Internet Draft E. Allman draft-ietf-msgtrk-smtpext-02.txt Sendmail, Inc. Valid for six months T. Hansen Updates: RFC 1891 AT&T Laboratories July 6, 2001 SMTP Service Extension for Message Tracking Status of This Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also dis- tribute 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 This document is a submission by the MSGTRK Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted to the ietf-msgtrk@imc.org mailing list. An archive of the mailing list may be found at http://www.imc.org/ietf-msgtrk/index.html Distribution of this memo is unlimited. 1. Abstract This memo defines an extension to the SMTP service whereby a client may mark a message for future tracking. Internet Draft Message Tracking ESMTP Extension July 6, 2001 2. Other Documents and Conformance The model used for Message Tracking is described in [DRAFT- MTRK-MODEL]. Doing a Message Tracking query is intended as a "last resort" mechanism. Normally, Delivery Status Notifications (DSNs) [RFC- DSN-SMTP] and Message Disposition Notifications (MDNs) [RFC-MDN] would provide the primary delivery status. Only if the message is not received, or there is no response from either of these mecha- nisms should a Message Tracking query be issued. The definition of the base64 token is imported from section 6.8 of [RFC-MIME]. Syntax notation in this document conforms to [RFC-ABNF]. 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 RFC 2119 [RFC-KEYWORDS]. 3. SMTP Extension Overview The Message Tracking SMTP service extension uses the SMTP ser- vice extension mechanism described in [RFC-ESMTP]. The following service extension is hereby defined: (1) The name of the SMTP service extension is "Message Track- ing". (2) The EHLO keyword value associated with this extension is "MTRK". (3) No parameters are allowed with this EHLO keyword value. Future documents may extend this specification by specifying options. (4) One optional parameter using the keyword "MTRK" is added to the MAIL FROM command. In addition, the ENVID and ORCPT parameters (as defined in RFC 1891 sections 5.4 and 5.2 respectively) MUST be supported, with extensions as described below. (5) The maximum length of a MAIL FROM command line is increased by 40 characters by the possible addition of the MTRK key- word and value. Note that a further extension of 614 char- acters for the ORCPT and ENVID parameters is required by [RFC-DSN-EXT]. (6) No SMTP verbs are defined by this extension. Allman & Hansen [Page 2] Internet Draft Message Tracking ESMTP Extension July 6, 2001 4. The Extended MAIL FROM Command The extended MAIL FROM command is issued by an SMTP client when it wishes to inform an SMTP server that message tracking information should be retained for future querying. The extended MAIL FROM command is identical to the MAIL FROM command as defined in [RFC-SMTP], except that MTRK, ORCPT, and ENVID parameters appear after the address. 4.1. The MTRK parameter to the ESMTP MAIL command Any sender wishing to track a message must first tag that message as trackable by creating two values A and B: A = some-large-random-number B = SHA1(A) The large random number A is calculated on a host-dependent basis. See [RFC-RANDOM] for a discussion of choosing good ran- dom numbers. This random number MUST be at least 128 bits but MUST NOT be more than 1024 bits. The 128-bit hash B of A is then computed using the SHA-1 algorithm as described in [NIST-SHA1]. The sender then base64 encodes value B and passes that value as the mtrk-certifier on the MAIL FROM command: mtrk-parameter = "MTRK=" mtrk-certifier [ ":" mtrk-timeout ] mtrk-certifier = base64 ; authenticator mtrk-timeout = 1*9digit ; seconds until timeout A is stored in the originator's tracking database to vali- date future tracking requests as described in [DRAFT-MTRK-MTQP]. B is stored in tracking databases of compliant MTAs and used to authenticate future tracking requests. The mtrk-timeout field indicates the number of seconds that the client requests that this tracking information be retained on intermediate servers, as measured from the initial receipt of the message at that server. Servers MAY ignore this value if it violates local policy. In particular, servers MAY silently enforce an upper limit to how long they will retain tracking data; this limit MUST be at least one day. If no mtrk-timeout field is specified then the server should use a local default. This default SHOULD be 8-10 days and MUST be at least one day. Notwithstanding this clause, the information MUST NOT be expired while the message remains in the queue for this server: that is, an MTQP server MUST NOT deny knowledge of a message while that same message sits in the MTA queue. If the message is relayed to another compliant SMTP server, the MTA acting as the client SHOULD pass an mtrk-timeout field Allman & Hansen [Page 3] Internet Draft Message Tracking ESMTP Extension July 6, 2001 equal to the remaining life of that message tracking informa- tion. Specifically, the tracking timeout is decremented by the number of seconds the message has lingered at this MTA and then passed to the next MTA. If the decremented tracking timeout is less than or equal to zero, the entire MTRK parameter MUST NOT be passed to the next MTA; essentially, the entire tracking path is considered to be lost at that point. See [RFC-DELIVERYBY] section 4 for an explanation of why a timeout is used instead of an absolute time. 4.2. Use of ENVID To function properly, Message Tracking requires that each message have a unique identifier that is never reused by any other message. For that purpose, if the MTRK parameter is given, an ENVID parameter MUST be included, and the syntax of ENVID from RFC 1891 section 5.4 is extended as follows: envid-parameter = "ENVID=" unique-envid unique-envid = local-envid "@" fqhn local-envid = xtext fqhn = xtext The unique-envid MUST be chosen in such a way that the same ENVID will never be used by any other message sent from this system or any other system. In most cases, this means setting fqhn to be the fully qualified host name of the system generat- ing this ENVID, and local-envid to an identifier that is never re-used by that host. Any resubmissions of this message into the message trans- mission system MUST assign a new ENVID. In this context, "resubmission" includes forwarding or resending a message from a user agent, but does not include MTA-level aliasing or forward- ing where the message does not leave and re-enter the message transmission system. 4.3. Forwarding Tracking Certifiers MTAs SHOULD forward unexpired tracking certifiers to com- pliant mailers as the mail is transferred during regular hop-to- hop transfers. If the "downstream" MTA is not MTRK-compliant, then the MTRK= parameter MUST be deleted. If the downstream MTA is DSN-compliant, then the ENVID and ORCPT parameters MUST NOT be deleted. If aliasing, forwarding, or other redirection of messages to a single recipient occurs, then the MTA SHOULD treat this as an ordinary hop-to-hop transfer and forward the MTRK=, ENVID=, and ORCPT= values; these values MUST NOT be modified. MTAs MUST NOT copy MTRK certifiers when relaying a message to multiple recipients. An MTA MAY designate one recipient in a multi-recipient alias as the "primary" recipient to which track- ing requests shall be forwarded; other addresses SHALL NOT Allman & Hansen [Page 4] Internet Draft Message Tracking ESMTP Extension July 6, 2001 receive tracking certifiers. MTAs MUST NOT forward MTRK certi- fiers when doing mailing list expansion. 5. Security Issues 5.1. Denial of service An attacker could attempt to flood the database of a server by submitting large numbers of small, tracked messages. In this case, a site may elect to lower its maximum retention period retroactively. 5.2. Confidentiality The mtrk-authenticator value (``A'') must be hard to pre- dict and not reused. The originating client must take reasonable precautions to protect the secret. For example, if the secret is stored in a message store (e.g., a "Sent" folder), the client must make sure the secret isn't accessible by attackers, particularly on a shared store. Many site administrators believe that concealing names and topologies of internal systems and networks is an important security feature. MTAs need to balance such desires with the need to provide adequate tracking information. In some cases site administrators may want to treat deliv- ery to an alias as final delivery in order to separate roles from individuals. For example, sites implementing ``postmas- ter'' or ``webmaster'' as aliases may not wish to expose the identity of those individuals by permitting tracking through those aliases. In other cases, providing the tracking informa- tion for an alias is important, such as when the alias points to the user's preferred public address. 6. References [DRAFT-MTRK-MODEL] T. Hansen, ``Message Tracking Model and Requirements.'' draft-ietf-msgtrk-model-03.txt. November 2000. [DRAFT-MTRK-MTQP] T. Hansen, ``Message Tracking Query Protocol.'' draft-ietf- msgtrk-mtqp-01.txt. November 2000. [RFC-ABNF] Crocker, D., Editor, and P. Overell, ``Augmented BNF for Syn- tax Specifications: ABNF'', RFC 2234, November 1997. [RFC-DELIVERYBY] D. Newman, ``Deliver By SMTP Service Extension.'' RFC 2852. June 2000. Allman & Hansen [Page 5] Internet Draft Message Tracking ESMTP Extension July 6, 2001 [RFC-DSN-REPT] G. Vaudreuil, ``The Multipart/Report Content Type for the Reporting of Mail System Administrative Messages.'' RFC 1892. January 1996. [RFC-DSN-SMTP] K. Moore, ``SMTP Service Extension for Delivery Status Notifi- cations.'' RFC 1891. January 1996. [RFC-DSN-STAT] K. Moore and G. Vaudreuil, ``An Extensible Message Format for Delivery Status Notifications.'' RFC 1894. January 1996. [RFC-EMSSC] G. Vaudreuil, ``Enhanced Mail System Status Codes.'' RFC 1893. January 1996. [RFC-ESMTP] Rose, M., Stefferud, E., Crocker, D., Klensin, J. and N. Freed, ``SMTP Service Extensions.'' STD 10, RFC 1869. Novem- ber 1995. [RFC-KEYWORDS] S. Bradner, ``Key words for use in RFCs to Indicate Require- ment Levels.'' RFC 2119. March 1997. [RFC-MDN] R. Fajman, ``An Extensible Message Format for Message Disposi- tion Notifications.'' RFC 2298. March 1998. [RFC-MIME] N. Freed and N. Borenstein, ``Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bod- ies.'' RFC 2045. November 1996. [RFC-MSGFMT] D. Crocker, ``Standard for the Format of ARPA Internet Text Messages.'' RFC 822. August 1982. [RFC-RANDOM] D. Eastlake, S. Crocker, and J. Schiller, ``Randomness Recom- mendations for Security.'' RFC 1750. December 1994. [RFC-RELATED] E. Levinson, ``The MIME Multipart/Related Content-type.'' RFC 2387. August 1998. [NIST-SHA1] NIST FIPS PUB 180-1, ``Secure Hash Standard.'' National Institute of Standards and Technology, U.S. Department of Com- merce. May 1994. DRAFT. [RFC-SMTP] J. Postel, ``Simple Mail Transport Protocol.'' RFC 821. August 1982. Allman & Hansen [Page 6] Internet Draft Message Tracking ESMTP Extension July 6, 2001 7. Authors' Addresses Eric Allman Sendmail, Inc. 6603 Shellmound Emeryville, CA 94608 U.S.A. E-Mail: eric@Sendmail.COM Phone: +1 510 594 5501 Fax: +1 510 594 5411 Tony Hansen AT&T Laboratories Lincroft, NJ 07738 U.S.A. Phone: +1 732 576 3207 E-Mail: tony@att.com Allman & Hansen [Page 7]