Internet Relay Chat

IRC is one of the earliest network protocols for text messaging and multi-participant chatting. It was created in 1988 and, despite the emergence of more sophisticated messaging protocols (including open standards like XMPP and SIP/SIMPLE, and proprietary protocols such as Microsoft’s MSNP, AOL’s OSCAR, and Skype), IRC remains a popular standard and still sees heavy use in certain communities, specially the open source software community.

Basic IRC architecture

Basic IRC architecture

The basic architecture of IRC, shown in the figure above, is fairly straightforward. In the simplest case, there is a single IRC server to which multiple IRC clients can connect to. An IRC client connects to the server with a specific identity. Most notably, each client must choose a unique nickname, or “nick”. Once a client is connected, it can communicate one-to-one with other users. Additionally, clients can run commands to query the server’s state (e.g., to obtain a list of connected users, or to obtain additional details about a specific nick). IRC also supports the creation of chat rooms called channels for one-to-many communication. Users can join channels and send messages to the channel; these messages will, in turn, be sent to every user in the channel.

Multi-server IRC architecture

Multi-server IRC architecture

IRC also supports the formation of server networks, where multiple servers form a tree of connections to support more clients and provide greater capacity. Servers in the same network share information about local events (e.g., a new client connects, a user connected to a given server joins a channel, etc.) so that all servers will have a copy of the same global state. In this project, we will only consider the case where there is a single IRC server.

The IRC Protocol

The IRC protocol used by IRC servers and clients is a text-based TCP protocol. Originally specified in 1993 [RFC1459], it was subsequently specified in more detail in 2000 through the following RFCs:

  • [RFC2810] Internet Relay Chat: Architecture. This document describes the overall architecture of IRC.
  • [RFC2811] Internet Relay Chat: Channel Management. This document describes how channels are managed in IRC.
  • [RFC2812] Internet Relay Chat: Client Protocol. This document describes the protocol used between IRC clients and servers (sometimes referred to as the “client-server” protocol)
  • [RFC2813] Internet Relay Chat: Server Protocol. This document describes the “server-server” protocol used between IRC servers in the same network.

You are not expected to read all of these documents. More specifically:

  • We recommend you do read all of [RFC2810], as it will give you a good sense of what the IRC architecture looks like. You may want to give it a cursory read at first, and revisit it as you become more familiar with the finer points of the IRC protocol.
  • In the second assignment you will implement a subset of [RFC2812]. We suggest you read [RFC2812 §1] and [RFC2812 §2]. For the remainder of the RFC, you should only read the sections relevant to the parts of the IRC protocol you will be implementing.
  • In the third assignment you will implement a subset of the functionality described in [RFC2811], which will require implementing additional parts of [RFC2812]. We suggest you hold off on reading [RFC2811] until we reach the third assignment; if you do want to read the introductory sections, take into account that we will only be supporting “standard channels” in the “#” namespace, and that we will not be supporting server networks.
  • We will not be implementing any part of [RFC2813].

Finally, you should take into account that, although IRC has an official specification, most IRC servers and clients do not conform to these RFCs. Most (if not all) servers do not implement the full specification (and even contradict it in some cases), and there are many features that are unique to specific implementations. In this project, we will produce an implementation that is partially compliant with these RFCs, and sufficiently compliant to work with some of the main IRC clients currently available.

In the remainder of this section, we will see an overview of the message format used in IRC. Then, in the next section, we will see several example communications (involving multiple messages between a client and a server).

Message format

IRC clients and servers communicate by sending plain ASCII messages to each other over TCP. The format of these messages is described in [RFC2812 §2.3], and can be summarized thusly:

  • The IRC protocol is a text-based protocol, meaning that messages are encoded in plain ASCII. Although not as efficient as a pure binary format, this has the advantage of being fairly human-readable, and easy to debug just by reading the verbatim messages exchanged between clients and servers.

  • A single message is a string of characters with a maximum length of 512 characters. The end of the string is denoted by a CR-LF (Carriage Return - Line Feed) pair (i.e., “\r\n”). There is no null terminator. The 512 character limit includes this delimiter, meaning that a message only has space for 510 useful characters.

  • The IRC specification includes no provisions for supporting messages longer than 512 characters, although many servers and clients support non-standard solutions (including ignoring the 512 limit altogether). In our implementation, any message with more than 510 characters (not counting the delimiter) will be truncated, with the last two characters replaced with “\r\n”.

  • A message contains at least two parts: the command and the command parameters. There may be at most 15 parameters. The command and the parameters are all separated by a single ASCII space character. The following are examples of valid IRC messages:

    NICK amy
    WHOIS doctor
    MODE amy +o
    JOIN #tardis
  • When the last parameter is prefixed with a colon character, the value of that parameter will be the remainder of the message (including space characters). The following are examples of valid IRC messages with a “long parameter”:

    PRIVMSG rory :Hey Rory...
    PRIVMSG #cmsc23300 :Hello everybody
    QUIT :Done for the day, leaving
  • Some messages also include a prefix before the command and the command parameters. The presence of a prefix is indicated with a single leading colon character. The prefix is used to indicate the origin of the message. For example, when a user sends a message to a channel, the server will forward that message to all the users in the channel, and will include a prefix to specify the user that sent that message originally. We will explain the use of prefixes in more detail in the next section.

    The following are examples of valid IRC messages with prefixes:

    :borja! PRIVMSG #cmsc23300 :Hello everybody
    :doctor! QUIT :Done for the day, leaving


The IRC protocol includes a special type of message called a reply. When a client sends a command to a server, the server will send a reply (except in a few special commands where a reply should not be expected). Replies are used to acknowledge that a command was processed correctly, to indicate errors, or to provide information when the command performs a server query (e.g., asking for the list of users or channels).

A reply is a message with the following characteristics:

  • It always includes a prefix.
  • The command will be a three-digit code. The full list of possible replies is specified in [RFC2812 §5].
  • The first parameter is always the target of the reply, typically a nick.

The following are examples of valid IRC replies: 001 borja :Welcome to the Internet Relay Network borja! 433 * borja :Nickname is already in use. 332 borja #cmsc23300 :A channel for CMSC 23300 students