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Transcript
Chapter 7.B and 7.C
Link Layer & Network Layer
Prof. Dina Katabi
Some slides are from lectures by Nick Mckeown, Ion Stoica, Frans
Kaashoek, Hari Balakrishnan, and Sam Madden
1
Previous Lecture


We learned how to share the network
infrastructure between many
connections/flows
We also learned about the implications
of the sharing scheme (circuit or packet
switching) on the service that the
traffic receives
2
This Lecture
An Example: HTTP
 Layering
 Link Layer

3
Example: HTTP over the Internet
Using TCP/IP and Ethernet
1
2
3
4
App
“A” MIT
“B”(CNN.Com)
OS
Ethernet
20
App
19
18
17
OS
Ethernet
5
R1 6
7 8
9 R2
10
R4
14
R5
15
11 16
R3 12
13
4
This Lecture
An Example: HTTP
 Layering
 Link Layer

15
Network Architecture
Problem
 Networks are complex (heterogeneity,
distributed, delay, losses, reordering, …)
 How do we organize a network implementation?
Solution
 To deal with complexity  use layering
16
Layering



Layering is a particular form of abstraction
The system is broken into a vertical
hierarchy of logically distinct entities
(layers)
The service provided by one layer is based
solely on the service provided by layer below
17
Layering: Our HTTP Example
HTTP
Application
End-to-End
Layer
TCP
Transport
IP
Network
Ethernet
Link
The 4-layer Internet model
18
Where are these layers?


Link and network layers are implemented everywhere
The end-to-end layer (i.e., transport and application)
is implemented only at hosts
Host
Application
Transport
Network
Datalink
Peer-layer communication
Router
Network
Datalink
Physical medium
Host
Application
Transport
Network
Datalink
20
Encapsulation


A layer can use only the service provided by the layer
immediate below it
Each layer may change and add a header to data packet
data
data
data
data
data
data
data
data
21
Interface

Higher layer calls lower layer


e.g., Link_Send(this_data, this_link)
Lower layer uses an up-call function to inform the
higher layer of data arrival

e.g., Network_Handle()
22
Multiplexing in the Internet


Many possible applications, transports, and link layers
But they all use IP at the network layer
Telnet
Application
Transport
Internet
Link
FTP HTTP
TCP
UDP
IP
Ethernet
Packet
radio
23
This Lecture
An Example: HTTP
 Layering
 Link Layer

24
This Lecture


To cope with the complexity, the
network architecture is organized into
layers
The link layer delivers data between two
machines that are directly connected
using a link
29