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Architecture of an intelligent Quality-of-Service aware Peer-to-Peer Multimedia Network Christoph Loeser, Michael Ditze, Peter Altenbernd C-LAB Paderborn University, Siemens Business Services Paderborn, Germany {Christoph.Loeser | Michael.Ditze | Peter.Altenbernd}@c-lab.de

Abstract

Peer-to-Peer networks have gained a lot of popularity in the past few years. Each participant offers own resources and occupies resources of other peers. This paper proposes an architecture for a P2P streaming network that considers structured data placement strategies of replicated multimedia content and QoS routing (RSVP). We consider point to point streaming within an autonomous system. So called active Rendezvous Servers do not just act as lookup services but rather perform the content distribution within the network. By analyzing access frequency time series it is possible to perform an intelligent replica method. Thus replica creation depends on access frequency, storage location, and each peers’ admission control. Keywords: Peer-to-Peer, Streaming, Video on Demand, Content Replication, Quality of Service, RSVP 1 INTRODUCTION In the last years peer-to-peer (P2P) applications became extremely popular for file sharing applications like Gnutella, eDonkey / Overnet, Kazaa, etc. When considering P2P applications, users typically talk about the sharing of resources like computation power, instant messaging, videoconferences, and MP3 / video files. Each peer is client and server at the same time: it offers resources and is also able to occupy other peers resources. Though the idea of peer-topeer communication is not new, application layer architectures in the Intra- or Internet mainly base on client server techniques: e.g. there are Mail-, HTTP-, FTP-, LDAPServer. However, P2P is much more and it denotes a general communication strategy, that avoids bottlenecks and single points of failure by reducing the number of centralized instances. In this field we distinguish between pure P2P and hybrid P2P architectures. Pure P2P systems do not have any central server. Hybrid architectures, in contrast, use at least one directory or Rendezvous server, that administrate references of all connected peers and their content. There are existing distributed P2P indexing and lookup services like Chord [30], CAN [26] or Pastry [27] which do not need Rendezvous Server at all and seem to be more fault tolerant than hybrid architectures. Though there has been much research activity in general P2P applications (e.g. content distribution, lookup services,

or groupware support) there has been just little work in P2P streaming networks. The main difference between conventional P2P and P2P streaming applications is the instant time when content is used. Considering the first item, content is completely downloaded before files are opened. In contrast to the P2P streaming where content is decoded, personated immediately and afterwards (probably) is discarded. The outline of this paper is as follows: In the next section we describe the characters of our network architecture, followed by the used protocols and techniques in section 3. In section 4 we outline the work of the active Rendezvous Server in a realm. In the last section we give an overview of related work and how this architectures differs from existing ones. 2 THE VIDEO PEER-TO-PEER NETWORK The base idea of sharing resources and (controlled) content distribution in a P2P environment can also be adapted to a video streaming network. Up to now most Video on Demand networks base on the traditional Client-Server architecture: One (or more) server with extreme resource capabilities offers streaming content to a set of clients (via streaming). The disadvantage of this architecture is that the scaling is not satisfying: Especially the Quality of Service (QoS) capabilities are in need of extreme resource demands and thus bound the number of possible simultaneous streams. But there have also been some proposals concerning hierarchal architectures: Here are some main-servers (also called backup-server or video-warehouse), which distribute their video content to some sub-servers that are located below in the hierarchy (often proxy server). The individual streaming takes place from the sub-server to the end-consumer. The use of P2P techniques transforms each computer to a small video server in the network that is able to store a few movies and offers them to other peers. In [19] we presented the base concept of a VoD peer-to-peer network. Also here we consider large institutes: e.g. a large hotel with set-top boxes in each hotel room. The base architecture of the network is shown in fig. 1. A set of peers interconnected with layer-2 Switches are forming several sub-networks (circles / ellipses). Each peer contains a P2P middleware, which ensures the base communication to other peers. The individual sub-networks are interconnected with routers.

Before we go further into technical details we describe some characteristics of the network: 1. We consider closed environments (i.e. universities, large companies, large hotels, etc.) that form an autonomous system (AS), containing several layer-2 subnetworks interconnected with routers. Additionally, we assume peers with different resource capabilities and heterogenous bandwidth constrains. For optional efficient streaming path selection we assume the routing protocol to be OSPF. 2. Standard connections between two peers are just point to point connections. We do not consider many-to-one or multicast connections. 3. We do not consider (distributed stored) segmentation of movie content. However, the source peer may be changed if a (significant) more efficient alternative source peer is recognized. 4. The use of centralized instances, i.e. a hybrid P2P architecture, mainly results from the task for efficient path selection, similar to "designated routers" in OSPF. 5. The AS network is segmented into so-called realms. Each realm has one active Rendezvous-Server (ARS) and each peer belongs to just one realm. Realms are segmented into groups. All peers interconnected with one layer2 switch represents one group. The system has to solve an optimization problem. A set of video content is distributed redundantly over the peers. The two main targets of this architecture are: • Efficient path selection for streaming within the

realm: Minimize the sum of all reserved bandwidth within the individual routers. Thereby it is necessary to verify that some routes will not become bottlenecks because of too many bandwidth reservations per edge. • Proactive workload management / data placement:

By analysis access frequency time series it is possible to determine when and which video content has been accessed. Basing on these values describing the past it is possible to create forecasts for the future. Thus replicas can be created and distributed before users actually send a query. Video content which has to be copied from another realm or which has to be replicated within the realm has to be placed on the destination peer itself (then there is no further path reservation necessary), or as close as possible to the destination peer (HOP count) to minimize the amount of all path reservations. Thereby the current workload of all possible peers have to be taken into account. Autonomous Systems (AS) We assume, that all peers and routers are administrated by one central instance. I.e. system administrators can choose which interior gateway routing protocol is used within the AS (e.g. RIP, OSPF, IGRP, EIGRP, etc). We assume the

Paderborn University to be an AS. The individual departments may then be the individual realms (fig. 1). For efficient streaming path selection OSPF as routing protocol is necessary. P2P Rendezvous Server A P2P rendezvous sever (RS) stores content references of individual peers and their content for all peers within its realm. Each peer in the network is assigned a unique RS. When a peer seeks for a specific movie content, the RS determines all possible source peers and then filters them according to their current workload and the edge congestion. In this architecture the rendezvous service is not bounded to a specific computer: To reduce the probability that the ARS acts as a single point of failure, each peer can become the leading ARS. As soon as a failure of the ARS is recognized the Bully-algorithm by Garcia-Molina as described in [32] elects the new ARS instance. Concerning the transport mechanisms of video content we distinguish between the following two techniques: • Streaming with Quality of Service guarantees: If the

content a peer requests is available within the own realm, the ARS determines a source peer and initiates the RSVP bandwidth reservation. • Copy with Best-Effort: If the requested content is not

available within the own realm the ARS seeks in other realms by contacting other ARSs. If the content is found it is copied with best-effort into the own realm. 3 PROTOCOLS AND TECHNIQUES Quality-of-Service & RSVP The Quality-of-Service demands are realized with the IntServ technique: The Resource Reservation Protocol (RSVP) determines a fixed path for IP-packet in the networks, reserves the demanded bandwidth and thereby guarantees an upper bound for the packet latency and the abberation of the average latency (jitter). The RSVP protocol consists of two local decision procedures: the admission control and the policy control. The admission control determines whether the nodes have sufficient available resources to supply the requested QoS (see next section). The policy control determines whether the user has administrative permissions to make the reservation. Establishing a QoS connection with RSVP occurs as follows (see [4]): The (RSVP) client sends a PATH message to the server. On its way the message logs all HOPs (host on path) which are on the route. As soon as the server receives the PATH message, it checks whether local resources are sufficient to assure the requested QoS. This is done by the local admission Control instance. In the positive case a RESVmessage is sent back to the client on the same path. Thus the message allocates the demanded bandwidth in the router. All packets of the following media stream, encapsulated in RTP packets, are then not sent due to the local routing tables of the routers but rather due to the fixed path. Bandwidth reservation in each router has a timeout (soft state).

  

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Figure 1: Base architecture of the P2P video network.(solid lines - QoS connections; dashed lines - best effort connections) The path which is reserved by the RSVP protocol depends on the routing protocol in the IP layer which is nondeterministic due to the possibility of all modern routing protocols to adapt the path selection to current link workload. To achieve an efficient path reservation in the realm it is necessary for higher layers to get influence on the path selection. Open Shortest Path First (OSPF) OSPF is the most common link-state routing protocol [17] [21]. Each router contains a database of all available edges that connect all available routers in the autonomous system. In fixed periods each router publishes Link State Advertisements (LSA) that describe the status of all edges. These LSAs are published by the flooding protocol to all other routers. OSPF standard metric is bandwidth. The advantage of of OSPF in contrast to other routing protocols (e.g. RIP) is the guarantee of being free of circles and a very fast convergence speed. 4

ACTIVE RENDEZVOUS-SERVER

The rendezvous server does not only act as a directory server, however it acts active in its realm: The ARS is creating replicas basing on access frequency observations and initiates also the RSVP connections.

A realm is defined as a graph G = (V, E). A path system W defines for each pair of nodes (x, y) ∈ V 2 a path from node x to node y . Let c(e) be the bandwidth capacity of edge e and cresi (e) a reservation i of bandwidth for edge e. For each e ∈ E is cres (e) for the sum of all reserved bandwidth of the path P w ∈ W containing the edge e. I.e. cres (e) := i cresi (e) where P i stands for the number of reservations. It is c(x, y) := e∈path(x,y) cres (e) the sum of all reserved bandwidth of the route from x to y . The ARS ∈ V may just run on peers. Efficient Path Selection for the RSVP-Reservation. Content queries which have been sent to the ARS are not just answered by determining all possible sources but rather by filtering this list with regard to the peer workload. Furthermore the ARS also determines the RTP streaming path and also initiates the reservation. Thus the ARS has to be aware of the realm topology, the different bandwidth capacities, and how much bandwidth has been reserved for which link (cres (e)). Therefore the OSPF and the RSVP protocol have to be adapted. The ARS algorithm works as follows: 1. Receive the actual link state database from the ARS

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Figure 2: Seasonal access frequency (with trend) of one content item. Forecasting after Wednesday (dashed). assigned gateway. This presumes a marginal modification of the standard OSPF algorithm. 2. When receiving a content request from a destination peer t, determine all possible source peers where the movie content is located. Thereby the ARS has to take the peer workload into account. This bases on the known in- & out- streams. s is the so determined source peer. 3. Then the ARS creates a shortest path tree with source s (resp. its gateway router) as root using the Dijkstra algorithm (O(E log N)). This tree differs from the standard tree that the routers create because it does not just takes the shortest path/bandwidth as metric but rather also the actual reserved/available bandwidth c(e) − cres (e). Thus it is possible to optimize the path to min(c(s, t)), and to guarantee a minimum of remaining bandwidth because the ARS knows all current streams and their reservations. The determined s and the thereby predefined RTP streaming path is then sent to t. When receiving the message from the ARS, t initiates a RSVP-PATH message to s. The now deterministic path selection does not rely on the individual tables of the routers but is rather predefined (QoS routing). Similar to the later relaying RTP video packets also the route of the PATH message is predefined. As soon as the RSVP connection is established t sends a notification message to the ARS. Active Data Distribution by the ARS It is essential to take a bottleneck avoiding parameter into account when assessing the RSVP path: Assuming the number of P2P connections may be high we must avoid that best effort IP traffic is affected. If the network contains x identical copies of a movie the number of possible streaming connection of this content probably will not exceed a number of 2 ∗ x or 3 ∗ x. However it is important to assure that there remain resources to be able to create replicas via best effort.

Admission Control (AC) Each peer that requests and offers a QoS-oriented video stream delivery processes an Admission Control (AC). The job of the AC is to analyze the real-time scheduling behavior of the peer in terms of processing power. It determines if further tasks can be processed on this peer without interfering already executing tasks, thus decreasing the QoS. Such tasks could be responsible for the decoding of compressed video information or the transmission of stored video data. Their necessity for real-time processing derives from the time constrains of video streams that require to display one video frame every 33ms according to the NTSC standard. The AC hence determines in advance if such a given taskset is feasible. A task-set is called feasible if all tasks in the set finish execution before their deadline even in worst-case conditions, i.e. when all tasks are released simultaneously and compete for CPU-resources [2]. An Admission Control Manager (ACM) then assigns resources to every task in the feasible task-set. Thus, the ACM determines if an additional peers request for resources can be granted. Otherwise, the stream is rejected. We presented such a method for Admission Control and real-time scheduling of MPEG streams in [13]. Proactive load-management / Caching Dealing with proactive workload management we consider a time dependent counters for each movie content managed by the ARS. For a cyclic content request it is possible to create time specific forecasts. Content is then replicated in advance. An important class of methods for modelling stochastic processes / timeseries is the ARIMA model. It is a set of filters which are applied to a white noise. These filters consist of an autocorrelation (AR), an integration (I), and a moving average method (MA). Generally the ARIMA method is described by a triple (a,b,c) which describes the order of the individual components (AR,I,MA). Moreover Box and Jenkins [6] added a further seasonal component. Thereby it is possible to make forecasts to achieve a appraisal for future values [22][7] (SARIMA). To analyze the white noise the autocorrelation function (ACF) and the partial autocorrelation function (PACF) are

used. The ACF characterizes the linear dependency between the values of a time series. The PACF determines the correlation of two values at the borders of a lag, not considering the correlation within the period. The ACF and PACF are used with the original time series. Peaks and seasonal cycles are then detected and can be included in the model by creating differences. Due to this removals the time series is stationary (no trend, no seasons). In fig. 2 you can see the access frequency of a video content. Here it looses popularity (trend) with proceeding time. Furthermore a seasonal behavior can be recognized. Basing on these past values it is then possible to create forecasts for (in this example) the next two days. 5

RELATED WORK

Within this chapter we give an overview of existing architectures and techniques. There has been quite a lot of work in general P2P networks, streaming network architectures, Video on Demand, multicast/broadcast algorithms, and quality of service techniques. However, these papers differ in several points from our architecture or they concentrate on just one a few of these topics. When considering some architectures of video networks there are quite a lot covering hierarchal structures in WANnetworks: In [15] authors propose a Central Office (CO) that delivers movie content to the end-user. [8] suggest an architecture with several backup-servers, that deliver their content to local-cache-server that are located in the middle of this hierarchy. They offer the videos with QoS support to the end-user. Also [35] proposes a central instance, a so-called Video Warehouse, which provides videos to Intermediate Storages which are located closer to the end-user. Here are no IP-QoS strategies considered, but rather some scheduling strategies for Video-on-Reservation. Also [20] uses a hierarchal structure and uses besides several VideoRoot- and Proxy-Server also a central directory server. The directory server administrates just references on the video content and is hence not an active part of the content replication and distribution. A lot of work has been done in the last few years in the P2P area: On the one hand there are P2P storage projects like e.g. CFS [11], Oceanstore [18] or PAST (bases on Pastry), on the other hand there are projects for indexing distributed content (some with efficient high level routing) like CAN [26], Pastry [27] [28], Chord [30], and Tapestry [37]. The aim of these projects is to eliminate the need of any central instance. These kind of indexing problems can also be found outside the P2P world. Lately there are also several proposals refering to P2P streaming: [12] introduces SpreadIt to build a multicast tree and to ensure good Quality of Service (not on IP-level). Also [33] proposes algorithms for the creation of a multicast tree. Using these multicast trees peers receive a media stream (from a unique root source or from other peers) and relay it to other peers. [36] proposes an architecture of heterogenous peers with "many to one" streaming. Moreover the authors say that peers do not act as servers. Also [23] uses multiple senders with just one receiver. However this architecture is more a client-server system.

The Resource-Mapping-Problem within a distributed, replica containing/creating storage environment is considered in [10]. Here the actual workload of different nodes is taken into account when replicas shall be created. However the authors just describe the problem and does not give a real solution. [9] proposes a replication algorithm for generic Content Distribution Networks (CDN). When creating replicas the algorithm builds up a Dissemination Tree. They distinguish between the original source, a replica, and a cache instance. But the QoS-constrains (neither IntServ nor DiffServ) that shall be taken into account are only marginally described. In [25] and [24] authors present a P2P QoS-aware replication service of distributed movie content for teaching purposes in a network. The communication bases on multicast. This has direct impact on the scalability of the architecture. To avoid excessive use of multicast-addresses authors propose to organize IP-addresses to smaller sub-groups. Regarding OSPF extensions for QoS routing [16] and [3] propose three techniques: Two of them consider the precalculation of possible QoS routes. They base on the Bellman-Ford algorithm and a three step quantization of possible edge workload. The third method proposes a ondemand calculation of QoS-routes. The advantage in contrast to the other two algorithms is the more exact adaption of the actual workload of the individual edges. The authors assume a pure distributed architecture without a central instance. Due to this the edge workload has to be propagated too often to all other routers and thereby create a strong coordination overhead. Thus authors propose a model where the link status is propagated in large periodical intervals or at significant modification. Due to the authors this technique may lead to some wrong routing decision. Further, not so close, topic related papers are the following: [14] describes pure P2P multicast problems and proposes a practical IP implementation. [5] elucidates QoS within end-devices and describes the architecture of a server with internal data replication. In [29] the NGISec protocol is described. This is a secure P2P protocol which is due to the authors superior to IPSec and SSL/TLS. Furthermore it offers QoS support. A more general description of P2P media streaming can be found in [31]. No concrete proposals can be found in this document. 6 Conclusion & Future work In this paper we proposed an architecture of an intelligent P2P-QoS aware video network for autonomous system networks (e.g. universities, hotels, large companies or institutes). We considered direct RSVP enabled point to point streaming connections between two peers. The whole network is segmented in realms, each controlled by an active Rendezvous Server which is not just acting as a directory server. Rather, the ARS initiates the creation of replicas within the realm according to the access frequency. By analyzing seasonal access time series (SARIMA), content can be distributed in advance with a standard IP best-effort strategy. Additionally the ARS determines the most efficient source peer by considering both the current workload of all possible source peers (admission control) and the cur-

rent workload of all edges. The latter assumes the use of the OSPF routing protocol within the autonomous system. Thus we proposed an application specific QoS routing. We have created a test-bed with several Linux machines running RedHat Linux (Kernel 2.4.20), XFree 4.2, Gstreamer 0.7.0.1 (developer version from sourceforge CVS), libmpeg2 for decoding. However the ARS runs on a Sun machine which acts as a QoS-enabled router. Therebye it is able to gain the topology information. Moreover we created a P2P Streaming Simulation Environment (which partly bases on [34] and uses the LEDA library [1]). With this application we are able to evaluate the content distribution and path selection algorithms of the ARS. This will be a major part of our future work. References [1] LEDA - Library of efficient data types and algorithms, http://www.algorithmic-solutions.de/leda.htm. [2] A. W. A. Burns. Real-Time Systems and Programming Languages (second edition), Addison-Wesley. 2097. [3] G. Apostolopoulos, R. Guerin, S. Kamat, A. Orda, T. Przygienda, and D. Williams. QoS routing mechanisms and OSPF extensions.Internet Draft, December 1998. [4] G. Armitage. Quality of Service in IP Networks, ISBN 1578701899. Macmillan Technical Publishing, 2000. [5] M. Billot, V. Issarny, I. Puaut, and M. Banatre. A proposal for Ensuring High Availability of Distributed Multimedia Applications. In Symposium on Reliable Distributed Systems, pages 220–227, 1996. [6] E. Box and G. Jenkins. Forecasting & Time Series: An Applied Approach, PWS Publishers, Belmont, 1976. [7] P. Brockwell and R. Davis. Time Series: Theory and Methods, Springer Verlag, New York, 1991. [8] L.-O. Burchard and R. Lüling. An Architecture for a Scalable Video-on-Demand Server Network with Quality-of-Service Guarantees. In Proc. 7th International Workshop on Interactive Distributed Multimedia Systems and Telecommunication Services, LNCS, pages 132–143. Springer, 2000. [9] Y. Chen, R. Katz, and J. Kubiatowicz. Dynamic Replica Placement for Scalable Content Delivery. In International Workshop on Peer-to-Peer Systems, 2002. [10] J. Chuang. Resource allocation for Stor-Serv: Network storage services with QoS guarantees. In Proceedings of NetStore’99 Symposium, 1999. [11] F. Dabek, M. Kaashoek, D. Karger, R. Morris, and I. Stoica. Wide-area cooperative storage with CFS. In Proceedings of ACM SOSP, 2001. [12] H. Deshpande, M. Bawa, and H. Garcia-Molina. Streaming Live Media over a Peer-to-Peer Network. Stanford Database Group Technical Report 2001-30. [13] M. Ditze. A new method for the real time scheduling and admission control of mpeg-2 streams, m.sc. thesis, paderborn university, 2001. [14] N. Eng, I. A. Rahman, and W. Suit. An Initial Approach of a Scalable Multicast-based pure Peer-to-Peer System. In Proceedings of the First International Conference on Peer-to-Peer Computing (P2P 01), 2001. [15] A. Gelman, H. Kobrinski, L. Smoot, and S. Weinstein. A Storeand-Forward Architecture for Video-On-Demand Service. In Proceedings of the International Communications Conference, page 27, 1991.

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