The 7Layer OSI Model

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The 7-Layer OSI Model
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Network Software

• An important component of any network is the
  software that controls communication.
• This software can be very complicated. Breaking
  the software into distinct modules helps simplify
  its design and enables reusability.
• Suppose we want to write software to allow us to
  transfer a file from one computer to another.
• One way is to write a single program that
  includes a windows interface, the file transfer
  protocols and the software for interacting with
  the network hardware
• This is a lot of work and chances are that it
  will take a long time to debug this program.
• A more sensible approach is to use an existing
  (and well tested) network access module.

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Network Software Modules

• The file transfer program can call upon the
  services of the Network Access Module.
• The Network Access Module performs all the tricky
  network communications stuff, thus simplifying
  the implementation of the file transfer program.
• Furthermore, if the Network Access Module on a
  different network has the same interface, then
  your file transfer program will work on that
  network too.

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Network Software Modules

• The Network Access Module will only provide a raw
  data transmission service. If some data is
  corrupted, it is still left to the file transfer
  program to send a request for the data to be
  retransmitted.
• There is no reason why we couldnt have a module
  that did this for us. Let us call this the
  Transport Module.
• Now the file transfer program calls upon the
  services of the Transport Module, which in turn
  calls upon the services of the Network Access
  Module.

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Peers

• Each module has a counterpart in the other
  computer. These counterparts are sometimes
  called peers.
• In a logical sense, each module effectively talks
  to its peer using the same type of protocol data
  units (PDUs).
• The file transfer programs talk to each other by
  sending each other Application Protocol Data
  Units.
• Similarly, the Transport Modules talk to each
  other by sending each other Transport Protocol
  Data Units that encapsulate the APDUs.

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The Open Systems Interconnection Model

• Further modularization brings helps to simplify
  the design of network software more. The OSI
  model has 7-layers.
• Each layer has a different level of abstraction.
• Each layer performs a well defined function.
• Layers require minimum data flow across the
  boundaries.
• Designed to encourage international protocol
  standards.

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The Open Systems Interconnection Model

• Modern networks offer end-to-end connectivity
  however the offered services may not fulfil the
  requirements of distributed applications and must
  therefore be enriched by an end-to-end
  communication support. This leads to a
  communication system consisting of 3 layers. End
  systems inter-communicate through layer T, the
  transport infrastructure. The service of layer T
  is a generic service corresponding to layer 2, 3
  or 4 services in the OSI Reference Model. In
  layer C the end-to-end communication support adds
  functionality to the services in layer T. This
  allows the provision of services at the layer A
  for distributed applications (A-C Interface).
• Layer C is decomposed into protocol functions,
  which encapsulate typical protocol tasks such as
  error and flow control, encryption and
  decryption, presentation coding and decoding
  among others. A protocol graph is an abstract
  protocol specification, where independence
  between protocol functions is expressed in the
  protocol graph. If multiple T services can be
  used, there is one protocol graph for each T
  service to realise a layer C service. Protocol
  functions (modules) can be accomplished in
  multiple ways, by different protocol mechanisms,
  as software or hardware solutions with each
  protocol configuration in a protocol graph being
  instantiated by one of its modules.

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The Open Systems Interconnection Model
3 layer model
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The Open Systems Interconnection Model
Layering is a form of information hiding where a
lower layer presents only a service interface to
an upper layer, hiding the details of how it
provides the service. Information hiding has
numerous benefits however it can sometimes lead
to poor performance especially in cases where
optimisations are possible given knowledge of
what the lower layers are doing. A flow control
protocol that is responsible for throttling a
source when it thinks that the network is
overloaded may assume that overloads are
correlated with packet loss thus that protocol
could throttle the source-sending rate when it
detects a packet loss (TCP). However, a flow
control protocol might not know how packets are
transferred across the network if the flow
control protocol were layered above a protocol
that is actually responsible for data transfer.
The end system may be connected to a network over
a lossy wireless link so that packet losses are
mostly due to link errors rather than network
overload. Unfortunately, even in this situation,
the flow control protocol thinks that the network
is congested and throttles a source even when
there is no need to do so
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The Open Systems Interconnection Model

• The information that a packet is lost on the
  link, which is available to the lower layer, is
  hidden from the higher (flow control) layer,
  which results in inefficient flow control.
• A solution would be for the lower layers to
  inform the upper layer concerning packet loss so
  that the flow control layer could distinguish
  between the link and congestive losses and
  perform a better job.
• However this violates layering, as the flow
  control layer now knows about the details of data
  transfer over its local link so if the data
  transfer layer changes because the end system is
  using a different link technology (e.g. Wireless
  link versus Wired link), the flow control layer,
  which ought to be independent of the link
  technology, must also change.
• This illustrates the tension between information
  hiding on one hand and performance on the other
  hand. Intelligent protocol stack designs should
  leak enough information between layers to allow
  efficient performance, but not so much that it is
  difficult to change the implementation of a
  layer.

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The Open Systems Interconnection Model
As layers are self-contained small pieces of
functionality, independent from other layers and
relatively small they can be verified for
correctness more easily than large chunks of
interdependent code. Layering forces better
structuring as code for a certain type of
functionality is localised in one place and
layers can be easily replaced/upgraded with new
versions. A protocol layer typically either
modifies a message (e.g. by adding a header), or
it may delay its delivery, (e.g. to preserve
ordering in a FIFO layer). Each message has a
layer type tag, describing the layer from which
it originated. Layers also generate messages
stamped with its unique type. Each layer checks
whether a message's layer type matches its own
and if so, the layer processes the message
otherwise it will be passed on to the next layer.
The protocol layer interface has methods to
process messages from layers above or below (Down
or Up) which are called when a message passes
through the layer.
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The Open Systems Interconnection Model

• Most protocols allow for some variation in
  packet sizes, timeouts, among others however the
  defaults provided may not be optimal for all
  situations. TCP uses window sizes from 1 to 7,
  and packet sizes in powers of 2 ranging from 64
  through 4096 however if over a noisy link it
  drops more than 10 percent of all packets, losses
  may be reduced by lowering the packet size and
  shrinking the window.
• On non-lossy links however, the overhead of
  sending ACKs for every 128 bytes may prove
  wasteful, therefore the packet size could be
  increased to 512 or whatever is closer to the
  maximum packet size. Interchanging ACK and NAK
  schemes may also lead to improved performance as
  in large scale multicast reliable communications.
  
• When the ACK-based scheme is used, an
  ACK-implosion problem can occur, as the number of
  participating receivers is large. In the
  NAK-based approach, a NAK-suppression mechanism
  can be applied to improve its scalability.
• The NAK-based approach with the NAK-suppression
  mechanism is suitable for scalable multicast
  communications.

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The Open Systems Interconnection Model
A flexible protocol system allows the dynamic
selection, configuration and reconfiguration of
protocol modules to dynamically shape the
functionality of a protocol in order to satisfy
application requirements or adapt to changing
service properties of the underlying network.
Flexible end-to-end protocols are configured to
include only the necessary functionality required
to satisfy an application QoS requirements for
the particular connection. Some uses that
dynamic stacks may be used for include increasing
throughput where environmental conditions are
analysed heuristics are applied to decide if
change would bring about optimal performance
interoperability in that dynamic stacks can
simplify the interoperability process, by
allowing code for protocol stacks to be written
once and placed on repositories where they can be
downloaded onto end systems so they can adopt the
same stack. Security can be increased at
run-time, for example, when an intrusion
detection system dynamically responds to unusual
behaviour and robustness, where faulty components
can be detected and replaced to improve
robustness (e.g. a mobile computer connected to
an Ethernet LAN may automatically detect that its
wired connection is broken forcing a switch to a
Wireless LAN or GSM connection. Here it is
profitable to dynamically load a new stack module
optimised for the different characteristics of
wireless connections).
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Dynamic Reconfigurable Middleware
This diagram represents the protocol graph of a
steaming video application in a dynamically
reconfigurable middleware framework. When a
network device is switched from a low speed
cellular network to a high speed wireless LAN, a
session manager will detect that the underlying
network has changed. The protocol stack control
mechanism could install a new stack module, which
contains an algorithm for conversion of a colour
video stream to a monochrome video stream.
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1. The Physical Layer

• The Physical Layer performs bit by bit
  transmission of the frames given to it by the
  Data Link Layer.
• The specifications of the Physical Layer include
• Mechanical and electrical interfaces
• Sockets and wires used to connect the host to the
  network
• Voltage levels uses (e.g. -5V and 5V)
• Encoding techniques (e.g. Manchester encoding)
• Modulation techniques used (e.g. square wave)
• The bit rate and the baud rate.

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2. The Data Link Layer
Network Layer
Data Link Layer

• The Data Link Layer tries to make the unreliable
  transmission of frames appear to be reliable.
• The Data Link Layer deals with
• Inserting and extracting Network Packets
  into/from frames
• Adding error checking information to frames.
  Also checking received frames for errors
• Sending requests for frames to be retransmitted
  if they are found to contain errors. Also
  responding to such requests
• Ensuring the Network Layer receives data in the
  order in which it was sent over the link
• Frame synchronisation
• Flow Control over the link

Physical Layer
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3. The Network Layer
Transport Layer
Network Layer

• The Network Layer controls the operation of the
  local part of the network.
• The Network Layer
• Examines packets given to it by the Data Link
  Layer to see if they are destined for this host
  or should be forwarded to another host
• Decides on the best way to route packets destined
  for other hosts
• Performs accounting tasks (such as counting the
  number of packets to determine congestion levels)
• Performs protocol conversions when packets are
  routed over different types of networks

Data Link Layer
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4. The Transport Layer
Session Layer
Transport Layer

• The Transport Layer controls the delivery of data
  between two hosts.
• The Transport Layer
• Fragments data streams from multiple sessions
  into packet sized portions and passes them to the
  Network Layer
• Reassembles data streams from data in packets
  given to it by the Network Layer
• Ensures delivery of data streams to appropriate
  sessions
• Performs host-to-host flow control to ensure data
  does not arrive faster than the host can cope with

Network Layer
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5. The Session Layer
Presentation Layer
Session Layer

• The Session Layer maintains a logical connection
  between two processes.
• The Session Layer
• Performs dialogue control functions and token
  management to keep track of who is to transmit
  data next
• Synchronisation functions (e.g. inserting
  checkpoints into data stream so that
  communication can be resumed from the last
  checkpoint after a failure)
• There may be multiple sessions (i.e. one for each
  communicating process). Each session is
  identified by a SAP (Service Access Point) number.

Transport Layer
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6. The Presentation Layer
Application Layer
Presentation Layer

• The Presentation Layer performs data
  representation and syntax conversions.
• The Presentation Layer
• Enables cross-platform communication by
  converting data representations (strings,
  integers, floating-point numbers etc.) to and
  from network standard formats
• Example the sending host uses the EBDIC
  character set and the receiving host uses ASCII.
  The Presentation Layer in the sending host takes
  the EBDIC code and coverts it to Unicode. The
  Presentation Layer in the receiving host gets the
  Unicode and coverts it to ASCII.

Session Layer
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7. The Application Layer
Process
Application Layer

• The Application Layer provides an interface to a
  range high-level network services used by the
  communicating process.
• Access to application services is through a set
  of calls to routines in a communications library.
• There are routines for identifying a host,
  determining if that host is available for
  communication and establishing a connection
• Data is typically sent and received in the same
  way as writing or reading to and from a file
• There may also be routines for handling e-mail
  and file transfers

Presentation Layer
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The Communicating Process

• The process provides the user interface and makes
  use of the routines provided by the Application
  Layer to perform communication.
• The communicating process is not considered to be
  part of the OSI 7-layer model.
• The process can just use the services provided by
  the Application Layer or it can implement its own
  high-level protocols that it can send and receive
  using the routines provided by the Application
  Layer.
• Of course, if a process does implement its own
  high-level protocol then the process with which
  it is communicating must be able to understand
  that protocol too.

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The Network Dependent Layers

• The lowest three layers of the OSI7-layer model
  are network dependentmeaning that they will be
  different for different typesof network.
• The layers above the network layer are network
  independent meaning that the software from the
  transport layer upwards can be moved from one
  network to another and still work.
• In the case of IEEE 802 networks, even the
  Network Layers can be the same since the Data
  Link Layers in these networks use a common
  interface called Logical Link Control (LLC).