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Client-To-Client Protocol

Written by sojge@docs.uu.se (October 27, 1991)

Client-To-Client Protocol

This client-to-client protocol is meant to be used as a way to

  • 1. in general send structured data (such as graphics, voice and different font information) between users' clients, and in a more specific case.
  • 2. place a query to a user's client and getting an answer.
As of now, only a simple text encryption scheme is implemented in category 1, and a few query/reply pairs in category 2. This paper will concentrate on the latter category.

Basic Protocol Between Clients and Server

Characters between client and server are 8-bit bytes (also known as octets) and can have numeric values from octal 0 up to 0377 inclusive (0 up to 255 decimal). Some characters are special.

CHARS ::= '\000' .. '\377'
NUL ::= '\000'
NL ::= '\n'
CR ::= '\r'

A line sent to a server, or received from a server (here called "low-level messages") consist or zero or more octets (except NUL, NL or CR) with either a NL or CR appended.

L-CHARS ::= '\001' .. '\011' | '\013' | '\014' |
'\016' .. '\377'

A NUL is never sent over to the server.

Low-Level Quoting

As messages to and from servers can't contain NUL, NL and CR, but it still might be desirable to send ANY character (in so called "middle level messages") between clients, those three characters have to be quoted. Therefore a quote character is needed. Of course, the quote character itself has to be quoted too.

M-QUOTE ::= '\020'

(Ie a CNTRL/P).

When sending a middle-level message, if finding a character being one of NUL, NL, CR or M-QUOTE, that character is replaced by a two-character sequence according to the following table.


When receiving a low-level message and seeing a M-QUOTE, look at the next character, and replace those two according to the following table to get the corresponding middle-level message.


If the character following M-QUOTE isn't any of the listed characters, that is an error, so drop the M-QUOTE character from the message, optionally warning the user about it. Ie, a string 'x' M-QUOTE 'y' 'z' from a server dequotes into 'x 'y' 'z'.

Before low-level quoting, a message to the server (and in the opposite direction, after low-level dequoting, a message from the server) looks like


Tagged Data

To send both extended data and query/reply pairs between clients, an extended data format is needed. The extended data are sent in the text part of a middle-level message (and efter low-level quoting of course also in the text part of the low-level message).

To send extended data inside the text, we need some way to delimit it. This is done by starting and ending extended data with a delimiter character.

X-DELIM ::= '\001'

As both the starting and ending delimiter looks the same, every second X-DELIM is called the odd, and every second the even delimiter. The first one in a message is odd.

When having being quoted (and conversely, before having been dequoted) any number of characters of any kind except X-DELIM can be used in the extended data, i.e., inside the X-DELIM pair.

X-CHR ::= '\000' | '\002' .. '\377'

An extended message is either empty (i.e., nothing between the odd and even delimiter), has one or more non-space characters (i.e., any character but '\040') or has one or more non-space characters followed by a space followed by zero or more characters.

X-N-AS ::= '\000' | '\002' .. '\037' | '\041' .. '\377' SPC ::= '\040' X-MSG ::= | X-N-AS+ | X-N-AS+ SPC X-CHR*

The first characters up until the first SPC (or if no SPC, all of the X-MSG) is called the tag of the extended message. The tag is used to know what kind of extended data is used.

The tag can be any string of characters, and if they happen to be letters, they are case-sensitive, so upper- and lowercase matters.

Extended data is only valid in PRIVMSG and NOTICE commands. If the extended data is a reply to a query, it is sent in a NOTICE, or else it is sent in a PRIVMSG. Both PRIVMSG and NOTICE to a user and to a channel may contain extended data.

The text part of a PRIVMSG or NOTICE might contain zero or more extended messages, intermixed with zero or more chunks of non-extended data.

CTCP Level Quoting

In order to be able to send the delimiter X-DELIM inside an extended data message, it has to be quoted. This introduces another quote character (which should differ from the low-level quote character so it won't have to be quoted yet again).

X-QUOTE ::= '\134'

(i.e., a backslash).

When quoting on the CTCP level, only actual CTCP messages (ie extended data, queries, replies) are quoted. This enables users to actually send X-QUOTE characters at will. The following translations should be used:


When dequoting on the CTCP level, only CTCP messages are dequoted, whereby the following table is used:


If a X-QUOTE is seen with another the character following it than the ones above, that constitutes an error, and the X-QUOTE character should be dropped. For example, the CTCP-quoted string 'x' X-QUOTE 'y' 'z' becomes after dequoting the three character string 'x' 'y' 'z'.

If a X-DELIM is found outside a CTCP message, the message will contain the X-DELIM. (This should only happen with the last X-DELIM when there are an odd number of X-DELIM's in a middle-level message.

Quoting Examples

There are basically three levels of messages. The highest level (H) is the text on the user-to-client level. The middle layer (M) is on the level where CTCP quoting has been applied to the H-level message. The lowest level (L) is on the client-to-server level, where low-level quoting has been applied to the M-level message.

The following relations are true, with lowQuote(message) being a function doing the low-level quoting, lowDequote(message) the low level dequoting, ctcpQuote(message) the CTCP level quoting, ctcpDequote(message) the CTCP level dequoting, and ctcpExtract(message) the removing of all CTCP messages from a message. The operator || denotes string concatenation.

L = lowQuote(M) M = ctcpDequote(L)

M = ctcpQuote(H) H = ctcpDequote(ctcpExtract(M))

When sending CTCP message imbedded in normal text

M = ctcpQuote(H1) || '\001' || ctcpQuote(X) || '\001' || ctcpQuote(H2)

Of course, there might be zero or more normal text messages and zero or more CTCP messages mixed.

Example 1

A user (called actor) wanting to send the string

Hi there!\nHow are you?

to a user called victim, or in other words, a message where the user has entered an inline newline (how this is done, if at all, differs from client to client), will result internally in the client in the command:

PRIVMSG victim :Hi there!\nHow are you? \K?

which will be CTCP quoted into

PRIVMSG victim :Hi there!\nHow are you? \\K?

which in turn will be low-level quoted into

PRIVMSG victim :Hi there!\020nHow are you? \\K?

and sent to the server after appending a newline at the end.

This will arrive on victim's side as

:actor PRIVMSG victim :Hi there!\020nHow are you? \\K?

(where the \\K would look similar to OK in SIS D47, et al) which after low-level dequoting becomes

:actor PRIVMSG victim :Hi there!\nHow are you? \\K?

and after CTCP dequoting

:actom PRIVMSG victim :Hi there!\nHow are you? \K?

How this is displayed differs from client to client, but it is suggested that a line break should occour between the words "there" and "How".

Example 2

If actor's client wants to send the string "Emacs wins," this might become the string "\n\t\big\020\001\000\\:" when being SED-encrypted using some key, so the client starts by CTCP-quoting this string into the string "\n\t\big\020\\a\000\\\\:" and builds the M-level command

PRIVMSG victim :\001SED \n\t\big\020\\a\000\\\\:\001

which after low-level quoting becomes

PRIVMSG victim :\001SED \020n\t\big\020\020\\a\0200\\\\:\001

which will be sent to the server, with a newline tacked on.

On victim's side, the string

:actor PRIVMSG victim :\001SED \020n\t\big\020\020\\a\0200\\\\:\001

will be received from the server and low-level dequoted into

:actor PRIVMSG victim :\001SED \n\t\big\020\\a\000\\\\:\001

whereafter the string "\n\t\big\020\\a\000\\\\:" will be extracted and first CTCP dequoted into "\n\t\big\020\001\000\\:" and then SED decoded getting back "Emacs wins" when using the same key.

Example 3

If the user actor wants to query the USERINFO of user victim, and is in the middle of a conversation, the client may decide to tack on USERINFO request on a normal text message. Then the client wants to send the textmessage "Say hi to Ron\n\t/actor" and the CTCP request "USERINFO" to victim.

PRIVMSG victim :Say hi to Ron\n\t/actor



which after CTCP quoting become

PRIVMSG victim :Say hi to Ron\n\t/actor



which gets merged into

PRIVMSG victim :Say hi to Ron\n\t/actor\001USERINFO\001

and after low-level quoting

PRIVMSG victim :Say hi to Ron\020n\t/actor\001USERINFO\001

and sent off to the server.

On victim's side, the message

:actor PRIVMSG victim :Say hi to Ron\020n\t/actor\001USERINFO\001

arrives. This gets low-level dequoted into

:actor PRIVMSG victim :Say hi to Ron\n\t/actor\001USERINFO\001

and thereafter split up into

:actor PRIVMSG victim :Say hi to Ron\n\t/actor



After CTCP dequoting both, the message

:actor PRiVMSG victim :Say hi to Ron\n\t/actor

gets displayed, while the CTCP command


gets replied to. The reply might be

USERINFO :CS student\n\001test\001

which gets CTCP quoted into

USERINFO :CS student\n\\atest\\a

and sent in a NOTICE as it is a reply:

NOTICE actor :\001USERINFO :CS student\n\\atest\\a\001

and low-level quoted into

NOTICE actor :\001USERINFO :CS student\020n\\atest\\a\001

after which is it sent to victim's server.

When arriving on actor's side, the message

:victim NOTICE actor :\001USERINFO :CS student\020n\\atest\\a\001

gets low-level dequoted into

:victim NOTICE actor :\001USERINFO :CS student\n\\atest\\a\001

At this point, all CTCP replies get extracted, giving 1 CTCP reply and no normal NOTICE

USERINFO :CS student\n\\atest\\a

The remaining reply gets CTCP dequoted into

USERINFO :CS student\n\001test\001

and presumably displayed to user actor.

Known Request/Reply Pairs

A request/reply pair is sent between the two clients in two phases. The first phase is to send the request. This is done with a "privmsg" command (either to a nick or to a channel -- it doesn't matter).

The second phase is to send a reply. This is done with a "notice" command.

The known request/reply pairs are for the following commands.

CLIENTINFODynamic master index of what a client knows
ERRMSGUsed when an error needs to be replied with
FINGERMainly used to get a user's idle time
USERINFOA string set by the user (never client coder)
VERSIONThe version and type of the client


This is used to get some data stored locally at a user's system about the user and also the idle time of the user. The request is in a "privmsg" and looks like


while the reply is in a "notice" and looks like

\001FINGER :#\001

where the # denotes contains information about the user's real name, login name at clientmachine and idle time, and is of type X-N-AS.


This is used to get information about the name of the other client and the version of it. The request in a "privmsg" is simply


and the reply

\001VERSION #:#:#\001

where the first # denotes the name of the client, the second # denotes the version of the client, and the third # the environment the client is running in.


X-N-CLN ::= '\000' .. '\071' | '\073' .. '\377'

the client name is a string of type X-N-CLN saying things like "Kiwi" or "ircII", the version saying things like "5.2" or "2.1.5c", the environment saying things like "GNU Emacs 18.57.19 under SunOS 4.1.1 on Sun SLC" or "Compiled with gcc -ansi under Ultrix 4.0 on VAX-11/730".


This is used to get information about where to get a copy of the client. The request in a "privmsg" is simply


and the reply is zero or more CTCP replies of the form

\001SOURCE #:#:#\001

followed by an end marker


where the first # is the name of an Internet host where the client can be gotten from with anonymous FTP, the second # a directory name, and the third # a space separated list of files to be gotten from that directory.


X-N-SPC ::= '\000' .. '\037' | '\041' .. '\377'

the name of the FTP site is to be given by name like "cs.bu.edu" or "funic.funet.fi".

The file name field is a directory specification optionally followed by one or more file names, delimited by spaces. If only a directory name is given, all files in that directory should be copied when retrieving the client's source. If some files are given, only those files in that directory should be copied. Note that the specification allows for all characters but space in the names, this includes allowing :. Examples are "pub/emacs/irc/" to get all files in directory pub/emacs/irc/, the client should be able to first login as user "ftp" and the give the command "CD pub/emacs/irc/", followed by the command "mget *". (It of course has to take care of binary and prompt mode too). Another example is "/pub/irc Kiwi.5.2.el.Z" in which case a "CD /pub/irc" and "get Kiwi.5.2.el.Z" is what should be done.


This is used to transmit a string which can be set by the user, but should never be set by the client. The query is simply


with the reply

\001USERINFO :#\001

where the # is the value of the string the client's user has set.


This is for client developers use to make it easier to show other client hackers what a certain client knows when it comes to CTCP. The replies should be fairly verbose explaining what CTCP commands are understood, what arguments are expected of what type, and what replies might be expected from the client.

The query is the word CLIENTINFO in a "privmsg" optionally followed by a colon and one or more specifying words delimited by spaces, where the word CLIENTINFO by itself,


should be replied to by giving a list of known tags (see above in section TAGGED DATA). This is only intended to be read by humans.

With one argument, the reply should be a description of how to use that tag; with two arguments, a description of how to use that tag's subcommand, and so on.


This is used as a reply whenever an unknown query is seen. Also, when used as a query, the reply should echo back the text in the query, together with an indication that no error has happened. Should the query form be used, it is

\001ERRMSG #\001

where # is a string containing any character, with the reply

\001ERRMSG # :#\001

where the first # is the same string as in the query and the second # a short text notifying the user that no error has occurred.

A normal ERRMSG reply which is sent when a corrupted query or some corrupted extended data is received, looks like

\001ERRMSG # :#\001

where the first # is the the failed query or corrupted extended data and the second # a text explaining what the problem is, like "unknown query" or "failed decrypting text".



PRIVMSG victim :\001FINGER\001

might return

:victim NOTICE actor :\001FINGER :Please check my USERINFO instead :Klaus Zeuge (sojge@mizar) 1 second has passed since victim gave a command last.\001

(this is only one line) or why not

:victim NOTICE actor :\001FINGER :Please check my USERINFO instead :Klaus Zeuge (sojge@mizar) 427 seconds (7 minutes and 7 seconds) have passed since victim gave a command last.\001

if Klaus Zeuge happens to be lazy? :-)



might return

:victim NOTICE actor :CLIENTINFO :You can request help of the commands CLIENTINFO ERRMSG FINGER USERINFO VERSION by giving an argument to CLIENTINFO.



might return

:victim NOTICE actor :CLIENTINFO :CLIENTINFO with 0 arguments gives a list of known client query keywords. With 1 argument, a description of the client query keyword is returned.

while sending

PRIVMSG victim :clientinfo clientinfo

probably will return something like

:victim NOTICE actor :ERRMSG clientinfo clientinfo :Query is unknown

as tag "clientinfo" isn't known.



might return

:victim NOTICE actor :CLIENTINFO :ERRMSG is the given answer on seeing an unknown keyword. When seeing the keyword ERRMSG, it works like an echo.


PRIVMSG victim :\001USERINFO\001

might return the somewhat pathetically long

:victim NOTICE actor :USERINFO :I'm studying computer science in Uppsala, I'm male (somehow, that seems to be an important matter on IRC :-) and I speak fluent swedish, decent german, and some english.


PRIVMSG victim :\001VERSION\001

might return

:victim NOTICE actor :\001VERSION Kiwi:5.2:GNU Emacs 18.57.19 under SunOS 4.1.1 on Sun SLC:FTP.Lysator.LiU.SE:/pub/emacs Kiwi-5.2.el.Z Kiwi.README\001

if the client is named Kiwi of version 5.2 and is used under GNU Emacs 18.57.19 running on a Sun SLCwith SunOS 4.1.1. The client claims a copy of it can be found with anonymous FTP on FTP.Lysator.LiU.SE after giving the FTP command "cd /pub/emacs/". There, one should get files Kiwi-5.2.el.Z and Kiwi.README; presumably one of the files tells how to proceed with building the client after having gotten the files.

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