ch6-layering - WINSLab

warmersafternoonΔίκτυα και Επικοινωνίες

23 Οκτ 2013 (πριν από 4 χρόνια και 17 μέρες)

74 εμφανίσεις

1

Protocol Layering

Myungchul Kim

mckim@icu.ac.kr

Tel: 042
-
866
-
6127

2


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.

3


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.


4


The seven layers











physical layer


coding scheme, connector shape and size, bit
-
level
synchronization

5

6

7


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.

8


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 flow
-
controlled, 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.

9


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


10


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.



11

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

12

The Internet Protocol Suite

Application

Presentation

Session

Transport

Network

Data link

Physical

UDP

TCP

Data Link

Physical

Applications

Presentation

Session

The Hourglass Model

13

3

3

7

6

5

7

6

5

7

6

5

7

6

5

7

6

5

7

6

5

7

6

5

7

6

5

Internetworking Options

4

3

2

1

4

3

2

1

1

4

3

2

1

4

3

2

1

2

1

1

4

3

2

1

4

3

2

1

3

repeater

bridge

(e.g. 802 MAC)

router

physical

data link

network

4

3

2

1

4

3

2

1

2

2

gateway

. . .

2

2

1

1

1

1

14

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

15

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