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Unformatted text preview: Disclaimer: All the materials provided by the NotesMirchi are obtained from external resources. NotesMirchi will not take the responsibility in case of any plagiarism. If the users find the materials to be an electronic reproduction of any copyrighted materials, they are advised to delete it immediately and report the same to NotesMirchi. The NotesMirchi logo and website address provided in the footer are added for only promotional purpose and are in no way associated with the content of the material. www.NotesMirchi.com UNIT I IT 1001 - DISTRIBUTED SYSTEMS 1.1 I TRODUCTIO TO DISTRIBUTED SYSTEMS Definition : A distributed System is a collection of independent computers that appeals to its users as a single coherent system. Important Characteristics of distributed System : ( i) In distributed systems, the differences between the various computers and the ways in which they communicate are hidden from users. Internal Organization of distributed system also hidden from the users. ( ii) Users and applications can interact with a distributed system in a consistent and uniform way, regardless of where and when interaction takes place. A distributed system will normally be continuously available, although perhaps certain parts may be temporarily out of order. The motivation for constructing and using distributed system is resource sharing. Resources such as printers, files, web pages or database records are managed by servers of appropriate t ype. The purpose of this chapter is to convey a clear view of the nature of distributed systems and the challenge that must be addressed in order to ensure that they are successful. We look at some key examples of distributed systems, the components from which they are constructed and their purposes. Next, we explore the design of resource sharing systems in the context of world wide web and finally, we look at the key challenges faced by the designers of distributed systems. 1.2 Examples of distributed Systems : Let us now have a look at several examples of distributed systems. T he Internet is an example of distributed system. The internet is an interconnected collection of computer networks of many different types. Figure 1.1 Illustrates the typical portion of the Internet. www.NotesMirchi.com —Q Figure 1.1 A typical portion of the internet It enables the users, wherever they are, to make use of services such as world wide web, e-mail and file transfer. The services of internet can be extended by addition of server and new type of services. The above figure shows a collection of intranets. Intranets are subnetwork operated by companies and other organizations. ISPs (Internet Service Providers) are companies that provide Connections to individual users and small organizations. The intranets are linked together by back bones. A back bone is a network link with a high transmission Capacity. It employs satellite connections, f iber optical cables and other high band width cables. Multimedia services are available in the internet. This service enable the users to access audio and video data. As a second example, consider a work flow information system that supports the automatic processing of orders. Typically, such a system is used by people from several departments, at different l ocations. For example, people from the sales department may be spread across a large region or an entire country. Orders are placed by means of laptop computers that are connected to the system through the telephone networks. Incoming orders are automatically forwarded to the planning department. The system will automatically forward orders to an appropriate and available person. www.NotesMirchi.com As a final example, consider the world wide web. The web offers a simple, consistent, and uniform model of distributed documents. To see a document, a user need merely activate a reference, and the document appears on the screen. P ublishing a document is also simple: you only have to give it a unique name in the form of uniform Resource Locator that refers to a local file containing the document's content. 1.3. CHALLENGES : The key challenges faced by the designers of Distributed system arc heterogeneity, openness, security, Scalability, failure handling, concurrency and the need for transparency. 1.3.1 Heterogeneity : They must be constructed from a variety of different networks, operating systems, computer hardware and programming languages. The internet Communication protocols mask the difference in networks, and middleware can deal with other differences. The term Middleware applies to a software layer that provides a programming abstraction as well as masking the heterogeneity of the underlying networks, hardware, operating systems and programming languages. Example : Common Object Request Broker Architecture (CORBA). 1.3.2 Openness The openness of a computer system is the characteristic that determines whether the system can be extended and re-implemented in various ways. The openness of distributed systems is determined primarily by the degree to which new resource sharing services can be added and be made available for use by a variety of client programs. The first step to provide openness is to publish the inter faces of the components, but the integration of components written by different programmers is a real challenge. • Open systems are characterized by the fact that their key interfaces are published. • O pen distributed systems are based the provision of a uniform communication mechanism and published interfaces for access to shared resources. • Open systems can be constructed from heterogeneous hardware and software. www.NotesMirchi.com 1.3.3 Security Security for information resources has there components ; (i) Confidentiality Protection against disclosure to unauthorized individuals. (ii) Integrity Protection against alteration or corruption. (iii) Availability Protection against interference with the means to access the resources. The challenge is to send sensitive information in a message over a network in a secure manner. But security is not just a matter of concealing the contents of messages, it also involves knowing for sure the identity of the user. The second challenge is to identify a remote user. Both of these challenges can be met by the use of encryption techniques. Encryption can be used to provide adequate protection of shared resource and to keep sensitive information secret when it is transmitted in messages over a network. Denial of service attacks are still a problem. 1.3.4 Scalability A distributed system is scalable if the cost of adding a user is a constant amount in terms of the resources that must be added. In other words, A system is described as scalable if it will remain effective when there is significant increase in the number of resources and the number of users. The design of scalable distributed system present the following challenges: Controlling the cost of physical resources : As the demand for a resource grows, it should be possible to extend the system, at reasonable cost, to meet it. Controlling the performance loss: Consider the management of a set of data whose size is proportional to the number of users or resources in them. In this case, Algorithms that use hierarchic structures scale better than linear structures. But, even with hierarchic structures, an increase in size will result in some loss in performance. www.NotesMirchi.com Size : When a system needs to scale, we must consider scaling with respect to size. If more users or resources need be supported, we are confronted the limitations of centralized services, data and algorithms. For example, many services are centralized in the sense that they are implemented by means of only a single server running on specific machine in the distributed system. The problem now is that the server become a bottleneck as the number of users grows. E ven if we have virtually unlimited processing and storage capacity, communication with that server will prohibit further growth. P reventing software resources running out: As an Example, the supply of available internet addresses is running out. There is no correct solution to this problem. It is difficult to predict the demand that will be put on a s ystem years ahead. Avoiding performance bottle necks : Algorithms should be decentralized to avoid having performance bottlenecks. Because, In a large distributed system, enormous number of messages have to be routed over many lines. The optimal way to do this is collect complete information about the load on all machines and lines, and t hen run a graph theory algorithm to compute all the optional. The trouble is that collecting and t ransporting all the Input and output information would again be a bad idea because these messages world overload part of the network. 1.3.5 Failure handling Failures fall into two obvious categories : hardware and software. If failure occurs, program may produce incorrect results on they may stop before they have completed and intended computation. Failures in a distributed system are partial, i.e., some components fail while others continue to function. Therefore, the handling of failures is particularly difficult. The techniques dealing with failures are l i st e d below. Detecting failures - Some failures can be detected. E.g. checksums can be used to detect corrupted data in a message or a file Masking failures - Some failures that have been detected can be hidden or made less severe. E.g. messages can be retransmitted when they fail to arrive. T olerating failures - I t is not possible to detect and hide all the failures that might occur in large network. Their clients can be designed to tolerate failures, which generally involve the users tolerating them as well. Services can be made to tolerate failures by the use of redundant components. R ecovery from failures - R ecovery involves the design of software so that the state of permanent data can be recovered or rolled back after a server has crashed. www.NotesMirchi.com 1.3.6 Concurrency Both services and applications provide resources that can be shared by clients in a distributed system. There is therefore a possibility that several clients will attempt to access a shared resource at the same time. The process that manages a shared resource could take one client request at a time. But that approach limits throughput. Therefore, services and applications generally allow multiple client requests to be processed concurrently. For an object to be safe in concurrent environment, its operations must be synchronized in such a way that its data remains consistent. This can be achieved by standard techniques. such as semaphores. 1.3.7 Transparency It is defined as the concealment from the user and the application programmer of the separation of components in a distributed system. Hence, the system is perceived as a whole rather than as a collection of independent components. The main aim of the transparency is to make certain aspects of distribution invisible to the application programmers. • A ccess transparency enables local and remote resources to be accessed using identical operations. • L ocation transparency enables resources to be accessed without knowledge of their location. • C oncurrency t ransparency enables several processes to operate concurrently using shared resources. The resources will not interfere among themselves. • Replication transparency enables multiple instances of resources to be used to increase reliability and performance without knowledge of the replicas by users or application programmers. • F ailure transparency e nables the concealment of faults. It allows users and application programs to complete their tasks even though they have the failure of hardware or software components. • M obility t ransparency allows the movement of resources and clients within a s ystem. The movement will not affect the operation of users or programs. • P erformance transparency a llows the system to be reconfigured to improve performance as loads vary. • S caling transparency a llows the system and applications to expand in scale. However it will not change to the system structure or the application algorithms. T here are two important transparencies available. They are access and location transparency; their presence or absence most strongly affects the utilization of distributed resources. They are referred together as network transparency. www.NotesMirchi.com 1.4 ARCHITECTURAL MODELS An architectural model of a distributed system is concerned with placement of its parts and the relationships between them. An architectural model defines the way in which the components of systems interact with one another and the way in which they are mapped onto an underlying network of computers. The overall goal is to ensure that the structure will meet present and likely future demands on it. Major concerns are to make the system reliable, manageable, adaptable and cost effective. An architectural model first simplifies and abstracts the functions of the individual components of a distributed system and then it considers : • The placement of the components across a network of computers, seeking to definite useful patterns for the distribution of data and workload. • The inter relationships between the components. An initial simplification is achieved by classifying processes as server processes, client processes and peer processes. This classification identifies the responsibilities of each and hence helps us to assess their workloads and to determine the impact of failures in each of them. 1.4.1 Software layers: The term 'software architecture' refers to the structuring of software as layers or modules in a single computer and in terms of services offered and requested between processes located in the same or different computers. This process and service-oriented view can be expressed in terms of service layers. Fig:1.3 software and hardware services www.NotesMirchi.com Platform The lowest level hardware and software layers are known as a platform for distributed systems and applications. These low level layers provide services to the layers above them. They are implemented independently in each computer. They bring the system's programming interface upto a level that facilitates communication and coordination between processes, eg., Intel X86/windows, Intel X86/Linux, Sun SPARC/Sun OS, Intel X86/Solaris, etc. Middleware Middleware is defined as a layer of software whose purpose is to mask heterogeneity and to provide convenient programming model to application programmers. Middleware is concerned with providing useful building blocks for the construction of software components that can work with one another in a distributed system. In particular, it raises the level of the communication activities of application programs through the support of abstractions such as remote method invocation, communication between a group of processes, notification of events, replication of shared data and transmission of multimedia data in real time. 1.4.2 System Architectures: In this section, Let us discuss the principal architectural models on which distribution of responsibilities is based. The main types of architectural models are given below. • Client - Server model: In the basic client-server model, processes in a distributed system are divided into two groups. (1) A Server (li) A Client A server is a process implementing a specific service. For example, a file system service or a database service. A client is a process that requests a service from a server by sending it a request and subsequently waiting for the server's reply. Figure 1.4 illustrates the simple structure in which client processes interact with individual server processes in separate host computers in order to access the shared resources that t hey manage. Figure 1.4: Clients invoke individual servers Servers may be clients of other servers. For example, a web server is a client of a local file server that manages the files in which the web pages are stored. Web servers and other Internet services are clients of the DNS service, which translates Internet Domain Names to network addresses. www.NotesMirchi.com • Services provided by multiple Servers: Services may be implemented as several server processes in separate host computers interacting as necessary to provide a service to client processes as shown in Figure 1.5. The servers may partition The set of objects based on the service and distribute them between themselves, or they may rv-smtain replicated copies of them on several hosts. The web provides a common example of partitioned data in which each web server manages its own set of resources. Replication is used to increase performance and availability and to improve fault tolerance. It provides multiple consistent copies of data in processes running in different computers. Service Figure 1.5: A service provided by multiple servers • Proxy servers and caches: A cache is a store of recently used data objects. When a new object is received at a computer it is added to the cache store, replacing some existing objects, if necessary. When an object is needed by a client process, the caching service first checks the cache and supplies the object from there if an up-to-date copy is available. If not, an up-to-date copy is fetched from the server. Caches may be co-located with each client or in a proxy server that can be shared by several clients. Web proxy servers provide a shared cache of web resources for the client machines at a site or across several sites. The purpose of proxy servers is to increase availability and performance of the service by reducing the load on the WAN and web servers. www.NotesMirchi.com Figure 1.6: Web proxy Server • Peer Processes In this architecture, all of the processes play similar roles, interacting cooperatively as peers to perform a distributed activity without any distinction between clients and servers. In this model, code in the peer processes maintains consistency of application-level resources and synchronizes application-level actions when necessary. The elimination of server processes reduces inter-process communication delays for access to local objects. 1.4.3 Variations on the client-server model Several variations on the client-server mode' can be derived from the consideration of the following factors: The use of mobile code and mobile agents www.NotesMirchi.com User's need low cost computers with limited hardware resource that are simple to manage. The requirement to add and remove mobile devices in a convenient manner. • Mobile code - Applets are a well.known and widely used example of mobile code. The user running a browser selects a link to an applet whose code is stored on a web server, the code is downloaded to the browser and runs there. An advantage of running the downloaded code locally ss thai is can give good interactive response since it does not suffer from delays or variability of band width associated with network communication. Mobile agents; A mobile agent is a running program including both code and data that travels from one computer to another m a network carrying out a task on someone's behalf such as collecting information, eventually returning with the result. Mobile agents might be used to install and mainioin software on thu* computers within ar; organization or to compare t he prices of products from a number of vendors by visiting the site of each vendor and performing a series of database operations. Mobile agents are a potential security threat to the resources in computers that they visit. The environment receiving a mobile agent should decide on which of the local resources it should be allowed to use, based on the identity of the user on whose behalf the agent is acting. • • Thin clients: T he term thin clients refers to a software layer that supports a windowb ased user interface on a computer that is local to the user while executing application programs on a remote computer. Instead of downloading the code of applications into the user's computer, it runs them on a compute server (a powerful computer that has the capacity to run large numbers of applications simultaneously). The main drawback of the thin client architecture is in highly interactive graphical activities such as CAD, and image processing, where the delays experienced by users are increased by the need to transfer image and vector information between the thin client and the application process, incurring both network and operating system latencies. Mobile devices and spontaneous networking: The world is increasingly populated by small and portable computing devices, including laptops, handheld devices such as PDAS, mobile phone and digital cameras, wearable computers such as smart watches, and devices embedded in everyday appliances such as washing machines. With appropriate integration into our distributed systems, these devices provide support for m obile computing, whereby users carry their mobile devices between network environments and take advantage of local and remote services as they do so. The form of distribution that integrates mobile devices and other devices into a given network is best described by the term spontaneous networking. • The key features of spontaneous networking are • E asy Connection t o a local n etwork - wireless links avoid the need for pre-installed cabling and avoid the inconvenience and reliability issues surrounding plugs and s ockets. A device brought into a new network environment is transparently www.NotesMirchi.com reconfigured to obtain connectivity. • Easy integration with local services: Devices that find themselves inserted into existing networks of devices automatically discover the services that are provided there, with no special configuration actions performed by the user. Limitations of spontaneous networking: Internet addressing and routing algorithms are difficult to implement L imited connectivity - users are not always connected as they more around (eg. through tunnels) Security and Privacy Discovery services: Spontaneous networking allows client processes running on portable devices to access services on t he networks to which they are connected. The clients may not know about the services that are available i n the network to which they are connected. The purpose of a discovery system is to accept and store details of services that are available on the network and to respond to queries from clients about them. A discovery service offers two interfaces: • A r egistration service - a ccepts registration requests from servers and records the details that t hey contain in the discovery service's database of currently available services. • A lookup service - accepts queries concerning available services and searches its database for registered services that match the queries. The result returned includes sufficient details to enable clients to select between several similar services based on their attributes and to make a connection to one or more of them. Network Computers The OS and application software for desktop computers typically require much of the active code and data to be located on a local disk. But the management of application files and the ma;r.fenance of a local software base require considerable technical effort of a nature that most users are not qualified to provide. www.NotesMirchi.com The network computer is a response to this problem. It downloads its operating system and any application software needed by the user from a remote file server. Applications are run locally but the files are managed by a remote file server. Since all the application data and code is stored by a file server, the users may migrate from one network computer to another. 1 .5 FU DAME TAL MODELS Fundamental models are concerned with a more formal description of the properties that are common in all of the architectural models. All communication between processes is achieved by means of messages. Message communication over a computer network can be affected by delays. It may suffer from a variety of failures. It is vulnerable to security attacks. These issues are addressed by three models. • The i nteraction model deals with performance and with the difficulty of setting time limits in a distributed system. • The failure model attempts to give a precise specification of the faults that can be exhibited by processes and communication channels. • The security model discusses the possible threats to processes and communication c hannels. It introduces the concept of a secure channel, which is secure against these threats. Let us discuss these models in detail: 1.5.1 Interaction model Interacting processes perform all activities in a distributed system. Each process has its own state, consisting of the set of data that it can access and update, including the variables in its program. The state belonging to each process is completely private, i.e., it cannot be accessed or updated by any other process. Two significant factors affecting i nteracting processes in a distributed system are: • • Communication performance is often a limiting characteristic It is impossible to maintain a single global notion of time Performance of communication Channels: Communication over a computer network has the following performance characteristics relating to latency, bandwidth and jitter. Delay between the start of a message is transmission from one process and the beginning of its receipt by another is referred to as l atency, t he latency includes: o The delay in accessing the network. o The time taken by the OS communication services which may vary according to the load on the OS. www.NotesMirchi.com Bandwidth is the total amount of information that can be transmitted in a given time. When a large number of communication channels are using the same network, they have to share the available bandwidth. Jitter i s the variation in the time taken to deliver a series of messages. Jitter is relevant to multimedia data. E.g., if consecutive samples of audio data are played witli different time intervals then the sound will be badly distorted. • Computer clocks and timing events Each computer in a distributed system has its own internal clock, which can be used by local processes to obtain the value of the current time. Therefore, two processes running on different computers can associate timestamps with their events. However, even if two processes read their clocks at the same time, their local clocks may supply different time values. This is because computer clocks drift from perfect time and their drift rates differ from one another. The term clock drift rate refers to the relative amount that a computer clock differs from a perfect reference clock. Even if the clocks on all computers are set to the same time initially, their clock would eventually vary quite significantly unless corrections are applied. • Two variants of the interaction model In a distributed system it is hard to set time limits for process execution, message delivery or clock drift. T wo opposing extreme positions provide a pair of simple models, the first has a strong assumption of time and the second makes no assumption about time. (i) Synchronous distributed systems: It is defined as one in which the following bounds are specified: o The time to execute each step of a process has known lower and upper bounds. o Each message transmitted over a channel is received within a known bounded time. o Each process has a local clock whose drift rate from real time has a known bound. It is possible to suggest likely upper and lower bounds for process execution time message delivery and clock drift rates in a distributed system. But it is difficult to arrive at realistic values and to p rovide guarantees of the chosen values. Unless the values are guaranteed, any design based on the chosen values will not be reliable. (ii) Asynchronous distributed system An asynchronous distributed system is one in which there are no bounds on process execution speeds, message transmission delays and clock drift rates. This exactly models the Internet, in which there is no intrinsic bound on server or network load. For example, to transfer a file using ftp. Actual distributed systems are very often asynchronous because of the need for processes to share the processors and for communication channels to share the network. www.NotesMirchi.com 1.5.2 The failure model The failure model defines the ways in which failure may occur in order to provide an understanding of the effects of failures. There are three types of failures and are explained below: • O mission failures: The faults classified as omission failures refer to cases when a process or communication channel fails to perform actions that it is supposed to do. There are two t ypes of omission failures. They are: (i) (ii) P rocess omission failures : The chief omission failure of a process is to crash. The design of services that can survive in the presence of faults can be simplified if it can be assumed that the services on which it depends crash cleanly, i.e. the processes either function correctly or else stop. Other processes may be able to detect such a crash by the fact that the process repeatedly fails to respond to invocation messages. However this method of crash detection relies on the use of timeouts. A process crash is called fail-stop if other processes can detect certainly that the process has crashed. Communication omission failures: As an example, consider the two communication primitives namely send and receive. A process P performs a send by inserting the message m in its outgoing message buffer.The communication channel transports m to q's incoming message buffer. Process q performs a receive by taking m from its incoming message buffer and then delivers it. www.NotesMirchi.com T he outgoing and incoming message buffers are typically provided by the operating system. The communication channel produces an omission failure if it does not transport a message from p's outgoing message buffer to q's incoming message buffer. This is known as 'dropping messages' and is generally caused by lack of buffer space. The loss of message between the sending process and the outgoing message buffer is called sending omission failure. The loss of message between the incoming message buffer and the receiving process is called receive-omission failure and the loss of messages in between is called channel omission failure • Arbitrary failures: The term arbitrary or B yzantine failure is used to describe the worst possible failure semantics, in which any type of error may occur. An arbitrary failure of a process is one in which it arbitrarily omits intended processing steps or takes unintended processing steps. Communication channels can suffer from arbitrary failures, e.g. message contents may be corrupted or non-existent messages may be delivered or real messages may be delivered more than once. • Timing failures: Timing failures are applicable in synchronous distributed system where time limits are set for all operations. Class of failure Affects Description Clock Process Local clock exceeds the bounds Performance Process Process exceeds the bounds Performance Channel Message transmission takes longer than the bound Timing is particularly relevent to multimedia computers with video and audio channels. Video i nformation may require a large amount of data to be transferred. To deliver such information without timing failures will make demands on the operating system as well as communication system. • M asking failure: Each component in a distributed system is generally constructed from a collection of other components. It is possible to construct reliable services from components that exhibit failures. For example, multiple servers that hold replicas of data can continue to p rovide a service when one of them crashes. A Knowledge of the failure characteristics can enable a new service to be designed to mask the failure of the components on which it depends. A service masks a failure, either by hiding it altogether or by converting it into a more acceptable type of failure. www.NotesMirchi.com 1.5.3 Security Model The security of a distributed system can be achieved by securing the processes and the channels used for can h their interactions and by protecting the obje s that they encapsulate against e object unauthorized access. Protecting objects : Protection is describe in terms of objects. Objects are intended to b used in different scribed ed be ways by different users. For example, some objects may hold a user's private d ata and other objects may hold , other shared data such as Web pages. To sup o support this, access rights specify who is allowed to perform the allow operations (read/write) of an object. The server is responsible for veri r verifying the identity of the client behind each d each invocation and checking their access righ on the requested object. The clients in turn ss rights in can check the authenticity of the server. Fig : Obje and Principals bjects Securing processes and their interaction actions Distributed systems are deployed and used in tasks that are likely to be subject to e ed ct external attacks by hostile users. This is true for applications such as financial transactions. ations For these applications, secrecy or int or integrity is crucial. Integrity is threatened by security violations and y secu communication failures. In order to ident identify and defeat threats we have to analyze those threats. We will e thos explore a model for the analysis of securit threats in the following paragraph. ecurity Processes interact by sending messages. The messages are exposed to attack because t he network mess ck bec and the communication service are open Servers and peer processes expose their interfaces, enabling e open. heir in invocations to be sent to them by any ot other process. To secure the messages passed over the network, assed cryptographic algorithms (encryption) is a n) applied to the message, to provide confidential entiality, authentication and integrity. The threats from a potential enemy are discusse d under the following headings: emy Threats to processes: A process that is designed to handle incoming requests may receive a message s that may from any other process in the distributed system and it cannot necessarily determine t he identity of the ibuted ine www.NotesM otesMirchi.com sender. This lack of reliable knowledge of the source of a message is a threat to the correct functioning of both servers and clients. E.g. Spoofing Threats to communication channels: An enemy can copy, alter or inject messages as they travel across the network and its intervening gateways. Such attacks present a threat to the privacy and integrity of information as it travels over the network and to the integrity of the system. All these threats can be defeated by the use of secure channels, which is described below: Secure Channels: A secure channel ensures the privacy and integrity of the data transmitted across it. Encryption and authentication are used to build secure channels as a service layer on top of existing communication services. A secure channel is a communication channel connecting a pair of processes with the following properties: Each of the processes knows reliably the identity of other processes. It ensures the privacy and integrity of the data transmitted across it. Each message includes a physical or logical time stamp to prevent messages from being replayed or reordered. Other possible threats: Denial of service - This is a form of attack in which the enemy interferes with the activities of authorized users by making excessive invocations on services or message transmission in a network, resulting in overloading of physical resources. Mobile code - Mobile code raise new and interesting security problems for any process that receives and executes program code from elsewhere, such as the e-mail attachments, such code may easily play a Trojan horse role. Drawbacks of security techniques: The use of security techniques such as encryption and access control incurs substantial processing overhead and management costs. 1.6 1.6.1 INTER PROCESS COMMUNICATION INTRODUCTION Inter process communication is concerned with the communication between processes in a distributed system, both in its own right and as support for communication between distributed objects. The Java API for inter process communication in the internet provides both datagram and stream communication. The Application Program Interface (API) to UDP provides a message passing abstraction - the simplest form of interprocess communication. This enables a sending process to transmit a single message to a www.NotesMirchi.com receiving process. The independent packets containing these messages are called datagrams. In the Java and UNIX APIs, the sender specifies the destination using a socket - an indirect reference to a particular port used by the destination process at a destination computer. The API to TCP provides the abstraction of a two-way stream between pairs of processes. 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