This document summarizes a lecture on computer networks from IIT Kanpur in 2006. It introduces concepts like layering, encapsulation, and end-to-end vs hop-by-hop communication in networking. Layering structures networks into logical layers that isolate functions and allow for modularity. Encapsulation involves each layer adding a header to packets. Communication can be end-to-end between applications or hop-by-hop between layers at each network node. These concepts provide the foundation for how networks are designed and operate.

1. CS425: Computer Networks
Bhaskaran Raman
[braman] AT [cse.iitk.ac.in]
http://www.cse.iitk.ac.in/users/braman/cs425/
Department of CSE, IIT Kanpur
Jul-Dec 2006
Lecture 02: Fri, 04 Aug 2006

2. Updates/Announcements
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Course web-page is up
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Lecture times (CS101):
– Mon 11-12, Fri 5-6:30
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Tutorial times (CS101):
– Tue 4:30-6, Wed 11-12:30, Thu 5:30-7
– May be reduced to one hour
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Will have to move to LHC if needed

3. Recap and Lecture Outline
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Recap:
– What is a network?
– What is a protocol?
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Lecture outline:
– What are the required/desirable functionalities?
– How to structure them?
– Concepts of layering, encapsulation
– Physical layer encoding (Kameswari Chebrolu)

4. Required/Desired Functionalities
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Modulation, ●
Multiplexing,
demodulation demultiplexing
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Encoding ●
Quality-of-Service
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Framing ●
Security
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Detect, correct errors ●
Compression
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Medium Access Control ●
Naming, addressing
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Routing ●
Application
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Reliability
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In-order delivery

5. OSI Layering
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What is layering? Application
– “ Structuring technique Presentation
which permits the
network... to be viewed Session
as logically composed of Transport
a succession of layers,
each wrapping the lower Network
layers and isolating them Link-Layer-Ctrl
from higher layers” Medium-Access
[Zim80] Physical

6. Advantages of Layering
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Handle heterogeneity
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Software reuse, modularity
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Allows extensibility, new technologies

7. Inter-Layer Communication
Layer N+1 Layer N+1
Downcall Upcall
Layer N Layer N
Layer N-1 Layer N-1
PHY
Host-1 Host-2
Interface definition between layers is crucial

8. Encapsulation & Decapsulation
Layer N+1 Layer N+1
Layer N Layer N
Layer N-1 Layer N-1
PHY
Host-1 Host-2
Each layer adds/removes its header

9. Logical Communication between
Layers at the Same Level
Application Application
Logical
Transport Transport
Logical
Network Network
Logical
LLC/MAC LLC/MAC
Logical
PHY PHY
PHY
Host-1 Host-2

10. End-to-end versus Hop-by-hop
Application Application
Logical
Transport Transport
Logical
Network Network Network Network
Logical Logical Logical
LLC/MAC LLC/MAC LLC/MAC LLC/MAC
Logical Logical Logical
PHY PHY PHY PHY
PHY PHY PHY
End-Host-1 Intermediate- Intermediate- End-Host-2
Host-1 Host-2

11. Summary of Concepts
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Layering
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Inter-layer communication
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Encapsulation
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End-to-end versus hop-by-hop communication
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Concepts not covered from Chapter-1 of
Peterson book:
– Packet switching
– Sockets
– Protocol graph
– Bandwidth-Delay Product (BDP)