02 Protocol Architecture Chapter 2

Chapter 2 Protocols and Architecture

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Need For Protocol Architecture • E.g. File transfer – Source must activate comms. Path or inform network of destination – Source must check destination is prepared to receive – File transfer application on source must check destination file management system will accept and store file for his user – May need file format translation

• • • •

Task broken into subtasks Implemented separately in layers in stack Functions needed in both systems Peer layers communicate

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Key Elements of a Protocol • Syntax – Data formats – Signal levels

• Semantics – Control information – Error handling

• Timing – Speed matching – Sequencing

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Protocol Architecture • Task of communication broken up into modules • For example file transfer could use three modules – File transfer application – Communication service module – Network access module

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Simplified File Transfer Architecture

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A Three Layer Model • Network Access Layer • Transport Layer • Application Layer

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Contd… Network Access Layer • Exchange of data between the computer and the network • Sending computer provides address of destination • May invoke levels of service • Dependent on type of network used (LAN, packet switched etc.) 7

Contd… • • • •

Transport Layer : Reliable data exchange Independent of network being used Independent of application

• Application Layer : • Support for different user applications • e.g. e-mail, file transfer 8

Protocol Architectures and Networks

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Addressing Requirements • Two levels of addressing required • Each computer needs unique network address • Each application on a (multi-tasking) computer needs a unique address within the computer – The service access point or SAP – The port on TCP/IP stacks 10

Protocols in Simplified Architecture

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• • • • •

Protocol Data Units (PDU) PDU = actual data + control information At each layer, protocols are used to communicate Control information is added to user data at each layer Transport layer may fragment user data Each fragment has a transport header added – Destination SAP – Sequence number – Error detection code

• This gives a transport protocol data unit

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Protocol Data Units

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Network PDU • Adds network header – network address for destination computer – Facilities requests

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Operation of a Protocol Architecture

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Standardized Protocol Architectures • Required for devices to communicate • Vendors have more marketable products • Customers can insist on standards based equipment • Two standards: – OSI Reference model • Never lived up to early promises

– TCP/IP protocol suite • Most widely used

• Also: IBM Systems Network Architecture (SNA)

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OSI • Open Systems Interconnection • Developed by the International Organization for Standardization (ISO) • Seven layers • A theoretical system delivered too late! • TCP/IP is the de facto standard

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OSI - The Model • A layer model • Each layer performs a subset of the required communication functions • Each layer relies on the next lower layer to perform more primitive functions • Each layer provides services to the next higher layer • Changes in one layer should not require changes in other layers 18

OSI Layers

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The OSI Environment

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OSI as Framework for Standardization

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Layer Specific Standards

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Elements of Standardization • Protocol specification – Operates between the same layer on two systems – May involve different operating system – Protocol specification must be precise • Format of data units • Semantics of all fields • allowable sequence of PCUs

• Service definition – Functional description of what is provided

• Addressing – Referenced by SAPs

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Service Primitives and Parameters • Services between adjacent layers expressed in terms of primitives and parameters • Primitives specify function to be performed • Parameters pass data and control info

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Primitive Types REQUEST

INDICATION

RESPONSE

A primitive issued by a service user to invoke some service and to pass the parameters needed to specify fully the requested service 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 providerinitiated action A primitive issued by a service user to 25 acknowledge or complete some procedure

Timing Sequence for Service Primitives

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OSI Layers (1) • Physical – Physical interface between devices • • • •

Mechanical Electrical Functional Procedural

• Data Link – Means of activating, maintaining and deactivating a reliable link – Error detection and control – Higher layers may assume error free

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Physical Layer

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Data Link Layer

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Data Link Layer Example

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OSI Layers (2) • Network – Transport of information – Higher layers do not need to know about underlying technology – Not needed on direct links

• Transport – – – – – –

Exchange of data between end systems Error free In sequence No losses No duplicates

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Network Layer

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Network Layer Example

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Network Layer Example

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Transport Layer

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Transport Layer Example

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Transport Layer Example

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OSI Layers (3) • Session – Control of dialogues between applications – Dialogue discipline – Grouping – Recovery

• Presentation – Data formats and coding – Data compression – Encryption

• Application –

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Session Layer

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Presentation Layer

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Application Layer

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Summary of Layer Functions

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TCP/IP Layers • 2. 3. 4. 5. 6.

5 independent layers Physical Network access Internet Transport / host-host Application

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1.Physical layer • Covers the interface b/w a data trans.device & Trans.medium or a nw • Is concerned with specifying the characteristics of the trans.medium, nature of the signals(what encoding tech is used),data rate(duration of a bit), topology(star,ring,mesh,bus) of the nw & related matters(point-point link/multipoint links) 44

Contd…  Computer network connects two or more autonomous computers.

 The computers can be geographically located anywhere. 45

Introduction to Computer Networks

Contd…  The network topology defines the way in which computers, printers, and other devices are connected. A network topology describes the layout of the wire and devices as well as the paths used by data transmissions.

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Introduction to Computer Networks

Bus Topology Commonly referred to as a linear bus, all the devices on a bus topology are connected by one single cable.

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Introduction to Computer Networks

Star & Tree Topology

The star topology is the most commonly used architecture in Ethernet LANs. When installed, the star topology resembles spokes in a bicycle wheel. Larger networks use the extended star topology also called tree topology. When used with network devices that filter frames or packets, like bridges, switches, and routers, this topology significantly reduces the traffic on the wires by sending packets only to the wires of the destination host.

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Ring Topology A frame travels around the ring, stopping at each node. If a node wants to transmit data, it adds the data as well as the destination address to the frame. The frame then continues around the ring until it finds the destination node, which takes the data out of the frame. Single ring – All the devices on the network share a single cable Dual ring – The dual ring topology allows data to be sent in both directions.

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Mesh Topology The mesh topology connects all devices (nodes) to each other for redundancy and fault tolerance. It is used in WANs to interconnect LANs and for mission critical networks like those used by banks and financial institutions. Implementing the mesh topology is expensive and difficult.

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Applications  E-mail  Searchable Data (Web Sites)  E-Commerce  News Groups  Internet Telephony (VoIP)  Video Conferencing  Chat Groups  Instant Messengers  Internet Radio

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2.Network access layer • Is concerned with the xchange of data b/w an endsystem(server,workstation) & the nw to which it is attached • Sender must invoke certain services like priority & other switching technologies • s/w at this layer depends upon the nw used • For Lans (ethernet), & for circuit,packet switching(frame relay) is used • Is concerned with access to & routing data across a nw for 2 endsystems attached to the same nw 52

3.Internet layer • When 2 devices are attached to diff nws, procedures are needed to allow data to traverse across multiple interconnected nws • This is the function of Internet layer • IP(internet protocol) is used at this layer to provide the routing func across multiple nw • This protocol is not only implemented in end systems but also in routers • A router is a processor that connects 2 nws & its primary func is to relay data from one nw to the other on its route from source to dest 53

4.Transport layer • Collect the mechanisms into a common layer shared by all applications • Reliable trans.of the data

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5.Application layer • Contains the logic needed to support various user appns • FTP,mail transfer etc

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OSI vs TCP/IP

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OSI v TCP/IP

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TCP • Usual transport layer is Transmission Control Protocol – Reliable connection to transfer the data b/w appns

• Connection – Is simply a temporary logical association between 2 entities in different systems

• TCP PDU – Called TCP segment – Includes source and destination port (c.f. SAP) • Identify respective users (applications) • Connection refers to pair of ports

• TCP tracks segments between entities on each connection 58

UDP • Alternative to TCP is User Datagram Protocol • Not guaranteed delivery • No preservation of sequence • No protection against duplication • Minimum overhead-enables a proc to send msgs to other proc with a min.of protocol mechanism • Adds a port addressing capability to IP • Ex: SNMP(simple nw management protocol) 59

Operation of TCP & IP • Total comm.system may consist of multiple nws , the constituent nw are subnetworks • Ethernet can be used to connect a computer to a subnetwork which enables the host to send data across the subnetwork, to a router that will forward the data • IP is implemented in all endsystems & routers that acts as a relay to move a block of data from one host thru 1/m routers to another host • TCP is also implemented in endsystems,it keeps track of the blocks to assure that all are delivered reliably to the appropriate appn 60

TCP/IP Concepts

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Addressing level • Level in architecture at which entity is named • Unique address for each end system (computer) and router • Network level address – IP or internet address (TCP/IP) – Network service access point or NSAP (OSI)

• Process within the system – Port number (TCP/IP) – Service access point or SAP (OSI) 62

Trace of Simple Operation • Process associated with port 1 in host A sends message to port 2 in host B • Process at A hands down message to TCP to send to port 2 • TCP hands down to IP to send to host B • IP hands down to network layer (e.g. Ethernet) to send to router J • Generates a set of encapsulated PDUs 63

PDUs in TCP/IP

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Example Header Information • Destination port • Sequence number • Checksum-sending TCP segment includes a code that is a func of the contents of the remainder of the segment. The receiver TCP performs the same calculation & compares the result with the incoming code 65

Some Protocols in TCP/IP Suite

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TCP/IP applications • SMTP(Simple mail transfer protocol) • FTP • TELNET

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