Protocols/MSNP/General/Connections
Overview
All connections to MSN servers take place over TCP/IP. The client always makes the outgoing connections to the servers. The official port for MSN Messenger is 1863, although there are many places in the protocol where alternate ports could be specified, so this may be subject to change.
The connection to the server must be considered asynchronous - you can send many commands to the server without waiting for a reply, and the server won't necessarily reply to your commands in the order you sent them. The server may also send messages that are not in reply to any particular message from the client. However, sometimes (for example, when logging into a notification server) the protocol requires you to send one command then receive one command, and so on.
There are also several OOB (out-of-band) protocols that take place directly between clients and do not involve the server. These protocols are described in their respective sections, and are not necessary for basic functionality of a client.
Encoding
At the lowest level, computers can only send 1s and 0s to each other. In order for two computers to communicate, they must agree on what the 1s and 0s represent. In MSN Messenger (except in file transfer), they represent characters, such as "Latin Capital letter A", "Digital Four", or "Runic letter short-twig-sol". In other words, MSN Messenger is a text-based protocol.
In theory
There are many different standards for representing characters using series of 1s and 0s. ASCII (the American Standard Code for Information Interchange) is the most famous and most widely used. It says that every byte with a decimal value between 0 and 127 inclusive represents a character - for example, a byte with value 90 represents "Roman capital letter Z", and a byte with a value 32 represents "Space". ASCII says nothing about what to do with bytes whose value is greater than 127. The complete ASCII table can be found many places on-line, including http://www.asciitable.com.
Because ASCII was primarily designed for use in America, people in other parts of the world have defined other standards - mostly, these are just extensions of ASCII to use bytes with values higher than 127, but some languages are so unlike English (e.g. Japanese) that they've started over from scratch.
One popular standard is ISO 8859-1, which extends ASCII to make it more useful in Western Europe - for example, it says that a byte with value 163 represents the British currency symbol