CTCP (Client To Client Protocol) is a way of sending arbitrary data over an IRC network, which may include bytes not allowed in standard IRC messages. CTCPs are sent as a PRIVMSG or NOTICE, where the first and last characters as '\001' (SOH), and special bytes are escaped by encoding them into a two-byte sequence beginning with '\020' (DLE). CTCPs consist of command name (typically in upper-case) followed by list of space-separated arguments, which may be empty.
One use of CTCPs supported by the vast majority of IRC clients today is the ACTION command, typically invoked with /me. For example, if the user 'foo' in the channel '#bar' were to issue
> /me dances
everyone in the channel would receive the message
> :foo PRIVMSG #bar :001ACTION dances001
Other common uses of CTCP include requesting the name and version of a user's IRC client, their local time, determining ping times, and initiating file transfers (DCC).
Characters are escaped as follows:
['\000' (NUL)] '\020 \060' ("0")
['\012' (NL)] '\020 \156' ("n")
['\015' (CR)] '\020 \162' ("r")
['\020' (DLE)] '\020 \020'
All other appearences of the escape character are errors, and are dropped.
See <http://www.irchelp.org/irchelp/rfc/ctcpspec.html> for more details.