Chapter 2: System Model Introduction Architecture Models Fundamental Models Summary

Introduction Why do we need model? Architecture model Each model is intended to provide an abstract, simplified but consistent description of a relevant aspect of distributed system design Architecture model defines the way in which the components of systems interact with one another and the way in which they are mapped onto the underlying network of computers Client/Server vs. Peer to Peer variants of C/S partition of data or replication as cooperating servers caching of data by proxy servers and clients use of mobile code and mobile agents requirement to add and remove mobile devices in a convenient manner

Introduction …continued Fundamental model concerned with a more formal description of the properties that are common in all of the architectural models The interaction 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 channels

Chapter 3: System Model Introduction Architecture Models Fundamental Models Summary

Concepts What does the architecture model consider? Software layers The placement of the components across a network of computers – seeking to define useful patterns for the distribution of data and workload The interrelationships between the components – their functional roles and patterns of communication between them Software layers Originally refer to the structuring of software as layers or modules in a single computer Recently in terms of services offered and requested between processes located in the same or different computers

Software and hardware service layers in distributed systems Platform Software layer Software and hardware service layers in distributed systems Platform The lowest-level hardware and software layers, e.g., Intel x86/Windows, SPARC/SunOS, PowerPC/MacOS Middleware A layer of software, mask heterogeneity, provide a convenient programming model to application programmers Examples: RPC, RMI, CORBA, DCOM, Isis(group communication system) Limitations: some systems require support at the application level e.g., In case of transferring large email message, TCP provides some error detection and correction, but it can’t recover from major network interruptions. So the mail transfer service at the application layer is required. ‘ the end-to-end argument’ [1984] some communication-related functions can be completely and reliably implemented only with the knowledge and help of the application standing at the end points of the communication system Counter to the view of middleware supporters (transparency entirely)

Software and hardware service layers in distributed systems

What is system architecture? System architectures What is system architecture? The division of responsibilities between system components (applications, server and other processes) and the placement of the components on computers in the network Main distributed system architectures 1. Client-Server model Be Historically the most important and remain the most widely employed Servers may in turn be clients of other servers 2. Services provided by multiple servers Partition the set of service objects on different servers, e.g. workflow system Maintain replicated service objects on several hosts, e.g. Sun NIS 3. Proxy servers and caches A cache is a store of recently used data objects that is closer than the objects themselves E.g., web page cache at web browser or web proxy server

Clients invoke individual servers

A service provided by multiple servers

Web Proxy Server

System architectures … continued 4. Peer processes ( peer to peer ) All of the processes play similar roles, interacting cooperatively as peers to perform a distributed activity or computation without any distinction between clients and servers Maintain consistency of application-level resources and synchronize application level action when necessary E.g., a peer-to-peer whiteboard

A distributed application based on peer processes

Variations on the client-server model Reasons of variation The use of mobile code and mobile agents Users need for low-cost computers with limited hardware resources The requirement to add and remove mobile devices in a convenient manner Several variations: 1. Mobile code good interactive response, e.g., applet 2. Mobile agent A running program that travels from one computer to another in a network carrying out a task on someone’s behalf, e.g., agilet[IBM], worm[Xerox PARC] 3. Network Computers Download its operating system and any application software from a remote file server All the application data and code is stored by a file server, so users may migrate

Web applets

Variations on the client-server model … continued 4. Thin client A software layer that supports a window-based user interface on a computer that is local to the user while executing application programs on a remote computer Drawback : high latencies Implementation: X-11, VNC[AT&T 1998] 5. Spontaneous networking The form of distribution that integrates mobile devices and other devices into a given network Key features: easy connection to a local network, easy integration with local services Key design issues Convenient connection and integration Limited connectivity: mobile device move around continuously, disconnection Security and privacy Discovery Services: registration service, lookup service

Thin clients and compute servers Application Process Network computer or PC Compute server network

Spontaneous networking in a hotel Internet gateway PDA service Music Discovery Alarm Camera Guests devices Laptop TV/PC Hotel wireless network

Design requirements for distributed architectures Performance issues Responsiveness: determined by the load and performance of the server and network, delays in the client and server operating system’s communication and middleware services as well as code of the service Throughput: the rate at which computational work is done, the throughput of the intervening software layers is important Balancing computational loads Dependability issues Fault tolerance: redundancy, e.g., data and processes be replicated, messages be retransmitted Security: e.g. locate sensitive data in computers that can be secured effectively against attack Quality of service Reliability, security Performance: ability to meet timeliness guarantees Adaptability: meet changing system configurations and resource Availability: have necessary computing and network resources at the appropriate times ( the abbreviation QoS) Use of caching and replication, e.g. Web caching protocol

Chapter 3: System Model Introduction Architecture Models Fundamental Models Summary

A system model should address the following questions Fundamental models A system model should address the following questions What are the main entities in the system? How do they interact? What are the characteristics that affect their individual and collective behavior? Purpose of a model Make explicit all the relevant assumptions about the system we are modeling Make generalizations concerning what is possible or impossible by logical analysis and mathematical proof Fundamental models intend to discuss Interaction Failure Security

Interaction model Examples of interaction in distributed system DNS, NIS: multiple server processes cooperate with one another P2P voice conference system: with strict real-time constraints Distributed algorithm: a definition of the steps to be taken by each of the processes of which the system is composed, including the transmission of messages between them Difficult to describe all the states, because of failures of processes and message transmissions Two significant factors affecting interacting processes Communication performance is often a limiting characteristic Latency – the delay between the sending of a message by one process and its receipt by another. Including: Network delay, Accessing delay, OS delay Bandwidth – total amount of information that can be transmitted over it in a given time Jitter – variation in the time taken to deliver a series of messages Impossible to maintain a single global notion of time Clock drift rate – the relative amount that a computer clock differs from a perfect reference clock Timing event: e.g., GPS, Logical time

Two variants of the interaction model Synchronous distributed system The time to execute each step of a process has know lower and upper bounds Each message transmitted over a channel is received within a known bounded time Each process has a local clock whose drift rate from real time has a known bound Asynchronous distributed system – no bounds on process execution speeds: e.g. each step may take an arbitrarily long time Message transmission delays: e.g. a message may be received after an arbitrarily long time Clock drift rates: the drift rate of a clock is arbitrary Examples of synchronous DS and asynchronous DS Asynchronous DS: email, ftp Synchronous DS: VOD, voice conference system

Example: disorder of messages Event ordering Example: disorder of messages A group including X, Y, Z and A X send “Meeting” to all; Y and Z reply “Re: Meeting” to all At A, the messages received are Z.”Re: Meeting”, X.”Meeting”, Y.”Re: Meeting” Logical time[Lamport 1978] Provide an ordering among the events at processes running in different computers in a distributed system

Failure model Failure model: defines the ways in which failure may occur in order to provide an understanding of the effects of failures and defeat the failures Failure models: 1. Omission failures A process or communication channel fails to perform actions that it is supposed to do Process omission failure: Crash Fail-stop: Crash that can be detected by other processes certainly, e.g., by timeouts in synchronous DS Communication omission failures: dropping messages Send omission, receive omission, channel omission Benign failures

Failure model …continued 2. Arbitrary (Byzantine) failures the worst possible failure semantics Arbitrarily omit intended processing steps or take unintended processing steps. E.g., return a wrong value in response to an invocation Arbitrary failures in process is hard to be detected, Arbitrary failures in communication channel exist but rare, by recognize and reject the faulty msgs Class of failure Affects Description Fail-stop Process Process halts and remains halted. Other processes may detect this state. Crash not be able to detect this state. Omission Channel A message inserted in an outgoing message buffer never arrives at the other end’s incoming message buffer. Send-omission A process completes a send, but the message is not put in its outgoing message buffer. Receive-omission A message is put in a process’s incoming message buffer, but that process does not receive it. Arbitrary (Byzantine) Process or channel Process/channel exhibits arbitrary behaviour: it may send/transmit arbitrary messages at arbitrary times, commit omissions; a process may stop or take an incorrect step.

Failure model …continued 3. Timing failures Applicable in synchronous distributed system, but not asynchronous DS Masking failures Hide, e.g., replicated servers convert, e.g., Checksum: arbitrary failure -> omission failure Reliability of one-to-one communication Validity – any message in the outgoing message buffer is eventually delivered to the incoming message buffer Integrity – the message received is identical to one sent, and no messages are delivered twice, against retransmit protocols and malicious messages Reliable communication is defined in terms of validity and integrity Class of Failure Affects Description Clock Process Process’s local clock exceeds the bounds on its rate of drift from real time. Performance Process exceeds the bounds on the interval between two steps. Channel A message’s transmission takes longer than the stated bound.

The security of a distributed system Security model The security of a distributed system The processes The communication channels The objects Protecting the objects Access rights: who is allowed to perform the operations of an object Principal: the authority who has some rights on the object

Securing processes and their interactions The enemy Threats to processes To servers: invocate with a false identity, e.g. cheating a mail server To clients: receive false result, e.g. stealing account password Threats to communication channels Copy, alter or inject messages Save and replay, e.g., retransfer money from one account to another Denial of service: excessive and pointless invocation on services or message transmissions in a network, resulting in overloading of physical resources (network bandwidth, server processing capacity) Mobile code: malicious mobile program, e.g. Trojan horse attachment Communication channel Copy of m Process p q The enemy m’

Securing processes and their interactions … continued Defeating security threats Cryptography and shared secrets Identify each other by the shared secrets that are only known by themselves. Cryptography is the base. Authentication – proving the identities supplied by their senders Secure channels Each process knows reliably the identities of the principal on whose behalf the other process is executing Ensure the privacy and integrity of the data transmitted across it Each message includes physical or logical time stamp Principal A Secure channel Process p q B

Chapter 3: System Model Introduction Architecture Models Fundamental Models Summary

Summary Architecture models Fundamental models Client / Server, e.g. Web, FTP, NEWS Multiple Servers, e.g. DNS Proxy and Cache, e.g. Web Cache Peer process Variations of C/S Mobile code, mobile agent, network computer, thin client, spontaneous networks Fundamental models Interaction models – synchronous DS and asynchronous DS Failure models – omission failures, arbitrary failures and timing failures Security model - the enemy and the approaches of defeating them