Network Working Group N. Freed
Request for Comments: 2442 D. Newman
Category: Informational Innosoft
J. Belissent
Sun Microsystems
M. Hoy
Mainbrace
November 1998
The
Batch SMTP
Media Type
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This document defines a MIME content type suitable for tunneling an
ESMTP [RFC-821, RFC-1869] transaction through any MIME-capable
transport. This type can be used for a variety of purposes,
including: Extending end-to-end MIME-based security services (e.g.,
[RFC-1847]) to cover message envelope information as well as message
content. Making it possible to use specific SMTP extensions sUCh as
NOTARY [RFC-1891] over unextended SMTP transport infrastructure.
Enabling the transfer of multiple separate messages in a single
transactional unit.
Requirements Notation
This document occasionally uses terms that appear in capital letters.
When the terms "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
appear capitalized, they are being used to indicate particular
requirements of this specification. A discussion of the meanings of
the terms "MUST", "SHOULD", and "MAY" appears in [RFC-1123]; the
terms "MUST NOT" and "SHOULD NOT" are logical extensions of this
usage.
The Application/batch-SMTP Content Type
The "application/batch-SMTP" MIME content type is a container for the
client side of an SMTP or ESMTP transaction. In keeping with
traditional SMTP, the contents are line oriented and CRLF line
terminators MUST be used.
The "application/batch-SMTP" type is defined as follows:
Media type name: application
Media suBType name: batch-SMTP
Required parameters: none
Optional parameters: required-extensions
Encoding considerations:
8bit material may appear, so quoted-printable or base64
encoding may be necessary on transports that do not
support 8bit. While the content of this type is
line-oriented and uses conventional CR/LF terminators,
lines longer than 7bit and 8bit encodings allow (998
octets) may appear, hence quoted-printable or
base64 encoding may be necessary even in conjunction
with 8bit transports.
Security considerations:
Discussed in the Security Considerations Section.
How application/batch-SMTP is used
The following diagram illustrates how the application/batch-SMTP type
is intended to be used:
application/batch-SMTP object
+----------------+
+-----------+ v +----------+ v +-----------+
batch MIME- batch
=> SMTP => capable => SMTP =>
generator transport processor
^ +-----------+ +----------+ +-----------+ ^
+-- conventional SMTP/RFC822 message transaction --+
A conventional SMTP message transaction is converted into an
application/batch-SMTP object by the batch SMTP generator. This
object is then carried over some type of MIME-capable transport. Once
the destination is reached the object is presented to a batch SMTP
processor, which converts the application/batch-SMTP object back into
a conventional SMTP message transaction.
Generation of application/batch-SMTP material
Application/batch-SMTP material is generated by a specially modified
SMTP client operating without a corresponding SMTP server. The client
simply assumes a successful response to all commands it issues. The
resulting content then consists of the collected output from the SMTP
client.
Honoring SMTP restrictions
Most batch SMTP processors will be constructed by modifying and
extending existing SMTP servers. As such, all of the restrictions on
SMTP constructs imposed by RFC821, RFC1123, and RFC1869 MUST be
observed. In particular, restrictions on command and data line
lengths, number of recipients, and so on still exist and apply to
batch SMTP.
Use of SMTP Extensions
Since no SMTP server is present the client must be prepared to make
certain assumptions about which SMTP extensions can be used. The
generator MAY assume that ESMTP [RFC-1869] facilities are available,
that is, it is acceptable to use the EHLO command and additional
parameters on MAIL FROM and RCPT TO. If EHLO is used MAY assume that
the 8bitMIME [RFC-1652], SIZE [RFC-1870], and NOTARY [RFC-1891]
extensions are available. In particular, NOTARY SHOULD be used. MAY
create private bilateral agreements which specify the availability of
additional SMTP extensions. Additional SMTP extensions MUST NOT be
used in the absence of such an agreement, and, perhaps more
importantly, a conformant generation of application/batch-SMTP
objects MUST be able to produce objects restricted to use of the
extensions listed above.
The "required-extensions" content type parameter MAY be used to
communicate a list of the extensions actually used, specified as a
comma-separated list of EHLO responses. If absent it defaults to the
list "8bitMIME,SIZE,NOTARY". Any use by private bilateral agreement
of additional or different extensions MUST be noted in the
"required-extensions" parameter.
Note that many SMTP extensions simply do not make sense in the
context of batch SMTP. For example, the pipelining extension [RFC-
2197] makes no sense in the absence of a network connection.
Handling Multiple Messages
Generators SHOULD attempt to minimize the number of messages placed
in a single application/batch-SMTP object. Ideally a single
application/batch-SMTP object will be created for each message. Note,
however, that some uses of application/batch-SMTP (e.g., mail
bagging) may exist solely to take advantage of the multiple messages
in a single container capability of batch SMTP, so requiring one
message per container is not possible.
DISCUSSION: The SMTP protocol provides for the transfer of a series
of messages over a single connection. This extends in a natural way
to batch SMTP. However, the issues in batch SMTP are somewhat
different. Suppose, for example, that a batch SMTP processor receives
an application/batch-SMTP object containing two messages but is
unable to process the second message because of a storage allocation
failure. But suppose that not only does this failure preclude
processing of the second message, it also precludes recording that
the first message has already been processed. Subsequent reprocessing
of the application/batch-SMTP could then lead to duplication of the
first message.
This issue is not materially different from the well-known problems
with SMTP synchronization that in practice often lead to duplicated
messages. Since this behavior is inherent in SMTP to begin with it is
not incumbent on application/batch-SMTP to completely address the
issue. Nevertheless, it seems prudent for application/batch-SMTP to
try and not make matters even worse.
Transport of application/batch-SMTP objects
Application/batch-SMTP objects may be transported by any transport
capable of preserving their MIME labelling, e.g., HTTP or SMTP.
Transports MUST remain cognizant of the special nature of
application/batch-SMTP. An application/batch-SMTP object contains one
or more "frozen" SMTP message transactions. SMTP message transactions
typically carry with them various assumptions about quality of
service, e.g., that messages will either be delivered successfully or
a nondelivery notification will be returned, that a nondelivery
notification will be returned if delivery cannot be accomplished in a
timely fashion, and so on. It is vital that the encapsulation of
these objects for carriage over other forms of transport not
interfere with these capabilities.
Processing of application/batch-SMTP material
Processing of application/batch-SMTP material is considerably more
complex than generating it. As might be eXPected, a modified
SMTP/ESMTP processor is used. However, since it cannot return
information to the client, it must handle all error conditions that
arise itself. In other Words, a batch SMTP processor assumes both the
responsibilities of a traditional SMTP server as well as part of the
responsibilities of a traditional SMTP client.
As such, a conforming processor: MUST check MIME content type
information to insure that the material it has been presented with is
labelled as application/batch-SMTP and doesn't specify any extensions
the processor doesn't support in the "required-extensions" parameter.
Application/batch-SMTP objects that employ an unsupported extension
SHOULD be forwarded to the local postmaster for manual inspection and
handling. MUST accept any syntactically valid EHLO or HELO command.
MUST accept any syntactically valid MAIL FROM command. A conforming
processor, MAY, if it so desires, note the unacceptability of some
part of a given MAIL FROM command and use this information to
subsequently generate non-delivery notifications for any or all
recipients. MUST accept any syntactically valid RCPT TO command. A
conforming processor SHOULD note the unacceptability of some part of
a given RCPT TO command and subsequently use this information to
generate a non-delivery notification for this recipient in lieu of
actually delivering the message. MUST accept any of the additional
parameters defined by the 8bitMIME, SIZE, and NOTARY SMTP extensions
on the MAIL FROM and RCPT TO commands. MUST accept the DATA command
even when no valid recipients are present. 8bit MIME messages MUST be
accepted. MUST accept the RSET command and handle multiple messages
in a single application/batch-SMTP object. Processors MUST process
each message in an application/batch-SMTP object once and SHOULD take
whatever steps are necessary to avoid processing a message more than
once. For example, if processing of an application/batch-SMTP object
containing multiple messages is interrupted at an intermediate point
it should subsequently be restarted at the end of the last message
that was completely processed. SHOULD forward any syntactically
invalid application/batch-SMTP message to the local postmaster for
manual inspection and handling.
Security Considerations
Application/batch-SMTP implements a tunneling mechanism. In general
tunneling mechanisms are prone to abuse because they may provide a
means of bypassing existing security restrictions. For example, an
application/batch-SMTP tunnel implemented over an existing SMTP
transport may allow someone to bypass relay restrictions imposed to
block redistribution of spam.
Application/batch-SMTP processors SHOULD implement Access
restrictions designed to limit access to the processor to authorized
generators only. (Note that this facility may be provided
automatically if application/batch-SMTP is being used to secure
message envelope information.)
Acknowledgements
The general concept of batch SMTP has been around for a long time.
One particular type of batch SMTP was defined by Alan Crosswell and
used on BITNET to overcome BITNET's native 8 character limit on user
and host names. However, this form of batch SMTP differed from the
current proposal in that it envisioned having the server return the
status code responses to the client. In this case the client bore the
burden of correlating responses with the original SMTP dialogue after
the fact.
Unfortunately this approach proved not to work well in practice.
BITNET eventually switched to the same basic form of batch SMTP that
has been defined here. Unfortunately that definition was, to the best
of the present authors' knowledge, never captured in a formal
specification. It should also be noted that the definition given here
also differs in that it takes SMTP extensions into account.
Einar Stefferud had previously considered the problem of carrying
extended SMTP messages over unextended SMTP transports. He proposed
that some form of "double enveloping" technology be developed to
address this problem. The mechanism presented here effectively
implements the type of solution he proposed.
References
[RFC-821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
RFC821, August 1982.
[RFC-822] Crocker, D., "Standard for the Format of ARPA Internet
Text Messages", STD 11, RFC822 August 1982.
[RFC-1123] Braden, B., "Requirements for Internet Hosts --
Application and Support", STD 3, RFC1123, October 1989.
[RFC-1652] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D.
Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
RFC1652, July 1994.
[RFC-1847] Galvin, J., Murphy, S., Crocker, S. and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC1847, October 1995.
[RFC-1869] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D.
Crocker, "SMTP Service Extensions", STD 10, RFC1869,
November 1995.
[RFC-1870] Klensin, J., Freed, N., Moore, K., "SMTP Service Extension
for Message Size Declaration", STD 10, RFC1870, November,
1995.
[RFC-2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC2045, December 1996.
[RFC-2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC2046,
December 1996.
[RFC-2197] Freed, N. and A. Cargille, "SMTP Service Extension for
Command Pipelining", RFC2197, September 1997.
Authors' Addresses
Ned Freed
Innosoft International, Inc.
1050 Lakes Drive
West Covina, CA 91790
USA
Phone: +1 626 919 3600
Fax: +1 626 919 3614
EMail: ned.freed@innosoft.com
Dan Newman
Innosoft International, Inc.
1050 Lakes Drive
West Covina, CA 91790
USA
Phone: +1 626 919 3600
Fax: +1 626 919 3614
EMail: dan.newman@innosoft.com
Mark Hoy
Mainbrace Corporation
1136 West Evelyn Avenue
Sunnyvale, CA 94086
tel: +1 408 774 5265
fax: +1 408 774 5290
email: mark.hoy@mainbrace.com
Jacques Bellisent
SunSoft
Phone: +1 650 786 3630
EMail: jacques.belissent@eng.sun.com
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