Movatterモバイル変換

Network Working Group                                          E. AllmanRequest for Comments: 3885                                Sendmail, Inc.Updates:3461                                                  T. HansenCategory: Standards Track                              AT&T Laboratories                                                          September 2004SMTP Service Extensionfor Message TrackingStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2004).Abstract   This memo defines an extension to the SMTP service whereby a client   may mark a message for future tracking.1.  Other Documents and Conformance   The model used for Message Tracking is described in [RFC-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 mechanisms   should a Message Tracking query be issued.   The definition of the base64 token is imported from section 6.8 of   [RFC-MIME].  Formally,      base64 =  %x2b / %x2f / %x30-39 / %x41-5a / %x61-7aAllman & Hansen             Standards Track                     [Page 1]

RFC 3885            Message Tracking ESMTP Extension      September 2004   The definition of the DIGIT token is imported from [RFC-MSGFMT].   Formally,      DIGIT =        %x30-39   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 inBCP 14,RFC 2119   [RFC-KEYWORDS].2.  SMTP Extension Overview   The Message Tracking SMTP service extension uses the SMTP service   extension mechanism described in [RFC-ESMTP].  The following service   extension is hereby defined:   (1)   The name of the SMTP service extension is "Message Tracking".   (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         parameters to this keyword value.   (4)   One optional parameter using the keyword "MTRK" is added to the         MAIL command.  In addition, the ENVID parameter of the MAIL         command (as defined inRFC 3461) MUST be supported, with         extensions as described below.  The ORCPT parameter of the RCPT         command (as defined inRFC 3461) MUST also be supported.  All         semantics associated with ENVID and ORCPT described inRFC 3461         MUST be supported as part of this extension.   (5)   The maximum length of a MAIL command line is increased by 40         characters by the possible addition of the MTRK keyword and         value.  Note that the 507 character extension of RCPT commands         for the ORCPT parameter and the 107 character extension of MAIL         commands for the ENVID parameter as mandated byRFC 3461 [RFC-         DSN-SMTP] must also be included.   (6)   No SMTP verbs are defined by this extension.Allman & Hansen             Standards Track                     [Page 2]

RFC 3885            Message Tracking ESMTP Extension      September 20043.  The Extended MAIL Command   The extended MAIL 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 command is identical   to the MAIL command as defined in [RFC-SMTP], except that MTRK,   ORCPT, and ENVID parameters appear after the address.3.1.  The MTRK parameter to the ESMTP MAIL command   Any sender wishing to request the retention of data for further   tracking of 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 random 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 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 validate future   tracking requests as described in [RFC-MTRK-MTQP].  B is stored in   tracking databases of compliant receiver 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 beAllman & Hansen             Standards Track                     [Page 3]

RFC 3885            Message Tracking ESMTP Extension      September 2004   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 equal to the   remaining life of that message tracking information.  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.3.2.  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 fromRFC 3461 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 generating this ENVID, and local-   envid to an identifier that is never re-used by that host.   In some cases, the total length of (local-envid + fqhn + 1) (for the   `@' sign) may exceed the total acceptable length of ENVID (100).  In   this case, the fqhn SHOULD be replaced by the SHA1(fqhn) encoded into   BASE64.  After encoding, the 160 bit SHA-1 will be a 27 octet string,   which limits local-envid to 72 octets.  Implementors are encouraged   to use an algorithm for the local-envid that is reasonably unique.   For example, sequential integers have a high probability of   intersecting with sequential integers generated by a different host,   but a SHA-1 of the current time of day concatenated with the host's   IP address and a random number are unlikely to intersect with the   same algorithm generated by a different host.Allman & Hansen             Standards Track                     [Page 4]

RFC 3885            Message Tracking ESMTP Extension      September 2004   Any resubmissions of this message into the message transmission   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 forwarding where the message   does not leave and re-enter the message transmission system.3.3.  Forwarding Tracking Certifiers   MTAs SHOULD forward unexpired tracking certifiers to compliant   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 a recipient occurs,   and the result of the redirection is exactly one recipient, 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 except for decrementing the mtrk-timeout field of the MTRK=   value, which MUST be modified as described insection 4.1 above.   MTAs MUST NOT copy MTRK certifiers when a recipient is aliased,   forwarded, or otherwise redirected and the redirection results in   more than one recipient.  However, an MTA MAY designate one of the   multiple recipients as the "primary" recipient to which tracking   requests shall be forwarded; other addresses MUST NOT receive   tracking certifiers.  MTAs MUST NOT forward MTRK certifiers when   doing mailing list expansion.4.  Security Considerations4.1.  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.4.2.  Confidentiality   The mtrk-authenticator value ("A") must be hard to predict 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.Allman & Hansen             Standards Track                     [Page 5]

RFC 3885            Message Tracking ESMTP Extension      September 2004   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 delivery to an   alias as final delivery in order to separate roles from individuals.   For example, sites implementing "postmaster" 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 information for an alias is important, such as when the   alias points to the user's preferred public address.   Therefore, implementors are encouraged to provide mechanisms by which   site administrators can choose between these alternatives.5.  IANA Considerations   IANA has registered the SMTP extension defined insection 3.6.  Acknowledgements   Several individuals have commented on and enhanced this document,   including Philip Hazel, Alexey Melnikov, Lyndon Nerenberg, Chris   Newman, and Gregory Neil Shapiro.7.  References7.1.  Normative References   [RFC-MTRK-MODEL]   Hansen, T., "Message Tracking Model and                      Requirements",RFC 3888, September 2004.   [RFC-MTRK-MTQP]    Hansen, T., "Message Tracking Query Protocol",RFC3887, September 2004.   [RFC-ABNF]         Crocker, D., Ed. and P. Overell, "Augmented BNF                      for Syntax Specifications: ABNF",RFC 2234,                      November 1997.   [RFC-ESMTP]        Klensin, J., Freed, N., Rose, M., Stefferud, E.,                      and D. Crocker, "SMTP Service Extensions", STD 10,RFC 1869, November 1995.   [RFC-KEYWORDS]     Bradner, S., "Key words for use in RFCs to                      Indicate Requirement Levels",BCP 14,RFC 2119,                      March 1997.Allman & Hansen             Standards Track                     [Page 6]

RFC 3885            Message Tracking ESMTP Extension      September 2004   [RFC-MIME]         Freed, N. and N. Borenstein, "Multipurpose                      Internet Mail Extensions (MIME) Part One: Format                      of Internet Message Bodies",RFC 2045, November                      1996.   [NIST-SHA1]        NIST FIPS PUB 180-1, "Secure Hash Standard"                      National Institute of Standards and Technology,                      U.S. Department of Commerce, May 1994.   [RFC-SMTP]         Klensin, J., Ed., "Simple Mail Transfer Protocol",RFC 2821, April 2001.   [RFC-MSGFMT]       Resnick, P., Ed., "Internet Message Format",RFC2822, April 2001.7.2.  Informational References   [RFC-DELIVERYBY]   Newman, D., "Deliver By SMTP Service Extension",RFC 2852, June 2000.   [RFC-DSN-SMTP]     Moore, K., "Simple Mail Transfer Protocol (SMTP)                      Service Extension for Delivery Status                      Notifications (DSNs)",RFC 3461, January 2003.   [RFC-MDN]          Hansen, T. and G. Vaudreuil, Eds., "Message                      Disposition Notification",RFC 3798, May 2004.   [RFC-RANDOM]       Eastlake, D., Crocker, S., and J. Schiller,                      "Randomness Recommendations for Security",RFC1750, December 1994.Allman & Hansen             Standards Track                     [Page 7]

RFC 3885            Message Tracking ESMTP Extension      September 20048.  Authors' Addresses   Eric Allman   Sendmail, Inc.   6425 Christie Ave, 4th Floor   Emeryville, CA  94608   U.S.A.   Phone: +1 510 594 5501   Fax: +1 510 594 5429   EMail: eric@Sendmail.COM   Tony Hansen   AT&T Laboratories   Middletown, NJ 07748   U.S.A.   Phone: +1 732 420 8934   EMail: tony+msgtrk@maillennium.att.comAllman & Hansen             Standards Track                     [Page 8]

RFC 3885            Message Tracking ESMTP Extension      September 20049.  Full Copyright Statement   Copyright (C) The Internet Society (2004).  This document is subject   to the rights, licenses and restrictions contained inBCP 78, and   except as set forth therein, the authors retain all their rights.   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.Intellectual Property   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 inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Allman & Hansen             Standards Track                     [Page 9]