What is Network protocol?

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

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 different 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

networking—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

acknowledges 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

conventional 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 exchanging

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

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 connection

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 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.