Direct Client-to-Client

Direct Client-to-Client (DCC) is an IRC-related sub-protocol enabling peers to interconnect using an IRC server for handshaking in order to exchange files or perform non-relayed chats. Once established, a typical DCC session runs independently from the IRC server. Originally designed to be used with ircII it is now supported by many IRC clients. Some peer-to-peer clients on napster-protocol servers also have DCC send/get capability, including TekNap, SunshineUN and Lopster. A variation of the DCC protocol called SDCC (Secure Direct Client-to-Client), also known as DCC SCHAT supports encrypted connections. An RFC specification on the use of DCC does not exist.

DCC connections can be initiated in two different ways:

ircII was the first IRC client to implement the CTCP and DCC protocols. The CTCP protocol was implemented by Michael Sandrof in 1990 for ircII version 2.1. The DCC protocol was implemented by Troy Rollo in 1991 for version 2.1.2, but was never intended to be portable to other IRC clients.

The CHAT service enables users to chat with each other over a DCC connection. The traffic will go directly between the users, and not over the IRC network. When compared to sending messages normally, this reduces IRC network load, allows sending of larger amounts of text at once, due to the lack of flood control, and makes the communication more secure by not exposing the message to the IRC servers (however, the message is still in plaintext).

DCC CHAT is normally initiated using a handshake. The user wishing to establish the connection sends the following CTCP to the target:

<ip> and <port> are those of the sender, and are expressed as integers. <protocol> is "chat" for standard DCC CHAT. The receiving party can then connect to the given port and address.

Once a connection is established, the protocol used for DCC CHAT is very simple: users exchange CRLF-terminated messages. Messages that begin with an ASCII 001 (control-A, represented below by ^A) and the word "ACTION", and are terminated by another ASCII 001, are interpreted as emotes:

...
Wikipedia