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+Network Working Group                        Marshall T. Rose (Delaware)
+Request for Comments: 934                       Einar A. Stefferud (NMA)
+                                                            January 1985
+
+              Proposed Standard for Message Encapsulation
+
+
+STATUS OF THIS MEMO
+
+   This RFC suggests a proposed protocol for the ARPA-Internet
+   community, and requests discussion and suggestions for improvements.
+   Distribution of this memo is unlimited.
+
+Introduction, Scope, and Motivation
+
+   The services that a user agent (UA) can offer are varied.  Although
+   all outgoing mail may be thought of as going through a single posting
+   slot to connect to the message transport system (MTS), it is possible
+   to consider a message draft being posted as described by one of the
+   following four types of postings:
+
+      Originate - a new message is composed from scratch, which, to the
+      knowledge of the UA, is unrelated to any message previously
+      handled by the user.
+
+      Reply - a message is composed as a reply to a message previously
+      received by the user.  In most circumstances, the UA aids the user
+      in composing the reply by constructing the header portion of the
+      message draft, using components extracted from the received
+      message headers.
+
+      Forward - one more more messages previously received by the user
+      are formatted by the UA as a part of the body portion of the
+      draft.  In this sense, a "digest" for an interest group may be
+      considered as forwarding.  Similarly, an argument may be made that
+      "blind-carbon-copies" should also be handled in this fashion.
+
+      Distribute - a message previously received by the user is
+      re-posted to the MTS.  The draft being re-posted is identical to
+      the original message with the exception that certain "ReSent-XXX"
+      headers are appended to the headers portion of the draft, and the
+      "Return-Path" header is reset to reference the re-sender's
+      address.  (See [RFC-821] for a discussion of the Return-Path
+      header.)
+
+   Most user agents support the first two of these activities, many
+   support the first three, and a few support all four.
+
+   This memo concerns itself only with the third type, which is message
+   forwarding.  (For a brief treatment of the semantics of message
+   components with respect to replies, see [RFC-822].) In many ways,
+
+
+Rose & Stefferud                                                [Page 1]
+
+
+
+RFC 934                                                     January 1985
+Message Encapsulation
+
+
+   forwarding can be thought of as encapsulating one or more messages
+   inside another.  Although this is useful for transfer of past
+   correspondence to new recipients, without a decapsulation process
+   (which this memo terms "bursting"), the forwarded messages are of
+   little use to the recipients because they can not be distributed,
+   forwarded, replied-to, or otherwise processed as separate individual
+   messages.
+
+      NOTE: RFC-822 mistakenly refers to distribution as forwarding
+      (section 4.2).  This memo suggests below, that these two
+      activities can and should be the same.
+
+   In the case of an interest group digest, a bursting capability is
+   especially useful.  Not only does the ability to burst a digest
+   permit a recipient of the digest to reply to an individual digested
+   message, but it also allows the recipient to selectively process the
+   other messages encapsulated in the digest.  For example, a single
+   digest issue usually contains more than one topic.  A subscriber may
+   only be interested in a subset of the topics discussed in a
+   particular issue.  With a bursting capability, the subscriber can
+   burst the digest, scan the headers, and process those messages which
+   are of interest.  The others can be ignored, if the user so desires.
+
+   This memo is motivated by three concerns:
+
+      In order to burst a message it is necessary to know how the
+      component messages were encapsulated in the draft.  At present
+      there is no unambiguous standard for interest group digests.  This
+      memo proposes such a standard for the ARPA-Internet.  Although
+      interest group digests may appear to conform to a pseudo-standard,
+      there is a serious ambiguity in the implementations which produce
+      digests.  By proposing this standard, the authors hope to solve
+      this problem by specifically addressing the implementation
+      ambiguity.
+
+      Next, there is much confusion as to how "blind-carbon-copies"
+      should be handled by UAs.  It appears that each agent in the
+      ARPA-Internet which supports a "bcc:" facility does so
+      differently. Although this memo does not propose a standard for
+      the generation of blind-carbon-copies, it introduces a formalism
+      which views the "bcc:" facility as a special case of the
+      forwarding activity.
+
+      Finally, both forwarding and distribution can be accomplished with
+      the same forwarding procedure, if a distributed message can be
+      extracted as a separate individually processable message.  With a
+      proper bursting agent, it will be difficult to distinguish between
+
+
+Rose & Stefferud                                                [Page 2]
+
+
+
+RFC 934                                                     January 1985
+Message Encapsulation
+
+
+      a message which has been distributed and a message which has been
+      extracted from a forwarded message. This memo argues that there is
+      no valuable distinction to be made, between forwarding and
+      distribution, and that in the interests of simplicity,
+      distribution facilities should not be generally available to the
+      ordinary users of a message system.  However, this memo also
+      argues that such facilities should be available to certain trusted
+      entities within the MTS.
+
+         NOTE: this memo does not propose that the distribution facility
+         be abolished.  Rather it argues the case forcefully in the hope
+         that other interested parties in the ARPA-Internet will join
+         this discussion.
+
+Message Encapsulation
+
+   This memo proposes the following encapsulation protocol: two agents
+   act on behalf of the user, a forwarding agent, which composes the
+   message draft prior to posting, and a bursting agent which decomposes
+   the message after delivery.
+
+   Definitions: a draft forwarding message consists of a header portion
+   and a text portion.  If the text portion is present, it is separated
+   from the header portion by a blank line.  Inside the text portion a
+   certain character string sequence, known as an "encapsulation
+   boundary", has special meaning.  Currently (in existing
+   digestification agents), an encapsulation boundary (EB) is defined as
+   a line in the message which starts with a dash (decimal code 45,
+   "-").  Initially, no restriction is placed on the length of the
+   encapsulation boundary, or on the characters that follow the dash.
+
+   1. The Header Portion
+
+   This memo makes no restriction on the header portion of the draft,
+   although it should conform to the RFC-822 standard.
+
+   2. The Text Portion
+
+   The text of the draft forwarding message consists of three parts: an
+   initial text section, the encapsulated messages, and the final text
+   section.
+
+      2.1. The Initial Text Section
+
+      All text (if any) up to the first EB comprises the initial text
+      section of the draft.  This memo makes no restrictions on the
+
+
+
+Rose & Stefferud                                                [Page 3]
+
+
+
+RFC 934                                                     January 1985
+Message Encapsulation
+
+
+      format of the initial text section of the draft.  In the case of a
+      digest, this initial text is usually the "table of contents" of
+      the digest.
+
+      2.2. The Final Text Section
+
+      All text (if any) after the last EB composes the final text
+      section of the draft.  This memo makes no restrictions on the
+      format of the final text section of the draft.  In the case of a
+      digest, this final text usually contains the sign-off banner for
+      the digest (e.g., "End of FOO Digest").
+
+      2.3. Encapsulated Messages
+
+      Each encapsulated message is bounded by two EBs: a pre-EB, which
+      occurs before the message; and, a post-EB, which occurs after the
+      message.  For two adjacent encapsulated messages, the post-EB of
+      the first message is also the pre-EB of the second message.
+      Consistent with this, two adjacent EBs with nothing between them
+      should be treated as enclosing a null message, and thus two or
+      more adjacent EBs are equivalent to one EB.
+
+      Each encapsulated message consists of two parts: a headers portion
+      and a text portion.  If the text portion is present, it is
+      separated from the header portion by a blank line.
+
+         2.3.1. The Header Portion
+
+         Minimally, there must be two header items in each message being
+         forwarded, a "Date:" field and a "From:" field. This differs
+         from RFC-822, which requires at least one destination address
+         (in a "To:" or "cc:" field) or a possibly empty "Bcc:" field.
+         Any addresses occuring in the header items for a message being
+         forwarded must be fully qualified.
+
+         2.3.2. The Text Portion
+
+         This memo makes no restrictions on the format of the text
+         portion of each encapsulated message.  (Actually, this memo
+         does restrict the format of the text portion of each
+         encapsulated message, but these restrictions are discussed
+         later.)
+
+   Before summarizing the generation/parsing rules for message
+   encapsulation, two issues are addressed.
+
+
+
+
+Rose & Stefferud                                                [Page 4]
+
+
+
+RFC 934                                                     January 1985
+Message Encapsulation
+
+
+Compatibility with Existing User Agents
+
+   The above encapsulation protocol is presently used by many user
+   agents in the ARPA-Internet, and was specifically designed to
+   minimize the amount of changes to existing implementations of
+   forwarding agents in the ARPA-Internet.
+
+   However, the protocol is not exactly like the pseudo-standard used by
+   those forwarding agents that compose digests.  In particular, the
+   post-EB of all messages encapsulated in a digest is preceeded and
+   followed by by a blank line.  In addition, the first message
+   encapsulated in a digest has a pre-EB that is followed by a blank
+   line, but usually isn't preceeded by a blank line (wonderful).
+
+   This memo recommends that implementors of forwarding agents wishing
+   to remain compatible with existing bursting agents consider
+   surrounding each EB with a blank line.  It should be noted that blank
+   lines following a pre-EB for an encapsulated message must be ignored
+   by bursting agents.  Further, this memo suggests that blank lines
+   preceeding a post-EB also be ignored by bursting agents.
+
+      NOTE: This recommendation is made in the interest of
+      backwards-compatibility.  A forwarding agent wishing to strictly
+      adhere to this memo, should not generate blank lines surrounding
+      EBs.
+
+Character-Stuffing the Encapsulation Boundary
+
+   It should be noted that the protocol is general enough to support
+   both general forwarding of messages and the specific case of digests.
+   Unfortunately, there is one issue of message encapsulation which
+   apparently is not addressed by any forwarding agent (to the authors'
+   knowledge) in the ARPA-Internet: what action does the forwarding
+   agent take when the encapsulation boundary occurs within a the text
+   portion of a message being forwarded?  Without exception, this
+   circumstance is ignored by existing forwarding agents.
+
+   To address this issue, this memo proposes the following
+   character-stuffing scheme: the encapsulation boundary is defined as a
+   line which starts with a dash.  A special case is made for those
+   boundaries which start with a dash and are followed by a space
+   (decimal code 32, " ").
+
+      During forwarding, if the forwarding agent detects a line in the
+      text portion of a message being forwarded which starts with the
+      encapsulation boundary, the forwarding agent outputs a dash
+      followed by a space prior to outputting the line.
+
+
+Rose & Stefferud                                                [Page 5]
+
+
+
+RFC 934                                                     January 1985
+Message Encapsulation
+
+
+      During bursting, if the bursting agent detects an encapsulation
+      boundary which starts with a dash followed by a space, then the
+      bursting agent does not treat the line as an encapsulation
+      boundary, and outputs the remainder of the line instead.
+
+   This simple character-stuffing scheme permits recursive forwardings.
+
+Generation/Parsing Rules for Message Encapsulation
+
+   The rules for forwarding/bursting are described in terms of regular
+   expressions.  The first author originally derived simple finite-state
+   automata for the rules, but was unable to legibly represent them in
+   this memo.  It is suggested that the implementors sketch the automata
+   to understand the grammar.
+
+   The conventions used for the grammar are simple.  Each state is
+   followed by one or more alternatives, which are separated by the "|"
+   character.  Each alternative starts with a character that is received
+   as input. (CRLF, although two characters is treated as one character
+   herein.)  The last alternative for a state is the character "c",
+   which represents any character not specified in the preceeding
+   alternatives.  Optionally following the input character is an output
+   string enclosed by curly-braces.  Following this is the state that
+   the automata enters.  The reader should note that these grammars are
+   extremely simple to implement (and, in most cases, can be implemented
+   quite efficiently).
+
+   When the forwarding agent encapsulates a message, it should apply the
+   following finite-state automaton.  The initial state is S1.
+
+      S1 ::   CRLF {CRLF} S1
+            | "-" {"- -"} S2
+            | c {c} S2
+
+      S2 ::   CRLF {CRLF} S1
+            | c {c} S2
+
+   This simply says that anytime a "-" is found at the beginning of a
+   line, a "- " is output prior to outputting the line.
+
+
+
+
+
+
+
+
+
+
+Rose & Stefferud                                                [Page 6]
+
+
+
+RFC 934                                                     January 1985
+Message Encapsulation
+
+
+   When the bursting agent decapsulates the text portion of a draft, it
+   should apply the following finite-state automaton.  The initial state
+   is S1.
+
+      S1 ::   "-" S3
+            | CRLF {CRLF} S1
+            | c {c} S2
+
+      S2 ::   CRLF {CRLF} S1
+            | c {c} S2
+
+      S3 ::   " " S2
+            | c S4
+
+      S4 ::   CRLF S5
+            | c S4
+
+      S5 ::   CRLF S5
+            | c {c} S2
+
+   Although more complicated than the grammar used by the forwarding
+   agent to encapsulate a single message, this grammer is still quite
+   simple.  Let us make the simplifying assumption that both the initial
+   and final text sections of the draft are messages in addition to the
+   encapsulated messages.
+
+   To begin, the current message being burst is scanned at state S1. All
+   characters are output until the EB is found (state S3).  If "- " is
+   found, the automaton enters state S2 and characters from the current
+   message are continued to be output.  Finally, a true EB is found
+   (state S4).  As the automaton traverses from state S3 to S4, the
+   bursting agent should consider the current message ended.  The
+   remainder of the EB is discarded (states S4 and S5).  As the
+   automaton traverses from state S5 to S2, the bursting agent should
+   consider a new message started and output the first character.  In
+   state S2, all characters are output until the EB is found.
+
+Blind Carbon Copies
+
+   Many user agents support a blind-carbon-copy facility.  With this
+   facility a draft has two types of addressees: visible and blind
+   recipients.  The visible recipients are listed as addresses in the
+   "To:" and "cc:" fields of the draft, and the blind recipients are
+   listed as addresses in the "Bcc:" fields of the draft.  The basis of
+   this facility is that copies of the draft which are delivered to the
+   recipients list the visible recipients only.
+
+
+
+Rose & Stefferud                                                [Page 7]
+
+
+
+RFC 934                                                     January 1985
+Message Encapsulation
+
+
+   One method of achieving this is to post a single draft, which lacks
+   any "Bcc:" fields, and, during posting, to interact with the MTS in
+   such a way that copies are sent to both the visible and blind
+   recipients.
+
+   Unfortunately, a key problem with this arrangement is that the blind
+   recipients can accidently reply to the draft in such a way that the
+   visible recipients are included as addressees in the reply. This is
+   socially unacceptable!  To avoid this problem, the message which the
+   visible recipients receive must be different than the message which
+   the blind recipients receive.
+
+   A second method is to post two drafts.  The first, which goes to the
+   visible recipients, is simply the draft without any "Bcc:" fields.
+   The second, which goes to the blind recipients, is simply the draft
+   with some string prepended to any "To:" and "cc:" field. For example,
+   the user agent might prepend "BCC-" to these fields, so that the
+   blind recipients get a draft with "BCC-To:" and "Bcc-cc:" fields and
+   no "To:" or "cc:" fields. Unfortunately, this is often very confusing
+   to the blind recipients.  Although accidental replies are not
+   possible, it is often difficult to tell that the draft received is
+   the result of a blind-carbon-copy.
+
+   The method which this memo suggests is to post two drafts, a visible
+   draft for the visible recipients, and a blind draft for the blind
+   recipients.  The visible draft consists of the original draft without
+   any "Bcc:" fields.  The blind draft contains the visible message as a
+   forwarded message.  The headers for the blind draft contain the
+   minimal RFC-822 headers and, if the original draft had a "Subject:"
+   field, then this header field is also included.  In addition, the
+   user agent might explicitly show that the blind draft is the result
+   of a blind-carbon-copy, with a "Bcc" header or prior to the first
+   encapsulating boundary in the body.
+
+Message Distribution
+
+   The main purpose of message distribution (often called redistribution
+   or resending) is to provide to a secondary recipient, perhaps not
+   included among the original addressees, with a "true original" copy
+   that can be treated like an original in every respect.
+
+   Such distribution is most often done by discussion group moderators
+   who use automated agents to simply repost received messages to a
+   distribution list.  The better automatic distribution agents insert a
+   new "Return-Path" header field to direct address failure notices to
+   the discussion group address list maintainer, rather than to the
+   original author.  This form of distribution is encouraged because it
+
+
+Rose & Stefferud                                                [Page 8]
+
+
+
+RFC 934                                                     January 1985
+Message Encapsulation
+
+
+   most simply serves to deliver messages to discussion group recipients
+   as processable originals.  It is performed by trusted pseudo-MTS
+   agents.
+
+   A second kind of distribution is that done by individuals who wish to
+   transfer a processable copy of a received message to another
+   recipient. This second form is discouraged in various new standards
+   for message transfer.  These include the NBS Standard for Mail
+   Interchange [FIPS-98], and the recent CCITT draft MHS (Mail Handling
+   Systems) X.400 standards [X.400]. In place of direct reposting of
+   received messages as though they are new drafts, the recommendation
+   is to forward the received message in the body of a new draft from
+   which is can be extracted by its secondary recipient for further
+   processing.
+
+   It is in support of this recommendation that this standard for
+   encapsulation/decapsulation is proposed.
+
+
+
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+
+Rose & Stefferud                                                [Page 9]
+
+
+
+RFC 934                                                     January 1985
+Message Encapsulation
+
+
+References
+
+   [RFC-822]    D.H. Crocker.  "Standard for the Format of ARPA-Internet
+                Text Messages", University of Delaware.  (August, 1982)
+
+   [RFC-821]    J.B. Postel.  "Simple Mail Transfer Protocol",
+                USC/Information Sciences Institute.  (August, 1982).
+
+   [FIPS-98]    National Bureau of Standards.  "Specification for
+                Message Format for Computer Based Message Systems."
+                (January, 1983).
+
+   [X.400]      Consultative Committee on International Telephone and
+                Telegraph.  "DRAFT Recommendation X.400.  Message
+                Handling Systems: System Model-Service Elements."
+
+Authors' Addresses
+
+   Marshall T. Rose
+
+      Department of Computer and Information Sciences
+      University of Delaware
+      Newark, DE 19716
+
+      MRose@UDel.ARPA
+
+   Einar A. Stefferud
+
+      Network Management Associates, Inc.
+      17301 Drey Lane
+      Huntington Beach, CA 92647
+
+      Stef@UCI.ARPA
+
+
+
+
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+Rose & Stefferud                                               [Page 10]
+