EECS 122: Introduction to Computer Networks Network Service and Applications

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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

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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
SwitchedCommunication Network
BroadcastCommunication Network
Packet-SwitchedCommunication Network
Circuit-SwitchedCommunication Network
Virtual Circuit Network
Datagram 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
SwitchedCommunication Network
BroadcastCommunication Network
Packet-SwitchedCommunication Network
Circuit-SwitchedCommunication Network
Virtual Circuit Network
Datagram Network
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Circuit Switching

• Three phases
• circuit establishment
• data transfer
• 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
Host 2
Node 1
Node 2
DATA
processing delay at Node 1
propagation delay between Host 1 and Node 1
propagation delay between Host 2 and Node 1
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Circuit Switching

• Node (switch) in a circuit switching network

Node
incoming links
outgoing links
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Circuit Switching Multiplexing/Demultiplexing
Frames
0
1
2
3
4
5
0
1
2
3
4
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Slots

• 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
  receiversurprisingly 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
SwitchedCommunication Network
BroadcastCommunication Network
Packet-SwitchedCommunication Network
Circuit-SwitchedCommunication Network
Virtual Circuit Network
Datagram Network
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Packet Switching

• Data sent as formatted bit-sequences (Packets)
• Packets have following structure
• 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

Header
Data
Trailer
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Packet Switching

• Node in a packet switching network

Node
incoming links
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
SwitchedCommunication Network
BroadcastCommunication Network
Packet-SwitchedCommunication Network
Circuit-SwitchedCommunication Network
Virtual Circuit Network
Datagram 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
Host 2
Node 1
Node 2
propagation delay between Host 1 and Node 2
transmission time of Packet 1 at Host 1

processing delay of Packet 1 at Node 2
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Datagram Packet Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Host E
Node 7
Node 6
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
SwitchedCommunication Network
BroadcastCommunication Network
Packet-SwitchedCommunication Network
Circuit-SwitchedCommunication Network
Virtual Circuit Network
Datagram 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
• VC establishment
• data transfer
• VC disconnect
• Note packet headers dont need to contain the
  full destination address of the packet

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Timing of Virtual-Circuit Packet Switching
Host 1
Host 2
Node 1
Node 2
propagation delay between Host 1 and Node 1
VC establishment
Data transfer
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
Host E
Node 7
Node 6
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 31 for audio, and 151 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
Destination
Source
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IP-centric View
Host C
Host D
Host A
Router 1
Router 2
Router 3
Router 5
Host B
Host E
Router 7
Router 6
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
Host E
Router 7
Router 6
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
  mappingsa 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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Question

• Why isnt the network in this picture?
• Network just delivers (or not) packets to their
  destination
• It plays no other role in application
• Obvious concept now, but novel at the time
• Compare to phone switches
• Makes it both harder and easier for applications
• Hosts more complex, applications less efficient
• Long-term flexibility

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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
Proxy
Your PC
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
  requesters IP address

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Gnutella (P2P)
Overlay Network
Dashed lines are virtual links
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Gnutella (contd)

• 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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Different Application Requirements

• Delay
• Interactive audio/video low per-packet delay
• Large file transfer per-packet delay unimportant
• Bandwidth
• High-quality video needs lots of bandwidth
• Low-quality audio needs little
• Loss
• Applications respond to loss very differently

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Requirements for File Transfer

• Size of transfers differ wildly
• Vast majority of transfers are small
• But most bytes are in the large transfers
• Mice vs elephants
• Small transfers
• Care about per-packet delay, not about bandwidth
• Large transfers
• Care about bandwidth, not per-packet delay

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Host-centric vs Data-centric

• Most early Internet applications were
  host-centric
• FTP file exchange between hosts
• Email exchange between email servers
• Audio/Video stream between hosts
• Users knew the host they wanted to contact, and
  used DNS to figure out the IP address of the
  relevant host.
• Some modern applications are data-centric
• Web I want www.cnn.com, but I dont care which
  host it comes from
• Gnutella I know the song title, but not who has
  it
• What is the right data-centric architecture?