has inherent key escrow. Lots of variant systems have been proposed which remove the escrow including certificate-based encryption and secure key issuing cryptography. 10

22 III 2-tier P2P-SIP Architecture with ID-based signature We have studied on the overview of Peer-to-Peer SIP architecture and some critical issues in P2P-SIP now. Still, there are some problems to be addressed. Current P2P-SIP model for message authentication has high latency of lookup and verification procedures for multimedia services. Thus, we propose 2-tier P2P-SIP architecture, using ID-based signature which can decrease the session setup latency, so as to meet the low latency requirement in case of multimedia services and prove that the proposed architecture achieves proper network scalability. First of all, we build 2-tier hierarchy and utilize localized caches in the P2P-SIP system in order to improve the performance in aspects of session setup latency. According to storage capacity and network connectivity of a node, a P2P-SIP network is comprised of super nodes and child nodes. 2-tier P2P-SIP architecture makes the system more scalable and the session setup faster. Additionally, super nodes preserve the resource for a certain time of period once the resource is requested. Secondly, we adopt ID-based signature to the P2P-SIP system in the first time to reduce the session setup latency. It allows users to verify digital signatures using only public information such as the user's identity, so that it reduces the complexity and costs for establishing and managing the public key of nodes. With this 2-tier P2P-SIP architecture using ID-based signature, users can make any kind of media session with other users. Based on 2-tier hierarchy, localized cache and ID-based signature, latency for lookup is faster than existing secure P2P-SIP system. 11

23 3.1 Building 2-tier Hierarchy and Utilizing Localized Cache In order to meet the requirement of low latency for multimedia services, we build 2-tier hierarchy in the P2P-SIP system first of all. According to storage capacity and network connectivity of a node, there are two types of nodes in the P2P-SIP system, super node and child node. Super nodes perform lookup service and resource storage service on behalf of child nodes. Child nodes stick to their super nodes, while they only request necessary services to super nodes. Usually, super nodes are stable nodes, while child nodes are weak and dynamic nodes in the P2P-SIP network. However, child nodes are allowed to promote to the role of super nodes whenever they satisfy the criteria on storage capacity and network connectivity. In the lookup service, super nodes can perform both local lookup within their child nodes and DHT-based lookup among them, and preserve the resource as a localized cache for a certain time of period once it is requested. Figure 1 below shows overall picture about 2-tier P2P-SIP architecture with localized cache. Performing two types of lookup under 2-tier hierarchy and utilizing a localized cache make session setup faster than current P2P-SIP system. 12

24 Figure 1. 2-tier P2P-SIP with localized cache 13

25 3.2 Adopting the ID-based Signature to P2P-SIP system ID-based signature scheme is firstly proposed in [11], which is signature scheme, based on the bilinear pairings on elliptic curves. ID-based signature allows users to verify digital signatures using only public information such as the user's identity, while PKI-based signature requires verification of each user s public key from CAs. It can utilize public identifier such as ID or SIP- URI as a public key, so that it reduces the complexity and costs for establishing and managing the public key of nodes. Thus, we make an efficient resource registration and location scheme, using ID-based signature. In order to authenticate resources in P2P-SIP, the user needs to generate ID-based signature and attach the signature to the end of message. In our work, we make use of Hess s ID-based signature algorithm [17]. First, the user creates a public-private key pair. Then the user signs the resource with a private key and registers signed resource to the peer-to-peer network. In turn of resource location, the user gets the signed resource from the peer-to-peer network and verifies that resource with public key, which can be his ID or SIP-URI. In detail, the scheme works as follows: 14

26 3.2.1 Key generation In this process, the user creates a public-private key pair and generates a specific ID for resource to be stored in peer-to-peer network. First of all, the user calculates the public key by using a hash function such as SHA-1. The generation of private key is performed by Private Key Generator, which is one of the trusted third parties. PKG computes the private key, using public key and one temporary random number by Hess s ID-based signature algorithm [17]. The specific procedure for generating private key is out of scope in this thesis. Then, the user can get the private key from PKG directly. And, the user needs to generate the resource ID by using another hash function within network size, N. This is the first step to make an ID-based signature, which attaches to the SIP message. Public Key = hash_func1 (SIP-URI) Private Key = multiplicative pairing operation (t, public key) Resource ID = hash_func2 (SIP-URI) / N 15

27 3.2.2 Resource registration In this process, the user creates two types of signature u and v, and calls put method of peer-to-peer network to store the signed resource. The creation of two signatures requires several operations of Hess s ID-based signature scheme. The specific procedure for generating two types of signature, u and v is out of scope in this thesis. After generating two signature, u and v, those are stored at other super nodes in the P2P-SIP network with resource itself. r = [pairing (P1, P)]k Q id = multiplicative operation (t, P) v = hash_func3 (resource information, r) u = additive operation [multiplicative operation (v, private key), multiplicative operation (k, P1)] P2P.put (id, resource information, u, v) register! 16

28 Figure 2. Registration with ID-based signature 17

29 3.2.3 Resource location and verification In this process, the user calls get method of peer-to-peer network to locate the signed resource and verify the integrity of the resource with two types of signature u and v. The verification of signed resource also requires several operations of Hess s ID-based signature scheme. The specific procedure for verifying the integrity of resource with two signatures, u and v is out of scope in this thesis. However, the user can identify the authenticity through the comparison of signature values P2P.get (SIP-URI) = resource information, u, v locate! r = multiplicative operation {pairing (u, P), [pairing (public key, Qid)]v} v = hash_func3 (resource information, r) compare v with v verify! 18

30 Figure 3. Resource location and verification of reource integrity 19

31 IV Performance Evaluation This chapter describes the performance evaluation in two aspects of the proposed architecture by simulating with Peer-to-Peer simulator (p2psim) [18]. The p2psim can evaluate and investigate various peer-to-peer protocols. It runs in several UNIX-like operating systems. We evaluated the proposed architecture through this tool. Additionally, we implemented SIP protocol over Chord protocol of p2psim and signature generation procedures for PKI-based model and ID-based model. We are interested in lookup latency as a performance metric. The lookup latency is very significant since much delay in P2P-SIP for multimedia communication cannot happen to establish a session with others successfully. The p2psim tool shows statistics about latency of each lookup event during simulations. We make use of those statistics for measurement. 20

32 4.1 Implementation of 2-tier P2P-SIP model, using ID-based signature Our proposed 2-tier P2P-SIP model is implemented in p2psim simulation tool. Basically, our model follows SIP over P2P approach, which means replacing SIP location service by any possible P2P protocol. In p2psim simulation tool, we choose the Chord protocol [8] as an underlying P2P protocol and add the SIP functionality over Chord implementation in p2psim. Figure 4 shows the typical P2P-SIP node architecture used in p2psim simulation tool. In this tool, we define two type of node, which consists of super peer and ordinary peer. Both of nodes are based on the structure illustrated at Figure 4. The super peers build the overall network structure in P2P-SIP and each of them has storage capacity regulating how many ordinary peers can be controlled. They control the promotion of their ordinary peers based on predefined criteria. We set the criteria of promotion as network connectivity for a certain time of period and storage capacity which can take over more than 10 ordinary peers. And ordinary peers assign their super peer when they try to join in the P2P-SIP network by the specific entry point. In the P2P module of node, each peer gets its own peer ID, which is generated by consistent hash function such as SHA-1. Also, resource ID is generated using consistent hash function. In our simulation, the space of these identifiers ranges over 32bit. 21

33 Application SIP User Agent SIP Proxy SIP message P2P-SIP peer P2P module (Chord) Lookup Network Data link & Physical Figure 4. P2P-SIP node architecture The interaction between P2P module and SIP module is operated by the following sequences. Firstly, the SIP user agent constructs REGISTER SIP message and send it over P2P module. Then the P2P module gets the SIP message and identifies the destination of SIP message through parsing. After identifying the destination of SIP message, the lookup method is performed by the P2P module and receives the corresponding IP address of destination node. With identified location information, SIP proxy sends INVITE SIP message over IP network, so as to setup the session with destined node. 22

34 Figure 5. Session setup over P2P-SIP As illustrated at Figure 5, the REGISTER SIP message is first sent by the ordinary peer and its super peer parses the message and sent P2P message over the P2P network by performing lookup method. 23

35 4.2 Simulation environment Table 1 shows the parameter of simulation to evaluate the P2P-SIP protocol based on p2psim. Network size(n) 100, 500, 1000, 3000, 5000, 10000, 30000, Network topology P2P protocol Euclidean model Chord Number of lookup event 30, 100, 1000 Total execution cycle Table 1. Simulation parameters The topology used for evaluation of the proposed P2P-SIP architecture is Euclidean model, which is a metric topology induced by the Euclidean metric. The basic peer-to-peer protocol used for P2P-SIP is Chord, which is one of the original distributed hash table protocols. For different network sizes (100 to 50000) and different lookup trials (10 to 40), we performed simulations to quantize the performance of the proposed scheme, using ID-based signature and the existing scheme, using PKI-based signature. 24

36 4.3 Metrics for evaluation and Simulation result Metrics for evaluation are followings: i) routing hops as session setup latency, and ii) node bandwidth as network overhead. Routing hops from source user to destination user shows the extent of session setup latency indirectly. Overall bandwidth of each node shows the extent of network overhead. 25

37 Figure 6. Effect of 2-tier hierarchy & localized cache 10% We have evaluated the proposed P2P-SIP model with ID-based signature in comparison with PKI-based P2P-SIP model in aspects of session setup latency. Figure 6 shows the evaluation result under 30 lookup events. Firstly, we construct a 2-tier P2P-SIP architecture given 20 super nodes and assume that there exists 10% hit ratio in cache. Then we attach ID-based signature to SIP message. As a result of evaluation with this design, the proposed model represents lower session setup latency than PKI-based model. In this simulation, session setup latency is represented by the number of hops needed to perform a successful lookup. As nodes increase in the P2P-SIP network, the gap of lookup cost between the proposed model with ID-based signature and PKI-based model is getting bigger. As a result of quantitative comparison, we identify average 22.98% of improvement in aspect of session setup latency from 2-tier P2P-SIP with 10% hit ratio cache. 26

38 Figure 7. Effect of 2-tier hierarchy & localized cache 20% In this evaluation, we assume that there exists 20% hit ratio in cache under 2- tier P2P-SIP model with 20 super nodes. Figure 7 shows the result of evaluation under 30 lookup events. As a result of evaluation with this design, the proposed model also plots lower lookup cost than PKI-based model. The network size varies from 100 to As a result of quantitative comparison, we identify average 32.56% of improvement in aspect of session setup latency from 2-tier P2P-SIP with 20% hit ratio cache. In case of the system with high hit ratio, session setup latency is decreased remarkably. 27

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