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Networking, Part 1: Introduction
Caveat: It should be obvious that page is not a complete description of IP, UDP or TCP! Instead it is a short introduction and is sufficient so that we can build upon these concepts in later lectures.
The following is the "30 second" introduction to internet protocol (IP) - which is the primary way to send packets ("datagrams") of information from one machine to another.
"IP4", or more precisely, "IPv4" is version 4 of the Internet Protocol that describes how to send packets of information across a network from one machine to another . Roughly 95% of all packets on the Internet today are IPv4 packets. A significant limitation of IPv4 is that source and destination addresses are limited to 32 bits (IPv4 was designed at a time when the idea of 4 billion devices connected to the same network was unthinkable - or at least not worth making the packet size larger)
Each IPv4 packet includes a very small header - typically 20 bytes, or more precisely 20 "octets" that includes a source and destination address.
Conceptually the source and destination addresses can be split into two: a network number (the upper bits) and a host number on that network.
A newer packet protocol "IPv6" solves many of the limitations of IPv4 (e.g. makes routing tables simpler and 128 bit addresses) however less than 5% of web traffic is IPv6 based.
A machine can have a IPv6 address and a IPv4 address.
A special IPv4 address is 127.0.0.1 also known as localhost. Packets sent to 127.0.0.1 will never leave the machine; the address is specified_ to be the same machine.
Notice that the 32 bits address is split into 4 octets i.e. each number in the dot notation can be 0-255 inclusive.
To send a datagram (packet) to a host on the Internet you need to specify the host address and a port. The port is an unsigned 16 bit number (i.e. the maximum port number is 65535). A process can listen for incoming packet on a particular port. However only processes with super-user (root) access can listen on ports < 1024. Any process can listen on ports 1024 or higher.
An often used port is port 80: Port 80 is used for unencrypted http requests (i.e. web pages). For example, if a web browser connects to http://www.bbc.com/, then it will be connecting to port 80.
UDP is a connectionless protocol. It's very simple to use: Decide the destination address and port and send your data packet! However the network makes no guarantee about whether the packets will arrive. Packets (aka Datagrams) may be dropped if the network is congested. Packets may be duplicated or arrive out of order.
Between two distant data-centers it's typical to see 3% packet loss.
A typical use case for UDP is when receiving uptodate data is more important than receiving all of the data. For example, a game may send continuous updates of player positions.
TCP is a connection-based protocol. TCP creates a pipe between two machines and abstracts away the low level packet-nature of the Internet: Thus, under most conditions, bytes sent from one machine will eventually arrive at the other end without duplication or data loss.
TCP will invisibly manage resending packets, ignoring duplicate packets, re-arranging out-of-order packets and changing the rate at which packets are sent.
Most services on the Internet today (e.g. a web service) use TCP because it hides the complexity of lower, packet-level nature of the Internet.
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