Comput e r Ne Two r k s And The Int e Rne t

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, 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, protocols in routersdetermine 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 network interface cards; congestion-control protocols in end systems control the rate at which packets are transmitted between sender and receiver.
Protocols are running everywhere in theInternet, and consequently much of this
book is about computer network protocols.
02-068 C01 pp4 6/14/02 5:45 PM Page 7As an example of a computer network protocol with which you are probablyfamiliar, consider what happens when you make a request to a Web server, that is,
when you type in the URL of a Web page into your Web browser. The scenario is illustrated in the right half of Figure1.2. 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 Webserver 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 thekey 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...