Protocols

A network protocol is a formal set of rules, conventions and data structure that governs how computers and other network devices exchange information over a network.

The OSI (open system for interconnection) specifies that the standards that allows many different types of computer hardware and software to have one framework to use in order to facilitate the communication between devices/programs.

The OSI specifies 7 layers, with the bottom layer being layer 1 (the physical layer) and the top layer, 7 being the application layer.

All of the protocols explained here fit into one or more of the layers because of the way in which they were written. In order to simplify the explanation there are two main divisions in the layers; layer 1 to 3 are considered the "media layers" which have to do with the type of network card and they type of cabling between cards and layer 4 to 7 are the "Host Layers" and have to do with the applications on the computer communicating with each other

OSI comparision with TCP/IP Protocol Stack

This table is copied from http://www.vlsm-calc.net/models.php

TCP/IP (Transmission Control Protocol / Internet Protocol)

TCP/IP is a set of protocols developed in the early days of the Internet by the U.S. military. The purpose was to allow computers to communicate over long distance networks. The TCP part has to do with the verifying delivery of the packets. The IP part refers to the moving of data packets between nodes. TCP/IP has since then become the foundation of the Internet. Therefore, TCP/IP software is built into all major operating systems, such as Unix, Windows, and the Mac OS and has become the main protocol on many networks.

Each computer or device on a tcp/ip network is given a unique address which consists of four numbers between 0 and 255 separated by dots.  E.g.  192.168.1.34.  This is known as the devices IP Address.

The term TCP/IP has come to represent a whole set of protocols including the following:

UDP (User Datagram Protocol)

User Datagram Protocol is an Internet protocol which transmits data packets without error checking.  This is useful where speed is more important than accuracy such as stream live video.

Transmission Control Protocol (TCP)

The Transmission Control Protocol (TCP) is one of the core protocols of the Internet Protocol Suite.  TCP provides reliable, ordered delivery of a stream of octets (bytes) from a program on one computer to another program on another computer. TCP is the protocol used by major Internet applications such as the World Wide Web, email, remote administration and file transfer. Other applications, which do not require reliable data stream service, may use UDP, which reduced latency over reliability.

If you are interested in these things I have added a file to this page that you can download and read.  It is the original paper that specified the creation of the TCP\IP protocol.  It makes for fascinating reading.

Sockets

In order for data to be sent from the sender device to the receiver each packet must be sent to a particular socket.  A socket is created by combining a unique IP address of the receiver with a port on that receiver. If you are looking for a web page on a server that has the IP address 196.23.168.165 the port would be 80 because it is http traffic so the socket would be 196.23.168.165:80.  Each packet sent to the server on behalf of this request would have this socket data as part of the header.

Ports

A port is a "place" that a receiver will listen for traffic of a particular type.  One of the jobs of a firewall is to lock ports except those that the device expects to get data on.  Servers have ports that are open to traffic, so an SMTP server for example would have port 25 open for traffic but other ports like port 80 would be closed so that no web traffic could enter the computer.

FTP (File Transfer Protocol) Port 20/21

File Transfer Protocol is used to transfer files between two computers. Generally used over a TCP/IP-based network.

SMTP (Simple Mail Transfer Protocol) Port 25

The Simple Mail Transfer Protocol is the set of procedures needed for users to send mail from their networked computer to remote users.

POP3(Post Office Protocol 3) Port 110

In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection.

Internet message access protocol (IMAP) Port 143

Internet message access protocol (IMAP) is one of the two most prevalent Internet standard protocols for e-mail retrieval, the other being the Post Office Protocol (POP).[1] Virtually all modern e-mail clients and mail servers support both protocols as a means of transferring e-mail messages from a server.

DHCP (Dynamic Host Configuration Protocol) Port 546/547

Dynamic Host Configuration Protocol (DHCP) is a method for automating the assignment of IP addresses and other networking information. It is used on campus in residence halls and libraries, as well as over the wireless network.

DNS (Domain Name System) Port 53

The protocol used to match names of computers and their IP addresses on a network.

HTTP (Hyper-Text Transfer Protocol) Port 80

HTTP is the set of communication standards that together enable different kinds of computers to communicate with each other over the worldwide web.

HTTPS Port 443

Hypertext Transfer Protocol Secure (HTTPS) is a combination of the Hypertext Transfer Protocol with the SSL/TLS protocol to provide encryption and secure identification of the server.

SSL (Secure Socket Layer)

SSL (Secure Sockets Layer) is the standard security technology for establishing an encrypted link between a web server and a browser. This link ensures that all data passed between the web server and browsers remain private and integral. SSL is an industry standard and is used by millions of websites (e.g. Facebook, banks etc.) in the protection of their online transactions with their customers.

To be able to create an SSL connection a web server requires an SSL Certificate. When you choose to activate SSL on your web server you will be prompted to complete a number of questions about the identity of your website and your company. Your web server then creates two cryptographic keys - a Private Key and a Public Key. 

The Public Key does not need to be secret and is placed into a Certificate Signing Request (CSR) - a data file also containing your details. You should then submit the CSR. During the SSL Certificate application process, the Certification Authority will validate your details and issue an SSL Certificate containing your details and allowing you to use SSL. Your web server will match your issued SSL Certificate to your Private Key. Your web server will then be able to establish an encrypted link between the website and your customer's web browser. 

The complexities of the SSL protocol remain invisible to your customers. Instead their browsers provide them with a key indicator to let them know they are currently protected by an SSL encrypted session - the lock icon in the lower right-hand corner, clicking on the lock icon displays your SSL Certificate and the details about it. All SSL Certificates are issued to either companies or legally accountable individuals. 

Typically an SSL Certificate will contain your domain name, your company name, your address, your city, your state and your country. It will also contain the expiration date of the Certificate and details of the Certification Authority responsible for the issuance of the Certificate. When a browser connects to a secure site it will retrieve the site's SSL Certificate and check that it has not expired, it has been issued by a Certification Authority the browser trusts, and that it is being used by the website for which it has been issued. If it fails on any one of these checks the browser will display a warning to the end user letting them know that the site is not secured by SSL.

WAP (Wireless Application Protocol)

Wireless Application Protocol (WAP) is an internet based protocol for transferring data to small handheld devices.