EXTRA M. Slusarz
Internet-Draft Open-Xchange Inc.
Intended status: Standards Track December 10, 2019
Expires: June 12, 2020
IMAP4 Extension: Message Preview Generation
draft-ietf-extra-imap-fetch-preview-07
Abstract
This document specifies an Internet Message Access Protocol (IMAP)
protocol extension allowing a client to request a server-generated
abbreviated representation of message data useful as a contextual
preview of the entire message.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on June 12, 2020.
Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used In This Document . . . . . . . . . . . . . . 3
3. FETCH Data Item . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Command . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Response . . . . . . . . . . . . . . . . . . . . . . . . 4
4. PREVIEW Algorithms . . . . . . . . . . . . . . . . . . . . . 5
4.1. Description . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. text/imap-fetch-preview . . . . . . . . . . . . . . . . . 6
5. PREVIEW Priority Modifiers . . . . . . . . . . . . . . . . . 7
5.1. LAZY . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Change History (To be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 12
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
Many modern mail clients display small extracts of the body text as
an aid to allow a user to quickly decide whether they are interested
in viewing the full message contents. Mail clients implementing the
Internet Message Access Protocol [RFC3501] would benefit from a
standardized, consistent way to generate these brief previews of
messages.
Generation of a preview on the server has several benefits. First,
it allows consistent representation of previews across all clients.
This standardized display can reduce user confusion when using
multiple clients, as abbreviated message representations in clients
will show identical message contents.
Second, server-side preview generation is more efficient. A client-
based algorithm needs to issue, at a minimum, a FETCH BODYSTRUCTURE
command in order to determine which MIME [RFC2045] body part(s)
should be represented in the preview. Subsequently, at least one
FETCH BODY command may be needed to retrieve body data used in
preview generation. These FETCH commands cannot be pipelined since
the BODYSTRUCTURE query must be parsed on the client before the list
of parts to be retrieved via the BODY command(s) can be determined.
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Additionally, it may be difficult to predict the amount of body data
that must be retrieved to adequately represent the part via a
preview, therefore requiring inefficient fetching of excessive data
in order to account for this uncertainty. For example, a preview
algorithm to display data contained in a text/html [RFC2854] part
will likely strip the markup tags to obtain textual content.
However, without fetching the entire content of the part, there is no
way to guarantee that sufficient non-tag content will exist unless
either 1) the entire part is retrieved or 2) an additional partial
FETCH is executed when the client determines that it does not possess
sufficient data from a previous partial FETCH to display an adequate
representation of the preview.
Finally, server generation allows caching in a centralized location.
Using server-generated previews allows global generation once per
message, and then cached for the retention period of the source
message. Retrieval of message data may be expensive within a server,
for example, so a server can be configured to reduce its storage
retrieval load by pre-generating preview data.
A server indicates support for this extension via the "PREVIEW"
capability name.
2. Conventions Used In This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
"User" is used to refer to a human user, whereas "client" refers to
the software being run by the user.
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.
As with all IMAP extension documents, the case used in writing IMAP
protocol elements herein is chosen for editorial clarity, and
implementations must pay attention to the numbered rules at the
beginning of [RFC3501] Section 9.
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3. FETCH Data Item
3.1. Command
To retrieve a preview for a message, the "PREVIEW" FETCH attribute is
used when issuing a FETCH command.
If no algorithm list is provided, the server MUST return data using
the text/imap-fetch-preview algorithm.
The client may explicitly indicate which algorithm(s) should be used
to generate the preview list via a parenthesized list of algorithm
names output after the PREVIEW attribute. Unsupported algorithms in
the list MUST be ignored. If the list contains no supported
algorithms, the server MUST return a tagged BAD response to the FETCH
command.
The order of the algorithms in the parenthesized list (from left to
right) defines the client's priority decision. Duplicate algorithms
in the list SHOULD be ignored. For purposes of duplicate detection,
priority modifiers (Section 5) should be ignored. A server MUST
honor a client's algorithm priority decision.
A server should return preview data for the first algorithm that
returns non-empty preview text. Empty preview text is defined as
either the NIL response or an empty (zero length) string. If no
algorithm produces non-empty preview text, the server should respond
with the preview data generated by the final algorithm in the list.
3.2. Response
The algorithm used by the server to generate the preview is returned
preceding the preview string.
The server returns a variable-length string that is the generated
preview for that message.
Example: Retrieving preview information in a SELECTed mailbox
C: A1 FETCH 1 (PREVIEW)
S: * 1 FETCH (PREVIEW (text/imap-fetch-preview "Preview text!"))
S: A1 OK FETCH complete.
A server SHOULD strive to generate the same string for a given
message for each request. However, since previews are understood to
be a representation of the message data and not a canonical view of
its contents, a client MUST NOT assume that a message preview is
immutable for a given message. This relaxed requirement permits a
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server to offer previews as an option without requiring potentially
burdensome storage and/or processing requirements to guarantee
immutability for a use case that does not require this strictness.
For example, the underlying IMAP server may change for an account due
to a software upgrade; account state information may be retained in
the migration, but the new server may support a different PREVIEW
generation algorithm. Thus, message state may remain the same but
the FETCH PREVIEW response may change, if that data was not part of
the data migration.
It is possible that preview text is not available now, but might be
available later -- perhaps the server's preview-generating resources
are overloaded, there is a server-imposed timeout during preview
generation, or there is some transient issue with fetching the
message body. In such cases, the server will return NIL as the
preview response, and the client can try to retrieve the preview
later.
On the other hand, it is possible that the server has determined that
no meaningful preview text can be generated for a particular message,
and that decision won't change later. Examples of this involve
encrypted messages, content types the server does not support
previews of, and other situations where the server is not able to
extract information for a preview. In such cases, the server will
return a zero-length string. Clients should not send another FETCH
for a preview for such messages.
4. PREVIEW Algorithms
4.1. Description
Algorithms MUST be named in media type [RFC6838] format.
This document defines an extension specific algorithm, text/imap-
fetch-preview, that outlines precise semantics regarding the
generated preview data. Future documents may define additional
PREVIEW specific algorithms.
In the absence of a PREVIEW specific algorithm definition, preview
generation semantics are server-dependent. If a server supports an
algorithm it MUST return preview data in the media type format
requested by the client. A client MUST NOT assume that a server
supports a preview algorithm not specifically defined in this
document.
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4.2. text/imap-fetch-preview
The text/imap-fetch-preview algorithm directs the server to use any
internal algorithm it desires, subject to the below limitations, to
generate a textual preview for a message.
The text/imap-fetch-preview algorithm MUST be implemented by any
server that supports the PREVIEW extension.
The generated preview text MUST be treated as text/plain [RFC2046]
media type data by the client.
The generated string MUST NOT be content transfer encoded and MUST be
encoded in UTF-8 [RFC3629]. The server SHOULD remove any formatting
markup and do what other processing might be useful in rendering the
preview as plain text.
For purposes of this section, a "preview character" is defined as a
single UCS character encoded in UTF-8. Note: a single preview
character may compromise multiple octets, so any buffers implemented
to conform to the string limitations identified in this document
should be sized to prevent possible overflow errors.
The server SHOULD limit the length of the preview text to 200 preview
characters. This length should provide sufficient data to generally
support both various languages (and their different average word
lengths) and diverse client display size requirements.
The server MUST NOT output preview text longer than 256 preview
characters.
If the FUZZY algorithm generates a preview that is not based on the
body content of the message and the LANGUAGE [RFC5255] extension is
supported by the server, the preview text SHOULD be generated
according to the language rules that apply to human-readable text.
For example, a message that consists of a single image MIME part has
no human-readable text from which to generate preview information.
Instead, the server may wish to output a description that the message
contains an image and describe some attributes of the image, such as
image format, size, and filename. This descriptive text is not a
product of the message body itself but is rather auto-generated data
by the server, and should thus use the rules defined for human-
generated text described in the LANGUAGE extension (if supported on
the server).
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5. PREVIEW Priority Modifiers
5.1. LAZY
The LAZY modifier directs the server to return the preview
representation only if that data can be returned without undue delay
to the client.
The LAZY modifier alters preview generation semantics of the specific
algorithm it is applied to. If no algorithm is provided as an
argument, the LAZY modifier applies to the text/imap-fetch-preview
algorithm.
This modifier allows a client to inform the server that preview data
is nice-to-have, but the server SHOULD NOT block the return of other
FETCH information at the expense of generating the preview data.
For example, a client displaying the initial mailbox listing to a
user may want to display preview information associated with messages
in that listing. However, this information is secondary to providing
the mailbox listing, with message details, and the client is willing
to load any unavailable previews in the background and display them
as they are provided by the server. In this case, the client would
apply the LAZY modifier to the desired algorithm(s) to direct the
server to only return pre-generated preview data so that retrieval of
the other FETCH information is not blocked by possibly expensive
preview generation.
Generally, the LAZY modifier will only be used once per mailbox load
during the initial listing. If preview information is not available
during this initial FETCH, the expectation is that a second non-LAZY
FETCH will take place after mailbox listing activities are complete.
Thus, a maximum of 2 PREVIEW FETCH queries should occur for any
message in a selected mailbox. A client SHOULD NOT continually issue
LAZY PREVIEW FETCH commands in a selected mailbox as the server is
under no requirement to return preview information for this command,
which could lead to an unnecessary waste of system and network
resources. See Example 4 in the Examples section for how this can be
implemented.
The LAZY modifier MUST be implemented by any server that supports the
PREVIEW extension.
6. Examples
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Example 1: Requesting FETCH without explicit algorithm selection.
C: A1 CAPABILITY
S: * CAPABILITY IMAP4rev1 PREVIEW
S: A1 OK Capability command completed.
[...a mailbox is SELECTed...]
C: A2 FETCH 1 (RFC822.SIZE PREVIEW)
S: * 1 FETCH (RFC822.SIZE 5647 PREVIEW
(text/imap-fetch-preview {200}
S: Lorem ipsum dolor sit amet, consectetur adipiscing elit.
S: Curabitur aliquam turpis et ante dictum, et pulvinar dui congue.
S: Maecenas hendrerit, lorem non imperdiet pellentesque, nulla
S: ligula nullam
S: ))
S: A2 OK FETCH complete.
Example 2: Requesting FETCH with explicit algorithm selection.
C: B1 FETCH 1 (RFC822.SIZE PREVIEW (text/imap-fetch-preview))
S: * 1 FETCH (RFC822.SIZE 91377 PREVIEW
(text/imap-fetch-preview {53}
S: Preview text generated from message body text data.
S: ))
S: B1 OK FETCH complete.
Example 3: Requesting FETCH with unknown explicit algorithm
selection.
C: C1 CAPABILITY
S: * CAPABILITY IMAP4rev1 PREVIEW
S: C1 OK Capability command completed.
[...a mailbox is SELECTed...]
C: C2 FETCH 1 (RFC822.SIZE PREVIEW (example/unknown))
S: C2 BAD FETCH contains unknown preview algorithm name.
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Example 4: Use explicit algorithm priority selection, with LAZY
modifier, to obtain previews during initial mailbox listing if
readily available; otherwise, load previews in background.
C: D1 FETCH 1:3 (ENVELOPE PREVIEW (LAZY))
S: * 1 FETCH (ENVELOPE ("Wed, 25 Oct 2017 15:03:11 +0000" [...])
PREVIEW (text/imap-fetch-preview "Preview text for message 1."))
S: * 2 FETCH (PREVIEW (text/imap-fetch-preview "") ENVELOPE
("Thu, 26 Oct 2017 12:17:23 +0000" [...]))
S: * 3 FETCH (ENVELOPE ("Fri, 27 Oct 2017 22:19:21 +0000" [...])
PREVIEW (text/imap-fetch-preview NIL))
S: D1 OK FETCH completed.
[...Client knows that message 2 has a preview that is empty;
therefore, client only needs to request message 3 preview again
(e.g. in background)...]
C: D2 FETCH 3 (PREVIEW (text/imap-fetch-preview))
S: * 3 FETCH (PREVIEW (text/imap-fetch-preview {30}
S: Message data from message 3.
S: ))
S: D2 OK Fetch completed.
Example 5: Retrieve preview information for search results within a
single mailbox. Use SEARCHRES [RFC5182] extension to save a round-
trip.
C: E1 CAPABILITY
S: * CAPABILITY IMAP4rev1 PREVIEW SEARCHRES
S: E1 OK Capability command completed.
[...a mailbox is SELECTed...]
C: E2 SEARCH RETURN (SAVE) FROM "FOO"
C: E3 FETCH $ (UID PREVIEW (LAZY=text/imap-fetch-preview))
S: E2 OK SEARCH completed.
S: * 5 FETCH (UID 13 PREVIEW (text/imap-fetch-preview "Preview!"))
S: * 9 FETCH (UID 23 PREVIEW (text/imap-fetch-preview NIL))
S: E3 OK FETCH completed.
[...Retrieve message 9 preview in background...]
C: E4 UID FETCH 23 (PREVIEW (text/imap-fetch-preview))
S: * 9 FETCH (UID 23 PREVIEW (text/imap-fetch-preview
"Another preview!"))
S: E4 OK FETCH completed.
7. Formal Syntax
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in ABNF [RFC5234]. It includes definitions
from IMAP [RFC3501].
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capability =/ "PREVIEW"
fetch-att =/ "PREVIEW" [SP "(" preview-alg-fetch *(SP
preview-alg-fetch) ")"]
msg-att-dynamic =/ "PREVIEW" SP "(" preview-alg SP nstring ")"
preview-alg = "text/imap-fetch-preview" / preview-alg-ext
preview-alg-ext = preview-atom ; New algorithm names SHOULD be
; registered with IANA and MUST
; conform with the media type
; format described in [RFC6838]
preview-alg-fetch = preview-alg / preview-mod ["=" preview-alg]
preview-atom = 1*<any ATOM-CHAR except "=">
preview-mod = "LAZY"
8. IANA Considerations
IMAP4 [RFC3501] capabilities are registered by publishing a standards
track or IESG-approved experimental RFC. The registry is currently
located at:
http://www.iana.org/assignments/imap-capabilities
This document requests that IANA adds the "PREVIEW" capability to the
IMAP4 [RFC3501] capabilities registry.
This document requests that IANA create a new "IMAP FETCH PREVIEW
Algorithms" registry, which registers preview algorithms by IETF
Review [RFC8126]. An assignment consists of the algorithm name (as
defined by the preview-alg-ext ABNF entry) and the document that
defines the algorithm. This document constitutes registration of the
text/imap-fetch-preview algorithm in that registry.
9. Security Considerations
Implementation of this extension might enable denial-of-service
attacks against server resources, due to excessive memory or CPU
usage during preview generation or increased storage usage if preview
results are stored on the server after generation. Servers MAY limit
the resources that preview generation uses. In order to mitigate
such attacks, servers SHOULD log the client authentication identity
on FETCH PREVIEW operations in order to facilitate tracking of
abusive clients.
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Just as the messages they summarize, preview data may contain
sensitive information. If generated preview data is stored on the
server, e.g. for caching purposes, these previews MUST be protected
with equivalent authorization and confidentiality controls as the
source message.
10. References
10.1. Normative References
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/info/rfc2046>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5255] Newman, C., Gulbrandsen, A., and A. Melnikov, "Internet
Message Access Protocol Internationalization", RFC 5255,
DOI 10.17487/RFC5255, June 2008,
<https://www.rfc-editor.org/info/rfc5255>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
10.2. Informative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
[RFC2854] Connolly, D. and L. Masinter, "The 'text/html' Media
Type", RFC 2854, DOI 10.17487/RFC2854, June 2000,
<https://www.rfc-editor.org/info/rfc2854>.
[RFC5182] Melnikov, A., "IMAP Extension for Referencing the Last
SEARCH Result", RFC 5182, DOI 10.17487/RFC5182, March
2008, <https://www.rfc-editor.org/info/rfc5182>.
Appendix A. Change History (To be removed by RFC Editor before
publication)
Changes from draft-slusarz-imap-fetch-snippet-00:
o Added standardized language to Section 2 regarding IMAP ABNF
conventions
o Changed draft name to draft-ietf-extra-imap-fetch-snippet-##
Changes from draft-ietf-extra-imap-fetch-snippet-00:
o Changed nomenclature from "snippet" to "preview"
o Changed draft name to draft-ietf-extra-imap-fetch-preview-##
o Update to RFC 8174 boilerplate
o Updated length requirements for PREVIEW=FUZZY
o Added preview-atom ABNF to limit use of "=" character
o UTF-8 is a normative reference
o Clarify that characters for purpose of length limitations are
defined as UCS characters as encoded by UTF-8
o Fix some incorrect literal lengths in examples
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Changes from draft-ietf-extra-imap-fetch-preview-00:
o Updated postal address
o Added example to FETCH response section
o Added example on how LANGUAGE extension may influence preview
generation
o Added recommendation that only one LAZY FETCH be executed for a
message per mailbox
o Added request to create algorithm and modifier registries
o Added requirement that algorithm and modifier names conform to RFC
6648
o Added DoS attack info to security considerations
o Distinguish between NIL response and zero-length string
o Don't use deprecated "X-" convention in example
o Spelling and nits
Changes from draft-ietf-extra-imap-fetch-preview-01:
o Fix capability ABNF
o Removed CAPABILITY string for examples where it did not add
valuable context
o Altered preview data in examples to cover a variety of potential
server return scenarios
o Added "SHOULD be registered" language to algorithm names and
priority modifiers
Changes from draft-ietf-extra-imap-fetch-preview-02:
o Move Acknowledgments to un-numbered appendix
o Improved abstract text
o Consistently use "priority modifiers" instead of "modifiers"
o Update example to conform with RFC 3501 UID FETCH requirements
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Changes from draft-ietf-extra-imap-fetch-preview-03:
o Remove preview modifier registry request
o Improve instructions for registration of algorithms
o Add storage information to security considerations
o Clarify parsing of algorithm list in FETCH command
o Clarify difference between NIL response and zero-length string
o Add normative reference for text/plain
o Add warning regarding buffers and multiple octet preview
characters
o Clarify how to handle preview data return when using an explicit
algorithm list
o Various editorial fixes
Changes from draft-ietf-extra-imap-fetch-preview-04:
o Make clear that preview caching is tied to retention period of the
source message
Changes from draft-ietf-extra-imap-fetch-preview-05:
o Clarify "zero-length string" preview data vs. NIL preview data
o MIME data -> media type
o Capability registration should not include the algorithm name
o Give example of how PREVIEW data might change over time
Changes from draft-ietf-extra-imap-fetch-preview-06:
o Change algorithm names to media types
o FUZZY algorithm changed to text/imap-fetch-preview
o Remove server broadcast of PREVIEW algorithm extensions from
capability
o Default, fallback algorithm in absence of client selection now
MUST be text/imap-fetch-preview
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o LAZY modifier should work on default algorithm if no specific
algorithm is provided as an argument
Acknowledgments
The author would like to thank the following people for their
comments and contributions to this document: Stephan Bosch, Bron
Gondwana, Teemu Huovila, Neil Jenkins, Steffen Lehmann, Barry Leiba,
Alexey Melnikov, Chris Newman, Pete Resnick, Jeff Sipek, Timo
Sirainen, Steffen Templin, and Aki Tuomi.
Author's Address
Michael M. Slusarz
Open-Xchange Inc.
530 Lytton Avenue
Palo Alto, California 94301
US
Email: michael.slusarz@open-xchange.com
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