© Chinese University, CSE Dept. Distributed Systems / Distributed Systems Topic 4: Naming, Trading, and Peer-to-Peer Systems Dr. Michael R. Lyu Computer Science & Engineering Department The Chinese University

© Chinese University, CSE Dept. Distributed Systems / Outline 1. Naming 2. Trading 3. P2P Systems 4. Summary

© Chinese University, CSE Dept. Distributed Systems / The Need: Location Transparency  Avoid using physical locations for locating components!  Naming: –Locating components by external names –Similar to white pages  Trading: –Locating components by service characteristics –Similar to yellow pages

© Chinese University, CSE Dept. Distributed Systems / Naming 1. Names, Addresses and Other Attributes 2. Naming Service Examples - Network File System (NFS) - Internet Domain Service 3. Common Characteristics 4. Limitations

© Chinese University, CSE Dept. Distributed Systems / Names and Their Attributes  Names are used to refer to a wide variety of resources (computers, services, remote files, or users).  Names are not the only useful means of identification: descriptive attributes are another.  A name is resolved when it is translated into data about the named resource or object, often in order to invoke an action upon it.  The association between a name and an object is called a binding.  In general, names are bound to attributes of the named objects. An attribute is the value of a property associated with an object.

© Chinese University, CSE Dept. Distributed Systems / Name Services  Name service: To store a collection of one or more naming contexts, which are sets of bindings between textual names and attributes for objects such as users, computers, services and remote objects.  Name service operations: binding a name and resolve a name  Name management is separated from other services for openness: –Unification: using the same naming scheme. –Integration: sharing name resources in different administrative domains.  Name space: the collection of all valid names recognized by a name service.  Hierarchic name space: names are hierarchically organized so each part of a name is resolved relative to a separate context.  Name service examples: Network File Service (NFS) Directories, Internet Domain Name Service, and X.500 Directory Service.

© Chinese University, CSE Dept. Distributed Systems / NFS Directories lec lyu teaching course_web www usr sbin bin inetd ls rlogin uac king cprj Example1: /uac/lec/lyu/course_web Example2: /uac/cprj/king/www

© Chinese University, CSE Dept. Distributed Systems / Internet Domain Name Service  The object named by DNS are IP addresses of computers as (naming) domains.  In DNS, any name can be resolved by any client, due to –Hierarchical partitioning of name database –Replication of naming data –Caching  Internet DNS name space is partitioned both organizationally and according to geography.

© Chinese University, CSE Dept. Distributed Systems / Internet Domain Name Service a.root-servers.net (root) edu.hk ac.uk cse.cuhk.edu.hk se.cuhk.edu.hk ns1.cuhk.edu.hk (edu.hk) ns1.cs.ucl.ac.uk (ac.uk) ic.ac.uk qmw.ac.uk city.ac.uk nameserv.city.ac.uk (city.ac.uk) *.city.ac.uk *.cse.cuhk.edu.hk beryl.cuhk.edu.hk (cse.cuhk.edu.hk)

© Chinese University, CSE Dept. Distributed Systems / Composed Naming Domains from URL URL Resource ID (IP number, port number, pathname) Network address 2:60:8c:2:b0:5a file Web server WebExamples/earth.html8888 DNS lookup Socket

© Chinese University, CSE Dept. Distributed Systems / Uniform Resource Identifier (URI)  URL is a particular type of URI in identifying web resources.  URLs scale up to an unlimited set of web resources for efficient access.  URLs are essentially addresses of web resources, which do not support mobility transparency.  Uniform Resource Name (URN) allows web resources to move around.  Uniform Resource Characteristic (URC), a subset of URN, describes attributes of web resources.

© Chinese University, CSE Dept. Distributed Systems / Navigation  Navigation is the process of locating naming data from among more than one name server in order to resolve a name.  Iterative navigation: a client presents the name to the local name server which attempts to resolve it.  Multicast navigation: a client multicasts the name to be resolved to the group of name servers.  Server-controlled navigation: A name server coordinates the resolution of the name and passes the result back to the user agent. –Non-recursive vs. recursive

© Chinese University, CSE Dept. Distributed Systems / Iterative Navigation Iterative navigation: A client iteratively contacts name servers NS1–NS3 in order to resolve a name In multicast navigation, the client multicasts the name to be resolved to a group of name servers. Client NS2 NS1 NS3 Name servers

© Chinese University, CSE Dept. Distributed Systems / Server-Controlled Navigation A name server NS1 communicates with other name servers on behalf of a client client Recursive server-controlled NS2 NS1 NS3 NS2 NS1 NS3 Non-recursive server-controlled

© Chinese University, CSE Dept. Distributed Systems / Name Caching  Caching is a general computing scheme to improve performance of data access.  Name caching is a process in maintaining the results of previous name resolutions in server memory space.  In addition to name service’s performance, caching helps to maintain the availability of both the name service and other services.  Caching is widely applicable because naming data are changed relatively rarely.

© Chinese University, CSE Dept. Distributed Systems / Common Characteristics Concerns to be addressed by a naming service:  Names.  Name Spaces.  Naming service provides operations for –Defining names of components (bind). –Lookup components by name (resolve).  Persistence of bindings.

© Chinese University, CSE Dept. Distributed Systems / Common Characteristics Qualities of service:  Distribution of name spaces  Performance profile –Caching –Replication  Transaction properties of concurrent naming operations

© Chinese University, CSE Dept. Distributed Systems / Limitations  Limitation of Naming: Client always has to identify the server by name.  Inappropriate if client just wants to use a service at a certain quality but does not know who is the service provider. Example: –Automatic cinema ticketing, –Video on demand, –Electronic commerce.

© Chinese University, CSE Dept. Distributed Systems / Trading 1. Characteristics 2. Example 3. A Typical Trading Service

© Chinese University, CSE Dept. Distributed Systems / Trading Characteristics  Trader operates as broker between client and server.  Enables client to change perspective from ´who?´ to ´what?´  Selection between multiple service providers.  Similar ideas in: –yellow pages –insurance broker –stock brokerage.

© Chinese University, CSE Dept. Distributed Systems / Trading Characteristics  Common language between client and server: –Service types –Qualities of service  Server registers service with trader.  Server defines assured quality of service: –Static QoS definition –Dynamic QoS definition.

© Chinese University, CSE Dept. Distributed Systems / Trading Characteristics  Clients ask trader for –a service of a certain type –at a certain level of quality  Trader supports –service matching –service shopping

© Chinese University, CSE Dept. Distributed Systems / Example  Distributed system for video-on-demand: Trader Video-on- demand provider MGM Warner Independent User Server

© Chinese University, CSE Dept. Distributed Systems / Typical Trading Service Trader Client Server (1) Register (2) Lookup (2a) Monitor QoS (3) Application

© Chinese University, CSE Dept. Distributed Systems / Peer-to-Peer Systems 1.Introduction 2.Napster and its legacy 3.Peer-to-peer middleware 4.Routing overlays

© Chinese University, CSE Dept. Distributed Systems / Introduction  The goal of peer-to-peer systems is to enable the sharing of data and resources.  It aims to support useful distributed services and applications using data and computing resources available in the personal computers and workstations as application.  Shirky: “ Applications that exploit resources available at the edges of the Internet – storage, cycles, content, human presence. ”  Discuss the general techniques that simplify the construction of peer-to-peer applications and enhance their scalability, reliability and security.

© Chinese University, CSE Dept. Distributed Systems / Characteristics of Peer-to-Peer Systems  Ensure each user contributes resources to the systems.  All nodes have the same functional capabilities and responsibilities.  Their correct operation does not depend on the existence of any centrally-administered systems.  Offer limited degree of anonymity to the providers and users of resources.  Choice of algorithm for the placement of data.

© Chinese University, CSE Dept. Distributed Systems / Example: Distributed Video Streaming –Traditional client/server solution is not appropriate for video streaming –Bottlenecks »Server load l The server bandwidth is the major bottleneck »Edge capacity l One connection to one client l The connection may degrade »End to end bandwidth –Scalability?

© Chinese University, CSE Dept. Distributed Systems / Example: P2P-Based Solution –Collaboration between client peers –Source server alleviated –Better scalability –Better reliability

© Chinese University, CSE Dept. Distributed Systems / Peer-to-Peer Systems  Three generations of peer-to-peer system and application development can be identified.  The first generation was launched by the Napster music exchange service  The second generation of file sharing applications offering greater scalability, anonymity and fault tolerance quickly.  The third generation is characterized by emergence of middleware layers for the application-independent management.

© Chinese University, CSE Dept. Distributed Systems / Napster  The first application in which a demand for a globally-scalable information storage and retrieval service emerged.  Popular for music exchange.  Napster ’ s architecture included centralized indexes but users supplied the files.

© Chinese University, CSE Dept. Distributed Systems / Napster ’ s Method of Operation

© Chinese University, CSE Dept. Distributed Systems / Napster ’ s Legacy  The original Napster was shut down as a result of legal proceedings.  Anonymity for the receivers and the providers of shared data and other resources is a concern for the designers of peer-to-peer systems.  Napster demonstrated the feasibility of building a useful large-scale service which depends almost wholly on data and computers owned by ordinary Internet users.

© Chinese University, CSE Dept. Distributed Systems / Peer-to-Peer Middleware  Provide a mechanism to enable clients to access data resources quickly and dependably whenever they are located throughout the network.  Designed specially to meet the need for the automatic placement and subsequent location of the distributed objects.

© Chinese University, CSE Dept. Distributed Systems / Functional Requirement  Simplify the construction of services that are implemented across many hosts in a widely distributed network.  Enable clients to locate and communicate with any individual resources.  Ability to add new resources (or hosts) and to remove them.

© Chinese University, CSE Dept. Distributed Systems / Non-Function Requirements  Global scalability  Load balancing  Optimization for local interactions between neighboring peers  Accommodating to highly dynamic host availability  Security of data in an environment with heterogeneous trust  Anonymity, deniability and resistance to censorship

© Chinese University, CSE Dept. Distributed Systems / Routing Overlays versus IP Routing  Routing overlays share many characteristics with IP packet routing infrastructure.  Additional application-level routing mechanism is required.

© Chinese University, CSE Dept. Distributed Systems / Routing Overlay Information Distribution Object: Node: D C’s routing knowledge D’s routing knowledgeA’s routing knowledge B’s routing knowledge C A B

© Chinese University, CSE Dept. Distributed Systems / Routing Overlays  The distributed algorithm known as a routing overlay takes responsibility for locating nodes and objects.  The routing overlay ensures that any node can access any object by routing each request through a sequence of nodes.  Peer-to-peer systems usually store multiple replicas of objects to ensure availability.

© Chinese University, CSE Dept. Distributed Systems / Routing Overlays  The main task: –Routes the request to a node at which a replica of the object resides.  Other tasks –Ensures that a new object is reachable by all other clients. –Make the object unavailable when its client requests a removal. –When a node leaves, its responsibilities are distributed amongst the other nodes.

© Chinese University, CSE Dept. Distributed Systems / Summary  Naming services provide facilities to give external names to components.  Trading services match service types requested by clients to servers that can satisfy them.  P2P systems exploit existing naming, routing, data replication and security techniques for resource sharing.  Read Chapters 10 and 13 of the textbook.