path: root/doc/rfc/rfc3391.txt

Network Working Group                                         R. Herriot
Request for Comments: 3391                                 December 2002
Category: Informational


         The MIME Application/Vnd.pwg-multiplexed Content-Type

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

IESG Note

   The IESG believes use of this media type is only appropriate in
   situations where the producer is fully aware of the capabilities and
   limitations of the consumer.  In particular, this mechanism is very
   dependent on the producer knowing when the consumer will need a
   particular component of a multipart object.  But consumers
   potentially work in many different ways and different consumers may
   need different things at different times.  This mechanism provides no
   means for a producer to determine the needs of a particular consumer
   and how they are to be accommodated.

   Alternative mechanisms, such as a protocol based on BEEP which is
   capable of bidirectional communication between the producer and
   consumer, should be considered when the capabilities of the consumer
   are not known by the producer.

Abstract

   The Application/Vnd.pwg-multiplexed content-type, like the
   Multipart/Related content-type, provides a mechanism for representing
   objects that consist of multiple components.  An
   Application/Vnd.pwg-multiplexed entity contains a sequence of chunks.
   Each chunk contains a MIME message or a part of a MIME message.  Each
   MIME message represents a component of the compound object, just as a
   body part of a Multipart/Related entity represents a component.  With
   a Multipart/Related entity, a body part and its reference in some
   other body part may be separated by many octets.  With an
   Application/Vnd.pwg-multiplexed entity, a message and its reference
   in some other message can be made quite close by chunking the message
   containing the reference.  For example, if a long message contains



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   references to images and the producer does not know of the need for
   each image until it generates the reference, then
   Application/Vnd.pwg-multiplexed allows the consumer to process the
   reference to the image and the image before it consumes the entire
   long message.  This ability is important in printing and scanning
   applications.  This document defines the Application/Vnd.pwg-
   multiplexed content-type. It also provides examples of its use.

Table of Contents

   1. Introduction....................................................2
   2. Terminology.....................................................7
   3. Details.........................................................9
   3.1  Syntax of Application/Vnd.pwg-multiplexed Contents...........10
   3.2  Parameters for Application/Vnd.pwg-multiplexed...............12
   3.2.1  The "type" Parameter.......................................12
   3.2.2  Syntax.....................................................12
   4. Handling Application/Vnd.pwg-multiplexed Entities..............12
   5. Examples.......................................................13
   5.1  Example With Multipart/Related...............................14
   5.2  Examples with Application/Vnd.pwg-multiplexed................15
   5.2.1  Example Where Each Chunk Has a Complete Message............15
   5.2.2  Example of Chunking the Root Message.......................17
   5.2.3  Example of Chunking the Several Messages...................18
   5.2.4  Example of Chunks with Empty Payloads......................20
   6. Security Considerations........................................22
   7. Registration Information for Application/Vnd.pwg-multiplexed...22
   8. Acknowledgments................................................23
   9. References.....................................................23
   10. Author's Address..............................................24
   11. Full Copyright Statement......................................25

1. Introduction

   The simple MIME content-types, such as "text/plain" provide a
   mechanism for representing a simple object, such as a text document.
   The Multipart/Related [RFC2387] content-type provides a mechanism for
   representing a compound object, such as a text document with two gif
   images.

   A compound object consists of multiple components.  One such
   component is the root component, which contains references to other
   components of the compound object.  These components may, in turn,
   contain references to other components of the compound object.  For
   example, a compound object could consist of a root html text
   component and two gif image components -- each referenced by the root
   component.




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   A compound object and a component are both abstractions.  For
   transmission over the wire or writing to storage, each needs a
   representation.  A "Multipart/Related entity" is one possible
   representation of a compound object, and a "body part" is one
   possible representation of a component.

   However, the Multipart/Related content-type is not a good solution
   for applications that require each component to be close to its
   corresponding reference in the root component.  This document defines
   a new MIME content-type Application/Vnd.pwg-multiplexed that provides
   a better solution for some applications.  The Application/Vnd.pwg-
   multiplexed content-type, like the Multipart/Related content-type,
   provides a common mechanism for representing a compound object.  A
   Multipart/Related entity consists of a sequence of body parts
   separated by boundary strings.  Each body part represents a component
   of the compound object.  An Application/Vnd.pwg-multiplexed entity
   consists of a sequence of chunks, each of whose length is specified
   in the chunk header.  Each chunk contains a message or a part of a
   message.  Each message represents a component of the compound object.
   Chunks from different messages can be interleaved.  HTTP is the
   typical transport for an Application/Vnd.pwg-multiplexed entity over
   the wire.  An Application/Vnd.pwg-multiplexed entity could be stored
   in a Microsoft HTML (message/rfc822) file whose suffix is .mht.

   The following paragraphs contain three examples of applications.  For
   each application, there is a discussion of its solution with the
   Application/Vnd.pwg-multiplexed content-type, the Multipart/Related
   content-type and BEEP [RFC3080].

   Example 1: a printing application.  A Producer creates a print stream
   that consists of a very long series of page descriptions, each of
   which references one or more images.  The root component is the long
   series of page descriptions.  An image may be referenced from
   multiple pages descriptions, and there is a mechanism to indicate
   when there are no additional references to an image (i.e., the image
   is out of scope).  The Producer does not know what images to include
   with a page until it generates that page.  The Consumer is presumed
   to have enough storage to hold all in-scope images and enough of the
   root component to process at least one page.  The Producer doesn't
   need any knowledge of the Consumer's storage capabilities in order to
   create an entity that the Consumer can successfully process.
   However, the Producer needs to be prudent about the number of images
   that are in-scope at any time.  Of course, a malicious Producer may
   try to exceed the storage capabilities of the Consumer, and the
   Consumer must guard against such entities (see section 6).  Here are
   ways to represent this compound object.





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      With the Application/Vnd.pwg-multiplexed content-type, each image
      is a message and the root component is a message.  The Producer
      breaks the root component message into chunks with each image
      message occurring shortly before its first reference.  When the
      Consumer encounters a reference, it can assume that it has already
      received the referenced image in an earlier chunk.

      With the Multipart/Related content-type, each image must either
      precede or follow the root component.

         If images follow the root component, the Consumer must read all
         remaining pages of the root component before it can print the
         first page that references such images.  The Consumer must wait
         to print such a page until it has received the entire root
         component, and the Consumer may not have the space to hold the
         remaining pages.

         If images precede the root component, the Producer must
         determine and send all such images before it sends the root
         component.  The Consumer must, in the best case, wait some
         additional time before it receives the first page of the root
         component.  In the worse case, the Consumer may not have enough
         storage for all the images.

         The Multipart/Related solution is not a good solution because
         of the wait time and because, in some cases, the Consumer may
         not have sufficient storage for all of the images.

      With BEEP, the images and root component can be sent in separate
      channels.  The Producer can push each image when it encounters the
      first reference or the Consumer can request it when it encounters
      the first reference.  The over-the-wire stream of octets is
      similar to an Application/Vnd.pwg-multiplexed entity.  However,
      there is a substantial difference in behavior for a printing
      application.  With the Application/Vnd.pwg-multiplexed content-
      type, the Producer puts each image message before its first
      reference so that when the Consumer encounters a reference, the
      image is guaranteed to be present on the printer.  With BEEP, if
      the Consumer pulls the image, the Consumer has to wait while the
      image comes over the network.  If the Producer pushes the image,
      BEEP may put the image message after its first reference and the
      Consumer may still have to wait for the image.  A high-speed
      printer should not have to risk waiting for images; otherwise it
      cannot run at full speed.

   Example 2: a scanning (fax-like) application.  The Producer is a
   scanner, which scans pages and sends them along with a vnd.pwg-
   xhtml-print+xml root component that contains references to each page



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   image.  Each page is referenced exactly once in the root-component.
   The Consumer is a printer that consumes vnd.pwg-xhtml-print+xml
   entities and their attachments.  That is, the Consumer is not limited
   to print jobs that come from scanners.  A Producer and Consumer are
   each presumed to have enough storage to hold a few page images and
   most if not all of the root component.  The Producer doesn't need any
   additional knowledge of the Consumer's storage capabilities in order
   to create an entity that the Consumer can successfully process.  Of
   course, a malicious Producer may try to exceed the storage
   capabilities of the Consumer and the Consumer must guard against such
   entities (see section 6).  Here are ways to represent this compound
   object.

      With the Application/Vnd.pwg-multiplexed content-type, each page
      image is a message and the root component is a message.  The
      Producer breaks the root component message into chunks with each
      image message just before or just after its reference.

      With the Multipart/Related content-type, the images cannot precede
      the root component because the Consumer might not have enough
      space to store them until the root component arrived.  In this
      case, the printer could fail to print the job correctly and the
      Producer might not know.  Therefore the images must follow the
      root component, and the Producer must scan all pages before it can
      send the first page.  At the very least, this solution delays the
      printing of the pages until all have been scanned.  In the worst
      case, the Producer does not have sufficient memory to buffer the
      images, and the job fails.

      With BEEP, the issues are the same as in the previous example,
      except that speed is not as important in this case.  So BEEP is a
      viable alternative for this example.

   Example 3: a printing application.  A Producer creates a print stream
   that consists of a series of pages, each of which references zero or
   more images.  Each image is referenced exactly once.  The Producer
   does not know what images to include with a page until it generates
   that page, and the Producer doesn't know the layout details; the
   Consumer handles layout.  The Producer has enough storage to send the
   root component and all images.  However, it may not have enough
   storage to hold the entire root component or all octets of any of the
   images.  The Consumer is presumed to have enough storage to render
   the root component and to render each image.  It may not have enough
   storage to hold the entire root component or all octets of any of the
   images.  The Producer doesn't determine the Consumer's storage
   capabilities.  Rather it arranges the components so that the Consumer
   is mostly likely to succeed.  Of course, a malicious Producer may try




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   to exceed the storage capabilities of the Consumer, and the Consumer
   must guard against such entities (see section 6).  Here are ways to
   represent this compound object.

      With the Application/Vnd.pwg-multiplexed content-type, each image
      is a message and the root component is a message.  The Producer
      breaks the root component message into chunks with each image
      message just after its reference.  The references appear first so
      that the Consumer knows the location of each image before it
      processes the image.  This strategy minimizes storage needs for
      Producer and Consumer and provides a good strategy in case of
      failure.  Here are the cases to consider.

      a) When the document consists of vertically aligned blocks where
         each block contains either lines of text or a single image, the
         sequence of chunks is the same as the sequence of printable
         blocks, thus minimizing Consumer buffering needs.

      b) When a block can contain N side-by-side images, the Consumer
         must buffer N-1 images unless the Producer interleaves the
         images.  If the Producer doesn't interleave the images, and the
         Consumer runs out of storage before it has received N-1,
         images, it can print what it has and print the remaining images
         below; not what the Producer intended, but better than nothing.
         If the Producer interleaves images, and the Consumer runs out
         of storage before it has received the bands of N images, the
         Consumer would print portions of images interleaved with
         portions of other images.  So, a Producer should not interleave
         images.

      c) When a block contains text and image side-by-side (i.e., run-
         around text), there are additional buffering requirements.
         When the Consumer processes the text that follows the
         reference, it will place some of it next to the image (run-
         around text) and will place the remaining text after the image.
         The Producer doesn't know where the run-around ends, and thus
         doesn't know where to end the text chunk and start the image
         chunk.  If the Producer ends the text too soon, then the
         Consumer either has to process the entire image (if it has
         enough storage) in order to get the remaining run-around text,
         or it ends the run-around text prematurely.  If the Producer
         ends the text too late, then the Consumer may have to store too
         much text and possibly put the image later than the Producer
         requested.  Because text data requires significantly less
         storage than image data, a good strategy for Producer is to err
         on the side of sending too much rather than too little text
         before the image data.




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      d) When a block contains text and multiple side-by-side images,
         the problem becomes a combination of items b) and c) above.

      The Application/Vnd.pwg-multiplexed content-type can be made to
      work in this example, but a Consumer must have failure strategies
      and the result may not be quite what the producer intended.  With
      the Multipart/Related content-type, the images cannot precede the
      root component because the Consumer might not have enough space to
      store them until the root component arrived.  Also, the images
      cannot follow the root component because the Consumer might not
      have enough storage for the root component before the first image
      arrives.  So the Multipart/Related content-type is not an
      acceptable solution for this example.

      With BEEP, the Producer can send the root component on channel 1
      and the Consumer can request images on even numbered channels when
      it encounters a reference.  This solution allows more flexibility
      than the Application/Vnd.pwg-multiplexed content-type.  If there
      are side-by-side images and/or run-around text, the Consumer can
      request bands of each image or run-around text over separate
      channels.

   In all of these examples, the Application/Vnd.pwg-multiplexed
   content-type provides a much better solution than Multipart/Related.
   However, it is evenly matched with BEEP.  For applications where
   speed is important and ordering of the chunks is important in order
   to avoid printing delays, the Application/Vnd.pwg-multiplexed
   content-type is best.  For applications, where the Consumer needs
   more control over the ordering of received octets, BEEP is best.

2. Terminology

   This document uses some of the MIME terms that are defined in
   [RFC2045].  The following are the terms used in this document:

      Entity: the headers and the content.  In this document, the term
      "entity" describes all the octets that represent a compound
      object.

      Message: an entity as in [RFC2045].  In this document, the term
      "message" describes all octets that represent one component of a
      compound object.  That is, it has MIME headers and content.

      Body Part: an entity inside a multipart.  That is, a body part is
      the headers and content (i.e., octets) between the multipart
      boundary strings not including the CRLF at the beginning and end.
      This document never uses "entity" to mean "body part".




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      Headers: the initial lines of an entity, message or body part.  An
      empty line (i.e., two adjacent CRLFs) terminates the headers.
      Sometimes the term "MIME header" is used instead of just "header".

      Content: the part of an entity, message or body part that follows
      the headers (i.e., follows the two adjacent CRLFs).  The content
      of a body part ends at the octet preceding the CRLF before the
      multipart boundary string.  The content of a message ends at the
      octets specified by the length field in the Chunk Header.

   This document uses the following additional terms.

      Chunk: a chunk of data, consisting of a chunk header, a chunk
      payload and a CRLF.

      Chunk Header: the first line of a chunk.  The line consists of the
      "CHK" keyword, the message number, the length and the continuation
      indicator, each separated by a single space character (ASCII 32).
      A CRLF terminates the line.  Each message in an
      Application/Vnd.pwg-multiplexed entity has a message number that
      normally differs from the message numbers of all other messages in
      the Application/Vnd.pwg-multiplexed entity.  The message number 0
      is reserved for final Chunk Header in the Application/Vnd.pwg-
      multiplexed entity.

      Chunk Payload: the octets between the Chunk Header and the Chunk
      Header of the next chunk.  The length field in the header's length
      field specifies the number of octets in the Chunk Payload.  The
      Chunk Payload is either a complete message or a part of a message.
      The continuation field in the header specifies whether the chunk
      is the last chunk of the message.

      CRLF: the sequence of octets corresponding to the two US-ASCII
      characters CR (decimal value 13) and LF (decimal value 10) which,
      taken together, in this order, denote a line break.  A CRLF
      terminates each chunk in order to provide visual separation from
      the next chunk header.

      Consumer: the software that receives and processes a MIME entity,
      e.g., software in a printer or software that reads a file.

      Producer: the software that creates and sends a MIME entity, e.g.,
      software in a scanner or software that writes a file.








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3. Details

   The Application/Vnd.pwg-multiplexed content-type, like
   Multipart/Related, is intended to represent a compound object
   consisting of several inter-related components.  This document does
   not specify the representation of these relationships, but [RFC2557]
   contains examples of Multipart/Related entities that use the
   Content-ID and Content-Location headers to identify body parts and
   URLs (including the "cid" URL) to reference body parts.  It is
   expected that Application/Vnd.pwg-multiplexed entities would use the
   patterns described in [RFC2557].

   For an Application/Vnd.pwg-multiplexed entity, there is one parameter
   for the Content-Type header.  It is a "type" parameter, and it is
   like the "type" parameter for the Multipart/Related content-type.
   The value of the "type" parameter must be the content-type of the
   root message and it effectively specifies the type of the compound
   object.

   An Application/Vnd.pwg-multiplexed entity contains a sequence of
   chunks.  Each chunk consists of a chunk header, a chunk payload and a
   CRLF.

     - The chunk header consists of a "CHK" keyword followed by the
       message number, the chunk payload length, whether the chunk is
       the last chunk of a message and, finally, a CRLF.  The length
       field removes the need for boundary strings that Multipart uses.
       (See section 3.1 for the syntax of a chunk header).

     - The chunk payload is a sequence of octets that is either a
       complete message or a part of a message.

     - The CRLF provides visual separation from the following chunk.

   Each message represents a component of the compound object, and a
   message is intended to have exactly the same representation, octet
   for octet, as a body part of a Multipart/Related entity that
   represents the same component.  When a message is split across
   multiple chunks, the chunks need not be contiguous.

   The contents of an Application/Vnd.pwg-multiplexed entity have the
   following properties:

      1) The first chunk contains a complete or partial message that (in
         either case) represents the root component of the compound
         object.





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      2) Additional chunks contain messages or partial messages that
         represent some component of the compound object.

      3) The final chunk's header contains a message number of 0, a
         length of 0 and a last-chunk-of-message mark (i.e., the chunk
         header line is "CHK 0 0 LAST").  The final chunk contains no
         chunk payload.

      4) A message can be broken into multiple parts and each break can
         occur anywhere within the message.  Each part of the message is
         zero or more bytes in length and each part of the message is
         the contents of its own chunk.  The order of the chunks within
         the Application/Vnd.pwg-multiplexed entity must be the same as
         the order of the parts within the message.

      5) A message represents a component of a compound object, and it
         is intended that it have exactly the same representation, octet
         for octet, as a body part of a Multipart/Related entity that
         represents the same component.  In particular, the message may
         contain a Content-Type header to specify the content-type of
         the message content.  Also, the message may contain a Content-
         ID header and/or Content-Location header to identify a message
         that is referenced from within another message.  If a message
         contains no Content-Type header, then the message has an
         implicit content-type of  "text/plain; charset=us-ascii", cf.
         [RFC2045].

   See section 4 for a discussion displaying an Application/Vnd.pwg-
   multiplexed entity.

3.1 Syntax of Application/Vnd.pwg-multiplexed Contents

   The ABNF [RFC2234] for the contents of an Application/Vnd.pwg-
   multiplexed entity is:

   contents = *chunk finalChunk
   chunk      = header payload CRLF
   header     = "CHK" SP messageNumber SP length SP isMore CRLF
   messageNumber   = 1..2147483647
   length   = 0..2147483647
   isMore       = "MORE" / "LAST"
   payload    = *OCTET
   finalChunk = finalHeader CRLF
   finalHeader  = "CHK" SP "0" SP "0" SP "LAST" CRLF







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   The messageNumber field specifies the message that the chunk is
   associated with.  See the end of this section for more details.

   The length field specifies the number of octets in the chunk payload
   (represented in ABNF as "payload").  The first octet of the chunk
   payload is the one immediately following the LF (i.e., final octet)
   of the chunk header.  The last octet of the chunk payload is the one
   immediately preceding the two octets CRLF that end the chunk.

   The isMore field has a value of "LAST" for the last chunk of a
   message and "MORE" for all other chunks of a message.

   Normally each message in an Application/Vnd.pwg-multiplexed entity
   has a unique message number, and a message consists of the
   concatenation of all the octets from the one or more chunks with the
   same message number.  The isMore field of the chunk header of the
   last chunk of each message must have a value of "LAST" and the isMore
   field of the chunk header of all other chunks must have a value of
   "MORE".

   Two or more messages may have the same message number, though such
   reuse of message numbers is not recommended.  The chunks with the
   same message number represent a sequence of one or more messages
   where the isMore field of the chunk header of the last chunk of each
   message has a value of "LAST".  All chunks whose isMore field of the
   chunk header has the value of "MORE" belong to the same message as
   the next chunk (in sequence) whose isMore field of the chunk header
   has the value of "LAST".  In other words, if two messages have the
   same message number, the last chunk of the first message must occur
   before the first chunk of the second message.

   The behavior of the Consumer is undefined if the final Chunk (i.e.,
   the Chunk whose chunk header is "CHK 0 0 LAST") occurs before the
   last chunk of every message occurs.

   Two adjacent chunks usually have different message numbers.  However,
   they may have the same message number.  If two adjacent chunks have
   the same message number, the two chunks could be combined into a
   single chunk, but they need not be combined.

   The number of octets in a chunk payload may be zero, and an
   Application/Vnd.pwg-multiplexed entity may contain any number of
   chunks with zero octets of chunk payload.  For example, the last
   chunk of each message may contain zero octets for programming
   convenience.  As another example, suppose that a particular compound
   object format requires that referenced messages occur before the root
   message.  This document requires that the first chunk of an
   Application/Vnd.pwg-multiplexed entity contain the root message or a



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   part of it.  So, the first chunk contains a chunk payload of zero
   octets with the first octet of the root message in the second chunk.
   That is, all of the message headers of the root message are in the
   second chunk.  As an extreme but unlikely example, it would be
   possible to have a message broken into ten chunks with zero octet
   chunk payloads in all chunks except for chunks 4 and 7.

3.2 Parameters for Application/Vnd.pwg-multiplexed

   This section defines additional parameters for Application/Vnd.pwg-
   multiplexed.

3.2.1 The "type" Parameter

   The type parameter must be specified.  Its value is the content-type
   of the "root" message.  It permits a Consumer to determine the
   content-type without reference to the enclosed message.  If the value
   of the type parameter differs from the content-type of the root
   message, the Consumer's behavior is undefined.

3.2.2 Syntax

   The syntax for "parameter" is:

     parameter   := "type"  "=" type "/" subtype ; cf. [RFC2045]

4. Handling Application/Vnd.pwg-multiplexed Entities

   The application that handles the Application/Vnd.pwg-multiplexed
   entity has the responsibility for displaying the entity.  However,
   Application/Vnd.pwg-multiplexed messages may contain Content-
   Disposition headers that provide suggestions for the display and
   storage of a message, and in some cases the application may pay
   attention to such headers.

   As a reminder, Content-Disposition headers [RFC1806] allow the sender
   to suggest presentation styles for MIME messages.  There are two
   presentation styles, 'inline' and 'attachment'.  Content-Disposition
   headers have a parameter for specifying a suggested file name for
   storage.

   There are three cases to consider for handling Application/Vnd.pwg-
   multiplexed entities:

      a) The Consumer recognizes Application/Vnd.pwg-multiplexed and the
         content-type of the root.  The Consumer determines the
         presentation style for the compound object; it handles the
         display of the components of the compound object in the context



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         of the compound object.  In this case, the Content-Disposition
         header information is redundant or even misleading, and the
         Consumer shall ignore them for purposes of display.  The
         Consumer may use the suggested file name if the entity is
         stored.

      b) The Consumer recognizes Application/Vnd.pwg-multiplexed, but
         not the content-type of the root.  The Consumer will give the
         user the choice of suppressing the entire Application/Vnd.pwg-
         multiplexed entity or treating the Application/Vnd.pwg-
         multiplexed entity as a Multipart/Mixed entity where each
         message is a body part of the Multipart/Mixed entity.  In this
         case (where the entity is not suppressed), the Consumer may
         find the Content-Disposition information useful for displaying
         each body part of the resulting Multipart/Mixed entity.  If a
         body part has no Content-Disposition header, the Consumer
         should display the body part as an attachment.

      c) The Consumer does not recognize Application/Vnd.pwg-
         multiplexed.  The Consumer treats the Application/Vnd.pwg-
         multiplexed entity as opaque and can do nothing with it.

5. Examples

   This section contains five examples.  Each example is a different
   representation of the same compound object.  The compound object has
   four components: an XHTML text component and three image components.
   The images are encoded in binary.  The string "<<binary data>>" and
   "<<part of binary data>>" in each example represents all or part of
   the binary data of each image.  Two of the images are potentially
   side by side and the third image is displayed later in the document.
   All of the images are identified by Content-Id and two of the images
   are also identified by a Content-Location.  One of the images
   references the Content-Location.

   The first example shows a Multipart/Related representation of the
   compound object in order to provide a representation that the reader
   is familiar with.  The remaining examples show Application/Vnd.pwg-
   multiplexed representations of the same compound object.  In the
   second example, each chunk contains a whole message.  In the third
   example, the XHTML message is split across 3 chunks, and these chunks
   are interleaved among the three image messages.  In the fourth
   example, the XHTML message is split across 4 chunks, and the two
   side-by-side images are each split across two chunks.  The XHTML
   chunks are interleaved among the image chunks.  In the fifth example,
   there are chunks with empty payloads and adjacent chunks with the
   same message number.




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   The last example may seem to address useless cases, but sometimes it
   is easier to write software if these cases are allowed.  For example,
   when a buffer fills, it may be easiest to write a chunk and not worry
   if the previous chunk had the same message number.  Likewise, it may
   be easiest to end a message with an empty chunk.  Finally, the
   Application/Vnd.pwg-multiplexed content-type requires that the first
   chunk be part of the root message.  Sometimes, it is more convenient
   for the Producer if the root message starts after the occurrence of
   some attachments.  Since a chunk can be empty, the first chunk of the
   root message can be empty, i.e., it doesn't even contain any headers.
   Then the first chunk contains a part of the root message, but the
   Producer doesn't generate any octets for that chunk.

   Each body part of the Multipart/Related entity and each message of
   the Application/Vnd.pwg-multiplexed entity contain a content-
   disposition, which the Consumer uses according to the rules in
   section 4.  Note the location of the content-disposition headers in
   the examples.

5.1 Example With Multipart/Related

   In this example, the compound object is represented as a
   Multipart/Related entity so that the reader can compare it with the
   Application/Vnd.pwg-multiplexed entities.

   Content-Type: multipart/related; boundary="boundary-example";
                 type="text/xhtml+xml"

   --boundary-example
   Content-ID: <49568.44343xxx@foo.com>
   Content-Type: application/vnd.pwg-xhtml-print+xml
   Content-Disposition: inline

   <?xml version="1.0"?>
   <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
       "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
   <html xmlns="http://www.w3.org/TR/xhtml1">
      <body>
         <p>some text
            <img src="cid:49568.45876xxx@foo.com"/>
            <img src="http://foo.com/images/image2.gif"/>
            some more text after the images
         </p>
         <p>some more text without images
         </p>
         <p>some more text
            <img src="cid:49568.47333xxx@foo.com"/>
         </p>



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         <p>some final text
         </p>
      </body>
   </html>
   --boundary-example
   Content-ID: <49568.45876xxx@foo.com>
   Content-Location: http://foo.com/images/image1.gif
   Content-Type: image/gif
   Content-Disposition: attachment

   <<binary data>>
   --boundary-example
   Content-ID: <49568.46000xxx@foo.com>
   Content-Location: http://foo.com/images/image2.gif
   Content-Type: image/gif
   Content-Disposition: attachment

   <<binary data>>
   --boundary-example
   Content-ID: <49568.47333xxx@foo.com>
   Content-Type: image/gif
   Content-Disposition: attachment

   <<binary data>>
   --boundary-example--

5.2 Examples with Application/Vnd.pwg-multiplexed

   The four examples in this section show Application/Vnd.pwg-
   multiplexed representations of the same compound object.  Note that
   each CRLF is represented by a visual line break.

5.2.1 Example Where Each Chunk Has a Complete Message

   In this example, the compound object is represented as an
   Application/Vnd.pwg-multiplexed entity.  Each chunk contains an
   entire message, i.e., none of the messages are split across multiple
   chunks.  Each message in this example is identical to the
   corresponding body part in the preceding Multipart/Relate example.

   Content-Type: application/vnd.pwg-multiplexed;
                 type="application/vnd.pwg-xhtml-print+xml"

   CHK 1 550 LAST
   Content-ID: <49568.44343xxx@foo.com>
   Content-Type: application/vnd.pwg-xhtml-print+xml
   Content-Disposition: inline




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   <?xml version="1.0"?>
   <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
       "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
   <html xmlns="http://www.w3.org/TR/xhtml1">
      <body>
         <p>some text
            <img src="cid:49568.45876xxx@foo.com"/>
            <img src="http://foo.com/images/image2.gif"/>
            some more text after the images
         </p>
         <p>some more text without images
         </p>
         <p>some more text
            <img src="cid:49568.47333xxx@foo.com"/>
         </p>
         <p>some final text
         </p>
      </body>
   </html>

   CHK 2 6346 LAST
   Content-ID: <49568.45876xxx@foo.com>
   Content-Location: http://foo.com/images/image1.gif
   Content-Type: image/gif
   Content-Disposition: attachment

   <<binary data>>
   CHK 3 6401 LAST
   Content-ID: <49568.46000xxx@foo.com>
   Content-Location: http://foo.com/images/image2.gif
   Content-Type: image/gif
   Content-Disposition: attachment

   <<binary data>>
   CHK 4 7603 LAST
   Content-ID: <49568.47333xxx@foo.com>
   Content-Type: image/gif
   Content-Disposition: attachment

   <<binary data>>
   CHK 0 0 LAST










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5.2.2 Example of Chunking the Root Message

   In this example, the compound object is represented as an
   Application/Vnd.pwg-multiplexed entity.  The message containing the
   XHTML component is split into 3 pieces so that the reference to an
   image is as close as possible to the beginning of the chunk.  The
   chunk containing the referenced image message occurs just before the
   chunk with the reference.  This minimizes the distance between
   reference and referenced message.

   Note that there are other possible arrangements (see the third
   example in section 5.2.3).  For example, a sender could split the
   XHTML message so that the reference to an image is as close as
   possible to the end of the chunk.  Then the chunk containing the
   referenced image message should occur just after the chunk with the
   reference.  The sender could mix this strategy with the one used in
   this example.

   Content-Type: application/vnd.pwg-multiplexed;
                 type=" application/vnd.pwg-xhtml-print+xml"

   CHK 1 267 MORE
   Content-ID: <49568.44343xxx@foo.com>
   Content-Type: application/vnd.pwg-xhtml-print+xml
   Content-Disposition: inline

   <?xml version="1.0"?>
   <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
       "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
   <html xmlns="http://www.w3.org/TR/xhtml1">
      <body>
         <p>some text

   CHK 2 6346 LAST
   Content-ID: <49568.45876xxx@foo.com>
   Content-Location: http://foo.com/images/image1.gif
   Content-Type: image/gif
   Content-Disposition: attachment

   <<binary data>>
   CHK 3 6401 LAST
   Content-ID: <49568.46000xxx@foo.com>
   Content-Location: http://foo.com/images/image2.gif
   Content-Type: image/gif
   Content-Disposition: attachment






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   <<binary data>>
   CHK 1 166 MORE
            <img src="cid:49568.45876xxx@foo.com"/>
            <img src="http://foo.com/images/image2.gif"/>
            some more text after the images
         </p>
         <p>some more text without images
         </p>
         <p>some more text

   CHK 4 7603 LAST
   Content-ID: <49568.47333xxx@foo.com>
   Content-Type: image/gif
   Content-Disposition: attachment

   <<binary data>>
   CHK 1 80 LAST
            <img src="cid:49568.47333xxx@foo.com"/>
         </p>
         <p>some final text
         </p>
      </body>
   </html>

   CHK 0 0 LAST

5.2.3 Example of Chunking the Several Messages

   In this example, the compound object is represented as an
   Application/Vnd.pwg-multiplexed entity.  The message containing the
   XHTML component is split into 4 pieces so that the reference to an
   image is as close as possible to either the beginning or the end of
   the chunk.  The references to the first and second images closely
   follow the referenced images.  The reference to the third image
   closely precedes the referenced image.  This minimizes the distance
   between reference and referenced message.  In addition, the first two
   image messages are split into two chunks each.

   Content-Type: application/vnd.pwg-multiplexed;
                 type=" application/vnd.pwg-xhtml-print+xml"

   CHK 1 303 MORE
   Content-ID: <49568.44343xxx@foo.com>
   Content-Type: application/vnd.pwg-xhtml-print+xml
   Content-Disposition: inline






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   <?xml version="1.0"?>
   <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
       "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
   <html xmlns="http://www.w3.org/TR/xhtml1">
      <body>
         <p>some text

   CHK 2 184 MORE
   Content-ID: <49568.45876xxx@foo.com>
   Content-Location: http://foo.com/images/image1.gif
   Content-Type: image/gif
   Content-Disposition: attachment

   <<part of binary data>>
   CHK 3 200 MORE
   Content-ID: <49568.46000xxx@foo.com>
   Content-Location: http://foo.com/images/image2.gif
   Content-Type: image/gif
   Content-Disposition: attachment

   <<part of binary data>>
   CHK 1 78 MORE
            <img src="cid:49568.45876xxx@foo.com"/>
            <img src="http://foo.com/images/image2.gif"/>

   CHK 2 6162 LAST
   <<part of binary data>>
   CHK 3 6201 LAST
   <<part of binary data>>
   CHK 1 127 MORE
            some more text after the images
         </p>
         <p>some more text without images
         </p>
         <p>some more text
            <img src="cid:49568.47333xxx@foo.com"/>

   CHK 4 7603 LAST
   Content-ID: <49568.47333xxx@foo.com>
   Content-Type: image/gif
   Content-Disposition: attachment










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   <<binary data>>
   CHK 1 41 LAST
         </p>
         <p>some final text
         </p>
      </body>
   </html>

   CHK 0 0 LAST

5.2.4 Example of Chunks with Empty Payloads

   This example is identical to the previous one, except that some
   chunks have a chunk payload of zero octets.  The root message starts
   with a chunk whose payload is empty and every message ends with a
   chunk whose payload is empty.  This example also shows two adjacent
   chunks that are from the same message.  These two chunks could be
   coalesced into a single chunk, but they might be kept separate for
   programming convenience.

   Content-Type: application/vnd.pwg-multiplexed;
                 type=" application/vnd.pwg-xhtml-print+xml"

   CHK 1 0 MORE

   CHK 2 184 MORE
   Content-ID: <49568.45876xxx@foo.com>
   Content-Location: http://foo.com/images/image1.gif
   Content-Type: image/gif
   Content-Disposition: attachment

   <<part of binary data>>
   CHK 3 200 MORE
   Content-ID: <49568.46000xxx@foo.com>
   Content-Location: http://foo.com/images/image2.gif
   Content-Type: image/gif
   Content-Disposition: attachment

   <<part of binary data>>
   CHK 1 303 MORE
   Content-ID: <49568.44343xxx@foo.com>
   Content-Type: application/vnd.pwg-xhtml-print+xml
   Content-Disposition: inline








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   <?xml version="1.0"?>
   <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
       "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
   <html xmlns="http://www.w3.org/TR/xhtml1">
      <body>
         <p>some text

   CHK 2 6162 MORE
   <<part of binary data>>
   CHK 3 6201 MORE
   <<part of binary data>>
   CHK 2 0 LAST

   CHK 3 0 LAST

   CHK 1 78 MORE
            <img src="cid:49568.45876xxx@foo.com"/>
            <img src="http://foo.com/images/image2.gif"/>

   CHK 4 7603 MORE
   Content-ID: <49568.47333xxx@foo.com>
   Content-Type: image/gif
   Content-Disposition: attachment

   <<binary data>>
   CHK 4 0 LAST

   CHK 1 127 MORE
            some more text after the images
         </p>
         <p>some more text without images
         </p>
         <p>some more text
            <img src="cid:49568.47333xxx@foo.com"/>

   CHK 1 41 MORE
         </p>
         <p>some final text
         </p>
      </body>
   </html>

   CHK 1 0 LAST

   CHK 0 0 LAST






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6. Security Considerations

   There are security considerations that pertain to each message of an
   Application/Vnd.pwg-multiplexed entity.  Those security
   considerations are described by the document that defines the
   content-type of the message.  They are not addressed in this
   document.

   There are also security considerations that pertain to the
   Application/Vnd.pwg-multiplexed entity as a whole.  A Producer that
   is buggy or malicious may send an Application/Vnd.pwg-multiplexed
   entity that could cause a Consumer to request more storage than it
   has, even if it has a large amount of storage.  A Consumer must be
   able to deal gracefully with the following scenarios and combinations
   of them:

     - The chunks of one or more messages are separated by a very large
       number of octets.  In the extreme case some or all of the
       messages don't terminate, i.e., they don't contain a closing
       chunk.
     - A very large number of messages are started and interleaved
       before their final chunk occurs.
     - A message contains one or more references to other messages that
       never occur or don't occur for a large number of octets.
     - A very large number of referenced messages occur before the
       Consumer knows that it can discard them.

7. Registration Information for Application/Vnd.pwg-multiplexed

   The following form is copied from RFC 1590, Appendix A.

     To: iana@iana.org

     Subject:           Registration of new Media Type
                        application/Vnd.pwg-multiplexed
     Media Type name:   Application
     Media subtype name:     Vendor Tree - vnd.pwg-multiplexed
     Required parameters:    Type, a media type/subtype.
     Optional parameters:    No optional parameters
     Encoding considerations:    Each message of an
                         Application/Vnd.pwg-multiplexed entity can be
                         encoded in any manner allowed by the Content-
                         Type of the message.  However, using the
                         reasoning of Multipart, the
                         Application/Vnd.pwg-multiplexed entity cannot
                         be encoded.  Otherwise, a message would be





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                         encoded twice, once at the message level and
                         once at the Application/Vnd.pwg-multiplexed
                         level.
     Security considerations:    See section 6 (Security
                                 Considerations) of RFC 3391.
     Published specification:    RFC 3391.
     Person & email address to contact for further information:

         Robert Herriot
         706 Colorado Ave.
         Palo Alto, CA 94303
         USA
         Phone: 1-650-327-4466
         Fax: 1-650-327-4466
         EMail: bob@herriot.com

8. Acknowledgments

   The author gratefully acknowledges the contributions of: Ugo Corda,
   Dave Crocker, Melinda Sue Grant, Graham Klyne, Carl-Uno Manros, Larry
   Masinter, Ira McDonald, Chris Newman, Henrik Frystyk Nielsen and Dale
   R. Worley.  In particular, Chris Newman provided invaluable help.

9. References

   [RFC1806] Troost, R. and S. Dorner, "Communicating Presentation
             Information in Internet Messages: The Content-Disposition
             Header", RFC 1806, June 1995.

   [RFC1873] Levinson, E. and J. Clark, "Message/External-Body Content-
             ID Access Type",  RFC 1873, December 1995.
             Levinson, E., "Message/External-Body Content-ID Access
             Type", Work in Progress.

   [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
             Extensions (MIME) Part One: Format of Internet Message
             Bodies", RFC 2045, November 1996.

   [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
             Extensions (MIME) Part Two: Media Types", RFC 2046,
             November 1996.

   [RFC2234] Crocker, D. and P. Overell, "Augmented BNF for
             SyntaxSpecifications: ABNF", RFC 2234, November 1997.

   [RFC2387] Levinson, E., "The MIME Multipart/Related Content-type",
             RFC 2387, August 1998.




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   [RFC2392] Levinson, E., "Content-ID and Message-ID Uniform Resource
             Locators", RFC 2392, August 1998.

   [RFC2557] Palme, J., "MIME Encapsulation of Aggregate Documents, such
             as HTML (MHTML", RFC 2557, March 1999.

   [RFC2822] Resnick, P., Editor, "Internet Message Format", RFC 2822,
             April 2001.

   [RFC3080] Rose, M., "The Blocks Extensible Exchange Protocol Core",
             RFC 3080, March 2001.

10. Author's Address

   Robert Herriot
   706 Colorado Ave.
   Palo Alto, CA 94303
   USA

   Phone: 1-650-327-4466
   Fax: 1-650-327-4466
   EMail: bob@herriot.com





























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11. Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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