1 - Interconnecting heterogeneous networks
Sergi Robles
Sergi.Robles@uab.es
Department of Information and Communication Engineering
Universitat Autònoma de Barcelona
Computer Networks, year 2011/12
Intro Technologies Internets History TCP/IP
Contents
1
Computer Networks
2
Network technologies
3
Interconnection of networks
4
History of the Internet
5
TCP/IP protocol family
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What is a network?
A system or group of interconnected things
→ people: social network
→ places: railroad network
→ homes: power grid
→ ...
→ computers: . . . ?
computer network!
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Computer network
A form of communication network
The nodes (things) are computers and the like: general
purpose computers, smartphones, printers, TVs, tablets,
supercomputers, car, toaster, fridge, . . .
Linked with: coax, optical fiber, twisted pair, radio,
microwaves, . . .
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Point-to-point:
Computers use the link (network) to exchange data.
Data is usually split in small packets to be transmitted
Making use of protocols
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Protocol
Hi
connection request
Hi
Connection response
Got the
time?
Get nice-picture
2:00
<file>
time
1
Defines the syntactic and semantic rules for
communication.
Details: message format, how to respond to given
messages, how to handle errors, . . .
1
Figure from J.F Kurose and K.W. Ross, 2007.
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What if we want more than 2 computers?
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What if we want more than 2 computers?
Network Infrastructure
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Intro Technologies Internets History TCP/IP
Network technologies Ethernet ATM
Contents
1
Computer Networks
2
Network technologies
Network technologies
Ethernet
ATM (Asynchronous Transfer Mode)
3
Interconnection of networks
4
History of the Internet
5
TCP/IP protocol family
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Network technologies Ethernet ATM
Network technologies
Network technologies that will be used as examples during the
subject.
Network technologies:
Ethernet (IEEE 802.3)
ATM (Asynchronous Transfer Mode)
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Network technologies Ethernet ATM
Contents
1
Computer Networks
2
Network technologies
Network technologies
Ethernet
ATM (Asynchronous Transfer Mode)
3
Interconnection of networks
4
History of the Internet
5
TCP/IP protocol family
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Network technologies Ethernet ATM
Ethernet
Ethernet
Bus in base bandwidth broadcast network.
Medium Access: CSMA/CD (Carrier Sense, Multiple
Access, Collision Detect).
Bus topology, normally star like topography.
Important elements
Hubs
Switches
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Network technologies Ethernet ATM
Ethernet addresses
Addresses
48 bit addresses. Example: 00:10:5A:3C:D3:66
Types:
Unicast: Addresses only one station.
Broadcast: Addresses all station in the network. All 1’s.
Multicast: Limited broadcast: only a number of stations.
A station will receive all frames sent to its physical address
(unicast), those sent to broadcast, and to a multicast address if
it belongs to the right group.
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Network technologies Ethernet ATM
Ethernet Frame Format
Preamble
Destination
Source
Type
8 octets
6 octets
6 octets
2 octets
Data
Max. 1500 octets
CRC
4 octets
Maximum size: 1518 octets.
→ Free space for an IP datagram is 1500 octets.
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Intro Technologies Internets History TCP/IP
Network technologies Ethernet ATM
Contents
1
Computer Networks
2
Network technologies
Network technologies
Ethernet
ATM (Asynchronous Transfer Mode)
3
Interconnection of networks
4
History of the Internet
5
TCP/IP protocol family
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Network technologies Ethernet ATM
ATM (Asynchronous Transfer Mode)
Features ATM
There is no broadcast (!)
Multiple Access.
Connection oriented. Many interconnected stations.
HW: Many interconnected switches. Each one allowing
only a limited number of connections.
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Network technologies Ethernet ATM
ATM Addresses
Connection paradigms in ATM:
Permanent Virtual Circuits (PVC)
Switched Virtual Circuits (SVC).
Every circuit has a 24 bit identifier:
VPI (8bits)
VCI (16 bits)
The Virtual Path Identifier (VPI) is used to optimise routing.
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Network technologies Ethernet ATM
ATM Cells
ATM cells are 53 bytes long: a header of 5 bytes and 48 bytes
more for data.
Control (4 bits)
Type (3 bits)
VPI (8 bits)
Priority (1 bit)
VCI (16 bits)
CRC (8 bits)
Cells are not used directly: ATM Adaptation Layer.
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Network technologies Ethernet ATM
Adaptation Layer
When establishing a new connection the host must specify
which adaptation layer protocol will be used.
Once the protocol is agreed, it can not be changed.
Adaptation layer manages detection and correction of
errors (corrupted cells, lost cells, ...).
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Network technologies Ethernet ATM
AAL 5 (ATM Adaptation Layer 5)
AAL 5 packet format:
Data (1 - 65535 bytes)
trailer (8 bytes)
In the trailer we have:
Not used (2 bytes)
Size (2 bytes)
CRC (4 bytes)
→ TCP/IP limits datagram size over an ATM to 9180 octets per
datagram.
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Intro Technologies Internets History TCP/IP
Reasons for a interconnection of networks Services
Contents
1
Computer Networks
2
Network technologies
3
Interconnection of networks
Reasons for a interconnection of networks
Services
4
History of the Internet
5
TCP/IP protocol family
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Reasons for a interconnection of networks Services
Introduction
Why do we want an interconnection of networks?...
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Reasons for a interconnection of networks Services
Introduction
Why do we want an interconnection of networks?...
Users want to have a universal network, providing global
communication services.
Physical networks are quite different (optimised for
different parameters). It is interesting to make
communication independent from network.
We want an open system, which is not tied to a specific
vendor.
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Reasons for a interconnection of networks Services
The interconnection of networks (internetworking) can be
achieved by using the TCP/IP protocol family.
TCP/IP
TCP/IP is a suite of protocols used for communication through
different interconnected networks (internet).
Internet has shown the feasibility of these protocols, their
scalability, and their flexibility.
Internet 6= internet
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Intro Technologies Internets History TCP/IP
Reasons for a interconnection of networks Services
Contents
1
Computer Networks
2
Network technologies
3
Interconnection of networks
Reasons for a interconnection of networks
Services
4
History of the Internet
5
TCP/IP protocol family
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Reasons for a interconnection of networks Services
Services
Services are normally provided through protocols. Protocols
specify the syntactic and semantic rules for communication
(message format, behaviour, HW independence, etc.)
Application level services: www, e-mail, ssh, file
transfer, ...
Network level services: Connectionless packet delivery
(no reliability assured), Reliable flow transport (connection
oriented).
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Reasons for a interconnection of networks Services
Aren’t out there enough protocols yet offering similar things?
Couldn’t we recycle some of them?
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Reasons for a interconnection of networks Services
Aren’t out there enough protocols yet offering similar things?
Couldn’t we recycle some of them?
No! TCP/IP protocols are designed with different requirements.
Here we highlight the main differences:
TCP/IP protocol suit
All these services are independent from the network
technology.
They provide a universal interconnection.
There is an acknowledgment only end-to-end.
Application protocols are standard.
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Intro Technologies Internets History TCP/IP
History Internet organization
Contents
1
Computer Networks
2
Network technologies
3
Interconnection of networks
4
History of the Internet
History
Internet organization
5
TCP/IP protocol family
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History Internet organization
History of the Internet
1957-8
1961
1962
1965
1970
1971
1975
1977-79
1980-83
1991
1994/98
2004/06
2012
−→
−→
−→
−→
−→
−→
−→
−→
−→
−→
−→
−→
−→
Sputnik satellite → DARPA program
Kleinrock, MIT (packet switching)
Licklider, MIT (Galactic Network)
Roberts (Practical interconnection of networks)
NCP protocol. Begins the devel. of apps.
Email: Protocol and application
Designing of new protocols (Kahn & Cerf)
Current protocols
ARPA installs TCP/IP (ARPANET/MILNET)
Beginning of the World Wide Web
Amazon and Google
Facebook and Twitter
We learn TCP/IP in Computer Networks II ;)
Table: Significant milestones in the history of the Internet
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History Internet organization
ARPANET asks the University of California (at Berkeley)
for the integration of TCP/IP within its operating system
(BSD UNIX).
BSD integrates TCP/IP in the model of their OS and
develops network utilities consistent with UNIX.
Spectacular sprawl. In 2000 there were more than 50˙106
computers. In 2010, the estimated number is 1.97˙109 !
There will be scalability problems (in the 80s domain name
administration was still performed manually!!). Soon it will
run out of addresses.
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History Internet organization
Contents
1
Computer Networks
2
Network technologies
3
Interconnection of networks
4
History of the Internet
History
Internet organization
5
TCP/IP protocol family
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History Internet organization
Internet organization
Internet Society (ISOC)
Established in 1992 for promoting the participation in the
Internet.
Internet Architecture Board (IAB) Technical steering of Internet.
Internet Research Task Force (IRTF)
Research (long term Internet)
Internet Engineering Task Force (IETF)
Engineering (Short-mid term Internet)
→ Request For Comments (RFC) Technical reports about
the protocols of Internet (new or revised).
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Intro Technologies Internets History TCP/IP
Families OSI Model Model TCP/IP Architecture
Contents
1
Computer Networks
2
Network technologies
3
Interconnection of networks
4
History of the Internet
5
TCP/IP protocol family
Protocol families
OSI Model
Model TCP/IP
TCP/IP Architecture
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Families OSI Model Model TCP/IP Architecture
Protocol family TCP/IP
Objective:
To build a system for the unified interconnection of networks,
cooperative, hiding HW details from the network, and
supporting a universal communication service.
Concept of layered protocols:
→ Why don’t having only one protocol doing it all??
At first sight it could seem the simplest solution!
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Families OSI Model Model TCP/IP Architecture
There are too much problems to be all considered at a single
level. The complexity would be excessive:
Problems to take into consideration:
Network HW faults
Congestion management
Packet delay and loss
Duplication of data and out of order arrivals
Moreover, it would not be rather reliable: programs could also
fail.
→ We need a layered scheme of protocols.
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Families OSI Model Model TCP/IP Architecture
Models
There are mainly two layered models of network architecture:
Open System Interconnection (OSI) from ISO
Model of 7 layers. A paradigm of this model is X.25.
TCP/IP
Model of 5 layers, with one layer devoted to routing
between different networks.
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Families OSI Model Model TCP/IP Architecture
Contents
1
Computer Networks
2
Network technologies
3
Interconnection of networks
4
History of the Internet
5
TCP/IP protocol family
Protocol families
OSI Model
Model TCP/IP
TCP/IP Architecture
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Families OSI Model Model TCP/IP Architecture
OSI Model
Application
Presentation
Session
Transport
Network
Link
Physical
Principle of layered protocols:
Layer n at destination receives exactly
the same as it was sent by layer n at
origin.
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Families OSI Model Model TCP/IP Architecture
OSI Model
Example in X.25:
Physic: Physical interconnection from computers to
switches. Procedures for packet transmission at bit level.
Link: How information is sent between the computer and
the switch. Frame format. Error detection. Reception
acknowledgment
Network: Computer-network interaction. Destination
address, routing. adjustment of information units size.
Network congestion.
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Families OSI Model Model TCP/IP Architecture
Transport: End-to-end reliability. End-to-end
acknowledgment to be sure no middle computer has failed.
Session: Organisation of protocol SW to provide
functionality to application programs. Remote terminal
access (connection from terminal to host).
Presentation: Functions needed by programs to access
the network e.g. Text compression, conversion from
images to bit flows.
Application: Support to application programs using the
network.
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Families OSI Model Model TCP/IP Architecture
Contents
1
Computer Networks
2
Network technologies
3
Interconnection of networks
4
History of the Internet
5
TCP/IP protocol family
Protocol families
OSI Model
Model TCP/IP
TCP/IP Architecture
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Families OSI Model Model TCP/IP Architecture
Model TCP/IP
Everything has its own name...
Conceptual layer
Objects between layers
Applications
Transport
Internet
Network Interface
Hardware
−→ Messages or flows
−→ Segments / User Datagrams
−→ Datagrams
−→ Frames
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Families OSI Model Model TCP/IP Architecture
Network interconnection does not happen at application level
(expensive, unreliable), but at network level.
Instead of interchanging big messages, only small messages
are sent.
Advantages:
It uses the HW just beneath, very efficient.
Network activity is differentiated from application activity.
Flexibility. It allows the development of general purpose
applications.
Unnoticeable network configuration changes can be done.
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Families OSI Model Model TCP/IP Architecture
The main protocols:
Transport: TCP (Transmission Control Protocol).
Provides reliability and control of the transmission.
Interconnection: IP (Internet Protocol).
Unreliable transport, connectionless, best effort delivery.
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Families OSI Model Model TCP/IP Architecture
Differences between OSI and TCP/IP
Two basic differences:
1
Reliability
2
“Intelligence” location
Reliability:
X.25 → Reliable transfer service at all levels.
TCP/IP → Reliability is only end-to-end. Best effort
network. Easier to implement.
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Families OSI Model Model TCP/IP Architecture
“Intelligence” and decision making.
X.25 → The network controls and it is the authority. The
connected elements do almost nothing.
TCP/IP → Hosts take part in almost all protocols
(reliability, selection of the next router, ...). Simple network
with “intelligent” elements.
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– Sergi Robles
Families OSI Model Model TCP/IP Architecture
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Intro Technologies Internets History TCP/IP
Families OSI Model Model TCP/IP Architecture
Contents
1
Computer Networks
2
Network technologies
3
Interconnection of networks
4
History of the Internet
5
TCP/IP protocol family
Protocol families
OSI Model
Model TCP/IP
TCP/IP Architecture
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Families OSI Model Model TCP/IP Architecture
TCP/IP Architecture
We cannot directly connect elements from different networks:
They won’t understand each other!
We need elements being in different networks at the same time.
These elements are the routers.
XR 1
R1
XR 2
R2
XR 3
Routers
Routers are special computers allowing the interconnection of
different physical networks.
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Families OSI Model Model TCP/IP Architecture
TCP/IP Architecture
Routers use networks, and do not care about computers.
From the user’s point of view this is totally transparent: she
does not need to know the internals of networks.
All networks are dealt with in the same way (LANs, WANs,
point-to-point links, ...).
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Families OSI Model Model TCP/IP Architecture
Physical
Network
host
Router
internet
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Intro Technologies Internets History TCP/IP
Host A
Families OSI Model Model TCP/IP Architecture
Host B
Router
Application
Application
Same message
Transport
Transport
Segment / Same user DG
Internet
Network 1
Interface
Same
Datagram
Int.
Int.
NI 1
NI 2
Same
Frame
Physical
Network 1
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Internet
Same
Datagram
Same
Frame
Network 2
Interface
Physical
Network 2
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Fair enough, the principle of layered protocols is clearly applied
in the uper layers of TCP/IP...
...But, what happens with the interconnection layer??
We have just said that the host has the view of only one big
virtual network. Haven’t we?
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Families OSI Model Model TCP/IP Architecture
Every datagram has a header that is slightly changed in every
middle router it goes through...
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Families OSI Model Model TCP/IP Architecture
Every datagram has a header that is slightly changed in every
middle router it goes through...
...therefore, the destination host will not receive an exact copy
of the datagram that it was sent. However, it will be mostly the
same.
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Intro Technologies Internets History TCP/IP
Families OSI Model Model TCP/IP Architecture
Every datagram has a header that is slightly changed in every
middle router it goes through...
...therefore, the destination host will not receive an exact copy
of the datagram that it was sent. However, it will be mostly the
same.
The principle of layered protocols only applies between
direct transmissions. Thus, we say that the internet layer
does not provide a end-to-end service.
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Families OSI Model Model TCP/IP Architecture
Two important boundaries
Protocol address boundary. Between the high level
address scheme and the low level address scheme.
Application, transport and internet layers use only IP
addresses.
Operating system boundary. Between the operating
system and application programs.
This boundary may vary between TCP/IP implementations,
but most of them place the suit of protocols inside the
system.
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Conceptual layers
Families OSI Model Model TCP/IP Architecture
Boundaries
Application
Programs outside the OS
Programs inside the OS
Transport
Internet
Network
Interface
Only IP addresses
Physical addresses
Hardware
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Families OSI Model Model TCP/IP Architecture
Some inconveniences about protocol layering
The layering concept is crystal clear, but it is often
inefficient.
Hiding low level details makes it very difficult to optimise
data transfers.
→ In practice, and against elegant designing, lower layers
propagate information to upper layer in order to improve
efficiency.
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Families OSI Model Model TCP/IP Architecture
Building a unified, cooperative network interconnection,
supporting a global communication service, requires solving a
few problems before:
Addressing
Address translation
Transport
Routing
Control
→ We will see solutions to all these problems along the term.
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