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Outline
 Origins of TCP/IP
 OSI Stack & TCP/IP Architecture
 Client Server Architecture
 IP Addressing & Numbering Rules
 IP Forwarding and default route
 Network Troubleshooting Tools

Origins of TCP/IP
 1950’s – 1960’s – US Govt. requirement for
“rugged” network that would continue to work
in case of a nuclear attack
 RAND Corporation (America’s leading think
thank) & DoD formed ARPA (Advanced
Research Project Agency)
 1968 – ARPA engineers proposed Distributed
network design for ARPANET Network

Distributed Network Design
 Pre-ARPANET networks
– “connection oriented”
– Management & control was centralized
 “New” Network – ARPANET
– Connectionless
– Decentralised
 Modern Internet has evolved from the ARPANET

Simplified view of the Internet

Internetworks
 Start with lots of little networks
 Many different types
– Ethernet, dedicated leased lines, dialup, ATM, Frame Relay,
FDDI
 Each type has its own idea of addressing and protocols
 Want to connect them all together and provide a
unified view of the whole lot (i.e. act as a single large
network)

A small internetwork or “Internet”

The unifying effect of the network
layer
 Define a protocol that works in the same way with
any underlying network
 Call it the network layer (IP)
 IP routers operate at the network layer
 There are defined ways of using:
» IP over Ethernet
» IP over ATM
» IP over FDDI
» IP over serial lines (PPP)
» IP over almost anything

OSI Stack & TCP/IP Architecture

What is TCP/IP?
 In simple terms is a language that enables
communication between computers
 A set of rules (protocol) that defines how two
computers address each other and send data to
each other
 Is a suite of protocols named after the two most
important protocols TCP and IP but includes
other protocols such as UDP, RTP, etc

Open Systems & TCP/IP
 TCP/IP formed from standardized communications
procedures that were platform independent and open
 Open systems
– open architecture - readily available to all
 What is open system networking?
– network based on well known and standardized protocols
– standards readily available
– networking open systems using a network protocol

OSI - Layered Model Concept
 Divide-and-conquer approach
 Dividing requirements into groups, e.g transporting of
data, packaging of messages, end user applications
 Each group can be referred to as a layer
– Upper layers are logically closer to the user and deal with
more abstract data, relying on lower layer protocols to
translate data into forms that can eventually be physically
transmitted.
 Open Systems Interconnection Reference Model (OSI-
RM) adopted as a standard for networking

OSI Model
APPLICATION
• Upper Layers
• Application oriented
• Independent of layers below
TRANSPORT
• Lower Layers
• Transmission of data
• No differentiation of upper layers
1
3
2
4
5
6
7 Application
Presentation
Session
Transport
Network
Data Link
Physical

Layers 7, 6, 5
 7: Application layer
– Provides different services to the applications
– Uses the underlying layers to carry out work
» e.g. SMTP (mail), HTTP (web), Telnet, FTP, DNS
 6: Presentation layer
– Converts data from applications into common format and
vice versa
 5: Session layer
– organizes and synchronizes the exchange of data between
application processes

Layer 4
 4: Transport layer
– Provides end to end transportation of segments
– E.g. TCP
» encapsulates TCP segments in network layer packets
» adds reliability by detecting and retransmitting lost packets
» uses acknowledgements and sequence numbers to keep
track of successful, out-of-order, and lost packets
» timers help differentiate between loss and delay
– UDP is much simpler: no reliability features

Layer 3
 3: Network layer
– Routes the information in the network
– E.g. IP is a network layer implementation which
defines addresses in such a way that route selection
can be determined.
» Single address space for the entire internetwork
» adds an additional layer of addressing, e.g. IP address,
which is different from MAC address.

Layer 3
 3: Network layer (e.g. IP)
– Unreliable (best effort)
» if packet gets lost, network layer doesn’t care for higher layers can
resend lost packets
– Forwards packets hop by hop
» encapsulates network layer packet inside data link layer frame
» different framing on different underlying network types
» receive from one link, forward to another link
» There can be many hops from source to destination

Layer 3
 3: Network layer (e.g. IP)
– Makes routing decisions
» how can the packet be sent closer to its destination?
» forwarding and routing tables embody “knowledge” of
network topology
» routers can talk to each other to exchange information
about network topology

Layer 2
 2: Data Link layer
– Provides reliable transit of data across a physical
network link
– bundles bits into frames and moves frames between
hosts on the same link
– a frame has a definite start, end, size
– often also a definite source and destination link-layer
address (e.g. Ethernet MAC address)
– some link layers detect corrupted frames while other
layers re-send corrupted frames (NOT Ethernet)

Layer 1
 1: Physical layer
– moves bits using voltage, light, radio, etc.
– no concept of bytes or frames
– bits are defined by voltage levels, or similar
physical properties
1101001000

OSI and TCP/IP
TCP/UDP – end to end reliability
IP - Forwarding (best-effort)
Framing, delivery
Raw signal
Mail, Web, etc.
1
3
2
4
5
6
7 Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Transport
Network
Data Link &
Physical
OSI TCP/IP

Protocol Layers:
The TCP/IP Hourglass Model
Network layer
Token
Ring
ATM X.25 PPP
Frame
Relay
HDLC
Ethernet
IP
UDP
TCP
HTTP FTP Telnet DNS
SMTP Audio Video
RTP
Data link layer
Transport layer
Application layer

Layer Interaction
 Application, Presentation and Session protocols are
end-to-end
 Transport protocol is end-to-end
– encapsulation/decapsulation over network protocol on end
systems
 Network protocol is throughout the internetwork
– encapsulation/decapsulation over data link protocol at each
hop
– Link and physical layers may be different on each hop

Layer Interaction:
OSI 7-Layer Model
Host Router Router Host
Application
Presentation
Session
Transport
Network
Link
Physical
Network
Link Link
Network
Link Link
Application
Presentation
Session
Transport
Network
Link
Physical
Physical
Hop
by
hop
End
to
end

Layer Interaction:
TCP/IP Model
Host Router Router Host
Application
TCP or UDP
IP
Link
Physical
IP
Link Link
IP
Link Link
Application
TCP or UDP
IP
Link
Physical
Physical
Hop
by
hop
End
to
end
No session or presentation layers in TCP/IP model

Encapsulation & Decapsulation
 Lower layers add headers (and sometimes
trailers) to data from higher layers
Application
Transport
Network
Data Link
Data Link
Network
Data
Transport Layer Data
Header
Network Layer Data
Header
Data
Header
Header
Link Layer Data
Data
Header
Header
Header
Header
Trailer
Trailer

Frame, Datagram, Segment, Packet
 Different names for packets at different layers
– Ethernet (link layer) frame
– IP (network layer) datagram
– TCP (transport layer) segment
 Terminology is not strictly followed
– we often just use the term “packet” at any layer

 Destination and source are 48-bit MAC
addresses
 Type 0x0800 means that the data portion of the
Ethernet frame contains an IP datagram. Type
0x0806 for ARP.
Layer 2 - Ethernet frame
Preamble Dest
6 bytes
Source
6 bytes
Length
2 bytes
Data
46 to 1500
bytes
CRC
4 bytes
Type
2 bytes

 Protocol = 6 means data
portion contains a TCP
segment. Protocol = 17
means UDP.
Layer 3 - IP datagram
IHL Type of Service Total Length
Version
Fragment Offset
Identification Flags
Time to Live Protocol Header Checksum
Source Address
Destination Address
 Version = 4
 If no options, IHL = 5
 Source and Destination
are 32-bit IP addresses
Data
Padding
Options

Source and Destination are 16-bit TCP port numbers (IP
addresses are implied by the IP header)
If no options, Data Offset = 5 (which means 20 octets)
Layer 4 - TCP segment
Source Port Destination Port
Sequence Number
Acknowledgement Number
Data
Offset
Window
Reserved A
C
K
U
R
G
E
O
L
R
S
T
S
Y
N
F
I
N
Checksum Urgent Pointer
Data
Padding
Options

Client Server Architecture
 simple example layer 7 protocol: HTTP
 Client makes requests, Server serves requests – e.g
HTTP for transferring “websites”. This is the easiest
way to provide services on demand and provides a
means of sharing resources more effectively.
 Example: Mimicking the browser with telnet (client)
talking to a web server (server)
– telnet www.google.com 80
– GET / HTTP/1.1
– Host: www.google.com

Purpose of an IP address
 Unique Identification of
– Source
Sometimes used for security or policy-based
filtering of data
– Destination
So the networks know where to send the data
 Network Independent Format
– IP over anything

Purpose of an IP Address
 Identifies a machine’s connection to a network
 Physically moving a machine from one network
to another requires changing the IP address
 TCP/IP uses unique 32-bit addresses

Basic Structure of an IP Address
133 27 162 125
10000101 00011011 10100010 01111101
85 1B A2 7D
 32 bit number (4 octet number):
(e.g. 133.27.162.125)
 Decimal Representation:
 Binary Representation:
 Hexadecimal Representation:

IP Address Allocation
 Private IP address ranges:
– 10/8 (10.0.0.0 – 10.255.255.255)
– 192.168/16 (192.168.0.0 – 192.168.255.255)
– 172.16/12 (172.16.0.0 – 172.31.255.255)
 Public IP address space
– Assigned by an appropriate authority such as RIPE, ARIN, AFRINIC,
etc. or Local Internet Registries (LIRs)
– Public Address space for the Africa Region available from AfriNIC
 Choose a small block from whatever range you have, and
subnet your networks (to avoid problems with broadcasts)

Addressing in Internetworks
 The problem we have
– More than one physical network
– Different Locations
– Larger number of computers
 Need structure in IP addresses
– network part identifies which network in the
internetwork (e.g. the Internet)
– host part identifies host on that network

Address Structure Revisited
 Hierarchical Division in IP Address:
– Network Part (Prefix)
» describes which physical network
– Host Part (Host Address)
» describes which host on that network
– Boundary can be anywhere
» very often NOT at a multiple of 8 bits
Network Host
205 . 154 . 8 1
11001101 10011010 00001000 00000001

Network Masks
 Network Masks help define which bits are used to
describe the Network Part and which for hosts
 Different Representations:
– decimal dot notation: 255.255.224.0
– binary: 11111111 11111111 11100000 00000000
– hexadecimal: 0xFFFFE000
– number of network bits: /19
 Binary AND of 32 bit IP address with 32 bit netmask
yields network part of address

Classless Addressing
 IP address with the subnet mask defines the
range of addresses in the block
– E.g 10.1.1.32/28 (subnet mask 255.255.255.240)
defines the range 10.1.1.32 to 10.1.1.47
– 10.1.1.32 is the network address
– 10.1.1.47 is the broadcast address
– 10.1.1.33 ->46 assignable addresses

Forwarding
 Computers can only send packets directly to other
computers on their subnet
 If the destination computer is not on the same subnet,
packets are sent via a “gateway”
 defaultrouter option in /etc/rc.conf sets the default
gateway for this system.
 IP forwarding on a FreeBSD box
– turned on with the gateway_enable option in /etc/rc.conf
otherwise the box will not forward packets from one
interface to another.

How DNS fits
 Computers use IP Addresses but Humans find
names easier to remember
 DNS provides a mapping of IP Addresses to
names and vice versa
 Computers may be moved between networks,
in which case their IP address will change BUT
their names can remain the same

Network Troubleshooting Tools
 ping
 traceroute
 tcpdump

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