Chapter 2: Protocols and Architecture

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COE 341 Data Computer Communications (Term
061)Dr. Radwan E. Abdel-Aal

• Chapter 2 Protocols and Architecture

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Agenda

• Need for protocols and for a layered protocol
  architecture
• Key Elements of a Protocol
• Framework for Standardization
• Simplified File Transfer Architecture
• A Three-Layer Model
• Protocol Architectures and Networks Addressing
• Primitives and Parameters
• Protocol operation Protocol Data Units (PDUs)
• Standard Protocol Architectures
• The OSI Protocol Architecture
• The Model
• OSI Layers
• The OSI versus the TCP/IP (the Internet) Protocol
  Architecture

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What is a Protocol?

• A set of rules or conventions agreed upon and
  observed by two communicating entities for
  successful exchange of data

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Why we Need Protocols?

• Equipment come from different vendors and run
  different operating systems Need common standard
  procedures and language to communicate properly
• Example File transfer
• Source must
• Activate communications link
• Check if destination is prepared to receive data
• Source file transfer application must
• Check if destination file management system will
  accept and can store file
• May need file format translation, etc

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Why we need Architecture (hierarchy) for
Protocols?

• A large task is better handled through splitting
  it into smaller subtasks (divide Conquer)
• Can be developed simultaneously by different
  teams using different platforms (hardware,
  software)
• Hierarchical layered architecture
• The subtasks are implemented separately in layers
  forming a stack
• The stack is implemented in all communicating
  end systems
• Higher layers handle higher-level tasks
• A layer provides/requests services to/from an
  adjacent layer on a system
• Peer layers in the two communicating systems
  interact with each other using a protocol
• Examples from everyday life

High Level, e.g. Applications
Low Level, e.g. Signals, Bits
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Guidelines for layering

• Each layer performs a subset of related subtasks
• Each layer provides services to the higher layer
  and requests services from the lower layer
• Ideally, changes in one layer should not require
  changes to other layers,
• e.g. changing the link from wire to optical
  fibre
• should require changing only the physical layer
• Information hiding Each layer sees only what it
  needs to see
• Not too few layers
• Not enough splitting of functionality
• Not too many layers
• Difficult to manage
• Large communication overhead between them

Physical Link
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Protocol Architecture Layered Structure
High-level functions
Communicating Entity
By exchanging formatted data blocks that obey a
protocol (PDUs)
Services Provided
Services Requested
Highest Level
Communication with peer layers on other entities
Lowest Level
Most primitive functions
Physical Link
Protocol stack on an end system
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Framework for Standardization
Well-defined interfaces at layer boundaries
Standards/code for various layers can be
developed Independently, Simultaneously, Using
different platforms (Advantage)
Requests
From/To
Services
To/From
? Changes and upgrades in a layer need not affect
other layers
WK 2
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Scope for Standardization at the Layers

• Service Definitions
• State only as a functional description- not how
  to implement it (for flexibility)
• For internal use only within this communicating
  entity (not a protocol)

(at the layer interfaces)
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SAP 2

SAP 1
Here we are dealing with other (foreign) systems!
?Need a strict protocol
Addressing within the same system Services are
requested from /provided by a layer through
Service Access Points (SAPs) (or Ports). Use the
SAP (port) number for addressing
1
3
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So, three standardization elements

• Addressing within the same system
• Referencing by SAP numbers We name the entity in
  a layer requesting a service from a lower layer
  by the SAP used on that lower layer
• Service definitions (vertically within a system)
• Only Functional description of what is required
  between layers within the same system
• Protocol specification (Horizontally, more
  strict)
• Operates between the same layer on the two
  communicating entities (peer layers)
• May involve different operating systems
• So, protocol specifications must be defined
  precisely
• Format of data units
• Semantics (meanings) of all fields

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Example Simplified Protocol Architecture

• The task of file transfer is broken down into
  three modules (layers) that handle
• File transfer application ? Application layer
  (Top)
• Communication services ? Transport layer
• Network access ? Network access
  layer

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Simplified Architecture for File Transfer
A Three-Layer Model
End System
Peer layers on the two systems communicate using
Protocols
End System
Services Requested/ Provided On the same system
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Network Access Layer

• Function Exchange of data between a computer and
  the network
• Depends on type of network used
• (LAN, packet switched, etc.)
• (should be the only layer that worries about
  such details)
• Should specify
• Address of destination computer on the network
• Possibly the level of service required from the
  network, e.g. priority, delay, etc.

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Transport Layer (communication services)

• Function Reliable exchange of data (error and
  flow etc.)
• Independent of network used (significance)
• Independent of application
  (serves all the higher-level applications-
  sharing of resources)

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Application Layer

• Function Supports different user applications
  running on the communicating entity
• e.g. e-mail, file transfer, etc.

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Protocol Architecture and Networks
Applications Layer contains modules, each
supporting an application running on this computer
(SAP)
Each application accesses the services of the
transport layer through a SAP or a Port
3 computers communicating over a network Using a
3-layer protocol architecture
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Addressing Requirements

• Two levels of addressing
• Each computer on the network needs a unique
  computer address
• Each application on a (multi-tasking) computer
  needs a unique application address within that
  computer
• We identify the application on a computer by the
  SAP number it uses
• Service access point or SAP in OSI, e.g. SAP 3
• Port in TCP/IP, e.g. Port 5
• Each of the two addressing levels is handled only
  by the appropriate layer (information hiding)
• Computer address is handled by the Network Access
  layer
• Application address (SAP ) is handled by the
  Transport layer

Example KFUPM mail bag
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Protocol Data Units (PDU)

• Peer layers communicate (through the lower
  layers of course!) by exchanging
  PDU data blocks
• At source Each layer adds its relevant control
  information (header) to the data it receives
  from a higher layer, thus forming its PDU, and
  pushes it down to the lower layer
  Encapsulation
• At destination that PDU is handled by the
  corresponding peer layer The relevant control
  header is removed, used, and the remaining part
  of the PDU is pushed up to the higher layer
  Decapsulation
• Example
• Transport layer may fragment user data into
  smaller packets
• A transport header is appended to each packet,
  which would include
• Destination SAP
• Sequence number of the data fragment
• Possibly, error detection/correction code
• This makes the transport layer protocol data unit
  (PDU)

Example The mail service
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Protocol Data Units (PDUs)
Application Layer
User Data
Transport Layer
What to include in the header?

• Address of destination application on the
  destination computer
• Packet sequence
• Error detection code?

Network Access Layer

• Address of destination computer on the network
• Network facilities required, e.g. delay

Network
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Generating PDUs at Source layers

• Layer L generates its PDU by appending a control
  part to data received from the next higher layer
  (L1)
• Appended control part takes the form of a header
• That header will be used by the peer layer L on
  the destination entity
• Both the new PDU and header are labeled
  (identified) by the layer generating/using them,
  i.e.
• Header (L) PDU (L1) ? PDU (L)

. . .
Lower Layers
L1
L
. . .
Source End System
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Operation of a Protocol Architecture
User Data
Destination SAP
TFER (record, DSAP, Dhost)
Destination Host
PDU Decapsulation
PDU Encapsulation
(Transport PDU)
TFER (transport_PDU, Dhost)
Network
(Network Access PDU)
Concept of overhead
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Standardizing Vertical Communication Service
Primitives and Parameters

• Services between vertically adjacent layers
  within the same system are expressed in terms of
  primitives and parameters
• Primitives specify the action to be performed
• Parameters pass data and control information
  required to perform the action
• Services are requested by a service user layer
• and performed by a service provider layer

TFER (record, DSAP, Dhost)
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Types of Primitives
Source (X)
Destination (Y)
D Service User
A Service User
2
3
1
4
C Service Provider
B Service Provider

REQUEST A primitive issued by a service user to invoke some service and to pass the parameters needed to specify fully the requested service
INDICATION A primitive issued by a service provider, either to indicate that a procedure has been invoked by the peer service user on the connection and to provide the associated parameters, or Notify the service user of a provider-initiated action
RESPONSE A primitive issued by a service user to acknowledge or complete some procedure previously invoked by an indication to that user
CONFIRM A primitive issued by a service provider to acknowledge or complete some procedure previously invoked by a request by the service user
Time Sequence
A to B Deliver this packet to Y
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C to D Here is a packet for you!
2

D to C Thanks!
3
B to A Done it!
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Which pair of primitives is more important?
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Timing Sequence for Service Primitives
Destination System
Destination System
Source System
Source System
Provider
Provider
Provider
Provider
A
D
B
C
No Response or Confirm (Assumed Done!) Saves time
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Standardized Protocol Architectures

• Required to allow devices from different
  manufacturers to communicate (inter-operability)
• A win-win environment
• Helps different vendors market their products
  better
• Helps customers shop around for best deals from
  different manufacturers (assured that equipment
  will work together)
• Main standard networking architectures
• OSI Reference model (X.200, 1977)
• A theoretical system delivered too late! Never
  lived up to early promises
• TCP/IP protocol suite (Developed for early forms
  of the internet)
• (The Internet protocol) Most widely used A de
  facto Standard

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OSI - The Model

• Developed by the International Standardization
  Organization (ISO) in 1977
• A 7-layer model
• Each layer
• performs a subset of the required communication
  functions
• relies on the next lower layer to perform more
  primitive functions
• provides services to the next higher layer
• Changes in one layer should not require changes
  in other layers

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The OSI Seven Layers
Internetworking (network of networks) (Internet)
End to End Virtual Connection
The Physical Medium
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The OSI Environment
Encapsulation
Decapsulation
Hardware or software
Trailer Marking end of frame
Note information hiding! e.g. DL layer does not
look inside the N-DPU
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OSI Layers with examples (1)

• Physical Example 10Base-T for Ethernet LAN
• Physical interface between devices
• Mechanical Connectors, etc.
• Electrical Bit representation, V and I levels,
  Data rates,...
• Functional Functions of individual interface
  circuits
• Procedural Sequence of events for exchanging bit
  streams
• Data Link Example HDLC (High level data link
  control) (Ch 7)
• Activating, maintaining and deactivating the data
  link (link management)
• Reliable data transfer across the link
  synchronization, error detection and control,
  flow control, so that higher layers may assume
  error free transmission over the link

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OSI Layers with examples (2)

• Network Example X.25 (Packet switching network)
• Data transportation over a network
• Relieves higher layers from worrying about
  underlying network details (routing, switching
  technology, etc)
• Only needs info on destination host address and
  required facilities
• Not needed (bypassed) on direct (point-to-point)
  connections
• Transport Example TCP (Transmission Control
  Protocol)
• Reliable exchange of data between end systems
• (possibly across multiple networks!)
• Without errors
• Without loss
• Without duplication
• Allows different levels of Quality of Service
  (QoS) Regarding acceptable error rates, maximum
  delay, priority, and security

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OSI Layers (3)

• Session
• Control of dialogues between applications
  running on end systems
• Dialogue disciplines, e.g. full duplex or half
  duplex
• Grouping Marking of data to indicate different
  groups
• Recovery Use of data check points
• Presentation
• Data formats and coding
• Data compression
• Data Encryption
• Application
• Allows user applications running on a host to
  access the OSI network environment
• Support for e-mail, file transfer, terminal
  access to remote computers (telnet), etc.

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Do we always need all the seven protocol layers?

• Only hosts at end points need to have all 7
  layers
• Point-to-Point on a single link
• - Network layer is bypassed
• Intermediate nodes in network are not interested
  in user data!-
• - Will have only the minimum number of required
  lower layers
• Within the same network An intermediate node
  needs only the bottom 3 layers (3-layer switch)
• Between multiple networks A node linking two
  networks (router) needs only the bottom layers

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Layers for a Relay node in a network
A 3-Layer node
(Network node)
?

• Relay Node, Joins
• Different physical links
• Different data links
• In the same network

What about a router that connects two networks?
(Intermediate Network node is not interested in
the data content- so, only the lower layers)
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TCP/IP Protocol Architecture

• Developed by the US Defense Advanced Research
  Project Agency (DARPA) for its packet switched
  network ARPANET (ancestor of the present
  Internet, 1966)
• Used today by the Internet
• Did not start as a formal model.. but as a
  working one
• Five Layers
• Application layer
• Host to host (end to end) Transport layer
• Internet layer (multiple interconnected networks)
• Network access layer (Data link/Network)
• Physical layer

Same layer exists also in OSI
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OSI vs. TCP/IP
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Summary (Chapter 2)

• Need for protocols and for a layered protocol
  architecture
• Key Elements of a Protocol
• Framework for Standardization
• Simplified File Transfer Architecture
• A Three-Layer Model
• Protocol Architectures and Networks Addressing
• Primitives and Parameters
• Protocol operation PDUs
• Standard Protocol Architectures
• The OSI Protocol Architecture
• The Model
• OSI Layers
• The OSI versus the TCP/IP (the Internet) Protocol
  Architecture)