Open-Xchange Inc.

530 Lytton Avenue Palo Alto California 94301 US michael.slusarz@open-xchange.com

ART EXTRA IMAP4 FETCH PREVIEW This document specifies an Internet Message Access Protocol (IMAP) protocol extension that allows a client to request a server-generated abbreviated text representation of message data that is useful as a contextual preview of the entire message.

Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

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Table of Contents

• .  Introduction
• .  Conventions Used in This Document
• .  FETCH Data Item
  • .  Command
  • .  Response
  • .  Preview Text Format
• .  LAZY Priority Modifier
  • .  LAZY
  • .  Client Implementation Advice
• .  Examples
• .  Formal Syntax
• .  IANA Considerations
• .  Security Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgments
• Author's Address

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 would benefit from a 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. While different clients might generate quite different preview text, having common preview text generated by the server can give a more consistent user experience to those who use multiple clients. 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 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. 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 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 that preview can be 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.

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 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 .

FETCH Data Item

Command To retrieve a preview for a message, the PREVIEW FETCH attribute is used when issuing a FETCH command.

Response The server returns a variable-length string that is the generated preview for that message. This string is intended to be viewed by the user as a contextual preview of the entire message and is not intended to be interpreted in any way by the client software. Example: Retrieving preview information in a SELECTed mailbox. C: A1 FETCH 1 (PREVIEW) 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 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 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 the server has determined that no meaningful preview text can be generated for a particular message. 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 MUST 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 detect this infrequent occurrence.) If the LAZY modifier is used, the server MAY return NIL for the preview response, indicating that preview generation could not be completed without causing undue delay. A server MUST NOT return NIL to a FETCH PREVIEW request made without the LAZY modifier.

Preview Text Format The generated preview text MUST be treated as text/plain media type data by the client. The generated string MUST NOT be content transfer encoded and MUST be encoded in UTF-8. The server SHOULD remove any formatting 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 Universal Character Set (UCS) character encoded in UTF-8. Note: a single preview character may comprise 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 preview is not generated based on the body content of the message, and the LANGUAGE 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; it should thus use the rules defined for human-readable text described in the LANGUAGE extension (if supported on the server).

LAZY Priority Modifier

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. If this modifier is used, and the server is unable to return preview data without undue delay, the server MUST return NIL as the preview response. The LAZY modifier MUST be implemented by any server that supports the PREVIEW extension.

Client Implementation Advice Upon opening a mailbox, a client generally performs a FETCH of message details in order to create a listing to present to the user (e.g., ENVELOPE data). Using this extension, a client may want to additionally display preview information as part of this listing. Quickly providing the base mailbox listing with basic message details is the primary goal of this command as this is required to allow the user to begin interacting with the mailbox. Preview data is likely to be of secondary importance; it provides useful context, but it is not necessary to perform message actions. A client can load unavailable previews in the background and display them asynchronously to the user as the preview data is provided by the server. In this scenario, the client would add the PREVIEW data item, with the LAZY modifier, to the list of FETCH items needed to generate the mailbox listing. This allows the server to advantageously return preview data without blocking the primary goal of quickly returning the basic message details used to generate the mailbox listing. Once this initial FETCH is complete, the client can then issue FETCH requests, without the LAZY modifier, to load the PREVIEW data item for the messages in which preview data was not returned. It is RECOMMENDED that these FETCH requests be issued in small batches, e.g., 50 messages per FETCH command, since preview generation may be expensive and a single large request may exceed server resource limits. See Example 2 in for an implementation of this strategy. A client SHOULD NOT continually issue FETCH PREVIEW requests with the LAZY modifier 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.

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 {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 preview with LAZY modifier, to obtain previews during initial mailbox listing if readily available; otherwise, load previews in background. C: B1 FETCH 1:4 (ENVELOPE PREVIEW (LAZY)) S: * 1 FETCH (ENVELOPE ("Wed, 23 Sep 2020 15:03:11 +0000" [...]) PREVIEW "Preview text for message 1.") S: * 2 FETCH (PREVIEW "" ENVELOPE ("Thu, 24 Sep 2020 12:17:23 +0000" [...])) S: * 3 FETCH (ENVELOPE ("Fri, 25 Sep 2020 09:13:45 +0000" [...]) PREVIEW NIL) S: * 4 FETCH (ENVELOPE ("Sat, 26 Sep 2020 07:11:18 +0000" [...]) PREVIEW NIL) S: B1 OK FETCH completed. [...Client has preview for message 1 and knows that message 2 has a preview that is empty; only need to request preview of messages 3 & 4 (e.g., in background)...] C: B2 FETCH 3:4 (PREVIEW) S: * 3 FETCH (PREVIEW {30} S: Message data from message 3. S: ) S: * 4 FETCH (PREVIEW "Message 4 preview") S: B2 OK Fetch completed. Example 3: Requesting preview for search results within a single mailbox. Use the SEARCHRES extension to save a round-trip. C: C1 CAPABILITY S: * CAPABILITY IMAP4rev1 PREVIEW SEARCHRES S: C1 OK Capability command completed. [...a mailbox is SELECTed...] C: C2 SEARCH RETURN (SAVE) FROM "FOO" C: C3 FETCH $ (UID PREVIEW (LAZY)) S: C2 OK SEARCH completed. S: * 5 FETCH (UID 13 PREVIEW "Preview!") S: * 9 FETCH (UID 23 PREVIEW NIL) S: C3 OK FETCH completed. [...Retrieve message 9 preview in background...] C: C4 UID FETCH 23 (PREVIEW) S: * 9 FETCH (UID 23 PREVIEW "Another preview!") S: C4 OK FETCH completed.

Formal Syntax The following syntax specification uses the Augmented Backus-Naur Form (ABNF) as described in . It includes definitions from IMAP. capability =/ "PREVIEW" fetch-att =/ "PREVIEW" [SP "(" preview-mod *(SP preview-mod) ")"] msg-att-dynamic =/ "PREVIEW" SP nstring preview-mod = "LAZY"

IANA Considerations IMAP capabilities are registered by publishing a Standards Track or IESG-approved Experimental RFC in the "IMAP Capabilities" registry located at . IANA has added the "PREVIEW" capability to this registry.

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. 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. Servers MAY limit the resources that preview generation uses. Such resource limitations might, in an extreme example, cause a server to return a preview that is the empty string for a message that otherwise would have had a non-empty preview. However, it is recommended that at least some preview text be provided in this situation, even if the quality of the preview is degraded. 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.

References Normative References This second document defines the general structure of the MIME media typing system and defines an initial set of media types. [STANDARDS-TRACK] In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The Internet Message Access Protocol, Version 4rev1 (IMAP4rev1) allows a client to access and manipulate electronic mail messages on a server. IMAP4rev1 permits manipulation of mailboxes (remote message folders) in a way that is functionally equivalent to local folders. IMAP4rev1 also provides the capability for an offline client to resynchronize with the server. IMAP4rev1 includes operations for creating, deleting, and renaming mailboxes, checking for new messages, permanently removing messages, setting and clearing flags, RFC 2822 and RFC 2045 parsing, searching, and selective fetching of message attributes, texts, and portions thereof. Messages in IMAP4rev1 are accessed by the use of numbers. These numbers are either message sequence numbers or unique identifiers. IMAP4rev1 supports a single server. A mechanism for accessing configuration information to support multiple IMAP4rev1 servers is discussed in RFC 2244. IMAP4rev1 does not specify a means of posting mail; this function is handled by a mail transfer protocol such as RFC 2821. [STANDARDS-TRACK] ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo obsoletes and replaces RFC 2279. Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] Internet Message Access Protocol (IMAP) version 4rev1 has basic support for non-ASCII characters in mailbox names and search substrings. It also supports non-ASCII message headers and content encoded as specified by Multipurpose Internet Mail Extensions (MIME). This specification defines a collection of IMAP extensions that improve international support including language negotiation for international error text, translations for namespace prefixes, and comparator negotiation for search, sort, and thread. [STANDARDS-TRACK] RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This initial document specifies the various headers used to describe the structure of MIME messages. [STANDARDS-TRACK] This document summarizes the history of HTML development, and defines the "text/html" MIME type by pointing to the relevant W3C recommendations. This memo provides information for the Internet community. Many IMAP clients use the result of a SEARCH command as the input to perform another operation, for example, fetching the found messages, deleting them, or copying them to another mailbox. This can be achieved using standard IMAP operations described in RFC 3501; however, this would be suboptimal. The server will send the list of found messages to the client; after that, the client will have to parse the list, reformat it, and send it back to the server. The client can't pipeline the SEARCH command with the subsequent command, and, as a result, the server might not be able to perform some optimizations. This document proposes an IMAP extension that allows a client to tell a server to use the result of a SEARCH (or Unique Identifier (UID) SEARCH) command as an input to any subsequent command. [STANDARDS-TRACK]

Acknowledgments The author would like to thank the following people for their comments and contributions to this document: , , , , , , , , , , , , and .

Author's Address Open-Xchange Inc.

530 Lytton Avenue Palo Alto California 94301 US michael.slusarz@open-xchange.com