Computer
Networks
The Internet Design Goals
 Internet over anything and anything on top of the
Internet
 Enable creativity on “top” of network layer and “below” the
network layer
Internet Architecture
Access Networks
Summer 2013
History of the Internet
 Enable connectivity between heterogeneous
networks
 Robust in face of failures
Barring complete network partition, failures should not
interfere with end-to-end communication
 Support multiple types of services and applications
 Evolvable as technology changes and new applications are
created
Internet Evolution
 70’s: A research project started at DARPA: 56 kbps and less
than 100 computers
 80-83: Split of ARPANET and MILNET
 85-86: NSF builds NSFNET as backbone to link six
Supercomputer centers: 1.5 Mbps, 10,000 computers
 87-90: Emergence of regional networks, NSI (NASA),
ESNet(DoE), DARTnet, TWBNet (DARPA): 100,000
computers
 90-92: NSFNET moves to 45 Mbps and 16 mid-level networks
 94: NSF backbone dismantled, multiple private backbones
 07: Backbones run at more than 10 Gbps data rate, supporting
more than 300 millions computers in more than 150 countries
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Internet Hierarchical
Structure
The Internet is roughly a multi-tiered hierarchy
Tier-1 ISP: e.g., Sprint
Sprint US backbone network
Tier-1 ISPs are at the center of the hierarchy
UUNet, Sprint, AT&T
National and International Coverage
Tier-1 service providers treat each other as equals
Tier-1
peer
privately
Tier 1 ISP
NAP
Tier 1 ISP
Tier-1 ISPS also
interconnect at
public Network
Access Points
(NAPs)
Tier 1 ISP
Network Access Point
Internet Tier-2 ISP Structure
Tier-2 ISPs, mostly regional ISPs, form the second
level of the hierarchy
Connect to one or more Tier-1 ISPs for full access to the
Internet
May possibly connect to other Tier-2 ISPs
Tier-1 ISPs
charge Tier-2
ISPs for
connectivity to
rest of the
Internet
Tier-2 ISP
Tier-2 ISP
Tier 1 ISP
NAP
Tier 1 ISP
Tier-2 ISP
Tier 1 ISP
Tier-2 ISPs
also peer
privately with
each other,
interconnect
at NAP
Tier-2 ISP
Tier-2 ISP
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Internet Tier-3 ISP Structure
 Tier-3 ISPs, mostly local ISPs, form the third level of the
hierarchy and the last hop to end systems
Local
ISP
Tier-3 ISPs
connect to
higher level
hierarchies
to connect to
the rest of
the Internet
Local
ISP
Local
ISP
Tier-2 ISP
Tier1 ISP
Tier1 ISP
Tier-2 ISP
Local
Local
ISP
ISP
Local
Local
ISP
ISP
Tier-2 ISP
Internet Architecture and Protocols
DESIGN PRINCIPLES
NAP
Tier1 ISP
Tier-2 ISP
Tier-2 ISP
Local
ISP
Local
ISP
The Internet Hours Glass
Architecture
Internet Design Principle
End-to-End Argument
The IP network layer provides a simple, best
effort, datagram service
HTTP TFTP
FTP
NV
May not be reliable
TCP
 Reliable data delivery is provided by TCP, when required,
for example for HTTP, Telnet, FTP, etc
Support for reliable data deliver does not impact other
applications
UDP is another protocol of choice
Applications
UDP TCP
UDP
Hour Glass Model
IP
Data Link
NET1
NET2
NET2
NETn
Physical
Everything else is implemented at application
level
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Internet Architecture
Key Advantages
The service can be implemented by a large variety of
network technologies
Application
Internet over anything
Does not require routers to maintain any fined grained
state about traffic
Internet
Protocol
Robust and scalable network architecture
Network
A wide range of applications can be supported
Telnet, FTP, Web, Pervasive and Mobile Applications
Decentralized network administration
Physical
TCP Protocol Architecture
OSI Model
Internet Model
Internet Protocol Stack
Protocols
Application Layer
Presentation Layer
Application Layer
HTTP
FTP
SMTP
Others
IMAP
Application Layer
Session Layer
TCP, UDP
Transport Layer
Transport Layer
Network Layer
Network Layer
TCP
UDP
IP
IP
ICMP
Link
Data Link Layer
Physical Layer
Application
Network Access
Layer
Ethernet
PPP
Wireless
Application Protocol
Application
TCP, UDP Protocol
TCP, UDP
IP Protocol
Data Link
IP Protocol
IP
Link
Link
Data Link
IP
Link
X.25, ATM
Frame Relay
etc.
Transport layer provides ports, logical channels identified by number
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Internet Protocol Encapsulation
Application Layer
Data Packet
Multiplexing and Demultiplexing
 There may be multiple
implementations of each layer.
 How does the receiver know what
version of a layer to use?
Transport Layer
HDR Data Packet
Network Layer
HDR HDR Data Packet
Link Layer
HDR HDR HDR Data Packet TRL
 Each header includes a
demultiplexing field that is used
to identify the next layer.
 Filled in by the sender
 Used by the receiver
 Multiplexing occurs at multiple
layers. E.g., IP, TCP, …
TCP
TCP
IP
IP
V/HL TOS
Length
ID
Flags/Offset
TTL Prot. H. Checksum
Source IP address
Destination IP address
Options..
Home Networks
 Typical home network components:
 ADSL or cable modem
 Router/Firewall
 Ethernet
 Wireless Access Point
Residential and Institutional
INTERNET ACCESS
Cable
Headend
Cable
Modem
Wireless
Laptops
Router/
Firewall
Switched
Ethernet
Wireless
Access
Point
5
DSL Access
Residential Access – Cable
Modems
Hybrid Fiber Coax (HFC)
 Up to 25 Mbps per
customer
 Currently a few Mbps
downstream, and a few
Mbps upstream
Asymmetric Link
Up to 10Mbps upstream and 1 Mbps downstream
The service is available via cable companies
Network of cable and fiber attaches homes
to ISP router
Shared access to router among homes
Potential congestion
DSL Modem
Cable Access
Wireless Access Networks
Shared wireless access
network connects end
system to router
Wireless LANs:
Radio spectrum replaces
wire
Router
Base
Station
Wider-area Wireless
Access
 Fiber node: 500 - 1K homes
 Distribution hub: 20K - 40 K homes
 Regional headend: 200 K - 400 K homes
Cable Access is also called Hybrid
Fiber-coaxial Cable (HFC)
Wireless access to ISP
router via cellular
network
Mobile
Hosts
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Institutional Access – Local
Area Networks
Local Area Network (LAN)
connects end system to edge
router
Ethernet:
Public or
Private
Internet
Internet Physical Infrastructure
Residential access
 Cable  DSL
 Fiber  Wireless
ISP
Shared or dedicated cable
connects end system and
router
10 Mbps, 100Mbps, Gigabit
Ethernet
Campus access,



Ethernet
Wireless
…
Backbone ISP
ISP
 The Internet is a network of networks
 Each individually administrated
network is called an Autonomous
System (AS)
Conclusion
 Internet Objectives and Brief History
 Internet Design Principles
 End-to-End Argument
 Internet Architecture
 Internet Protocol Stack
 Internet Encapsulation
 Internet Access
 DSL, Fiber to the Premise, Hybrid Fiber-Coaxial Cable
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