Why is HTTP an application layer protocol

Data logs of the WWW

The Internet protocol family is a family of around 500 network protocols that form the basis for network communication on the Internet. The term TCP / IP protocol family is also often used, but other transport protocols are also used on the Internet outside of the World Wide Web.

To structure the communication tasks, functional levels, so-called layers (layer), differentiated. For the Internet protocol family, this is TCP / IP reference model authoritative. It describes the structure and interaction of the network protocols from the Internet protocol family and divides them into four layers that build on one another. TCP / IP stands for Transmission Control Protocol / Internet Protocol.

The individual layers fulfill the following functions:

Application layer
The application layer Application layer) includes all protocols that work together with application programs and use the network infrastructure for the exchange of application-specific data.
Transport layer
The transport layer Transport Layer) establishes an end-to-end connection. The most important protocol of this layer is the Transmission Control Protocol (TCP), which allows connections between two network participants to be reliable (not safeas the word for sure in the sense of 'forgery-proof, eavesdropping-proof' is used) sending data streams. However, datagram protocols - for example the User Datagram Protocol (UDP) - also belong in this layer, in which only the delivery to the correct service is made reliable and no connection is established.
Internet layer
The internet layer Internet Layer) is responsible for the forwarding of packets and the route selection (routing). Direct connections are considered on this layer and the layers below it. The task of this layer is to determine the next intermediate destination for a received packet and to forward the packet there. The core of this layer is the Internet Protocol (IP) version 4 or 6, which provides a package delivery service. So-called dual stacks, such as in Windows Vista or Windows Server 2008, can automatically recognize whether they can reach a communication partner via IPv6 or IPv4 and preferably use IPv6. This is transparent for appropriately programmed applications. The Internet layer corresponds to the network layer of the ISO / OSI reference model.
Network access layer
The network access layer Link Layer) is specified in the TCP / IP reference model, but does not contain any protocols from the TCP / IP family. Rather, it should be understood as a placeholder for various techniques for transferring data from point to point. The Internet protocols were developed with the aim of connecting different subnets. Therefore, the host-to-network layer can be supported by protocols such as Ethernet, FDDI, PPP (Point-to-point connection) or 802.11 (WIRELESS INTERNET ACCESS) fill out. The network access layer corresponds to the security and bit transmission layer of the ISO / OSI reference model.

The main internet protocols

Application layer (corresponds to OSI layer 5-7)

  • HTTP - Hypertext Transfer Protocol (WWW)
  • HTTPS - Hypertext Transfer Protocol Secure
  • FTP - File Transfer Protocol - file transfer
  • SMTP - Simple Mail Transfer Protocol - sending emails
  • POP3 - Post Office Protocol (Version 3) - E-mail on demand
  • IMAP - Internet Message Access Protocol - access to e-mail
  • NNTP - Network News Transfer Protocol - Discussion forums (Usenet)
  • Telnet - unencrypted login on remote computers (remote terminal)
  • DNS (Domain Name Service) - conversion between domain names and IP addresses
  • SNMP - Simple Network Management Protocol - management of devices in the network
  • SSH - Secure Shell (encrypted remote terminal)
  • NTP - Network Time Protocol

Transport layer (corresponds to OSI layer 4)

  • TCP (Transmission Control Protocol) - transmission of data streams (connection-oriented, reliable)
  • UDP (User Datagram Protocol) - transmission of data packets (connectionless, unreliable, low overhead)
  • SCTP (Stream Control Transmission Protocol) - transport protocol
  • TLS Transport Layer Security, formerly 'Secure Socket Layer (SSL)' - extension of TCP to include encryption

(slightly modified from: wikipedia)