From the Operating Systems course (CMPSCI 377) at UMass Amherst, Fall 2007.

1. Operating Systems
CMPSCI 377
Networks
Emery Berger
University of Massachusetts Amherst
UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science

2. Networks
Goal: Efficient, correct, robust message passing

between two separate nodes
Local area network (LAN) – nodes in single

building, fast & reliable (Ethernet)
Media: twisted-pair, coax, fiber

Bandwidth: 10-1,000MB/s

Wide area network (WAN) – nodes across large

geographic area (Internet)
Media: fiber, microwave links, satellite channels

Bandwidth: 1.544MB/s (T1), 45 MB/s (T3)

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3. Principles of Net Communication
Data broken into packets (~ 1Kb)

Basic unit of transfer

Computers & routers control packet flow

Road analogy:

Packets = cars

Network = roads

Computer = traffic lights (intersection)

Too many packets on shared link/node =

traffic jam
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4. Communication Protocols
Protocol: agreed-upon rules for

communication
Protocol stack: layers that comprise

networking software
Each layer N provides service to layer N+1

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5. Traditional Layers
Application layer – applications that use the net

Presentation layer – data format conversion

(big/little endian)
Session layer – implements communication strategy

(e.g., RPC)
Transport layer – reliable end-to-end communication

Network layer – routing & congestion control

Data link control layer – reliable point-to-point

communication over unreliable channel
Physical layer – electrical/optical signaling across “wire”

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6. TCP/IP Protocol Stack
Internet standard protocol stack

TCP: reliable protocol – packets received in order

UDP (user datagram protocol) – unreliable

 No guarantee of delivery
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7. Packet Format
All info needed to

recreate original
message
Packets may arrive

out of order
need sequence

number
Data segment

contains headers
for higher protocol
layers &
application data
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8. Sockets
Standard API for network programming

Duplex communication (2-way)

Can configure sockets to use UDP or TCP

UDP – best for lossy communication

Pings, video, audio, other multimedia

Sends datagrams (packets)

TCP – most stuff

Sends reliable stream of bytes

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9. Socket communication
Sockets (in TCP) send stream of bytes

Multiple sends may be combined

into one received message
To send multiple application-level

messages, must have application level
protocol
E.g., 4-bytes = message type, 4-bytes =

message length, plus message
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10. Client-side: using sockets
Creates socket

int fd = socket (AF_INET, SOCK_STREAM, 0)

Converts hostname to IP address with

getaddrinfo
getaddrinfo (name, portStr, NULL, portNum)

Connects to host

connect (fd, addr, len); // from getaddrinfo

Once connected, can send or recv

send (fd, data, amount, 0);

recv (fd, buf, len, 0);

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11. Server-side: using sockets
As before:

Create socket as before

Convert hostname to IP address with getaddrinfo

but in passive mode
hint.ai_flags = AI_PASSIVE

getaddrinfo (name, portStr, &hint, &addrinfo)

Binds socket to address:

bind (fd, addr, len); // from getaddrinfo

Listens for connection:

listen (fd, 1); // number of waiting msgs

Accepts connection ) new socket

int newfd = accept (fd, addr, len);

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12. Exercise
Lamest web server ever

Wait for connections on localhost port 80

Read message until first newline (‘n’)

Send <html><body>Hello

world</body></html>
Use:

int fd = socket (AF_INET, SOCK_STREAM, 0)

getaddrinfo (name, portStr, &hint, &addrinfo)

bind (fd, addr, addrlen)

int v = listen (fd, 1)

int newfd = accept (fd, addr, len)

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13. Network Topologies
Connection of nodes impacts:

Maximum & average communication time

Fault tolerance

Expense

Two basic topologies:

Bus (for LANs)

Point-to-point

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14. Bus Network Topologies
Bus nodes connect to common network

Linear bus – single shared link

Nodes connect directly to each other via bus

Inexpensive (linear in # of nodes)

Tolerant of node failures

Ethernet LAN

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15. Bus Network Topologies
Ring bus – single shared circular link

Same technology & tradeoffs as linear bus

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16. Point-to-Point Network Topologies
Fully-connected

Each message takes one “hop”

Node failure – no effect on communication with

others
Expensive – impractical for WANs

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17. Point-to-Point Network Topologies
Fully-connected

Each message takes one “hop”

Node failure – no effect on communication with

others
Expensive – impractical for WANs

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18. Point-to-Point Network Topologies
Fully-connected

Each message takes one “hop”

Node failure – no effect on communication with

others
Expensive – impractical for WANs

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19. Point-to-Point Network Topologies
Fully-connected

Each message takes one “hop”

Node failure – no effect on communication with

others
Expensive – impractical for WANs

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20. Point-to-Point Network Topologies
2
1 3
4
Partially connected

Links between some, but not all nodes

Less expensive, less tolerant to failures

Single node failure can partition network

Several hops – requires routing

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21. Point-to-Point Network Topologies
I want to go
2
to Node 4!
1 3
4
Partially connected

Links between some, but not all nodes

Less expensive, less tolerant to failures

Single node failure can partition network

Several hops – requires routing

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22. Point-to-Point Network Topologies
I want to go
2
to Node 4!
1 3
4
Partially connected

Links between some, but not all nodes

Less expensive, less tolerant to failures

Single node failure can partition network

Several hops – requires routing

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23. Point-to-Point Network Topologies
Tree structure: network hierarchy

Messages fast between direct descendants

Max message cost?

Not failure tolerant

Any interior node fails – network partitioned

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24. Point-to-Point Network Topologies
Star network: all nodes connect to central node

Each message takes how many hops?

Not failure tolerant

Inexpensive – sometimes used for LANs

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25. Point-to-Point Network Topologies
One-directional ring

Given n nodes, max hops?

Inexpensive

Fault-tolerant?

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26. Point-to-Point Network Topologies
Bi-directional ring

Given n nodes, max hops?

Inexpensive

Fault-tolerant? One node? Two?

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27. Point-to-Point Network Topologies
Doubly-connected ring: nodes connected to

neighbors & one more distant
Given n nodes, max hops?

Fault-tolerant?

More expensive

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28. The End
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29. Distributed Systems
Distributed system: physically separate processors

connected by one or more communication links
 no shared clock or memory
P2
P1
P4 P3
Most systems today distributed in some way

 e-mail, file servers, network printers, remote
backup, web...
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30. Parallel vs. Distributed Systems
Tightly coupled systems: “parallel

processing”
Processors share clock, memory, run one OS

Frequent communication

Loosely coupled systems: “distributed

computing”
Each processor has own memory, runs

independent OS
Infrequent communication

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31. Advantages of Distributed Systems
Resource sharing

Computational speedup

Reliability

Communication

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32. Advantages of Distributed Systems
Resource sharing

Resources need not be replicated

Shared files

Expensive (scarce) resources can be shared

Poster-size color laser printers

Processors present same environment to

user
Keeping files on file server

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33. Advantages, continued
Computational speedup

n processors = n times computational power

SETI@home

Problems must be decomposable into

subproblems
Trivial = embarrassingly parallel

Coordination & communication required

between cooperating processes
Synchronization

Exchange of results

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34. Advantages, continued
Reliability

Replication of resources provides fault

tolerance
One node crashes, user works on another one

Performance degradation but system available

Must avoid single point of failure

Single, centralized component of system

Example: central file servers

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35. Advantages, continued
Communication

Users/processes on different systems can

communicate
Mail, transaction processing systems like

airlines & banks, www
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