電子電氣自動(dòng)化通信外文翻譯--基于pastry點(diǎn)對(duì)點(diǎn)路由層的結(jié)構(gòu)化即時(shí)消息體系

1、 1中文 中文 3957 字出處： 出處：SNCNW 2003, Swedish National Computer Networking Workshop, Stockholm. 2003英文原文： 英文原文：A Distributed Instant Messaging Architecturebased on the

2、 Pastry Peer-To-Peer Routing SubstrateHenrik Lundgren, Richard Gold, Erik Nordstr¨ om, Mattias WiggbergDepartment of Information Technology,Uppsala University, Sweden.ABSTRACTCurrent Instant Messaging systems rely o

3、n archi-tectures that include central, or only partly dis-tributed, server(s). Although some systems can ex-change messages peer-to-peer, the user registration and lookup are still based on a centralized solution. We hav

4、e implemented a fully distributed instant mes-saging system by utilizing the Pastry peer-to-peer routing substrate. We use the Pastry object inser-tion functionality to insert user information when a user joins the netwo

5、rk and a novel search engine to effectively perform distributed user lookup. Cur-rently, our system runs in the FreePastry simulator and supports joining/leaving the network, searching for users and message exchange. 1.

6、INTRODUCTIONThe increasing availability of cable and DSL services has made it possible for users to be continuously connected to the Internet. Users now want to announce their presence to their friends so that they can “

7、meet” online and communi-cate. E-mail is still the common way to exchange messages on the Internet. It is a store-and-forward system where mes-sages are stored at intermediate nodes (mail servers) and only retrieved at u

8、ser request. With increased user pres-ence, Instant Messaging (IM) is gaining popularity. IM sys-tems deliver messages, as the name implies, instantly i.e., messages are sent directly to the recipient without any in-term

9、ediate storage and thus allow for real-time text-based communication.There exist several IM systems today and depending on their architectures, messages can be forwarded via dedicated servers or, preferably, directly bet

10、ween the communicating parties (peer-to-peer). Table 1 shows a side by side com-parison of the most common IM systems today. Among all IM systems, none is both fully distributed and decentral-ized in terms of registratio

11、n, lookup and message delivery.Although some of them, like Jabber and IRC (with DCC), have some of the desired characteristics, they all suffer from a single point of failure where a malfunction will close the service, e

12、ither by making it impossible to find clients or de-liver messages.Peer-to-peer systems provide powerful platforms for a va-riety of decentralized services, such 32.3 Node Arrival and Node DepartureA new node joining the

13、 overlay network has to initialize its state tables (i.e., the routing table and the leaf set) and then inform others of its presence. Assume a node with nodeId X. When node X wants to join the Pastry network it first lo

14、cates a node in the network using for example “expanding ring IP multicast”. Assume the found node has nodeId A. Now node X sends a special join message with key X to node A. As with normal messages, node A routes this m

15、essage to node Z which is numerically closest to X. Each node along the path to node Z sends one row from its routing table to node X. The i:th node sends its i:th row. Node Z sends its leaf set to node X. Finally, node

16、X informs any node that needs to be aware of its arrival.As nodes may fail or depart without warning, neighboring nodes in the key space (i.e., nodes in the leaf set) periodically exchange keep-alive messages. If a node

17、is unresponsive for a time T, it is considered to be failed and all the members of the failed node’s leaf set update their leaf sets. Stale entries in the routing table are repaired by first trying to find a new route vi

18、a its downstream node, and if not successful, starting its route table management mechanism.2.4 APastry-based Store-and-Forward systemPOST [2] is a messaging system based on Pastry which provides message storage, per-use

19、r meta-data (e.g., relating messages to user), and notification. In POST, messages are objects that are being inserted into to the peer-to-peer net-work and “message storage”-nodes are responsible for noti-fying the reci

20、pient when it has any messages pending. Such a system is well-suited for email, but does not fulfill the re-quirements for being a true instant messaging system as it is a store-and-forward architecture.3. DISTRIBUTED IN

21、STANT MESSAGING ARCHITECTURE (DIMA)We have designed a Distributed Instant Messaging Archi-tecture (DIMA) using Pastry and SCAN (Searching in Con-tent Addressable Networks) [3]. The Pastry overlay network is used for node

22、 insertion and message routing. SCAN is a search method for structured peer-to-peer networks, cur-rently being developed at Uppsala University. The current version of SCAN is implemented on top of Pastry, but could equal

23、ly well be implemented on top of other routing sub- strates like for example Chord [5]. We start this section with a short overview of SCAN.3.1 SCANSCAN implements a search mechanism for Pastry net-works that is capable

24、of identifying the nodes holding the most relevant information. A classification of relevance of each node that is returned by the system is made on the basis of the number of keywords successfully matched. 3.1.1 Key Gen

25、erationThe idea behind SCAN is that content is addressable. In SCAN, a Pastry key does not correspond to one physical host, but to a piece of content on a host. Therefore a host may insert more than one nodeId into the P

26、astry network. Content meta-data e.g., keywords, are encoded using the ASCII table, into a set of Pastry keys for NodeIds that are inserted into the network. A salt is added to the end of the Pastry key to make sure that

27、 similar/equal keywords do not generate the same Pastry key (figure 1). However, similar Pastry keys (generated from the same keyword) are inserted close to each other in the Pastry network since they share the same pref