Proxy Caching Strategies for Internet Media Streaming

Introduction

Media streaming consists in the viewing of dynamic media information while being downloaded by clients. With the explosive growth of the Web and the mature of digital video technology, media streaming has received a great deal of interest as a promising solution for multimedia delivery services. This approach allows that media objects can be accessed in a similar way to conventional text and images using a download-and-play mode. However, unlike static text-based content, proxy caching has difficulty in delivering streaming media content because media objects are usually very large and its transmission consumes a great amount of network resources, prolongs startup latency, and threatens the playback continuity. The size of a conventional Web object is typically on the order of 1–100 kbytes and, therefore, a decision regarding either caching or not an object in its totality is an easy task (Liu & Xu, 2004). However, the size of media objects is very large, reaching a size on the order of several hundreds of Mbytes or even Gbytes. Therefore, caching a whole media object at a Web proxy optimized for delivering conventional small-size Web objects is not feasible, since large streams would quickly exhaust the capacity of the proxy cache. Besides, the streaming of media objects requires a significant amount of resources such as disk space and network bandwidth, which need to be maintained during a long period of time. Moreover, the long playback duration of a streaming may allow several client-server interactions. Therefore, access rates might be different for different parts of a stream, which makes cache management potentially more complex, as pointed out by Liu and Xu (2004). On the other hand, a download-before-playing solution provides continuous playback, but it also introduces a large startup delay.

An effective solution to reduce client-perceived latencies and network congestion is to cache data at proxies widely deployed across the Internet. This solution, besides inexpensive, also leads to an improvement of both availability of objects and packet losses since redundant network transmission decreases while transmission efficiency increases. However, proxies are generally optimized for delivering conventional small-size Web objects, which may not satisfy the requirements of streaming applications. Due to these particular features of media objects, novel caching strategies have been proposed.

With the evolution of the Internet as the dominant architecture for applications, contents, and services, these are gradually migrating from the client-server paradigm to the edge services paradigm and to the peer-to-peer (P2P) computing paradigm. Recently, P2P system has received a great amount of interest as a promising scalable and cost-effective solution for next-generation multimedia content distribution. This kind of systems have advantages regarding systems based on the client-server paradigm, namely improved scalability and reliability, cheaper infrastructures due to direct communication among peers, and easiness of resource aggregation in order to provide, for instance, massive processing power (Ye, Makedon, & Ford, 2004). However, P2P systems also have some drawbacks, namely the considerably more complex searching and node organization and security issues (Aberer, Punceva, Hauswirth, & Schmidt., 2002). Therefore, this article limits the discussion to low-cost proxy caching strategies for media streaming over Internet.

Background

As discussed earlier, media caching has different requisites regarding conventional Web caching due to the special features of media streaming. Since the content of a media object is rarely updated, management issues like cache consistency and coherence are less critical in media caching. However, it requires an effective management of proxy cache resources due to the resource requirements of media objects (Liu & Xu, 2004).

Roughly, there are two main types of caching strategies: the strategies focused on homogeneous clients and the strategies focused on heterogeneous clients. Most of the proposed strategies are focused on homogeneous clients, which have identical or similar configurations and capabilities behind a proxy. Figure 1 presents an overview of caching strategies for media streaming. A brief description of these strategies is provided in the next sections.

Figure 1.

Overview of caching strategies for media streaming