CSomputer

cience

Foundations

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 1 / 21

COS 140: Foundations of Computer Science

Computer Networks

Fall 2017

CSomputer

cience

Foundations

Homework, announcements

Overview

Some Details

Network Protocols

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 2 / 21

• New chapter (23) online

• No homework assigned today, sorry!

CSomputer

cience

Foundations

What are Computer Networks?

Overview

• What are they?

• Types of Networks

Some Details

Network Protocols

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 3 / 21

• Multiple computers that are connected in some way

◦ Directly

◦ Via hubs, bridges, gateways, relays, routers, . . .

• The connected computers can communicate with each other

◦ Communication protocol

◦ Formal or ad hoc

◦ Idiosyncratic or standard

CSomputer

cience

Foundations

Why have computer networks?

Overview

• What are they?

• Types of Networks

Some Details

Network Protocols

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 4 / 21

• General public: WWW, e-mail, e-commerce. . .

• Business: e-commerce, b-to-b transactions, data sharing,

communication, synchronization of business processes. . .

• Science: data sharing, communication (formal, informal), CPU

sharing/grids. . .

• Military: data sharing, synchronization, communication, control of

assets. . .

CSomputer

cience

Foundations

Origins

Overview

• What are they?

• Types of Networks

Some Details

Network Protocols

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 5 / 21

• Hard to imagine 1st-world life w/o Internet & Web

◦ But networks very young: 1st remote login in 1969

◦ The Internet is even younger: public use in late 80s

◦ The Web is younger still: mid-90s

• Why did computer scientists invent them?

◦ Resource sharing: programs, data, equipment

◦ Increase reliability: alternate sources of supply (CPU, disk)

◦ Economics:

• Make full use of expensive mainframes

• Later: network of small computers cheaper than single,

powerful machine (e.g., Beowulf cluster)

◦ Communication between computer scientists

CSomputer

cience

Foundations

From networks → the Internet

Overview

• What are they?

• Types of Networks

Some Details

Network Protocols

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 6 / 21

• DoD:

◦ Better communication within military

◦ National security demands nationwide communication, even

in face of nuclear war

◦ Existing communication infrastructure: vulnerable

◦ Develop a distributed communication scheme that can survive

attack

◦ ⇒ ARPANET (early 70s)

• Academics/NSF ⇒ CSNET, NSFNET (early to mid-80s)

• “The Internet”/Commercial access (early to mid-90s; ARPANET,

NSFNET retired)

CSomputer

cience

Foundations

Types of Networks

Overview

• What are they?

• Types of Networks

Some Details

Network Protocols

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 7 / 21

• One extreme: Local Area Networks (LAN)

◦ Computers are in the same building

◦ Have control over the way the computers are set up

◦ Can have dedicated lines which connect the computers

• The other: Wide Area Networks (WAN)

◦ Great phys. distance might separate computers

◦ May not have a lot of control over all of the computers (may

be from several companies, universities, etc.)

◦ Creating the connection can be much more expensive

• Between the two:

◦ Intranets

◦ Metropolitan Area Networks

CSomputer

cience

Foundations

What is a network, really?

Overview

Some Details

• What is a network,

really?

• Interfacing with the

Network

• Store-and-Forward

Networks

• Network Topology

• Broadcast &

point-to-point

• Buses and Broadcast

Channels

Network Protocols

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 8 / 21

• A network is a set of computers connected by communication

channels

• Physical channel may be: wires (twisted pair, coax), fiber, radio

(Wi-Fi, e.g.), other

• One computer communicates with another via causing changes

to the channel such that the other can detect them.

• Examples:

◦ Voltages

◦ Properties of the voltage (e.g.) in the wire: amplitude,

frequency

◦ Light pulses

◦ Properties of radio waves

CSomputer

cience

Foundations

Interfacing with the Network

Overview

Some Details

• What is a network,

really?

• Interfacing with the

Network

• Store-and-Forward

Networks

• Network Topology

• Broadcast &

point-to-point

• Buses and Broadcast

Channels

Network Protocols

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 9 / 21

• Computers on the network ≡ hosts

• A host is connected to the network (to the channel) via its

network controller – e.g., a network card

• Controller puts information from host into the network (by

modulating something about the channel) and getting information

from the network

• Originally, several hosts might share their connection to the

network; now single host per network controller card

CSomputer

cience

Foundations

Beyond the controller

Overview

Some Details

• What is a network,

really?

• Interfacing with the

Network

• Store-and-Forward

Networks

• Network Topology

• Broadcast &

point-to-point

• Buses and Broadcast

Channels

Network Protocols

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 10 / 21

• Hubs: connect group of computers to network – repeater, or with

an internal network

• Bridge: between LANs of same type (e.g., Ethernets)

• Router (original ARPANET name: Interface Message Processors,

IMPS): route information packets between hosts or LANs

CSomputer

cience

Foundations

Store-and-Forward Networks

Overview

Some Details

• What is a network,

really?

• Interfacing with the

Network

• Store-and-Forward

Networks

• Network Topology

• Broadcast &

point-to-point

• Buses and Broadcast

Channels

Network Protocols

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 11 / 21

• A unit of information to be transferred is called a packet

• Packet arrives at router, which stores it until the output line is free

and it can be forwarded to the next router

• Ultimately, final router delivers it to the recipient host

• Also called point-to-point or packet switched.

CSomputer

cience

Foundations

Network Topology

Overview

Some Details

• What is a network,

really?

• Interfacing with the

Network

• Store-and-Forward

Networks

• Network Topology

• Broadcast &

point-to-point

• Buses and Broadcast

Channels

Network Protocols

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 12 / 21

• Which hosts and routers are connected to each other.

• Possible topologies include: bus, ring, tree, complete, irregular –

possibly with hub topology overlaid

• In LANs, often can design the topology so hosts are connected

for maximum efficiency.

• In WANs, have irregular topologies because may have little

control over connections, or may add in subnets eliminating

ability to make a global design.

• Difference between internets and the Internet

CSomputer

cience

Foundations

Broadcast vs Point-to-Point Channels

Overview

Some Details

• What is a network,

really?

• Interfacing with the

Network

• Store-and-Forward

Networks

• Network Topology

• Broadcast &

point-to-point

• Buses and Broadcast

Channels

Network Protocols

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 13 / 21

• Broadcast channel (or protocol):

◦ All hosts get all of the messages.

◦ Messages marked with intended recipient so can be ignored

by others.

• Point-to-point: targeted toward particular host

• Multicast: subscribers get all packets

CSomputer

cience

Foundations

Buses and Broadcast Channels

Overview

Some Details

• What is a network,

really?

• Interfacing with the

Network

• Store-and-Forward

Networks

• Network Topology

• Broadcast &

point-to-point

• Buses and Broadcast

Channels

Network Protocols

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 14 / 21

• Networks with broadcast channels can be buses

• They are different from what we have studied about system

buses because:

◦ Routers and host computers linked to bus, not devices

associated with computers.

• They are like what we have studied about system buses because:

◦ Need to protect against more than one computer trying to

communicate at a time.

CSomputer

cience

Foundations

Protocols

Overview

Some Details

Network Protocols

• Protocols

• Example: Business

Correspondence

• Layered Architectures

for Networks

• ISO OSI Standard

• TCP/IP

• TCP/IP Layers

• Standards

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 15 / 21

• Specifications for interaction between entities.

• If two computers use the same protocols, they can interact.

• Allows the processing of the computers to be hidden.

◦ Abstracts a computer to the protocol.

◦ What is sent/received and when: defined by protocol

◦ How the computer implements the protocol – solely

responsibility of computer/OS designer

CSomputer

cience

Foundations

Example: Business Correspondence

Overview

Some Details

Network Protocols

• Protocols

• Example: Business

Correspondence

• Layered Architectures

for Networks

• ISO OSI Standard

• TCP/IP

• TCP/IP Layers

• Standards

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 16 / 21

• Several levels of interaction on both sides:

◦ Correspondents

◦ Secretaries

◦ Post offices/mail rooms

• Can consider each level to be communicating with the same level

in other correspondent.

• Communication really goes to bottom layer and then back up.

• At each level, communication happens in a way particular to that

level.

• Communication between layers is also specified.

CSomputer

cience

Foundations

Layered Architectures for Networks

Overview

Some Details

Network Protocols

• Protocols

• Example: Business

Correspondence

• Layered Architectures

for Networks

• ISO OSI Standard

• TCP/IP

• TCP/IP Layers

• Standards

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 17 / 21

• Way to modularize the problem so can focus on a section at a

time and ignore other details.

• Separate protocols for each layer, so the same layer appears to

communicate between machines.

• Interfaces between layers are also specified so the proper

information is passed up and down the hierarchy.

• Physical layer at the bottom actually passes the bits.

CSomputer

cience

Foundations

ISO OSI Standard

Overview

Some Details

Network Protocols

• Protocols

• Example: Business

Correspondence

• Layered Architectures

for Networks

• ISO OSI Standard

• TCP/IP

• TCP/IP Layers

• Standards

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 18 / 21

• Application layer - provides user with services that are needed,

such as e-mail, file transfer, remote job entry

• Presentation layer - Concerned with syntax and semantics of the

bits; interpret the bits as characters, etc.; data compression

• Session layer - Allows users on a different machine to establish a

session which has some services like file transfer or remote log in

• Transport layer - prepare data for the network layer and make

sure it arrives error-free

• Network layer - operation of subnet; packet routing

• Data link layer - find data intended to be sent; remove errors from

transmissions

• Physical layer - transmitting bits over the communication channel

CSomputer

cience

Foundations

TCP/IP

Overview

Some Details

Network Protocols

• Protocols

• Example: Business

Correspondence

• Layered Architectures

for Networks

• ISO OSI Standard

• TCP/IP

• TCP/IP Layers

• Standards

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 19 / 21

• Started in USA; DoD model

• The major worldwide protocol – the Internet protocol

• Missing some of the layers in ISO/OSI

CSomputer

cience

Foundations

TCP/IP Layers

Overview

Some Details

Network Protocols

• Protocols

• Example: Business

Correspondence

• Layered Architectures

for Networks

• ISO OSI Standard

• TCP/IP

• TCP/IP Layers

• Standards

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 20 / 21

• Application layer – Same as OSI’s

◦ Sometimes considered as part of TCP/IP◦ Ex: HTTP, FTP, ...

• Transport layer – OSI’s transport and session layers

◦ Transport Control Protocol (TCP) – connection-oriented◦ Guarantees packets arrive in order, retransmitting as needed◦ Also User Datagram Protocol (UDP; connectionless), others

• Internetwork layer – OSI’s network layer

◦ Internet Protocol, or IP

• Link layer – Corresponds to OSI’s physical and data link layers

◦ E.g.: Ethernet, Wi-Fi

CSomputer

cience

Foundations

A Word About Standards

Overview

Some Details

Network Protocols

• Protocols

• Example: Business

Correspondence

• Layered Architectures

for Networks

• ISO OSI Standard

• TCP/IP

• TCP/IP Layers

• Standards

Copyright c© 2002–2017 UMaine School of Computing and Information Science – 21 / 21

• Especially important when have issues of interoperability

• Often have de facto standards that are set by large groups of

users of corporations

• Standards need to be set between research and industry interest

groups