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 'ome 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 igure 1.2. Human protocol (or good manners, at
least) dictates that one fir st offer a greeting (the first "Hi" in Figure 1.2) to initiate
communication with someone el e. 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,
ancl actions taken when these messages are sent or received or other events
occur, playa 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 , econd 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 tran smitted 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 encouragofnetw
ing you to ask your question-teachers lo ve 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

network protocol is similar to a human protocol, except that the entities exchangfamilial
ing messaoes and raking actions are hardware or software components of some device (for example, computer, POA, cellphone, 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
the network interface cards of two physically connected computers control the flow
of bits on the "wire" between the two network interface cards ; congcstion-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 recei ved are the key defining
elements of a protocol:
A protocol defines the format and the order (4 mess(/ges exchanged between
two or more communicating entities, as well as the actions taken on the trunsmission
andlor receipt oIll 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.