CE693: Adv.

Computer Networking

L-1 Intro to Computer Networks

Fall 1391
Acknowledgments: Lecture slides are from the graduate level Computer
Networks course thought by Srinivasan Seshan at CMU. When slides are
obtained from other sources, a a reference will be noted on the bottom of
that slide. A full list of references is provided on the last slide.
Outline

• Administrivia

• Layering

2
Objectives
• Understand the state-of-the-art in network
protocols, architectures and applications
• Understand how networking research is
done
• Teach the typical constraints and thought
processes used in networking research
• How is class different from undergraduate
networking (CE-443)
• Training network programmers vs. training
network researchers

4
Class Info
• Check under:
• sharif.edu/~kharrazi/
• kharrazi@sharif.edu
• Check class webpage regularly
• Course schedule
• Reading list
• Lecture notes
• Announcements
• Assignments
• Subscribe to class mailing list
5
Course Materials
• Research papers
• Links to ps or pdf on Web page
• Combination of classic and recent work
• ~40 papers
• Optional readings
• Recommended textbooks
• In you want to review background material
• Peterson & Davie or Kurose & Ross

7
Grading (Tentative)
• Reading/Participation 20%
• Assignments 45%
• Midterm ??
• Final 35%
• At most 3 students/groups will be allowed
to undertake a class project, pending
approval by me.

8
Class Coverage
• Little coverage of physical and data link
layer
• Little coverage of undergraduate material
• Students expected to know this
• Focus on network to application layer
• We will deal with:
• Protocol rules and algorithms
• Investigate protocol trade-offs
• Why this way and not another?

10
Lecture Topics

Traditional Recent Topics

• Layering • Multicast
• Internet architecture • Mobility/wireless
• Routing (IP) • Active networks
• Transport (TCP) • QoS
• Queue management • Security
(FQ, RED) • Network measurement
• Naming (DNS) • Overlay networks
• P2P applications

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Outline

• Administrivia

• Layering

12
This/Next Lecture: Design
Considerations
• How to determine split of functionality
• Across protocol layers
• Across network nodes
• Assigned Reading
• [SRC84] End-to-end Arguments in System
Design
• [Cla88] Design Philosophy of the DARPA
Internet Protocols
• Optional Reading
• [Cla02] Tussle in Cyberspace: Defining
Tomorrow’s Internet

39
What is the Objective of Networking?
• Communication between applications on
different computers
• Must understand application needs/
demands
• Traffic data rate
• Traffic pattern (bursty or constant bit rate)
• Traffic target (multipoint or single destination,
mobile or fixed)
• Delay sensitivity
• Loss sensitivity

13
Back in the Old Days…

14
Packet Switching (Internet)

Packets

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Packet Switching
• Interleave packets from different sources
• Efficient: resources used on demand
• Statistical multiplexing
• General
• Multiple types of applications
• Accommodates bursty traffic
• Addition of queues

16
Characteristics of Packet Switching
• Store and forward
• Packets are self contained units
• Can use alternate paths – reordering
• Contention
• Congestion
• Delay

17
internet[work]
• A collection of Internet[work]
interconnected
networks
• Host: network
endpoints (computer,
PDA, light switch, …)
• Router: node that
connects networks
• Internet vs. internet

18
Challenge
• Many differences between networks
• Address formats
• Performance – bandwidth/latency
• Packet size
• Loss rate/pattern/handling
• Routing
• How to translate between various network
technologies?

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internet[work]
• A collection of Internet[work]
interconnected
networks
• Host: network
endpoints (computer,
PDA, light switch, …)
• Router: node that
connects networks
• Internet vs. internet

18
How To Find Nodes?

Internet

Computer 1 Computer 2

Need naming and routing

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Naming

What’s the IP address for www.cmu.edu?

It is 128.2.11.43

Computer 1 Local DNS Server

Translates human readable names to logical endpoints

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Routing

Routers send
packet towards
destination H R

H
R
H R

R
H
R
R H: Hosts
H R: Routers

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Meeting Application Demands
• Reliability
• Corruption
• Lost packets
• Flow and congestion control
• Fragmentation
• In-order delivery
• Etc…

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What if the Data gets Corrupted?

Problem: Data Corruption

GET index.html GET windex.html

Internet

Solution: Add a checksum

0,9 9 6,7,8 21
X
4,5 7 1,2,3 6

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What if Network is Overloaded?

Problem: Network Overload

Solution: Buffering and Congestion Control

• Short bursts: buffer
• What if buffer overflows?
• Packets dropped
• Sender adjusts rate until load = resources  “congestion control”

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What if the Data gets Lost?

Problem: Lost Data

GET index.html
Internet

Solution: Timeout and Retransmit

GET index.html
GET index.html
Internet
GET index.html

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What if the Data Doesn’t Fit?

Problem: Packet size

• On Ethernet, max IP packet is 1.5kbytes

• Typical web page is 10kbytes

Solution: Fragment data across packets

ml x.ht inde GET

GET index.html

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What if the Data is Out of Order?

Problem: Out of Order

ml inde x.ht GET

GET x.htindeml

Solution: Add Sequence Numbers

ml 4 inde 2 x.ht 3 GET 1

GET index.html

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Lots of Functions Needed
• Link
• Multiplexing
• Routing
• Addressing/naming (locating peers)
• Reliability
• Flow control
• Fragmentation
• Etc….

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What is Layering?
• Modular approach to network functionality
• Example:

Application

Application-to-application channels

Host-to-host connectivity

Link hardware

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Protocols
• Module in layered structure
• Set of rules governing communication
between network elements (applications,
hosts, routers)
• Protocols define:
• Interface to higher layers (API)
• Interface to peer
• Format and order of messages
• Actions taken on receipt of a message

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Layering Characteristics
• Each layer relies on services from layer
below and exports services to layer above
• Interface defines interaction
• Hides implementation - layers can change
without disturbing other layers (black box)

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Layering

User A User B

Application

Transport

Network

Link

Host Host

Layering: technique to simplify complex systems

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E.g.: OSI Model: 7 Protocol Layers

• Physical: how to transmit bits

• Data link: how to transmit frames
• Network: how to route packets
• Transport: how to send packets end2end
• Session: how to tie flows together
• Presentation: byte ordering, security
• Application: everything else

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OSI Layers and Locations

Application

Presentation

Session

Transport

Network

Data Link

Physical

Host Switch Router Host

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Is Layering Harmful?
• Sometimes..
• Layer N may duplicate lower level functionality
(e.g., error recovery)
• Layers may need same info (timestamp, MTU)
• Strict adherence to layering may hurt
performance

38
Next Lecture: Design Considerations
• How to determine split of functionality
• Across protocol layers
• Across network nodes
• Assigned Reading
• [SRC84] End-to-end Arguments in System
Design
• [Cla88] Design Philosophy of the DARPA
Internet Protocols
• Optional Reading
• [Cla02] Tussle in Cyberspace: Defining
Tomorrow’s Internet

39