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Network Working Group                                         M. Crispin
INTERNET DRAFT: Message Submission              University of Washington
Document: draft-crispin-lemonade-pull-01.txt               February 2004


                           Message Submission

Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC 2026.

    Internet-Drafts are working documents of the Internet Engineering
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Abstract

   This document describes a set of strategies to allow clients to
   submit new email messages incorporating content which resides on
   locations external to the client.  This is the so-called "IMAP
   Pull" approach currently being considered by the LEMONADE working
   group as one solution to the "forward without download" problem
   (that is, as a means for clients to send new messages consisting of
   or containing all or parts of previously received messages without
   having to download and upload the data).  Some of the strategies
   in this document rely upon extensions to other protocols;
   specifically URLAUTH and CATENATE in the IMAP protocol (RFC 3501)
   and BURL in the SUBMIT protocol (RFC 2476).

   This document describes the strategies in multiple variants, as well
   as the implications on authorization and the "fcc problem".


Conventions Used in this Document

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
   in this document are to be interpreted as described in [KEYWORDS].

   The formal syntax use the Augmented Backus-Naur Form (ABNF)
   notation including the core rules defined in Appendix A of [ABNF].

   In examples, "C:" and "S:" indicate lines sent by the client and
   server respectively.  If a single "C:" or "S:" label applies to
   multiple lines, then the line breaks between those lines are for
   editorial clarity only and are not part of the actual protocol
   exchange.


Introduction

   The advent of client/server email using the [IMAP] and [SMTP]
   protocols has changed what formerly were local disk operations to
   become excessive and repetitive network data transmissions.  As
   noted in [LEMONADE], this is an onerous burden for clients
   operating over low-bandwidth or high-latency links.

   Two major approach were identified in [LEMONADE] to address this
   problem: "pull" and "push".  The strategy used to implement the push
   approach is the subject of another document ([PUSH]).  This document
   discusses the pull approach, and describes three strategies in which
   the pull approach can be implemented.

   All three of the pull strategies discussed below make use of the [BURL]
   SUBMIT extension to enable access to external sources during the
   submission of a message.  In combination with the IMAP [URLAUTH]
   extension, inclusion of message parts or even entire messages from the
   IMAP mail store is possible with a minimal trust relationship between
   the IMAP and SUBMIT servers.

   Pull has the distinct advantage of maintaining one submission protocol,
   and thus avoids the risk of having multiple parallel and possible
   divergent mechanisms for submission.  Furthermore, by keeping the
   details of message submission in the SUBMIT server, the pull
   strategies are prepared to work with other message retrieval protocols
   such as POP, NNTP, or whatever else may be designed in the future.


Message Sending Overview

   The act of sending an email message is best thought of as involving
   multiple steps: initiation of a new draft, draft editing, message
   assembly, and message submission.

   Initiation of a new draft and draft editing take place on the MUA.
   Frequently, users choose to save more complex and/or time-consuming
   messages on an [IMAP] server (via the APPEND command with the \Draft
   flag) for later recall by the MUA and resumption of the editing process.

   Message assembly is the process of producing a complete message from
   the final revision of the draft and external sources.  At assembly
   time, external data is retrieved and inserted in the message.

   Message submission is the process of inserting the assembled message
   into the [SMTP] infrastructure, typically using the [SUBMIT] protocol.

   It should be noted that another way of handling external data is to
   send a pointer to the data, rather than the data itself, to the
   recipient.  To be useful, this pointer must be something that the
   recipient can resolve and has authorization to reference.  Common
   examples of this are the MESSAGE/EXTERNAL-BODY subtype defined in
   [MEDIA] and the practice of including public-access URLs in message
   texts.


Traditional Strategy

   Traditionally, messages are initiated, edited, and assembled entirely
   within an MUA, although drafts may be saved to an [IMAP] server and
   later retrieved from the server.  The completed text is then
   transmitted to an MSA for delivery.

   There is often no clear boundary between the editing and assembly
   process.  If a message is forwarded, its content is often retrieved
   immediately and inserted into the message text.  Similarly, once
   external content is inserted or attached, the content is usually
   retrieved immediately and made part of the draft.

   As a consequence, each save of a draft and subsequent retrieve of
   the draft transmits that entire (possibly large) content, as does
   message submission.

   In the past, this was not much of a problem, because drafts,
   external data, and the message submission mechanism were typically
   located on the same system as the MUA.  The most common problem was
   running out of disk quota.


[PUSH] Strategy

   The [PUSH] strategy is the topic of another document, but bears brief
   mention for comparison purposes.  In the [PUSH] strategy, messages are
   initiated and edited within an MUA.  The proposed [CATENATE] extension
   to [IMAP] is then used to upload the message to the IMAP server and
   assemble it, and finally the proposed SUBMIT extension in [PUSH] is
   used to pass the message to an MSA.

   [PUSH] explicitly limits external content to other messages in the
   currently selected mailbox on the IMAP server, although there is some
   discussion about allowing other content in [CATENATE] via URLs.  The
   result of such an extension could be thought of as a hybrid pull-push
   model, since the IMAP server is used both to pull external references
   and push the resulting message to the MSA.

   From the point of view of the "fcc problem" (see below) an advantage
   (and disadvantage) of this strategy is that a fully-assembled copy of
   the message is left on the IMAP server.


Pull Strategy -- [BURL] Variant

   In the [BURL] variant of the pull strategy, messages are initiated
   and edited within an MUA.  The MUA then uploads the message
   to a [SUBMIT] server with the [BURL] extension.  Among the types of
   URIs that can be passed to the [BURL] command are [URLAUTH] format
   URLs, this satisfying the [LEMONADE] forward-without-download
   requirement.

   Note that the assembly and submission operations are done in a single
   step, as opposed to the two-step process in the [PUSH] strategy; and
   that external content is not limited to other messages in the currently
   selected mailbox on the IMAP server.

   The main disadvantage of this variant is that it does not address the
   "fcc problem" (see below) intrinsically, although it is suitable with
   a generalized solution to the fcc problem.


Pull Strategy -- [BURL]/[CATENATE] Variant

   In the [BURL]/[CATENATE] variant of the pull strategy, messages are
   initiated and edited within an MUA.  The proposed [CATENATE] extension
   to [IMAP] is then used to upload the message to the IMAP server and
   assemble it, and finally a [URLAUTH] format URL is given to a [SUBMIT]
   server with the [BURL] extension for submission.

   If this strategy is chosen over the pure [BURL] variant, it is possible
   to simplify the [BURL] specification so that a single BURL command
   defines the message data without any BDAT.

   This is another form of a hybrid pull-push model.  As with the [PUSH]
   strategy, from the point of view of the "fcc problem" (see below) an
   advantage (and disadvantage) of this strategy is that a fully-assembled
   copy of the message is left on the IMAP server.

   As noted under the [PUSH] strategy, to be fully useful the proposed
   [CATENATE] extension has to allow arbitrary URLs.


Pull Strategy -- [BURL]/Embedded URI Variant

   In the embedded URI variant of the pull strategy, messages are initiated
   and edited within an MUA, and saved on the [IMAP] server via an APPEND
   command.  External data is indicated by URIs which are embedded in the
   message in some easily-identifiable form, perhaps as an added parameter
   to the [MIME] Content-Type header.  A [URLAUTH] format URL is given to
   a [SUBMIT] server with the [BURL] extension for assembly and submission.

   If this strategy is chosen over the pure [BURL] variant, it is possible
   to simplify the [BURL] specification so that a single BURL command
   defines the message data without any BDAT.

   The advantage of this strategy is that the client does not deal with
   message assembly at all; the entire burden is on the [SUBMIT] server.

   This is the only strategy which is specifically draft-friendly, since
   drafts with pointers to the external data are suitable for submission
   without requiring the MUA to break up the draft between texts and
   pointers for message assembly.  The client can simply download the draft
   from the IMAP server and re-edit it, without the burden of downloading
   the external data (since it hasn't yet been incorporated into the
   message) or fetching the message structure, parsing it, and then fetching
   the editable parts.

   From the point of view of the "fcc problem" (see below) an advantage
   (and disadvantage) of this strategy is that a copy of the message
   with references to external content (but not the external content itself)
   is left on the IMAP server.


Authorization Strategies

   In any of these strategies, authorization is necessary.  It should be
   noted that in all strategies (including [PUSH]), the [SUBMIT] server
   is trusted with the data.

   In the traditional strategy, authorization comes from authentication;
   the client authenticates to the [IMAP] server as a userid that is
   authorized to access the data, fetches the data from the [IMAP] server,
   authenticates to the [SUBMIT] server as a userid that is authorized to
   submit a message, and then sends the data to the [SUBMIT] server.

   The [PUSH] strategy multiplexes authorization from a single credential
   derived from authentication to the [IMAP] server; that credential is
   assumed to be sufficient to access all data and to submit the data.
   There is no provision in [PUSH] for differential authorization; for
   example, it is not possible in [PUSH] to have authentication to the
   [IMAP] server using a general "role" account (e.g., "helpdesk") for
   access authorization, but submit as a specific individual.

   The Pull strategies allow a wide range of authorization mechanisms to
   obtain data from the pulled URI:

   (1) The simplest mechanism is a public-access URI.  External data is
   commonly handled today this way, e.g., the MESSAGE/EXTERNAL-BODY subtype
   defined in [MEDIA] and the practice of including public-access URLs in
   message texts.

   (2) Another simple mechanism is one in which the [IMAP] server trusts the
   [SUBMIT] server.  The [SUBMIT] server is authenticated by the [IMAP]
   server by one of several means (e.g., IP address, Kerberos, SSL/TLS
   certificate validation, or special account), and is subsequently
   authorized by the [IMAP] server to access any data on the server.
   This is the trust model that the [PUSH] model uses, only in reverse.

   (3) A variant of the trust mechanism is one in which the [SUBMIT]
   server uses a [SASL] authentication identity on the [IMAP] server that
   is permitted to use another identity as the authorization identity.  The
   authorization identity passed to the [IMAP] server is the authentication
   identity used by the client to access the [SUBMIT] server.  The advantage
   of this variant is that the [SUBMIT] server is only responsible for
   asserting the client's identity but not the client's authorization.  Many
   existing webmail systems work this way today.

   (4) The so-called "pawn-ticket" authorization mechanism uses a URI which
   contains its own authorization credentials using a scheme such as
   [URLAUTH].  The advantage of this mechanism is that the [SUBMIT] server
   can not access any data on the [IMAP] server without a "pawn-ticket"
   created by the client.  The "pawn-ticket" grants acces only to the specific
   data that the [SUBMIT] server is authorized to access, can be revoked by
   the client, and can have a time-limited validity.

   (5) A variant of the "pawn-ticket" authorization mechanism adds a
   requirement that a trust relationship, as described in mechanism (2)
   above, be established in order for the pawn-ticket to be accepted.  The
   advantage of this variant is that even if an attacker learns the
   "pawn-ticket", it is useless to him because the attacker does not have
   the trust relationship.  The AUTHID and AUTHROLE extensions in [URLAUTH]
   provide a means for doing this.

   Of the above mechanisms, (5) is recommended, since it requires that an
   attacker must both discover a "pawn-ticket" and use the trust relationship
   of the [SUBMIT] server.

   The "pawn-ticket" can be further secured by using the optional userid
   argument in AUTHROLE=compose in a [URLAUTH] format URL.   For example, if
   example.com's [SUBMIT] server is given a URL of
      imap://joe@example.com/INBOX;uid=20;section=1.2;authrole=submit:smith
   then the [SUBMIT] server MUST require that smith is the userid
   authenticated to the [SUBMIT] server.  This allows the [SUBMIT] and the
   [IMAP] servers to be in different authentication realms; in the above
   example, the user is "joe" on the [IMAP] server and "smith" on the
   [SUBMIT] server.

   Note that, even if the user doesn't authenticate to the [SUBMIT] server,
   there is no added risk over the push strategy.  The "pawn-ticket" ensures
   authorized access to the desired data, and only that data.


The fcc Problem

   The "fcc problem" refers to a frequent need to deliver a copy of the
   message to a "file carbon copy" receipient.  By far, the most common
   case of fcc is a client leaving a copy of outgoing mail in a "sent
   messages" or "outbox" mailbox.

   In the traditional strategy, the MUA duplicates the effort spent in
   transmitting to the MSA by writing the message to the fcc destination
   in a separate step.  This may be a write to a local disk file or an
   APPEND to a mailbox on an IMAP server.  The latter is one of the
   "excessive and repretitive network data transmissions" which [LEMONADE]
   seeks to reduce or eliminate; and represents the "problem" aspect of
   the "fcc problem".

   The proposed [CATENATE] extension to [IMAP] is a potentional solution
   to the fcc problem.  It requires making several simplifying assumptions:
    (1a) there is one, and only one, fcc destination on a single server
    (2a) the server which holds the fcc is the same as the server which
          stages the outgoing message for submission
    (3a) it is desired that the fcc be a copy of the complete message text
          with all external data inserted in the message
   As noted above, [CATENATE] is used by the [PUSH] strategy and by the
   [BURL]/[CATENATE] variant of the pull strategy.

   The Embedded URI variant of the pull strategy represents an alternative
   potentional solution to the fcc problem.  It requires making several
   simplifying assumptions:
    (1b) there is one, and only one, fcc destination on a single server
    (2b) the server which holds the fcc is the same as the server which
          stages the outgoing message for submission
    (3b) it is desired that the fcc be a copy of the message with all
          external data pointed to by URI references.

   It can be noted that the first two simplifying assumptions are the same
   in both potential solutions.  The difference is in the third assumption,
   which are complete opposites.  Thus, the preferred solution to the fcc
   problems depends upon one's preferences in the third assumption.

   Neither of these two potential solutions are satisfactory if the first
   two simplifying assumptions can not be made, or if it is a requirement
   that the third assumption be a matter of user preference.  The developers
   of at least one MUA (Pine) found that it is very important for users to
   control whether the sent messages mailbox contains external data.

   A more general solution to the fcc problem is possible with the [BURL]
   and Embedded URI variants of the pull strategy.  The agent which does
   the message assembly is given the fcc (or fccs) as part of the list of
   message recipients, along with authorization information and whether the
   fcc should include external data or pointers.  This simplifies to "no
   action needed" in the Embedded URI variant if the user wants pointers
   in the fcc.


Security Considerations

   Security considerations are discussed throughout this memo.


Acknowledgements

   Thanks to Randall Gellens and Rob Siemborski for numerous helpful
   suggestions.

References

   The following references are normative:

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

   [BURL]     Newman, C., "Message Composition",
              draft-newman-lemonade-compose-00.txt (work in progress),
              June 2003.

   [CATENATE] Resnick, P., "Internet Message Access Protocol (IMAP)
              CATENATE Extension", draft-ietf-lemonade-catenate-01
              (work in progress), January 2004.

   [IMAP]     Crispin, M., "Internet Message Access Protocol - Version
              4rev1", RFC 3501, March 2003.

   [IMAPURL]  Newman, C., "IMAP URL Scheme", RFC 2192, September 1997.

   [LEMONADE] Gellens, R., "IMAP Submit Without Download",
              draft-ietf-lemonade-submit-01.txt (work in progress),
              October 2003.

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

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

   [SASL]     Myers, J., "Simple Authentication and Security Layer (SASL)",
              RFC 2222, October 1997.

   [SUBMIT]   Gellens, R. and Klensin, J., "Message Submission", RFC 2476,
              December 1998.

   The following references are informative:

   [PUSH]     Gellens, R., "IMAP Message Submission",
              draft-gellens-lemonade-push-00.txt (work in progress),
              December 2003.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.


Author's Address

   Mark R. Crispin
   Networks and Distributed Computing
   University of Washington
   4545 15th Avenue NE
   Seattle, WA  98105-4527

   Phone: (206) 543-5762
   EMail: MRC@CAC.Washington.EDU


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