2. The IRC Specification

2.1 Overview

The protocol as described herein is for use both with server to server and client to server connections. There are, however, more restrictions on client connections (which are considered to be untrustworthy) than on server connections.

2.2 Character codes

No specific character set is specified. The protocol is based on a a set of codes which are composed of eight (8) bits, making up an octet. Each message may be composed of any number of these octets; however, some octet values are used for control codes which act as message delimiters.

Regardless of being an 8-bit protocol, the delimiters and keywords are such that protocol is mostly usable from USASCII terminal and a telnet connection.

Because of IRC's scandanavian origin, the characters {}| are considered to be the lower case equivalents of the characters []\, respectively. This is a critical issue when determining the equivalence of two nicknames.

2.3 Messages

Servers and clients send eachother messages which may or may not generate a reply. If the message contains a valid command, as described in later sections, the client should expect a reply as specified but it is not advised to wait forever for the reply; client to server and server to server communication is essentially asynchronous in nature.

Each IRC message may consist of up to three main parts: the prefix (optional), the command, and the command parameters (of which there may be up to 15). The prefix, command, and all parameters are separated by one (or more) ASCII space character(s) (0x20).

The presence of a prefix is indicated with a single leading ASCII colon character (':', 0x3b), which must be the first character of the message itself. There must be no gap (whitespace) between the colon and the prefix. The prefix is used by servers to indicate the true origin of the message. If the prefix is missing from the message, it is assumed to have originated from the connection from which it was received. Clients should not use prefix when sending a message from themselves; if they use a prefix, the only valid prefix is the registered nickname associated with the client. If the source identified by the prefix cannot be found from the server's internal database, or if the source is registered from a different link than from which the message arrived, the server must ignore the message silently.

The command must either be a valid IRC command or a three (3) digit number represented in ASCII text.

IRC messages are always lines of characters terminated with a CR-LF (Carriage Return - Line Feed) pair, and these messages shall not exceed 512 characters in length, counting all characters including the trailing CR-LF. Thus, there are 510 characters maximum allowed for the command and its parameters. There is no provision for continuation message lines. See Section 8 for more details about current implementations.

15-441 Note: Your server must handle message termination as specified in Section 8.

2.3.1 Message format in 'pseudo' BNF

The protocol messages must be extracted from the contiguous stream of octets. The current solution is to designate two characters, CR and LF, as message separators. Empty messages are silently ignored, which permits use of the sequence CR-LF between messages without extra problems.

The extracted message is parsed into the components <prefix>, <command> and list of parameters matched either by <middle> or <trailing> components.

The BNF representation for this is:

<message> ::=

[':' <prefix> <SPACE> ] <command> <params> <crlf>

<prefix> ::=

<servername> | <nick> | <extended prefix>

<extended prefix> ::=

<nick>'!'<user>'@'<host>

<command> ::=

<letter> { <letter> } | <number> <number> <number>

<SPACE> ::=

' ' { ' ' }

<params> ::=

<SPACE> [ ':' <trailing> | <middle> <params> ]

<middle> ::=

<Any *non-empty* sequence of octets not including SPACE or NUL or CR or LF, the first of which may not be ':'>

<trailing> ::=

<Any, possibly *empty*, sequence of octets not including NUL or CR or LF>

<crlf> ::=

CR LF

NOTES:

1. <SPACE> is consists only of SPACE character(s) (0x20). Specially notice that TABULATION, and all other control characters are considered NON-WHITE-SPACE.
2. After extracting the parameter list, all parameters are equal, whether matched by <middle> or <trailing>. <Trailing> is just a syntactic trick to allow SPACE within parameter.
3. The fact that CR and LF cannot appear in parameter strings is just artifact of the message framing. This might change later.
4. The NUL character is not special in message framing, and basically could end up inside a parameter, but as it would cause extra complexities in normal C string handling. Therefore NUL is not allowed within messages.
5. The last parameter may be an empty string.
6. The extended prefix (['!' <user> ] ['@' <host> ]) must not be used in server to server communications. However, it must be used for server to client messages.

Most protocol messages specify additional semantics and syntax for the extracted parameter strings dictated by their position in the list. For example, many server commands will assume that the first parameter after the command is the list of targets, which can be described with:

<target> ::=

<to> [ "," <target> ]

<to> ::=

<channel> | <user> '@' <servername> | <nick> | <mask>

<channel> ::=

('#' | '&') <chstring>

<servername> ::=

<host>

<host> ::=

see RFC 952 [DNS:4] for details on allowed hostnames

<nick> ::=

<letter> { <letter> | <number> | <special> }

<mask> ::=

('#' | '$') <chstring>

<chstring> ::=

<any 8bit code except SPACE, BELL, NUL, CR, LF and comma (',')>

Other parameter syntaxes are:

<user> ::=

<nonwhite> { <nonwhite> }

<letter> ::=

'a' ... 'z' | 'A' ... 'Z'

<number> ::=

'0' ... '9'

<special> ::=

'-' | '[' | ']' | '\' | '`' | '^' | '{' | '}'

<nonwhite> ::=

<any 8bit code except SPACE (0x20), NUL (0x0), CR (0xd), and LF (0xa)>

15-441 Note: All targets will be nicks. Channels, masks and user@servername will not be used in the project.

2.4 Numeric replies

Most of the messages sent to the server generate a reply of some sort. The most common reply is the numeric reply, used for both errors and normal replies. The numeric reply must be sent as one message consisting of the sender prefix, the three digit numeric, and the target of the reply. In all other respects, a numeric reply is just like a normal message, except that the keyword is made up of 3 numeric digits rather than a string of letters. A list of different replies is supplied in section 6.

Example:

    :irc-server.example.edu 421 nickname BOGUSCOMMAND :Unknown command
    

15-441 Note: For the sender prefix on messages originated by the server, you should use the hostname of the server, as returned by gethostname().