TCP/IP Analysis and Troubleshooting Toolkit
Contents
1. t quite that simple but the concept is basically the same Different digital signaling methods create ones and zeros on the media Now with the ability to have only two kinds of signals to recognize it is much easier for amplifiers to pick out the digital ones and zeros from the background noise With this ability to tell signals apart from noise it became much easier to build networks capable of carrying computerized binary data over long distances 10 Chapter 1 NOTE There are many types of digital signaling One of the factors that drives the type of digital signaling used in a specific technology is its efficiency and method of bit representation For example 10 Mb Ethernet uses what is called Manchester encoding a type of digital signaling but for 100 Mb Fast Ethernet Manchester was inefficient if not impossible to use because the cabling available at the time the late 1980s couldn t support its high bandwidth Instead Fast Ethernet uses what is called Non Return to Zero Inverted NRZI encoding and in certain configurations Multi Level Three MLT 3 Other data link technologies use different digital signaling methods Token Ring uses Differential Manchester and T1 circuits use AMI or B8ZS encoding Layer 2 The Data Link Layer So how do a bunch of ones and zeros become IP packets that traverse the net work For the network interface card NIC to put bits on the wire it first must have a method of accessing the media Th2. Display Notification Logging Packet Generator Configuring and Using Your Analyzer Capture Configuration Filtering Expert Analysis Measuring Performance Analysis Tips Placing Your Analyzers Using Proper Filters Troubleshooting from the Bottom Up Knowing Your Protocols Comparing Working Traces Analyzing after Each Change Summary Part Il The Core Protocols Chapter 3 Inside the Internet Protocol Reviewing Layer 2 Communications Multiplexing Error Control Addressing Case Study NetBEUI Communications Name Resolution Reliable Connection Setup NetBIOS Session Setup Application Process Limitations of Layer 2 Communication Networks Network Layer Protocols Internet Protocol Addressing IP Addressing Reserved Addressing Classful Addressing Classless Addressing IP Communications Address Resolution Protocol ARP ARP Packet Format Case Study Troubleshooting IP Communications with ARP and PING ARP Types ARP in IP Communication Case Study Incomplete ARP 32 33 33 35 36 37 37 42 42 44 45 45 45 45 48 52 56 61 61 62 63 63 63 65 65 67 69 70 70 71 71 72 73 74 75 75 76 77 79 81 85 85 88 92 93 94 97 100 101 101 Contents vii IP Routing 104 The Routing Table 104 Route Types 108 Router Routing Tables 110 The Forwarding Process 112 Case Study Local Routing 114 IP Packet Format 117 Version 117 Header Length 117 Type of Service 117 Datagram Length 119 Fragment ID 119 Fragmentation
3. Flags 119 Fragment Offset 119 Time to Live 120 Protocol 120 Header Checksum 121 Source IP Address 121 Destination IP Address 121 Options 121 Data 121 Case Study TTL Expiring 122 Case Study Local Routing Revisited 124 A Word about IP Version 6 126 The IPv6 Header 128 IPv6 Address Format 129 Other Changes to IPv6 130 Summary 130 Chapter 4 Internet Control Message Protocol 131 Reliability in Networks 132 Connection Oriented versus Connectionless Networks 132 Feedback 133 Exploring the Internet Control Messaging Protocol 134 ICMP Header 134 ICMP Types and Codes 135 ICMP Message Detail 137 Destination Unreachable Type 3 137 Diagnostic Messages 144 Redirect Codes type 5 146 Time Exceeded Type 11 151 Informational Messages 151 Network Diagnostics with ICMP 152 Summary 154 Chapter 5 User Datagram Protocol 155 Revisiting the Transport Layer 156 UDP Header 157 Source Port 157 Destination Port 157 UDP Length 158 UDP Checksum 158 Data 159 UDP Communication Process 160 viii Contents Case Studies in UDP Communications Name Resolution Services Routing Information Protocol Simple Network Management Protocol UDP and Firewalls Case Study Failed PCAnywhere Session Case Study NFS Failures Traceroute Caveats Summary Chapter 6 Transmission Control Protocol Introduction to TCP Requirements for a Reliable Transport Protocol Fast Sender and Slow Receiver Packet Loss Data Duplication Priority Data Out of Order Data The TCP H
4. Token Ring Data Link FDDI Frame Relay HDLC Physical X 21 RS 232 DS1 DS3 Figure 1 1 The OSI model Defining the Layers Because almost all protocols are based on the OSI model it is important to completely understand how the model operates and to understand the proto cols you must first understand the framework The following sections explain the seven layers in more detail and Figure 1 1 gives examples of protocols that reside at each layer Layer 1 Physical Layer The simplest definition of the physical layer is that it deals with how binary data is translated into signals and transmitted across the communications medium I talk more about media in the Detailed Layer Analysis section later in this chapter The physical layer also comprises the functions and pro cedures that are responsible for the transmission of bits Examples would be procedures such as RS 232 handshaking or zero substitution functions on B8ZS T1 circuits The physical layer concerns itself only with sending a stream of bits between two devices over a network Introduction to Protocol Analysis Layer 2 Data Link Layer Layer 2 the data link layer handles the functions and procedures necessary for coordinating frames between devices At the data link layer zeros and ones are logically grouped into frames with a defined beginning and end Unlike the physical layer the data link layer contains a measure of intelligence Ethernet a co
5. may not be available in electronic books Library of Congress Cataloging in Publication Data is available from the publisher ISBN 0 471 42975 9 Printed in the United States of America 100987654321 To my parents who always believed in me Acknowledgments About the Author Introduction Part I Chapter 1 Chapter 2 Foundations of Network Analysis Introduction to Protocol Analysis A Brief History of Network Communications OSI to the Rescue Defining the Layers Layer 1 Physical Layer Layer 2 Data Link Layer Layer 3 Network Layer Layer 4 Transport Layer Layer 5 Session Layer Layer 6 Presentation Layer Layer 7 Application Layer Protocol Analysis of the Layers Layer 1 The Physical Layer Layer 2 The Data Link Layer Layer 3 Network Layer Layer 4 Transport Layer Layer 5 Session Layer Layer 6 Presentation Layer Layer 7 Application Layer Putting It All Together History of TCP IP Summary Analysis Tools and Techniques Reviewing Network Management Tools Categorizing Network Management Tools by Function Fault Management Systems Performance Management and Simulation Contents xi xiii p SFCWADWAMDRANNNNDDATWW 3 NNNNNNNE COD RP WWE CO vuuunNn aR5ADON0D vi Contents Protocol Analyzers Application Specific Tools Classifying Tools by How They Perform Functions Protocol Analyzers Problem Solving Tools Why Protocol Analysis Protocol Analyzer Functions Data Capture Network Monitoring Data
6. the physical layer concerns itself with how communications signals are transmitted across a medium Appropriately a medium is defined as a path where communication signals can be carried A path is anything from copper water or air to even barbed wire if you can get the signals to successfully transmit over it Media carry communication Introduction to Protocol Analysis signals In wireless networks signals travel over air as RF radio frequency radio waves On 10BaseT Ethernet networks they are carried as electrical volt age In Fiber Distributed Data Interface FDDI networks glass is used as the medium the signals travel as pulses of light over glass fiber optic cables Many reasons exist as to why specific types of media are used in different tech nologies Theoretically you should be able to use whatever medium you want to carry the signals unfortunately the way those signals are represented places limitations on the types of media you can use Analog Signaling Communications signals are transmitted in two ways The first method ana log is used to transmit signals that have values that vary over time Sound is a perfect example of an analog signal Sound is measured as an analog signal in cycles per second or hertz The range of the human voice varies from about 100 Hz to 1 500 Hz When early telephone networks were developed it was difficult to create good quality long distance communications using analog signals because
7. About the Author Kevin Burns is the founder of Tracemasters Inc of Philadelphia Pennsyl vania a consulting organization specializing in network analysis and training Kevin s 10 years of experience consist of the design implementation and analysis of various multiprotocol multivendor networks This book comprises the techniques he has used in diagnosing complex network and application problems which he also teaches to students at various seminars and corporate settings Kevin can be reached at kburns tracemasters com Introduction Why I Wrote This Book Network engineers face difficult challenges on a daily basis Servers can crash WAN links can become saturated and for unknown reasons an application s performance can come to a crawl pitting network engineers against applica tion developers in a complicated blame game usually without facts Without the proper tools and training when something breaks network engineers often have to ask why Why can t users obtain DHCP addresses why can t users log into the server and the ever so bothersome question why is the network slow During all of this commotion upper management is usually also asking why Why haven t these problems been resolved Most large net work infrastructures have a mix of troubleshooting tools at their disposal but more often than not the wrong tools are selected for the wrong job How can you best use the tools at your disposal and the knowledge
8. TCP IP Analysis and Troubleshooting Toolkit Kevin Burns WILEY Wiley Publishing Inc TCP IP Analysis and Troubleshooting Toolkit Kevin Burns WILEY Wiley Publishing Inc Executive Publisher Robert Ipsen Vice President and Publisher Joe Wikert Editor Carol A Long Developmental Editor Kevin Kent Editorial Manager Kathryn Malm Production Editor Pamela M Hanley Text Design amp Composition Wiley Composition Services This book is printed on acid free paper Copyright 2003 by Kevin Burns All rights reserved Published by Wiley Publishing Inc Indianapolis Indiana Published simultaneously in Canada No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording scanning or otherwise except as permitted under Section 107 or 108 of the 1976 United States Copyright Act without either the prior written permission of the Publisher or authorization through payment of the appropriate per copy fee to the Copyright Clearance Center Inc 222 Rose wood Drive Danvers MA 01923 978 750 8400 fax 978 646 8700 Requests to the Pub lisher for permission should be addressed to the Legal Department Wiley Publishing Inc 10475 Crosspoint Blvd Indianapolis IN 46256 317 572 3447 fax 317 572 4447 E mail permcoordinator wiley com Limit of Liability Disclaimer of Warranty While the publisher and autho
9. ands SMB Responses SMB Operations Analysis Initial Connection File Transfer File Locking Interprocess Communication Named Pipes Mailslots DCE RPC Microsoft Applications NetLogon Browser Protocol Summary Appendix A What s on the Web Site System Requirements What s on the Web Site Standards and RFCs Author Created Materials Applications Using the Flash Video Examples Troubleshooting Appendix B BSMB Status Codes Index 289 290 291 293 294 298 299 299 300 302 306 308 311 312 312 315 316 317 319 320 321 326 326 329 330 332 336 338 339 344 351 352 353 353 354 359 359 362 368 369 369 370 370 371 371 371 372 373 399 Acknowledgments This book never would have been a reality without the following people Emily Roche who helped me open the door to writing and took me to my first book proposal seminar Toni Lopopolo who taught the seminar and put me in con tact with my great agent Jawahara Saidullah I want to thank Tony Fortunato for patiently reviewing my book for technical accuracy Thanks also goes out to everyone at Wiley Publishing who worked so hard on this book including my great development editor Kevin Kent who held me to task on making sure readers would be able to easily understand the complex case studies and examples in the book Last but not least I want to thank my parents who have given me everything and asked for nothing in return This book is for you xi
10. ayer Protocols Analyzing Upper Layer Protocols Chapter Goals Domain Name System DNS DNS Database DNS Message Format Using NSLookup Name Servers ROOT Name Servers Name Server Caching Resource Records Analyzing DNS IPCONFIG CyberKit DNS Expert Common DNS Configuration Mistakes File Transfer Protocol FTP FTP Commands and Responses Case Study Active Transfer Failure Case Study Passive Transfer Failure Case Study FTP Failures through Firewall Case Study Revisiting FTP Transfer Failures Hypertext Transport Protocol HTTP HTTP Requests HTTP Responses HTTP Headers and Messages Host Header Redirection Cookies Cache Control Headers 207 209 212 213 215 215 217 218 218 218 219 220 221 222 224 225 225 227 229 229 231 233 233 235 238 240 242 244 247 249 250 254 254 260 260 261 262 264 265 265 269 272 273 276 278 278 281 284 285 285 285 288 x Contents HTTP Proxies Measuring Proxy Latency Analyzing Advanced Web Architectures Case Study Web Site Failure Simple Mail Transport Protocol Summary Chapter 8 Microsoft Related Protocols Dynamic Host Configuration Protocol DHCP Header DHCP Process DHCP Messages DHCP Options DHCP Leases NetBIOS over TCP IP NetBIOS Names NetBIOS Services Datagram Service Session Service Name Service NetBIOS Operations Name Service Operations NetBIOS Datagram Operations NetBIOS Session Operations Server Message Block SMB Header SMB Comm
11. book which can be found by pointing your browser to www wiley com compbooks burns contains protocol standards such as RFCs Requests for Comment IETF Internet Engineering Task Force standards and other resources concerning the protocols discussed in the book It also contains online videos of most of the books example materials and trace files from the actually case studies which you can load and examine for your self Finally it includes several freeware and shareware utilities that are a must in the network analyst s toolkit For more specific information as to what is on the Web site see Appendix A PART Foundations of Network Analysis Introduction to Protocol Analysis What is protocol analysis A protocol is defined as a standard procedure for regulating data transmission between computers Protocol analysis is the process of examining those procedures The way we go about this analysis is with special tools called protocol analyzers Protocol analyzers decode the stream of bits flowing across a network and show you those bits in the struc tured format of the protocol Using protocol analysis techniques to understand the procedures occurring on your network is the focus of this book In my 10 years of analyzing and implementing networks I have learned that in order to understand how a vendor s hardware platform such as a router or switch functions you need to understand how the protocols that the hardware imple ments ope
12. e transport layer may retransmit lost data perform flow control between end systems and many times add an extra layer of error protection to application data While the network layer delivers data between two endpoints the transport layer can guarantee that it gets to its destination Layer 5 Session Layer The session layer provides the ability to further control communications between end systems by providing another layer of abstraction between trans port protocols and the application If an application layer protocol possesses this functionality a session layer protocol may not be needed NetBIOS as you will see later in this chapter is a perfect example of a session layer protocol Sometimes the session layer does not reveal itself as a protocol but rather as a Chapter 1 procedure performed to allow a protocol to continue its functions Even though a protocol will exist at a certain layer a procedure of that protocol can sometimes perform functions that normally reside in another layer I will note instances in later chapters where this anomaly takes place Layer 6 Presentation Layer The presentation layer is another layer that sometimes does not manifest itself in obvious ways The presentation layer handles making sure that data for mats used by application layer protocols are compatible between end systems Some examples of Layer 6 would be ASCII JPG and ASN 1 Just as I indicated was the case with Layer 5 some protoc
13. eader Source Port Destination Port Sequence Number Acknowledgment Number Header Offset Reserved Bits Connection Flags Window Size TCP Checksum Urgent Pointer Options Data TCP Implementation Multiplexing Data Sequencing and Acknowledgment Flow Control TCP Connection Management TCP Open Initial Sequence Number ISN TCP Connection States TCP Options TCP Close Half Close TCP Reset Case Study Missing Drive Mappings Case Study No Telnet Case Study Dropped Sessions TCP Data Flow Management Data Sequencing and Acknowledgment TCP Retransmissions Retransmission Time Out Case Study Bad RTO Delayed Acknowledgments Case Study Slow Surfing 164 165 166 169 169 170 171 173 174 175 175 176 177 177 178 178 179 179 180 180 181 181 181 181 182 182 182 182 183 183 183 183 183 183 184 185 185 189 189 192 193 194 194 196 197 200 200 202 202 203 204 206 Contents The Push Flag TCP Sliding Windows Slow Start and Congestion Avoidance Nagle Algorithm Data Protection Case Study TCP Checksum Errors TCP Expert Symptoms TCP Application Analysis TCP and Throughput Segment Size Latency Window Size Case Study Slow Web Server Case Study Bad Windowing Case Study Inefficient Applications High Performance Extensions to TCP Selective Acknowledgments Window Scale Option Timestamp Option Summary Part Ill Related TCP IP Protocols Chapter 7 Upper Layer Protocols Introduction to Upper L
14. ed processing was complicated It required addressing error control and synchronized coordination between systems Unfortunately the communication architectures designed to meet those requirements were not compatible across vendors boundaries Many closed proprietary systems were developed most notably IBM s System Network Architecture SNA and Digi tal Equipment Corporation s DECNet Down the road other companies such as Novell and Apple followed suit In order to open up these closed systems a Introduction to Protocol Analysis framework was needed which would allow interoperability between various vendors systems OSI to the Rescue OSI Open System Interconnection developed by the International Organization for Standardization ISO was the solution designed to promote interoperability between vendors It defines an architecture for communications that support dis tributed processing The OSI model describes the functions that allow systems to communicate successfully over a network Using what is called a layered approach communications functions are broken down into seven distinct layers The seven layers beginning with the bottom layer of the OSI model are as follows Layer 1 Physical layer Layer 2 Data link layer Layer 3 Network layer Layer 4 Transport layer Layer 5 Session layer Layer 6 Presentation layer Layer 7 Application layer Each layer provides a service to the layers above it bu
15. he gap left by these two books by taking the most impor tant concepts on the protocols and applying them to the most common problems a network analyst sees on TCP IP networks For the more curious interested in the intricate details and inner workings of the protocol I have provided an appendix further detailing the website The goal behind the TCP IP Analysis and Troubleshooting Toolkit is to give the reader the information needed to successfully maintain the protocol in real world networks Since TCP IP is the most common protocol in use today this made the decision to concentrate an entire book on the subject of its analysis and troubleshooting methods easy Rather than write a book about the many intricate and often mundane details of the protocol I attempt to empower you with the knowledge to understand and diagnose problems related to the TCP IP protocol You will quickly notice that many of the examples in the book are either Cisco or Microsoft specific Since those are the two most prevalent vendors in use today I have chosen to use examples pertaining to their systems The examples are by no means exclusive to either Cisco or Microsoft In almost all cases you can take the examples and apply them to any vendor s hardware or software Specific examples that apply to a certain vendor are noted Along this line you might also notice several analysis tools mentioned or used in the examples The type of tool is not typically important jus
16. is method is called the media access method All data link protocols designed for use in shared networks have one One function of the media access method is letting the destination station recog nize which bit is the first bit of the Media Access Control MAC frame Once the first bit of the frame is found the NIC can start grouping the ones and zeros into a Data Link Control DLC frame Just as there are different methods of digital signaling there are different types of DLC frames In Ethernet the IP protocol is carried by Ethernet II frames On Token Ring IP is carried by Token_Ring_ SNAP frames Since the objective of this book is to learn how best to analyze TCP IP networks I won t detail the many frame types that exist For more information on the various frame types refer to Data Link Protocols by Ulysses Black Prentice Hall Professional 1993 It is important however to understand the basic details of Layer 2 framing Each DLC frame has five basic parts Media access portion Addressing m Service access points m Upper layer data ES Frame protection These five basic parts are illustrated in Figure 1 2 and discussed in detail in the sections that follow
17. mmon Layer 2 protocol contains detection algorithms for controlling collision detection corrupted frames and address recognition Higher layers depend on the data link layer not only to provide an error free path but also to detect errors that may occur Corrupted data should never be passed to upper layers Layer 3 Network Layer Layer 3 is the end to end communications provider Whereas the data link layer s responsibility ends at the next Layer 2 device the network layer is responsible for routing data from the source to the destination over multiple Layer 2 paths Applications utilizing a Layer 3 protocol do not need to know the details of the underlying Layer 2 network Layer 3 networks such as those using the Internet Protocol will span many different Layer 2 technologies such as Ethernet Token Ring Frame Relay and Asynchronous Transfer Mode ATM Some examples of Layer 3 protocols are IP IPX and AppleTalk Data gram Delivery Protocol DDP Although the network layer is responsible for the addressing and routing of data from source to destination it is not respon sible for guaranteeing its delivery Layer 4 Transport Layer Networks are not reliable On Ethernet networks collisions can occur resulting in data loss switches can drop packets due to congestion and networks them selves can lose data due to overloaded links the Internet itself experiences anomalies such as these on a daily basis Protocols that operate in th
18. of your networks to assist you in quickly and decisively solving problems on your network infra structures The answer to that question is the subject of this book I wrote this book for the people on the front lines the network field engi neers I have a great respect for field engineers They are the doers the people that make things work they are also the first people whose pagers start beep ing when things don t work In my over 10 years of experience supporting desktops servers and large complex network infrastructures I ve come to the conclusion that the best field engineers are the ones who can solve the really tough problems People who are good problem solvers are usually tenacious and curious These two qualities drive these people to stay up all night to try to solve a prob lem They know the answer is there somewhere waiting to be uncovered and xv xvi Introduction they are tenacious enough to dig until they find it The truly curious will most likely have read many good books on the TCP IP protocol including W Richard Stevens TCP IP Illustrated Addison Wesley January 1994 and Dou glas Comer s Internetworking with TCP IP Prentice Hall January 2000 To date these books are the flagship manuscripts on understanding TCP IP but they focus intensely on theory and lack in practical examples That said I still rec ommend every analyst have a copy of them on their bookshelves I have attempted to bridge t
19. ol functions performed in other layers fit nicely into the description of the presentation layer Layer 7 Application Layer Many people confuse Layer 7 with the applications used on servers or work stations Application layer protocols are not user applications but instead the protocols that allow those applications to operate over a network A user browsing the Internet with Internet Explorer utilizes an application layer pro tocol called HTTP Microsoft Word users saving files to a network server make use of the Server Message Block SMB protocol To a user a network drive simply appears as G but in the background there are powerful application layer protocols that allow G to represent a location on a remote server Other examples of application layer protocols are FTP and Telnet Protocol Analysis of the Layers The following sections comprise a protocol analysis approach to the OSI model They explain what each layer does and more importantly why How each layer performs its function is left up to the protocol designers I discuss how TCP IP performs its functions in Chapters 3 through 6 More advanced readers may notice some vague or overly generic descriptions of packet descriptions in the following sections I have written the descriptions this way to provide a generic blueprint for describing the layer s functionality the details follow later in the book Layer 1 The Physical Layer As I indicated earlier in the chapter
20. oncepts of network communi cation that are needed to benefit from the material presented in the later chapters m Part II The Core Protocols builds the foundation for understanding the protocols that TCP IP is built upon It is these protocols that provide the support for all other application layer protocols m Part III Related TCP IP Protocols extends the search for understand ing by revealing the inner workings of standard and vendor indepen dent protocol implementations Applications such as DNS Domain Name System HTTP Hypertext Transport Protocol and FTP File Transport Protocol are thoroughly analyzed and a deep investigation is conducted into Microsoft s TCP IP implementation including the ever so mysterious Server Message Block protocol In each chapter the material is complemented with numerous case studies and examples from real live networks These examples and case studies are given to illustrate how the knowledge and techniques discussed can be put to use Tools This book uses several different analysis tools to illustrate the troubleshooting examples While the tools are not necessary to understand the examples you do need them to view the trace files included on the companion Web site The Web site includes instructions for downloading the freeware version of the Ethereal protocol analyzer which can be used to view the traces xviii Introduction The Companion Web Site The companion Web site to this
21. r have used their best efforts in preparing this book they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representatives or written sales materials The advice and strategies contained herein may not be suitable for your situation You should consult with a professional where appropriate Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages including but not limited to special inci dental consequential or other damages For general information on our other products and services please contact our Customer Care Department within the United States at 800 762 2974 outside the United States at 317 572 3993 or fax 317 572 4002 Trademarks Wiley the Wiley Publishing logo and related trade dress are trademarks or registered trademarks of Wiley Publishing Inc in the United States and other countries and may not be used without written permission All other trademarks are the property of their respective owners Wiley Publishing Inc is not associated with any product or ven dor mentioned in this book Screenshot s Copyright 2002 Wildpackets Inc All rights reserved Wiley also publishes its books in a variety of electronic formats Some content that appears in print
22. rate Routers switches hubs gateways and so on are simply nothing without the protocols Protocols make networks happen Routers and other devices implement those protocols Understand the protocol and you can largely understand what happens inside the box A Brief History of Network Communications For years complex processing needs have been the driving factors behind the development of computer systems Early on these needs were met by the devel opment of supercomputers Supercomputers were designed to service a single Chapter 1 application at a very high speed thus saving valuable time in performing manual calculations Supercomputers with their focus on servicing a single application couldn t fully meet the business need for a computing system supporting multiple users Applications designed for use by many people required multiple input output systems for which supercomputers were not designed These systems were known as time sharing systems because each user was given a small slice of time from the overall processing system The earliest of these sys tems were known as mainframes Although not as fast as supercomputers mainframes could service the business needs of many users running multiple applications simultaneously This feature made them far more effective at servicing multiple business needs The advent of mainframes thus led to the birth of centralized computing With its debute centralized computing could p
23. rovide all aspects of a net worked communications system within a tightly controlled cohesive system Such systems as IBM s S 390 provided the communication paths applications and storage systems within a large centralized processing system Client work stations were nothing more than text screens that let users interact with the applications running on the centralized processing units Distributed computing followed on the heels of centralized computing Distributed computing is characterized by the division of business processes on separate computer systems In the late 80 s and early 90 s the dumb terminal screens used in centralized computing architectures started to be replaced by computer workstations that had their own processing power and memory and more importantly the ability to run applications separate from the mainframe Early distributed systems were nothing more than extensions of a single vendor solution bought from a single vendor over modem or dedicated leased lines Because the vendor controlled all aspects of the system it was easy for that vendor to develop the communication functions that were needed to make their centralized systems distributed These types of systems are known as closed systems because they only interoperate with other systems from the same manufacturer Apple Computer and Novell were among the first compa nies to deliver distributed although still proprietary networking systems Distribut
24. t also depends on ser vices from the layers below it The model also provides a layer of abstraction because upper layers do not need to know the details of how the lower layers operate they simply must possess the ability to use the lower layers services The model was created so that in a perfect world any network layer protocol such as IP Internet Protocol IPX Internet Packet Exchange or X 25 could operate regardless of the physical media it runs over This concept applies to all of the layers and in later chapters you can see how some application protocols function identically over different network protocols and sometimes even dif ferent vendors Server Message Block SMB is a perfect example of this as it is used by Microsoft IBM and Banyan s server operating systems Most commu nication protocols map very nicely to the OSI model OSI actually consists of not only the model but also a suite of complex protocols Although the protocols are rarely used today their original purpose was to provide a single protocol suite that all vendors could adopt into their systems allowing for interoperability The model survived but unfortunately the protocols did not 6 Chapter 1 The OSI Model Example Protocols Anplieski SMB HTTP FTP SMTP cal NCP TELNET Presentation JPG GIF MPEG ASN 1 SMB Negotiation Sector NetBIOS TCP 3 way handshake Transport TCP SPX Network IP IPX DDP f Ethernet
25. t as long as it provides the functionality needed or described An understanding of the technology is what s important and that is what this book concentrates on Who Should Read This Book Although this book does provide an introduction to network analysis tech niques and the TCP IP protocol it is not for beginners A basic understanding of the OSI model is important as well as a decent level of experience manag ing server operating systems running TCP IP More advanced readers already familiar with the protocol will benefit greatly from the case studies presented in each chapter This book will help you become a better network analyst If you are a network administrator eager Introdution xvii to learn more about understanding communications between clients and servers this is a good place to start If you are already familiar with configur ing routers and switches this book will teach you the technology behind the configuration commands it will help you learn to think outside the box This book is about technology and how to best use tools at your disposal to keep your networks running smoothly How This Book Is Organized The book is organized into three parts m Part I Foundations of Network Analysis answers such questions as Why protocol analysis and What tools do I use It explains the process of capturing and manipulating trace files It also provides a refresher of the OSI model and the basic c
26. when these analog signals were amplified there was no way to distinguish the noise from the voice signal As the analog voice signal was amplified so was the noise Converting analog voice signals to digital signals was one way to solve this problem Digital Signaling Unlike analog signals digital signals have only discrete values either a one or a zero Early digital telephone engineers figured out a way to modulate an ana log signal onto a digital carrier using something called pulse code modulation or PCM PCM lets the instantaneous frequency of an analog signal be repre sented by a binary number Instead of an amplifier having to guess at which signal to amplify now it just had to repeat either a zero or a one Using this method greatly improved the quality of long distance communications When computer data needed to be transmitted across network links the decision to use digital signaling was easy Since computers already represented data using zeros and ones these zeros and ones could very easily be transmitted across networks digitally How these ones and zeros are represented is what digital signaling is all about On 10BaseT Ethernet networks data is represented by electrical volt age a one is represented by a transition from 2 05 V to 0 V and a zero is rep resented by a transition from 0 V to 2 05 V Over fiber optic networks a one might be represented by a pulse of light and a zero by the absence of light The process isn
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