--- 1/draft-ietf-extra-imap-fetch-preview-07.txt 2020-07-01 18:13:08.949565658 -0700 +++ 2/draft-ietf-extra-imap-fetch-preview-08.txt 2020-07-01 18:13:08.977566374 -0700 @@ -1,90 +1,88 @@ EXTRA M. Slusarz Internet-Draft Open-Xchange Inc. -Intended status: Standards Track December 10, 2019 -Expires: June 12, 2020 +Intended status: Standards Track July 1, 2020 +Expires: January 2, 2021 IMAP4 Extension: Message Preview Generation - draft-ietf-extra-imap-fetch-preview-07 + draft-ietf-extra-imap-fetch-preview-08 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. + abbreviated text 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. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on June 12, 2020. + This Internet-Draft will expire on January 2, 2021. Copyright Notice - Copyright (c) 2019 IETF Trust and the persons identified as the + Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions Used In This Document . . . . . . . . . . . . . . 3 - 3. FETCH Data Item . . . . . . . . . . . . . . . . . . . . . . . 4 + 3. FETCH Data Item . . . . . . . . . . . . . . . . . . . . . . . 3 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 + 3.3. Preview Text Format . . . . . . . . . . . . . . . . . . . 5 + 4. PREVIEW Priority Modifiers . . . . . . . . . . . . . . . . . 5 + 4.1. LAZY . . . . . . . . . . . . . . . . . . . . . . . . . . 5 + 5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6 + 6. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 7 + 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 + 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 + 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 + 9.1. Normative References . . . . . . . . . . . . . . . . . . 8 + 9.2. Informative References . . . . . . . . . . . . . . . . . 9 Appendix A. Change History (To be removed by RFC Editor before - publication) . . . . . . . . . . . . . . . . . . . . 12 - Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 15 - Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15 + publication) . . . . . . . . . . . . . . . . . . . . 9 + Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12 + Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12 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. + standardized, consistent way to generate these brief textual 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) @@ -133,341 +131,249 @@ 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. 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: * 1 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 + be an approximation 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 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. + For example, the underlying IMAP server may change due to a system + software upgrade; an account's state information may be retained in + the migration but the new server may generate different PREVIEW text + than the old server. 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. - -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. + return a zero-length string. Clients SHOULD NOT send another FETCH + for a preview for such messages. (As discussed previously, preview + data is not immutable so there is chance that at some point in the + future the server would be able to generate meaningful text. + However, this scenario is expected to be rare so a client should not + continually send out requests to try to capture this infrequent + occurrence. - The text/imap-fetch-preview algorithm MUST be implemented by any - server that supports the PREVIEW extension. +3.3. Preview Text Format 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 + markup and do whatever 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). + If the preview is not generated 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). -5. PREVIEW Priority Modifiers +4. PREVIEW Priority Modifiers -5.1. LAZY +4.1. 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. + send the LAZY modifier directing 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 + resources. See Example 2 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 - Example 1: Requesting FETCH without explicit algorithm selection. +5. Examples + + Example 1: Requesting PREVIEW without LAZY modifier. 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: * 1 FETCH (RFC822.SIZE 5647 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: ) 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. - - 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. + Example 2: Requesting PREVIEW 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 + PREVIEW "Preview text for message 1.") + S: * 2 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)) + 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} + C: D2 FETCH 3 (PREVIEW) + S: * 3 FETCH (PREVIEW {30} S: Message data from message 3. - S: )) + S: ) S: D2 OK Fetch completed. - Example 5: Retrieve preview information for search results within a + Example 3: 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)) + C: E3 FETCH $ (UID PREVIEW (LAZY)) 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: * 5 FETCH (UID 13 PREVIEW "Preview!") + S: * 9 FETCH (UID 23 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!")) + C: E4 UID FETCH 23 (PREVIEW) + S: * 9 FETCH (UID 23 PREVIEW "Another preview!") S: E4 OK FETCH completed. -7. Formal Syntax +6. Formal Syntax The following syntax specification uses the augmented Backus-Naur Form (BNF) as described in ABNF [RFC5234]. It includes definitions from IMAP [RFC3501]. 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] + fetch-att =/ "PREVIEW" [SP "(" preview-mod *(SP + preview-mod) ")"] - preview-atom = 1*. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . @@ -483,35 +389,25 @@ [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . [RFC5255] Newman, C., Gulbrandsen, A., and A. Melnikov, "Internet Message Access Protocol Internationalization", RFC 5255, DOI 10.17487/RFC5255, June 2008, . - [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type - Specifications and Registration Procedures", BCP 13, - RFC 6838, DOI 10.17487/RFC6838, January 2013, - . - - [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, - . - [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . -10.2. Informative References +9.2. 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, . [RFC2854] Connolly, D. and L. Masinter, "The 'text/html' Media Type", RFC 2854, DOI 10.17487/RFC2854, June 2000, . @@ -639,27 +536,31 @@ 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 o LAZY modifier should work on default algorithm if no specific algorithm is provided as an argument + Changes from draft-ietf-extra-imap-fetch-preview-07: + + o Remove algorithm selection; PREVIEW always returns text in format + defined in Section 3.3 + 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