gujiaxi · October 17, 2012 14:04
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 <h1>
 <font color="#FF0000">1.2.What is a Protocol?</font></h1></center>
 Now that we've got a bit of a feel for what the "Internet" is, let's consider
 another important word is the title of this book: "protocol."&nbsp; What
 <i>is</i>
 a protocol?&nbsp; What does a protocol <i>do</i>? How would you recognize
 a protocol if&nbsp; you met one?
 <h3>
 A Human Analogy</h3>
 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.&nbsp; A typical exchange is shown in Figure 1.2-1. Human
 protocol (or good manners, at least) dictates that one first offers a greeting
 (the first "Hi" in Figure 1.2-1) to initiate communication with someone
 else. The typical response to a "Hi" message (at least outside of New York
 City)&nbsp; is a returned "Hi" message.&nbsp; Implicitly, one then takes
 a cordial "Hi" response as an indication that one can proceed ahead and
 ask for the time of day. A different response to the initial "Hi" (such
 as&nbsp; "Don't bother me!", or "I don't speak English," or an unprintable
 reply that one might receive in New York City) might indicate an unwillingness
 or inability to communicate.&nbsp; In this case, the human protocol would
 be to not ask for the time of day. Sometimes one gets no reponse at all
 to a question, in which case one typically gives up asking that person
 for the time. Note that in our human protocol,<i> 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).</i>&nbsp; Clearly, transmitted and received messages, and actions
 taken when these message are sent or received or other events occur, play
 a central role in a human protocol.&nbsp; If people run different protocols
 (e.g., if one person has manners but the other does not, or if one understands
 the concept of time and the other does not)&nbsp; the protocols do not&nbsp;
 interoperate and no useful work can be accomplished.&nbsp; The same is
 true in networking -- it takes two (or more) communicating entities running
 the same&nbsp; protocol in order to accomplish a task.
 <p>Let's consider a second human analogy.&nbsp; Suppose you're in a college
 class (a computer networking class, for example!).&nbsp; The teacher is
 droning on about protocols and you're confused.&nbsp; The teacher stops
 to ask,&nbsp; "Are there any questions?" (a message that is transmitted
 to, and received by, all students who are not sleeping).&nbsp; You raise
 your hand (transmitting an implicit message to the teacher).&nbsp; Your
 teacher acknowledges you with a smile, saying "Yes ......." (a transmitted
 message encouraging you to ask your question - teachers <i>love</i> to
 be asked questions) and you then ask your question (i.e., transmit your
 message to your teacher).&nbsp; Your teacher hears your question (receives
 your question message) and answers (transmits a reply to you).&nbsp; Once
 again, we see that the transmission and receipt of messages, and a set
 of conventional actions taken when these mesages are sent and received,
 are at the heart of this question-and-answer protocol.
 <br>&nbsp;
 <h3>
 <b>Network Protocols</b></h3>
 A network protocol is similar to a human protocol, except that the entities
 exchanging messages and taking actions are hardware or software components
 of a computer network, components that we will study shortly in the following
 sections.&nbsp; All activity in the Internet that involves two or more
 communicating remote entities is governed by a protocol.&nbsp; Protocols
 in&nbsp; routers determine a packet's path from source to destination;
 hardware-implemented protocols in the network interface cards of two physically
 connected computers control the flow of bits on the "wire" between the
 two computers; a congestion control protocol controls the rate at which
 packets are transmitted between sender and receiver. Protocols are running
 everywhere in the Internet, and consequently much of this book is about
 computer network protocols.
 <center><img SRC="../../../../../www.seas.upenn.edu/~ross/book/overview/protocol.GIF" tppabs="http://www.seas.upenn.edu/~ross/book/overview/protocol.GIF" ALT="What is a protocol?" HSPACE=20 VSPACE=20 height=224 width=558>
 <br><b>Figure 1.2-1:</b> A human protocol and a computer network protocol</center>

 <p>As an example of a computer network protocol with which you are probably
 familiar, consider what happens when you make a request to a WWW server,&nbsp;
 i.e., when you type in the URL of a WWW page into your web browser. The
 scenario is illustrated in the right half of Figure 1.2-1. First, your
 computer will send a so-called "connection request" message to the WWW
 server and wait for a reply.&nbsp; The WWW server will eventually receive
 your connection request message and return a "connection reply" message.
 Knowing that it is now OK to request the WWW document, your computer then
 sends the name of the WWW page it wants to fetch from that WWW server in
 a "get" message.&nbsp; Finally, the WWW server returns the contents of
 the WWW document to your computer.
 <p>Given the human and networking examples above,&nbsp; the exchange of
 messages and the actions taken when these messages are sent and received
 are the key defining elements of a protocol:
 <p>&nbsp;&nbsp;&nbsp;&nbsp;<i> A <b>protocol </b>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.</i>
 <p>The Internet, and computer networks in general, make extensive use of
 protocols. 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.
 <br>&nbsp;
 <p>&nbsp;<a href="../../Contents.htm" tppabs="http://kia.etel.ru/lib/Networking/Contents.htm">Return
 to Table Of Contents</a>
 <p>
 <hr WIDTH="100%"><font color="#0000FF">Copyright Keith W. Ross and Jim
 Kurose 1996-2000</font>
 </body>
 </html>
	<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
	<html>
	<head>
	<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
	<meta name="Author" content="Jim Kurose">
	<meta name="GENERATOR" content="Mozilla/4.61 [en] (Win98; I) [Netscape]">
	<title>What is a protocol?</title>
	</head>
	<body>

	<center>
	<h1>
	<font color="#FF0000">1.2.What is a Protocol?</font></h1></center>
	Now that we've got a bit of a feel for what the "Internet" is, let's consider
	another important word is the title of this book: "protocol."  What
	<i>is</i>
	a protocol?  What does a protocol <i>do</i>? How would you recognize
	a protocol if  you met one?
	<h3>
	A Human Analogy</h3>
	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 1.2-1. Human
	protocol (or good manners, at least) dictates that one first offers a greeting
	(the first "Hi" in Figure 1.2-1) to initiate communication with someone
	else. The typical response to a "Hi" message (at least outside of New York
	City)  is a returned "Hi" message.  Implicitly, one then takes
	a cordial "Hi" response as an indication that one can proceed ahead 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 an unprintable
	reply that one might receive in New York City) might indicate an unwillingness
	or inability to communicate.  In this case, the human protocol would
	be to not ask for the time of day. Sometimes one gets no reponse at all
	to a question, in which case one typically gives up asking that person
	for the time. Note that in our human protocol,<i> 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).</i>  Clearly, transmitted and received messages, and actions
	taken when these message are sent or received or other events occur, play
	a central role in a human protocol.  If people run different protocols
	(e.g., 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 networking -- it takes two (or more) communicating entities running
	the same  protocol in order to accomplish a task.
	<p>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 acknowledges you with a smile, saying "Yes ......." (a transmitted
	message encouraging you to ask your question - teachers <i>love</i> to
	be asked questions) and you then ask your question (i.e., 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 conventional actions taken when these mesages are sent and received,
	are at the heart of this question-and-answer protocol.
	<br>
	<h3>
	<b>Network Protocols</b></h3>
	A network protocol is similar to a human protocol, except that the entities
	exchanging messages and taking actions are hardware or software components
	of a computer network, components that we will study shortly in the following
	sections.  All activity in the Internet that involves two or more
	communicating remote entities is governed by a protocol.  Protocols
	in  routers determine a packet's path from source to destination;
	hardware-implemented protocols in the network interface cards of two physically
	connected computers control the flow of bits on the "wire" between the
	two computers; a congestion control protocol controls the rate at which
	packets are transmitted between sender and receiver. Protocols are running
	everywhere in the Internet, and consequently much of this book is about
	computer network protocols.
	<center><img SRC="../../../../../www.seas.upenn.edu/~ross/book/overview/protocol.GIF" tppabs="http://www.seas.upenn.edu/~ross/book/overview/protocol.GIF" ALT="What is a protocol?" HSPACE=20 VSPACE=20 height=224 width=558>
	<br><b>Figure 1.2-1:</b> A human protocol and a computer network protocol</center>

	<p>As an example of a computer network protocol with which you are probably
	familiar, consider what happens when you make a request to a WWW server,
	i.e., when you type in the URL of a WWW page into your web browser. The
	scenario is illustrated in the right half of Figure 1.2-1. First, your
	computer will send a so-called "connection request" message to the WWW
	server and wait for a reply.  The WWW server will eventually receive
	your connection request message and return a "connection reply" message.
	Knowing that it is now OK to request the WWW document, your computer then
	sends the name of the WWW page it wants to fetch from that WWW server in
	a "get" message.  Finally, the WWW server returns the contents of
	the WWW document to your computer.
	<p>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:
	<p>    <i> A <b>protocol </b>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.</i>
	<p>The Internet, and computer networks in general, make extensive use of
	protocols. 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.
	<br>
	<p> <a href="../../Contents.htm" tppabs="http://kia.etel.ru/lib/Networking/Contents.htm">Return
	to Table Of Contents</a>
	<p>
	<hr WIDTH="100%"><font color="#0000FF">Copyright Keith W. Ross and Jim
	Kurose 1996-2000</font>
	</body>
	</html>