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Protocols and Reference Models

Babu Ram dawadi

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Protocol Hierarchies

• A protocol is an agreement that standardizes the way communication will be handled.

• Differing from the standard protocol may result in the other party not understanding the communication.

• Networks are organized as a stack of levels or layers.• We build each layer on the one below it.• Layers differ in number and function from one network to

another.• Each layer hides underlying details from the one above it

– sort of like a virtual machine.• Each layer talks to the ones above & below it

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Example Hierarchy

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Data Transfer• No data is transferred directly from one machine

to another on that layer – the layers can only talk to the ones above or below them on their host.

• A message from layer 5 will have to travel to layer 1, move across the physical medium, and then back up to layer 5 on the different machine.

• Layer 1 is the only layer able to move data from one machine to another, through the physical medium.

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Network Architecture

• A set of layers and protocols is called a network architecture.

• The architecture contains enough information for a programmer to be able to write code to interface with the protocol at any layer.

• A list of protocols used by a system (with one protocol per layer) is called a protocol stack.

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Abstraction

• The process of abstracting the work done at each layer is important to networking.

• This allows us to break up a big problem (networking) and reduce it to smaller problems that can more easily be solved (the design of the individual layers).

• Many of these layers are now implemented in hardware or firmware.

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Layer Design Issues• Some form of addressing needed.

– which machine should get the data?

• Some form of error control needed.– electronics are not perfect….

• Some form of flow control needed.– for when the order of the messages becomes mixed up.

• What happens when we want to send multiple unrelated packets to the same address?– we multiplex and demultiplex the messages

• How do we determine that which path data should follow?– we use routing algorithms.

Interfaces & Services

• The function of each layer is to provide services to its upper layer.

• Active elements on each layer are called entities.

• The entities in layer n implement a service used by layer n+1 where layer n is called service provider and layer n+1 is called service user

• At a typical interface, the layer n+1 entity passes an IDU to the layer n through SAP.

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Interfaces and services (contd’)• IDU=SDU+control information• The SDU is passed across the network to the

peer entity and then upto layer n+1• Layer n fragment SDU into small pieces and add

header and send as separate PDU (packet)

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ICIICI SDUSDU

ICIICI SDUSDU

IDUIDU

SAPSAP

hdrhdr PDUPDULayer nLayer n

Interface Interface

Layer n+1Layer n+1

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Connection Types• Connection-Oriented

service– like the phone system– a “live” connection must be

established– data is sent in “real time”

and received by the other end

– the other end will respond back as data is received

– Example: a telnet connection

• Connectionless Service – like the postal system– data is packaged and sent

to the destination– no time limit is placed on the

transport of the data– each packet of data is

determined to be a single message in and of itself with no relation to other messages

– Example: DNS

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Quality of Service

• We may need to know if the data arrived okay.

• There are two types of reliable connection-oriented service:– message sequencing (a series of messages)– byte streams (a long stream of data)

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QoS (cont’d)

• For the most part, connectionless protocols do not worry about QoS.

• We do not get a response when they are received because they are connectionless.

• We can ask for an acknowledgement if so desired.

• We can also just wait for a reply to the initial message.

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Protocol Examples

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Reference Models

• We will look at two important network architectures, the OSI Reference Model and the TCP/IP model.

• The OSI reference model will be used as the basis for the rest of the book (and this class)

• The OSI Reference Model– The ISO have divided up these issues over a layered hierarchy

of 7 levels called the Reference Model for Open Systems Interconnection (OSI/RM).

– The OSI model is not a network architecture as it does not specify the exact services and protocols to be used by each layer. It simply says what each layer should do.

– The OSI model deals with connecting open systems i.e. systems that are open for communication to other systems.

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The OSI Model

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Ideas behind the model

• Layers should be created where abstraction is needed.

• Each layer should perform a well-defined function.

• The function of the layer should be chosen while defining protocols.

• The layer boundaries should be chosen to minimize the information flow across the interfaces.

• There should not be too many or too few layers.

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The Physical Layer• Converts bits into signals for outgoing messages

& signals into bits for incoming messages

• Issues are mainly mechanical, electrical and timing interfaces, and the physical medium itself.

• Transmits bits. (bitstream)

10 Base 2 – Co Ax 10 Base 5 – Thicknet Fibre Optic Connectors UTP

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Layer 2: Data Link• Defines psychical addressing, network topology, and is

also concerned with error notification, sequencing of frames and flow control.

• Deals with transforming a raw line into a line that appears free of errors to the network layer.

• This layer deals with breaking up the data, sending it and then receiving acknowledgements back.

• This layer also deals with different hardware speeds by slowing some transmissions if the other end can’t keep up.

Layer 2: Data Link

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• Examples of Data Link WAN specifications are:

• Frame Relay (operates also on the Physical layer)

• PPP (operates also on the Physical layer)

• X.25 (operates also on the Physical and Network layer)

• Transmits Frames.

• Bridges and Switches operate at this layer.

• The Data Link layer consists of two sublayers:

• LCC (Logical Link Control) sublayer

• Manages communication between devices over a single link of a network.

• Enables multiple higher-layer protocols to share a single physical data link.

• MAC sublayer

• Manages protocol access to the physical network medium.

• Determines hardware addresses.

Layer 3 – Network Layer• Defines logical addressing for nodes and networks/segments.

• Enables internetworking, passing data from one network to another.

• Defines the logical network layout so routers can determine how to forward packets trough an internetwork.

• Routing occurs at this layer, hence Routed and Routing protocols reside on this layer.

• Routed protocols are used to encapsulate data into packets. The header added by the Network layer contains a network address so it can be routed trough an internetwork.

• Examples of Network layer Routed protocols are:

• IP, IPX, AppleTalk

• Routing protocols are used to create routing tables; routing tables are used to determine the best path / route. Routing protocols provide periodic communication between routers in an internetwork to maintain information on network links in a routing table.

• Examples of Network layer Routing protocols are:

• OSPF, IGRP/EIGRP, RIP, BGP.

• Transmits Packets.

• Routers operate at this layer.

Layer 4 – Transport Layer• The main purpose of this layers is making sure that the data is delivered error-free and in the correct sequence.

• Establishes, maintains and terminates virtual circuits.

• Provides error detection and recovery.

• Is concerned with reliable and unreliable transport. When using a connection-oriented, reliable transport protocol, such as TCP, acknowledgments are send back to the sender to confirm that the data has been received.

• Provides Flow Control and Windowing.

• Provides multiplexing; the support of different flows of data to different applications on the same host.

• Examples of Transport layer protocols are:

• TCP (connection-oriented, reliable, provides guaranteed delivery.)

• UDP (connectionless, unreliable, less overhead, reliability can be provided by the Application layer)

• SPX

•Transmits Segments.

Layer 5 – Session Layer

• The session layer establishes, manages, maintains and terminates communication channels between software programs on network nodes. • Provides error reporting for the Application and Presentation layer. • Examples of Session layer protocols are:

• NFS • SQL • RPC

•Transmits Data.

Layer 6 – Presentation Layer

• Defines coding and conversion functions. • Ensures that information sent from the application layer of one system is readable by the application layer of another system. • Includes common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes, common examples of these formats and schemes are:

• MPEG, QuickTime • ASCII, EBCDIC • GIF, TIFF, JPEG

• Transmits Data.

Layer 7 – Application Layer

• Provides network services directly to applications. Software programs itself are not part of the OSI model. • Determines the identity and availability of communication partners, and determines if sufficient resources are available to start program-to-program communication. • This layer is closest to the user. • Examples of Application layer protocols are:

• Telnet • SMTP • FTP • SNMP

• Transmits Data.

Data Encapsulation

• Data Encapsulation is the process of adding a header to wrap the data that flows down the OSI model.

• Each OSI layer may add it's own header to the data received from above. (from the layer above or from the software program 'above' the Application layer.)

• The 5 Steps of Data Encapsulation are:

1. The Application, Presentation and Session layers create DATA from users‘ input.

2. The Transport layer converts the DATA to SEGMENTS

3. The Network layer converts the SEGMENTS to PACKETS (or datagrams)

4. The Data Link layer converts the PACKETS to FRAMES

5. The Physical layer converts the FRAMES to BITS.

• At the sending computer the information goes from top to bottom while each layers divides the information received from upper layers in to smaller pieces and adds a header. At the receiving computer the information flows up the model discarding the corresponding header at each layer and putting the pieces back together.

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Data encapsulation

Each layer contains a Protocol Data Unit (PDU). PDU’s are used for peer-to-peer conversations.

Why Layered Model?

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Reduces Complexity

Standardzes Interface

Facilitates Modular Engineering

Ensures Interoperable Tech.

Accelerates Evolution

Simplifies teaching and learning