PRISM Architecture: Supporting Enhanced Streaming Services in a Content Distribution Network
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1 PRISM Architecture: Supporting Enhanced Streaming Services in a Content Distribution Network C. D. Cranor, M. Green, C. Kalmanek, D. Shur, S. Sibal, C. Sreenan and J.E. van der Merwe ¡ AT&T Labs - Research ¢ University College Cork Florham Park, NJ, USA Cork, Ireland Abstract IP content distribution networks (CDNs) provide scal- PRISM is an architecture for distributing, storing, and ability by distributing many servers across the Internet delivering high quality streaming content over IP networks. “close” to consumers. Consumers obtain content from In this paper we present the design of the PRISM architec- these edge servers directly rather than from the origin ture and show how it can support an enhanced set of stream- server. Current CDNs support traditional Web content ing services. In particular, PRISM supports the distribu- fairly well, however support for streaming content is typ- tion and delivery of live television content over IP, as well ically less sophisticated and often limited to the deliv- as the storage of such content for subsequent on-demand ery of fairly low quality live streams (so called “web- access. In addition to accommodating the unique aspects casting”) and distribution of low-to-medium quality video of these types of services, PRISM also offers the frame- clips (“clip-acceleration”). In order for Internet-based work and architectural components necessary to support streaming to approach the same level of entertainment other streaming content distribution networks (CDN) ser- value as broadcast media, CDNs must drastically improve vices such as video-on-demand. We present an overview the quality of streaming that can be supported. Addition- of the PRISM architecture focusing on components unique ally, streaming CDNs will become more sophisticated in to PRISM including content management, content discov- terms of services that can be supported in order to accom- ery, and content-aware redirection. We also describe the modate new business models and new types of service of- PRISM testbed that we are using for experimental lab ser- ferings. vice trials. In this paper we present PRISM, a Portal Infrastruc- 2 Keywords: streaming content distribution network (CDN), ture for Streaming Media. PRISM is an architecture for content management, content discovery, content-aware distributing, storing, and delivering high quality streaming redirection media over IP networks. PRISM enables services that are more sophisticated than those currently available on the In- ternet. Our approach has been to consider in detail how 1 Introduction such an architecture will support these types of stream- ing services by considering a TV-like service as an exam- With the emergence of broadband access networks ple. In particular, PRISM supports both the distribution and powerful personal computer systems, the demand and delivery of live television content over IP, as well as for network-delivered full-motion streaming video con- the storage of such content for subsequent on-demand ac- tent is growing. Indeed, a number of companies cess, i.e. stored-TV (STV). We are not aware of any ex- have already made rudimentary efforts to deliver live isting streaming CDN architectures with the latter capabil- (www.icravetv.com) and stored (www.recordtv.com) tele- ities. The PRISM-based STV service allows users to view 1 vision over the Internet. While the traditional Web service content based on the name of the content as well as the model can be applied to IP-based streaming content, the time at which it was “aired.” For example, users can re- user experience does not compare favorably with the qual- quest programming that aired on CNN at 1PM on January ity of cable, satellite, or broadcast television. On the other 1, 2001. Content stored inside the network is made acces- hand, compared to the Web, current television broadcasting sible throughout the whole PRISM infrastructure. A user technologies offer high quality but provide limited choice based in the U.S. can access European sporting events or and allow little or no on-demand access to video content. other TV content — both live or on-demand. PRISM also £ Contact author: Kobus van der Merwe, AT&T Labs - Research, 180 Park Avenue, Florham Park, NJ 07932, USA, [email protected]. 2An preliminary version of this paper was presented as a work-in- 1Both have since been silenced because of legal problems. progress report at the NOSSDAV 2000 workshop [1]. 2 allows individual users to specify content to be stored in a “network-VCR” type service. In addition, a managed Backbone Network Portal Access Network STV service where certain “bundles” of content are always Portal Live stored by the service provider for user access is feasible. Live Source Client Source Client An example service bundle could include the storage of Client Portal the three most recent hours of content for a large number Live Source of channels, the storage of the peak viewing hours for a Client smaller set of channels stored for a week, and the archival Live of a set of popular programs for several months. This type Source of PRISM service makes available significantly more con- Content distribution: from live sources to portals tent to users compared to emerging personal/digital video and portal-to-portal recorders. Content delivery: from portals to clients Our goal is to provide content at a quality level compa- rable to existing broadcast TV. Coupled with the potential Figure 1: PRISM data plane on-demand nature of PRISM services, this translates into significant access bandwidth requirements. Technically, broadband access technologies such as cable and xDSL Clients: receive content from a portal and display it to offer enough bandwidth for PRISM services at entertain- end-users. Clients are networked set-top boxes or PCs ment quality. An actual service offering would also depend connected to the backbone using broadband access. on business considerations both in terms of broadband ac- A client normally interacts with a local portal that is cess economics and business models that are attractive to close to it in the network. Note that a client’s local content providers. In the near future advances in encod- portal may act as a proxy when it receives a request ing technology might also help to reduce the bandwidth re- for content that it does not have locally stored. This quirements for PRISM-like services. allows the local portal to provide VCR-like controls to While we focus on applying the PRISM architecture to the client even when the content being viewed is com- services involving live and stored television content, the ing from a remote portal, and allows the local portal resulting framework and architectural components are ap- to cache the recently viewed content. plicable to other content-based streaming services such as video-on-demand. The components of the PRISM architec- The three types of PRISM elements communicate ture are introduced in Section 2. These components include through network interactions. There are two classes of content naming, content management, content discovery, PRISM network interactions: those that occur on the data and content-aware redirection. In Section 3 we present a plane and those that occur on the control plane. sampling of related work. The PRISM architecture is be- The data path functionality is required by any streaming ing realized as a trial within our laboratories. We briefly CDN. Since it is not unique to PRISM it is only briefly con- describe the status of this effort as well as future plans in sidered below. Figure 1 shows PRISM’s data plane compo- Section 4. nents: Content distribution: transferring content from a live 2 PRISM Architecture source to one or more portals, or transferring content between portals. Content should be transfered effi- PRISM facilitates the creation of services that make both ciently while maintaining an appropriate balance be- live and on-demand TV content available over an IP net- tween timeliness and reliability. work. The PRISM architecture is built around three types of basic elements, as shown in Figure 1: Content delivery: streaming content from a portal to one or more clients. In order to provide acceptable per- Live sources: receive content from a content provider, en- ceived performance for latency-sensitive operations code and packetize it, and then stream it into the such as VCR-like functions it is important that portals PRISM IP network infrastructure. be topologically close to clients. Portals: receive multimedia content from live sources and The remainder of the architectural discussion focuses on other portals and transmit it to PRISM clients. Por- the PRISM control plane. A unique aspect of the PRISM tals can store and archive live content, thus allowing control plane comes from storing live content inside the content to be viewed on-demand. Portals also provide network for subsequent on-demand access. This differs VCR-like functions such as fast-forward and rewind from on-demand access in conventional streaming CDNs to clients. Portals are positioned between clients and in that there is no well-known origin server where con- live sources, typically at or upstream of a bandwidth tent is known to reside. This has a major impact on the discontinuity such as a cable head-end. control plane architecture. The role of PRISM’s control 3 Inputs: name := "stv:" channel_name - content location | "stv:" channel_name "?" spec Inputs: - client - service location type - load spec := "start=" utctime - SLA - client QoS - load - policy | "start=" utctime ";end=" utctime - policy | "program=" name content location Content | "program=" name ";offset=" time Redirector Content Discovery Access Manager channel_name := "<" brand ";" channel ";" Portal Client distributor ";" location ">" Backbone Figure 3: Naming scheme URI syntax Portal Portal Portal Content Management Content Discovery Content Aware Redirection scheme to allow us to uniquely identify all or parts of a Figure 2: PRISM control plane broadcast stream.