Networking concepts - TCP/IP layer model

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Understanding the TCP/IP layer model and explain the significance of each layer with real world packet flow.

 Explanation:-
TCP/IP is normally considered to be a 4 layer system. The 4 layers are as follows :


  1. Application layer
  2. Transport layer
  3. Internet/Network layer
  4. Network Access layer/network interface layer
Application layer also has various protocols that applications use to communicate with the second layer, the transport layer. Some of the popular application layer protocols are :
  • HTTP (Hypertext transfer protocol)
  • FTP (File transfer protocol)
  • SMTP (Simple mail transfer protocol)
  • SNMP (Simple network management protocol) etc
Transport layer provides backbone to data flow between two hosts. This layer receives data from the application layer . There are many protocols that work at this layer but the two most commonly used protocols at transport layer are TCP and UDP.
TCP is used where a reliable connection is required while UDP is used in case of unreliable connections.
TCP divides the data(coming from the application layer) into proper sized chunks and then passes these chunks onto the network. It acknowledges received packets, waits for the acknowledgments of the packets it sent and sets timeout to resend the packets if acknowledgements are not received in time. The term ‘reliable connection’ is used where it is not desired to loose any information that is being transferred over the network through this connection. So, the protocol used for this type of connection must provide the mechanism to achieve this desired characteristic. For example, while downloading a file, it is not desired to loose any information(bytes) as it may lead to corruption of downloaded content.
UDP provides a comparatively simpler but unreliable service by sending packets from one host to another. UDP does not take any extra measures to ensure that the data sent is received by the target host or not. The term ‘unreliable connection’ are used where loss of some information does not hamper the task being fulfilled through this connection. For example while streaming a video, loss of few bytes of information due to some reason is acceptable as this does not harm the user experience much.
Network layer is also known as Internet layer. The main purpose of this layer is to organize or handle the movement of data on network. By movement of data, we generally mean routing of data over the network. The main protocol used at this layer is IP. While ICMP(used by popular ‘ping’ command) and IGMP are also used at this layer.
This layer is also known as network interface layer. This layer normally consists of device drivers in the OS and the network interface card attached to the system. Both the device drivers and the network interface card take care of the communication details with the media being used to transfer the data over the network. In most of the cases, this media is in the form of cables. Some of the famous protocols that are used at this layer include ARP(Address resolution protocol), PPP(Point to point protocol) etc.
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TCP/IP CONCEPT EXAMPLE

One thing which is worth taking note is that the interaction between two computers over the network through TCP/IP protocol suite takes place in the form of a client server architecture.
Client requests for a service while the server processes the request for client.

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PORTS, SERVERS AND STANDARDS

On a particular machine, a port number coupled with the IP address of the machine is known as a socket. A combination of IP and port on both client and server is known as four tuple. This four tuple uniquely identifies a connection. In this section we will discuss how port numbers are chosen.
You already know that some of the very common services like FTP, telnet etc run on well known port numbers. While FTP server runs on port 21, Telent server runs on port 23. So, we see that some standard services that are provided by any implementation of TCP/IP have some standard ports on which they run. These standard port numbers are generally chosen from 1 to 1023. The well known ports are managed by Internet Assigned Numbers Authority(IANA).
While most standard servers (that are provided by the implementation of TCP/IP suite) run on standard port numbers, clients do not require any standard port to run on.
Client port numbers are known as ephemeral ports. By ephemeral we mean short lived. This is because a client may connect to server, do its work and then disconnect. So we used the term ‘short lived’ and hence no standard ports are required for them.
Also, since clients need to know the port numbers of the servers to connect to them, so most standard servers run on standard port numbers.
The ports reserved for clients generally range from 1024 to 5000. Port number higher than 5000 are reserved for those servers which are not standard or well known.
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What Is A Domain Name Server (DNS) And How Does It Work


DNS (Domain Name Server) resolution is the process of translating IP addresses to domain names.

 https://www.geeksforgeeks.org/domain-name-server-dns-in-application-layer/

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