What is the “Internet”?

Internet = the world’s network of networks
Two possible views of the internet:
1. The core infrastructure linking the computing devices that are connected to it. This includes the network design and protocols: HTTP, SMTP, TCP, IP, Ethernet, etc.
2. All the network applications running on hosts (end systems) that make use of the services provided by the infrastructure above: google search, firefox, facebook, etc.
This course is about the first definition.

What does the internet do?

Simply put, the internet (in fact, any network) exists for exactly one purpose: to allow one host to send data to another host.
By design, the internet is a packet-switched network. (Cf. transportation network).
- Data is chopped into packets at the transmitting host.
- Each p cket is sent from the transmitting host to the receiving host via a route independently of any other packets.
- Data is assembled & reconstructed from the packets at the receiving end.
Everything else (infrastructure design, protocols) exists just to make this possible, fault-tolerant, robust, scalable, etc.

The internet consists of diverse networks:
- LANs
- WiFi networks
- telephone network
- cellular networks
- etc.
We can’t study them all in this course. We will focus on the big picture of the internet, and not on these different networks.

Network architecture deals with the what and where questions.
- in what form to send data, packets or files?
- where should a particular work be done, at the hosts or switches?
Network engineering deals with the how question.
- how to implement the routing algorithm?
- how to design a LAN?

For networks in general
- speed (measured in bits per second (bps))
- cost
- reliability (measured in mean time between failure (mtbf))
- others, specific to that particular kind of network
These are mainly engineering questions.
For the internet
- connectivity of different networks
- scalability to the entire world
- fault-tolerance (including recovery)
- support varied applications
These are mainly architecture questions.

A protocol is the steps for conducting the communication pre-agreed upon by both parties.

Example phone conversation protocol:

    Alice: Hi
    Bob: Hi
    Alice: Got the time?
    Bob: 1:30

Example HTTP conversation protocol:

    Alice: request connection
    Bob: connected
    Alice: give me https://gustavus.edu/mcs/cs-guide.php
    Bob: here you are: ...

sender:

    while (condition) {
        message = createMessage()
        send(message, ...)
    }

receiver:

    while (condition) {
        message = receive(message, ...)
    }