1. System Models Chapter 2: Coulouris +
Chapter notes from K. Birman’s that in turn was based on Professor Paul Francis, Cornell University
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2. 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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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.
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 look at architectural models + Interaction models in this discussion; We will deal with failure and security models later in the semester.
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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. 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: CORBA spec by OMG, Java RMI, grid software (Globus, Open grid Services), Web services.
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7. Clients invoke individual servers
EX: 1. File server,
2. Web crawler
EX: Web server
EX: browser,
web client
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8. A service provided by multiple servers
EX: akamai, altavista, Sun’s NIS (data replication)
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9. 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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10. A distributed application based on peer processes
Ex: distributed
Whiteboard
Application;
Music sharing
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11. Web applets
EX: Code streaming; mobile code
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12. 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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13. 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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14. Classic view of network API
Start with host name (maybe)
foo.bar.com
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15. Classic view of network API
Start with host name
Get an IP address
foo.bar.com
gethostbyname()
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16. 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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17. 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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18. 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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19. 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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20. 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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21. 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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22. 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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23. 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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24. 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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25. 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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26. 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 L2
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27. 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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28. 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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29. 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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30. 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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31. 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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32. 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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33. 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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34. Summary
When designing systems or analyzing systems, you want to examine at the high level the architectural model.
Subsequent steps will explore functional models such as interaction model, security model, failure model, reliability model etc.
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