软件体系结构第7-2讲.ppt

7) Remote Access 远程访问 The whole concept of Peer-to-Peer systems runs on the principle of remote access.远程访问原则. Users are scattered all of the world Remote Access is an important quality requirement of Peer-to-Peer systems. 9. Some Issues of Further P2P Evolution 6. Pure Peer-to-Peer architecture: Gnutella 工作步骤-Steps: 1. Node A is first connected to the network. 2. It Pings to the other Nodes (B and C) to discover new nodes on the network. 3. A pong message is sent as a reply to a ping and provides information on a network node, including IP address, port number, and number of files shared. 4. A query message is used to search for files shared by other nodes on the network. It contains a query string and a minimum requested link speed. 6. Pure Peer-to-Peer architecture: Gnutella 5. A query reply message contains a list of one or more files which match a given query, the size of each file, and the link speed of the responding node. 6. A push message is used to upload file to clients behind a firewall who can not download files themselves. 6. Pure Peer-to-Peer architecture: Gnutella Note: For the connection, a node has to know the IP of the other node that is already online. once it is connected, it makes an announcement that it is online. Each of the other nodes reply to the message of information like the number of files it contains and the other details. 6. Pure Peer-to-Peer architecture: Gnutella For the file sharing to happen, the node acts as a HTTP Web Server. If the search has to download, it connects to the node in the same way as a browser would connect to a web server. 6. Pure Peer-to-Peer architecture: Gnutella How long will a specific request live“? 存活时间. TTL: time to live. It determines how long a specific request will live on the network. The number is actually how many layers each request will pass through, where a layer is when one servant has sent the request to all of the servants connected to it.