# Summary of the internet architecture
## Learning goals
## Script

By creating the Internet we created a decentralized system to give us a fast way of communication.
In order to achieve this the Internet Protocol Suite (commonly known as "TCP/IP") has been introduced. It is an open, and standardized collection of protocols that act on four different Layers. There exist various other layered models for creating global communication networks. Besides the internet protocol suite there is the OSI model. Even though most people will tell you that you can map various layers of the OSI model directly to the internet protocol suite this is not completely true and subtle differences exist.
Let us now summarize the 4 layers of the Internet Protocol suite.
The first Layer - the link layer - is a group of methods or protocols that only operate on a host's link. The link is the physical and logical network component used to interconnect hosts or nodes in the network and a link protocol is a suite of methods and standards that operate only between adjacent network nodes of a Local area network segment or a wide area network connection. As an example we learned how ethernet worked on a Local area network.
The 2nd layer - the Internet Layer - is a group of internetworking methods, protocols, and specifications that are used to transport datagrams (also called packets) from the originating host across network boundaries, if necessary, to the destination host specified by a network address (IP address) which is defined for this purpose by the Internet Protocol (IP). The internet layer derives its name from its function of facilitating internetworking, which is the concept of connecting multiple networks with each other through gateways and routers.
The 3rd layer - Transport Layer - provides end-to-end communication services for applications within a layered architecture of network components and protocols. It also provides convenient services such as connection-oriented data stream support, reliability, flow control, and multiplexing. As its most prominent example we have been introduced to the Transmission Control Protocol in this course which serves exactly the above mentioned purposes.
The fourth layer is called the Application layer. It is an abstraction layer reserved for communications protocols and methods designed for process-to-process communications across an Internet Protocol computer network. Application layer protocols use the underlying transport layer protocols to establish process-to-process connections via ports. We have seen the Domain Name system as one (very central) application of the Internet. A much older example would be the [[[Wikipedia:Simple Mail Transfer Protocol|SMTP - Simple Mail Transfer Protocol]]] used for email.
Even though the layers are separated in theory there are many services and protocols that act on various layers. The address resolution protocol is an example which connects the link layer and the Internet layer. [[[Wikipedia:Network address translation|Network address translation (NAT)]]] is using information from Linked Layer (like mac addresses) Internet layer (IP addresses) and from the Transport layer (Port numbers) in order to work properly.
* When transfering data like a file from one computer to another computer we have to have a protocol on the application layer which enables a process to process communication channel.
* The file is usually split into segments which can are encapsulated into an TCP segment on the transport layer. The transport layer protocol is used to established a host to host connection.
* Each TCP segment is packed inside an IP packet which then can be routed between various routers and across network network boundaries. The routing happens on the internet layer which interconnects different networks on the path of the packet.
* In order to send data from one router to another in a network a linked layer protocol that supports the network is used. IP packets are then encapsulated inside frames. Ethernet or DSL are typical linked layer protocols.
* As the data arrives the encapsulation process is reversed in order to assemble the original file.
DNS plays such an important role in the Internet protocol suite since it gives us humans a meaningful scheme of addressing hosts on the Internet. Other addressing schemes like Ip addresses or mac addresses exist on lower level protocols. With these technologies and protocols we can address computers but we do not have a way of addressing files on the web yet. This is done by using a URL which is a concept introduced and used by Tim Berners-Lee while creating the World Wide Web.
The World Wide Web will be the application on the Internet on which we will focus for the rest of the course. Besides the URL it has two other important components.
* The Hypertext markup language (HTML) is a mark up language for Hypertext - a concept for interlinking documents and information and not only hosts on a network. Even though the idea of Hypertext existed longer than computers its combination with URLs first enabled humans to build a decentralized hypertext system which would scale around the entire planet and enable us to connect as many documents as we wanted to.
* The last component is called the Hypertext transfer protocol (HTTP). It is a request response protocol primarily designed to transfer HTML documents but could also be used to transfer any other file. HTTP is kind of the opposite of email as information is requested (pulled) and not send (pushed)
Having this said the World Wide Web and the Internet are not the same thing. The Internet builds a global network like the street system which enables data to travel between hosts - as streets enable people to travel between various places. The World Wide Web is an application on the internet. Even though the internet is a core building block of the internet I want to stress that it would would have been possible to create the World Wide Web without the invention of the Internet by just using the street system and mail as an underlying networking and communication system even though this would have been pretty boring, useless and probably very expensive.