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RFC 864 INTERNET STANDARD TESTING · APPLICATION

Chargen: Character Generator Protocol

A service that streams meaningless but recognizable text, useful for testing networks.

On rfc-editor.org ↗
PUBLISHED
May 1983
AUTHORS
J. Postel
STREAM
Legacy
PORT
19 (TCP and UDP)
UDP REPLY SIZE
random, between 0 and 512 characters
PATTERN LINE LENGTH
72 characters
ASCII PRINTING CHARACTERS
95
DOI
10.17487/RFC0864
ERRATA
Reported errata

RFC text

Original text · RFC Editor View on rfc-editor.org ↗ 
Network Working Group                                          J. Postel
Request for Comments: 864                                            ISI
                                                                May 1983



                      Character Generator Protocol




This RFC specifies a standard for the ARPA Internet community.  Hosts on
the ARPA Internet that choose to implement a Character Generator
Protocol are expected to adopt and implement this standard.

A useful debugging and measurement tool is a character generator
service.  A character generator service simply sends data without regard
to the input.

TCP Based Character Generator Service

   One character generator service is defined as a connection based
   application on TCP.  A server listens for TCP connections on TCP port
   19.  Once a connection is established a stream of data is sent out
   the connection (and any data received is thrown away).  This
   continues until the calling user terminates the connection.

   It is fairly likely that users of this service will abruptly decide
   that they have had enough and abort the TCP connection, instead of
   carefully closing it.  The service should be prepared for either the
   carfull close or the rude abort.

   The data flow over the connection is limited by the normal TCP flow
   control mechanisms, so there is no concern about the service sending
   data faster than the user can process it.

UDP Based Character Generator Service

   Another character generator service is defined as a datagram based
   application on UDP.  A server listens for UDP datagrams on UDP port
   19.  When a datagram is received, an answering datagram is sent
   containing a random number (between 0 and 512) of characters (the
   data in the received datagram is ignored).

   There is no history or state information associated with the UDP
   version of this service, so there is no continuity of data from one
   answering datagram to another.

   The service only send one datagram in response to each received
   datagram, so there is no concern about the service sending data
   faster than the user can process it.

Character Generator Protocol


Data Syntax

   The data may be anything.  It is recommended that a recognizable
   pattern be used in tha data.

      One popular pattern is 72 chraracter lines of the ASCII printing
      characters.  There are 95 printing characters in the ASCII
      character set.  Sort the characters into an ordered sequence and
      number the characters from 0 through 94.  Think of the sequence as
      a ring so that character number 0 follows character number 94.  On
      the first line (line 0) put the characters numbered 0 through 71.
      On the next line (line 1) put the characters numbered 1 through
      72.  And so on.  On line N, put characters (0+N mod 95) through
      (71+N mod 95).  End each line with carriage return and line feed.

Character Generator Protocol


Example

!"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefgh
"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghi
#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghij
$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijk
%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijkl
&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklm
'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmn
()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmno
)*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnop
*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopq
+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqr
,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrs
-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrst
./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstu
/0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuv
0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvw
123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwx
23456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxy
3456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz
456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{
56789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|
6789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}
789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~
89:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~
9:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !
:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"
;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#
<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$
=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%
>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&
?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'
@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'(
ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'()
BCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'()*
CDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'()*+
DEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'()*+,
EFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'()*+,-
FGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'()*+,-.
GHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'()*+,-./
HIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'()*+,-./0
IJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'()*+,-./01
JKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'()*+,-./012
KLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'()*+,-./0123
LMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'()*+,-./01234
MNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'()*+,-./012345
NOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ !"#$%&'()*+,-./0123456

AI ELI5

On this page
PROBLEMEngineers need a simple source of data to debug and measure a network connection, without having to supply input.
MECHANISMA server on port 19 sends data and ignores whatever it receives. Over TCP it streams until the user disconnects; over UDP it answers each datagram with a random number of characters between 0 and 512.
WHY IT MATTERSThe RFC is an Internet Standard for hosts that choose to implement it. It also recommends a rotating pattern of the 95 ASCII printing characters in 72-character lines.

§1A Character Generator Service

RFC 864 defines the Character Generator Protocol (chargen). The RFC calls a character generator a useful debugging and measurement tool: it simply sends data without regard to the input.

Hosts on the ARPA Internet that choose to implement it are expected to adopt this standard. Implementing it is optional.

It is a machine that just keeps talking, whatever you tell it. That makes it handy for testing whether a line is working.

§2The TCP and UDP Services

Over TCP, a server listens on TCP port 19. Once a connection is established it sends a stream of data and throws away anything it receives. This continues until the calling user terminates the connection. Users may abruptly abort instead of closing carefully, so the service should be prepared for either a careful close or a rude abort. Normal TCP flow control keeps the service from sending faster than the user can process.

Over UDP, a server listens on UDP port 19. For each datagram received it sends one answering datagram containing a random number (between 0 and 512) of characters, and the received data is ignored. The UDP version keeps no history or state, so there is no continuity of data from one answering datagram to the next.

With TCP the machine pours out letters until you hang up. With UDP it sends back one random-sized handful of letters each time you knock.

§3Data Syntax and the Rotating Pattern

The data may be anything, but the RFC recommends a recognizable pattern. One popular pattern uses 72-character lines of the ASCII printing characters. There are 95 printing characters; sort them and number them 0 through 94, treating the sequence as a ring so that character 0 follows character 94.

Line 0 holds characters 0 through 71, line 1 holds characters 1 through 72, and so on. Line N holds characters (0+N mod 95) through (71+N mod 95). Each line ends with a carriage return and line feed. The effect is a diagonal pattern that makes dropped or garbled data easy to spot.

Each line is the line before it, slid over by one letter, wrapping around at the end. If anything goes missing, the pattern looks wrong.