CS 335 - Computer Networks

CS 335
-

Computer Networks


Dr. Randy L. Ribler


103 Hobbs


ribler@lynchburg.edu


Introduction


Read Chapter 1 in Kurose and Ross

How do we design networks?

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Use layers of “abstractions”

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Each layer provides a set of “services”

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Upper layers implement the services in terms of
lower layer services

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Each layer treats the layer above it and below it
as an abstraction

Sentence

Word

Net

Talk

Sentence

Net

Talk

Send sentence

Send Word


Route Word

Alphanumeric
Transmission

Word

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Classroom Demonstration Protocol

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Talk

•
Talk

•
Talk

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Talk

•
Talk

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Talk

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Net

•
Word

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Sen
-
tence

•
Net

•
Net

•
Net

•
Net

•
Net

•
Word

•
Word

•
Word

•
Word

•
Word

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Sen
-
tence

•
Sen
-
tence

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Sen
-
tence

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Sen
-
tence

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Sen
-
tence

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Host 0

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Host 1

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Host 2

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Host 3

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Host 4

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Host 5

ISO OSI Reference Model


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International Standards Organization

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Open Systems Interconnection (OSI)

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The ISO OSI contains 7 layers


OSI Reference Model

Application

Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Presentation
Layer

Application

Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Presentation
Layer

Application

Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Presentation
Layer

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Adapted from p34, Tanenbaum

Sending Process

Receiving Process

OSI Reference Model
Terminology

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Services
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What the layer does

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like public member functions in OOP

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Interface
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Parameters and results

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like the parameter lists and return values in
OOP

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Protocols
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the communication between
peer levels

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like the hidden implementation of class in OOP.


Physical Layer

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Transmission of “raw” bits

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Hardware level concerns

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voltages, pins

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mechanical, electrical, and procedural
interfaces

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Communicates between a pair of nodes

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Provided by modems and other point
-
to
-
point transmission hardware


Data Link Layer

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Imposes a structure on the bit stream

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data
frames

(100’s or 1000’s of bytes long)

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Acknowledge receipt of frames

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Handle damaged, lost, or duplicate frames

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Flow regulation

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Don’t overflow buffers


The Network Layer

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Routing “packets”

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Congestion Control

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Accounting

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Heterogeneous network interconnects

The Transport Layer

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Provides multiplexing/demultiplexing
between network traffic and individual
processes.

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Establishes/deletes network connections

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End
-
to
-
end flow control.

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Not needed on routers (unless router is also
a host).

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Often provides a reliable bit stream.

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Transport layer protocols handle
“messages.”



The Session Layer

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Information about where to obtain service

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Access Rights/permissions

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establish “sessions” (login or ftp)

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Traffic Control ( half/full duplex
connections)

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The Session Layer is generally under
utilized

Presentation Layer

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General Purpose Application Utilities

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Utilities that operate on data

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Data word sizes

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Encryption

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Data Compression

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Code conversions

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e.g., ASCII <
-
> EBCDIC

Application Layer


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Application specific code

–
ftp

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telnet

–
http

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Anything not implemented elsewhere in the
protocol stack.

Internet Protocol (IP)

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Internet protocol has been revised

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IP version 4 (IPv4) is the most widely used.

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IP version 6 (IPv6) will eventually replace it.

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IPv4 uses 32
-
bit host addresses

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IPv6 uses 128
-
bit host addresses


TCP/IP Reference Model

Application

Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Presentation
Layer

Application

Layer

Transport Layer

Internet Layer

Host
-
to
-
network

Layer

OSI Model

TCP/IP Model

IP

TCP or UDP

Talk

Authorize Talk

Break Message

Apart

Route Message

Manage Frames

Send Bit Stream

ASCII
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EBCDIC

Talk

Authorize Talk

Reassemble

Message

Route Message

Manage Frames

Receive

Bit Stream

You

Your Friend

Application

Presentation

Session

Transport

Network

Data Link

Physical

Internet Protocol (IPv4)

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32
-
bit IP addresses

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Store internally as a single word

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Usually expressed to user’s using “dotted
decimal notation” (e.g., 161.115.147.1) where
each of the four numbers range from 0 to 255.

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Usually aliased to host name of the system (e.g.
mail.lynchburg.edu


Domain Name System (DNS)

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How can we map host names to IP
addresses?

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Domain Name Servers provide names in
response to client requests.

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A hierarchical representation provides name
management at each level of domain name.


Client/Server Models

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Network interaction is based on an
asymmetrical relationship

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Server

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provides services to authorized clients


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typically handles many clients simultaneously

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works passively, driven by client requests

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Client

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requests services

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typically initiates


Port Numbers

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IP addresses identify host machines only.

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Port numbers identify services on machines.

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16
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bit port numbers provide values (0
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65535)

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“Well
-
known ports” (0
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1023)

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Registered ports (1024
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49151) (49152 is 75% of
available port numbers)

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Dynamic or private ports

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Internet Assigned Number Authority (IANA)

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Determines (0
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1023)

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Registers (1024
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49151)

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Does not control (49151
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65535)




Some Well Known Port Numbers

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23 (telnet)

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21 (ftp)

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13 (daytime)

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25 (smtp (mail))

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43 (whois)

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144 (news)

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80 (http web server)


Telnet Client

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telnet <hostname> <port number>

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provides a general interface to request a service
and return a text response.



Ping

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See if there is a connection between two
machines

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See if a particular host is running


ping <hostname>

Trace Route

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traceroute <hostname> (unix)

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tracert <hostname> (dos/windows)

Internetworking

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161.115

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lynchburg.edu

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161.115.100.22

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acavax.lynchburg.edu

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161.115.147.1

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161.115.100.27

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161.115.100.29

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208.22.66

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networkvirginia.net

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128.100.200.123

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128.173

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cs.vt.edu

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128.100

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cns.vt.edu

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128.173.133.44

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161.115.144.
2

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128.100.200.145

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208.22.66.1

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208.22.66.145

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208.22.66.100

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208.22.66.101

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208.22.66.102

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128.100.200.165

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128.100.201.155

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128.100.230.156

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128.173.133.155

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128.173.133.102

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lasi
-
main

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150.123.23.1

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GATEWAYS

Network Protocols Must Adopt
One of the Two Byte Orders

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Network Byte Order

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The protocol byte order

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Host Byte Order

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The native machine byte order

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Conversion Functions

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htons : convert 16
-
bit value from host byte order to
network byte order. (ntohs provides the inverse)

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htonl: convert 32
-
bit value from host byte order to
network byte order. (ntohl provides the inverse)


Domain Name Management

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Each level is responsible for assigning its
own names

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.edu is the top
-
level domain for educational
institutions in the US. We need to get
permission from the managers of the .edu
domain before we can use lynchburg.edu

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lynchburg.edu is the Lynchburg College
domain name. We can name our computers
anything we want, (e.g., acavax.lynchburg.edu).

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If we wanted to we could create a
cs.lynchburg.edu subdomain


Lavender
Army

Magenta

Army

Magenta

Army

Distributed processing
problem from Data
Networks, by Dimitri
Bertsekas and Robert
Gallager