Transcript Review of Underlying Network Technologies

Internetworking Concept
and Architectural Model
Chapter 3
Looked at low-level details of transmission
across individual data networks
Raise level of abstraction
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System that hides underlying details while
providing universal communication services
Result: high level abstraction; framework for
all design decisions
Later chapters:
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Use the abstraction to build layers of internet
communication software
Show how applications use the comm system
Application-Level
Interconnection
Two approaches to hiding details
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Have applications do the work
Hide details in the operating system
Application-level connections require
application gateways
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Application running on each computer in the
network
Understands details of network connections
Seems natural at first, but:
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New functionality means new application
program for every computer
New network hardware means modifying all
existing programs
With hundreds or thousands of networks, no way
to write all possible application programs
Intermediate application programs between the
source and destination must operate correctly
Network-Level Interconnection
Alternative to application-level
Provides a mechanism to deliver small
data packets from source to destination
No intermediate application programs
Several advantages
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Efficient (scheme maps onto underlying HW)
Separates data communication from applications
Flexible (allows general-purpose comm facilities)
Modifying network does not affect applications
Key is abstraction of internetworking
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Internetwork (internet) concept is powerful
Detaches comm from network technologies
Hides low-level details from user
Drives software design decisions
Helps handle physical addresses and routes
Motivation for internetworking comes from
two observations:
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No single network hardware technology can
satisfy all needs
Example: LAN vs WAN
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Users desire universal interconnection
Goal:
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Build unified interconnection of networks to
support universal communication
Each network has own technologies
Software between networks and application
programs gives impression of single large network
Internet Properties
Universal service
Hidden underlying architecture
No forced network interconnection topology
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Adding network should be easy
Intermediate networks not directly connected to
source or destination computers
Set of shared, universal machine identifiers
Operations and applications independent of
underlying network technologies
Routers
Use a special computer to connect
networks
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Called internet gateways or internet routers
IP router & IP gateway used interchangeably
Routers must know the topology beyond
the networks to which it connects
Key:
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Routers use the destination network, not the
destination computer, for forwarding packets
Will study in detail in later chapters
For now:
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Assume it is possible and practical for
routers to do their job
Also assume that only routers connect
physical networks
User’s View
Want user to see only one network to
which all machines are connected
Network level interconnections means:
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Applications that communicate run without
change on any computer
Internet SW worries about physical
connections
TCP/IP internet protocols treat all
networks equally
Internetworking Summary
Internet is more than collection of networks
interconnected by computers
Need conventions that allow communication
Need universal identifiers and set of
procedures for moving data to destination
IP routers (or gateways) connect networks
and forward packets
Details As Yet Unanswered
Form of internet addresses?
How addresses relate to Ethernet, ATM, etc.?
What does a packet look like?
What if packets come too fast for a computer
or router to handle?
How do routers learn routes?
How do multiple applications executing concurrently on one computer send/receive
packets to multiple destinations without
problems?