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.

1. 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.

2. 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,

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   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

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   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.

2.1. Required Header lines

2.1.1. 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:

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             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.

2.1.2. 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.

2.1.3. 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.

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   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.

2.1.4. 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.

2.1.5. 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).

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   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.

2.1.6. 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

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   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.

2.2. Optional Headers

2.2.1. 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.

2.2.2. 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.

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   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.

2.2.3. 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.

2.2.4. 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.

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2.2.5. 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.

2.2.6. 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.

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   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.

2.2.7. 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.

2.2.8. 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.

2.2.9. 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

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   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.

3. 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

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   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.

3.1. 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.

3.2. 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

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   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.

3.3. 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.

3.4. 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.

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3.5. 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

3.6. 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").

3.7. Checkgroups

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   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.

4. 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.

4.1. 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

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   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.

4.2. 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:

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               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.

4.3. 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:

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RFC 1036 Standard for USENET Messages December 1987

               #! 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.

5. 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

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RFC 1036 Standard for USENET Messages December 1987

   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.

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