【導(dǎo)讀】對(duì)面向網(wǎng)絡(luò)的研究集中于協(xié)議的設(shè)計(jì)，并且在網(wǎng)絡(luò)中傳輸數(shù)據(jù)，無論怎樣，在應(yīng)用程序軟件的開發(fā)過程中已對(duì)這個(gè)問題引起了一定的注意。顯然，面向網(wǎng)絡(luò)的網(wǎng)絡(luò)協(xié)議的復(fù)雜性要求合適應(yīng)用程序設(shè)計(jì)接口和抽象觀念，不需要面向網(wǎng)絡(luò)協(xié)議的詳細(xì)知識(shí)，因此，簡化了應(yīng)用程序員的任務(wù)。在這篇文章里，我們提出作為聯(lián)系在一個(gè)覆蓋網(wǎng)絡(luò)內(nèi)的終點(diǎn)的新程序設(shè)計(jì)抽象觀念的一個(gè)面向網(wǎng)絡(luò)上的Socket的概念。面向網(wǎng)絡(luò)上的Socket預(yù)防不依賴選擇的一基于Socket的API涂上拓?fù)鋵W(xué)，并且可能成形為不同的面向網(wǎng)絡(luò)工作上的拓?fù)鋵W(xué)。面向網(wǎng)絡(luò)上的Socket能在TCP，UDP或者其他傳送協(xié)議上方支持應(yīng)用數(shù)據(jù)傳輸。目前可得到的網(wǎng)絡(luò)轉(zhuǎn)接器是TCP，UDP和UDP多路傳送。Socket交換二類消息，消息和消息applica-tion協(xié)議。一個(gè)面向網(wǎng)絡(luò)上的Socket由一次在面向網(wǎng)絡(luò)上的Socket被建立時(shí)，成形的零部件的收集組成，使用被提供的套屬性。

　　

【正文】 nstantiation of a new overlay socket. 4 Overlay Network Programming An application developer does not need to be familiar with the details of the ponents of an overlay socket as described in the previous section. The developer is exposed only to the API of the overlay socket and to a file with configuration parameters. The configuration file is a text file which stores all attributes needed to configure an overlay socket. The configuration file is modified whenever a change is needed to the transport protocol, the overlay protocol, or some other parameters of the overlay socket. In the following, we summarize only the main features of the API, and we refer to [12] for detailed information on the overlay socket API. Overlay Socket API Since the overlay topology and the forwarding of applicationlayer data is transparent to the application program, the API for overlay network programming can be made simple. Applications need to be able to create a new overlay network, join and leave an existing overlay network, send data to and receive data from other members in the overlay. The API of the overlay socket is messagebased, and intentionally stays close to the familiar Berkeley socket API [3]. Since space considerations do not permit a description of the full API, we sketch the API with the help of a simplified example. Figure 4 shows the fragment of a Java program that uses an overlay socket. An application program configures and creates an overlay socket with the help of an overlay manager (o m). The overlay manager reads configuration parameters for the overlay socket from a configuration file ( p), which can look similarly as shown in Figure 5. The application program reads the overlay ID with mand (“OverlayID”) from the file, and creates an configuration object (confi g) for an overlay socket with the.// Generate the configuration object OverlayManager om = new OverlayManager()。 String MyOverlay = (OverlayID)。 OverlaySocketConfig config = new (MyOverlay)。 // create an overlay socket OL Socket socket = (callback)。 // Join an overlay ()。 // Create a message OL Message msg = (byte[] data, int length)。 // Send the message to all members in overlay network (msg)。 // Receive a message from the socket OL Message msg = ()。 Fig. 4. Program with overlay sockets. OVERLAY Server: OverlayServer = OVERLAY ID: OverlayID = 1234 KeyAttributes= Socket,Node,SocketAdapter SOCKET: Socket = HCast20 = 255 = 200 SOCKET ADAPTER: SocketAdapter = TCP = 16384 NODE: Node = DT20 = 400 NODE ADAPTER: NodeAdapter = NodeAdptUDPServer = 8192 = :8081 Fig. 5. Configuration file (simplified) given overlay ID. The configuration object also loads all configuration information from the configuration file, and then creates the overlay socket(OverlaySocke t). Once the overlay socket is created, the socket joins the overlay network ( p). When a socket wants to multicast a message, it instantiates a new message () and transmits the message using the sendToAll method. Other transmission options are sendToParent, sendToChildren, sendToNeighbors, and sendToNode, which, respectively, send a message to the upstream neighbor with respect to a given root (see Figure 2), to the downstream neighbors, to all neighbors, or to a particular node with a given logical address. Overlay Network Properties Management As seen, the properties of an overlay socket are configured by setting attributes in a configuration file. The overlay manager in an application process uses the attributes to create a new overlay socket. By modifying the attributes in the configuration file, an application programmer can configure the overlay protocol or transport protocol that is used by the overlay socket. Changes to the file must be done before the socket is created. Figure 5 shows a (simplified) example of a configuration file. Each line of the configuration file assigns a value to an attribute. The plete list of attributes and the range of values is documented in [12]. Without explaining all entries in Figure 5, the file sets, among others, the overlay ID to ‘1234 ’, selects version of the DT protocol as overlay protocol (‘Node=DT20 ’), and it sets the transport protocol of the socket adaptor to TCP(‘SocketAdapter=TCP ’). Each overlay network is associated with a set of attributes that characterize the properties of the overlay sockets that participate in the overlay network. As mentioned earlier, the most important attribute is the overlay ID, which is used to identify an network, and which can be used as a key toaccess all other attributes of an overlay network. The overlay ID should be a globally unique identifier. A new overlay network is created by generating a new overlay ID and associating a set of attributes that specify the properties of the overlay sockets in the overlay network. To join an overlay network, an overlay socket must know the overlay ID and the set of attributes for this overlay ID. This information can be obtained from a configuration file, as shown in Figure 5. All attributes have a name and a value, both of which are strings. For example, the overlay protocol of an overlay socket can be determined by an attribute with name NODE. If the attribute is set to NODE=DT2 0, then the overlay node in the overlay socket runs the DT (version 2) overlay protocol. The overlay socket distinguishes between two types of attributes: key attributes and configurable attributes. Key attributes are specific to an overlay network with a given overlay ID. Key attributes are selected when the overlay ID is created for an overlay network, and cannot be modified afterwards. Overlay sockets that participate in an overlay network must have identical key attributes, but can have different configurable attributes. The attributes OverlayID and KeyAttributes are key attributes by default in all overlay networks. Configura