【正文】 changed by the overlay socket. Currently available work adapters are TCP, UDP, and UDP multicast. Fourth, the overlay socket provides mechanisms for bootstrapping new overlay works. In this paper, we provide an overview of the overlay socket design and discuss overlay work programming with the overlay socket. The overlay socket has been implemented in Java as part of the HyperCast software distribution [12]. The software has been used for various overlay applications, and has been tested in both localarea as well as widearea settings. The HyperCast software implements the overlay topologies described in [15] and [16]. This paper highlights important issues of the overlay socket, additional information can be found in the design documentation available from[12]. Several studies before us have addressed overlay work programming issues. Even 專業(yè)外文翻譯 3 early overlay work proposals, such as Yoid [9], Scribe [4], and Scattercast [6], have presented APIs that aspire to achieve independence of the API from the overlay work topology used. Particularly, Yoid and Scattercast use a socketlike API, however, these APIs do not address issues that arise when the same API is used by different overlay work topologies. Several works on applicationlayer multicast overlays integrate the application program with the software responsible for maintaining the overlay work, without explicitly providing generalpurpose include Narada [5], Overcast [13], ALMI [17], and NICE [2]. A recent study [8] has proposed a mon API for the class of socalled structured overlays, which includes Chord [19], CAN [18], and Bayeux [20], and other overlays that were originally motivated by distributed hash tables. Our work has a different emphasis than [8], since we assume a scenario where an application programmer must work with several, possibly fundamentally different, overlay work topologies and different transmission modes (UDP, TCP), and, therefore, needs mechanisms that make it easy to change the configuration of the underlying overlay work..Inter Overlay socket Application Overlay socket Application Application Overlay socket Application Application Overlay socket Application Overlay Network. Fig. 1. The overlay work is a collection of overlay sockets. Root (sender) Root (receiver) (a) Multicast (b) Unicast. Fig. 2. Data forwarding in overlay works. The rest of the paper is anized as following. In Section 2 we introduce concepts, abstractions, and terminology needed for the discussion of the overlay socket. In Section 3 we present the design of the overlay socket, and discuss its ponents. In Section 4 we show how to write programs using the overlay socket. We present brief conclusions in Section 5. 2 Basic Concepts An overlay socket is an endpoint for munication in an overlay work, and an overlay work is seen as a collection of overlay sockets that selfanize using an overlay protocol (see Figure 1). An overlay socket offers to an application programmer a Berkeley socketstyle API [3] for sending and receiving data over an overlay work. 專業(yè)外文翻譯 4 Each overlay socket executes an overlay protocol that is responsible for maintaining the membership of the socket in the overlay work topology. Each overlay socket has a logical address and a physical address in the overlay work. The logical address is dependent on the type of overlay protocol used. In the overlay protocols currently implemented in HyperCast , the logical addresses are 32 bit integers or _ x _ y _ coordinates, where x and y are positive 32bit positive integers. The physical address is a transport layer address where overlay sockets receive messages from the overlay work. On the Inter, the physical address is an IP address and a TCP or UDP port number. Application programs that use overlay sockets only work with logical addresses, and do not see physical addresses of overlay nodes. When an overlay socket is created, the socket is configured with a set of configuration parameters, called attributes. The application program can obtain the attributes from a configuration file or it downloads the attributes from a server. The configuration file specifies the type of overlay protocol and the type of transport protocol to be used,.but also more detailed information such as the size of internal buffers, and the value of protocolspecific timers. The most important attribute is the overlay identifier (overlay ID) which is used as a global identifier for an overlay work and which can be used as a key to access the other attributes o f the overlay work. Each new overlay ID corresponds to the creation of a new overlay work. Overlay sockets exchange two types of messages, protocol messages and application messages. Protocol messages are the messages of the overlay protocol that maintain the overlay topology. Application messages contain applicationdata that is encapsulatedn an overlay message header. An application message uses logical addresses in the header to identify source and, for unicast, the destination of the message. If an overlay socket receives an application message from one of its neighbors in the overlay 專業(yè)外文翻譯 5 work, it determines if the message must be forwarded to other overlay sockets, and if the message needs to be passed to the local application. The transmission modes currently supported by the overlay sockets are unicast, and multicast. In multicast, all members in the overlay work are both unicast and multicast,the mon abstraction for data forwarding is that of passing data in spanning trees that are embedded in the overlay topology. For example, a multicast message is