Transcript Introduction and Layering

Computer Networking
Eliezer Dor (eliezer [email protected])
Teaching Assistant: Allon Wagner
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Course Information
Lectures:
Recitation:
Wed
Mon
12 – 15
10 – 11 / 11 – 12
Orenstein 103
Kaplun 118
Web site:
http://www.cs.tau.ac.il/~allonwag/comnet2012A/index.html
Main Book:
• Kurose-Ross: A Top-down Approach to Computer Networking
Additional Books:
1. Keshav : An Engineering Approach to Computer Networking
2. Tanenbaum : Computer Networks
3. Bertsekas and Gallager : Data Networks
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Practical Information
Homework assignments:
Mandatory
Both theoretical and programming
Grades:
Final Exam:
theory exercises:
Programming exercises:
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60%
20%
20%
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Chapter 1
Introduction
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Chapter 1 Contents
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Course Info
General introduction
Introduction to Layering
The 5 layers of the Internet: basics
What is a protocol
Layer models: more detail
Extra slides
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4-18
19-31
32-36
37-42
43-63
64-83
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Motivation
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1st stage society: Agriculture, handicraft
2nd stage society: Industrial, labor intensive
Today’s society:
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automated industry with sophisticated logistics
information intensive:
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business, knowledge, advertising, news, social
interaction, recreation
Future society is likely to be even more
information-dominated
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The Purpose of the Network
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serves network applications residing in hosts
applications at distinct hosts need to
co-ordinate actions / co-operate
thus they need to communicate information to
each other
network must deliver that information
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to the right host
to the right application process / thread
network serves applications which serve users
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Information
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A representation of knowledge
Examples:
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Can be represented in two ways
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text, music, video, technical specifications
software, instructions, reports, alarms
analog (pictures / ideograms)
digital (bits)
the Digital Revolution
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convert information as pictures to information as bits
networks move around bits instead of pictures
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Challenges
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make order in the jungle of applications
organize information into manageable units
keep track of info units sent/ moving/ received
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take account of errors / misunderstandings etc.
move the bits through the network
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find the destination host in the network jungle
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using an efficient path
learn automatically the current network topology
make efficient use of link / router capacities
resolve competition for use of same resource
Cheaply, Securely, with Quality of Service,
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Internet Physical Infrastructure
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HOST =
Workstation OR Server
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This course’s Challenge
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To discuss this complexity in an organized
way, so that we
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understand the issues / alternatives
can follow/design/troubleshoot processes
Need to divide the job into functional layers
Understand the interrelation between them
These problems are beyond a specific
technology
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Early communications systems
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telegraph, telephone
first used direct point to point links
when number of users grew:
introduced switching points/ configurable circuits
each call had a dedicated circuit for its duration
phone line
Switched connection
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Data Networks
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set of interconnected nodes exchanging information
links are common usage
switching node must:
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choose for each data unit a link bringing it closer to dest.
schedule their transmission on the common usage links
(resolve the competition for the usage of the link)
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Qwest backbone
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http://www.qwest.com/largebusiness/enterprisesolutions/networkMaps/preloader.swf
Networking Tasks – phone net. sol’n
Addressing - Identify the end user

phone number 1-201-222-2673
= country code + region code + exchange + number
Routing – Find route from source to destination.
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determined from phone number by static routing tables
Forwarding – How information is moved
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circuit switching::a fixed circuit along path to destination
Information Units - How information is sent
voice samples; no addressing attached
 samples sent continuously , 8000/sec
 network must prepare source-dest. circuit in advance

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Networking Tasks – Internet Solution
Addressing - Identify the end user
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IP addresses 132.66.48.37, = network number || host #
Routing- Find route from source to destination
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routers learn automatically network topology
build routing tables / updated frequently
Forwarding – How information is moved
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packet switching: move packets 1 by 1 through routers.
Information Units - How information is sent.
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self-descriptive packet = data + header
header contains destination address
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Telephone network supports a high end-to-end
quality of service, but is expensive
Internet supports no quality of service but is
flexible and cheap
Future networks will have to support a wide
range of service qualities at a reasonable cost
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Cerf and Kahn’s internetworking principles:
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autonomy - no internal changes required to
interconnect networks
best effort service model
stateless routers
decentralized control
Defines today’s Internet architecture
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Why Layering
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Communication is a very complex task
What we need is:
communication btw applications at distant hosts
What is reasonably feasible in one piece is:
the ability to transfer a series of bits over a link
We need to bridge between very sophisticated
applications and very primitive physical layer
What is needed is to divide the task’s functionality
into well chosen parts
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each part should be reasonably ‘easy’ to do
they should work well together
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How to do Layering
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Define a conceptual Layering Model
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Set principles for proper usage of the model
Build protocols for each layer
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means: what is the function of each layer
how they cooperate / use each other’s services
protocol is between same layer entities @ distinct nodes
there may be several protocols in each layer providing
different type service for the layer’s function
Define interfaces between layers
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interface (here) is between distinct layer entities at same
node (computing device)
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Layering Principles
Modularity
 each layer works independently of the others
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this means:
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information exchange only according to Interfaces
defined in the Model
analogous to the Object Oriented principle in S/W eng.
don’t change/peek into internal variables of other layers
modularity is bypassed very seldom
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only when there is no other solution to a problem
Transparency
 layering should be invisible to user
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Layering Benefits
Layering enables:
 discussion/understanding of the issues
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efficient development of protocols
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enables clear visualizing of relationships btw. functions
it’s impossible to think about all layers @ once
each layer has a different functional focus
no need to think other layers when designing it
easy replacement/maintenance of protocols
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as long as modularity & interfaces are adhered to
Layering is a good reference model for discussion
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A mail system layering model
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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How do we Communicate?
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Send a mail from Alice to Bob
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Bob
Alice in Champaign, Bob in Hollywood
Example:
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US Postal Service
Alice
Hollywood, California
Champaign, Illinois
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What does Alice do?
Alice
200 Cornfield Rd.
Champaign, IL 61820
Bob
100 Santa Monica Blvd.
Hollywood, CA 90028
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Bob’s address (his mailbox)
Bob’s name – in case people share mailbox
Postage – have to pay!
Alice’s own name and address
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in case Bob wants to return a message
In case the mail has to be returned.
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What does Bob do?
Alice
200 Cornfield Rd.
Champaign, IL 61820
Bob
100 Santa Monica Blvd.
Hollywood, CA 90028
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Install a mailbox
Receive the mail
Get rid of envelope
Read the message
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Layers
User
Post office (P.O)
Ground transfer:
Airport transfer:
Airplane routing
Peer entities
Champaign
Hollywood
give parcel to P.O
pick up parcel at P.O
counter handling
put parcel in mailbox
on truck to airport
from airport to dest. P.O
loading on airplane
take off the airplane
from source to destination
each layer implements a service
 via its own internal-layer actions
 relying on services provided by layer below
Qn: Find scenarios justifying adding extra layers to the
mail model.
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Name the layers and specify their place in model
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What Layers are Necessary?
msg
Application: I received your msg …
I want you to do …
Host A
Router
Transmitter: 1011001…
Physical Layer
2.
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how to
get it
to B ?
Application
NETWORK
1.
how to
make
sense?
Host B
Receiver
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What Layers are Necessary?
msg
Application: I received your msg …
I want you to do …
Host A
Router
Frame
Link Layer
format a frame
Transmitter: 1011001…
Physical Layer
2.
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how to
get it
to B ?
Application
NETWORK
how to
make Host B
sense?
Frame
Link L.
V1.
1011001… Receiver
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What Layers are Necessary?
msg
Application: I received your msg …
too many
I want you to do …
details !!
NETWORK
3.
Net Layer: I can route it!
packet
Link Layer
Frame
1011001…
1011001…
Transmitter:
how to
Physical Layer
get it
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V 2. to B ?
4.
Host B
Router
Host A
Application
too many
pieces!!
packet Net L.
…
Frame Link L.
1011001… Receiver
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What Layers are Necessary?
msg
Application: I received your msg …
I want you to do …
Transport
NETWORK
I’ll look at details!
V
Net Layer
Net Layer
packet
Host A
Link Layer
Frame
packet
Router
…
Transmitter: 1011001…
1011001…
Physical Layer
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V
Application
Transport
I’ll reassemble it!
Net L.
Host B
Frame Link L.
1011001… Receiver
THE FIVE LAYER MODEL
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Application Layer
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Tasks:
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write messages serving needs of application
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L5
proper type of semantics (meaning, information)
appropriate syntax/format, so that semantics is understood
keep track of the interaction process / state machine
Focus: on needs of a specific application type
Data unit: Message
Peer:
the Application Layer at destination host
Uses:
the Transport Layer
Used by: the application itself
Run by: the application
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Transport Layer
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Main Tasks:
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prepare data for transfer
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L4
fragment data into proper size segments / reassemble at dest.
add header which enables delivery to the correct appl. process
optional: error- /flow- /congestion-control
Data Unit: Segment
Focus: on control of End-to-End data transfer
Peer:
the Transport Layer at destination host
Uses:
the Network Layer
Used by: the Application Layer
Run by: the OS of the host
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Network Layer
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Main Tasks:
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L3
learn network topology in real time
prepare routing tables for fast usage in forwarding data
network layer (WAN) addressing
forward data from source to destination
Data Unit: Datagram / “packet”
Focus: on network and data fowarding
Peers: the Network Layer along the whole path
Uses:
the Link Layer
Used by: the Transport Layer
Run by: the OS of the host, the router S/W
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Link Layer
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Main Tasks:
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insert delimiters so start/end of frame can be known
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L2
physical layer may transfer an endless stream of bits
this is part of the task of the Link header and Link trailer
in LAN, access control/ link layer addressing
Data Unit: Frame
Focus: data transfer over a link
Peer:
Link Layer at the other end of the link
Uses:
the Physical Layer
Used by: the Network Layer
Run by: the NIC (Network interface card, ‫)כרטיס רשת‬
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Physical Layer
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Main Tasks:
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L1
transmit signals that encode bits 1 and 0
receive such signals and decode bits from them
synchronize the bit rate clocks of the peer nodes
Data Unit: Bit
Focus: bit transfer over a link
Peer:
the Physical Layer @ other end of the link
Uses:
the raw media: cable/ space
Used by: the Link Layer
Run by: transmitter/ receiver /wave propagation
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Why do we need Protocols
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Communication is between applications or other
S/W entities
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Its objective: enable cooperation on a common task
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Need protocols to understand each other
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Semantics: what I report/ want of you to do
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Syntax/ format: how write/ read this info
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Open/ Proprietary Protocols
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Open protocol can be used by anyone
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it is published by a standards organization
or a public consortium
e.g. draft standard. standard
Proprietary protocol is owned by a company
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may be used subject to company’s agreement
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Why do we need Standards
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Communication happens between entities
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Hosts (personal computers, servers)
Routers
H/W entities produced by different vendors
S/W applications/ OS entities also
Need agreement to ensure correct, efficient and
meaningful communication
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this is called Interworking
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Organizations that Issue Standards
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IETF (Internet Engineering Task Force)
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IEEE (Institute for Electrical and Electronic Engineers)
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ITU (International Telecommunications Union)
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ISO (International Organization for Standardization)
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W3C (World Wide Web Consortium)
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Protocols
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A protocol is a set of rules and formats
that govern the communication
between communicating peers
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set of valid message formats - syntax
meaning of each message - semantics
Necessary for any function that requires
cooperation between peers
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Protocols
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A protocol provides a service
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Peer entities use a protocol to provide a
service to a higher-level peer entity
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For example: the post office “registered” protocol
for reliable parcel transfer service
for example, truck drivers use a protocol to present
post offices with the abstraction of an unreliable
parcel transfer service
In the layering model:
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each layer gives service to next higher layer
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ISO OSI reference model
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Reference model
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Service architecture
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formally defines what is meant by a layer, a service
etc.
describes the services provided by each layer and the
service access point
Protocol architecture
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set of protocols that implement the service
architecture
compliant service architectures may still use noncompliant protocol architectures
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The seven/five Layers
There are only 5 (!!) in most architectures
Application
Presentation
Session
Transport
Network
Data Link
Physical
End system
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Application
Transport
Network
Data Link
Physical
Intermediate
system
Application
Presentation
Session
Transport
Network
Data Link
Physical
End system
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The seven Layers - protocol stack
data
Application
Presentation
Session
AH
PH
Network
Data Link
Physical
Physical
Session
data
SH
Transport
Network
Data Link
TH
data
data
data
NH
data
DH+data+DT
bits
Application
Presentation
Session
Transport
Network
Data Link
Physical
and presentation layers are not so important, and are often ignored
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Postal network
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Application: people using the postal system
Session and presentation: chief clerk sends some
priority mail, and some by regular mail ;
translator translates letters going abroad.
Transport layer: mail clerk sends a message,
retransmits if not acked
Network layer: postal system computes a route and
forwards the letters
Datalink layer: letters loaded on planes, trains, trucks
Physical layer: the driver/pilot carrying letters in sack
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Internet protocol stack
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application: supporting network applications
 ftp, smtp, http
transport: host-host data transfer
 tcp, udp
network: routing of datagrams from source
to destination
 ip, routing protocols
link: data transfer between neighboring
network elements
 ppp, ethernet, WiFi, token ring
physical: bits “on the wire”
Network access
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application
transport
network
link
physical
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source host
message
M
segment Ht M
datagram/ H H
n t M
packet
frame Hl Hn Ht M Tl
application
transport
network
link
physical
Encapsulation
1011………
M – message
Ht – transport
header
Hn – network
header
Hl – link header
Tl – link trailer
destination host
M
Ht
M
Hn Ht
M
Hl’ Hn Ht
M
Tl’
application
transport
network
link
physical
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Hn Ht
Hl’ Hn Ht
M
M
Tl’
network
link
physical
Hn Ht
M
Hl Hn Ht
M
router
1-48
Tl
Protocols and Interfaces
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Service & protocol at layer k
to layer k+1
Service received by layer k from layer k-1
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Packet structure: sending host view
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L5: application layer generates a message and
passes it to transport layer
L4: transport layer adds its header (H4=Ht)
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L3: network layer adds its header (H3=Hn)
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this generates a datagram, which is passed to link layer
L2: link layer adds header (H2=Hl), trailer (T2=Tl)
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this generates a segment which is passed to netwk layer
(one message may be fragmented into several segments)
this generates a frame which is passed to physical layer
L1: physical layer sends the frame as a sequence
of bytes is on link
H2 H3 H4 Message T2 frame
datagram
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segment
1-51
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Packet structure: router view
Receiving stage:
• L1: physical layer receives frame, passes it to L2
• L2: link layer checks H2+T2 and removes them
– this makes a datagram which is passed to network layer
H2 H3
L3 payload
datagram
T2
frame
Sending stage:
L3: network layer decides on which link to send 
transfers datagram to L2

L2: link layer adds new H2+T2 and makes a frame 
frame is passed to L1 which sends its bits on link
H2 H3
datagram
*
L3 payload
T2*

frame
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Packet structure: destination view
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L1: physical layer receives frame from link
L2: link layer recognizes frame boundaries
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L3: network layer checks H3 and removes it
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this generates a segment, passed to transport layer
L4: transport layer checks H4 and removes it
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checks H2+T2 and removes them
this makes a datagram, passed to network layer
this leaves the message which is saved in receive buffer
L5: application layer takes message from buffer
H2 H3 H4 Message T2 frame
datagram
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segment
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Physical layer
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Link Types:
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shapes/sizes/material of connectors and cables/media
coding scheme to represent a bit
bit-level synchronization
Interconnecting Nodes:
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Point to Point (usually continuous transmission)
LAN/multiple access (intermittent transmission)
What is contained in a standard:
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L1
Repeater
Hub (on LAN only)
Located: in transmitter/receiver of NIC )‫(כרטיס רשת‬
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Datalink layer
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Protocol Types
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header & trailer format
indication of start & end frame (delimitation)
in cont. transmission links: filler frame/marker format
in LAN: media access (MAC) rules, addressing rules
Interconnecting Nodes (in LAN only)
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PTP protocols (HDLC, PPP, LAPD)
LAN protocols: MAC prot.’s: Ethernet, Token Ring, WiFI)
 Auxiliary protocols: STP, DHCP, ARP
What is contained in a protocol
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L2
Bridge, (L2-) Switch
Located: in NIC of hosts, routers, switches
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Network layer
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Network Types
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header format, address formats
forwarding rules (how to use routing tables)
What is contained in a routing protocol:
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Circuit-switching / Packet-switching(datagram or VC)
Protocol Types: Routing/Forwarding/Auxiliary(IPCP)
What is contained in a forwarding protocol:
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L3
rules/ messages for learning topology info
rules for building routing tables
Interconnecting Nodes: Router
Metaphor: Gives a Host To Host EndToEnd virtual channel
Located: in Host OS, in Router CPU (part may sit in NIC)
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Network layer: structure L3
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In datagram networks
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In connection-oriented* network
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same network forwards routing msgs & data msgs
separate data plane and control plane
data plane only schedules and forwards data
control plane prepares (virtual) circuits before data
is sent (routing rules required for control msgs)
Internet
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forwarding by IP protocol (a datagram protocol)
 best effort service (no reliability tools)
several routing protocols (RIP, OSFP, BGP)
* i.e Circuit Switching OR Virtual Circuit networks
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Network layer: action
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L3
At end-systems:
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mainly hides details of datalink layer from L4
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segments and reassembles

detects errors
At intermediate systems:
 participates in routing protocol to create routing
tables
 forwards packets (fragments them if needed)
 schedules the transmission order of packets
 chooses which packets to drop
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Transport layer
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Protocol Types:
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L4
Reliable stream protocols (TCP, SCTP, SSL)
Unreliable datagram protocols (UDP)
What is contained a protocol
header format
 user-process multiplexing rules (using port numbers)
in Reliable protocols, has also:
 error control (ack, seq. #s, retransmission, timers)
 flow control (don’t overwhelm destination)
 congestion control (don’t overload network)



Metaphor: Gives a Process to Process ETE channel
Location: Hosts only (located in OS)
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Application layer

Application Types:

User-oriented applications (Web, Mail, File xfer..)





Infrastructure applications (DNS, NTP)
Each specific application type has a separate protocol
What is contained a protocol


Protocols: HTTP, SMTP+POP, FTP
Protocols:


L5
header format
rules for mutual interaction of peer processes
Metaphor: Talks to peer application about common job
Location: Hosts only, run by the application S/W
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Layer Model at a Glance
L
Task
Unit Peer @
By
@
5
Tell peer what
Message
application needs
Destination
Application
Host only
4
ETE control of
data transfer
Segment
Destination
OS
Host only
3
Route data thru
Network
Datagram
All nodes
along path
OS
All nodes
2
Delimit packet;
Link xfer control
Frame
Neighbor
node
NIC
All nodes
1
Transfer bits over
Bit
link
Neighbor
node
NIC
All nodes
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Layer Model Issues & details
L
Types
Ex.
Node
5
1.User oriented
2.Infrastructure
1.http,ftp
2.Dns
Applicat’n
gateway
4
1.Reliable
2.Best effort
1.TCP
2.UDP
3
1.Circuit Sw.
2.Packet Sw.
1.Phone
2.tcp/ip
Router
Rtg algo., address,
forwarding rules
2
1.PTP protocol
2.LAN protocol
1.hdlc, ppp
2.ethernet
Bridge
L2-switch
Delimitation
1.filler, 2.MAC
1
1.PTP link
2.shared link
Repeater
Hub
Cable spec/ bit
coding /clock sync
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Defines
Interaction rules,
message semantic
and format
Appl.proc.multiplex
error/flow/cong ctrl
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Layer Model: Discussion


Studied (basically) the Internet 5 Layer Model
OSI model (defined earlier, by ISO)

Contains 2 more layers:





Duplex ctrl, Data priority, Special session controls
Presentation layer


Application Layer is pushed to Layer 7
Not used in the Internet
Session layer


Layer 5 (Sessiion)
Layer 6 (Presentation)
Data structure standardization, encoding, encryption
see Extra slides for more details
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EXTRA SLIDES
Ch. :1
noitcudortnI
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History
1961-1972: Early packet-switching principles
1961: Kleinrock - queuing theory shows effectiveness of
packet-switching
1964: Baran - packet-switching in military networks
1967: ARPAnet – conceived by Advanced Research
Projects Agency
1969: first ARPAnet node operational
1972: ARPAnet demonstrated publicly
– NCP (Netwk Control Protocol) 1st host-host protocol
– first e-mail program
– ARPAnet has 15 nodes
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History
1972-1980: Internetworking, new and
proprietary nets
1970: ALOHAnet satellite network in Hawaii
1973: Metcalfe’s PhD thesis proposes Ethernet
1974: Cerf and Kahn - architecture for interconnecting
networks
late70’s: proprietary architectures: DECnet, SNA, XNA
late 70’s: switching fixed length packets (ATM precursor)
1979: ARPAnet has 200 nodes
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Cerf and Kahn’s internetworking principles:
– minimalism, autonomy - no internal
changes required to interconnect networks
– best effort service model
– stateless routers
– decentralized control
Defines today’s Internet architecture
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History
1980-1990: new protocols,
proliferation of networks
1983: deployment of TCP/IP
1982: SMTP e-mail protocol defined
1983: DNS defined for name-to-IP-address translation
1985: FTP protocol defined
1988: TCP congestion control
new national networks: CSnet, BITnet, NSFnet, Minitel
100,000 hosts connected to confederation of networks
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History
1990 - : commercialization and WWW
early 1990’s: ARPAnet decommissioned
1991: NSF lifts restrictions on commercial use of NSFnet
(decommissioned, 1995)
early 1990s: WWW
hypertext [Bush 1945, Nelson 1960’s]
HTML, http: Berners-Lee
1994: Mosaic, later Netscape
late 1990’s: commercialization of WWW
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Demand and Supply
• Huge growth in users
– The introduction of the web
• Faster home access
– Better user experience.
• Infrastructure
– Significant portion of telecommunication.
• New evolving industries
– Although, sometimes temporary setbacks
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Internet: Users
Million users
1400
1200
1000
800
600
400
200
0
1995 1997 1999 2001 2003 2005 2007 2009
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year
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Penetration around the Globe (2009)
80
%Population
USA+Canada
%Penetration
70
Australia
60
Asia/Pacific
Europe
50
40
Latin America
30
20
Middle East
Asia/Pacific
Africa
10
Europe
Africa
USA+Canada
Middle East
Latin America
Australia
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Au
st
ra
lia
er
ic
a
Am
La
tin
SA
+C
an
ad
a
st
Ea
id
dl
e
M
Eu
ro
pe
cif
ic
As
ia
/P
a
Af
ric
a
0
U
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http://www.internetworldstats.com/stats.htm
Users around the Globe (2002/5/9)
800
Asia/Pacific
700
2009
600
2005
500
Europe
400
2002
300
USA+Canada
Latin America
200
100
Africa
Middle East
Australia
Au
st
ra
lia
er
ic
a
Am
La
tin
an
ad
a
st
SA
+C
Ea
id
dl
e
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U
M
Eu
ro
pe
cif
ic
As
ia
/P
a
Af
ric
a
0
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Technology: Modem speed
100000
56000
60000
40000
20000
300 1200 2400
33600
28800
14400
9600
0
19
79
19
80
19
84
19
87
19
91
19
93
19
95
19
97
20
08
bps
80000
year
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Today’s options
•
•
•
•
Modem: 56 K
ISDN: 64K – 128K
OBSOLETE
Frame Relay: 56K ++
Today High Speed Connections
– Cable, ADSL, Satellite.
– All are available at
• 5Mb (2005)
• 30 Mb (2009)
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Coming soon (1999)
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Today (2005)
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Session layer (L5 of OSI)
• Not common
• Provides full-duplex service, expedited data
delivery, and session synchronization
• Internet
– doesn’t have a standard session layer
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Session layer (cont.)
• Duplex
– if transport layer is simplex, concatenates two transport
endpoints together
• Expedited data delivery
– allows some messages to skip ahead in end-system queues,
by using a separate low-delay transport layer endpoint
• Synchronization
– allows users to place marks in data stream and to roll back
to a prespecified mark
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Presentation layer (OSI L6)
• Usually ad hoc
• Touches the application data
(Unlike other layers which deal with headers)
• Hides data representation differences between
applications
– characters (ASCII, unicode, EBCDIC.)
• Can also encrypt data
• Internet
– no standard presentation layer
– only defines network byte order for 2- and 4-byte
integers
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‫עיקרון השכבות‬
Destination
Source
VoIP
Email(smtp)
UDP
TCP
ftp
Transport
Network (IPv4)
Modem
Ethernet
Application
Network
WiFi
Data-Link
Network
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‫עיקרון השכבות‬
Destination
Source
app1
UDP
app2
app3
app1
TCP
UDP
Network (IPv4)
Modem Ethernet
app2
app3
TCP
Network (IPv4)
WiFi
Modem
Ethernet
Network
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WiFi
Discussion
• Layers break a complex problem into
smaller, simpler pieces.
• Why seven layers?
– Need a top and a bottom  2
– Need to hide physical link; so need datalink  3
– Need both end-to-end and hop-by-hop actions; so
need at least the network and transport layers  5
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