The Purpose of Network Protocols

Without protocols, devices would lack the ability to understand the electronic signals they send to each other over network connections. Network protocols serve these basic functions:

• address data to the correct recipient(s)
• physically transmit data from source to destination, with security protection if needed
• receive messages and send responses appropriately

Consider a comparison between network protocols with how a postal service handles physical paper mail. Just as the postal service manages letters from many sources and destinations, so too do network protocols keep data flowing along many paths continuously. Unlike physical mail, however, network protocols also provide some advanced capabilities like delivering a constant flow of messages to one destination (called streaming) and automatically making copies of a message and delivering it to multiple destinations at once (called broadcasting).

Common Types of Network Protocols

No one protocol exists that supports all the features every kind of computer network needs. Many different kinds of network protocols have been invented over the years, each attempting to support certain kinds of network communication.

Three basic characteristics that distinguish one type of protocol from another are:

1. Simplex vs. duplex. A simplex connection allows only one device to transmit on a network. Conversely, duplex network connections allow devices to both transmit and receive data across the same physical link.

2. Connection-oriented or connection-less.

A connection-oriented network protocol exchanges (a process called a handshake) address information between two devices that allows them to carry on a conversation (called a session) with each other. Conversely, connection-less protocols deliver individual messages from one point to another without regard for any similar messages sent before or after (and without knowing whether messages are even successfully received).

3. Layer. Network protocols normally work together in groups (called stacks because diagrams often depict protocols as boxes stacked on top of each other). Some protocols function at lower layers closely tied to how different types of wireless or network cabling physically works. Others work at higher layers linked to how network applications work, and some work at intermediate layers in 

The Internet Protocol Family

The most common network protocols in public use belong to the Internet Protocol (IP) family. IP is itself the basic protocol that enables home and other local networks across the Internet to communicate with each other.

IP works well for moving individual messages from one network to another but does not support the concept of a conversation (a connection over which a stream of messages can travel in one or both directions).

The Transmission Control Protocol (TCP) extends IP with this higher layer capability, and because point-to-point connections are so essential on the Internet, the two protocols are almost always paired together and known as TCP/IP.

Both TCP and IP operate in the middle layers of a network protocol stack. Popular applications on the Internet have sometimes implemented their own protocols on top of TCP/IP. HyperText Transfer Protocol (HTTP) is used by Web browsers and servers worldwide. TCP/IP, in turn, runs on top of lower-level network technologies like Ethernet. Other popular network protocols in the IP family include ARP, ICMPand FTP.

How Network Protocols Use Packets

The Internet and most other data networks work by organizing data into small pieces called packets. To improve communication performance and reliability, each larger message sent between two network devices is often subdivided into smaller packets by the underlying hardware and software. These packet switching networks require packets to be organized in specific ways according to the protocols the network supports. This approach works well with the technology of modern networks as these all handle data in the form of bits and bytes (digital ‘1’s and ‘0s’). 

Each network protocol defines rules for how its data packets must be organized (formatted). Because protocols like Internet Protocol often work together in layers, some data embedded inside a packet formatted for one protocol can be in the format of some other related protocol (a method called encapsulation).

Protocols typically divide each packet into three parts – header, payload and footer. (Some protocols, like IP, do not utilize footers.) Packet headers and footers contain contextual information required to support the network, including addresses of the sending and receiving devices, while payloads contain the actual data to be transmitted. Headers or footers also often include some special data to help improve the reliability and or performance of network connections, such as counters that keep track of the order in which messages were sent, and checksums that help network applications detect data corruption or tampering.

How Network Devices Use Protocols

The operating systems of network devices include built-in support for some lower level network protocols. All modern desktop computer operating systems support both Ethernet and TCP/IP, for example, while many smartphones support Bluetoothand protocols from the Wi-Fi family. These protocols ultimately connect to the physical network interfaces of a device, like its Ethernet ports and Wi-Fi or Bluetooth radios.

Network applications, in turn, support the higher level protocols which talk to the operating system. A Web browser, for example, is capable of translating addresses like http://vk.com/ into HTTP packets that contain the necessary data that a Web server can receive and in turn send back the correct Web page. The receiving device is responsible for re-assembling individual packets into the original message, by stripping off the headers and footers and concatenating packets in the correct sequence.