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James F. Kurose
University of Massachusetts, Amherst
Keith W. Ross
Polytechnic Institute of NYU
COMPUTER NETWORKING
A Top-Down Approach

iii
About the Authors
Jim Kurose
Jim Kurose is a Distinguished University Professor in the College of Information
and Computer Sciences at the University of Massachusetts Amherst, where
he has been on the faculty since receiving his PhD in computer science from
Columbia University. He received a BA in physics from Wesleyan University.
He has held a number of visiting scientist positions in the United States and
abroad, including IBM Research, INRIA, and the Sorbonne University in
France. He recently completed a five-year term as Assistant Director at the
US National Science Foundation, where he led the Directorate of Computer
and Information Science and Engineering in its mission to uphold the nation’s
leadership in scientific discovery and engineering innovation.
Jim is proud to have mentored and taught an amazing group of students,
and to have received a number of awards for his research, teaching, and
service, including the IEEE Infocom Award, the ACM SIGCOMM Lifetime
Achievement Award, the ACM Sigcomm Test of Time Award, and the
IEEE Computer Society Taylor Booth Education Medal. Dr. Kurose is a for-
mer Editor-in-Chief of IEEE Transactions on Communications and of IEEE/
ACM Transactions on Networking. He has served as Technical Program
co-Chair for IEEE Infocom, ACM SIGCOMM, ACM Internet Measurement
Conference, and ACM SIGMETRICS. He is a Fellow of the IEEE, the ACM
and a member of the National Academy of Engineering. His research
interests include network protocols and architecture, network measurement,
multimedia communication, and modeling and performance evaluation.
Keith Ross
Keith Ross is the Dean of Engineering and Computer Science at NYU
Shanghai and the Leonard J. Shustek Chair Professor in the Computer Science
and Engineering Department at NYU. Previously he was at University of
Pennsylvania (13 years), Eurecom Institute (5 years) and NYU-Poly (10 years).
He received a B.S.E.E from Tufts University, a M.S.E.E. from Columbia
University, and a Ph.D. in Computer and Control Engineering from The
University of Michigan. Keith Ross is also the co-founder and original CEO
of Wimba, which develops online multimedia applications for e-learning and
was acquired by Blackboard in 2010.
Professor Ross’s research interests have been in modeling and meaurement
of computer networks, peer-to-peer systems, content distribution networks,
social networks, and privacy. He is currently working in deep reinforcement

iv ABOUT THE AUTHORS
learning. He is an ACM Fellow, an IEEE Fellow, recipient of the Infocom
2009 Best Paper Award, and recipient of 2011 and 2008 Best Paper Awards
for Multimedia Communications (awarded by IEEE Communications Society).
He has served on numerous journal editorial boards and conference pro-
gram committees, including IEEE/ACM Transactions on Networking, ACM
SIGCOMM, ACM CoNext, and ACM Internet Measurement Conference.
He also has served as an advisor to the Federal Trade Commission on P2P
file sharing.

To Julie and our three precious
ones—Chris, Charlie, and Nina
JFK
A big THANKS to my professors, colleagues,
and students all over the world.
KWR

Preface
Welcome to the eighth edition of Computer Networking: A Top-Down Approach.
Since the publication of the first edition 20 years ago, our book has been adopted
for use at many hundreds of colleges and universities, translated into 14 languages,
and used by many hundreds of thousands students and practitioners worldwide. We’ve
heard from many of these readers and have been overwhelmed by the positive response.
What’s New in the Eighth Edition?
We think one important reason for this success has been that our book continues to
offer a fresh and timely approach to computer networking instruction. We’ve made
changes in this eighth edition, but we’ve also kept unchanged what we believe (and
the instructors and students who have used our book have confirmed) to be the most
important aspects of this book: its top-down approach, its focus on the Internet and a
modern treatment of computer networking, its attention to both principles and prac-
tice, and its accessible style and approach toward learning about computer network-
ing. Nevertheless, the eighth edition has been revised and updated substantially.
Readers of earlier editions of our book may recall that in moving from the sixth to
the seventh edition, we deepened our coverage of the network layer, expanding material
which had been previously covered in a single chapter into a new chapter focused
on the so-called “data plane” component of the network layer (Chapter 4) and a new
chapter focused on the network layer’s “control plane” (Chapter 5). That change
turned out to be prescient, as software-defined networking (SDN), arguably the most
important and exciting advance in networking in decades, has been rapidly adopted
in practice—so much so that it’s already hard to imagine an introduction to modern
computer networking that doesn’t cover SDN. SDN has also enabled new advances
in the practice of network management, which we also cover in modernized and deep-
er detail in this edition. And as we’ll see in Chapter 7 of this eighth edition, the separa-
tion of the data and control planes is now also deeply embedded in 4G/5G mobile
cellular network architectures, as is an “all-IP” approach to their core networks. The
rapid adoption of 4G/5G networks and the mobile applications they enable are un-
doubtedly the most significant changes we’ve seen in networking since the publication
of our seventh edition. We’ve thus significantly updated and deepened our treatment
of this exciting area. Indeed, the ongoing wireless network revolution is so important
that we think it has become a critical part of an introductory networking course.
In addition to these changes, we’ve also updated many sections throughout the
book and added new material to reflect changes across the breadth of networking.
In some cases, we have also retired material from the previous edition. As always,
vii

viii PREFACE
material that has been retired from the printed text can always be found on our book’s
Companion Website. The most important changes in this eighth edition are the following:
• Chapter 1 has been updated to reflect the ever-growing reach and use of the In-
ternet, and of 4G/5G networks.
• Chapter 2, which covers the application layer, has been significantly updated,
including material on the new HTTP/2 and HPPT/3 protocols for the Web.
• Chapter 3, has been updated to reflect advances in, and evolution in use of,
transport-layer congestion control and error-control protocols over the past five
years. While this material had remained relatively stable for quite some time,
there have been a number of important advances since the seventh edition. Several
new congestion-control algorithms have been developed and deployed beyond
the “classic” TCP algorithms. We provide a deeper coverage of TCP CUBIC, the
default TCP protocol in many deployed systems, and examine delay-based ap-
proaches to congestion control, including the new BBR protocol, which is de-
ployed in Google’s backbone network. We also study the QUIC protocol, which
is being incorporated into the HTTP/3 standard. Although QUIC is technically
not a transport-layer protocol—it provides application-layer reliability, conges-
tion control, and connection multiplexing services at the application layer—it
uses many of the error- and congestion-control principles that we develop in the
early sections of Chapter 3.
• Chapter 4, which covers the network-layer data plane, has general updates
throughout. We’ve added a new section on so-called middleboxes, which per-
form network-layer functions other than routing and forwarding, such as firewall-
ing and load balancing. Middleboxes build naturally on the generalized “match
plus action” forwarding operation of network-layer devices that we cover earlier
in Chapter 4. We’ve also added timely new material on topics such as the amount
of buffering that is “just right” in network routers, on net neutrality, and on the
architectural principles of the Internet.
• Chapter 5, which cover the network-layer’s control plane, contains updated ma-
terial on SDN, and a significantly new treatment of network management. The
use of SDN has evolved beyond management of packet-forwarding tables to in-
clude configuration management of network devices as well. We introduce two
new protocols, NETCONF and YANG, whose adoption and use have fueled this
new approach toward network management.
• Chapter 6, which covers the link layer, has been updated to reflect the continu-
ing evolution of link-layer technologies such as Ethernet. We have also updated
and expanded our treatment of datacenter networks, which are at the heart of the
technology driving much of today’s Internet commerce.
• As noted earlier, Chapter 7 has been significantly updated and revised to reflect
the many changes in wireless networking since the seventh edition, from short-
range Bluetooth piconets, to medium-range wireless 802.11 local area networks
(WLANs), to wide-area 4G/5G wireless cellular networks. We have retired our

PREFACE
ix
coverage of earlier 2G and 3G networks in favor of a broader and deeper treat-
ment of today’s 4G LTE networks and tomorrow’s 5G networks. We have also
updated our coverage of mobility issues, from the local issue of handover of mo-
bile devices between base stations to the global issue of identity management and
mobile device roaming among different global cellular networks.
• Chapter 8, which covers network security, has been updated to reflect changes
in wireless network security in particular, with new material on WPA3 security in
WLANs, and mutual device/network mutual authentication and confidentiality in
4G/5G networks.
We have also retired Chapter 9, on multimedia networking, from this edition. Over
time, as multimedia applications became more prevalent, we had already migrated
Chapter 9 material on topics such as video streaming, packet scheduling, and content
distribution networks into earlier chapters. As noted earlier, all retired material from
this and earlier editions can be found on our book’s Companion Website.
Audience
This textbook is for a first course on computer networking. It can be used in both
computer science and electrical engineering departments. In terms of programming
languages, the book assumes only that the student has experience with C, C++, Java,
or Python (and even then only in a few places). Although this book is more precise
and analytical than many other introductory computer networking texts, it rarely uses
any mathematical concepts that are not taught in high school. We have made a delib-
erate effort to avoid using any advanced calculus, probability, or stochastic process
concepts (although we’ve included some homework problems for students with this
advanced background). The book is therefore appropriate for undergraduate courses
and for first-year graduate courses. It should also be useful to practitioners in the
networking industry.
What Is Unique About This Textbook?
The subject of computer networking is enormously complex, involving many con-
cepts, protocols, and technologies that are woven together in an intricate manner.
To cope with this scope and complexity, many computer networking texts are often
organized around the “layers” of a network architecture. With a layered organization,
students can see through the complexity of computer networking—they learn about
the distinct concepts and protocols in one part of the architecture while seeing the
big picture of how all parts fit together. From a pedagogical perspective, our personal
experience has been that such a layered approach indeed works well. Nevertheless,
we have found that the traditional approach of teaching—bottom up; that is, from the
physical layer toward the application layer—is not the best approach for a modern
course on computer networking.

x PREFACE
A Top-Down Approach
Our book broke new ground 20 years ago by treating networking in a top-down

manner—that is, by beginning at the application layer and working its way down
toward the physical layer. The feedback we received from teachers and students alike
have confirmed that this top-down approach has many advantages and does indeed
work well pedagogically. First, it places emphasis on the application layer (a “high
growth area” in networking). Indeed, many of the recent revolutions in computer
networking—including the Web, and media streaming—have taken place at the

application layer. An early emphasis on application-layer issues differs from the

approaches taken in most other texts, which have only a small amount of material on
network applications, their requirements, application-layer paradigms (e.g., client-
server and peer-to-peer), and application programming interfaces. Second, our expe-
rience as instructors (and that of many instructors who have used this text) has been
that teaching networking applications near the beginning of the course is a powerful
motivational tool. Students are thrilled to learn about how networking applications
work—applications such as e-mail, streaming video, and the Web, which most stu-
dents use on a daily basis. Once a student understands the applications, the student
can then understand the network services needed to support these applications. The
student can then, in turn, examine the various ways in which such services might be
provided and implemented in the lower layers. Covering applications early thus pro-
vides motivation for the remainder of the text.
Third, a top-down approach enables instructors to introduce network applica-
tion development at an early stage. Students not only see how popular applica-
tions and protocols work, but also learn how easy it is to create their own network

applications and application-layer protocols. With the top-down approach, students
get early
exposure to the notions of socket programming, service models, and

protocols—important concepts that resurface in all subsequent layers. By providing
socket programming examples in Python, we highlight the central ideas without
confusing students with complex code. Undergraduates in electrical engineering
and computer science will have no difficulty following the Python code.
An Internet Focus
Although we dropped the phrase “Featuring the Internet” from the title of this book
with the fourth edition, this doesn’t mean that we dropped our focus on the Internet.
Indeed, nothing could be further from the case! Instead, since the Internet has become
so pervasive, we felt that any networking textbook must have a significant focus on
the Internet, and thus this phrase was somewhat unnecessary. We continue to use the
Internet’s architecture and protocols as primary vehicles for studying fundamental
computer networking concepts. Of course, we also include concepts and protocols
from other network architectures. But the spotlight is clearly on the Internet, a fact
reflected in our organizing the book around the Internet’s five-layer architecture: the
application, transport, network, link, and physical layers.

PREFACE
xi
Another benefit of spotlighting the Internet is that most computer science and
electrical engineering students are eager to learn about the Internet and its protocols.
They know that the Internet has been a revolutionary and disruptive technology and
can see that it is profoundly changing our world. Given the enormous relevance of
the Internet, students are naturally curious about what is “under the hood.” Thus, it
is easy for an instructor to get students excited about basic principles when using the
Internet as the guiding focus.
Teaching Networking Principles
Two of the unique features of the book—its top-down approach and its focus on the
Internet—have appeared in the titles of our book. If we could have squeezed a third
phrase into the subtitle, it would have contained the word principles. The field of
networking is now mature enough that a number of fundamentally important issues
can be identified. For example, in the transport layer, the fundamental issues include
reliable communication over an unreliable network layer, connection establishment/
teardown and handshaking, congestion and flow control, and multiplexing. Three fun-
damentally important network-layer issues are determining “good” paths between two
routers, interconnecting a large number of heterogeneous networks, and managing the
complexity of a modern network. In the link layer, a fundamental problem is sharing a
multiple access channel. In network security, techniques for providing confidentiality,
authentication, and message integrity are all based on cryptographic fundamentals.
This text identifies fundamental networking issues and studies approaches toward

addressing these issues. The student learning these principles will gain knowledge
with a long “shelf life”—long after many of today’s network standards and protocols
have become obsolete, the principles they embody will remain important and rel-
evant. We believe that the combination of using the Internet to get the student’s foot in
the door and then emphasizing fundamental issues and solution approaches will allow
the student to quickly understand just about any networking technology.
Student Resources
Student resources are available on the Companion Website (CW) at pearson.com/
cs-resources. Resources include:
• Interactive learning material. The book’s Website contains
VideoNotes—
video presentations of important topics throughout the book done by the authors,
as well as walkthroughs of solutions to problems similar to those at the end
of the chapter. We’ve seeded the Website with VideoNotes and online prob-
lems for Chapters 1 through 5. As in earlier editions, the Website contains
the interactive animations that illustrate many key networking concepts. Pro-
fessors can integrate these interactive features into their lectures or use them
as mini labs.

xii PREFACE
• Additional technical material. As we have added new material in each edition of
our book, we’ve had to remove coverage of some existing topics to keep the book
at manageable length. Material that appeared in earlier editions of the text is still
of
interest, and thus can be found on the book’s Website.
• Programming assignments. The Website also provides a number of detailed
programming assignments, which include building a multithreaded Web
server,
building an e-mail client with a GUI interface, programming the sender and

receiver sides of a reliable data transport protocol, programming a distributed
routing algorithm, and more.
• Wireshark labs. One’s understanding of network protocols can be greatly

deepened by seeing them in action. The Website provides numerous Wireshark
assignments that enable students to actually observe the sequence of messages
exchanged between two protocol entities. The Website includes separate Wire-
shark labs on HTTP, DNS, TCP, UDP, IP, ICMP, Ethernet, ARP, WiFi, TLS and
on tracing all protocols involved in satisfying a request to fetch a Web page. We’ll
continue to add new labs over time.
Authors’ Website. In addition to the Companion Website, the authors maintain
a public Website, http://gaia.cs.umass.edu/kurose_ross, which contains additional
interactive material for students and mirrors publically available material from the
Website, such as PowerPoint slides and Wireshark lab materials. Of particular interest
is http://gaia.cs.umass.edu/kurose_ross/interactive, containing interactive exercises
that create (and present solutions for) problems similar to selected end-of-chapter
problems. Since students can generate (and view solutions for) an unlimited number
of similar problem instances, they can work until the material is truly mastered.
Pedagogical Features
We have each been teaching computer networking for more than 30 years. Together,
we bring more than 60 years of teaching experience to this text, during which time
we have taught many thousands of students. We have also been active researchers
in computer networking during this time. (In fact, Jim and Keith first met each other
as master’s students in a computer networking course taught by Mischa Schwartz
in 1979 at Columbia University.) We think all this gives us a good perspective on
where networking has been and where it is likely to go in the future. Nevertheless,
we have resisted temptations to bias the material in this book toward our own pet
research projects. We figure you can visit our personal Websites if you are interested
in our research. Thus, this book is about modern computer networking—it is about
contemporary protocols and technologies as well as the underlying principles behind
these protocols and technologies. We also believe that learning (and teaching!) about
networking can be fun. A sense of humor, use of analogies, and real-world examples
in this book will hopefully make this material more fun.

PREFACE
xiii
Supplements for Instructors
We provide a complete supplements package to aid instructors in teaching this
course. This material can be accessed from Pearson’s Instructor Resource Center
(http://www.pearsonhighered.com/irc). Visit the Instructor Resource Center for

information about accessing these instructor’s supplements.
• PowerPoint® slides. We provide PowerPoint slides for all eight chapters. The
slides have been completely updated with this eighth edition. The slides cover
each chapter in detail. They use graphics and animations (rather than relying only
on monotonous text bullets) to make the slides interesting and visually appealing.
We provide the original PowerPoint slides so you can customize them to best suit
your own teaching needs. Some of these slides have been contributed by other
instructors who have taught from our book.
• Homework solutions. We provide a solutions manual for the homework prob-
lems in the text, programming assignments, and Wireshark labs. As noted

earlier, we’ve introduced many new homework problems at each chapter’s end.
For additional interactive problems and solutions, an instructor (and students)
can consult this books Companion Website at Pearson, or the authors’ Website
of interactive problems at http://gaia.cs.umass.edu/kurose_ross/interactive.
Chapter Dependencies
The first chapter of this text presents a self-contained overview of computer net-
working. Introducing many key concepts and terminology, this chapter sets the stage
for the rest of the book. All of the other chapters directly depend on this first chapter.
After completing Chapter 1, we recommend instructors cover Chapters 2 through 6
in sequence, following our top-down philosophy. Each of these five chapters lever-
ages material from the preceding chapters. After completing the first six chapters,
the instructor has quite a bit of flexibility. There are no interdependencies among the
last two chapters, so they can be taught in any order. However, the last two chapters
depends on the material in the first six chapters. Many instructors first teach the first
six chapters and then teach one of the last two chapters for “dessert.”
One Final Note: We’d Love to Hear from You
We encourage students and instructors to e-mail us with any comments they might
have about our book. It’s been wonderful for us to hear from so many instructors and
students from around the world about our first seven editions. We’ve incorporated
many of these suggestions into later editions of the book. We also encourage instructors

xiv PREFACE
to send us new homework problems (and solutions) that would complement the cur-
rent homework problems. We’ll post these on the instructor-only portion of the Web-
site. We also encourage instructors and students to create new interactive animations
that illustrate the concepts and protocols in this book. If you have an animation that
you think would be appropriate for this text, please submit it to us. If the animation
(including notation and terminology) is appropriate, we’ll be happy to include it on
the text’s Website, with an appropriate reference to the animation’s authors.
So, as the saying goes, “Keep those cards and letters coming!” Seriously, please
do continue to send us interesting URLs, point out typos, disagree with any of our
claims, and tell us what works and what doesn’t work. Tell us what you think should
or shouldn’t be included in the next edition. Send your e-mail to kurose@cs.umass
.edu and keithwross@nyu.edu.
Acknowledgments
Since we began writing this book in 1996, many people have given us invaluable
help and have been influential in shaping our thoughts on how to best organize and
teach a networking course. We want to say A BIG THANKS to everyone who has
helped us from the earliest first drafts of this book, up to this eighth edition. We are
also very thankful to the thousands of readers from around the world—students,
faculty, practitioners—who have sent us thoughts and comments on earlier editions
of the book and suggestions for future editions of the book. Special thanks go out to:
Al Aho (Columbia University)
Hisham Al-Mubaid (University of Houston-Clear Lake)
Pratima Akkunoor (Arizona State University)
Paul Amer (University of Delaware)
Shamiul Azom (Arizona State University)
Lichun Bao (University of California at Irvine)
Paul Barford (University of Wisconsin)
Bobby Bhattacharjee (University of Maryland)
Steven Bellovin (Columbia University)
Pravin Bhagwat (Wibhu)
Supratik Bhattacharyya (Amazon)
Ernst Biersack (Eurécom Institute)
Shahid Bokhari (University of Engineering & Technology, Lahore)
Jean Bolot (Technicolor Research)
Daniel Brushteyn (former University of Pennsylvania student)
Ken Calvert (University of Kentucky)
Evandro Cantu (Federal University of Santa Catarina)
Jeff Case (SNMP Research International)
Jeff Chaltas (Sprint)
Vinton Cerf (Google)

PREFACE
xv
Byung Kyu Choi (Michigan Technological University)
Bram Cohen (BitTorrent, Inc.)
Constantine Coutras (Pace University)
John Daigle (University of Mississippi)
Edmundo A. de Souza e Silva (Federal University of Rio de Janeiro)
Philippe Decuetos (former Eurecom Institute student)
Christophe Diot (Google)
Prithula Dhunghel (Akamai)
Deborah Estrin (Cornell University)
Michalis Faloutsos (University of California at Riverside)
Wu-chi Feng (Oregon Graduate Institute)
Sally Floyd (ICIR, University of California at Berkeley)
Paul Francis (Max Planck Institute)
David Fullager (Netflix)
Lixin Gao (University of Massachusetts)
JJ Garcia-Luna-Aceves (University of California at Santa Cruz)
Mario Gerla (University of California at Los Angeles)
David Goodman (NYU-Poly)
Yang Guo (Alcatel/Lucent Bell Labs)
Tim Griffin (Cambridge University)
Max Hailperin (Gustavus Adolphus College)
Bruce Harvey (Florida A&M University, Florida State University)
Carl Hauser (Washington State University)
Rachelle Heller (George Washington University)
Phillipp Hoschka (INRIA/W3C)
Wen Hsin (Park University)
Albert Huang (former University of Pennsylvania student)
Cheng Huang (Microsoft Research)
Esther A. Hughes (Virginia Commonwealth University)
Van Jacobson (Google)
Pinak Jain (former NYU-Poly student)
Jobin James (University of California at Riverside)
Sugih Jamin (University of Michigan)
Shivkumar Kalyanaraman (IBM Research, India)
Jussi Kangasharju (University of Helsinki)
Sneha Kasera (University of Utah)
Parviz Kermani (U. Massachusetts)
Hyojin Kim (former University of Pennsylvania student)
Leonard Kleinrock (University of California at Los Angeles)
David Kotz (Dartmouth College)
Beshan Kulapala (Arizona State University)
Rakesh Kumar (Bloomberg)
Miguel A. Labrador (University of South Florida)
Simon Lam (University of Texas)

xvi PREFACE
Steve Lai (Ohio State University)
Tom LaPorta (Penn State University)
Tim-Berners Lee (World Wide Web Consortium)
Arnaud Legout (INRIA)
Lee Leitner (Drexel University)
Brian Levine (University of Massachusetts)
Chunchun Li (former NYU-Poly student)
Yong Liu (NYU-Poly)
William Liang (former University of Pennsylvania student)
Willis Marti (Texas A&M University)
Nick McKeown (Stanford University)
Josh McKinzie (Park University)
Deep Medhi (University of Missouri, Kansas City)
Bob Metcalfe (International Data Group)
Vishal Misra (Columbia University)
Sue Moon (KAIST)
Jenni Moyer (Comcast)
Erich Nahum (IBM Research)
Christos Papadopoulos (Colorado Sate University)
Guru Parulkar (Open Networking Foundation)
Craig Partridge (Colorado State University)
Radia Perlman (Dell EMC)
Jitendra Padhye (Microsoft Research)
Vern Paxson (University of California at Berkeley)
Kevin Phillips (Sprint)
George Polyzos (Athens University of Economics and Business)
Sriram Rajagopalan (Arizona State University)
Ramachandran Ramjee (Microsoft Research)
Ken Reek (Rochester Institute of Technology)
Martin Reisslein (Arizona State University)
Jennifer Rexford (Princeton University)
Leon Reznik (Rochester Institute of Technology)
Pablo Rodrigez (Telefonica)
Sumit Roy (University of Washington)
Catherine Rosenberg (University of Waterloo)
Dan Rubenstein (Columbia University)
Avi Rubin (Johns Hopkins University)
Douglas Salane (John Jay College)
Despina Saparilla (Cisco Systems)
John Schanz (Comcast)
Henning Schulzrinne (Columbia University)
Mischa Schwartz (Columbia University)
Ardash Sethi (University of Delaware)
Harish Sethu (Drexel University)

PREFACE
xvii
K. Sam Shanmugan (University of Kansas)
Prashant Shenoy (University of Massachusetts)
Clay Shields (Georgetown University)
Subin Shrestra (University of Pennsylvania)
Bojie Shu (former NYU-Poly student)
Mihail L. Sichitiu (NC State University)
Peter Steenkiste (Carnegie Mellon University)
Tatsuya Suda (University of California at Irvine)
Kin Sun Tam (State University of New York at Albany)
Don Towsley (University of Massachusetts)
David Turner (California State University, San Bernardino)
Nitin Vaidya (Georgetown University)
Michele Weigle (Clemson University)
David Wetherall (Google)
Ira Winston (University of Pennsylvania)
Di Wu (Sun Yat-sen University)
Shirley Wynn (former NYU-Poly student)
Raj Yavatkar (Google)
Yechiam Yemini (Columbia University)
Dian Yu (former NYU-Shanghai student)
Ming Yu (State University of New York at Binghamton)
Ellen Zegura (Georgia Institute of Technology)
Honggang Zhang (Suffolk University)
Hui Zhang (Carnegie Mellon University)
Lixia Zhang (University of California at Los Angeles)
Meng Zhang (former NYU-Poly student)
Shuchun Zhang (former University of Pennsylvania student)
Xiaodong Zhang (Ohio State University)
ZhiLi Zhang (University of Minnesota)
Phil Zimmermann (independent consultant)
Mike Zink (University of Massachusetts)
Cliff C. Zou (University of Central Florida)
We also want to thank the entire Pearson team—in particular, Carole Snyder and
Tracy Johnson—who have done an absolutely outstanding job on this eighth edition
(and who have put up with two very finicky authors who seem congenitally unable
to meet deadlines!). Thanks also to artists, Janet Theurer and Patrice Rossi Calkin,
for their work on the beautiful figures in earlier editions of our book, and to Manas
Roy and his team at SPi Global for their wonderful production work on this edition.

Finally, a most special thanks go to our previous editors at
Addison-Wesley and
Pearson—Matt Goldstein, Michael Hirsch, and Susan Hartman. This book would not
be what it is (and may well not have been at all) without their graceful management,
constant encouragement, nearly infinite patience, good humor, and perseverance.

xix
1
81
181
303
377
449
531
Chapter 1 Computer Networks and the Internet
Chapter 2 Application Layer
Chapter 3 Transport Layer
Chapter 4 The Network Layer: Data Plane
Chapter 5 The Network Layer: Control Plane
Chapter 6 The Link Layer and LANs
Chapter 7 Wireless and Mobile Networks
Chapter 8 Security in Computer Networks 607
References 691
Index 731
Brief Contents

Chapter 1 Computer Networks and the Internet 1
1.1 What Is the Internet? 2
1.1.1 A Nuts-and-Bolts Description 2
1.1.2 A Services Description 5
1.1.3 What Is a Protocol? 7
1.2 The Network Edge 9
1.2.1 Access Networks 12
1.2.2 Physical Media 18
1.3 The Network Core 22
1.3.1 Packet Switching 23
1.3.2 Circuit Switching 27
1.3.3 A Network of Networks 31
1.4 Delay, Loss, and Throughput in Packet-Switched Networks 35
1.4.1 Overview of Delay in Packet-Switched Networks 35
1.4.2 Queuing Delay and Packet Loss 39
1.4.3 End-to-End Delay 41
1.4.4 Throughput in Computer Networks 43
1.5 Protocol Layers and Their Service Models 47
1.5.1 Layered Architecture 47
1.5.2 Encapsulation 52
1.6 Networks Under Attack 54
1.7 History of Computer Networking and the Internet 58
1.7.1 The Development of Packet Switching: 1961–1972 58
1.7.2 Proprietary Networks and Internetworking: 1972–1980 59
1.7.3 A Proliferation of Networks: 1980–1990 61
1.7.4 The Internet Explosion: The 1990s 62
1.7.5 The New Millennium 63
1.8 Summary 64
Homework Problems and Questions 66
Wireshark Lab 76
Interview: Leonard Kleinrock 78
Table of Contents
xxi

xxii TABLE OF CONTENTS
Chapter 2 Application Layer 81
2.1 Principles of Network Applications 82
2.1.1 Network Application Architectures 84
2.1.2 Processes Communicating 85
2.1.3 Transport Services Available to Applications 88
2.1.4 Transport Services Provided by the Internet 90
2.1.5 Application-Layer Protocols 94
2.1.6 Network Applications Covered in This Book 95
2.2 The Web and HTTP 95
2.2.1 Overview of HTTP 96
2.2.2 Non-Persistent and Persistent Connections 98
2.2.3 HTTP Message Format 101
2.2.4 User-Server Interaction: Cookies 105
2.2.5 Web Caching 108
2.2.6 HTTP/2 113
2.3 Electronic Mail in the Internet 116
2.3.1 SMTP 118
2.3.2 Mail Message Formats 121
2.3.3 Mail Access Protocols 121
2.4 DNS—The Internet’s Directory Service 122
2.4.1 Services Provided by DNS 123
2.4.2 Overview of How DNS Works 125
2.4.3 DNS Records and Messages 131
2.5 Peer-to-Peer File Distribution 136
2.6 Video Streaming and Content Distribution Networks 143
2.6.1 Internet Video 143
2.6.2 HTTP Streaming and DASH 144
2.6.3 Content Distribution Networks 145
2.6.4 Case Studies: Netflix and YouTube 149
2.7 Socket Programming: Creating Network Applications 152
2.7.1 Socket Programming with UDP 154
2.7.2 Socket Programming with TCP 159
2.8 Summary 165
Socket Programming Assignments 175
Wireshark Labs: HTTP, DNS 177
Interview: Tim Berners-Lee 178

TABLE OF CONTENTS xxiii
Chapter 3 Transport Layer 181
3.1 Introduction and Transport-Layer Services 182
3.1.1 Relationship Between Transport and Network Layers 182
3.1.2 Overview of the Transport Layer in the Internet 185
3.2 Multiplexing and Demultiplexing 187
3.3 Connectionless Transport: UDP 194
3.3.1 UDP Segment Structure 198
3.3.2 UDP Checksum 198
3.4 Principles of Reliable Data Transfer 200
3.4.1 Building a Reliable Data Transfer Protocol 202
3.4.2 Pipelined Reliable Data Transfer Protocols 211
3.4.3 Go-Back-N (GBN) 215
3.4.4 Selective Repeat (SR) 220
3.5 Connection-Oriented Transport: TCP 227
3.5.1 The TCP Connection 227
3.5.2 TCP Segment Structure 230
3.5.3 Round-Trip Time Estimation and Timeout 235
3.5.4 Reliable Data Transfer 238
3.5.5 Flow Control 246
3.5.6 TCP Connection Management 249
3.6 Principles of Congestion Control 255
3.6.1 The Causes and the Costs of Congestion 255
3.6.2 Approaches to Congestion Control 262
3.7 TCP Congestion Control 263
3.7.1 Classic TCP Congestion Control 263
3.7.2 Network-Assisted Explicit Congestion Notification and
Delayed-based Congestion Control 274
3.7.3 Fairness 276
3.8 Evolution of Transport-Layer Functionality 279
3.9 Summary 282
Programming Assignments 300
Wireshark Labs: Exploring TCP, UDP 300
Interview: Van Jacobson 301
Chapter 4 The Network Layer: Data Plane 303
4.1 Overview of Network Layer 304
4.1.1 Forwarding and Routing: The Data and Control Planes 304
4.1.2 Network Service Model 309
4.2 What’s Inside a Router? 311
4.2.1 Input Port Processing and Destination-Based Forwarding 314
4.2.2 Switching 317

xxiv TABLE OF CONTENTS
4.2.3 Output Port Processing 319
4.2.4 Where Does Queuing Occur? 319
4.2.5 Packet Scheduling 325
4.3 The Internet Protocol (IP): IPv4, Addressing, IPv6, and More 330
4.3.1 IPv4 Datagram Format 331
4.3.2 IPv4 Addressing 333
4.3.3 Network Address Translation (NAT) 344
4.3.4 IPv6 347
4.4 Generalized Forwarding and SDN 353
4.4.1 Match 355
4.4.2 Action 356
4.4.3 OpenFlow Examples of Match-plus-action in Action 357
4.5 Middleboxes 360
4.6 Summary 364
Wireshark Lab: IP 374
Interview: Vinton G. Cerf 375
Chapter 5 The Network Layer: Control Plane 377
5.1 Introduction 378
5.2 Routing Algorithms 380
5.2.1 The Link-State (LS) Routing Algorithm 383
5.2.2 The Distance-Vector (DV) Routing Algorithm 388
5.3 Intra-AS Routing in the Internet: OSPF 395
5.4 Routing Among the ISPs: BGP 399
5.4.1 The Role of BGP 399
5.4.2 Advertising BGP Route Information 400
5.4.3 Determining the Best Routes 402
5.4.4 IP-Anycast 406
5.4.5 Routing Policy 407
5.4.6 Putting the Pieces Together: Obtaining Internet Presence 410
5.5 The SDN Control Plane 411
5.5.1 The SDN Control Plane: SDN Controller and
SDN Network-control Applications 414
5.5.2 OpenFlow Protocol 416
5.5.3 Data and Control Plane Interaction: An Example 418
5.5.4 SDN: Past and Future 419
5.6 ICMP: The Internet Control Message Protocol 423
5.7 Network Management and SNMP, NETCONF/YANG 425
5.7.1 The Network Management Framework 426
5.7.2 The Simple Network Management Protocol (SNMP)
and the Management Information Base (MIB) 428
5.7.3 The Network Configuration Protocol (NETCONF) and YANG 432
5.8 Summary 436

TABLE OF CONTENTS xxv
Socket Programming Assignment 5: ICMP Ping 443
Programming Assignment: Routing 444
Wireshark Lab: ICMP 445
Interview: Jennifer Rexford 446
Chapter 6 The Link Layer and LANs 449
6.1 Introduction to the Link Layer 450
6.1.1 The Services Provided by the Link Layer 452
6.1.2 Where Is the Link Layer Implemented? 453
6.2 Error-Detection and -Correction Techniques 454
6.2.1 Parity Checks 456
6.2.2 Checksumming Methods 458
6.2.3 Cyclic Redundancy Check (CRC) 459
6.3 Multiple Access Links and Protocols 461
6.3.1 Channel Partitioning Protocols 463
6.3.2 Random Access Protocols 465
6.3.3 Taking-Turns Protocols 474
6.3.4 DOCSIS: The Link-Layer Protocol for Cable Internet Access 475
6.4 Switched Local Area Networks 477
6.4.1 Link-Layer Addressing and ARP 478
6.4.2 Ethernet 484
6.4.3 Link-Layer Switches 491
6.4.4 Virtual Local Area Networks (VLANs) 497
6.5 Link Virtualization: A Network as a Link Layer 501
6.5.1 Multiprotocol Label Switching (MPLS) 502
6.6 Data Center Networking 505
6.6.1 Data Center Architectures 505
6.6.2 Trends in Data Center Networking 509
6.7 Retrospective: A Day in the Life of a Web Page Request 512
6.7.1 Getting Started: DHCP, UDP, IP, and Ethernet 512
6.7.2 Still Getting Started: DNS and ARP 514
6.7.3 Still Getting Started: Intra-Domain Routing to the DNS Server 515
6.7.4 Web Client-Server Interaction: TCP and HTTP 516
6.8 Summary 518
Wireshark Labs: 802.11 Ethernet 527
Interview: Albert Greenberg 528
Chapter 7 Wireless and Mobile Networks 531
7.1 Introduction 532
7.2 Wireless Links and Network Characteristics 536
7.2.1 CDMA 539

xxvi TABLE OF CONTENTS
7.3 WiFi: 802.11 Wireless LANs 542
7.3.1 The 802.11 Wireless LAN Architecture 544
7.3.2 The 802.11 MAC Protocol 548
7.3.3 The IEEE 802.11 Frame 553
7.3.4 Mobility in the Same IP Subnet 556
7.3.5 Advanced Features in 802.11 559
7.3.6 Personal Area Networks: Bluetooth 560
7.4 Cellular Networks: 4G and 5G 563
7.4.1 4G LTE Cellular Networks: Architecture and Elements 564
7.4.2 LTE Protocols Stacks 570
7.4.3 LTE Radio Access Network 571
7.4.4 Additional LTE Functions: Network Attachment and
Power Management 572
7.4.5 The Global Cellular Network: A Network of Networks 574
7.4.6 5G Cellular Networks 575
7.5 Mobility Management: Principles 578
7.5.1 Device Mobility: a Network-layer Perspective 578
7.5.2 Home Networks and Roaming on Visited Networks 579
7.5.3 Direct and Indirect Routing to/from a Mobile Device 580
7.6 Mobility Management in Practice 587
7.6.1 Mobility Management in 4G/5G Networks 587
7.6.2 Mobile IP 592
7.7 Wireless and Mobility: Impact on Higher-Layer Protocols 594
7.8 Summary 596
Wireshark Lab: WiFi 602
Interview: Deborah Estrin 603
Chapter 8 Security in Computer Networks 607
8.1 What Is Network Security? 608
8.2 Principles of Cryptography 610
8.2.1 Symmetric Key Cryptography 612
8.2.2 Public Key Encryption 618
8.3 Message Integrity and Digital Signatures 624
8.3.1 Cryptographic Hash Functions 625
8.3.2 Message Authentication Code 626
8.3.3 Digital Signatures 628
8.4 End-Point Authentication 634
8.5 Securing E-Mail 639
8.5.1 Secure E-Mail 640
8.5.2 PGP 643

TABLE OF CONTENTS xxvii
8.6 Securing TCP Connections: TLS 644
8.6.1 The Big Picture 646
8.6.2 A More Complete Picture 649
8.7 Network-Layer Security: IPsec and Virtual Private Networks 651
8.7.1 IPsec and Virtual Private Networks (VPNs) 651
8.7.2 The AH and ESP Protocols 653
8.7.3 Security Associations 653
8.7.4 The IPsec Datagram 655
8.7.5 IKE: Key Management in IPsec 658
8.8 Securing Wireless LANs and 4G/5G Cellular Networks 659
8.8.1 Authentication and Key Agreement in 802.11 Wireless LANs 659
8.8.2 Authentication and Key Agreement in 4G/5G Cellular Networks 664
8.9 Operational Security: Firewalls and Intrusion Detection Systems 667
8.9.1 Firewalls 667
8.9.2 Intrusion Detection Systems 675
8.10 Summary 679
Wireshark Lab: SSL 688
IPsec Lab 688
Interview: Steven M. Bellovin 689
References 691
Index 731

1
1
Today’s Internet is arguably the largest engineered system ever created by mankind,
with hundreds of millions of connected computers, communication links, and
switches; with billions of users who connect via laptops, tablets, and smartphones;
and with an array of new Internet-connected “things” including game consoles, sur-
veillance systems, watches, eye glasses, thermostats, and cars. Given that the Inter-
net is so large and has so many diverse components and uses, is there any hope of
understanding how it works? Are there guiding principles and structure that can
provide a foundation for understanding such an amazingly large and complex sys-
tem? And if so, is it possible that it actually could be both interesting and fun to
learn about computer networks? Fortunately, the answer to all of these questions is
a resounding YES! Indeed, it’s our aim in this book to provide you with a modern
introduction to the dynamic field of computer networking, giving you the princi-
ples and practical insights you’ll need to understand not only today’s networks, but
tomorrow’s as well.
This first chapter presents a broad overview of computer networking and the
Internet. Our goal here is to paint a broad picture and set the context for the rest
of this book, to see the forest through the trees. We’ll cover a lot of ground in this
introductory chapter and discuss a lot of the pieces of a computer network, without
losing sight of the big picture.
We’ll structure our overview of computer networks in this chapter as follows.
After introducing some basic terminology and concepts, we’ll first examine the basic
hardware and software components that make up a network. We’ll begin at the net-
work’s edge and look at the end systems and network applications running in the
network. We’ll then explore the core of a computer network, examining the links
1
CHAPTER
Computer
Networks and
the Internet
1

2 CHAPTER 1 • COMPUTER NETWORKS AND THE INTERNET
and the switches that transport data, as well as the access networks and physical
media that connect end systems to the network core. We’ll learn that the Internet is
a network of networks, and we’ll learn how these networks connect with each other.
After having completed this overview of the edge and core of a computer net-
work, we’ll take the broader and more abstract view in the second half of this chap-
ter. We’ll examine delay, loss, and throughput of data in a computer network and
provide simple quantitative models for end-to-end throughput and delay: models
that take into account transmission, propagation, and queuing delays. We’ll then
introduce some of the key architectural principles in computer networking, namely,
protocol layering and service models. We’ll also learn that computer networks are
vulnerable to many different types of attacks; we’ll survey some of these attacks and
consider how computer networks can be made more secure. Finally, we’ll close this
chapter with a brief history of computer networking.
1.1 What Is the Internet?
In this book, we’ll use the public Internet, a specific computer network, as our prin-
cipal vehicle for discussing computer networks and their protocols. But what is the
Internet? There are a couple of ways to answer this question. First, we can describe
the nuts and bolts of the Internet, that is, the basic hardware and software components
that make up the Internet. Second, we can describe the Internet in terms of a network-
ing infrastructure that provides services to distributed applications. Let’s begin with
the nuts-and-bolts description, using Figure 1.1 to illustrate our discussion.
1.1.1 A Nuts-and-Bolts Description
The Internet is a computer network that interconnects billions of computing devices
throughout the world. Not too long ago, these computing devices were primarily
traditional desktop computers, Linux workstations, and so-called servers that store
and transmit information such as Web pages and e-mail messages. Increasingly,
however, users connect to the Internet with smartphones and tablets—today, close
to half of the world’s population are active mobile Internet users with the percentage
expected to increase to 75% by 2025 [Statista 2019]. Furthermore, nontraditional
Internet “things” such as TVs, gaming consoles, thermostats, home security systems,
home appliances, watches, eye glasses, cars, traffic control systems, and more are
being connected to the Internet. Indeed, the term computer network is beginning to
sound a bit dated, given the many nontraditional devices that are being hooked up to
the Internet. In Internet jargon, all of these devices are called hosts or end systems.
By some estimates, there were about 18 billion devices connected to the Internet in
2017, and the number will reach 28.5 billion by 2022 [Cisco VNI 2020].

1.1 • WHAT IS THE INTERNET? 3
Figure 1.1 ♦ Some pieces of the Internet
Key:
Traffic light Thermostat Fridge
Datacenter Workstation
Host
(= end system)
Mobile
Computer
Base
station
Router Cell phone
tower
Smartphone
or tablet
Link-layer
switch
Server
Content Provider Network
National or
Global ISP
Datacenter Network
Datacenter Network
Mobile Network
Enterprise Network
Home Network
Local or
Regional ISP

End systems are connected together by a network of communication links and
packet switches. We’ll see in Section 1.2 that there are many types of communica-
tion links, which are made up of different types of physical media, including coaxial
cable, copper wire, optical fiber, and radio spectrum. Different links can transmit
data at different rates, with the transmission rate of a link measured in bits/second.
When one end system has data to send to another end system, the sending end system
segments the data and adds header bytes to each segment. The resulting packages
of information, known as packets in the jargon of computer networks, are then sent
through the network to the destination end system, where they are reassembled into
the original data.
A packet switch takes a packet arriving on one of its incoming communication
links and forwards that packet on one of its outgoing communication links. Packet
switches come in many shapes and flavors, but the two most prominent types in
today’s Internet are routers and link-layer switches. Both types of switches forward
packets toward their ultimate destinations. Link-layer switches are typically used in
access networks, while routers are typically used in the network core. The sequence
of communication links and packet switches traversed by a packet from the send-
ing end system to the receiving end system is known as a route or path through
the network. Cisco predicts annual global IP traffic will reach nearly five zettabytes
(1021
bytes) by 2022 [Cisco VNI 2020].
Packet-switched networks (which transport packets) are in many ways
similar to transportation networks of highways, roads, and intersections (which
transport vehicles). Consider, for example, a factory that needs to move a large
amount of cargo to some destination warehouse located thousands of kilometers
away. At the factory, the cargo is segmented and loaded into a fleet of trucks.
Each of the trucks then independently travels through the network of highways,
roads, and intersections to the destination warehouse. At the destination ware-
house, the cargo is unloaded and grouped with the rest of the cargo arriving
from the same shipment. Thus, in many ways, packets are analogous to trucks,
communication links are analogous to highways and roads, packet switches are
analogous to intersections, and end systems are analogous to buildings. Just as
a truck takes a path through the transportation network, a packet takes a path
through a computer network.
End systems access the Internet through Internet Service Providers (ISPs),
including residential ISPs such as local cable or telephone companies; corpo-
rate ISPs; university ISPs; ISPs that provide WiFi access in airports, hotels, cof-
fee shops, and other public places; and cellular data ISPs, providing mobile access
to our smartphones and other devices. Each ISP is in itself a network of packet
switches and communication links. ISPs provide a variety of types of network access
to the end systems, including residential broadband access such as cable modem
or DSL, high-speed local area network access, and mobile wireless access. ISPs
also provide Internet access to content providers, connecting servers directly to
the Internet. The Internet is all about connecting end systems to each other, so the

ISPs that provide access to end systems must also be interconnected. These lower-
tier ISPs are thus interconnected through national and international upper-tier ISPs
and these upper-tier ISPs are connected directly to each other. An upper-tier ISP
consists of high-speed routers interconnected with high-speed fiber-optic links. Each
ISP network, whether upper-tier or lower-tier, is managed independently, runs the
IP protocol (see below), and conforms to certain naming and address conventions.
We’ll examine ISPs and their interconnection more closely in Section 1.3.
End systems, packet switches, and other pieces of the Internet run protocols that
controlthesendingandreceivingofinformationwithintheInternet.TheTransmission
Control Protocol (TCP) and the Internet Protocol (IP) are two of the most impor-
tant protocols in the Internet. The IP protocol specifies the format of the packets
that are sent and received among routers and end systems. The Internet’s principal
protocols are collectively known as TCP/IP. We’ll begin looking into protocols in
this introductory chapter. But that’s just a start—much of this book is concerned with
networking protocols!
Given the importance of protocols to the Internet, it’s important that everyone
agree on what each and every protocol does, so that people can create systems and
products that interoperate. This is where standards come into play. Internet standards
are developed by the Internet Engineering Task Force (IETF) [IETF 2020]. The IETF
standards documents are called requests for comments (RFCs). RFCs started out
as general requests for comments (hence the name) to resolve network and protocol
design problems that faced the precursor to the Internet [Allman 2011]. RFCs tend
to be quite technical and detailed. They define protocols such as TCP, IP, HTTP (for
the Web), and SMTP (for e-mail). There are currently nearly 9000 RFCs. Other bod-
ies also specify standards for network components, most notably for network links.
The IEEE 802 LAN Standards Committee [IEEE 802 2020], for example, specifies
the Ethernet and wireless WiFi standards.
1.1.2 A Services Description
Our discussion above has identified many of the pieces that make up the Internet.
But we can also describe the Internet from an entirely different angle—namely, as
an infrastructure that provides services to applications. In addition to traditional
applications such as e-mail and Web surfing, Internet applications include mobile
smartphone and tablet applications, including Internet messaging, mapping with
real-time road-traffic information, music streaming movie and television streaming,
online social media, video conferencing, multi-person games, and location-based
recommendation systems. The applications are said to be distributed applications,
since they involve multiple end systems that exchange data with each other. Impor-
tantly, Internet applications run on end systems—they do not run in the packet
switches in the network core. Although packet switches facilitate the exchange of
data among end systems, they are not concerned with the application that is the
source or sink of data.

Let’s explore a little more what we mean by an infrastructure that provides
services to applications. To this end, suppose you have an exciting new idea for a dis-
tributed Internet application, one that may greatly benefit humanity or one that may
simply make you rich and famous. How might you go about transforming this idea
into an actual Internet application? Because applications run on end systems, you are
going to need to write programs that run on the end systems. You might, for example,
write your programs in Java, C, or Python. Now, because you are developing a dis-
tributed Internet application, the programs running on the different end systems will
need to send data to each other. And here we get to a central issue—one that leads
to the alternative way of describing the Internet as a platform for applications. How
does one program running on one end system instruct the Internet to deliver data to
another program running on another end system?
End systems attached to the Internet provide a socket interface that speci-
fies how a program running on one end system asks the Internet infrastructure to
deliver data to a specific destination program running on another end system. This
Internet socket interface is a set of rules that the sending program must follow so
that the Internet can deliver the data to the destination program. We’ll discuss the
Internet socket interface in detail in Chapter 2. For now, let’s draw upon a simple
analogy, one that we will frequently use in this book. Suppose Alice wants to send
a letter to Bob using the postal service. Alice, of course, can’t just write the letter
(the data) and drop the letter out her window. Instead, the postal service requires
that Alice put the letter in an envelope; write Bob’s full name, address, and zip
code in the center of the envelope; seal the envelope; put a stamp in the upper-
right-hand corner of the envelope; and finally, drop the envelope into an official
postal service mailbox. Thus, the postal service has its own “postal service inter-
face,” or set of rules, that Alice must follow to have the postal service deliver her
letter to Bob. In a similar manner, the Internet has a socket interface that the pro-
gram sending data must follow to have the Internet deliver the data to the program
that will receive the data.
The postal service, of course, provides more than one service to its custom-
ers. It provides express delivery, reception confirmation, ordinary use, and many
more services. In a similar manner, the Internet provides multiple services to its
applications. When you develop an Internet application, you too must choose one
of the Internet’s services for your application. We’ll describe the Internet’s ser-
vices in Chapter 2.
We have just given two descriptions of the Internet; one in terms of its hardware
and software components, the other in terms of an infrastructure for providing ser-
vices to distributed applications. But perhaps you are still confused as to what the
Internet is. What are packet switching and TCP/IP? What are routers? What kinds of
communication links are present in the Internet? What is a distributed application?
How can a thermostat or body scale be attached to the Internet? If you feel a bit over-
whelmed by all of this now, don’t worry—the purpose of this book is to introduce
you to both the nuts and bolts of the Internet and the principles that govern how and

why it works. We’ll explain these important terms and questions in the following
sections and chapters.
1.1.3 What Is a Protocol?
Now that we’ve got a bit of a feel for what the Internet is, let’s consider another
important buzzword in computer networking: protocol. What is a protocol? What
does a protocol do?
A Human Analogy
It is probably easiest to understand the notion of a computer network protocol by
first considering some human analogies, since we humans execute protocols all of
the time. Consider what you do when you want to ask someone for the time of day.
A typical exchange is shown in Figure 1.2. Human protocol (or good manners, at
Figure 1.2 ♦ A human protocol and a computer network protocol
GET http://www.pearsonhighered.com/
cs-resources/
TCP connection request
Time Time
TCP connection reply
<file>
Hi
Got the time?
Time Time
Hi
2:00

least) dictates that one first offer a greeting (the first “Hi” in Figure 1.2) to initiate
communication with someone else. The typical response to a “Hi” is a returned
“Hi” message. Implicitly, one then takes a cordial “Hi” response as an indication
that one can proceed and ask for the time of day. A different response to the initial
“Hi” (such as “Don’t bother me!” or “I don’t speak English,” or some unprintable
reply) might indicate an unwillingness or inability to communicate. In this case,
the human protocol would be not to ask for the time of day. Sometimes one gets no
response at all to a question, in which case one typically gives up asking that person
for the time. Note that in our human protocol, there are specific messages we send,
and specific actions we take in response to the received reply messages or other
events (such as no reply within some given amount of time). Clearly, transmitted
and received messages, and actions taken when these messages are sent or received
or other events occur, play a central role in a human protocol. If people run differ-
ent protocols (for example, if one person has manners but the other does not, or if
one understands the concept of time and the other does not) the protocols do not
interoperate and no useful work can be accomplished. The same is true in network-
ing—it takes two (or more) communicating entities running the same protocol in
order to accomplish a task.
Let’s consider a second human analogy. Suppose you’re in a college class (a
computer networking class, for example!). The teacher is droning on about protocols
and you’re confused. The teacher stops to ask, “Are there any questions?” (a message
that is transmitted to, and received by, all students who are not sleeping). You raise
your hand (transmitting an implicit message to the teacher). Your teacher acknowl-
edges you with a smile, saying “Yes . . .” (a transmitted message encouraging you
to ask your question—teachers love to be asked questions), and you then ask your
question (that is, transmit your message to your teacher). Your teacher hears your
question (receives your question message) and answers (transmits a reply to you).
Once again, we see that the transmission and receipt of messages, and a set of con-
ventional actions taken when these messages are sent and received, are at the heart
of this question-and-answer protocol.
Network Protocols
A network protocol is similar to a human protocol, except that the entities exchang-
ing messages and taking actions are hardware or software components of some
device (for example, computer, smartphone, tablet, router, or other network-capable
device). All activity in the Internet that involves two or more communicating remote
entities is governed by a protocol. For example, hardware-implemented protocols in
two physically connected computers control the flow of bits on the “wire” between
the two network interface cards; congestion-control protocols in end systems control
the rate at which packets are transmitted between sender and receiver; protocols in
routers determine a packet’s path from source to destination. Protocols are running

1.2 • THE NETWORK EDGE 9
everywhere in the Internet, and consequently much of this book is about computer
network protocols.
As an example of a computer network protocol with which you are probably
familiar, consider what happens when you make a request to a Web server, that
is, when you type the URL of a Web page into your Web browser. The scenario is
illustrated in the right half of Figure 1.2. First, your computer will send a connec-
tion request message to the Web server and wait for a reply. The Web server will
eventually receive your connection request message and return a connection reply
message. Knowing that it is now OK to request the Web document, your computer
then sends the name of the Web page it wants to fetch from that Web server in a
GET message. Finally, the Web server returns the Web page (file) to your computer.
Given the human and networking examples above, the exchange of messages
and the actions taken when these messages are sent and received are the key defining
elements of a protocol:
A protocol defines the format and the order of messages exchanged between two
or more communicating entities, as well as the actions taken on the transmission
and/or receipt of a message or other event.
The Internet, and computer networks in general, make extensive use of pro-
tocols. Different protocols are used to accomplish different communication tasks.
As you read through this book, you will learn that some protocols are simple and
straightforward, while others are complex and intellectually deep. Mastering the
field of computer networking is equivalent to understanding the what, why, and how
of networking protocols.
1.2 The Network Edge
In the previous section, we presented a high-level overview of the Internet and
networking protocols. We are now going to delve a bit more deeply into the com-
ponents of the Internet. We begin in this section at the edge of the network and
look at the components with which we are most familiar—namely, the computers,
smartphones and other devices that we use on a daily basis. In the next section, we’ll
move from the network edge to the network core and examine switching and routing
in computer networks.
Recall from the previous section that in computer networking jargon, the com-
puters and other devices connected to the Internet are often referred to as end sys-
tems. They are referred to as end systems because they sit at the edge of the Internet,
as shown in Figure 1.3. The Internet’s end systems include desktop computers

Figure 1.3 ♦ End-system interaction
National or
Global ISP
Datacenter Network
Datacenter Network
Mobile Network
Enterprise Network
Home Network
Local or
Regional ISP
(e.g., desktop PCs, Macs, and Linux boxes), servers (e.g., Web and e-mail servers),
and mobile devices (e.g., laptops, smartphones, and tablets). Furthermore, an
increasing number of non-traditional “things” are being attached to the Internet as
end systems (see the Case History feature).
End systems are also referred to as hosts because they host (that is, run) appli-
cation programs such as a Web browser program, a Web server program, an e-mail

client program, or an e-mail server program. Throughout this book we will use the
terms hosts and end systems interchangeably; that is, host = end system. Hosts
are sometimes further divided into two categories: clients and servers. Infor-
mally, clients tend to be desktops, laptops, smartphones, and so on, whereas
servers tend to be more powerful machines that store and distribute Web pages,
stream video, relay e-mail, and so on. Today, most of the servers from which we
receive search results, e-mail, Web pages, videos and mobile app content reside
in large data centers. For example, as of 2020, Google has 19 data centers on four
continents, collectively containing several million servers. Figure 1.3 includes
two such data centers, and the Case History sidebar describes data centers in
more detail.
DATA CENTERS AND CLOUD COMPUTING
Internet companies such as Google, Microsoft, Amazon, and Alibaba have built
massive data centers, each housing tens to hundreds of thousands of hosts. These
data centers are not only connected to the Internet, as shown in Figure 1.1, but also
internally include complex computer networks that interconnect the datacenter’s hosts.
The data centers are the engines behind the Internet applications that we use on a
daily basis.
Broadly speaking, data centers serve three purposes, which we describe here in
the context of Amazon for concreteness. First, they serve Amazon e-commerce pages
to users, for example, pages describing products and purchase information. Second,
they serve as massively parallel computing infrastructures for Amazon-specific data
processing tasks. Third, they provide cloud computing to other companies. Indeed,
today a major trend in computing is for companies to use a cloud provider such as
Amazon to handle essentially all of their IT needs. For example, Airbnb and many
other Internet-based companies do not own and manage their own data centers but
instead run their entire Web-based services in the Amazon cloud, called Amazon
Web Services (AWS).
The worker bees in a data center are the hosts. They serve content (e.g., Web
pages and videos), store e-mails and documents, and collectively perform massively
distributed computations. The hosts in data centers, called blades and resembling
pizza boxes, are generally commodity hosts that include CPU, memory, and disk
storage. The hosts are stacked in racks, with each rack typically having 20 to
40 blades. The racks are then interconnected using sophisticated and evolving data
center network designs. Data center networks are discussed in greater detail in
Chapter 6.
CASE HISTORY

1.2.1 Access Networks
Having considered the applications and end systems at the “edge of the network,”
let’s next consider the access network—the network that physically connects an end
system to the first router (also known as the “edge router”) on a path from the end
system to any other distant end system. Figure 1.4 shows several types of access
Figure 1.4 ♦ Access networks
National or
Global ISP
Datacenter Network
Datacenter Network
Mobile Network
Enterprise Network
Home Network
Local or
Regional ISP

networks with thick, shaded lines and the settings (home, enterprise, and wide-area
mobile wireless) in which they are used.
Home Access: DSL, Cable, FTTH, and 5G Fixed Wireless
As of 2020, more than 80% of the households in Europe and the USA have Internet
access [Statista 2019]. Given this widespread use of home access networks let’s begin
our overview of access networks by considering how homes connect to the Internet.
Today, the two most prevalent types of broadband residential access are
digital subscriber line (DSL) and cable. A residence typically obtains DSL
Internet access from the same local telephone company (telco) that provides its
wired local phone access. Thus, when DSL is used, a customer’s telco is also
its ISP. As shown in Figure 1.5, each customer’s DSL modem uses the existing
telephone line exchange data with a digital subscriber line access multiplexer
(DSLAM) located in the telco’s local central office (CO). The home’s DSL
modem takes digital data and translates it to high-frequency tones for transmis-
sion over telephone wires to the CO; the analog signals from many such houses
are translated back into digital format at the DSLAM.
The residential telephone line carries both data and traditional telephone signals
simultaneously, which are encoded at different frequencies:
• A high-speed downstream channel, in the 50 kHz to 1 MHz band
• A medium-speed upstream channel, in the 4 kHz to 50 kHz band
• An ordinary two-way telephone channel, in the 0 to 4 kHz band
This approach makes the single DSL link appear as if there were three separate
links, so that a telephone call and an Internet connection can share the DSL link at
Figure 1.5 ♦ DSL Internet access
Home PC
Home
phone
DSL
modem
Internet
Telephone
network
Splitter
Existing phone line:
0-4KHz phone; 4-50KHz
upstream data; 50KHz–
1MHz downstream data
Central
office
DSLAM

the same time. (We’ll describe this technique of frequency-division multiplexing
in Section 1.3.1.) On the customer side, a splitter separates the data and telephone
signals arriving to the home and forwards the data signal to the DSL modem. On the
telco side, in the CO, the DSLAM separates the data and phone signals and sends
the data into the Internet. Hundreds or even thousands of households connect to a
single DSLAM.
The DSL standards define multiple transmission rates, including downstream
transmission rates of 24 Mbs and 52 Mbs, and upstream rates of 3.5 Mbps and
16 Mbps; the newest standard provides for aggregate upstream plus downstream
rates of 1 Gbps [ITU 2014]. Because the downstream and upstream rates are dif-
ferent, the access is said to be asymmetric. The actual downstream and upstream
transmission rates achieved may be less than the rates noted above, as the DSL
provider may purposefully limit a residential rate when tiered service (different
rates, available at different prices) are offered. The maximum rate is also limited
by the distance between the home and the CO, the gauge of the twisted-pair line
and the degree of electrical interference. Engineers have expressly designed DSL
for short distances between the home and the CO; generally, if the residence is not
located within 5 to 10 miles of the CO, the residence must resort to an alternative
form of Internet access.
While DSL makes use of the telco’s existing local telephone infrastructure,
cable Internet access makes use of the cable television company’s existing cable
television infrastructure. A residence obtains cable Internet access from the same
company that provides its cable television. As illustrated in Figure 1.6, fiber optics
Figure 1.6 ♦ A hybrid fiber-coaxial access network
Fiber
cable
Coaxial cable
Hundreds
of homes
Cable head end
Hundreds
of homes
Fiber
node
Fiber
node
Internet
CMTS

connect the cable head end to neighborhood-level junctions, from which tradi-
tional coaxial cable is then used to reach individual houses and apartments. Each
neighborhood junction typically supports 500 to 5,000 homes. Because both fiber
and coaxial cable are employed in this system, it is often referred to as hybrid fiber
coax (HFC).
Cable internet access requires special modems, called cable modems. As
with a DSL modem, the cable modem is typically an external device and con-
nects to the home PC through an Ethernet port. (We will discuss Ethernet in
great detail in Chapter 6.) At the cable head end, the cable modem termination
system (CMTS) serves a similar function as the DSL network’s DSLAM—
turning the analog signal sent from the cable modems in many downstream
homes back into digital format. Cable modems divide the HFC network into two
channels, a downstream and an upstream channel. As with DSL, access is typi-
cally asymmetric, with the downstream channel typically allocated a higher
transmission rate than the upstream channel. The DOCSIS 2.0 and 3.0 standards
define downstream bitrates of 40 Mbps and 1.2 Gbps, and upstream rates
of 30 Mbps and 100 Mbps, respectively. As in the case of DSL networks, the
maximum achievable rate may not be realized due to lower contracted data rates
or media impairments.
One important characteristic of cable Internet access is that it is a shared broad-
cast medium. In particular, every packet sent by the head end travels downstream on
every link to every home and every packet sent by a home travels on the upstream
channel to the head end. For this reason, if several users are simultaneously down-
loading a video file on the downstream channel, the actual rate at which each user
receives its video file will be significantly lower than the aggregate cable down-
stream rate. On the other hand, if there are only a few active users and they are all
Web surfing, then each of the users may actually receive Web pages at the full cable
downstream rate, because the users will rarely request a Web page at exactly the
same time. Because the upstream channel is also shared, a distributed multiple access
protocol is needed to coordinate transmissions and avoid collisions. (We’ll discuss
this collision issue in some detail in Chapter 6.)
Although DSL and cable networks currently represent the majority of residential
broadband access in the United States, an up-and-coming technology that provides
even higher speeds is fiber to the home (FTTH) [Fiber Broadband 2020]. As the
name suggests, the FTTH concept is simple—provide an optical fiber path from
the CO directly to the home. FTTH can potentially provide Internet access rates in
the gigabits per second range.
There are several competing technologies for optical distribution from the CO
to the homes. The simplest optical distribution network is called direct fiber, with
one fiber leaving the CO for each home. More commonly, each fiber leaving the
central office is actually shared by many homes; it is not until the fiber gets rela-
tively close to the homes that it is split into individual customer-specific fibers.
There are two competing optical-distribution network architectures that perform

this splitting: active optical networks (AONs) and passive optical networks (PONs).
AON is essentially switched Ethernet, which is discussed in Chapter 6.
Here, we briefly discuss PON, which is used in Verizon’s FiOS service.
Figure 1.7 shows FTTH using the PON distribution architecture. Each home has
an optical network terminator (ONT), which is connected by dedicated optical
fiber to a neighborhood splitter. The splitter combines a number of homes (typi-
cally less than 100) onto a single, shared optical fiber, which connects to an optical
line terminator (OLT) in the telco’s CO. The OLT, providing conversion between
optical and electrical signals, connects to the Internet via a telco router. At home,
users connect a home router (typically a wireless router) to the ONT and access the
Internet via this home router. In the PON architecture, all packets sent from OLT to
the splitter are replicated at the splitter (similar to a cable head end).
In addition to DSL, Cable, and FTTH, 5G fixed wireless is beginning to be
deployed. 5G fixed wireless not only promises high-speed residential access, but
will do so without installing costly and failure-prone cabling from the telco’s
CO to the home. With 5G fixed wireless, using beam-forming technology, data
is sent wirelessly from a provider’s base station to the a modem in the home.
A WiFi wireless router is connected to the modem (possibly bundled together),
similar to how a WiFi wireless router is connected to a cable or DSL modem.
5G cellular networks are covered in Chapter 7.
Access in the Enterprise (and the Home): Ethernet and WiFi
On corporate and university campuses, and increasingly in home settings, a local
area network (LAN) is used to connect an end system to the edge router. Although
there are many types of LAN technologies, Ethernet is by far the most preva-
lent access technology in corporate, university, and home networks. As shown in
Figure 1.7 ♦ FTTH Internet access
Internet
Central office
Optical
splitter
ONT
ONT
ONT
OLT
Optical
fibers

Figure 1.8, Ethernet users use twisted-pair copper wire to connect to an Ethernet
switch, a technology discussed in detail in Chapter 6. The Ethernet switch, or a
network of such interconnected switches, is then in turn connected into the larger
Internet. With Ethernet access, users typically have 100 Mbps to tens of Gbps
access to the Ethernet switch, whereas servers may have 1 Gbps 10 Gbps access.
Increasingly, however, people are accessing the Internet wirelessly from lap-
tops, smartphones, tablets, and other “things”. In a wireless LAN setting, wireless
users transmit/receive packets to/from an access point that is connected into the
enterprise’s network (most likely using wired Ethernet), which in turn is connected
to the wired Internet. A wireless LAN user must typically be within a few tens of
meters of the access point. Wireless LAN access based on IEEE 802.11 technol-
ogy, more colloquially known as WiFi, is now just about everywhere—universities,
business offices, cafes, airports, homes, and even in airplanes. As discussed in detail
in Chapter 7, 802.11 today provides a shared transmission rate of up to more than
100 Mbps.
Even though Ethernet and WiFi access networks were initially deployed in
enterprise (corporate, university) settings, they are also common components of
home networks. Many homes combine broadband residential access (that is, cable
modems or DSL) with these inexpensive wireless LAN technologies to create pow-
erful home networks Figure 1.9 shows a typical home network. This home network
consists of a roaming laptop, multiple Internet-connected home appliances, as well
as a wired PC; a base station (the wireless access point), which communicates with
the wireless PC and other wireless devices in the home; and a home router that con-
nects the wireless access point, and any other wired home devices, to the Internet.
This network allows household members to have broadband access to the Internet
with one member roaming from the kitchen to the backyard to the bedrooms.
Figure 1.8 ♦ Ethernet Internet access
Ethernet
switch
Institutional
router
1 Gbps
1 Gbps
1 Gbps
Server
To Institution’s
ISP

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and burn the Heavens, and consume this world into hot Embers,
which, said they, could not be done, if the Sun were not fiery of it
self. Others again said, This opinion could not stand with reason; for
Fire being a destroyer of all things, the Sun-Stone after this manner
would burn up all the near adjoining Bodies: Besides, said they, Fire
cannot subsist without fuel; and the Sun-Stone having nothing to
feed on, would in a short time consume it self; wherefore they
thought it more probable that the Sun was not actually hot, but onely
by the reflection of its light; so that its heat was an effect of its light,
both being immaterial. But this opinion again was laught at by others,
and rejected as ridiculous, who thought it impossible that one
immaterial should produce another; and believed that both the light
and heat of the Sun proceeded from a swift Circular motion of the
Æthereal Globules, which by their striking upon the Optick nerve,
caused light, and their motion produced heat: But neither would this
opinion hold; for, said some, then it would follow, that the sight of
Animals is the cause of light; and that, were there no eyes, there
would be no light; which was against all sense and reason. Thus they
argued concerning the heat and light of the Sun; but, which is
remarkable, none did say, that the Sun was a Globous fluid body, and
had a swift Circular motion; but all agreed, It was fixt and firm like a
Center, and therefore they generally called it the Sun-Stone.
Then the Empress asked them the reason, Why the Sun and Moon
did often appear in different postures or shapes, as sometimes
magnified, sometimes diminished; sometimes elevated, otherwhiles
depressed; now thrown to the right, and then to the left? To which
some of the Bird-men answered, That it proceeded from the various
degrees of heat and cold, which are found in the Air, from whence did
follow a differing density and rarity; and likewise from the vapours
that are interposed, whereof those that ascend are higher and less
dense then the ambient air, but those which descend are heavier and
more dense. But others did with more probability affirm, that it was
nothing else but the various patterns of the Air; for like as Painters do
not copy out one and the same original just alike at all times; so, said
they, do several parts of the Air make different patterns of the

luminous Bodies of the Sun and Moon: which patterns, as several
copies, the sensitive motions do figure out in the substance of our
eyes.
This answer the Empress liked much better then the former, and
enquired further, What opinion they had of those Creatures that are
called the motes of the Sun? To which they answered, That they
were nothing else but streams of very small, rare and transparent
particles, through which the Sun was represented as through a glass:
for if they were not transparent, said they, they would eclipse the
light of the Sun; and if not rare and of an airy substance, they would
hinder Flies from flying in the Air, at least retard their flying motion:
Nevertheless, although they were thinner then the thinnest vapour,
yet were they not so thin as the body of air, or else they would not
be perceptible by animal sight. Then the Empress asked, Whether
they were living Creatures? They answered, Yes: Because they did
encrease and decrease, and were nourished by the presence, and
starved by the absence of the Sun.
Having thus finished their discourse of the Sun and Moon, the
Empress desired to know what Stars there were besides? But they
answer'd, that they could perceive in that World none other but
Blazing Stars, and from thence it had the name that it was called the
Blazing-World; and these Blazing-Stars, said they, were such solid,
firm and shining bodies as the Sun and Moon, not of a Globular, but
of several sorts of figures: some had tails; and some, other kinds of
shapes.
After this, The Empress asked them, What kind of substance or
creature the Air was? The Bird-men answered, That they could have
no other perception of the Air, but by their own Respiration: For, said
they, some bodies are onely subject to touch, others onely to sight,
and others onely to smell; but some are subject to none of our
exterior Senses: For Nature is so full of variety, that our weak Senses
cannot perceive all the various sorts of her Creatures; neither is there
any one object perceptible by all our Senses, no more then several
objects are by one sense. I believe you, replied the Empress; but if
you can give no account of the Air, said she, you will hardly be able

to inform me how Wind is made; for they say, that Wind is nothing
but motion of the Air. The Bird-men answer'd, That they observed
Wind to be more dense then Air, and therefore subject to the sense
of Touch; but what properly Wind was, and the manner how it was
made, they could not exactly tell; some said, it was caused by the
Clouds falling on each other; and others, that it was produced of a
hot and dry exhalation: which ascending, was driven down again by
the coldness of the Air that is in the middle Region, and by reason of
its leightness, could not go directly to the bottom, but was carried by
the Air up and down: Some would have it a flowing Water of the Air;
and others again, a flowing Air moved by the blaz of the Stars.
But the Empress, seeing they could not agree concerning the cause
of Wind, asked, Whether they could tell how Snow was made? To
which they answered That according to their observation, Snow was
made by a commixture of Water, and some certain extract of the
Element of Fire that is under the Moon; a small portion of which
extract, being mixed with Water, and beaten by Air or Wind, made a
white Froth called Snow; which being after some while dissolved by
the heat of the same spirit, turned to Water again. This observation
amazed the Empress very much; for she had hitherto believed, That
Snow was made by cold motions, and not by such an agitation or
beating of a fiery extract upon water: Nor could she be perswaded to
believe it until the Fish- or Mear-men had delivered their observation
upon the making of Ice, which, they said, was not produced, as some
hitherto conceived, by the motion of the Air, raking the Superficies of
the Earth, but by some strong saline vapour arising out of the Seas,
which condensed Water into Ice; and the more quantity there was of
that vapour, the greater were the Mountains of Precipices of Ice; but
the reason that it did not so much freeze in the Torrid Zone, or under
the Ecliptick, as near or under the Poles, was, that this vapour in
those places being drawn up by the Sun-beams into the middle
Region of the Air, was onely condensed into Water, and fell down in
showres of Rain; when as, under the Poles, the heat of the Sun being
not so vehement, the same vapour had no force or power to rise so

high, and therefore caused so much Ice, by ascending and acting
onely upon the surface of water.
This Relation confirmed partly the observation of the Bird-men
concerning the cause of Snow; but since they had made mention that
that same extract, which by its commixture with Water made Snow,
proceeded from the Element of Fire, that is under the Moon: The
Emperess asked them, of what nature that Elementary Fire was;
whether it was like ordinary Fire here upon Earth, or such a Fire as is
within the bowels of the Earth, and as the famous Mountains
Vesuvius and Ætna do burn withal; or whether it was such a sort of
fire, as is found in flints, &c. They answered, That the Elementary
Fire, which is underneath the Sun, was not so solid as any of those
mentioned fires; because it had no solid fuel to feed on; but yet it
was much like the flame of ordinary fire, onely somewhat more thin
and fluid; for Flame, said they, is nothing else but the airy part of a
fired Body.
Lastly, the Empress asked the Bird-men of the nature of Thunder and
Lightning? and whether it was not caused by roves of Ice falling upon
each other? To which they answered, That it was not made that way,
but by an encounter of cold and heat; so that an exhalation being
kindled in the Clouds, did dash forth Lightning, and that there were
so many rentings of Clouds as there were Sounds and Cracking
noises: But this opinion was contradicted by others, who affirmed
that Thunder was a sudden and monstrous Blaz, stirred up in the Air,
and did not always require a Cloud; but the Empress not knowing
what they meant by Blaz (for even they themselves were not able to
explain the sense of this word) liked the former better; and, to avoid
hereafter tedious disputes, and have the truth of the Phænomena's
of Cœlestial Bodies more exactly known, commanded the Bear-men,
which were her Experimental Philosophers, to observe them through
such Instruments as are called Telescopes, which they did according
to her Majesties Command; but these Telescopes caused more
differences and divisions amongst them, then ever they had before;
for some said, they perceived that the Sun stood still, and the Earth
did move about it; others were of opinion, that they both did move;

and others said again, that the Earth stood still, and Sun did move;
some counted more Stars then others; some discovered new Stars
never seen before; some fell into a great dispute with others
concerning the bigness of the Stars; some said, The Moon was
another World like their Terrestrial Globe, and the spots therein were
Hills and Vallies; but others would have the spots to be the Terrestrial
parts, and the smooth and glossie parts, the Sea: At last, the
Empress commanded them to go with their Telescopes to the very
end of the Pole that was joined to the World she came from, and try
whether they could perceive any Stars in it: which they did; and,
being returned to her Majesty, reported that they had seen three
Blazing-Stars appear there, one after another in a short time,
whereof two were bright, and one dim; but they could not agree
neither in this observation: for some said, It was but one Star which
appeared at three several times, in several places; and others would
have them to be three several Stars; for they thought it impossible,
that those three several appearances should have been but one Star,
because every Star did rise at a certain time, and appear'd in a
certain place, and did disappear in the same place: Next, It is
altogether improbable, said they, That one Star should fly from place
to place, especially at such a vast distance, without a visible motion;
in so short a time, and appear in such different places, whereof two
were quite opposite, and the third side-ways: Lastly, If it had been
but one Star, said they, it would always have kept the same splendor,
which it did not; for, as above mentioned, two were bright, and one
was dim. After they had thus argued, the Empress began to grow
angry at their Telescopes, that they could give no better Intelligence;
for, said she, now I do plainly perceive, that your Glasses are false
Informers, and instead of discovering the Truth, delude your Senses;
Wherefore I Command you to break them, and let the Bird-men trust
onely to their natural eyes, and examine Cœlestial Objects by the
motions of their own Sense and Reason. The Bear-men replied, That
it was not the fault of their Glasses, which caused such differences in
their Opinions, but the sensitive motions in their Optick organs did
not move alike, nor were their rational judgments always regular: To
which the Empress answered, That if their Glasses were true

Informers, they would rectifie their irregular Sense and Reason; But,
said she, Nature has made your Sense and Reason more regular then
Art has your Glasses; for they are meer deluders, and will never lead
you to the knowledg of Truth; Wherefore I command you again to
break them; for you may observe the progressive motions of
Cœlestial Bodies with your natural eyes better then through Artificial
Glasses. The Bear-men being exceedingly troubled at her Majesties
displeasure concerning their Telescopes, kneel'd down, and in the
humblest manner petitioned, that they might not be broken; for, said
they, we take more delight in Artificial delusions, then in Natural
truths. Besides, we shall want Imployments for our Senses, and
Subjects for Arguments; for, were there nothing but truth, and no
falshood, there would be no occasion to dispute, and by this means
we should want the aim and pleasure of our endeavors in confuting
and contradicting each other; neither would one man be thought
wiser then another, but all would either be alike knowing and wise, or
all would be fools; wherefore we most humbly beseech your Imperial
Majesty to spare our Glasses, which are our onely delight, and as
dear to us as our lives. The Empress at last consented to their
request, but upon condition, that their disputes and quarrels should
remain within their Schools, and cause no factions or disturbances in
State, or Government. The Bear-men, full of joy, returned their most
humble thanks to the Empress; and to make her amends for the
displeasure which their Telescopes had occasioned, told her Majesty,
that they had several other artificial Optick-Glasses, which they were
sure would give her Majesty a great deal more satisfaction. Amongst
the rest, they brought forth several Microscopes, by the means of
which they could enlarge the shapes of little bodies, and make a
Lowse appear as big as an Elephant, and a Mite as big as a Whale.
First of all they shewed the Empress a gray Drone-flye, wherein they
observed that the greatest part of her face, nay, of her head,
consisted of two large bunches all cover'd over with a multitude of
small Pearls or Hemispheres in a Trigonal order: Which Pearls were of
two degrees, smaller and bigger; the smaller degree was lowermost,
and looked towards the ground; the other was upward, and looked
sideward, forward and backward: They were all so smooth and

polished, that they were able to represent the image of any object,
the number of them was in all 14000. After the view of this strange
and miraculous Creature, and their several observations upon it, the
Empress asked them, What they judged those little Hemispheres
might be? They answered, That each of them was a perfect Eye, by
reason they perceived that each was covered with a Transparent
Cornea, containing a liquor within them, which resembled the watery
or glassie humor of the Eye. To which the Emperess replied, That
they might be glassie Pearls, and yet not Eyes; and that perhaps their
Microscopes did not truly inform them. But they smilingly answered
her Majesty, That she did not know the vertue of those Microscopes:
for they never delude, but rectifie and inform the Senses; nay, the
World, said they, would be but blind without them, as it has been in
former ages before those Microscopes were invented.
After this, they took a Charcoal, and viewing it with one of their best
Microscopes, discovered in it an infinite multitude of pores, some
bigger, some less; so close and thick, that they left but very little
space betwixt them to be filled with a solid body; and to give her
Imperial Majesty a better assurance thereof, they counted in a line of
them an inch long, no less then 2700 pores; from which Observation
they drew this following Conclusion, to wit, That this multitude of
pores was the cause of the blackness of the Coal; for, said they, a
body that has so many pores, from each of which no light is
reflected, must necessarily look black, since black is nothing else but
a privation of light, or a want of reflection. But the Empress replied,
That if all Colours were made by reflection of light, and that Black
was as much a colour as any other colour; then certainly they
contradicted themselves in saying that black was made by want of
reflection. However, not to interrupt your Microscopical Inspections,
said she, let us see how Vegetables appear through your Glasses;
whereupon they took a Nettle, and by the vertue of the Microscope,
discovered that underneath the points of the Nettle there were
certain little bags or bladders, containing a poysonous liquor, and
when the points had made way into the interior parts of the skin,
they like Syringe-pipes served to conveigh that same liquor into

them. To which Observation the Empress replied, That if there were
such poyson in Nettles, then certainly in eating of them, they would
hurt us inwardly, as much as they do outwardly? But they answered,
That it belonged to Physicians more then to Experimental
Philosophers, to give Reasons hereof; for they only made
Microscopical inspections, and related the Figures of the Natural parts
of Creatures according to the representation of their glasses.
Lastly, They shewed the Empress a Flea, and a Lowse; which
Creatures through the Microscope appear'd so terrible to her sight,
that they had almost put her into a swoon; the description of all their
parts would be very tedious to relate, and therefore I'le forbear it at
this present. The Empress, after the view of those strangely-shaped
Creatures, pitied much those that are molested with them, especially
poor Beggars, which although they have nothing to live on
themselves, are yet necessitated to maintain and feed of their own
flesh and blood, a company of such terrible Creatures called Lice;
who, instead of thanks, do reward them with pains, and torment
them for giving them nourishment and food. But after the Empress
had seen the shapes of these monstrous Creatures, she desir'd to
know, Whether their Microscopes could hinder their biting, or at least
shew some means how to avoid them? To which they answered, That
such Arts were mechanical and below the noble study of
Microscopical observations. Then the Empress asked them, Whether
they had not such sorts of Glasses that could enlarge and magnifie
the shapes of great Bodies as well as they had done of little ones?
Whereupon they took one of their best and largest Microscopes, and
endeavoured to view a Whale thorow it; but alas! the shape of the
Whale was so big, that its Circumference went beyond the
magnifying quality of the Glass; whether the error proceeded from
the Glass, or from a wrong position of the Whale against the
reflection of light, I cannot certainly tell. The Empress seeing the
insufficiency of those Magnifying-Glasses, that they were not able to
enlarge all sorts of Objects, asked the Bear-men, whether they could
not make Glasses of a contrary nature to those they had shewed her,
to wit, such as instead of enlarging or magnifying the shape or figure

of an Object, could contract it beneath its natural proportion: Which,
in obedience to her Majesties Commands, they did; and viewing
through one of the best of them, a huge and mighty Whale appear'd
no bigger then a Sprat; nay, through some no bigger then a Vinegar-
Eele; and through their ordinary ones, an Elephant seemed no bigger
then a Flea; a Camel no bigger then a Lowse; and an Ostrich no
bigger then a Mite. To relate all their Optick observations through the
several sorts of their Glasses, would be a tedious work, and tire even
the most patient Reader, wherefore I'le pass them by; onely this was
very remarkable and worthy to be taken notice of, that
notwithstanding their great skil, industry and ingenuity in
Experimental Philosophy, they could yet by no means contrive such
Glasses, by the help of which they could spy out a Vacuum, with all
its dimensions, nor Immaterial substances, Non-beings, and Mixt-
beings, or such as are between something and nothing; which they
were very much troubled at, hoping that yet, in time, by long study
and practice, they might perhaps attain to it.
The Bird- and Bear-men being dismissed, the Empress called both the
Syrens- or Fish-men, and the Worm-men, to deliver their
Observations which they had made, both within the Seas, and the
Earth. First, she enquired of the Fish-men whence the saltness of the
Sea did proceed? To which they answered, That there was a volatile
salt in those parts of the Earth, which as a bosom contain the Waters
of the Sea, which Salt being imbibed by the Sea, became fixt; and
this imbibing motion was that they call'd the Ebbing and Flowing of
the Sea; for, said they, the rising and swelling of the Water, is caused
by those parts of the volatile Salt as are not so easily imbibed, which
striving to ascend above the Water, bear it up with such a motion, as
Man, or some other Animal Creature, in a violent exercise uses to
take breath. This they affirmed to be the true cause both of the
saltness, and the ebbing and flowing-motion of the Sea, and not the
jogging of the Earth, or the secret influence of the Moon, as some
others had made the World believe.
After this, the Empress enquired, Whether they had observed, that all
Animal Creatures within the Seas and other waters, had blood? They

answered, That some had blood, more or less, but some had none.
In Crea-fishes and Lobsters, said they, we perceive but little blood;
but in Crabs, Oysters, Cockles, &c. none at all. Then the Empress
asked them, in what part of their Bodies that little blood did reside?
They answered, in a small vein, which in Lobsters went through the
middle of their tails, but in Crea-fishes was found in their backs: as
for other sorts of Fishes, some, said they, had onely blood about their
Gills, and others in some other places of their Bodies; but they had
not as yet observed any whose veins did spread all over their Bodies.
The Empress wondring that there could be living Animals without
Blood, to be better satisfied, desired the Worm-men to inform her,
whether they had observed Blood in all sorts of Worms? They
answered, That, as much as they could perceive, some had Blood,
and some not; a Moth, said they, had no Blood at all, and a Lowse
had, but like a Lobster, a little Vein along her back: Also Nits, Snails,
and Maggots, as well as those that are generated out of Cheese and
Fruits, as those that are produced out of Flesh, had no blood: But,
replied the Empress, If those mentioned creatures have no blood,
how is it possible they can live? for it is commonly said, That the life
of an Animal consists in the blood, which is the seat of the Animal
spirits. They answered, That blood was not a necessary propriety to
the life of an Animal; and that that which was commonly called
Animal spirits, was nothing else but corporeal motions proper to the
nature and figure of an Animal. Then she asked both the Fish- and
Worm-men, whether all those Creatures that have blood, had a
circulation of blood in their veins and arteries? But they answered,
That it was impossible to give her Majesty an exact account thereof,
by reason the circulation of blood was an interior motion, which their
senses, neither of themselves, nor by the help of any Optick
Instrument could perceive; but as soon as they had dissected an
Animal Creature, to find out the truth thereof, the interior corporeal
motions proper to that particular figure or creature, were altered.
Then said the Empress, If all Animal Creatures have not blood, it is
certain, they all have neither Muscles, tendons, nerves, &c. But, said
she, Have you ever observed Animal Creatures that are neither flesh,
nor Fish, but of an intermediate degree between both? Truly,

answered both the Fish- and Worm-men, We have observed several
Animal Creatures that live both in Water, and on the Earth,
indifferently, and if any, certainly those may be said to be of such a
mixt nature, that is, partly Flesh, and partly Fish: But how is it
possible, replied the Empress, that they should live both in Water,
and on the Earth, since those Animals that live by the respiration of
Air, cannot live within Water; and those that live in Water, cannot live
by the respiration of Air, as Experience doth sufficiently witness. They
answered her Majesty, That as there were different sorts of
Creatures, so they had also different ways of Respirations; for
Respiration, said they, is nothing else but a composition and division
of parts, and the motions of nature being infinitely various, it is
impossible that all Creatures should have the like motions; wherefore
it was not necessary, that all Animal Creatures should be bound to
live either by the Air, or by Water onely, but according as Nature had
ordered it convenient to their Species. The Empress seem'd very well
satisfied with their answer, and desired to be further informed,
Whether all Animal Creatures did continue their Species by a
successive propogation of particulars, and whether in every Species
the off-springs did always resemble their Generator or Producer, both
in their interior and exterior Figures? They answered, her Majesty,
That some Species or sorts of Creatures, were kept up by a
successive propagation of an off-spring that was like the producer,
but some were not. Of the first rank, said they, are all those Animals
that are of different sexes, besides several others; but of the second
rank are for the most part those we call Insects, whose production
proceds from such causes as have no conformity or likeness with
their produced Effects; as for example, Maggots bred out of Cheese,
and several others generated out of Earth, Water, and the like. But
said the Empress, there is some likeness between Maggots and
Cheese; for Cheese has no blood, nor Maggots neither; besides, they
have almost the same taste which Cheese has. This proves nothing,
answered they; for Maggots have a visible, local, progressive motion,
which Cheese hath not. The Empress replied, That when all the
Cheese was turned into Maggots, it might be said to have local,
progressive motion. They answered, That when the Cheese by its

own figurative motions was changed into Maggots, it was no more
Cheese. The Empress confessed that she observed Nature was
infinitely various in her works, and that though the species of
Creatures did continue, yet their particulars were subject to infinite
changes. But since you have informed me, said she, of the various
sorts and productions of Animal Creatures, I desire you to tell me
what you have observed of their sensitive perceptions? Truly,
answered they, Your Majesty puts a very hard question to us, and we
shall hardly be able to give a satisfactory answer to it; for there are
many different sorts of Creatures, which as they have all different
perceptions, so they have also different organs, which our senses are
not able to discover, onely in an Oystershell we have with admiration
observed, that the common sensorium of the Oyster lies just as the
closing of the shells, where the pressure and re-action may be
perceived by the opening and shutting of the shells every tide.
After all this, the Empress desired the Worm men to give her a true
Relation how frost was made upon the Earth? To which they
answered, That it was made much after the manner and description
of the Fish- and Bird-men, concerning the Congelation of Water into
Ice and Snow, by a commixture of saline and acid particles; which
relation added a great light to the Ape-men, who were the Chymists,
concerning their Chymical principles, Salt, Sulphur, and Mercury. But,
said the Empress, if it be so, it will require an infinite multitude of
saline particles to produce such a great quantity of Ice, Frost and
Snow: besides, said she, when Snow, Ice and Frost, turn again into
their former principle, I would fain know what becomes of those
saline particles? But neither the Worm-men, nor the Fish- and Bird-
men, could give her an answer to it.
Then the Empress enquired of them the reason, Why Springs were
not as salt as the Sea is? also, why some did ebb and flow? To which
it was answered, That the ebbing and flowing of some Springs, was
caused by hollow Caverns within the Earth, where the Seawater
crowding thorow, did thrust forward, and drew backward the Spring-
water, according to its own way of ebbing and flowing; but others
said, That it proceeded from a small proportion of saline and acid

particles, which the Spring-water imbibed from the Earth; and
although it was not so much as to be perceived by the sense of
Taste; yet it was enough to cause an ebbing and flowing-motion. And
as for the Spring- water being fresh, they gave, according to their
Observation, this following reason: There is, said they, a certain heat
within the Bowels of the Earth, proceeding from its swift circular
motion, upon its own axe, which heat distills the rarest parts of the
Earth into a fresh and insipid water, which water being through the
pores of the Earth, conveighed into a place where it may break forth
without resistance or obstruction, causes Springs and Fountains; and
these distilled Waters within the Earth, do nourish and refresh the
grosser and drier parts thereof. This Relation confirmed the Empress
in the opinion concerning the motion of the Earth, and the fixedness
of the Sun, as the Bird-men had informed her; and then she asked
the Worm-men, whether Minerals and Vegetables were generated by
the same heat that is within the Bowels of the Earth? To which they
could give her no positive answer; onely this they affirmed, That heat
and cold were not the primary producing causes of either Vegetables
or Minerals, or other sorts of Creatures, but onely effects; and to
prove this our assertion, said they, we have observed, that by change
of some sorts of Corporeal motions, that which is now hot, will
become cold; and what is now cold, will grow hot; but the hottest
place of all, we find to be the Center of the Earth: Neither do we
observe, that the Torrid Zone does contain so much Gold and Silver
as the Temperate; nor is there great store of Iron and Lead
wheresoever there is Gold; for these Metals are most found in colder
Climates towards either of the Poles. This Observation, the Empress
commanded them to confer with her Chymists, the Ape-men; to let
them know that Gold was not produced by a violent, but a temperate
degree of heat. She asked further, Whether Gold could not be made
by Art? They answered, That they could not certainly tell her Majesty,
but if it was possible to be done, they thought Tin, Lead, Brass, Iron
and Silver, to be the fittest Metals for such an Artificial Transmutation.
Then she asked them, Whether Art could produce Iron, Tin, Lead, or
Silver? They answered, Not, in their opinion. Then I perceive, replyed
the Empress, that your judgments are very irregular, since you

believe that Gold, which is so fixt a Metal, that nothing has been
found as yet which could occasion a dissolution of its interior figure,
may be made by Art, and not Tin, Lead, Iron, Copper or Silver, which
yet are so far weaker, and meaner Metals then Gold is. But the
Worm-men excused themselves, that they were ignorant in that Art,
and that such questions belonged more properly to the Ape-men,
which were Her Majesties Chymists.
Then the Empress asked them, Whether by their Sensitive
perceptions they could observe the interior corporeal, figurative
Motions both of Vegetables and Minerals? They answer'd, That their
Senses could perceive them after they were produced, but not
before; Nevertheless, said they, although the interior, figurative
motions of Natural Creatures are not subject to the exterior, animal,
sensitive perceptions, yet by their Rational perception they may judg
of them, and of their productions if they be regular: Whereupon the
Empress commanded the Bear-men to lend them some of their best
Microscopes. At which the Bear- men smilingly answered her Majesty,
that their Glasses would do them but little service in the bowels of
the Earth, because there was no light; for, said they, our Glasses do
onely represent exterior objects, according to the various reflections
and positions of light; and wheresoever light is wanting, the glasses
wil do no good. To which the Worm-men replied, that although they
could not say much of refractions, reflections, inflections, and the
like; yet were they not blind, even in the bowels of the Earth: for
they could see the several sorts of Minerals, as also minute Animals,
that lived there; which minute Animal Creatures were not blind
neither, but had some kind of sensitive perception that was as
serviceable to them, as sight, taste, smell, touch, hearing, &c. was to
other Animal Creatures: By which it is evident, That Nature has been
as bountiful to those Creatures that live underground, or in the
bowels of the Earth, as to those that live upon the surface of the
Earth, or in the Air, or in Water. But howsoever, proceeded the Worm-
men, although there is light in the bowels of the Earth, yet your
Microscopes will do but little good there, by reason those Creatures
that live under ground have not such an optick sense as those that

live on the surface of the Earth: wherefore, unless you had such
Glasses as are proper for their perception, your Microscopes will not
be any ways advantagious to them. The Empress seem'd well pleased
with this answer of the Worm-men; and asked them further, Whether
Minerals and all other Creatures within the Earth were colourless? At
which question they could not forbear laughing; and when the
Empress asked the reason why they laught? We most humbly beg
your Majesties pardon, replied they; for we could not chuse but
laugh, when we heard of a colourless Body. Why, said the Empress,
Colour is onely an accident, which is an immaterial thing, and has no
being of it self, but in another body. Those, replied they, that
informed your Majesty thus, surely their rational motions were very
irregular; For how is it possible, that a Natural nothing can have a
being in Nature? If it be no substance, it cannot have a being, and if
no being, it is nothing; Wherefore the distinction between subsisting
of it self, and subsisting in another body, is a meer nicety, and non-
sense, for there is nothing in Nature that can subsist of, or by it self,
(I mean singly) by reason all parts of Nature are composed in one
body, and though they may be infinitely divided, commixed, and
changed in their particular, yet in general, parts cannot be separated
from parts as long as Nature lasts; nay, we might as probably affirm,
that Infinite Nature would be as soon destroyed, as that one Atom
could perish; and therefore your Majesty may firmly believe, that
there is no Body without colour, nor no Colour without body; for
colour, figure, place, magnitude, and body, are all but one thing,
without any separation or abstraction from each other.
The Empress was so wonderfully taken with this Discourse of the
Worm-men, that she not only pardoned the rudeness they committed
in laughing at first at her question, but yielded a full assent to their
opinion, which she thought the most rational that ever she had heard
yet; and then proceeding in her questions, enquired further, whether
they had observed any seminal principles within the Earth free from
all dimensions and qualities, which produced Vegetables, Minerals,
and the like? To which they answered, That concerning the seeds of
Minerals, their sensitive perceptions had never observed any; but

Vegetables had certain seeds out of which they were produced. Then
she asked, whether those seeds of Vegetables lost their Species, that
is, were annihilated in the production of their off-spring? To which
they answered, That by an Annihilation, nothing could be produced,
and that the seeds of Vegetables were so far from being annihilated
in their productions, that they did rather numerously increase and
multiply; for the division of one seed, said they, does produce
numbers of seeds out of it self. But repli'd the Empress, A particular
part cannot increase of it self. 'Tis true, answer'd they: but they
increase not barely of themselves, but by joining and commixing with
other parts, which do assist them in their productions, and by way of
imitation form or figure their own parts into such or such particulars.
Then, I pray inform me, said the Empress, what disguise those seeds
put on, and how they do conceal themselves in their Transmutations?
They answered, That seeds did no ways disguise or conceal, but
rather divulge themselves in the multiplication of their off-spring;
onely they did hide and conceal themselves from their sensitive
perceptions so, that their figurative and productive motions were not
perceptible by Animal Creatures. Again, the Empress asked them,
whether there were any Non-beings within the Earth? To which they
answered, That they never heard of any such thing; and that, if her
Majesty would know the truth thereof, she must ask those Creatures
that are called Immaterial spirits, which had a great affinity with Non-
beings, and perhaps could give her a satisfactory answer to this
question. Then she desired to be informed, What opinion they had of
the beginning of Forms? They told her Majesty, That they did not
understand what she meant by this expression; For, said they, there
is no beginning in Nature, no not of Particulars; by reason Nature is
Eternal and Infinite, and her particulars are subject to infinite
changes and transmutations by vertue of their own Corporeal,
figurative self-motions; so that there's nothing new in Nature, not
properly a beginning of any thing. The Empress seem'd well satisfied
with all those answers, and enquired further, Whether there was no
Art used by those Creatures that live within the Earth? Yes, answered
they: for the several parts of the Earth do join and assist each other
in composition or framing of such or such particulars; and many

times, there are factions and divisions; which cause productions of
mixt Species; as, for example, weeds, instead of sweet flowres and
useful fruits; but Gardeners and Husbandmen use often to decide
their quarrels, and cause them to agree; which though it shews a
kindness to the differing parties, yet 'tis a great prejudice to the
Worms, and other Animal-Creatures that live under ground; for it
most commonly causes their dissolution and ruine, at best they are
driven out of their habitations. What, said the Empress, are not
Worms produced out of the Earth? Their production in general,
answered they, is like the production of all other Natural Creatures,
proceeding from the corporeal figurative motions of Nature; but as
for their particular productions, they are according to the nature of
their Species; some are produced out of flowers, some out of roots,
some out of fruits, some out of ordinary Earth. Then they are very
ungrateful Children, replied the Empress, that they feed on their own
Parents which gave them life. Their life, answered they, is their own,
and not their Parents; for no part or creature of Nature can either
give or take away life; but parts do onely assist and join with parts,
either in dissolution or production of other Parts and Creatures.
After this, and several other Conferences, which the Empress held
with the Worm-men, she dismissed them; and having taken much
satisfaction in several of their Answers, encouraged them in their
Studies and Observations. Then she made a Convocation of her
Chymists, the Ape-men; and commanded them to give her an
account of the several Transmutations which their Art was able to
produce. They begun first with a long and tedious Discourse
concerning the Primitive Ingredients of Natural bodies; and how, by
their Art, they had found out the principles out of which they consist.
But they did not all agree in their opinions; for some said, That the
Principles of all Natural Bodies were the four Elements, Fire, Air,
Water, Earth, out of which they were composed: Others rejected this
Elementary commixture, and said, There were many Bodies out of
which none of the four Elements could be extracted by any degree of
Fire whatsoever; and that, on the other side, there were divers
Bodies, whose resolution by Fire reduced them into more then four

different Ingredients; and these affirmed, That the only principles of
Natural Bodies were Salt, Sulphur, and Mercury: Others again
declared, That none of the forementioned could be called the True
Principles of Natural Bodies; but that by their industry and pains
which they had taken in the Art of Chymistry, they had discovered,
that all Natural Bodies were produced but from one Principle, which
was Water; for all Vegetables, Minerals, and Animals, said they, are
nothing else, but simple Water distinguished into various figures by
the vertue of their Seeds. But after a great many debates and
contentions about this Subject, the Empress being so much tired that
she was not able to hear them any longer, imposed a general silence
upon them, and then declared her self in this following Discourse.
I am too sensible of the pains you have taken in the Art of Chymistry,
to discover the Principles of Natural Bodies, and wish they had been
more profitably bestowed upon some other, then such experiments;
for both by my own Contemplation, and the Observations which I
have made by my rational & sensitive perception upon Nature, and
her works, I find, that Nature is but one Infinite Self-moving Body,
which by the vertue of its self-motion, is divided into Infinite parts,
which parts being restless, undergo perpetual changes and
transmutations by their infinite compositions and divisions. Now, if
this be so, as surely, according to regular Sense and Reason, it
appears no otherwise; it is in vain to look for primary Ingredients, or
constitutive principles of Natural Bodies, since there is no more but
one Universal Principle of Nature, to wit, self-moving Matter, which is
the onely cause of all natural effects. Next, I desire you to consider,
that Fire is but a particular Creature, or effect of Nature, and
occasions not onely different effects in several Bodies, but on some
Bodies has no power at all; witness Gold, which never could be
brought yet to change its interior figure by the art of Fire; and if this
be so, Why should you be so simple as to believe that Fire can shew
you the Principles of Nature? and that either the Four Elements, or
Water onely, or Salt Sulphur and Mercury, all which are no more but
particular effects and Creatures of Nature, should be the Primitive
Ingredients or Principles of all Natural Bodies? Wherefore, I will not

have you to take more pains, and waste your time in such fruitless
attempts, but be wiser hereafter, and busie your selves with such
Experiments as may be beneficial to the publick.
The Empress having thus declared her mind to the Ape-men, and
given them better Instructions then perhaps they expected, not
knowing that her Majesty had such great and able judgment in
Natural Philosophy, had several conferences with them concerning
Chymical Preperations, which for brevities sake, I'le forbear to
reherse: Amongst the rest, she asked, how it came that the Imperial
Race appear'd so young, and yet was reported to have lived so long;
some of them two, some three, and some four hundred years? and
whether it was by Nature, or a special Divine blessing? To which they
answered, That there was a certain Rock in the parts of that World,
which contained the Golden Sands, which Rock was hallow within,
and did produce a Gum that was a hundred years before it came to
its full strength and perfection; this Gum, said they, if it be held in a
warm hand, will dissolve into an Oyl, the effects whereof are
following: It being given every day for some certain time, to an old
decayed man, in the bigness of a little Pea, will first make him spit for
a week, or more; after this, it will cause Vomits of Flegm; and after
that it will bring forth by vomits, humors of several colours; first of a
pale yellow, then of a deep yellow, then of a green, and lastly of a
black colour; and each of these humours have a several taste, some
are fresh, some salt, some sower, some bitter, and so forth; neither
do all these Vomits make them sick, but they come out on a sudden,
and unawares, without any pain or trouble to the patient: And after it
hath done all these mentioned effects, and clear'd both the Stomack
and several other parts of the body, then it works upon the Brain,
and brings forth of the Nose such kinds of humors as it did out of the
Mouth, and much after the same manner; then it will purge by stool,
then by urine, then by sweat, and lastly by bleeding at the Nose, and
the Emeroids; all which effects it will perform within the space of six
weeks, or a little more; for it does not work very strongly, but gently,
and by degrees: Lastly, when it has done all this, it will make the
body break out into a thick Scab, and cause both Hair, Teeth, and

Nails to come off; which scab being arrived to its full maturity, opens
first along the back, and comes off all in a piece like armour, and all
this is done within the space of four months. After this the Patient is
wrapt into a Cere- cloth, prepared of certain Gums and Juices,
wherein he continues until the time of nine Months be expired from
the first beginning of the cure, which is the time of a Childs formation
in the Womb. In the mean while, his diet is nothing else but Eagles-
eggs, and Hinds-milk; and after the Cere-cloth is taken away, he will
appear of the age of Twenty, both in shape, and strength. The
weaker sort of this Gum is soveraign in healing of wounds, and curing
of slight distempers. But this is also to be observed, that none of the
Imperial race does use any other drink but Lime-water, or water in
which Lime-stone is immerged; their meat is nothing else but Fowl of
several sorts, their recreations are many, but chiefly Hunting.
This Relation amazed the Empress very much; for though in the
World she came from, she had heard great reports of the
Philosophers-stone, yet had she not heard of any that had ever found
it out, which made her believe that it was but a Chymera; she called
also to mind, that there had been in the same World a Man who had
a little Stone which cured all kinds of Diseases outward and inward,
according it was applied; and that a famous Chymist had found out a
certain Liquor called Alkahest, which by the vertue of its own fire,
consumed all Diseases; but she had never heard of a Medicine that
could renew old Age, and render it beautiful, vigorous and strong:
Nor would she have so easily believed it, had it been a medicine
prepared by Art; for she knew that Art, being Natures Changeling,
was not able to produce such a powerful effect; but being that the
Gum did grow naturally, she did not so much scruple at it; for she
knew that Nature's Works are so various and wonderful, that no
particular Creature is able to trace her ways.
The Conferences of the Chymists being finished, the Empress made
an Assembly of her Galenical Physicians, her Herbalists and
Anatomists; and first she enquired of her Herbalists the particular
effects of several Herbs and Drugs, and whence they proceeded? To
which they answered, that they could, for the most part, tell her

Majesty the vertues and operations of them, but the particular causes
of their effects were unknown; onely thus much they could say, that
their operations and vertues were generally caused by their proper
inherent, corporeal, figurative motions, which being infinitely various
in Infinite Nature, did produce infinite several effects. And it is
observed, said they, that Herbs and Drugs are as wise in their
operations, as Men in their words and actions; nay, wiser; and their
effects are more certain then Men in their opinions; for though they
cannot discourse like Men, yet have they Sense and Reason, as well
as Men; for the discursive faculty is but a particular effect of Sense
and Reason in some particular Creatures, to wit, Men, and not a
principle of Nature, and argues often more folly then wisdom. The
Empress asked, Whether they could not by a composition and
commixture of other Drugs make them work other effects then they
did, used by themselves? They answered, That they could make
them produce artificial effects, but not alter their inherent, proper
and particular natures.
Then the Empress commanded her Anatomists to dissect such kinds
of Creatures as are called Monsters. But they answered her Majesty,
That it would be but an unprofitable and useless work, and hinder
their better imployments; for when we dissect dead Animals, said
they, it is for no other end, but to observe what defects and
distempers they had, that we may cure the like in living ones, so that
all our care and industry concerns onely the preservation of Mankind;
but we hope your Majesty will not preserve Monsters, which are most
commonly destroyed, except it be for novelty: Neither will the
dissection of Monsters prevent the errors of Nature's irregular
actions; for by dissecting some, we cannot prevent the production of
others; so that our pains and labour will be to no purpose, unless to
satisfie the vain curiosities of inquisitive men. The Empress replied,
That such dissections would be very beneficial to Experimental
Philosophers. If Experimental Philosophers, answer'd they, do spend
their time in such useless Inspections, they waste it in vain, and have
nothing but their labour for their pains.

Lastly, her Majesty had some Conferences with the Galenick
Physicians about several Diseases, and amongst the rest, desired to
know the cause and nature of Apoplexies, and the spotted Plague.
They answered, That a deadly Apoplexy was a dead palsie of the
Brain, and the spotted Plague was a Gangrene of the Vital parts: and
as the Gangrene of outward parts did strike inwardly; so the
Gangrene of inward parts, did break forth outwardly: which is the
cause, said they, that as soon as the spots appear, death follows; for
then it is an infallible sign, that the body is throughout infected with a
Gangrene, which is a spreading evil; but some Gangrenes do spread
more suddenly than others, and of all sorts of Gangrenes, the Plaguy-
Gangrene is the most infectious; for other Gangrenes infect but the
next adjoining parts of one particular body, and having killed that
same Creature, go no further, but cease; when as, the Gangrene of
the Plague, infects not onely the adjoining parts of one particular
Creature, but also those that are distant; that is, one particular body
infects another, and so breeds a Universal Contagion. But the
Empress being very desirous to know in what manner the Plague was
propagated, and became so contagious, asked, Whether it went
actually out of one body into another? To which they answered, That
it was a great dispute amongst the Learned of their Profession,
Whether it came by a division and composition of parts; that is, by
expiration and inspiration; or whether it was caused by imitation:
some Experimental Philosophers, said they, will make us believe, that
by the help of their Microscopes, they have observed the Plague to
be a body of little Flies like Atoms, which go out of one body into
another, through the sensitive passages; but the most experienced
and wisest of our society, have rejected this opinion as a ridiculous
fancy, and do, for the most part, believe, that it is caused by an
imitation of Parts; so that the motions of some parts which are
sound, do imitate the motions of those that are infected and that by
this means, the Plague becomes contagions, and spreading.
The Empress having hitherto spent her time in the Examination of the
Bird- Fish- Worm- and Ape- men, &c. and received several
Intelligences from their several imployments; at last had a mind to

divert her self after her serious Discourses, and therefore she sent for
the Spider-men, which were her Mathematicians, the Lice-men which
were here Geometricians, and the Magpie- Parrot- and Jackdaw-men,
which were her Orators and Logicians. The Spider-men came first,
and presented her Majesty with a table full of Mathematical points,
lines, and figures of all sorts, of squares, circles, triangles, and the
like; which the Empress, notwithstanding that she had a very ready
wit, and quick apprehension, could not understand; but the more she
endeavoured to learn, the more was she confounded: Whether they
did ever square the Circle, I cannot exactly tell, nor whether they
could make imaginary points and lines; but this I dare say, That their
points and lines were so slender, small and thin, that they seem'd
next to Imaginary. The Mathematicians were in great esteem with the
Empress, as being not onely the chief Tutors and Instructors in many
Arts, but some of them excellent Magicians and Informers of spirits,
which was the reason their Characters were so abstruse and intricate,
that the Emperess knew not what to make of them. There is so much
to learn in your Art, said she, that I can neither spare time from other
affairs to busie my self in your profession; nor, if I could, do I think I
should ever be able to understand your Imaginary points, lines and
figures, because they are Non-beings.
Then came the Lice-men, and endeavoured to measure all things to a
hairs-breadth, and weigh them to an Atom; but their weights would
seldom agree, especially in the weighing of Air, which they found a
task impossible to be done; at which the Empress began to be
displeased, and told them, that there was neither Truth nor Justice in
their Profession; and so dissolved their society.
After this, the Empress was resolved to hear the Magpie- Parrot- and
Jackdaw-men, which were her professed Orators and Logicians;
whereupon one of the Parrot-men rose with great formality, and
endeavoured to make an Eloquent Speech before her Majesty; but
before he had half ended, his arguments and divisions being so
many, that they caused a great confusion in his brain, he could not
go forward, but was forced to retire backward, with great disgrace
both to himself, and the whole society; and although one of his

brethren endeavoured to second him by another speech, yet was he
as far to seek, as the former. At which the Empress appear'd not a
little troubled, and told them, That they followed too much the Rules
of Art, and confounded themselves with too nice formalities and
distinctions; but since I know, said she, that you are a people who
have naturally voluble tongues, and good memories; I desire you to
consider more the subject you speak of, then your artificial periods,
connexions and parts of speech, and leave the rest to your natural
Eloquence; which they did, and so became very eminent Orators.
Lastly, her Imperial Majesty being desirous to know what progress
her Logicians had made in the Art of disputing, Commanded them to
argue upon several Themes or Subjects; which they did; and having
made a very nice discourse of Logistical terms and propositions,
entred into a dispute by way of Syllogistical Arguments, through all
the Figures and Modes: One began with an Argument of the first
Mode of the first Figure, thus: Every Politician is wise: Every Knave is
a Politician, Therefore every Knave is wise.
Another contradicted him with a Syllogism of the second Mode of the
same Figure, thus: No Politician is wise: Every Knave is a Politician,
Therefore no Knave is wise.
The third made an Argument in the third Mode of the same Figure,
after this manner: Every Politician is wise: some Knaves are
Politicians, Therefore some Knaves are wise.
The Fourth concluded with a Syllogism in the fourth Mode of the
same Figure, thus; No Politician is wise: some Knaves are Politicians,
Therefore some Knaves are not wise.
After this they took another subject, and one propounded this
Syllogism: Every Philosopher is wise: Every Beast is wise, Therefore
every Beast is a Philosopher.
But another said that this Argument was false, therefore he
contradicted him with a Syllogism of the second Figure of the fourth
Mode, thus: Every Philosopher is wise: some Beasts are not wise,
Therefore some Beasts are not Philosophers.

Computer Networking: A Top-Down Approach 8th Edition James F. Kurose

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