Internet DRAFT - draft-crispin-lemonade-pull
draft-crispin-lemonade-pull
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
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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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