Lecture 1 networking & internetworking

Networking &
Internetworking
Connecting People, Places, and
Everything Else

Networks
 Any connection between two or more computers
 e.g. Even when you connect two computers via a USB
cable
 Networks use a set of low-level protocols (rules for
communication)
 e.g. TCP/IP, IPX/SPX (Internetwork Packet Exchange/Sequenced
Packet Exchange)
 Networks use standardized hardware
 e.g. Twisted pair cabling & Ethernet hubs, ATM
switches & optical fibre cabling

Network Speed
 A network’s speed can be summed up with two
values:
 Bit rate:
 How many bits can be placed on the network in a
given time interval (e.g. 1 second)?
 This is often called bandwidth, but this is a misnomer
since bandwidth has to do with the range of
frequencies to be used
 Bit rate becomes the dominant factor when sending
many packets (e.g. a large file)
 Latency:
 How long does it take a bit to be received by the
destination node?
 Latency becomes the dominant factor when sending
individual packets, or alternating sending/receiving

Networks: Purpose
 Sharing files
 FTP, NFS, SMB(server message block)
 Communicating
 E-Mail, instant messaging, games
 Executing programs remotely
 rlogin, telnet

Network Messaging
 Most local area networks use electrostatic
network hardware
 The wires transmit messages using electricity
 The transmission hardware charges the wire
positively or negatively to indicate 1 and 0
respectively
 The reception hardware senses the charge

Internetworking: internets (WANs)
 e.g. The Internet
 Any connection between two or more
networks
 e.g. An Ethernet network connected to another
Ethernet network by glass fibre cable and
ATM switches

An Internet
LAN A LAN B
LAN C LAN E
LAN D
Backbone A

Internets: Purpose
 Larger scope
 Access more shared files
 Communicate with more people
 Execute programs on more machines

Network Properties
Networking Fundamentals for Specific
Network Types

Important Network Properties
 Scope: A network should provide services to
several applications
 Scalability: A network should operate
efficiently when deployed on a small-scale as
well as on a large-scale
 Robustness: A network should operate in
spite of failures or lost data

 Self-Stabilization: A network, after a failure
or other problem, should return to normal (or
near normal) without human intervention
 Autoconfigurability: A network should
optimize its own parameters in order to
achieve better performance
 Safety: A network should prevent failures as
well as prevent failures from affecting other
areas of the network

 Configurability: A network’s parameters
should be configurable to improve
performance
 Determinism: Two networks with identical
conditions should yield identical results
 Migration: It should be possible to add new
features to a network without disruption of
network service

Network Usage
 Ideally, the network usage should be
maximized
 If network resources are unused, the network
is not being used efficiently
 Unused network resources could be used to
provide higher throughput to hosts
 This typically becomes a problem in routing
 If all routers choose the single optimal path,
some (less than optimal) regions of the
network will be unused

The Internet
The Information Age

Internet History
A Condensed Timeline of Internet
Development and Research Projects

The Birth of Arpanet
 Developed by ARPA (Advanced Research
Projects Agency)
 A packet-switched network connecting a
number of LANs, called Arpanet
 Used primarily for connecting the networks of
the U.S. Government’s defense initiative
(DARPA, which was a branch of the DoD)
 Became a useable internet in 1977

The Internet Split
 Originally, Arpanet was strictly military and defense-
oriented
 Arpanet was converted to use the new standard
TCP/IP protocol set (1980)
 The Defense Communication Agency (DCA) split
Arpanet into two networks (1983):
 Arpanet: To be used for internetworking research
projects
 Milnet: To be used strictly for military purposes

A Military & University Internet
 The University of California (at Berkeley)
incorporated TCP/IP programming into its
BSD UNIX operating system (1983)
 ARPA funded research projects at many
Universities in order to make then internet-
capable (1983-1989)
 BSD UNIX developed the socket network
programming model commonly used today
 It was now possible for anyone to write
internet applications
 This resulted in a boom of internet applications,
many of which survive to this day

A Public Internet
 It became practical for private organizations
to connect to the Internet (mid-late 1980s)
 Due to inexpensive hardware
 The Internet Architecture Board (IAB) was
empowered to manage research
 Coordinates and focuses research and
development with regards to the Internet and
TCP/IP

Internet Implementation
Under the Hood

TCP/IP
 A considerably large part of this course
 The underlying network protocols upon which
application-level protocols are built
 e.g. HTTP, SMTP, IMAP(Internet Message Access Protocol
 TCP/IP is the framework for the Internet

TCP/IP
 TCP/IP is actually two protocols:
 TCP: Transport control protocol
 Creates reliable transport (handles lost
messages), offers a logical stream of data
(reorders mixed up messages)
 IP: Internet protocol
 Defines addressing (e.g. 137.207.32.2), routing
protocols (how to get messages from source to
destination), etc.

Internet Messaging
 TCP is a reliable protocol
 If a message does not arrive, it is re-sent
 Messages must be acknowledged by their
recipients before a certain time expires
 The message’s time-to-live (TTL) value

Layered Architectures
Schemes for Organizing the
Responsibility of Networking Components

Network Service Models
 Provide a layered abstraction for networking
 Each layer performs specific tasks
 Between each layer is an interface
 e.g. The hardware access layer might interact directly with
the hardware, providing a hardware-independent interface
to higher layers
 The same layer at the source and the destination are known
as ‘peer’ layers
 e.g. A ‘transport’ layer may provide reliable messaging, so
the transport layer in the source and destination will
communicate to ensure each message arrived in tact

Network Service Model
Sender Receiver
Layer n Layer n
……
Layer 2 Layer 2
Layer 1Layer 1
Network
Lowerlevel
Higherlevel

The OSI Reference Model
 A layered service model developed by the
International Standardization Organization
(ISO)
 Defines 7 conceptual layers
 Each serves a very specific purpose
 OSI: Open System Interconnection
 Developed as a reference to be used for all
future protocols

 The 7 layers are (highest to lowest level):
1. Application
2. Presentation
3. Session
4. Transport
5. Network
6. Data link
7. Physical

Application Application
Presentation Presentation
Session Session
TransportTransport
Network Network
Data linkData link
Physical Physical
protocol
protocol
protocol
protocol
protocol
protocol
protocol

 Represents the actual network hardware
 Deals with problems such as:
 Sending signals across wires
 e.g. Charging a wire with a specific voltage
 Converting bits to signals
 Even two Ethernet cards may have different physical
layers, as this layer deals with hardware specific
concerns
Physical Layer

 Represents the interface to the network
hardware
 Transmission of groups of bits
 e.g. Groups of bits might represent an ASCII text
string, a floating point number, or a chunk of
binary data
 Verifying data integrity (using checksums)
Data Link Layer

 Handles the connection between sender and receiver
 Determining a path from the sender node to the
recipient node (i.e. routing)
 Determining the correct recipient (i.e. addressing)
 Network congestion
 Fragmenting data into packets
 Reassembly of packets
Network Layer

 Represents an end-to-end reliable
communication stream
 Lost (unacknowledged) packets
 Duplicate packets
 Reordering packets
Transport Layer

 Represents a dialogue between sender and receiver
 Somewhat irrelevant in today’s networks
 Handles the establishment of an authenticated
connection to the receiver
 Authentication of the sender node on the packet
assembler and dissembler (PAD)
 This is a remote computer which provided the lower
layers in a shared manner, which required
authentication
Session Layer

 Specifies data representations so that both sides can
determine how to read data
 e.g. How many bytes to use for floating point values
(including compressed as well as uncompressed
values, encryption)
 e.g. What is the order of the bytes?
 Uses an ISO-defined standard for these
representations: Abstract Syntax Notation 1 (ASN.1)
Presentation Layer

 Defines what data is stored in the message
(specific to each application)
 e.g. An E-Mail application would store such
things as recipient, subject, and body text into
an E-Mail application-level message
 e.g. A web server would put header
information (information about the server & the
document) as well as the document itself into
its application-level messages
Application Layer

Session Message:
•Session Header
•Recipient
•Subject
•Body
Message:
•Recipient – CHAR(9)
•Subject – CHAR (17)
•Body – CHAR (243)
Frame:
•Data Link Header
•Network Header
•Transport Header
•Session Header
•Recipient
•Subject
•Body
OSI Reference Model: An Example
Application
Presentation
Session
Transport
Network
Data link
Physical
E-Mail:
•Recipient
•Subject
•Body
Network01001101111010010011001…
Network Frame:
•Network Header
•Transport Header
•Session Header
•Recipient
•Subject
•Body
Transport Message:
•Transport Header
•Session Header
•Recipient
•Subject
•Body

OSI Reference Model: Routing
Application
Presentation
Session
Transport
Network
Data link
Physical
Application
Presentation
Session
Transport
Network
Data link
Physical
Network
Data link
Physical
Router

OSI Reference Model Overview
 Each layer provides some abstraction to the higher
levels
 e.g. The physical layer actually charges the wire
 Higher layers need not worry about how to charge the
wire
 e.g. The transport layer ensures that message arrive
 Higher layers can assume that messages will arrive,
and will not be lost
 The OSI reference model was used as the basis for
X.25 networks.

The TCP/IP Service Model
 Researchers developing the TCP/IP protocol
suite also developed a layered reference
model
 The TCP/IP reference model consists of 5
layers
 3 software layers
 1 software & hardware layer
 1 hardware layer

 The 5 layers:
1. Application
2. Transport
3. Internet
4. Network Interface
5. Hardware

 Defines what data is stored in the message (specific
to each application)
 e.g. An E-Mail application would store such things as
recipient, subject, and body text into an E-Mail
application-level message
 e.g. A web server would put header information
(information about the server & the document) as well
as the document itself into its application-level
messages
 Essentially, this layer is identical to the application
layer in the OSI reference model
Application Layer

 Handles end-to-end communication
 Divides the data into manageable chunks of
information (packets)
 Provides reliable communication
 Ensures that all packets are received
 Provides error-free communication
 Uses a checksum to verify data integrity
 Implemented by the TCP protocol
 Transport control protocol
Transport Layer

 Handles communication between machines
 The path of a message is determined (routing)
 The destination of a message is determined
(addressing)
 Implemented by the IP protocol
 Internet protocol
Internet Layer

 Handles low level interaction with hardware
 Issues commands to the hardware to transmit a
number of bits (1 or 0)
 Deals with hardware-specific concerns
 Implemented by the device drivers for the hardware
installed into the operating system
 Essentially, this layer is identical to the data link layer
in the OSI model
Network Interface Layer

 Actually transmits signals onto the network
 Deals with issues such as:
 How to transmit signals (e.g. electrify the wire)
 How to detect problems (e.g. collisions)
 Represents the actual network hardware
 Essentially this layer is identical to the physical layer
in the OSI model
Hardware Layer

TCP/IP Service Model: Example
Application
Transport
Internet
Network
Interface
Hardware
Network01001101111010010011001…
IP Datagrams:
•IP Header
•TCP Header
•Data Bytes
Transport Packet:
•TCP Header
•Data Bytes
E-Mail:
•Data Bytes
Network Frame:
•IP Header
•TCP Header
•Data Bytes

TCP/IP Service Model: Routing
Application
Transport
Internet
Network
Interface
Hardware
Application
Transport
Internet
Network
Interface
Hardware
Internet
Network
Interface
Hardware
Router

TCP/IP Service Model: Overview
 Major differences between OSI and TCP/IP:
 TCP/IP has no presentation layer
 The applications must agree on a data format (how
many bytes for a floating point, etc)
 Thus, presentation/encoding is handled by the
application layer
 TCP/IP has no session layer
 Not significant: It does little in modern networks
 In TCP/IP a session is typically managed by the
application layer

The TCP/IP Protocol in Action
 Consider the following simplified network
route
 The source (S) and destination (D) are
separated by two routers (R1, R2)
S DR1 R2

 Let’s consider a web browser, using HTTP
 The web browser on S sends a packet to the web
server on D
 The application layer (i.e. the browser) provides the
logical (IP) addresses for S (IPS) and D (IPD)
 The application layer also provides the port numbers
for the source (PortS) and destination (PortD)
S DR1 R2
HTTP Req

 The Transport layer (TCP) uses the port
numbers (e.g. 2765 and 80) to create a TCP
packet (sometimes called a segment):
S DR1 R2
Source Port: 2765
Destination Port: 80
HTTP Req

Source IP: 137.207.140.71
Dest IP: 24.87.204.16
 The Internet (i.e. IP) layer uses the IP
addresses specified by the application layer
to create an IP datagram
 e.g. 137.207.140.71, 24.87.204.16
 Next, a route is determined for the packet,
using S’s routing table
 S only needs one router’s address (R1)
S DR1 R2
TCP Segment
HTTP Req

Source MAC: MACS
Dest MAC: MACR1
IP Datagram
 The MAC addresses of S and R1 (MACS and
MACR1) are used to create a network frame
 If the MAC address of R1 is not known, ARP
(address resolution protocol) is used
S DR1 R2
TCP Segment
HTTP Req

Source MAC: MACS
Dest MAC: MACR1
IP Datagram
 Let’s simplify the picture (for clarity)
 In subsequent steps the IP datagram and its
contents will not change very much
S DR1 R2

Source MAC: MACS
Dest MAC: MACR1
IP Datagram
 The network frame is transmitted on the
network to R1
 This is possible since S and R1 are both
members of the same network
S DR1 R2

IP Datagram
 R1 will extract the IP datagram from the
payload of the network frame
 R1 looks up the destination IP address (IPD) in
it’s routing table, to determine which router
should get the datagram next (R2)
S DR1 R2

Source MAC: MACR1
Dest MAC: MACR2
IP Datagram
 R1 uses its own MAC address (MACR1) and
R2’s MAC address (MACR2) to create another
network frame
S DR1 R2

Source MAC: MACR1
Dest MAC: MACR2
IP Datagram
 The network frame is received by R2, and the
IP datagram is extracted from it’s payload
 R2 uses its routing table to lookup IPD
 In this case, R2 is directly connected to D
 This is called direct routing
S DR1 R2

ARP Request
IP: 24.87.204.16
MAC: ?
IP Datagram
 Most likely, R2 does not have the MAC
address of D (MACD)
 The address resolution protocol (ARP) is used
to determine the MAC address:
S DR1 R2

ARP Response
IP: 24.87.204.16
MAC: 08-7F-3C-90-0C-DF
IP Datagram
 D recognizes it’s IP address and responds
with its MAC address (MACD)
 e.g. 08-7F-3C-90-0C-DF
S DR1 R2

Source MAC: MACR2
Dest MAC: MACD
IP Datagram
 A network frame is created by R2 now that
the MAC address is known
 The frame is sent directly to D
S DR1 R2

Source MAC: MACR2
Dest MAC: MACD
IP Datagram
 D extracts the IP datagram from the network
frame (which is discarded)
 The IP datagram’s payload is passed to the
transport layer
S DR1 R2

 The Transport layer (within D’s operating
system), will use the port numbers specified
in the TCP segment to determine to which
application it should send the segment
 In this case, to the application bound to port
80 (the web server)
S DR1 R2
Source Port: 2765
Destination Port: 80
HTTP Req

 Now, the web server on D has the HTTP
request, and it processes it
 An HTTP response is sent back using the
same process
 The web server uses the same IP addresses
and logical addresses as the last message
S DR1 R2
HTTP Req

Lecture 1 networking & internetworking

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