Network protocol explained

The computer is started, the browser is opened, but instead of the start page, all you see is an error page telling you that the internet con­nec­tion couldn’t be es­tab­lished. Has this ever happened to you? Checking the router and Ethernet cable doesn’t turn up any problems, and only when you run an error di­ag­nos­tic do you find the answer: At least one network protocol is missing on the computer. In some cases, running an automatic repair and rebooting will fix the error. But often the protocol is also missing afterward, or the error message appears again after a certain amount of time. Possible causes can be, for example, outdated drivers, or com­pli­ca­tions with the anti-virus software or firewall being used.

So why do network protocols play such an important role in es­tab­lish­ing an internet con­nec­tion? In order to un­der­stand this, it helps to more closely identify the different types of network protocols and their various functions.

To combine computers in a network, it seems like it would be suf­fi­cient to just use an Ethernet cable. But without help, the computer systems aren’t capable of ex­chang­ing data packets and so can’t establish a data con­nec­tion. That task is instead carried out by the network protocols, which are connected to their re­spec­tive protocol family on the so-called switching layer, or network layer (layer 3 according to the OSI model). They contain agree­ments for the data exchange and so regulate the con­di­tions for its sub­se­quent transport, ad­dress­ing, routing, and error checking. For two computers to com­mu­ni­cate with each other, they need the same network protocols. They agree on the following con­di­tions for the trans­mis­sion, which are either placed in front of the re­spec­tive package as a header, or attached as a trailer:

• Size of the data package or packages
• Type of package
• Sender and addressee
• Other involved protocols

Not every data con­nec­tion between computer systems is knitted in the same pattern.  As a result, it makes a dif­fer­ence whether you are con­nect­ing two computers in your home network or over the internet as part of a huge computer network and sending data to multiple ad­dressees. Similarly, the com­mu­ni­ca­tion hierarchy of the par­tic­i­pants plays a big role. That’s why various network protocols exist for in­di­vid­ual forms of com­mu­ni­ca­tion. These protocols have the following possible ap­pli­ca­tion scenarios and dis­tin­guish­ing features:

1. Number of com­mu­ni­ca­tion partners: Different network protocols can be dis­tin­guished by the number of computers allowed to connect to them at any given time. If, for example, trans­mit­ted data is addressed to a single receiver, then it’s a unicast trans­mis­sion. An exchange between more than two com­mu­ni­ca­tion partners happens via multicast con­nec­tions. Sending data packets to all network users is called broad­cast­ing – this type of con­nec­tion is obviously best known for radio and tele­vi­sion.


2. Path of the data flow: The direction in which the data moves is another char­ac­ter­is­tic that separates different network protocols from one another. Protocols with simplex trans­mis­sion are one-sided com­mu­ni­ca­tions, where one computer acts only as the trans­mit­ter and the other only as the receiver. For half-duplex trans­mis­sions, com­mu­ni­ca­tion partners can exchange data al­ter­nate­ly. Full-duplex trans­mis­sions allow data to be sent si­mul­ta­ne­ous­ly and in both di­rec­tions.


3. Hierarchy of the com­mu­ni­ca­tion partners: Certain types of con­nec­tions, like the client-server model, are based on a clearly defined hi­er­ar­chi­cal structure. In that case, for example, different clients initiate the con­nec­tion to a single server, which then processes the requests. Sym­met­ri­cal com­mu­ni­ca­tion, the coun­ter­part to this example’s asym­met­ri­cal com­mu­ni­ca­tion, is marked by either peer-to-peer or computer-to-computer con­nec­tions. In this structure form, all computers have equal rights and so can both offer services and use them.


4. Syn­chro­niza­tion of com­mu­ni­ca­tion: Data exchange can also be dif­fer­en­ti­at­ed by whether in­di­vid­ual bits are syn­chro­nized between trans­mit­ter and receiver (syn­chro­nous com­mu­ni­ca­tion) or not (asyn­chro­nous com­mu­ni­ca­tion).


5. Type of con­nec­tion: Network protocols can also be divided into con­nec­tion-oriented and con­nec­tion­less protocols. Con­nec­tion-oriented protocols require the con­nec­tion between sender and receiver to exist for the entire duration of the trans­mis­sion, and try to ensure that the data arrives in a certain order and is re­trans­mit­ted in the event of a failure. Con­nec­tion­less protocols do without the con­nec­tion set-up and removal, so their data packets contain sub­stan­tial­ly less in­for­ma­tion. But the data can also arrive at the receiver in a random order, and is not au­to­mat­i­cal­ly re­trans­mit­ted in the event of incorrect trans­mis­sion.

Apart from the technical back­ground, the variety of network protocols also results from the fact that many man­u­fac­tur­ers have developed their own protocols or protocol stacks for their devices.

For the network layer, as for all the other layers, there are a number of stan­dard­ized but also pro­pri­etary protocols which are suitable for different ap­pli­ca­tion areas or are partially limited to specific operating systems or devices. Many of these protocols are no longer active today, mainly due to the increased dis­tri­b­u­tion of the internet protocol family. These stacks with more than 500 protocols also contain the most important and well-known network protocol IP (Internet Protocol), which is the basis of the internet.

The internet protocol has the job of trans­port­ing data packets from a sender to a receiver over multiple networks. To do this, it defines the guide­lines for ad­dress­ing and routing, or the finding of data packets. IP is not only the standard network protocol for wide area networks (WANs) – the in­di­vid­ual, worldwide networks that connect the internet to each other – but also for local networks. It’s supported by all man­u­fac­tur­ers and operating systems, but also requires the necessary know-how in terms of con­fig­u­ra­tion as well as the ap­pro­pri­ate hardware (router).

The following table shows a his­tor­i­cal­ly important network protocol overview:

After the protocols of the switching layer have es­tab­lished the basis for com­mu­ni­ca­tion, another protocol is needed so that the data packets reach the cor­re­spond­ing ap­pli­ca­tions. With the OSI model, this for­ward­ing is carried out on the transport layer (layer 4). Each stack also has its own protocols. For the internet protocol family, these are par­tic­u­lar­ly

• TCP (Trans­mis­sion Control Protocol)
• and UDP (User Datagram Protocol).

Since the great success of the internet, the first mentioned TCP is equal to IP as a standard for network con­nec­tions. In most cases it builds directly on IP, which is why TCP/IP networks are often used. As a con­nec­tion-oriented protocol, TCP requires an existing con­nec­tion between the com­mu­ni­ca­tion users for the transport of data packets. It guar­an­tees reliable transport of the data and that all of the packets will arrive complete and in the correct order. To do this, the protocol adds ad­di­tion­al in­for­ma­tion, such as a sequence number and proof sum to the data.

UDP is the TCP coun­ter­part of the internet protocol family for the simple and quick transfer of smaller data packets without a con­nec­tion. UDP con­nec­tions don’t offer any security for a packet arriving at the addressee, but thanks to the low ad­min­is­tra­tion data (ad­di­tion­al in­for­ma­tion in the header), there isn’t a clear speed advantage for data transfer where smaller trans­mis­sion errors aren’t a problem. For this reason, the User Datagram Protocol is used for audio and video streaming, DNS queries, and VPN (Virtual Private Network) con­nec­tions.

Like the internet protocol family, other protocol stacks also have specific trans­mis­sion protocols based on their network protocols and largely similar to TCP. Novell networks, for example, wait in the transport layer with the protocol SPX (Sequenced Packet Exchange). With the AppleTalk stack, the data packets can be trans­port­ed using the ATP (AppleTalk Trans­ac­tion Protocol).