• Introduction• Architecture Models• Fundamental Models• Summary

Chapter 2: System Model

Introduction

• Why do we need 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

• Introduction• Architecture Models• Fundamental Models• Summary

Chapter 3: System Model

Concepts

• What does the architecture model consider?– 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 layer

• Software and hardware service layers in distributed systems

• Platform– The lowest-level hardware and software layers, e.g., Intel x86/Wi

ndows, 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 relia

bly 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

Applications, services

Computer and network hardware

Platform

Operating system

Middleware

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 architectures1. Client-Server model

– Be Historically the most important and remain the most widely employed

– Servers may in turn be clients of other servers2. 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

Server

Client

Client

invocation

result

Serverinvocation

result

Process:Key:

Computer:

A service provided by multiple servers

Server

Server

Server

Service

Client

Client

Web Proxy Server

Client

Proxy

Web

server

Web

server

serverClient

System architectures … continued

4. Peer processes ( peer to peer )– All of the processes play similar roles, interacting cooperativel

y 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

Coordination

Application

code

Coordination

Application

code

Coordination

Application

code

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., applet2. 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

a) client request results in the downloading of applet code

Web server

ClientWeb serverApplet

Applet code

Client

b) client interacts with the applet

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

ThinClient

ApplicationProcess

Network computer or PCCompute server

network

Spontaneous networking in a hotel

Internet

gateway

PDA

service

Music service

serviceDiscovery

Alarm

Camera

Guestsdevices

LaptopTV/PC

Hotel wireless

network

Design requirements for distributed architectures

• Performance issues– Responsiveness: determined by the load and performance of the server a

nd 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 app

ropriate times ( the abbreviation QoS)• Use of caching and replication, e.g. Web caching protocol

• Introduction• Architecture Models• Fundamental Models• Summary

Chapter 3: System Model

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

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.”Meetin

g”, Y.”Re: Meeting”• Logical time[Lamport 1978]

– Provide an ordering among the events at processes running in different computers in a distributed system

send

receive

send

receive

m1 m2

2

1

3

4X

Y

Z

Physical time

Am3

receive receive

send

receive receive receivet1 t2 t3

receive

receive

m2

m1

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 failuresprocess p process q

Communication channel

send

Outgoing message buffer Incoming message buffer

receivem

Failure model …continued

2. Arbitrary (Byzantine) failures– the worst possible failure semantics– Arbitrarily omit intended processing steps or take unintended proce

ssing 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 msgsClass of failure Affects Description

Fail-stop Process Process halts and remains halted. Other processes maydetect this state.

Crash Process Process halts and remains halted. Other processes maynot be able to detect this state.

Omission Channel A message inserted in an outgoing message buffer neverarrives at the other end’s incoming message buffer.

Send-omission Process A process completes a send, but the message is not putin its outgoing message buffer.

Receive-omissionProcess A message is put in a process’s incoming messagebuffer, but that process does not receive it.

Arbitrary(Byzantine)

Process orchannel

Process/channel exhibits arbitrary behaviour: it maysend/transmit arbitrary messages at arbitrary times,commit omissions; a process may stop or take anincorrect 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 DescriptionClock Process Process’s local clock exceeds the bounds on its

rate of drift from real time.Performance Process Process exceeds the bounds on the interval

between two steps.Performance Channel A message’s transmission takes longer than the

stated bound.

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

Network

invocation

resultClient

Server

Principal (user) Principal (server)

ObjectAccess rights

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 Process qm

The enemym’

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 stampPrincipal A

Secure channelProcess p Process q

Principal B

• Introduction• Architecture Models• Fundamental Models• Summary

Chapter 3: System Model

Summary

• Architecture 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