Internet Draft: On-Demand Mail Relay                         R. Gellens
Document: draft-gellens-on-demand-05.txt                       QUALCOMM
Expires: 7 January 1999                                     7 July 1998

                      ON-DEMAND MAIL RELAY (ODMR)
                     SMTP with Dynamic IP Addresses

Status of this Memo:

    This document is an Internet Draft.  Internet Drafts are working
    documents of the Internet Engineering Task Force (IETF), its Areas,
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    A version of this draft document is intended for submission to the
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Copyright Notice

    Copyright (C) The Internet Society 1998.  All Rights Reserved.


    With the spread of low-cost computer systems and Internet
    connectivity, the demand for local mail servers had been rising.
    Many people now want to operate a mail server on a system which has
    only an intermittent connection to a service provider.  If the
    system has a static IP address, the [ESMTP] [ETRN] command can be
    used.  However, systems with dynamic IP addresses (which are very
    common with low-cost connections) have no widely-deployed solution.

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    This memo proposes a new service, On-Demand Mail Relay (ODMR), which
    is a profile of [ESMTP], providing for a secure, extensible, easy to
    implement approach to the problem.

1.  Conventions Used in this Document

    Because the client and server roles reverse during the session, to
    avoid confusion, the terms "customer" and "provider" will be used in
    place of "client" and "server", although of course this protocol may
    be useful in cases other than commercial service providers and

    In examples, "P:" is used to indicate lines sent by the provider,
    and "C:" indicates those sent by the customer.  Line breaks within a
    command are for editorial purposes only.

    The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
    in this document are to be interpreted as defined in [KEYWORDS].

2.  Description

    On-Demand Mail Relay is a restricted profile of SMTP.  Port number
    366 is reserved for On-Demand Mail Relay.  The initial client and
    server roles are short-lived, as the point is to allow the
    intermittently-connected host to request mail held for it by a
    service provider.

    The customer initiates a connection to the provider, authenticates,
    and requests its mail.  The roles of client and server then reverse,
    and normal [ESMTP] proceeds.

    The provider has an On-Demand Mail Relay process listening for
    connections on the ODMR port.  This process does not need to be a
    full SMTP server.  It does need to be an SMTP client with access to
    the outgoing mail queues, and as a server implement the EHLO, AUTH,
    ATRN, and QUIT commands.

    An MTA normally has a mail client component which processes the
    outgoing mail queues, attempting to send mail for particular
    domains, based on time or event (such as new mail being placed in
    the queue, or receipt of an ETRN command by the SMTP server
    component).  The On-Demand Mail Relay service processes the outgoing
    queue not on a timer or new mail creation, but on request.

    The provider side has normal SMTP server responsibilities, including
    generation of relay or failure DSNs as needed.

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3.  States

    The On-Demand Mail Relay service has three states: an initial state,
    an authenticated state, and a reversed state.  The state progression
    is illustrated in the following diagram:

!      initial state      !
  !             !
QUIT           AUTH
  !             !
  !             V
  !      -----------------------
  !      ! authenticated state !
  !      -----------------------
  !       !            !
  !      QUIT         ATRN
  !       !            !
  !       !            V
  !       !      ------------------
  !       !      ! reversed state !
  !       !      ------------------
  !       !         !
  !       !        QUIT
  !       !         !
  V       V         V
  !    termination    !

3.1.  Initial State

    In the initial state, the provider is the server and the customer is
    the client.  Three commands are valid:  EHLO, AUTH, and QUIT.

3.1.1.  EHLO

    The EHLO command is the same as in [ESMTP].  The response must
    include AUTH and ATRN.

3.1.2.  AUTH

    The AUTH command is specified in [AUTH].  The AUTH command uses a
    [SASL] mechanism to authenticate the session.  The session is not
    considered authenticated until a success response to AUTH has been

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    For interoperability, implementations MUST support the CRAM-MD5
    mechanism [CRAM].  Other SASL mechanisms may be supported.  A site
    MAY disable CRAM-MD5 support if it uses more secure methods.  The
    EXTERNAL mechanism [SASL] might be useful in some cases, for
    example, if the provider has already authenticated the client, such
    as during a PPP connection.

3.1.3.  QUIT

    The QUIT command is the same as in [SMTP].

3.2.  Authenticated State

    The authenticated state is entered after a successful AUTH command.
    Two commands are valid in the authenticated state:  ATRN and QUIT.

3.2.1.  ATRN (Authenticated TURN)

    Unlike the TURN command in [SMTP], the ATRN command optionally takes
    one or more domains as a parameter.  The ATRN command MUST be
    rejected if the session has not been authenticated.  Response code
    505 should be used for this.

    The timeout for this command MUST be at least 10 minutes to allow
    the provider time to process its mail queue.

    An ATRN command sent with no domains is equivalent to an ATRN
    command specifying all domains to which the customer has access.

    If the authentication used by the customer does not provide access
    to all of the domains specified in ATRN, the provider MUST NOT send
    mail for any domains to the customer; the provider MUST reject the
    ATRN command with a 450 code.

    If the customer does have access to all of the specified domains,
    but none of them have any queued mail, the provider normally rejects
    the ATRN command with response code 453.  The provider MAY use
    response code 450, to avoid disclosing information to unauthorized
    parties regarding the domains for which it provides ODMR service.
    Or, the provider MAY issue a 250 success code, and after the roles
    are reversed, send a QUIT following the EHLO.

    The provider MAY also reject the ATRN command with a 450 response to
    indicate refusal to accept multiple requests issued within a
    particular time interval.

    If the customer has access to all of the specified domains and mail
    exists in at least one of them, the provider issues a 250 success

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    If the server is unable to verify access to the requested domains
    (for example, a mapping database is temporarily unavailable),
    response code 451 is sent.

    [ABNF] for ATRN:

    atrn          = "ATRN" [domain *("," domain)]

    domain        = sub-domain 1*("." sub-domain)

    sub-domain    = (ALPHA / DIGIT) *(ldh-str)

    ldh-str       = *(ALPHA / DIGIT / "-") (ALPHA / DIGIT)

3.3.  Reversed State

    After the provider has sent a success reply to the ATRN command, the
    roles reverse, and the customer becomes the server, and the provider
    becomes the client.  At this point normal [ESMTP] commands are used.
    Typically the provider sends EHLO immediately following the success
    response to ATRN, to be followed by MAIL FROM and so on.

3.4.  Other Commands

    The provider MAY reject all commands other than EHLO, AUTH, ATRN,
    and QUIT with response code 502.

4.  Example On-Demand Mail Relay Session:
      P:  220 ISP.NET on-demand mail relay server ready
      C:  EHLO
      P:  250-AUTH CRAM-MD5 Kerberos-v5
      P:  250 ATRN
      C:  AUTH CRAM-MD5
      P:  334 MTg5Ni42OTcxNzA5NTJASVNQLkNPTQo=
      C:  Zm9vYmFyLm5ldCBiOTEzYTYwMmM3ZWRhN2E0OTViNGU2ZTczMzRkMzg5MAo=
      P:  235 now authenticated as
      C:  ATRN,
      P:  250 OK now reversing the connection
      P:  EHLO ISP.NET
      C:  250 SIZE
      P:  MAIL FROM: <>
      C:  250 OK
      P:  RCPT TO: <>
      C:  250 OK, recipient accepted
      P:  QUIT
      C:  221 closing connection

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5.  Response Codes

    The response codes used in this document are:

    250  Requested mail action okay, completed
    450  ATRN request refused
    451  Unable to process ATRN request now
    453  You have no mail
    502  Command not implemented
    505  Authentication required

6.  Security Considerations

    Because access to the On-Demand Mail Relay server is only useful
    with a prior arrangement between the parties (so the provider is the
    target of MX records for the customer's domains and thus has mail to
    relay), it may be useful for the provider to restrict access to the
    On-Demand Mail Relay port.  For example, a TCP wrapper or firewall
    could be used to block access to port 366 except within the
    provider's network.  This might be useful when the provider is the
    customer's ISP.  Use of such mechanisms does not reduce the need for
    the AUTH command, however, but can increase the security it

    Use of SASL in the AUTH command allows for substitution of more
    secure authentication mechanisms in the future.

    See sections 3.1.2 and 3.2.1 for additional security details.

7.  Acknowledgments

    This draft has been developed in part based on comments and
    discussions which took place on and off the IETF-disconn-smtp
    mailing list.  Special thanks to Chris Newman and Ned Freed for
    their comments.

8.  References

    [ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications:
    ABNF", RFC 2234, Internet Mail Consortium, Demon Internet Ltd.,
    November 1997. <>

    [AUTH] Myers, J., "SMTP Service Extension for Authentication", (work
    in progress),

    [CODES-EXTENSION] Freed, N., "SMTP Service Extension for Returning
    Enhanced Error Codes", RFC 2034, October 1996,

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    [CRAM] Klensin, Catoe, Krumviede, "IMAP/POP AUTHorize Extension for
    Simple Challenge/Response", RFC 2195, September 1997,

    [ESMTP] Klensin, Freed, Rose, Stefferud, E., and D. Crocker, "SMTP
    Service Extensions", RFC 1869, STD 10, November 1995,

    [ETRN] De Winter, J., "SMTP Service Extension for Remote Message
    Queue Starting", RFC 1985, August 1996,

    [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
    Requirement Levels", RFC 2119, March 1997,

    [SASL] Myers, J., "Simple Authentication and Security Layer (SASL)",
    RFC 2222, October 1997, <>

    [SMTP-CODES] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
    1893, January 1996, <>

    [SMTP] Postel, J., "Simple Mail Transfer Protocol", RFC 821, STD 10,
    August 1982, <>

9.  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 implementation 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

    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

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10.  Author's Address

    Randall Gellens                    +1.619.651.5115
    Qualcomm, Inc.                     +1.619.651.5334 (fax)
    6455 Lusk Blvd.                    Randy@Qualcomm.Com
    San Diego, CA  92121-2779

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