Internet Draft: POP3 Extension Mechanism R. Gellens Document: draft-gellens-pop3ext-00.txt QUALCOMM, Incorporated Expires: 12 August 1998 C. Newman Innosoft L. Lundblade QUALCOMM, Incorporated 12 February 1998 POP3 Extension Mechanism Status of this Memo: This document is an Internet Draft. 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. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress." To learn the current status of any Internet Draft, please check the "1id-abstracts.txt" listing contained in the Internet Drafts shadow directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). This document will expire before the end of August 1998. Distribution of this draft is unlimited. Copyright Notice Copyright (C) The Internet Society 1998. All Rights Reserved. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions Used in this Document . . . . . . . . . . . . . . . . 2 3. General Command Grammar . . . . . . . . . . . . . . . . . . . . . 2 4. Parameter and Response Lengths. . . . . . . . . . . . . . . . . . 3 5. The CAPA Command . . . . . . . . . . . . . . . . . . . . . . . . . 3 6. Initial Set of Capabilities . . . . . . . . . . . . . . . . . . . 4 6.1. TOP capability . . . . . . . . . . . . . . . . . . . . . . . . . 4 6.2. USER capability . . . . . . . . . . . . . . . . . . . . . . . . 5 6.3. SASL capability . . . . . . . . . . . . . . . . . . . . . . . . 5 6.4. LOGIN-DELAY capability. . . . . . . . . . . . . . . . . . . . . 5 6.5. PIPELINING capability . . . . . . . . . . . . . . . . . . . . . 6 6.6. LMOS capabilities . . . . . . . . . . . . . . . . . . . . . . . 7 Gellens, Newman, Lundblade [Page 1] Expires August 1998 Internet Draft POP3 Extension Mechanism February 1998 6.7. UIDL capability . . . . . . . . . . . . . . . . . . . . . . . . 8 6.8. IMPLEMENTATION capability . . . . . . . . . . . . . . . . . . . 8 7. Future Extensions to POP3 . . . . . . . . . . . . . . . . . . . . 9 8. Extended POP3 Response Codes. . . . . . . . . . . . . . . . . . . 9 8.1. Initial POP3 response codes . . . . . . . . . . . . . . . . . . 10 9. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . . 11 10. Security Considerations . . . . . . . . . . . . . . . . . . . . 12 11. References. . . . . . . . . . . . . . . . . . . . . . . . . . . 12 12. Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 13 13. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . 13 1. Introduction Post Office Protocol version 3 [POP3] is very widely used. However, while it includes some optional commands (and some useful protocol extensions have been published), it lacks a mechanism for advertising support for these extensions or for behavior variations. Currently these optional features and extensions can only be detected by probing, if at all. This is at best inefficient, and possibly worse. As a result, some clients have manual configuration options for POP3 server capabilities. Because one of the most important features of POP3 is its simplicity, it is not desirable to have a lot of extensions. However, some extensions are necessary (such as ones that provide improved security [POP-AUTH]), some are very desirable in certain situations, and a means for discovering server behavior is needed. This specification defines a mechanism to detect support for optional commands, extensions, and unconditional server behavior. Included is an initial set of currently implemented capabilities which vary between server implementations. This also extends POP3 error messages so that machine parsable codes can be provided to the client. 2. Conventions Used in this Document The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" in this document are to be interpreted as described in "Key words for use in RFCs to Indicate Requirement Levels" [KEYWORDS]. In examples, "C:" and "S:" indicate lines sent by the client and server respectively. 3. General Command Grammar Gellens, Newman, Lundblade [Page 2] Expires August 1998 Internet Draft POP3 Extension Mechanism February 1998 The general form of a POP3 command (described using [ABNF]): command ::= keyword *(SP param) CRLF ;255 octets maximum keyword ::= 3*4tchar param ::= 1*tchar tchar ::= %x30-39 / %x3C-7E ;"0"-"9" / "<"-"~" 4. Parameter and Response Lengths This specification increases the length restrictions on command parameters imposed by RFC 1939. The maximum length of a command is increased from 45 characters (4 character command, single space, 40 character argument) to 255 octets. The maximum length of a command response is 512 octets (including the terminating CRLF). 5. The CAPA Command The POP3 CAPA command returns a list of capabilities supported by the POP3 server. It is available in both the AUTHORIZATION and TRANSACTION states. Additional capabilities MAY become available in the TRANSACTION state, but all capabilities listed in AUTHORIZATION state MUST also be available. Each capability may enable additional protocol commands, additional parameters and responses for existing commands, or describe an aspect of server behavior. These details are specified in the description of the capability. CAPA Arguments: none Restrictions: none Discussion: An -ERR response indicates the capability command is not implemented and the client will have to probe for capabilities as before. An +OK response is followed by a list of capabilities, one per line. Each capability name MAY be followed by an "=" sign and arguments. The capability list is terminated by a line containing a termination octet (".") and a CRLF pair. Gellens, Newman, Lundblade [Page 3] Expires August 1998 Internet Draft POP3 Extension Mechanism February 1998 Possible Responses: +OK -ERR Examples: C: CAPA S: +OK Capability list follows S: TOP S: USER S: SASL=CRAM-MD5 KERBEROS_V4 S: LOGIN-DELAY=900 S: PIPELINING S: LMOS-NEW=60 S: LMOS-RETR=7 S: LMOS-TOP=20 S: UIDL S: IMPLEMENTATION="Shlemazle Plotz v302" S: . 6. Initial Set of Capabilities This section defines an initial set of POP3 capabilities. These include the optional POP3 commands, already published POP3 extensions, and behavior variations between POP3 servers which can impact clients. Note that there is no APOP capability, even though APOP is an optional command in [POP3]. Clients discover server support of APOP by the presence in the greeting banner of an initial challenge enclosed in angle brackets ("<>"). Therefore, an APOP capability would introduce two ways for a server to announce the same thing. 6.1. TOP capability CAPA tag: TOP Arguments: none Added commands: TOP Standard commands affected: none Discussion: The TOP capability indicates the optional TOP command is available. Gellens, Newman, Lundblade [Page 4] Expires August 1998 Internet Draft POP3 Extension Mechanism February 1998 6.2. USER capability CAPA tag: USER Arguments: none Added commands: USER PASS Standard commands affected: none Discussion: The USER capability indicates that the USER and PASS commands are supported, although they may not be available to all users. 6.3. SASL capability CAPA tag: SASL Arguments: Supported SASL mechanisms Added commands: AUTH Standard commands affected: none Discussion: The POP3 AUTHentication command [POP-AUTH] permits the use of [SASL] authentication mechanisms with POP3. The SASL capability indicates that the AUTH command is available and that it supports an optional base64 encoded second argument for an initial client response as described in the SASL specification. The argument to the SASL capability is a space separated list of SASL mechanisms which are supported. 6.4. LOGIN-DELAY capability CAPA tag: LOGIN-DELAY Arguments: minimum seconds between logins Added commands: Gellens, Newman, Lundblade [Page 5] Expires August 1998 Internet Draft POP3 Extension Mechanism February 1998 none Standard commands affected: none Discussion: POP3 clients often login frequently to check for new mail. Unfortunately, the process of creating a connection, authenticating the user, and opening the user's maildrop can be very resource intensive on the server. A number of deployed POP3 servers try to reduce server load by requiring a delay between logins. The LOGIN-DELAY capability includes an integer argument which indicates the number of seconds after an "+OK" response to a PASS, APOP, or AUTH command before another authentication will be accepted. Clients which permit the user to configure a mail check interval can use this capability to determine the minimum permissible interval. Servers which advertise LOGIN-DELAY SHOULD enforce it. 6.5. PIPELINING capability CAPA tag: PIPELINING Arguments: none Added commands: none Standard commands affected: none Discussion: The PIPELINING capability indicates the server is capable of accepting multiple commands at a time; the client does not have to wait for the response to a command before issuing a subsequent command. If a server supports PIPELINING, it MUST process each command in turn. If a client uses PIPELINING, it MUST keep track of which commands it has outstanding, and match server responses to commands in order. If either the client or server uses blocking writes, it MUST not exceed the window size of the underlying transport layer. Some POP3 clients have an option to indicate the server supports "Overlapped POP3 commands." This capability removes the need to configure this at the client. This is roughly synonymous with the ESMTP PIPELINING extension [PIPELINING]. Gellens, Newman, Lundblade [Page 6] Expires August 1998 Internet Draft POP3 Extension Mechanism February 1998 6.6. LMOS capabilities CAPA tags: LMOS-RETR LMOS-TOP LMOS-NEW Arguments: Retention period, in days, for a message category Added commands: none Standard commands affected: none Discussion: While POP3 allows clients to leave messages on the server, RFC 1939 warns about the problems that may arise from this, and allows servers to delete messages based on site policy. To quote from RFC 1939: Sites are free to establish local policy regarding the storage and retention of messages on the server, both read and unread. For example, a site might delete unread messages from the server after 60 days and delete read messages after 7 days. Such message deletions are outside the scope of the POP3 protocol and are not considered a protocol violation. ... It should be noted that enforcing site message deletion policies may be confusing to the user community, since their POP3 client may contain configuration options to leave mail on the server which will not in fact be supported by the server. One special case of a site policy is that messages may only be downloaded once from the server, and are deleted after this has been accomplished. This could be implemented in POP3 server software by the following mechanism: "following a POP3 login by a client which was ended by a QUIT, delete all messages downloaded during the session with the RETR command". ... Servers implementing a download-and-delete policy may also wish to disable or limit the optional TOP command, since it could be used as an alternate mechanism to download entire messages. The LMOS capabilities avoid the problems mentioned in RFC 1939, by allowing the server to inform the client as to the policy in effect. The argument to the LMOS capabilities indicate the server retention period, in days, for messages in a specific category. Zero indicates messages in that category are Gellens, Newman, Lundblade [Page 7] Expires August 1998 Internet Draft POP3 Extension Mechanism February 1998 immediately deleted. Each LMOS capability tag corresponds to a message category, that is, messages which have been the targets of a RETR or TOP command, or are new. The absence of a particular LMOS capability tag (in the presence of other LMOS capability tags) indicates no automatic deletion for messages in that category. Examples: LMOS-NEW=30 LMOS-RETR=5 LMOS-TOP=10 LMOS-NEW=60 LMOS-RETR=60 LMOS-TOP=60 LMOS-RETR=45 The first example set indicates the server deletes new (unseen) messages after 30 days, messages which have been downloaded using RETR after 5 days, and messages which have been examined using TOP after 10 days. The second example set specifies that the server deletes all messages after 60 days. The third example set indicates that messages downloaded using RETR are automatically deleted after 45 days, but messages in other categories are not automatically deleted. 6.7. UIDL capability CAPA tag: UIDL Arguments: none Added commands: UIDL Standard commands affected: none Discussion: The UIDL capability indicates that the UIDL command is supported. 6.8. IMPLEMENTATION capability CAPA tag: Gellens, Newman, Lundblade [Page 8] Expires August 1998 Internet Draft POP3 Extension Mechanism February 1998 IMPLEMENTATION Arguments: string giving server implementation information Added commands: none Standard commands affected: none Discussion: It is often useful to identify an implementation of a particular server (for example, when logging). This is commonly done in the welcome banner, but one must guess if a string is an implementation ID or not. The argument to the IMPLEMENTATION capability is a string, enclosed in double-quote marks, identifying the server. A server MAY include the implementation identification both in the welcome banner and in the IMPLEMENTATION capability. Clients MUST NOT modify their behavior based on the server implementation. Instead the server and client should agree on a private extension. 7. Future Extensions to POP3 Future extensions to POP3 are in general discouraged, as POP3's usefulness lies in its simplicity. Extensions which offer capabilities supplied by IMAP [IMAP4] or SMTP [SMTP] are strongly discouraged and unlikely to be permitted on the IETF standards track. Clients MUST NOT require the presence of any extension for basic functionality. Capabilities beginning with the letter "X" are reserved for experimental non-standard extensions and their use is discouraged. All other capabilities MUST be defined in a standards track or IESG approved experimental RFC. 8. Extended POP3 Response Codes POP3 is currently only capable of indicating success or failure to most commands. Unfortunately, clients often need to know more information about the cause of a failure in order to gracefully recover. This is especially important in response to a failed login (there are widely-deployed clients which attempt to decode Gellens, Newman, Lundblade [Page 9] Expires August 1998 Internet Draft POP3 Extension Mechanism February 1998 the error text of a PASS command result, to try and distinguish between "unable to get maildrop lock" and "bad login"). This specification amends the POP3 standard to permit an optional response code, enclosed in square brackets, at the beginning of the human readable text portion of a "+OK" or "-ERR" response. Clients supporting this extension MAY remove any information enclosed in square brackets prior to displaying human readable text to the user. Immediately following the open square bracket "[" character is a response code which is interpreted in a case-insensitive fashion by the client. The response code is hierarchical, with a "/" separating levels of detail about the error. Clients MUST ignore unknown hierarchical detail about the response code. This is important, as it could be necessary to provide further detail for response codes in the future. For example, ENCRYPT-NEEDED/TLS and ENCRYPT-NEEDED/SSH might indicate a suggestion to use the TLS or SSH protocols respectively for encryption. Examples: C: USER mrose S: -ERR [ENCRYPT-NEEDED] You need to activate encryption before logging in. 8.1. Initial POP3 response codes This specification defines some POP3 response codes which can be used to determine the reason for a failed login. Additional response codes MAY be defined by publication in an RFC (standards track or IESG approved experimental RFCs are preferred). LOGIN-DELAY This occurs on a -ERR response to an AUTH, USER, PASS or APOP command and indicates that the user has logged in recently and will not be allowed to login again until the login delay period has expired. PASS-EXPIRED This occurs on a -ERR response to an AUTH, USER, PASS or APOP command and indicates the user will not be allowed to login until his password/passphrase is changed. ENCRYPT-NEEDED This occurs on an -ERR response to an AUTH, USER or APOP command and indicates that the requested authentication mechanism is only permitted underneath a security layer. The client MAY take action to activate a security layer and repeat the same AUTH, USER or APOP command or try an AUTH command with a stronger mechanism. The client SHOULD record the fact that Gellens, Newman, Lundblade [Page 10] Expires August 1998 Internet Draft POP3 Extension Mechanism February 1998 encryption is needed for that user, server and mechanism combination. AUTH-TOO-WEAK This occurs on an -ERR response to an AUTH, USER or APOP command and indicates that the mechanism is too weak and is no longer permitted for that user by site policy. This allows a mechanism to be disabled on a per-user rather than a per-server level which is useful if different users have different security requirements or for transitioning from plaintext USER/PASS to a more secure mechanism. The client SHOULD record the fact that the user, server and mechanism combination is no longer permitted. TRANSITION-NEEDED This occurs on an -ERR response to an AUTH or APOP command. It indicates that the server has an entry for the specified user in a legacy authentication database but does not yet have credentials to offer the requested mechanism. A client which receives this error code MAY do a one-time login using the USER/PASS commands or another plaintext mechanism, which SHOULD be protected by a privacy layer, to initialize credentials for the requested mechanism. IN-USE This occurs on an -ERR response to an AUTH, APOP, or PASS command. It indicates the authentication was successful, but the user's maildrop is currently in use (probably by another POP3 client). 9. IANA Considerations This document requests that IANA maintain two new registries: POP3 capabilities and POP3 response codes. New POP3 capabilities MUST be defined in a standards track or IESG approved experimental RFC, and MUST NOT begin with the letter "X". New POP3 capabilities MUST include the following information: CAPA tag, arguments, added commands, standard commands affected, and discussion. In addition, new limits for POP3 command and response lengths may need to be included. New POP3 response codes MUST be defined in an IESG-approved RFC. Standards-track or experimental are preferred, but BCP or informational are permitted. New POP3 response codes MUST include the following information: the complete response code, for which responses (+OK or -ERR) and commands it is valid, and a definition of its meaning. Gellens, Newman, Lundblade [Page 11] Expires August 1998 Internet Draft POP3 Extension Mechanism February 1998 10. Security Considerations A capability list can reveal information about the server's authentication capabilities which can be used to determine if certain attacks will be successful. However, allowing clients to automatically detect availability of stronger mechanisms and alter their configurations to use them can improve overall security at a site. The TRANSITION-NEEDED error code can be inserted by an active attacker in an attempt to get the client to send the user's password unencrypted. Clients SHOULD prompt the user to get permission prior to transition. The additional error codes will allow gradual upgrading of security services on a per-user basis so they can improve overall security at a site. 11. References [ABNF] Crocker, D., Overell, P., "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, Internet Mail Consortium, Demon Internet Ltd., November 1997. <ftp://ds.internic.net/rfc/rfc2234.txt> [IMAP4] Crispin, M., "Internet Message Access Protocol - Version 4rev1", RFC 2060, University of Washington, December 1996. <ftp://ds.internic.net/rfc/rfc2060.txt> [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, Harvard University, March 1997. <ftp://ds.internic.net/rfc/rfc2119.txt> [PIPELINING] Freed, N., "SMTP Service Extension for Command Pipelining", RFC 2197, Innosoft, September 1997. <ftp://ds.internic.net/rfc/rfc2197.txt> [POP3] Myers, J., Rose, M., "Post Office Protocol -- Version 3", RFC 1939, Carnegie Mellon, Dover Beach Consulting, Inc., May 1996. <ftp://ds.internic.net/rfc/rfc1939.txt> [POP-AUTH] Myers, J., "POP3 AUTHentication command", work in progress, Netscape Communications, November, 1997. <ftp://ftp.isi.edu/internet-drafts/draft-myers-sasl-pop3-02.txt> [SASL] Myers, J., "Simple Authentication and Security Layer (SASL)", RFC 2222, Netscape Communications, October 1997. <ftp://ds.internic.net/rfc/rfc2222.txt> [SMTP] Postel, J, "Simple Mail Transfer Protocol", RFC 821, STD 10, Information Sciences Institute, August 1982. <ftp://ds.internic.net/rfc/rfc821.txt> Gellens, Newman, Lundblade [Page 12] Expires August 1998 Internet Draft POP3 Extension Mechanism February 1998 12. Full Copyright Statement Copyright (C) The Internet Society 1998. All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implmentation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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. 13. Authors' Addresses Randall Gellens +1 619 651 5115 QUALCOMM, Incorporated +1 619 651 5334 (fax) 6455 Lusk Blvd. randy@qualcomm.com San Diego, CA 92121-2779 USA Chris Newman chris.newman@innosoft.com Innosoft International, Inc. 1050 Lakes Drive West Covina, CA 91790 USA Laurence Lundblade +1 619 658 3584 QUALCOMM, Incorporated lgl@qualcomm.com 6455 Lusk Blvd. San Diego, Ca, 92121-2779 USA Gellens, Newman, Lundblade [Page 13] Expires August 1998