Christ Church -- FAQ: What are protocols?

Frequently Asked Questions (FAQ)
with answers

What are protocols?

In information technology, protocols are the special sets of rules that two computers "agree" to use to communicate. One basic purpose of protocols is to make it possible for computers and networks of differing platforms to communicate. The different platforms could refer to hardware (e.g. a mainframe computer at a large company, an individual PC, or an Apple computer) or to software (e.g. MS Windows, Unix, or MacOS). Each platform performs communications, and accomplishes most other tasks, in very different ways. Protocols which can be implemented across, or without regard to platforms make it possible to bridge these differences.

If you've been around computers, or read about them, you've probably heard about some protocols. Even if you didn't know it. If you've been on the Internet, or read about it, you can't help but to have heard about protocols. One you have probably seen mentioned is HTTP (Hyper-Text Transfer Protocol). This is the protocol that computers use to transfer web pages, and is one of the higher level protocols common on the Internet.

To understand protocols, you will want to understand that computer networking consists of multiple layers of protocols. We don't need much more detail than that for our purposes, but you should keep in mind the fact that there are different layers. We referred already to HTTP as a higher level protocol. This means that it, and others like it, are used at the level of your application software (e.g. Netscape Navigator, or MS Internet Explorer). Some of the others like it are SMTP (Simple Mail Transfer Protocol), FTP (File Transfer Protocol), NNTP (Network News Transfer Protocol), and Telnet (which you might be pleased to know, does not have an acronym).

You may recognize some of these from the beginning of URL's you've typed into a web browser. These protocols determine how specific information is requested, sent, and received between two computers. When you type a URL that begins with something like "http://", "ftp://", or "telnet://" into your browser, it is requesting that information be transfered using that protocol. (There are other ways besides a web browser to accomplish this, but the concept is the same.) If the requested service is available, the remote computer opens up a connection of that type between your (client) software and the remote (server) software. If the remote machine offers more than one of these services, there is server software running for each of them, and the protocol determines which one is going to handle your requests. Any client or server software can communicate with any other, regardless of the version, as long as they both adhere to the protocol. This is why you can use Netscape, Outlook, Eudora, Pine, Elm, or a host of other software packages to read and send e-mail. They all "talk" SMTP.

You may resonably wonder why one such protocol isn't enough, and it's because they each do different things. For instance, SMTP delivers e-mail from one specific user to other specific user(s). Usenet newgroups use NNTP, and these messages propogate themselves throughout the Usenet system so that anyone connecting to an NNTP server can view them. They are an open discusion forum, and access is not limited to any specifically targeted user(s). Simple file transfers use FTP (e.g. downloading a new version of your web browser software for installation). The Telnet protocol provides an interactive command shell (similar to a DOS command-line) that executes commands on the remote machine that you enter on your local machine. For example, you could make a telnet connection from home to your office and do things like renaming files, making and deleting directories (folders), and just about anything else you could do if you were actually sitting at the computer in your office. So you can see that each of these provides much different functionality to the user, and consequently needs different rules or protocols for accomplishing its own tasks.

Below this layer of networking protocols are others that operate at a lower level. Logically, we've presented these in reverse order since each higher level is built upon the lower ones. The order we chose here is becasue the higher ones are more important for the purposes of this discussion, but there is one set of lower level protocols to which you'll see frequent references. That is TCP/IP, which plays a fundamental role both in how the information is broken down into pieces (packets) which can physically be transferred over the network, and also in making sure those packets get to and from the proper locations. Basically, once your computer has made a connection with a remote computer and has agreed upon a higher (or application level) protocol, the baton passes to TCP/IP for the job of actually transferring the information on behalf of that particular protocol. That's kind of a mouthful, but the following might help if you've gotten this far, and care to continue a little bit further.

"TCP/IP (Transmission Control Protocol/Internet Protocol) is the basic communication language or protocol of the Internet. It can also be used as a communications protocol in the private networks called intranets and in extranets. When you are set up with direct access to the Internet, your computer is provided with a copy of the TCP/IP program just as every other computer that you may send messages to or get information from also has a copy of TCP/IP.
TCP/IP is a two-layered program. The higher layer, Transmission Control Protocol, manages the assembling of a message or file into smaller packets that are transmitted over the Internet and received by a TCP layer that reassembles the packets into the original message. The lower layer, Internet Protocol, handles the address part of each packet so that it gets to the right destination. Each gateway computer on the network checks this address to see where to forward the message. Even though some packets from the same message are routed differently than others, they'll be reassembled at the destination.
TCP/IP uses the client/server model of communication in which a computer user (a client) requests and is provided a service (such as sending a Web page) by another computer (a server) in the network. TCP/IP communication is primarily point-to-point, meaning each communication is from one point (or host computer) in the network to another point or host computer. TCP/IP and the higher-level applications that use it are collectively said to be "connectionless" because each client request is considered a new request unrelated to any previous one (unlike ordinary phone conversations that require a dedicated connection for the call duration). Being connectionless frees network paths so that everyone can use them continuously. (Note that the TCP layer itself is not connectionless as far as any one message is concerned. Its connection remains in place until all packets in a message have been received.)
Personal computer users usually get to the Internet through the Serial Line Internet Protocol (SLIP) or the Point-to-Point Protocol (PPP). These protocols encapsulate the IP packets so that they can be sent over a dial-up phone connection to an access provider's modem." (1)