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Types of Applications

Direct Network Applications Most applications that work in a networked

environment are classified as client-server applications. These applications, such as

FTP, web browsers, and e-mail, all have twocomponents, which allow them to function -the client side, and the server side. The clientside is located on the local computer and is

the requestor of the services. The server sideis located on a remote computer and providesservices in response to the client's requests.

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Direct Network Applications(cont.)

A client-server application works by constantly repeating the followinglooped routine: client-request, server-response; client-request, server-response; etc. For example, a web browser accesses a web page byrequesting a uniform resource locator (URL), or web address, on a

remote web server. After it locates the URL, the web server that isidentified by that URL responds to the request. Then, based on theinformation received from the web server, the client can request moreinformation from the same web server, or can access another webpage from a different web server.

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Types of Applications

Indirect Network Applications Within a LAN environment, indirect-application

network support is a client-server function. If aclient wants to save a file from a word processor to a network server, the redirector enables theword processing application to become a networkclient.

Redirector is a protocol that works with computer operating systems and network clients instead of specific application programs

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Indirect Network Applications(cont.)

Examples of redirectors Apple File Protocol NetBIOS Extended User Interface (NetBEUI)

Novell IPX/SPX Protocols Network File System (NFS) of the TCP/IP Protocol

Suite

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Indirect Network Applications(cont.)

The requester software is part of the network client software that isinstalled on the client computer of a network. The requester (redirector) part of the software works with client computersoperating system to determine if the operation that is to be performedneeds to use network resources or local computer resources.

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Indirect Network Applications(cont.)

A redirector (requester) will allow a local companyto use network storage devices as if they werelocally attached. This is done through the use of drive mappings.

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Presentation Layer (Layer 6)

The presentation layer is responsible for presenting data in a formthat the receiving device can understand. To better understand theconcept, use the analogy of two people speaking differentlanguages. The only way for them to understand each other is tohave another person translate. The presentation layer serves as thetranslator for devices that need to communicate over a network

Insures data is readable by receving system Deals with format of the data and data structures Negotiates data transfer syntax for application layer

Examples JPEG, ASCII, EBCDIC,TIFF, GIF, PICT, encryption, MPEG, MIDI, etc.

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Presentation Layer

Three Main Functions Data Formatting (Presentation) Data Encryption

Data Compression

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Data Formatting ASCII - Interchange EBCDIC - Extended Binary Coded

Decimal Interchange CodePICT - a picture format used totransfer QuickDraw graphics betweenprograms on the MAC operatingsystemTIFF (Tagged Image File Format) - aformat for high-resolution, bit-mappedimagesJPEG (Joint Photographic ExpertsGroup) - graphic format used mostoften to compress still images of complex pictures and photographsMIDI (Musical Instrument DigitalInterface) - for digitized musicMPEG (Motion Picture Experts Group)- standard for the compression andcoding of motion video for CDs anddigital storageQuickTime - a standard that handlesaudio and video for programs on bothMAC and PC operating system

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Session Layer (Layer 5) The session layer defines how to start, control, and end

conversations (called sessions). This includes the control andmanagement of multiple bidirectional messages so that theapplication can be notified if only some of a series of messagesare completed. This allows the presentation layer to have aseamless view of an incoming stream of data. The presentation

layer can be presented with data if all flows occur in somecases. For example, an automated teller machine transactionin which you withdraw cash from your checking account shouldnot debit your account, and then fail, before handing you thecash, recording the transaction even though you did notreceive money. The session layer creates ways to imply whichflows are part of the same session and which flows must

complete before any are considered complete. RPC, SQL, NFS, NetBIOS names, AppleTalk ASP, DECnet SCP

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Session Layer (cont.)

Accounting, conversation control who can talk when, and session parameter negotiation.

Dialogue control and seperation enable applications to communicate between the source and

destination

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Session Layer (cont.)Dialogue Control

Two-way alternate communication Communication partners take turns while sending messages to

avoid interrupting each other. For example; Internet Relay Chat (IRC)

Two-way simultaneous communication Communication partners send each other whatever they want

without waiting turns. Synchronization Problem

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Session Layer (cont.)Dialogue Seperation

Dialogue separation is the orderly initiation,termination, and managing of communication.The main graphic illustrates a minor synchronization. At the "Time Axis,

t = checkpoint", the host A session layer sends a synchronization message to host B, atwhich time both hosts perform the followingroutine:

1. back up the particular files2. save the network settings3. save the clock settings4. make note of the end point in the

conversation

A major synchronization would involve more back-and-forth steps and conversation than is shown in thisdiagram.

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Session Layer (cont.)Dialogue Seperation

Checkpointing is similar tothe way a word processor on a stand-alone computer pauses for a second when itperforms an AutoSave of the current document.However, these checkpointsare used, instead, toseparate parts of a sessionpreviously referred to asdialogues.

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Session Layer Protocols

Network File System (NFS) Structured Query Language (SQL) Remote Procedure Call (RPC)

X-Window System AppleTalk Session Protocol (ASP) Digital Network Architecture Session Control

Protocol (DNA SCP)

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The Transport Layer

Layer 4 of the OSI reference model. This layer isresponsible for reliable network communicationbetween end nodes. The transport layer providesmechanisms for the establishment, maintenance,

and termination of virtual circuits, transport faultdetection and recovery, and information flowcontrol.

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TCP/IP Protocol Suite has twoTransport Layer Protocols (TCP & UDP)

TCP (Transmission Control Protocol) supplies a virtual circuitbetween end-user applications. These are its characteristics: connection-oriented reliable divides outgoing messages into segments reassembles messages at the destination station re-sends anything not received reassembles messages from incoming segments. flow control

UDP (User Datagram Protocol) transports data unreliably betweenhosts. Following are the characteristics of UDP : connectionless unreliable transmit messages (called user datagrams) provides no software checking for message delivery (unreliable) does not reassemble incoming messages uses no acknowledgments provides no flow control

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TCP/IP Protocol Graph

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TCP Segment Format

source port -- number of the callingport

destination port -- number of thecalled port

sequence number -- number usedto ensure correct sequencing of the arriving data

acknowledgment number - nextexpected TCP octet

HLEN -- number of 32-bit words inthe header

reserved -- set to zerocode bits -- control functions (such

as setup and termination of asession)

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TCP Segment Format (cont.)

window specifies the size of the sender s receivewindow (that is, the buffer space available for incoming data)

checksum -- calculatedchecksum of the header and data fields

urgent pointer -- indicatesthe end of the urgent data

option-one option --maximum TCP segment

sizedata -- upper-layer protocol

data

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UDP Datagram Format User Datagram Protocol (UDP) is the connectionless transport

protocol in the TCP/IP protocol stack. UDP is a simple protocol thatexchanges datagrams, without acknowledgments or guaranteeddelivery. Error processing and retransmission must be handled byother protocols.

UDP uses no windowing or acknowledgments, therefore applicationlayer protocols provide reliability. UDP is designed for applicationsthat do not need to put sequences of segments together.

Protocols that use UDP include: TFTP (Trivial File Transfer Protocol) SNMP (Simple Network Management Protocol) DHCP (Dynamic Host Control Protocol)

DNS (Domain Name System)

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Port Numbers

Both TCP and UDP use port (or socket) numbers to pass information to the upper layers. Port numbers areused to keep track of different conversations thatcross the network at the same time.

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Port Numbers (cont.)

Application software developers have agreed to usethe well-known port numbers that are defined in

RFC1700. Windows 2000 c:\winnt\system32\drivers\etc Linux/Unix /etc/services

Reserved Port Numbers Numbers 0 - 255 for public applications Numbers from 256-1023 assigned to companies

for marketable applications Numbers above 1024 -65536 are unregulated

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TCP Mechanisms

Data transfer Multiplexing Error recovery (reliability)

Flow control using windowing Connection establishment and termination

Exam Certification Guide Page 217

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Ordered Data Transfer

TCP accomplishes data transfer by establishing aconnection between a socket on each of theendpoint computers.

Applications use TCP services by opening asocket; TCP manages the delivery of the data tothe other socket.

A socket source/destination pair uniquelyidentifies a relationship between two applicationsin a network. TCP manages the ordered transferof data between these two sockets, using IPservices to deliver the data.

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Multiplexing

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Error Recovery Reliability

TCP provides sequencing of segments with a forward referenceacknowledgment. Each datagram is numbered before transmission. At thereceiving station, TCP reassembles the segments into a completemessage. If a sequence number is missing in the series, that segment isre-transmitted. Segments that are not acknowledged within a given time

period result in re-transmission Positive acknowledgment and retransmission, or PAR, is a common

technique many protocols use to provide reliability. With PAR, the sourcesends a packet, starts a timer, and waits for an acknowledgment beforesending the next packet. If the timer expires before the source receives anacknowledgment, the source retransmits the packet and starts the timer over again.

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Flow control using windowing Window size determines the amount of data

that you can transmit at one time beforereceiving an acknowledgment from thedestination. The larger the window sizenumber (bytes), the greater the amount of data that the host can transmit. After a hosttransmits the window-sized number of bytes,the host must receive an acknowledgmentthat the data has been received before it cansend any more messages. For example, witha window size of 1, each individual (1)segment must be acknowledged before youcan send the next segment.

TCP uses expectational acknowledgments ,meaning that the acknowledgment number

refers to the octet that is next expected. The"sliding" part, of sliding window , refers to thefact that the window size is negotiateddynamically during the TCP session. Thisresults in inefficient use of bandwidth by thehosts.

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Connection Establishment andTermination

Three-Way Handshake / Connection EstablishmentSYN & ACK Flags

Two single-bit portions of the flags field of the TCP header areused to signal the three-step process for connectionestablishment. Called the SYN and ACK flags, these bits have aparticularly interesting meaning. SYN means, Synchronize thesequence numbers, which is one necessary component in

initialization for TCP. The ACK field means the acknowledgmentfield is valid in this header.

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Connection Establishment andTermination

This four-way termination sequence is straightforward and usesan additional flag, called the FIN bit. (FIN is short for finished,as you might guess.) One interesting note: Before the devicereceiving the first FIN segment sends the third flow in thesequence, TCP notifies the application that the connection iscoming down. TCP waits on an acknowledgment from theapplication before sending the segment. Thats why the secondflow is required: To acknowledge the first so that the sidetaking down the connection doesnt start resending the first

TCP segment.

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The Domain Name System (DNS)

Internet is built on a hierarchicaladdressing scheme. All protocols uses 32-bit IP addresses for networkcommunication. They are difficult to remember Do not provide a functional logical grouping

Domain Name System Host name IP Address Mapping

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The Domain Name System(cont.)

The Domain Name System (DNS) is a distributeddatabase used by TCP/IP applications to map betweenhostnames and IP addresses

Each site maintains its own database in its DomainName Server or Internet Service Providers provides thisservice to their customers.

DNS Servers also resolves hostnames to IP addressesupon a client request (resolver function)

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Fully Qualified Domain Name (FQDN)

When using the Domain Name System, it is common to work with onlya part of the domain hierarchy, for example, the ral.ibm.com domain.The Domain Name System provides a simple method of minimizing thetyping necessary in this circumstance. If a domain name ends in a dot(for example, www.btegitim.com. ), it is assumed to be complete. This is

termed a fully qualified domain name (FQDN) or an absolutedomain name . However, if it does not end in a dot (for example,www.btegitim), it is incomplete and the DNS resolver may completethis, for example, by appending a suffix such as .com or .com.tr to thedomain name. The rules for doing this are implementation-dependentand locally configurable.

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Hierarchical Name Space

TLDs (Top Level Domains)

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Root Servers

If the name server doesn't contain the information requested, it mustcontact another name server. Not every server, however, knows howto contact every other server. Instead, every name server must knowhow to contact the root name servers. The root servers then know thename and location (i.e. IP address) of each authoritative name servers

for all the second-level domains. There are six root servers in theworld and every primary name server has to know the address of oneof root server.

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Resource Record Types / Query Types