# 50.012 Lecture 1
# Overview of Internet

## Internet -- "nuts and bolts" view
There are billions of computing devices, or known as **hosts**. These are end systems that run **network apps**.

In order to facilitate communication between hosts, we need **communication links** (e.g. fiber, copper, radio, satellite). The transmission rate is known as **bandwidth**.

**Packet switches** are nodes that function to forward packets. They consist of routers and switches.


The **internet** is a network of networks.
There are **protocols** to control the sending and receiving of messages, and **Internet standards** that have to be adhered to.

## Internet -- service view

Infrastructure that provides services to applications.
It provides programming interface to apps.
* hooks that allow sending and receiving app programs to "connect" to Internet.
* provides service options, analogous to postal service.

## Packet-switching

Each packet is sent through the link, which is received by the router.
Most routers work in the store-and-forward manner, meaning that they will store the entirety of each packet when it arrives, before it forwards the packet to the destination.

## Alternative: Circuit-switching

End-to-end resources are reserved for one communication between source & destination

A circuit segment is idle when not used.


## Internet Protocol Stack
* Application: supporting network applications
* Transport: data transfer between processes
* Network: Routing of datagrams from source to destination
* Link: Data transfer between neighbouring network elements.
* Physical: bits "on the wire"



d~proc~:
d~queue~:
d~trans~: time taken to send out the packet to the link.
d~prop~: time taken to send the packet from source router to destination router.