RFC1036

Network Working Group M. Horton Request for Comments: 1036 AT&T Bell Laboratories Obsoletes: RFC-850 R. Adams

                                          Center for Seismic Studies
                                                       December 1987

          Standard for Interchange of USENET Messages

STATUS OF THIS MEMO

This document defines the standard format for the interchange of
network News messages among USENET hosts.  It updates and replaces
RFC-850, reflecting version B2.11 of the News program.  This memo is
disributed as an RFC to make this information easily accessible to
the Internet community.  It does not specify an Internet standard.
Distribution of this memo is unlimited.

Introduction

This document defines the standard format for the interchange of
network News messages among USENET hosts.  It describes the format
for messages themselves and gives partial standards for transmission
of news.  The news transmission is not entirely in order to give a
good deal of flexibility to the hosts to choose transmission
hardware and software, to batch news, and so on.

There are five sections to this document.  Section two defines the
format.  Section three defines the valid control messages.  Section
four specifies some valid transmission methods.  Section five
describes the overall news propagation algorithm.

Message Format

The primary consideration in choosing a message format is that it
fit in with existing tools as well as possible.  Existing tools
include implementations of both mail and news.  (The notesfiles
system from the University of Illinois is considered a news
implementation.)  A standard format for mail messages has existed
for many years on the Internet, and this format meets most of the
needs of USENET.  Since the Internet format is extensible,
extensions to meet the additional needs of USENET are easily made
within the Internet standard.  Therefore, the rule is adopted that
all USENET news messages must be formatted as valid Internet mail
messages, according to the Internet standard RFC-822.  The USENET
News standard is more restrictive than the Internet standard,

placing additional requirements on each message and forbidding use
of certain Internet features.  However, it should always be possible
to use a tool expecting an Internet message to process a news
message.  In any situation where this standard conflicts with the
Internet standard, RFC-822 should be considered correct and this
standard in error.

Here is an example USENET message to illustrate the fields.

          From: [email protected] (Jerry Schwarz)
          Path: cbosgd!mhuxj!mhuxt!eagle!jerry
          Newsgroups: news.announce
          Subject: Usenet Etiquette -- Please Read
          Message-ID: <[email protected]>
          Date: Fri, 19 Nov 82 16:14:55 GMT
          Followup-To: news.misc
          Expires: Sat, 1 Jan 83 00:00:00 -0500
          Organization: AT&T Bell Laboratories, Murray Hill

          The body of the message comes here, after a blank line.

  Here is an example of a message in the old format (before the
  existence of this standard). It is recommended that
  implementations also accept messages in this format to ease upward
  conversion.

           From: cbosgd!mhuxj!mhuxt!eagle!jerry (Jerry Schwarz)
           Newsgroups: news.misc
           Title: Usenet Etiquette -- Please Read
           Article-I.D.: eagle.642
           Posted: Fri Nov 19 16:14:55 1982
           Received: Fri Nov 19 16:59:30 1982
           Expires: Mon Jan 1 00:00:00 1990

           The body of the message comes here, after a blank line.

  Some news systems transmit news in the A format, which looks like
  this:

            Aeagle.642
            news.misc
            cbosgd!mhuxj!mhuxt!eagle!jerry
            Fri Nov 19 16:14:55 1982
            Usenet Etiquette - Please Read
            The body of the message comes here, with no blank line.

A standard USENET message consists of several header lines, followed
by a blank line, followed by the body of the message.  Each header

line consist of a keyword, a colon, a blank, and some additional
information.  This is a subset of the Internet standard, simplified
to allow simpler software to handle it.  The "From" line may
optionally include a full name, in the format above, or use the
Internet angle bracket syntax.  To keep the implementations simple,
other formats (for example, with part of the machine address after
the close parenthesis) are not allowed.  The Internet convention of
continuation header lines (beginning with a blank or tab) is
allowed.

Certain headers are required, and certain other headers are
optional.  Any unrecognized headers are allowed, and will be passed
through unchanged.  The required header lines are "From", "Date",
"Newsgroups", "Subject", "Message-ID", and "Path".  The optional
header lines are "Followup-To", "Expires", "Reply-To", "Sender",
"References", "Control", "Distribution", "Keywords", "Summary",
"Approved", "Lines", "Xref", and "Organization".  Each of these
header lines will be described below.

Required Header lines

From

The "From" line contains the electronic mailing address of the
person who sent the message, in the Internet syntax.  It may
optionally also contain the full name of the person, in parentheses,
after the electronic address.  The electronic address is the same as
the entity responsible for originating the message, unless the
"Sender" header is present, in which case the "From" header might
not be verified.  Note that in all host and domain names, upper and
lower case are considered the same, thus "[email protected]",
"[email protected]", and "[email protected]" are all equivalent.
User names may or may not be case sensitive, for example,
"[email protected]" might be different from
"[email protected]".  Programs should avoid changing the case of
electronic addresses when forwarding news or mail.

RFC-822 specifies that all text in parentheses is to be interpreted
as a comment.  It is common in Internet mail to place the full name
of the user in a comment at the end of the "From" line.  This
standard specifies a more rigid syntax.  The full name is not
considered a comment, but an optional part of the header line.
Either the full name is omitted, or it appears in parentheses after
the electronic address of the person posting the message, or it
appears before an electronic address which is enclosed in angle
brackets.  Thus, the three permissible forms are:

          From: [email protected]
          From: [email protected] (Mark Horton)
          From: Mark Horton <[email protected]>

Full names may contain any printing ASCII characters from space
through tilde, except that they may not contain "(" (left
parenthesis), ")" (right parenthesis), "<" (left angle bracket), or
">" (right angle bracket).  Additional restrictions may be placed on
full names by the mail standard, in particular, the characters ","
(comma), ":" (colon), "@" (at), "!" (bang), "/" (slash), "="
(equal), and ";" (semicolon) are inadvisable in full names.

Date

The "Date" line (formerly "Posted") is the date that the message was
originally posted to the network.  Its format must be acceptable
both in RFC-822 and to the getdate(3) routine that is provided with
the Usenet software.  This date remains unchanged as the message is
propagated throughout the network.  One format that is acceptable to
both is:

                  Wdy, DD Mon YY HH:MM:SS TIMEZONE

Several examples of valid dates appear in the sample message above.
Note in particular that ctime(3) format:

                      Wdy Mon DD HH:MM:SS YYYY

is not acceptable because it is not a valid RFC-822 date.  However,
since older software still generates this format, news
implementations are encouraged to accept this format and translate
it into an acceptable format.

There is no hope of having a complete list of timezones.  Universal
Time (GMT), the North American timezones (PST, PDT, MST, MDT, CST,
CDT, EST, EDT) and the +/-hhmm offset specifed in RFC-822 should be
supported.  It is recommended that times in message headers be
transmitted in GMT and displayed in the local time zone.

Newsgroups

The "Newsgroups" line specifies the newsgroup or newsgroups in which
the message belongs.  Multiple newsgroups may be specified,
separated by a comma.  Newsgroups specified must all be the names of
existing newsgroups, as no new newsgroups will be created by simply
posting to them.

Wildcards (e.g., the word "all") are never allowed in a "News-
groups" line.  For example, a newsgroup comp.all is illegal,
although a newsgroup rec.sport.football is permitted.

If a message is received with a "Newsgroups" line listing some valid
newsgroups and some invalid newsgroups, a host should not remove
invalid newsgroups from the list.  Instead, the invalid newsgroups
should be ignored.  For example, suppose host A subscribes to the
classes btl.all and comp.all, and exchanges news messages with host
B, which subscribes to comp.all but not btl.all.  Suppose A receives
a message with Newsgroups: comp.unix,btl.general.

This message is passed on to B because B receives comp.unix, but B
does not receive btl.general.  A must leave the "Newsgroups" line
unchanged.  If it were to remove btl.general, the edited header
could eventually re-enter the btl.all class, resulting in a message
that is not shown to users subscribing to btl.general.  Also,
follow-ups from outside btl.all would not be shown to such users.

Subject

The "Subject" line (formerly "Title") tells what the message is
about.  It should be suggestive enough of the contents of the
message to enable a reader to make a decision whether to read the
message based on the subject alone.  If the message is submitted in
response to another message (e.g., is a follow-up) the default
subject should begin with the four characters "Re:", and the
"References" line is required.  For follow-ups, the use of the
"Summary" line is encouraged.

Message-ID

The "Message-ID" line gives the message a unique identifier.  The
Message-ID may not be reused during the lifetime of any previous
message with the same Message-ID.  (It is recommended that no
Message-ID be reused for at least two years.)  Message-ID's have the
syntax:

                 <string not containing blank or ">">

In order to conform to RFC-822, the Message-ID must have the format:

                      <unique@full_domain_name>

where full_domain_name is the full name of the host at which the
message entered the network, including a domain that host is in, and
unique is any string of printing ASCII characters, not including "<"
(left angle bracket), ">" (right angle bracket), or "@" (at sign).

For example, the unique part could be an integer representing a
sequence number for messages submitted to the network, or a short
string derived from the date and time the message was created.  For
example, a valid Message-ID for a message submitted from host ucbvax
in domain "Berkeley.EDU" would be "<[email protected]>".
Programmers are urged not to make assumptions about the content of
Message-ID fields from other hosts, but to treat them as unknown
character strings.  It is not safe, for example, to assume that a
Message-ID will be under 14 characters, that it is unique in the
first 14 characters, nor that is does not contain a "/".

The angle brackets are considered part of the Message-ID.  Thus, in
references to the Message-ID, such as the ihave/sendme and cancel
control messages, the angle brackets are included.  White space
characters (e.g., blank and tab) are not allowed in a Message-ID.
Slashes ("/") are strongly discouraged.  All characters between the
angle brackets must be printing ASCII characters.

Path

This line shows the path the message took to reach the current
system.  When a system forwards the message, it should add its own
name to the list of systems in the "Path" line.  The names may be
separated by any punctuation character or characters (except "."
which is considered part of the hostname).  Thus, the following are
valid entries:

               cbosgd!mhuxj!mhuxt
               cbosgd, mhuxj, mhuxt
               @cbosgd.ATT.COM,@mhuxj.ATT.COM,@mhuxt.ATT.COM
               teklabs, zehntel, sri-unix@cca!decvax

(The latter path indicates a message that passed through decvax,
cca, sri-unix, zehntel, and teklabs, in that order.) Additional
names should be added from the left.  For example, the most recently
added name in the fourth example was teklabs.  Letters, digits,
periods and hyphens are considered part of host names; other
punctuation, including blanks, are considered separators.

Normally, the rightmost name will be the name of the originating
system.  However, it is also permissible to include an extra entry
on the right, which is the name of the sender.  This is for upward
compatibility with older systems.

The "Path" line is not used for replies, and should not be taken as
a mailing address.  It is intended to show the route the message
traveled to reach the local host.  There are several uses for this
information.  One is to monitor USENET routing for performance

reasons.  Another is to establish a path to reach new hosts.
Perhaps the most important use is to cut down on redundant USENET
traffic by failing to forward a message to a host that is known to
have already received it.  In particular, when host A sends a
message to host B, the "Path" line includes A, so that host B will
not immediately send the message back to host A.  The name each host
uses to identify itself should be the same as the name by which its
neighbors know it, in order to make this optimization possible.

A host adds its own name to the front of a path when it receives a
message from another host.  Thus, if a message with path "A!X!Y!Z"
is passed from host A to host B, B will add its own name to the path
when it receives the message from A, e.g., "B!A!X!Y!Z".  If B then
passes the message on to C, the message sent to C will contain the
path "B!A!X!Y!Z", and when C receives it, C will change it to
"C!B!A!X!Y!Z".

Special upward compatibility note:  Since the "From", "Sender", and
"Reply-To" lines are in Internet format, and since many USENET hosts
do not yet have mailers capable of understanding Internet format, it
would break the reply capability to completely sever the connection
between the "Path" header and the reply function.  It is recognized
that the path is not always a valid reply string in older
implementations, and no requirement to fix this problem is placed on
implementations.  However, the existing convention of placing the
host name and an "!"  at the front of the path, and of starting the
path with the host name, an "!", and the user name, should be
maintained when possible.

Optional Headers

Reply-To

This line has the same format as "From".  If present, mailed replies
to the author should be sent to the name given here.  Otherwise,
replies are mailed to the name on the "From" line. (This does not
prevent additional copies from being sent to recipients named by the
replier, or on "To" or "Cc" lines.)  The full name may be optionally
given, in parentheses, as in the "From" line.

Sender

This field is present only if the submitter manually enters a "From"
line.  It is intended to record the entity responsible for
submitting the message to the network.  It should be verified by the
software at the submitting host.

For example, if John Smith is visiting CCA and wishes to post a
message to the network, using friend Sarah Jones' account, the
message might read:

          From: [email protected] (John Smith)
          Sender: [email protected] (Sarah Jones)

If a gateway program enters a mail message into the network at host
unix.SRI.COM, the lines might read:

          From: [email protected]
          Sender: [email protected]

The primary purpose of this field is to be able to track down
messages to determine how they were entered into the network.  The
full name may be optionally given, in parentheses, as in the "From"
line.

Followup-To

This line has the same format as "Newsgroups".  If present, follow-
up messages are to be posted to the newsgroup or newsgroups listed
here.  If this line is not present, follow-ups are posted to the
newsgroup or newsgroups listed in the "Newsgroups" line.

If the keyword poster is present, follow-up messages are not
permitted.  The message should be mailed to the submitter of the
message via mail.

Expires

This line, if present, is in a legal USENET date format.  It
specifies a suggested expiration date for the message.  If not
present, the local default expiration date is used.  This field is
intended to be used to clean up messages with a limited usefulness,
or to keep important messages around for longer than usual.  For
example, a message announcing an upcoming seminar could have an
expiration date the day after the seminar, since the message is not
useful after the seminar is over.  Since local hosts have local
policies for expiration of news (depending on available disk space,
for instance), users are discouraged from providing expiration dates
for messages unless there is a natural expiration date associated
with the topic.  System software should almost never provide a
default "Expires" line.  Leave it out and allow local policies to be
used unless there is a good reason not to.

References

This field lists the Message-ID's of any messages prompting the
submission of this message.  It is required for all follow-up
messages, and forbidden when a new subject is raised.
Implementations should provide a follow-up command, which allows a
user to post a follow-up message.  This command should generate a
"Subject" line which is the same as the original message, except
that if the original subject does not begin with "Re:" or "re:", the
four characters "Re:" are inserted before the subject.  If there is
no "References" line on the original header, the "References" line
should contain the Message-ID of the original message (including the
angle brackets).  If the original message does have a "References"
line, the follow-up message should have a "References" line
containing the text of the original "References" line, a blank, and
the Message-ID of the original message.

The purpose of the "References" header is to allow messages to be
grouped into conversations by the user interface program.  This
allows conversations within a newsgroup to be kept together, and
potentially users might shut off entire conversations without
unsubscribing to a newsgroup.  User interfaces need not make use of
this header, but all automatically generated follow-ups should
generate the "References" line for the benefit of systems that do
use it, and manually generated follow-ups (e.g., typed in well after
the original message has been printed by the machine) should be
encouraged to include them as well.

It is permissible to not include the entire previous "References"
line if it is too long.  An attempt should be made to include a
reasonable number of backwards references.

Control

If a message contains a "Control" line, the message is a control
message.  Control messages are used for communication among USENET
host machines, not to be read by users.  Control messages are
distributed by the same newsgroup mechanism as ordinary messages.
The body of the "Control" header line is the message to the host.

For upward compatibility, messages that match the newsgroup pattern
"all.all.ctl" should also be interpreted as control messages.  If no
"Control" header is present on such messages, the subject is used as
the control message.  However, messages on newsgroups matching this
pattern do not conform to this standard.

Also for upward compatibility, if the first 4 characters of the
"Subject:" line are "cmsg", the rest of the "Subject:" line should
be interpreted as a control message.

Distribution

This line is used to alter the distribution scope of the message.
It is a comma separated list similar to the "Newsgroups" line.  User
subscriptions are still controlled by "Newsgroups", but the message
is sent to all systems subscribing to the newsgroups on the
"Distribution" line in addition to the "Newsgroups" line.  For the
message to be transmitted, the receiving site must normally receive
one of the specified newsgroups AND must receive one of the
specified distributions.  Thus, a message concerning a car for sale
in New Jersey might have headers including:

               Newsgroups: rec.auto,misc.forsale
               Distribution: nj,ny

so that it would only go to persons subscribing to rec.auto or misc.
for sale within New Jersey or New York.  The intent of this header
is to restrict the distribution of a newsgroup further, not to
increase it.  A local newsgroup, such as nj.crazy-eddie, will
probably not be propagated by hosts outside New Jersey that do not
show such a newsgroup as valid.  A follow-up message should default
to the same "Distribution" line as the original message, but the
user can change it to a more limited one, or escalate the
distribution if it was originally restricted and a more widely
distributed reply is appropriate.

Organization

The text of this line is a short phrase describing the organization
to which the sender belongs, or to which the machine belongs.  The
intent of this line is to help identify the person posting the
message, since host names are often cryptic enough to make it hard
to recognize the organization by the electronic address.

Keywords

A few well-selected keywords identifying the message should be on
this line.  This is used as an aid in determining if this message is
interesting to the reader.

2.2.10. Summary

This line should contain a brief summary of the message.  It is
usually used as part of a follow-up to another message.  Again, it

is very useful to the reader in determining whether to read the
message.

2.2.11. Approved

This line is required for any message posted to a moderated
newsgroup.  It should be added by the moderator and consist of his
mail address.  It is also required with certain control messages.

2.2.12. Lines

This contains a count of the number of lines in the body of the
message.

2.2.13. Xref

This line contains the name of the host (with domains omitted) and a
white space separated list of colon-separated pairs of newsgroup
names and message numbers.  These are the newsgroups listed in the
"Newsgroups" line and the corresponding message numbers from the
spool directory.

This is only of value to the local system, so it should not be
transmitted.  For example, in:

           Path: seismo!lll-crg!lll-lcc!pyramid!decwrl!reid
           From: [email protected] (Brian Reid)
           Newsgroups: news.lists,news.groups
           Subject: USENET READERSHIP SUMMARY REPORT FOR SEP 86
           Message-ID: <[email protected]>
           Date: 1 Oct 86 11:26:15 GMT
           Organization: DEC Western Research Laboratory
           Lines: 441
           Approved: [email protected]
           Xref: seismo news.lists:461 news.groups:6378

the "Xref" line shows that the message is message number 461 in the
newsgroup news.lists, and message number 6378 in the newsgroup
news.groups, on host seismo.  This information may be used by
certain user interfaces.

Control Messages

This section lists the control messages currently defined.  The body
of the "Control" header line is the control message.  Messages are a
sequence of zero or more words, separated by white space (blanks or
tabs).  The first word is the name of the control message, remaining
words are parameters to the message.  The remainder of the header

and the body of the message are also potential parameters; for
example, the "From" line might suggest an address to which a
response is to be mailed.

Implementors and administrators may choose to allow control messages
to be carried out automatically, or to queue them for annual
processing.  However, manually processed messages should be dealt
with promptly.

Failed control messages should NOT be mailed to the originator of
the message, but to the local "usenet" account.

Cancel

                 cancel <Message-ID>

If a message with the given Message-ID is present on the local
system, the message is cancelled.  This mechanism allows a user to
cancel a message after the message has been distributed over the
network.

If the system is unable to cancel the message as requested, it
should not forward the cancellation request to its neighbor systems.

Only the author of the message or the local news administrator is
allowed to send this message.  The verified sender of a message is
the "Sender" line, or if no "Sender" line is present, the "From"
line.  The verified sender of the cancel message must be the same as
either the "Sender" or "From" field of the original message.  A
verified sender in the cancel message is allowed to match an
unverified "From" in the original message.

Ihave/Sendme

               ihave <Message-ID list> [<remotesys>]
               sendme <Message-ID list> [<remotesys>]

This message is part of the ihave/sendme protocol, which allows one
host (say A) to tell another host (B) that a particular message has
been received on A.  Suppose that host A receives message
"<[email protected]>", and wishes to transmit the message to
host B.

A sends the control message "ihave <[email protected]> A" to
host B (by posting it to newsgroup to.B).  B responds with the
control message "sendme <[email protected]> B" (on newsgroup
to.A), if it has not already received the message.  Upon receiving

the sendme message, A sends the message to B.

This protocol can be used to cut down on redundant traffic between
hosts.  It is optional and should be used only if the particular
situation makes it worthwhile.  Frequently, the outcome is that,
since most original messages are short, and since there is a high
overhead to start sending a new message with UUCP, it costs as much
to send the ihave as it would cost to send the message itself.

One possible solution to this overhead problem is to batch requests.
Several Message-ID's may be announced or requested in one message.
If no Message-ID's are listed in the control message, the body of
the message should be scanned for Message-ID's, one per line.

Newgroup

                  newgroup <groupname> [moderated]

This control message creates a new newsgroup with the given name.
Since no messages may be posted or forwarded until a newsgroup is
created, this message is required before a newsgroup can be used.
The body of the message is expected to be a short paragraph
describing the intended use of the newsgroup.

If the second argument is present and it is the keyword moderated,
the group should be created moderated instead of the default of
unmoderated.  The newgroup message should be ignored unless there is
an "Approved" line in the same message header.

Rmgroup

                        rmgroup <groupname>

This message removes a newsgroup with the given name.  Since the
newsgroup is removed from every host on the network, this command
should be used carefully by a responsible administrator.  The
rmgroup message should be ignored unless there is an "Approved:"
line in the same message header.

Sendsys

                       sendsys (no arguments)

The sys file, listing all neighbors and the newsgroups to be sent to
each neighbor, will be mailed to the author of the control message
("Reply-To", if present, otherwise "From").  This information is
considered public information, and it is a requirement of membership
in USENET that this information be provided on request, either
automatically in response to this control message, or manually, by
mailing the requested information to the author of the message.
This information is used to keep the map of USENET up to date, and
to determine where netnews is sent.

The format of the file mailed back to the author should be the same
as that of the sys file.  This format has one line per neighboring
host (plus one line for the local host), containing four colon
separated fields.  The first field has the host name of the
neighbor, the second field has a newsgroup pattern describing the
newsgroups sent to the neighbor.  The third and fourth fields are
not defined by this standard.  The sys file is not the same as the
UUCP L.sys file.  A sample response is:

  From: cbosgd!mark  (Mark Horton)
  Date: Sun, 27 Mar 83 20:39:37 -0500
  Subject: response to your sendsys request
  To: [email protected]

  Responding-System: cbosgd.ATT.COM
  cbosgd:osg,cb,btl,bell,world,comp,sci,rec,talk,misc,news,soc,to,
        test
  ucbvax:world,comp,to.ucbvax:L:
  cbosg:world,comp,bell,btl,cb,osg,to.cbosg:F:/usr/spool/outnews
        /cbosg
  cbosgb:osg,to.cbosgb:F:/usr/spool/outnews/cbosgb
  sescent:world,comp,bell,btl,cb,to.sescent:F:/usr/spool/outnews
        /sescent
  npois:world,comp,bell,btl,ug,to.npois:F:/usr/spool/outnews/npois
  mhuxi:world,comp,bell,btl,ug,to.mhuxi:F:/usr/spool/outnews/mhuxi

Version

                       version (no arguments)

The name and version of the software running on the local system is
to be mailed back to the author of the message ("Reply-to" if
present, otherwise "From").

Checkgroups

The message body is a list of "official" newsgroups and their
description, one group per line.  They are compared against the list
of active newsgroups on the current host.  The names of any obsolete
or new newsgroups are mailed to the user "usenet" and descriptions
of the new newsgroups are added to the help file used when posting
news.

Transmission Methods

USENET is not a physical network, but rather a logical network
resting on top of several existing physical networks.  These
networks include, but are not limited to, UUCP, the Internet, an
Ethernet, the BLICN network, an NSC Hyperchannel, and a BERKNET.
What is important is that two neighboring systems on USENET have
some method to get a new message, in the format listed here, from
one system to the other, and once on the receiving system, processed
by the netnews software on that system.  (On UNIX systems, this
usually means the rnews program being run with the message on the
standard input. <1>)

It is not a requirement that USENET hosts have mail systems capable
of understanding the Internet mail syntax, but it is strongly
recommended.  Since "From", "Reply-To", and "Sender" lines use the
Internet syntax, replies will be difficult or impossible without an
Internet mailer.  A host without an Internet mailer can attempt to
use the "Path" header line for replies, but this field is not
guaranteed to be a working path for replies.  In any event, any host
generating or forwarding news messages must have an Internet address
that allows them to receive mail from hosts with Internet mailers,
and they must include their Internet address on their From line.

Remote Execution

Some networks permit direct remote command execution.  On these
networks, news may be forwarded by spooling the rnews command with
the message on the standard input.  For example, if the remote
system is called remote, news would be sent over a UUCP link
with the command:

                          uux - remote!rnews

and on a Berknet:

                          net -mremote rnews

It is important that the message be sent via a reliable mechanism,
normally involving the possibility of spooling, rather than direct
real-time remote execution.  This is because, if the remote system
is down, a direct execution command will fail, and the message will
never be delivered.  If the message is spooled, it will eventually
be delivered when both systems are up.

Transfer by Mail

On some systems, direct remote spooled execution is not possible.
However, most systems support electronic mail, and a news message
can be sent as mail.  One approach is to send a mail message which
is identical to the news message: the mail headers are the news
headers, and the mail body is the news body.  By convention, this
mail is sent to the user newsmail on the remote machine.

One problem with this method is that it may not be possible to
convince the mail system that the "From" line of the message is
valid, since the mail message was generated by a program on a
system different from the source of the news message.  Another
problem is that error messages caused by the mail transmission
would be sent to the originator of the news message, who has no
control over news transmission between two cooperating hosts
and does not know whom to contact.  Transmission error messages
should be directed to a responsible contact person on the
sending machine.

A solution to this problem is to encapsulate the news message into a
mail message, such that the entire message (headers and body) are
part of the body of the mail message.  The convention here is that
such mail is sent to user rnews on the remote system.  A mail
message body is generated by prepending the letter N to each line of
the news message, and then attaching whatever mail headers are
convenient to generate.  The N's are attached to prevent any special
lines in the news message from interfering with mail transmission,
and to prevent any extra lines inserted by the mailer (headers,
blank lines, etc.) from becoming part of the news message.  A
program on the receiving machine receives mail to rnews, extracting
the message itself and invoking the rnews program.  An example in
this format might look like this:

            Date: Mon, 3 Jan 83 08:33:47 MST
            From: [email protected]
            Subject: network news message
            To: [email protected]

            NPath: cbosgd!mhuxj!harpo!utah-cs!sask!derek
            NFrom: [email protected] (Derek Andrew)
            NNewsgroups: misc.test
            NSubject: necessary test
            NMessage-ID: <[email protected]>
            NDate: Mon, 3 Jan 83 00:59:15 MST
            N
            NThis really is a test.  If anyone out there more than 6
            Nhops away would kindly confirm this note I would
            Nappreciate it.  We suspect that our news postings
            Nare not getting out into the world.
            N

Using mail solves the spooling problem, since mail must always be
spooled if the destination host is down.  However, it adds more
overhead to the transmission process (to encapsulate and extract the
message) and makes it harder for software to give different
priorities to news and mail.

Batching

Since news messages are usually short, and since a large number of
messages are often sent between two hosts in a day, it may make
sense to batch news messages.  Several messages can be combined into
one large message, using conventions agreed upon in advance by the
two hosts.  One such batching scheme is described here; its use is
highly recommended.

News messages are combined into a script, separated by a header of
the form:

               #! rnews 1234

where 1234 is the length of the message in bytes.  Each such line is
followed by a message containing the given number of bytes.  (The
newline at the end of each line of the message is counted as one
byte, for purposes of this count, even if it is stored as <CARRIAGE
RETURN><LINE FEED>.)  For example, a batch of message might look
like this:

            #! rnews 239
            From: [email protected] (Jerry Schwarz)
            Path: cbosgd!mhuxj!mhuxt!eagle!jerry
            Newsgroups: news.announce
            Subject: Usenet Etiquette -- Please Read
            Message-ID: <[email protected]>
            Date: Fri, 19 Nov 82 16:14:55 EST
            Approved: [email protected]

            Here is an important message about USENET Etiquette.
            #! rnews 234
            From: [email protected] (Jerry Schwarz)
            Path: cbosgd!mhuxj!mhuxt!eagle!jerry
            Newsgroups: news.announce
            Subject: Notes on Etiquette message
            Message-ID: <[email protected]>
            Date: Fri, 19 Nov 82 17:24:12 EST
            Approved: [email protected]

            There was something I forgot to mention in the last
            message.

Batched news is recognized because the first character in the
message is #.  The message is then passed to the unbatcher for
interpretation.

The second argument (in this example rnews) determines which
batching scheme is being used.  Cooperating hosts may use whatever
scheme is appropriate for them.

The News Propagation Algorithm

This section describes the overall scheme of USENET and the
algorithm followed by hosts in propagating news to the entire
logical network.  Since all hosts are affected by incorrectly
formatted messages and by propagation errors, it is important
for the method to be standardized.

USENET is a directed graph.  Each node in the graph is a host
computer, and each arc in the graph is a transmission path from
one host to another host.  Each arc is labeled with a newsgroup
pattern, specifying which newsgroup classes are forwarded along
that link.  Most arcs are bidirectional, that is, if host A
sends a class of newsgroups to host B, then host B usually sends
the same class of newsgroups to host A.  This bidirectionality
is not, however, required.

USENET is made up of many subnetworks.  Each subnet has a name, such

as comp or btl.  Each subnet is a connected graph, that is, a path
exists from every node to every other node in the subnet.  In
addition, the entire graph is (theoretically) connected.  (In
practice, some political considerations have caused some hosts to be
unable to post messages reaching the rest of the network.)

A message is posted on one machine to a list of newsgroups. That
machine accepts it locally, then forwards it to all its neighbors
that are interested in at least one of the newsgroups of the
message.  (Site A deems host B to be "interested" in a newsgroup if
the newsgroup matches the pattern on the arc from A to B.  This
pattern is stored in a file on the A machine.)  The hosts receiving
the incoming message examine it to make sure they really want the
message, accept it locally, and then in turn forward the message to
all their interested neighbors.  This process continues until the
entire network has seen the message.

An important part of the algorithm is the prevention of loops.  The
above process would cause a message to loop along a cycle forever.
In particular, when host A sends a message to host B, host B will
send it back to host A, which will send it to host B, and so on.
One solution to this is the history mechanism.  Each host keeps
track of all messages it has seen (by their Message-ID) and
whenever a message comes in that it has already seen, the incoming
message is discarded immediately.  This solution is sufficient to
prevent loops, but additional optimizations can be made to avoid
sending messages to hosts that will simply throw them away.

One optimization is that a message should never be sent to a machine
listed in the "Path" line of the header.  When a machine name is
in the "Path" line, the message is known to have passed through the
machine.  Another optimization is that, if the message originated
on host A, then host A has already seen the message.  Thus, if a
message is posted to newsgroup misc.misc, it will match the pattern
misc.all (where all is a metasymbol that matches any string), and
will be forwarded to all hosts that subscribe to misc.all (as
determined by what their neighbors send them).  These hosts make up
the misc subnetwork.  A message posted to btl.general will reach all
hosts receiving btl.all, but will not reach hosts that do not get
btl.all.  In effect, the messages reaches the btl subnetwork.  A
messages posted to newsgroups misc.misc,btl.general will reach all
hosts subscribing to either of the two classes.

Notes

<1>  UNIX is a registered trademark of AT&T.