Transcript slides - My FIT (my.fit.edu)

Reference Models and Standards
Reference Models (1)
• A reference model is the formal name for a
protocol suite – a collection of protocols and layer
definitions that provide network service
• We discuss a few models in this lesson
– The 7-layer, Open Systems Interconnection (OSI) model
created primarily by the phone companies
– The 5-layer Transmission Control Protocol (TCP) /
Internet Protocol (IP) model created by the Internet
Engineering Task Force (IETF)
– The ATM model
– The Bluetooth model
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Reference Models (2): OSI
• Conceptual objects are central to the OSI model
– Services: define what the layer does, i.e., its semantics
– Interfaces: describe how to access the services
– Protocols: describe the interaction with the peer and
implement the offered service described by the interface
– The concepts are the only significant contributions of
this reference model
• Designed without any thought of internetworking
– Typical phone company arrogance:
• Everyone will use our technologies
• Therefore internetworking is not needed (WRONG!!!)
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Reference Models (3): OSI
• The model was devised before the protocols – so
there are problems with what service is put into
each layer
– Designers couldn’t agree whether there should be 6 or 8
layers, so they comprised and declared 7 layers
– As a joke people now often talk about layers 8 and 9 politics and economics - as two unspoken layers
– The lower layer names are often still used
• OSI is complex - initial implementations were poor
• TCP/IP was already established and as a result
TCP/IP became more popular
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Reference Models (4): OSI
“endian” issues
synchronization
end-to-end
boundary
routing
framing
voltages, cabling
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Reference Models (5): TCP/IP
• The principle TCP/IP design goals were
internetworking capability and survivability
– The US Department of Defense wanted connections to
continue working as long as the source and destination
machines continued to work
• Originally there were 4 layers with a Host-toNetwork layer below the Internet layer
– Now TCP/IP is a 5 layer model with a Network Interface
layer and a Physical layer similar to the Data-Link layer
and the Physical layer of the OSI model
– The OSI names are often used at layer 2 and even
sometimes at layer 3 without too much confusion...
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Reference Models (6): TCP/IP
Also called the
network-access layer
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Reference Models (7): TCP/IP
Layer 3 is often called the network layer and Layer 2 the
data-link layer – borrowing from the OSI model
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Reference Models (8): TCP/IP
• At the transport layer there were originally two
protocols
– Transmission Control Protocol (TCP): a reliable,
connection-oriented byte stream protocol
– User Datagram Protocol (UDP): an unreliable
connectionless protocol
– Other transport layer protocols are emerging
– Some “transport” protocols are constructed at the
application layer as a shim on top of UDP
• Session and presentation layers were not included
– The application must perform those roles in TCP/IP
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Reference Models (9): TCP/IP
• The focal point of TCP/IP is the Internet layer –
giving the protocol graph a “narrow waist”
– This is the layer responsible for routing datagrams in an
internetwork
– It is also responsible for congestion control and
fragmentation
– The key to the design is an arbitrary number of
transport protocols above and network technologies
below this layer
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Reference Models (10): TCP/IP
Application
Transport
Internet
Network
Interface
TCP/IP Initial protocol graph
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Reference Models (11): TCP/IP
• The descriptions provided in the OSI protocol stack
slide for layers 1 to 4 generally apply here as well
• We will study Layers 3 and 4 particularly with
Ethernet at Layer 2
• Although ubiquitous, TCP/IP has its problems
– TCP/IP did not initially distinguish between service,
protocol, and interface – so its protocols are more
coupled and therefore harder to replace
– TCP/IP isn’t really a model – just a description of
protocols that already had been created
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Reference Models (12): ATM
• Asynchronous Transfer Mode (ATM) is a phone
company promoted, packet-switched, connectionoriented technology with its own reference model
– The model is 3-dimensional with a user plane and a
control plane, and with layer and plane management
functions at all layers
• During the late 1990s it was promoted as the “onesize fits all” technology but it has since been
overtaken by the much simpler Ethernet for LAN
use
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Reference Models (13): ATM
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Reference Models (14): Bluetooth
• Bluetooth is a Personal Area Network (PAN)
technology for very small piconets
• It has its own protocol architecture that has been
modified somewhat by the IEEE 802.15 committee
to make it compatible with other 802 networks
• Perhaps most interesting is that there is no layer
corresponding to the network layer of the OSI
model – the piconet is too small to need routing
– But there has been interest in adding routing
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Reference Models (15): Bluetooth
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Reference Models (16): Hybrid Model
• The OSI model is useful but its protocols are not popular
(although DECNET is based on OSI)
• The TCP/IP model was never used but its protocols are
ubiquitous
• A hybrid reference model used by our textbook and others
is shown below.
5 Application layer
4 Transport layer
3 Network layer
2 Data link layer
1 Physical layer
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Standards (1)
• ITU – International Telecommunication Union
• ISO – International Standards Organization
– ANSI – American National Standards Institute is the US
member, but it is not government sponsored
• IEEE – Institute of Electrical and Electronic
Engineers
– Lots of standards at networking layer 2 (and 1) – some
now available for free due to sponsorship of vendors
• IETF – Internet Engineering Task Force
– Lots of Request For Comments (RFCs) and standards at
higher layers
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Standards (2)
• http://standards.ieee.org/getieee802/
– Free download of IEEE 802 standards
– Well-written and the final word on most things layer 2 – but they
can be difficult to digest
• http://www.ietf.org
• http://www.rfc-editor.org
– Searchable database of Requests For Comments (RFCs)
– Building networks requires reading RFCs and understanding how to
determine their status in the standardization process
– The quality is variable – but the important standards have been
revised several times and are generally readable
– Briefly examine the RFCs published on April 1 for some networking
humor 
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