1. System Models Chapter 2: Coulouris +
Chapter notes from K. Birman’s that in turn was based on Professor Paul Francis notes, Cornell University
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2. Distributed system models
“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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3. 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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4. 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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5. Software and hardware service layers in distributed systems
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6. Project 1 System Model 7

7. 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: Many web frameworks; the cloud enabled by advances in virtualization
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8. Clients invoke individual servers
EX: 1. File server,
2. Web crawler
EX: Web server
EX: browser,
web client
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9. A service provided by multiple servers
EX: akamai (data duplication), amazon aws (zones)
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10. Web proxy server and caches
Proxy servers + cache are used to provide increased
Availability and performance. They also play a major role
Firewall based security.
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11. A distributed application based on peer processes
Examples: Bit torrent based sharing;
IPFS: Interplanetary file system
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12. Case Study from Chapter 1: World wide web (www)
The world wide web (Tim Berner’s Lee: reading material for this week)
web/
world-changed-forever/
Web (web 1.0) was world wide web: a killer app for the Internet
Web Semantic was well received at the beginning but fluttered off
Web 3.0 – What web3.0 define it.
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13. Origins of WWW
Began as a application for document exchange among a community of physicists connected by the Internet
Web is an open system: It can be implemented in new ways without disturbing its existing functionality. Example: There are many types of browsers implemented on different platforms and operating systems
Web is also open with respect to types of resources that can be published and shared on it. Any media can be shared as long as you helper/plugin to handle that format of media.
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14. 2/15/20192/15/2019

15. Main Technical Standards
HTML : Hypertext Markup Language for specifying the contents and structure of contents
Uniform Resource Locator (URL) also known as URI (Uniform Resource Identification) that provide uniform identification for resource sin the web space: Scheme: scheme-specific identifier. Example: ftp://
A Client-server system architecture with standard rules for interaction (HTTP) in which browsers and other clients fetch documents and other resources from web servers.
Here is a question for you: Lets discuss this? How have these standards evolved since their beginnings?
I want some good answers.
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16. Web 2.0
HTML5 that include XHTML, well-formed, meaningful tags and interoperability through APIs.
HTTP did NOT come with security built-in, HTTP became HTTPS. The interacting parties agree to minimally encrypt the message and maintain a secure channel between them so that a middle person cannot access and misuse transactions and data.
and an important abstraction for the webpage called DOM or document object model (DOM)
What is next? What is in web3.0?
Reading: Read: Chapter 1 WWW case study: pages: 26-33
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17. 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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18. 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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19. Classic view of network API
Start with host name (maybe)
foo.bar.com
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20. Classic view of network API
Start with host name
Get an IP address
foo.bar.com
gethostbyname()
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21. Classic view of network API
Start with host name
Get an IP address
Make a socket (protocol, address)
foo.bar.com
gethostbyname()
socket();connect();…
sock_id
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22. 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.com
gethostbyname()
socket();connect();…
sock_id …
1,2,3,4,5,6,7,8,
TCP sock
UDP sock
Network
Eventually arrive in order
May or may not arrive
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23. 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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24. Protocol layering example
Browser
process
Web server
process
HTTP
HTTP
TCP
TCP
Router IP IP IP
Link1
Link1
Link2
Link1
Physical Link 1
Physical Link 2
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25. Protocol layering example
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.
Browser
process
Web server
process
HTTP
HTTP H
TCP
TCP
Router IP IP IP
Link1
Link1
Link2
Link1
Physical Link 1
Physical Link 2
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26. Protocol layering example
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.)
Browser
process
Web server
process
HTTP
HTTP
TCP
TCP
Router H T IP IP IP
Link1
Link1
Link2
Link1
Physical Link 1
Physical Link 2
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27. Protocol layering example
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.
Browser
process
Web server
process
HTTP
HTTP
TCP
TCP
Router IP IP IP H T I
Link1
Link1
Link2
Link1
Physical Link 1
Physical Link 2
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28. Protocol layering example
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”.
Browser
process
Web server
process
HTTP
HTTP
TCP
TCP
Router IP IP IP
Link1
Link1
Link2
Link1
Physical Link 1
Physical Link 2 H T I L1
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29. Protocol layering example
The router’s link layer receives the packet, strips the link header, and hands the result to the IP forwarding process.
Browser
process
Web server
process
HTTP
HTTP
TCP
TCP
Router IP IP IP H T I
Link1
Link1
Link2
Link1
Physical Link 1
Physical Link 2
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30. Protocol layering example
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.
Browser
process
Web server
process
HTTP
HTTP
TCP
TCP
Router IP IP IP H T I
Link1
Link1
Link2
Link1
Physical Link 1
Physical Link 2
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31. Protocol layering example
The packet goes over the link to the web server, after which each layer processes and strips its corresponding header.
Browser
process
Web server
process
HTTP
HTTP H
TCP
TCP
Router H T IP IP IP H T I
Link1
Link1
Link2
Link1
Physical Link 1
Physical Link 2 H T I
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32. 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 ( )) or binary (IP, TCP, Ethernet)
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33. Demuxing fields Ethernet: Protocol Number IP: 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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34. 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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35. IP(v4) address format In binary, a 32-bit integer
In text, this: “ ”
Each decimal digit represents 8 bits (0 – 255)
“Private” addresses are not globally unique:
Used behind NAT boxes
/8, /12, /16
Multicast addresses start with 1110 as the first 4 bits (Class D address) /4
Unicast and anycast addresses come from the same space
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36. 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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37. 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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38. UDP vs. TCP UDP is more real-time UDP has more of a “message” flavor
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 , 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
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39. Figure 2.13 Real-time ordering of events
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Pearson Education 2005

40. Figure 2.14 Processes and channels
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Pearson Education 2005

41. Figure 2.15 Omission and arbitrary failures
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.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Pearson Education 2005

42. Figure 2.16 Timing failures
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.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Pearson Education 2005

43. Figure 2.17 Objects and principals
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Pearson Education 2005

44. Figure 2.18 The enemy m m’ p q Copy of The enemy Process
Communication channel
Copy of m
Process p q
The enemy m’
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Pearson Education 2005

45. Figure 2.19 Secure channels
Principal B
Principal A
Process p
Secure channel
Process q
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn © Pearson Education 2005

46. 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.
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