6/14/20151 System Models Chapter 2: Coulouris + Chapter notes from K. Birman’s that in turn was...

transcript

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System ModelsChapter 2: Coulouris +

Chapter notes from K. Birman’s that in turn was based on Professor Paul

Francis notes, Cornell University

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Distributed system models Model: “a simplified representation of a

system or phenomenon, as in the sciences or economics, with any hypotheses required to describe the system or explain the phenomenon, often mathematically.”

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System Models Architectural model defines the way in which the

components of the system are placed and how they interact with one another and the way in which they are mapped onto the underlying network of computers.

Fundamental models: Interaction model deals with communication details among the

components and their timing and performance details. Failure model gives specification of faults and defines reliable

communication and correct processes. Security model specifies possible threats and defines the concept of

secure channels. We will discuss the various models at a high level in this

discussion and will elaborate on each of these as we discuss other systems.

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Architectural Model Concerned with placement of its parts and

relationship among them. Example: client-server model, peer-to-peer model Abstracts the functions of the individual

components. Defines patterns for distribution of data and

workload. Defines patterns of communication among the

components. Example: Definition of server process, client

process and peer process and protocols for communication among processes; definition client/server model and its variations.

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Software and hardware service layers in distributed systems

Applications, services

Computer and network hardware

Platform

Operating system

Middleware

National Weather Service Web Site

Data AggregatorRMI WeatherInfo

Server

RMI WeatherInfo Client

Application

RMIIP Socket API

Weather Web Service Web Client

AnalyticsWeather Web Service

Server

Relation Database

SOAP/RESTXML

Weather Google Map Client

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Middleware Layer of software whose purpose is to mask

the heterogeneity and to provide a convenient programming model for application programmers.

Middleware supports such abstractions as remote method invocation, group communications, event notification, replication of shared data, real-time data streaming.

Examples: Java RMI, grid software (Globus, Open grid Services), Web services.

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Clients invoke individual servers

Server

Client

invocation

result

Serverinvocation

result

Process:Key:

Computer:

EX: browser, web client

EX: Web server

EX: 1. File server, 2. Web crawler

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A service provided by multiple servers

Server

Service

Client

EX: akamai (data duplication), now amazon aws (zones)

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Web proxy server and caches

Client

server

serverClient

Proxy servers + cache are used to provide increased Availability and performance. They also play a major role Firewall based security. http://www.interhack.net/pubs/fwfaq/

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A distributed application based on peer processes

Coordination

Application

Coordination

Application

Coordination

Application

Ex: distributed Whiteboard Application;Music sharing

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Web appletsa) client request results in the downloading of applet code

Web server

ClientWeb serverApplet

Applet code

Client

b) client interacts with the applet

EX: Code streaming; mobile code

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Interaction Models Within address space (using path as

addresses) Socket based communication: connection-

oriented, connection-less Socket is an end-point of communication Lets look at some code + details

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Socket based communication

int sockfd; struct sockaddr_in addr;

addr.sin_family = AF_INET; addr.sin_addr.s_addr =

inet_addr(SERV_HOST_ADDR); addr.sin_port = htons(SERV_TCP_PORT);

sockfd = socket(AF_INET, SOCK_STREAM, 0);connect(sockfd, (struct sockaddr *) &addr,

sizeof(serv_addr));do_stuff(stdin, sockfd);

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Classic view of network API Start with host name

(maybe) foo.bar.com

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Classic view of network API Start with host name Get an IP address foo.bar.com

gethostbyname()

10.5.4.3

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Classic view of network API Start with host name Get an IP address Make a socket

(protocol, address)

foo.bar.comgethostbyname()

10.5.4.3

sock_id

socket();connect();…

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Classic view of network API Start with host name Get an IP address Make a socket

(protocol, address) Send byte stream

(TCP) or packets (UDP)

foo.bar.comgethostbyname()

10.5.4.3

sock_id

socket();connect();…

TCP sock UDP sock

Network

1,2,3,4,5,6,7,8,9 . . . …

Eventually arrive in order

May or may not arrive

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Protocol layering Communications stack consists of a set of services, each providing a service to the layer above, and using services of the layer below Each service has a programming API, just like any

software module Each service has to convey information one or

more peers across the network This information is contained in a header

The headers are transmitted in the same order as the layered services

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Protocol layering example

Browserprocess

Web serverprocess

Physical Link 1 Physical Link 2

Router

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Browserprocess

Web serverprocess

Router

Browser wants to request a page. Calls HTTP with the web address (URL).HTTP’s job is to convey the URL to the web server.HTTP learns the IP address of the web server, adds its header, and calls TCP.

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Browserprocess

Web serverprocess

TCP’s job is to work with server to make sure bytes arrive reliably and in order.TCP adds its header and calls IP.(Before that, TCP establishes a connection with its peer.)

T Router

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Browserprocess

Web serverprocess

IP’s job is to get the packet routed to the peer through zero or more routers.IP determines the next hop from the destination IP address.IP adds its header and calls the link layer (i.e. Ethernet) with the next hop address.

Router

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Browserprocess

Web serverprocess

The link’s job is to get the packet to the next physical box (here a router).It adds its header and sends the resulting packet over the “wire”.

Router

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Browserprocess

Web serverprocess

The router’s link layer receives the packet, strips the link header, and hands the result to the IP forwarding process.

Router

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Browserprocess

Web serverprocess

The router’s IP forwarding process looks at the destination IP address, determines what the next hop is, and hands the packet to the appropriate link layer with the appropriate next hop link address.

Router

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Browserprocess

Web serverprocess

The packet goes over the link to the web server, after which each layer processes and strips its corresponding header.

Router

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Basic elements of any protocol header Demuxing field

Indicates which is the next higher layer (or process, or context, etc.)

Length field or header delimiter For the header, optionally for the whole packet

Header format may be text (HTTP, SMTP (email)) or binary (IP, TCP, Ethernet)

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Demuxing fields Ethernet: Protocol Number

Indicates IPv4, IPv6, (old: Appletalk, SNA, Decnet, etc.)

IP: Protocol Number Indicates TCP, UDP, SCTP

TCP and UDP: Port Number Well known ports indicate FTP, SMTP, HTTP, SIP, many others Dynamically negotiated ports indicate specific processes (for these and

other protocols)

HTTP: Host field Indicates “virtual web server” within a physical web server

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IP (Internet Protocol) Three services:

Unicast: transmits a packet to a specific host Multicast: transmits a packet to a group of hosts Anycast: transmits a packet to one of a group of hosts

(typically nearest) Destination and source identified by the IP address (32

bits for IPv4, 128 bits for IPv6) All services are unreliable

Packet may be dropped, duplicated, and received in a different order

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IP(v4) address format In binary, a 32-bit integer In text, this: “128.52.7.243”

Each decimal digit represents 8 bits (0 – 255) “Private” addresses are not globally unique:

Used behind NAT boxes 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16

Multicast addresses start with 1110 as the first 4 bits (Class D address) 224.0.0.0/4

Unicast and anycast addresses come from the same space

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UDP (User Datagram Protocol) Runs above IP Same unreliable service as IP

Packets can get lost anywhere: Outgoing buffer at source Router or link Incoming buffer at destination

But adds port numbers Used to identify “application layer” protocols or

processes Also a checksum, optional

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TCP (Transmission Control Protocol) Runs above IP

Port number and checksum like UDP Service is in-order byte stream

Application does not absolutely know how the bytes are packaged in packets

Flow control and congestion control Connection setup and teardown phases Can be considerable delay between bytes in at source

and bytes out at destination Because of timeouts and retransmissions

Works only with unicast (not multicast or anycast)

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UDP vs. TCP UDP is more real-time

Packet is sent or dropped, but is not delayed UDP has more of a “message” flavor

One packet = one message But must add reliability mechanisms over it

TCP is great for transferring a file or a bunch of email, but kind-of frustrating for messaging Interrupts to application don’t conform to message boundaries No “Application Layer Framing”

TCP is vulnerable to DoS (Denial of Service) attacks, because initial packet consumes resources at the receiver

Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems:

Figure 2.8Real-time ordering of events

receive

Physical time

receive receive

receive receive receivet1 t2 t3

receive

Figure 2.9Processes and channels

process p process q

Communication channel

Outgoing message buffer Incoming message buffer

receivem

Figure 2.10Omission and arbitrary failuresClass of failure Affects DescriptionFail-stop ProcessProcess halts and remains halted. Other processes may

detect this state.Crash ProcessProcess halts and remains halted. Other processes may

not be able to detect this state.Omission ChannelA message inserted in an outgoing message buffer never

arrives at the other end’s incoming message buffer.Send-omission ProcessA process completes a send, but the message is not put

in its outgoing message buffer.Receive-omissionProcessA message is put in a process’s incoming message

buffer, 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 an

incorrect step.

Figure 2.11Timing failures

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.

Figure 2.12Objects and principals

Network

invocation

resultClient

Server

Principal (user) Principal (server)

ObjectAccess rights

Figure 2.13The enemy

Communication channel

Copy of m

Process p Process qm

The enemym’

Figure 2.14Secure channels

Principal A

Secure channelProcess p Process q

Principal B

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Summary When designing systems or analyzing

systems, you want to examine at the high level the architectural model.

Subsequent steps will explore fundamental models such as interaction model, security model, failure model, reliability model etc.

6/14/20151 System Models Chapter 2: Coulouris + Chapter notes from K. Birman’s that in turn was...

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