Transcript ch6-layering

Protocol Layering
Myungchul Kim
[email protected]
Tel: 042-866-6127
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• Protocols and protocol layering
– peer entities
– a protocol is a set of rules and formats that govern the
communication between communicating peers.
– layering: the lower layer provides a service used by the higher
layer in its execution.
– service access point (SAP): the interface between the lower and the
upper layer
– protocol data units (PDU) between peer entities in the same layer.
– service data unit (SDU): handed to a layer by an upper layer.
– protocol stack -> information hiding -> reduce system performance
• The importance of layering
– a complex problem into smaller, more manageable pieces
– the implementation details of a layer are hidden (abstracted) from
other layers.
– many upper layers can share the services provided by a lower layer.
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• Problems with layering
– layering is a form of information hiding
– lead to poor performance
– example of layers between transport and link: TCP over a lossy
wireless link
• ISO OSI reference model
– open system: a system that implements open protocol standards
– reference model: formally defines what is meant by a layer, a
service, a service access point, name, etc.
– service architecture: describes the services provided by each layer.
– protocol architecture: the set of protocols that implement the
service architecture.
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• The seven layers
– physical layer
• coding scheme, connector shape and size, bit-level
synchronization
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– datalink layer
• framing: insert markets in the bit stream
• frame packets
• medium access control (MAC): datalink-layer address and
access to the medium
• logical link control: link error control and flow control
• Ethernet card: physical and datalink layers
– network layer
• concatenate logically a set of links to form the abstraction of an
end-to-end link.
• compute a route
• segmentation and reassembly
• unique network-wide addresses
• in datagram network, routing and data forwarding
• in connection-oriented network, the data plane and the control
plane.
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• the beauty of IP is that we can layer it over practically any
datalink layer technology, because it makes very few
assumptions about the datalink layer.
• type-of-service in the IP
– transport layer
• create the abstraction of an error-controlled, and flowcontrolled, end-to-end link
• flow control
• multiplex multiple applications to the same end-to-end
connection
• port number
• User Datagram Protocol (UDP): multiplexing, but not error
recovery or flow control for audio, video, network file system.
• Transmission Control Protocol (TCP): WWW, file transfer.
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– session layer
• provides the abstraction of full-duplex service, expedited data
delivery and session synchronization.
• The Internet does not have a standard session layer protocol.
– presentation layer
• deals with data
• encrypt data
• The Internet does not support a standard presentation layer.
– application layer
• WWW: browser, server, navigation
• file transfer using TCP
• real-time audio and video retrieval using UDP
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• why seven layers ?
– the case for the session and presentation layers appears to be weak.
– error control can be placed at the application, transport, network,
or datalink layers.
– flow control can be at the network or transport layers.
• why did the ISO OSI protocol stack fail?
– standardization process was slow, cumbersome,…
– it was specified before there was much experience. (the Internet
philosophy is to standardize only after implementation)
– a game of user numbers.
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Limitations of the Layered Model
• Some layers are not always cleanly separated.
– Inter-layer dependencies in implementations for performance
reasons
– Some dependencies in the standards (header checksums)
• Higher layers not always well defined.
– Session, presentation, application layers
• Lower layers have “sublayers”.
– Sublayers well defined in the standards
• Interfaces are not really standardized.
– It would be hard to mix and match layers from independent
implementations
– Many cross-layer assumptions, e.g. buffer management
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The Internet Protocol Suite
Application
Presentation
Session
Transport
Applications
Presentation
Session
UDP TCP
Network
Data link
Data Link
Physical
Physical
The Hourglass Model
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Internetworking Options
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physical
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repeater
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network
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router
data link
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bridge
(e.g. 802 MAC)
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gateway
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Relevant Standardization
Bodies
• Trend toward a priori standards.
– a mixed blessing
• ITU-TS (formerly CCITT) - Telecommunications Sector of
the International Telecommunications Union.
– government representatives (PTTs/State Department)
– responsible for international “recommendations”
• T1 - telecom committee reporting to American National
Standards Institute.
– T1/ANSI formulate US positions
– interpret/adapt ITU standards for US use
– represents US in ISO
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More Bodies
• IEEE - Institute of Electrical and Electronics Engineers.
– responsible for many LAN physical layer and datalink layer
standards
• IETF - Internet Engineering Task Force.
– standards for network layer and “higher”
• ATM Forum.
– voting membership mostly manufacturers
– comparatively rapid evolution of recommendations
• ISO - International Standards Organization.
– covers a broad area
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