EECS 122:
Introduction to Computer Networks
Network Service and Applications
Computer Science Division
Department of Electrical Engineering and Computer Sciences
University of California, Berkeley
Berkeley, CA 94720-1776
Katz, Stoica F04
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Overview
•
•
Taxonomy of Communication Networks
Services and Applications
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Taxonomy of Communication
Networks

Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Switched
Communication
Network
Circuit-Switched
Communication
Network
Broadcast
Communication
Network
Packet-Switched
Communication
Network
Datagram
Network
Virtual Circuit Network
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Broadcast vs. Switched
Communication Networks

Broadcast Communication Networks
- Information transmitted by any node is received by every other
node in the network
• Examples: usually in LANs (Ethernet, WiFi)
- Problem: coordinate the access of all nodes to the shared
communication medium (Multiple Access Problem)

Switched Communication Networks
- Information transmitted to a sub-set of designated nodes
• Examples: WANs (Telephony Network, Internet)
- Problem: how to forward information to intended node(s)?
• Done by special nodes (e.g., routers, switches) executing
routing protocols
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Taxonomy of Communication
Networks

Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Switched
Communication
Network
Circuit-Switched
Communication
Network
Broadcast
Communication
Network
Packet-Switched
Communication
Network
Datagram
Network
Virtual Circuit Network
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Circuit Switching

Three phases
1. circuit establishment
2. data transfer
3. circuit termination


If circuit not available: “Busy signal”
Examples
-
Telephone networks
ISDN (Integrated Services Digital Networks)
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Telegraph Network

Alexander Graham Bell
- 1876: Demonstrates the telephone at US Centenary
Exhibition in Philadelphia
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Telephone Network

Almon Brown Strowger (1839 - 1902)
- 1889: Invents the “girl-less, cuss-less” telephone
system, aka the mechanical switching system
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Timing in Circuit Switching
Host 1
Node 1
Node 2
Host 2
processing delay at Node 1
propagation delay
between Host 1
and Node 1
Circuit
Establishment
propagation delay
between Host 2
and Node 1
Data
Transmission
DATA
Circuit
Termination
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Circuit Switching

Node (switch) in a circuit switching network
incoming links
Node
outgoing links
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Circuit Switching:
Multiplexing/Demultiplexing
Frames
Slots = 0 1 2 3 4 5


0 1 2 3 4 5
Time divided in frames and frames into slots
Relative slot position inside a frame determines to which
conversation data belongs
- E.g., slot 0 belongs to red conversation


Requires synchronization between sender and receiver—
surprisingly difficult!
In case of non-permanent conversations
- Needs to dynamically bind a slot to a conservation
- How to do this?

If a conversation does not use its circuit the capacity is lost!
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Taxonomy of Communication
Networks

Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Switched
Communication
Network
Circuit-Switched
Communication
Network
Broadcast
Communication
Network
Packet-Switched
Communication
Network
Datagram
Network
Virtual Circuit Network
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Packet Switching


Data sent as formatted bit-sequences (Packets)
Packets have following structure:
Header
Data
Trailer
• Header and Trailer carry control information
(e.g., destination address, check sum)



Each packet traverses the network from node to node
along some path (Routing)
At each node the entire packet is received, stored briefly,
and then forwarded to the next node (Store-and-Forward
Networks)
Typically no capacity is allocated for packets
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Packet Switching

Node in a packet switching network
incoming links
Node
outgoing links
Memory
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Packet Switching:
Multiplexing/Demultiplexing

Data from any conversation can be transmitted at
any given time
- Single conversation can use the entire link capacity if it
is alone

How to tell them apart?
- Use meta-data (header) to describe data
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Taxonomy of Communication
Networks

Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Switched
Communication
Network
Circuit-Switched
Communication
Network
Broadcast
Communication
Network
Packet-Switched
Communication
Network
Datagram
Network
Virtual Circuit Network
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Datagram Packet Switching

Each packet is independently switched
- Each packet header contains destination address


No resources are pre-allocated (reserved) in
advance
Example: IP networks
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Timing of Datagram Packet
Switching
Host 1
transmission
time of Packet 1
at Host 1
Node 1
Packet 1
Host 2
Node 2
propagation
delay between
Host 1 and
Node 2
Packet 2
Packet 1
Packet 3
processing
delay of
Packet 1 at
Node 2
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
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Datagram Packet Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Node 6
Node 7
Host E
Node 4
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Taxonomy of Communication
Networks

Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Switched
Communication
Network
Circuit-Switched
Communication
Network
Broadcast
Communication
Network
Packet-Switched
Communication
Network
Datagram
Network
Virtual Circuit Network
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Virtual-Circuit Packet Switching

Hybrid of circuit switching and packet switching
- Data is transmitted as packets
- All packets from one packet stream are sent along a
pre-established path (=virtual circuit)



Guarantees in-sequence delivery of packets
However, packets from different virtual circuits
may be interleaved
Example: ATM networks
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Virtual-Circuit Packet Switching

Communication with virtual circuits takes place
in three phases
1. VC establishment
2. data transfer
3. VC disconnect

Note: packet headers don’t need to contain the
full destination address of the packet
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Timing of Virtual-Circuit Packet
Switching
Host 1
Node 1
Host 2
Node 2
propagation delay
between Host 1
and Node 1
VC
establishment
Packet 1
Packet 2
Packet 1
Data
transfer
Packet 3
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
VC
termination
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Datagram Packet Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Node 6
Node 7
Host E
Node 4
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Packet-Switching vs. Circuit-Switching

Most important advantage of packet-switching
over circuit switching: Exploitation of statistical
multiplexing:
- Efficient bandwidth usage; ratio between peek and
average rate is 3:1 for audio, and 15:1 for data traffic

However, packet-switching must handle
congestion:
- More complex routers
- Harder to provide good network services (e.g., delay
and bandwidth guarantees)

In practice they are combined:
- IP over SONET, IP over Frame Relay
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Overview
•

Taxonomy of Communication Networks
Services and Applications
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The Internet Protocol (IP)

Problem:
- Many different network technologies
- e.g., Ethernet, Token Ring, ATM, Frame Relay, etc.
- How can you hook them together?
• n x n translations?

IP was invented to glue them together
- n translations
- Minimal requirements (datagram)

The Internet is founded on IP
- “IP over everything”
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Addressing

Every Internet host has an IP address
- e.g., 67.114.133.15

Packets include destination address
- Network is responsible for routing packet to address

Host-view:
Network
Sourc
e
Destination
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IP-centric View
Host C
Host D
Host A
Router 1
Router 2
Router 3
Router 5
Host B
Router 6
Router 7
Host E
Router 4
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Physical View

A big mess!

Every “link” could be a whole network of ATM,
frame relay, ethernet, DSL, etc.

Beauty of IP: you can ignore these different
network technologies

In many networks, IP is used only at the edge
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Back to IP
Host C
Host D
Host A
Router 1
Router 2
Router 3
Router 5
Host B
Router 6
Router 7
Host E
Router 4
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Routing

Routers have “routing tables”
- Tables mapping each destination with an outgoing link
- Requires that routing table is highly compressible!
- Implications for address assignment, mobility, etc.

Routing decisions made packet-by-packet
- Routers keep no connection state

Question: Why have the network do routing?
- Why not the hosts?
- Compare delivery-by-hand to FedEx
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Internet Service
•
“Best-Effort” service
- No guarantees about packet delivery
- Hosts must cope with loss and delay

Why this service model?
- Why not guarantee no loss and low delay?
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Domain Name Service (DNS)

Humans/applications use machine names
- e.g., www.cs.berkeley.edu

Network (IP) uses IP addresses
- e.g., 67.114.112.23

DNS translates between the two
- An overlay service in its own right
- Global distribution of name-to-IP address mappings—a
kind of content distribution system as well
- Unsung hero of the Internet
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File Transfer (FTP, SCP, etc.)
Get file from soup.cs.berkeley.edu
Get file
Your PC
file
soup.cs.berkeley.edu
Get address for
soup.cs.berkeley.edu
67.132.22.5
DNS
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Email
Email message exchange is similar to previous
example, except
 Exchange is between mail servers
 DNS gives name of mail server for domain
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Web
Get www.icir.org/file.html
Your PC
Proxy
Get file.html
file.html
www.icir.org
Get address for
www.icir.org
67.132.22.5
DNS
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Caching

Caches can be visible or transparent

Visible:
- Client is configured to ask cache

Transparent:
- Cache intercepts packet on its way to web server
- Example of a “application-aware middlebox”
- Violates purity of architecture, but are prevalent...
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Content Distribution Network (CDN)
How to get closest copy of replicated content?

CDNs have mirror servers distributed globally

CDN customers allow CDN to run their DNS

“Smart” DNS server returns results based on
requester’s IP address
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Gnutella (P2P)
Overlay Network
Dashed lines are “virtual links”
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Gnutella (cont’d)

User asks for file (by metadata)

Each host sends request to its “neighbors” in
overlay network

Responses sent back to original requesting node

Many variations on P2P file sharing.....
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Overlay Networks

Create a set of “virtual links” between hosts

Communication between neighbors on overlay is
done by IP

But the overlay can use different routing, or
application-specific processing, at overlays nodes

IP is often overlay on circuit-switched network
- App-specific networks increasingly overlaid on IP
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Architecture

The assignment of tasks and knowledge

Who does what, and where is the state kept?

How they do it: algorithms and implementation
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Internet Architecture

Routers do routing, and almost nothing else
- No application-specific functions

Hosts do all application-specific processing

Allowed wide variety of applications to flourish on
Internet
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