Internet Draft: Lemonade Profile S. H. Maes Document: draft-ietf-lemonade-profile-06.txt A. Melnikov Expires: May 2006 November 2005 Lemonade Profile Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document describes a profile (a set of required extensions, restrictions and usage modes) of the IMAP and mail submission protocols. This profile allows clients (especially those that are constrained in memory, bandwidth, processing power, or other areas) to efficiently use IMAP and Submission to access and submit mail. This includes the ability to forward received mail without needing to download and upload the mail, to optimize submission and to efficiently resynchronize in case of loss of connectivity with the server. The Lemonade profile relies upon extensions to IMAP and Mail Submission protocols; specifically URLAUTH and CATENATE IMAP protocol [RFC3501] extensions and BURL extension to the SUBMIT protocol [RFC2476]. Conventions used in this document In examples, "M:", "I:" and "S:" indicate lines sent by the client messaging user agent, IMAP e-mail server and SMTP submit server respectively. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Table of Contents Status of this Memo 1 Abstract 1 Conventions used in this document 2 Table of Contents 2 1. Introduction 3 2. Forward without download 4 2.1. Motivations 4 2.2. Message Sending Overview 4 2.3. Traditional Strategy 5 2.4. Step by step description 6 2.4.1. Message assembly using IMAP CATENATE extension 7 2.4.2. Message assembly using SMTP CHUNKING and BURL extensions 10 2.5. Normative statements related to forward without download 14 2.6. Security Considerations for pawn-tickets. 14 2.7. The fcc problem 14 2.8. Registration of $Forwarded IMAP keyword 15 3. Message Submission 15 3.1. Pipelining 15 3.2. DSN Support 16 3.3. Message size declaration 16 3.4. Enhanced status code Support 16 3.5. TLS 16 4. Quick resynchronization 16 5. Additional IMAP extensions 16 6. Summary of the required IMAP and SMTP extensions 17 7. Future work 18 8. Security Considerations 18 8.1. Confidentiality Protection of Submitted Messages 18 8.2. TLS 19 9. IANA Considerations 19 10. References 19 10.1. Normative References 19 10.2. Informative References 21 Open issues 21 Version History 21 Acknowledgments 22 Authors Addresses 23 Intellectual Property Statement 23 1. Introduction Lemonade provides enhancements to Internet email to support diverse service environments. This document describes the lemonade profile that includes: - "Forward without download" that describes exchanges between Lemonade clients and servers to allow to submit new email messages incorporating content which resides on locations external to the client. - Quick mailbox resynchronization using [CONDSTORE]. - Several IMAP and SMTP extensions that allow to save bandwidth and/or number of round trips required to send/receive data. The organization of this document is as follows. Section 2 describes the Forward without download. Section 3 describes additional SMTP extensions that must be supported by all Lemonade Submission servers. Section 4 describes IMAP quick resynchronization. 2. Forward without download 2.1. Motivations The advent of client/server email using the [RFC3501], [RFC2821] and [RFC2476] protocols has changed what formerly were local disk operations to become repetitive network data transmissions. Lemonade "forward without download" makes 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 SMTP SUBMIT servers. Lemonade "forward without download" has the advantage of maintaining one submission protocol, and thus avoids the risk of having multiple parallel and possibly divergent mechanisms for submission. The client can use Submit/SMTP [RFC2476] extensions without these being added to IMAP. Furthermore, by keeping the details of message submission in the SMTP SUBMIT server, Lemonade "forward without download" can work with other message retrieval protocols such as POP, NNTP, or whatever else may be designed in the future. 2.2. Message Sending Overview The act of sending an email message can be 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 takes place in the MUA. Frequently, users choose to save more complex messages on an [RFC3501] 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 [RFC2821] infrastructure, typically using the [RFC2476] protocol. 2.3. Traditional Strategy Traditionally, messages are initiated, edited, and assembled entirely within an MUA, although drafts may be saved to an [RFC3501] 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, when 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. 2.4. Step by step description The model distinguishes between a Messaging User Agent (MUA), an IMAPv4Rev1 Server ([RFC3501]) and a SMTP submit server ([RFC2476]), as illustrated in Figure 1. +--------------------+ +--------------+ | | <------------ | | | MUA (M) | | IMAPv4Rev1 | | | | Server | | | ------------> | (Server I) | +--------------------+ +--------------+ ^ | ^ | | | | | | | | | | | | | | | | | | | | | | | | v | | +--------------+ | |----------------------> | SMTP | | | Submit | |-----------------------------| Server | | (Server S) | +--------------+ Figure 1: Lemonade "forward without download" Lemonade "forward without download" allows a Messaging User Agent to compose and forward an e-mail combining fragments that are located in an IMAP server, without having to download these fragments to the client. There are two ways to perform "forward without download" based on where the message assembly takes place. The first uses extended APPEND command [CATENATE] to edit a draft message in the message store and cause the message assembly on the IMAP server. The second uses a succession of BURL and BDAT commands to submit and assemble through concatenation, message data from the client and external data fetched from the provided URL. The two subsequent sections provide step-by-step instructions on how "forward without download" is achieved. 2.4.1. Message assembly using IMAP CATENATE extension In the [BURL]/[CATENATE] variant of the Lemonade "forward without download" strategy, messages are initially composed and edited within an MUA. The [CATENATE] extension to [RFC3501] is then used to create the messages on the IMAP server by transmitting new text and assembling them. The [UIDPLUS] IMAP extension is used by the client in order to learn the UID of the created messages. Finally a [URLAUTH] format URL is given to a [RFC2476] server for submission using the [BURL] extension. The flow involved to support such a use case consists of: M: {to I -- Optional} The client connects to the IMAP server, optionally starts TLS (if data confidentiality is required), authenticates, opens a mailbox ("INBOX" in the example below) and fetches body structures (See [RFC3501]). Example: M: A0051 UID FETCH 25627 (UID BODYSTRUCTURE) I: * 161 FETCH (UID 25627 BODYSTRUCTURE (("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 1152 23)( "TEXT" "PLAIN" ("CHARSET" "US-ASCII" "NAME" "trip.txt") "<960723163407.20117h@washington.example.com>" "Your trip details" "BASE64" 4554 73) "MIXED")) I: A0051 OK completed M: {to I} The client invokes CATENATE (See [CATENATE] for details of the semantics and steps) -- this allows the MUA to create messages on the IMAP server using new data combined with one or more message parts already present on the IMAP server. Note that the example for this step doesn't use the LITERAL+ [LITERAL+] extension. Without LITERAL+ the new message is constructed using 3 round-trips. If LITERAL+ is used, the new message can be constructed using one round-trip. M: A0052 APPEND Sent FLAGS (\Seen $MDNSent) CATENATE (TEXT {475} I: + Ready for literal data M: Message-ID: <419399E1.6000505@caernarfon.example.org> M: Date: Thu, 12 Nov 2004 16:57:05 +0000 M: From: Bob Ar <bar@example.org> M: MIME-Version: 1.0 M: To: foo@example.net M: Subject: About our holiday trip M: Content-Type: multipart/mixed; M: boundary="------------030308070208000400050907" M: M: --------------030308070208000400050907 M: Content-Type: text/plain; format=flowed M: M: Our travel agent has sent the updated schedule. M: M: Cheers, M: Bob M: --------------030308070208000400050907 M: URL "/INBOX;UIDVALIDITY=385759045/; UID=25627;Section=2.MIME" URL "/INBOX; UIDVALIDITY=385759045/;UID=25627;Section=2" TEXT {44} I: + Ready for literal data M: M: --------------030308070208000400050907-- M: ) I: A0052 OK [APPENDUID 387899045 45] CATENATE Completed M: {to I} The client uses GENURLAUTH command to request a URLAUTH URL (See [URLAUTH]). I: {to M} The IMAP server returns a URLAUTH URL suitable for later retrieval with URLFETCH (See [URLAUTH] for details of the semantics and steps). M: A0054 GENURLAUTH "imap://bob.ar@example.org/Sent; UIDVALIDITY=387899045/;uid=45/;expire=2005-10- 28T23:59:59Z;urlauth=submit+bob.ar" INTERNAL I: * GENURLAUTH "imap://bob.ar@example.org/Sent; UIDVALIDITY=387899045/;uid=45/;expire= 2005-10-28T23:59:59Z;urlauth=submit+bob.ar: internal:91354a473744909de610943775f92038" I: A0054 OK GENURLAUTH completed M: {to S} The client connects to the mail submission server and starts a new mail transaction. It uses BURL to let the SMTP submit server fetch the content of the message from the IMAP server (See [BURL] for details of the semantics and steps -- this allows the MUA to authorize the SMTP submit server to access the message composed as a result of the CATENATE step). Note that the second EHLO command is required after a successful STARTTLS command. Also note that there might be a third required EHLO command if the second EHLO response doesn't list any BURL options. Section 2.4.2 demonstrates this. S: 220 owlry.example.org ESMTP M: EHLO potter.example.org S: 250-owlry.example.com S: 250-8BITMIME S: 250-BINARYMIME S: 250-PIPELINING S: 250-BURL imap S: 250-CHUNKING S: 250-AUTH PLAIN S: 250-DSN S: 250-SIZE 10240000 S: 250-STARTTLS S: 250 ENHANCEDSTATUSCODES M: STARTTLS S: 220 Ready to start TLS ...TLS negotiation, subsequent data is encrypted... M: EHLO potter.example.org S: 250-owlry.example.com S: 250-8BITMIME S: 250-BINARYMIME S: 250-PIPELINING S: 250-BURL imap S: 250-CHUNKING S: 250-AUTH PLAIN S: 250-DSN S: 250-SIZE 10240000 S: 250 ENHANCEDSTATUSCODES M: AUTH PLAIN aGFycnkAaGFycnkAYWNjaW8= S: 235 2.7.0 PLAIN authentication successful. M: MAIL FROM:<bob.ar@example.org> S: 250 2.5.0 Address Ok. M: RCPT TO:<foo@example.net> S: 250 2.1.5 foo@example.net OK. M: BURL imap://bob.ar@example.org/Sent;UIDVALIDITY=387899045/; uid=45/;urlauth=submit+bar:internal: 91354a473744909de610943775f92038 LAST S: {to I} The mail submission server uses URLFETCH to fetch the message to be sent (See [URLAUTH] for details of the semantics and steps. The so-called "pawn-ticket" authorization mechanism uses a URI which contains its own authorization credentials.). I: {to S} Provides the message composed as a result of the CATENATE step). Mail submission server opens IMAP connection to the IMAP server: I: * OK [CAPABILITY IMAP4REV1 STARTTLS NAMESPACE LITERAL+ CATENATE URLAUTH] imap.example.com IMAP server ready S: a000 STARTTLS I: a000 Start TLS negotiation now ...TLS negotiation, if successful - subsequent data is encrypted... S: a001 LOGIN submitserver secret I: a001 OK submitserver logged in S: a002 URLFETCH "imap://bob.ar@example.org/Sent; UIDVALIDITY=387899045/;uid=45/;urlauth=submit+bob.ar: internal:91354a473744909de610943775f92038" I: * URLFETCH "imap://bob.ar@example.org/Sent; UIDVALIDITY=387899045/;uid=45/;urlauth=submit+bob.ar: internal:91354a473744909de610943775f92038" {15065} ...message body follows... S: a002 OK URLFETCH completed I: a003 LOGOUT S: * BYE See you later S: a003 OK Logout successful Note that if the IMAP server doesn't send CAPABILITY response code in the greeting, the mail submission server must issue the CAPABILITY command to learn about supported IMAP extensions as described in RFC 3501. Also, if data confidentiality is not required the mail submission server may omit the STARTTLS command before issuing the LOGIN command. S2: {to M} OK (2XX) Submission server returns OK to the MUA: S: 250 2.5.0 Ok. M: {to I} The client marks the forwarded message on the IMAP server. M: A0053 UID STORE 25627 +FLAGS.SILENT ($Forwarded) I: A0053 OK STORE completed Note: the UID STORE command shown above will only work if the marked message is in the currently selected mailbox. This command can be omitted. The $Forwarded IMAP keyword is described in section 2.8. 2.4.2. Message assembly using SMTP CHUNKING and BURL extensions In the [BURL]/[CHUNKING] variant of the Lemonade "forward without download" strategy, messages are initially composed and edited within an MUA. The BURL [BURL] and BDAT [CHINKING] commands are then used to create the messages from multiple parts during submission [RFC2476], existing body parts are referenced using [URLAUTH] format URLs. The flow involved to support such a use case consists of: M: {to I -- Optional} The client connects to the IMAP server, optionally starts TLS (if data confidentiality is required), authenticates, opens a mailbox ("INBOX" in the example below) and fetches body structures (See [RFC3501]). Example: M: A0051 UID FETCH 25627 (UID BODYSTRUCTURE) I: * 161 FETCH (UID 25627 BODYSTRUCTURE (("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 1152 23)( "TEXT" "PLAIN" ("CHARSET" "US-ASCII" "NAME" "trip.txt") "<960723163407.20117h@washington.example.com>" "Your trip details" "BASE64" 4554 73) "MIXED")) I: A0051 OK completed M: {to I} The client uses GENURLAUTH command to request URLAUTH URLs (See [URLAUTH]) referencing pieces of the message to be assembled. I: {to M} The IMAP server returns URLAUTH URLs suitable for later retrieval with URLFETCH (See [URLAUTH] for details of the semantics and steps). M: A0054 GENURLAUTH "imap://bob.ar@example.org/INBOX; UIDVALIDITY=385759045/;UID=25627;Section=2.MIME; expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar" INTERNAL "imap://bob.ar@example.org/INBOX; UIDVALIDITY=385759045/;UID=25627;Section=2; expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar" INTERNAL I: * GENURLAUTH "imap://bob.ar@example.org/INBOX; UIDVALIDITY=385759045/;UID=25627;Section=2.MIME; expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar: internal:A0DEAD473744909de610943775f9BEEF" "imap://bob.ar@example.org/INBOX; UIDVALIDITY=385759045/;UID=25627;Section=2; expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar: internal:BEEFA0DEAD473744909de610943775f9" I: A0054 OK GENURLAUTH completed M: {to S} The client connects to the mail submission server and starts a new mail transaction. It uses BURL to let the SMTP submit server fetch pieces of the message to be sent from the IMAP server (See [BURL] for details of the semantics and steps). Note that the second EHLO command is required after a successful STARTTLS command. The third EHLO command is required if and only if the second EHLO response doesn't list any BURL options. See section 2.4.1 for an example of submission where the third EHLO command/response is not present. S: 220 owlry.example.org ESMTP M: EHLO potter.example.org S: 250-owlry.example.com S: 250-8BITMIME S: 250-BINARYMIME S: 250-PIPELINING S: 250-BURL S: 250-CHUNKING S: 250-AUTH DIGEST-MD5 S: 250-DSN S: 250-SIZE 10240000 S: 250-STARTTLS S: 250 ENHANCEDSTATUSCODES M: STARTTLS S: 220 Ready to start TLS ...TLS negotiation, subsequent data is encrypted... M: EHLO potter.example.org S: 250-owlry.example.com S: 250-8BITMIME S: 250-BINARYMIME S: 250-PIPELINING S: 250-BURL S: 250-CHUNKING S: 250-AUTH DIGEST-MD5 CRAM-MD5 PLAIN EXTERNAL S: 250-DSN S: 250-SIZE 10240000 S: 250 ENHANCEDSTATUSCODES M: AUTH PLAIN aGFycnkAaGFycnkAYWNjaW8= S: 235 2.7.0 PLAIN authentication successful. M: EHLO potter.example.org S: 250-owlry.example.com S: 250-8BITMIME S: 250-BINARYMIME S: 250-PIPELINING S: 250-BURL imap imap://imap.example.org S: 250-CHUNKING S: 250-AUTH DIGEST-MD5 CRAM-MD5 PLAIN EXTERNAL S: 250-DSN S: 250-SIZE 10240000 S: 250 ENHANCEDSTATUSCODES M: MAIL FROM:<bob.ar@example.org> BODY=BINARY S: 250 2.5.0 Address Ok. M: RCPT TO:<foo@example.net> S: 250 2.1.5 foo@example.net OK. M: BDAT 475 M: Message-ID: <419399E1.6000505@caernarfon.example.org> M: Date: Thu, 12 Nov 2004 16:57:05 +0000 M: From: Bob Ar <bar@example.org> M: MIME-Version: 1.0 M: To: foo@example.net M: Subject: About our holiday trip M: Content-Type: multipart/mixed; M: boundary="------------030308070208000400050907" M: M: --------------030308070208000400050907 M: Content-Type: text/plain; format=flowed M: M: Our travel agent has sent the updated schedule. M: M: Cheers, M: Bob M: --------------030308070208000400050907 S: 250 2.5.0 OK M: BURL imap://bob.ar@example.org/INBOX; UIDVALIDITY=385759045/;UID=25627;Section=2.MIME; expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar: internal:A0DEAD473744909de610943775f9BEEF S: 250 2.5.0 OK M: BURL imap://bob.ar@example.org/INBOX; UIDVALIDITY=385759045/;UID=25627;Section=2; expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar: internal:BEEFA0DEAD473744909de610943775f9 S: 250 2.5.0 OK M: BDAT 44 LAST M: M: --------------030308070208000400050907-- S: {to I} The mail submission server uses URLFETCH to fetch the pieces of the message to be sent (See [URLAUTH] for details of the semantics and steps. The so-called "pawn-ticket" authorization mechanism uses a URI which contains its own authorization credentials.). I: {to S} Returns the requested body parts. Mail submission server opens IMAP connection to the IMAP server: I: * OK [CAPABILITY IMAP4REV1 STARTTLS NAMESPACE LITERAL+ CATENATE URLAUTH] imap.example.com IMAP server ready S: a001 LOGIN submitserver secret I: a001 OK submitserver logged in S: a002 URLFETCH "imap://bob.ar@example.org/INBOX; UIDVALIDITY=385759045/;UID=25627;Section=2.MIME; expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar: internal:A0DEAD473744909de610943775f9BEEF" "imap:// bob.ar@example.org/INBOX; UIDVALIDITY=385759045/;UID=25627;Section=2; expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar: internal:BEEFA0DEAD473744909de610943775f9" I: * URLFETCH "imap://bob.ar@example.org/INBOX; UIDVALIDITY=385759045/;UID=25627;Section=2.MIME; expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar: internal:A0DEAD473744909de610943775f9BEEF" {84} ...message section follows... "imap://bob.ar@example.org/INBOX; UIDVALIDITY=385759045/;UID=25627;Section=2; expire=2006-10-28T23:59:59Z;urlauth=submit+bob.ar: internal:BEEFA0DEAD473744909de610943775f9" {15065} ...message section follows... S: a002 OK URLFETCH completed I: a003 LOGOUT S: * BYE See you later S: a003 OK Logout successful Note that if the IMAP server doesn't send CAPABILITY response code in the greeting, the mail submission server must issue the CAPABILITY command to learn about supported IMAP extensions as described in RFC 3501. Also, if data confidentiality is required the mail submission server should start TLS before issuing the LOGIN command. S2: {to M} OK (2XX) Submission server acknowledges acceptance of the message by sending 250 response to the last BDAT command: S: 250 2.5.0 Ok, message accepted. M: {to I} The client marks the forwarded message on the IMAP server. M: A0053 UID STORE 25627 +FLAGS.SILENT ($Forwarded) I: A0053 OK STORE completed Note: the UID STORE command shown above will only work if the marked message is in the currently selected mailbox. This command can be omitted. The $Forwarded IMAP keyword is described in section 2.8. 2.5. Normative statements related to forward without download Lemonade compliant IMAP servers MUST support IMAPv4Rev1 [RFC3501], CATENATE [CATENATE], UIDPLUS [UIDPLUS] and URLAUTH [URLAUTH]. This support MUST be declared via CAPABILITY [RFC3501]. Lemonade compliant submit servers MUST support the BURL [BURL], 8BITMIME [8BITMIME], BINARYMIME [CHUNKING] and CHUNKING [CHUNKING]. This support MUST be declared via EHLO [RFC2821]. BURL MUST support URLAUTH type URLs [URLAUTH], and thus MUST advertise the "imap" option following the BURL EHLO keyword. Additional normative statements are provided in other sections. 2.6. Security Considerations for pawn-tickets. The so-called "pawn-ticket" authorization mechanism uses a URI, which contains its own authorization credentials using [URLAUTH]. The advantage of this mechanism is that the SMTP submit [RFC2476] server cannot access any data on the [RFC3501] server without a "pawn- ticket" created by the client. The "pawn-ticket" grants access only to the specific data that the SMTP submit [RFC2476] server is authorized to access, can be revoked by the client, and can have a time-limited validity. 2.7. The fcc problem The "fcc problem" refers to delivering a copy of a message to a "file carbon copy" recipient. By far, the most common case of fcc is a client leaving a copy of outgoing mail in a "Sent Mail" 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 " repetitive network data transmissions" which represents the "problem" aspect of the "fcc problem". The [CATENATE] extension to [RFC3501] can be used to address the fcc problem. The final message is constructed in the mailbox designed for outgoing mail. Note that the [CATENATE] extension can only create a single message and only on the server which stages the outgoing message for submission. Additional copies of the message can be created on the same server using one or more COPY command. 2.8. Registration of $Forwarded IMAP keyword $Forwarded IMAP keyword is used by several IMAP clients to specify that the message was resent to another email address, embedded within or attached to a new message. A mail client sets this keyword when it successfully forwards the message to another email address. Typical usage of this keyword is to show a different (or additional) icon for a message that has been forwarded. Once set the flag SHOULD NOT be cleared. Lemonade compliant servers MUST be able to store the $Forwarded keyword. They MUST preserve it on the COPY operation. The servers MUST support the SEARCH KEYWORD $Forwarded. 3. Message Submission LEMONADE compliant mail submission servers are expected to implement the following set of SMTP extensions to make message submission efficient. Lemonade clients SHOULD take advantage of these features. 3.1. Pipelining Mobile clients regularly use networks with a relatively high latency. Avoidance of round-trips within a transaction has a great advantage for the reduction in both bandwidth and total transaction time. For this reason LEMONADE compliant mail submission servers MUST support the SMTP Service Extensions for Command Pipelining [REF2197]. Clients SHOULD pipeline SMTP commands when possible. 3.2. DSN Support LEMONADE compliant mail submission servers MUST support SMTP service extensions for delivery status notifications [RFC3461]. 3.3. Message size declaration LEMONADE compliant mail submission servers MUST support the SMTP Service Extension for Message Size Declaration [RFC2927]. Note a LEMONADE compliant mail submission server must perform message size limit enforcement after performing "expansion" of all BURL parts. A LEMONADE compliant client SHOULD use message size declaration. In particular it SHOULD NOT send a message to a mail submission server, if the client knows that the message exceeds the maximal message size advertised by the submission server. 3.4. Enhanced status code Support LEMONADE compliant mail submission servers MUST support SMTP Service Extension for Returning Enhanced Error Codes [RFC2034]. 3.5. TLS LEMONADE Compliant mail submission servers MUST support SMTP Service Extension for Secure SMTP over TLS [SMTP-TLS]. 4. Quick resynchronization LEMONADE Compliant IMAP servers MUST support the CONDSTORE [CONDSTORE] extension. It allows a client to quickly resynchronize any mailbox by asking the server to return all flag changes that have occurred since the last known mailbox synchronization mark. [IMAP-DISC] shows how to perform quick mailbox resynchronization. 5. Additional IMAP extensions Lemonade compliant IMAP servers MUST support the NAMESPACE [NAMESPACE] extension. The extension allows clients to discover shared mailboxes and mailboxes belonging to other users. Lemonade compliant IMAP servers MUST support the LITERAL+ [LITERAL+] extension. The extension allows clients to save a round trip each time a non-synchronizing literal is sent. Lemonade compliant IMAP servers MUST support the IDLE [IDLE] extension. The extension allows clients to receive instant notifications about changes in the selected mailbox, without the need for constant polling for changes. LEMONADE Compliant IMAP servers MUST support IMAP over TLS [RFC3501] as required by RFC 3501. 6. Summary of the required IMAP and SMTP extensions ------------------------------------------------------ |Name of an SMTP extension| Comment | |-------------------------|--------------------------| | PIPELINING | Section 3.1 | |-------------------------|--------------------------| | DNS | Section 3.2 | |-------------------------|--------------------------| | SIZE | Section 3.3 | |-------------------------|--------------------------| | ENHANCEDSTATUSCODES | Section 3.4 | |-------------------------|--------------------------| | STARTTLS | Section 3.5 | |-------------------------|--------------------------| | BURL | Forward without download,| | | Section 2 | |-------------------------|--------------------------| | URLAUTH support in BURL | Section 2.5 | |-------------------------|--------------------------| | CHUNKING, | Section 2.5 | | BINARYMIME | Section 2.5 | |-------------------------|--------------------------| | 8BITMIME, | Required by BURL | |-------------------------|--------------------------| | AUTH | Required by Submission, | | | See [SMTPAUTH]. | |-------------------------|--------------------------| ------------------------------------------------------ |Name of an IMAP extension| Comment | | or feature | | |-------------------------|--------------------------| | NAMESPACE | Section 5 | |-------------------------|--------------------------| | CONDSTORE | Section 4 | |-------------------------|--------------------------| | STARTTLS |Required by IMAP (RFC3501)| |-------------------------|--------------------------| | URLAUTH, | Forward without download,| | CATENATE, | Section 2 | | UIDPLUS | | |-------------------------|--------------------------| | LITERAL+ | Section 5 | |-------------------------|--------------------------| | IDLE | Section 5 | |-------------------------|--------------------------| | $Forwarded IMAP keyword | Section 2.8 | |-------------------------|--------------------------| 7. Future work The Lemonade Working Group is looking into additional issues related to access of email from mobile devices, possibly including: - Media conversion (static and possibly streamed) - Transport optimization for low or costly bandwidth and less reliable mobile networks (e.g. quick reconnect) - Server to client notifications, possibly outside of the traditional IMAP band - Dealing with firewall and intermediaries - Compression and other bandwidth optimization - Filtering - Other considerations for mobile clients 8. Security Considerations Security considerations on Lemonade "forward without download" are discussed throughout section 2. Additional security considerations can be found in [RFC3501] and other documents describing other SMTP and IMAP extensions comprising the Lemonade Profile. Note that the mandatory to implement authentication mechanism for SMTP submission is described in [RFC2476]. The mandatory to implement authentication mechanism for IMAP is described in [RFC3501]. 8.1. Confidentiality Protection of Submitted Messages When clients submit new messages, link protection such as TLS guards against an eavesdropper seeing the contents of the submitted message. It's worth noting, however, that even if TLS is not used, the security risks are no worse if BURL is used to reference the text than if the text is submitted directly. If BURL is not used, an eavesdropper gains access to the full text of the message. If BURL is used, the eavesdropper may or may not be able to gain such access, depending on the form of BURL used. For example, some forms restrict use of the URL to an entity authorized as a submission server or a specific user. 8.2. TLS When LEMONADE clients use the BURL extension to mail submission, an extension that requires sending a URLAUTH token to the mail submission server, such a token should be protected from interception to avoid a replay attack that will disclose the contents of the message to an attacker. TLS based encryption of the mail submission path will provide protection against this attack. LEMONADE clients SHOULD use TLS protected IMAP and mail submission channels when using BURL-based message submission to protect the URLAUTH token from interception. LEMONADE compliant mail submission servers SHOULD use TLS protected IMAP connection when fetching message content using the URLAUTH token provided by the LEMONADE client. When a client uses SMTP STARTTLS to send a BURL command which references non-public information, there is a user expectation that the entire message content will be treated confidentially. To meet this expectation, the message submission server should use STARTTLS or a mechanism providing equivalent data confidentiality when fetching the content referenced by that URL. 9. IANA Considerations This document doesn't require any IANA registration or action. 10. References 10.1. Normative References [BURL] Newman, C. "Message Composition", draft-ietf-lemonade-burl- XX.txt (work in progress). [8BITMIME] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker, "SMTP Service Extension for 8bit-MIMEtransport", RFC 1652, July 1994. [CHUNKING] Vaudreuil, G., "SMTP Service Extensions for Transmission of Large and Binary MIME Messages", RFC 3030, December 2000. [CATENATE] Resnick, P. "Internet Message Access Protocol (IMAP) CATENATE Extension", draft-ietf-lemonade-catenate-XX.txt, (work in progress). [UIDPLUS] Crispin, M., "Internet Message Access Protocol (IMAP) - UIDPLUS extension", work in progress, draft-crispin-imap- rfc2359bis-XX.txt. [RFC2119] Brader, S. "Keywords for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. http://www.ietf.org/rfc/rfc2119 [RFC2197] Freed, N. "SMTP Service Extension for Command Pipelining", RFC 2197, September 1997. http://www.ietf.org/rfc/rfc2197 [RFC2476] Gellens, R. and Klensin, J., "Message Submission for Mail", draft-gellens-submit-bis-02.txt. [SMTP-TLS] Hoffman, P. "SMTP Service Extension for Secure SMTP over TLS ", RFC 3207, February 2002. http://www.ietf.org/rfc/rfc3207 [RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [RFC3501] Crispin, M. "IMAP4, Internet Message Access Protocol Version 4 rev1", RFC 3501, March 2003. http://www.ietf.org/rfc/rfc3501 [RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications (DSNs)", RFC 3461, January 2003. http://www.ietf.org/rfc/rfc3461 [URLAUTH] Crispin, M. and Newman, C., "Internet Message Access Protocol (IMAP) - URLAUTH Extension", draft-ietf-lemonade-urlauth- XX.txt, (work in progress). [RFC2034] Freed, N., "SMTP Service Extension for Returning Enhanced Error Codes", RFC 2034, October 1996. [NAMESPACE] Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342, May 1998. [SMTPAUTH] Myers, J., "SMTP Service Extension for Authentication", RFC 2554, March 1999. [LITERAL+] Myers, J., "IMAP4 non-synchronizing literals", RFC 2088, January 1997. [CONDSTORE] Melnikov, A. and S. Hole, "IMAP Extension for Conditional STORE", work in progress. [IDLE] Leiba, B., "IMAP4 IDLE command", RFC 2177, June 1997. 10.2. Informative References [IMAP-DISC] Melnikov, A. "Synchronization Operations For Disconnected Imap4 Clients", IMAP-DISC, work in progress, draft- melnikov-imap-disc-XX.txt Version History This section will be deleted before publication. Version 06: [1] Updated the reference to SMTP STARTTLS. [2] Updated the CATENATE example as per comments from Dave Cridland (message context, missing additional EHLO, etc.). [3] Added a new section showing use of BURL/BDAT for message assembly. [4] Added a requirement to support IMAP IDLE extension. [5] Added description of the $Forwarded IMAP keyword. [6] Added a requirement to support URLAUTH in BURL. [7] Mentioned mandatory to implement authentication in the Security Considerations. [8] Other editorial fixes from Randy and Greg. Version 05: [1] Removed any references to POSTADDRESS and quick reconnect. [2] Added reference to LITERAL+. [3] Added a new section about CONDSTORE. [4] Split TLS text between 3 sections. [5] Added new text that security of BURL is no worse than sending in the clear. [6] Added ";expire" to the URLAUTHs in the forward without download example. Version 04: [1] Removed future delivery from the phase 1 of the profile. [2] Updated the list of required SMTP and IMAP extensions and associated normative statements. [3] Updated the references. [4] Moved (and updated) text about TLS to the Security Considerations section. [5] Removed most editor's notes. [6] Proposed terminology Lemonade profile phase 1 (and later phases) to distinguish current status from future work. Version 03: [1] Updated boilerplate. [2] Addressed most of the comments raised by Randy Gellens and some from Pete Resnick. [3] Purged and updated references. [4] Updated examples as per changes in CATENATE and other documents. [5] Replaced Lemonade Pull model by Lemonade "forward without download". [6] Qualified normative statement on future delivery. Version 02: [1] Improved abstract based on review comments as well as change to reflect the re-organized content of the present Lemonade profile. [2] Editorial improvement of section 2.1 [3] Addition of section 2.5 with normative statements for lemonade compliant clients and servers regarding forward without download. [4] Addition of section 3 on message submission. [5] Move of media conversion to future work [6] Add section 4.1 on normative statements related to quick reconnect scheme. [6] Addition of Binary and 8-bit MIME Transport to future work. [7] Addition of IANA statement. [8] Update and fix of the references. Version 01: [1] We removed the sections of the profile related to mobile e-mail as well as discussion. This will be part of the next version of the Lemonade profile work. [2] We added detailed examples for the different steps included in section 2.4. [3] We added section 3 on media conversion. [4] We added examples on Quick reconnect schemes in section 4. [5] We updated the security considerations. [6] We fixed references based on updates above. [7] We added a future work section. [8] We fixed the boiler plate statements on the "status of this memo" and "Copyright". Acknowledgments This document is a product of Lemonade WG. The editors' thanks the Lemonade WG members that contributed comments and corrections, in particular: Randy Gellens, Dave Cridland and Greg Vaudreuil. This document borrows some text from draft-crispin-lemonade-pull- xx.txt as well as the trio [BURL], [CATENATE] and [URLAUTH]. Authors' Addresses Stephane H. Maes Oracle Corporation 500 Oracle Parkway M/S 4op634 Redwood Shores, CA 94065 USA Phone: +1-650-607-6296 Email: stephane.maes@oracle.com Alexey Melnikov Isode Limited 5 Castle Business Village 36 Station Road Hampton, Middlesex TW12 2BX UK Email: Alexey.melnikov@isode.com Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. The IETF has been notified of intellectual property rights claimed in regard to some or all of the specification contained in this document. For more information consult the online list of claimed rights. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society.
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draft-ietf-lemonade-profile-06
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