EE 122: Lecture 25 (Review)

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EE 122 Lecture 25(Review)

• Ion Stoica
• December 6, 2001

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Overview

• A taxonomy of communication networks
• Router Architecture in Packet-Switching Networks
• Layering
• End-to-end argument

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A 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, Wavelan)
• Problem coordinate the access of all nodes to
  the shared communication medium (Multiple Access
  Problem)
• Switched communication networks
• Information is transmitted to a sub-set of
  designated nodes
• Examples WANs (Telephony Network, Internet)
• Problem how to forward information to intended
  node(s)
• this is done by special nodes (e.g., routers,
  switches) running routing protocols

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A 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
• Example telephone networks

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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

• A 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 divided in
  slots
• Relative slot position inside a frame determines
  which conversation the data belongs to
• E.g., slot 0 belongs to red conversation
• Need synchronization between sender and receiver
• In case of non-permanent conversations
• Need to dynamic 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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A 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 are sent as formatted bit-sequences,
  so-called packets.
• Packets have the following structure
• Header and Trailer carry control information
  (e.g., destination address, check sum)
• Each packet is passed through 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

• A 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
• A 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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A 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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A 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 Datagram 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 Ability to exploit statistical
  multiplexing
• Efficient bandwidth usage ratio between peek and
  average rate is 31 for audio, and 151 for data
  traffic
• However, packet-switching needs to deal with
  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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Final Exam

• Friday, December 14, 8am-11am
• Topics all lectures
• Format identical to the midterms
• Conceptual questions
• Problems
• Close books, no calculators

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Overview

• A taxonomy of communication networks
• Router Architecture in Packet-Switching Networks
• Layering
• End-to-end argument

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Router Architecture in Packet Switching Networks

• Set of input and output interfaces interconnected
  by a high speed fabric

fabric
input interface
output interface
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Data and Control Planes

• Data Plane all operations performed by a router
  on a packet as the packet propagates to its
  destination
• forwarding, buffer management, scheduling
• Control Plane all operation required to set and
  maintain state in a router state required to
  process packets on the data path
• routing protocols, signaling

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Typical Functions Performed by Input Interface on
Data Path

• Packet forwarding decide to which output
  interface to forward each packet based on the
  information in packet header

128.16.120.xxx 1
12.82.xxx.xxx 2

1
2
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Typical Functions Performed by Output Interface

• Buffer management decide when and which packet
  to drop
• Scheduler decide when and which packet to
  transmit

Buffer
Scheduler
1
2
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Typical Functions Performed by Output Interface

• Packet classification map each packet to a
  predefined flow
• use to implement more sophisticated services
  (e.g., QoS)
• Flow a subset of packets between any two
  endpoints in the network

flow 1
flow 2
Classifier
Scheduler
1
2
flow n
Buffer management
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Overview

• A taxonomy of communication networks
• Router Architecture in Packet-Switching Networks
• Layering
• End-to-end argument

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What is Layering?

• A technique to organize a network system into a
  succession of logically distinct entities, such
  that the service provided by one entity is solely
  based on the service provided by the previous
  (lower level) entity

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Why Layering?
(FTP File Transfer Protocol, NFS Network File
Transfer, HTTP World Wide Web protocol)
FTP
NFS
Telnet
Application
Coaxial cable
Fiber optic
Transmission Media

• No layering each new application has to be
  re-implemented for every network technology!

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Why Layering?

• Solution introduce an intermediate layer that
  provides a unique abstraction for various network
  technologies

FTP
NFS
Telnet
Application
Intermediate layer
Coaxial cable
Fiber optic
Transmission Media
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Layering

• Advantages
• Modularity protocols easier to manage and
  maintain
• Abstract functionality lower layers can be
  changed without affecting the upper layers
• Reuse upper layers can reuse the functionality
  provided by lower layers
• Disadvantages
• Information hiding inefficient implementations

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ISO OSI Reference Model

• Seven layers
• Lower three layers are peer-to-peer
• Next four layers are end-to-end

Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Network
Datalink
Datalink
Datalink
Physical
Physical
Physical
Physical medium
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OSI vs. TCP/IP

• OSI conceptually define services, interfaces,
  protocols
• Internet provide a successful implementation

Application
Application
Presentation
Session
Transport
Transport
Network
Internet
Datalink
Host-to- network
Physical
OSI
TCP
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Overview

• A taxonomy of communication networks
• Router Architecture in Packet-Switching Networks
• Layering
• End-to-end argument

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End-to-End Argument

• Think twice before implementing a functionality
  that you believe that is useful to an application
  at a lower layer
• If the application can implement a functionality
  correctly, implement it a lower layer only as a
  performance enhancement

J. H. Saltzer, D. P. Reed, D. D. Clark,
End-to-End Arguments in System Design, ACM
Transactions on Computer Systems, Vol. 2, No. 4,
1984, pp. 277-288
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Example Reliable File Transfer
Host A
Host B
Appl.
Appl.
OS
OS

• Solution 1 make each step reliable, and then
  concatenate them
• Solution 2 end-to-end check and retry

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Discussion

• Solution 1 not complete
• What happens if the sender or/and receiver
  misbehave?
• The receiver has to do the check anyway!
• Thus, full functionality can be entirely
  implemented at application layer no need for
  reliability from lower layers
• Is there any need to implement reliability at
  lower layers?

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Discussion

• Yes, but only to improve performance
• Example
• assume a high error rate on communication network
• then, a reliable communication service at
  datalink layer might help

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Trade-offs

• Application has more information about the data
  and the semantic of the service it requires
  (e.g., can check only at the end of each data
  unit)
• A lower layer has more information about
  constraints in data transmission (e.g., packet
  size, error rate)
• Note these trade-offs are a direct result of
  layering!

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Rule of Thumb

• Implementing a functionality at a lower level
  should have minimum performance impact on the
  application that do not use the functionality

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Internet End-to-End Argument

• At network layer provides one simple service
  best effort datagram (packet) delivery
• Only one higher level service implemented at
  transport layer reliable data delivery (TCP)
• performance enhancement used by a large variety
  of applications (Telnet, FTP, HTTP)
• does not impact other applications (can use UDP)
• Everything else implemented at application level

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Key Advantages

• The service can be implemented by a large variety
  of network technologies
• Does not require routers to maintain any fined
  grained state about traffic. Thus, network
  architecture is
• Robust
• Scalable