RFC 1459, IRC : The IRC Specification
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 7 for more details about
current implementations.
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> [ '!' <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) Use of the extended prefix (['!' <user> ] ['@' <host> ]) must
not be used in server to server communications and is only
intended for server to client messages in order to provide
clients with more useful information about who a message is
from without the need for additional queries.
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)>
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. A numeric reply is not allowed to originate
from a client; any such messages received by a server are silently
dropped. 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.
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