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RARE-DRAFT                                              Jeroen Houttuin
IESG Mail Applicability Design Team                    RARE Secretariat
<draft-rare-msg-a-bombs-00.txt>                              April 1994

              Bombs series: Behaviour of Mail Based Servers
                             Part 2: A-bombs
                            Answering servers


Abstract

   This document defines rules for the behaviour of Mail Based Echo
   Servers and Vacation Servers in the Internet. It is highly desirable
   that other e-mail networks connected to the Internet also implement
   these rules.

Status of this Memo

   This document is a RARE Draft. RARE Drafts form a subseries of the
   Internet Drafts, which 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. For example, RARE Drafts are produced by the RARE
   Working Groups.

   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."

   Please check the I-D abstract listing contained in each Internet
   Draft directory to learn the current status of this or any other
   Internet Draft.

   Distribution of this memo is unlimited.

   This document builds upon the classification of MBS types, which can
   be found in the Bombs series, part1: C-bombs [13].

   Within the context of the connectivity testing tool 'concord',
   initial work on the requirements for echo servers was done within
   SWITCH and XNREN ([7], [8]).

   The document was then integrated in the work of the IESG solicited
   Mail Applicability Design Team, consisting of: Ned Freed (INNOsoft),
   Jeroen Houttuin (RARE), John Klensin (INfoods, UN), Keith Moore
   (University of Tennessee).









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   Depending on the nature of your comments, please respond to one of
   the following addresses:

         The main discussion group:       wg-msg@rare.nl
         The design team:                 mail-as@infoods.unu.edu
         The author:                      houttuin@rare.nl


Contents

   1. Introduction                                                   2
   2. General approach                                               2
   3. Implementation levels and protocols                            4
   4. Rules                                                          4
      4.1. Input message restrictions                                5
      4.2. Output messages                                           6
           4.2.1. Relation to the input message                      6
           4.2.2. Restrictions                                      10
      4.3. Logging                                                  14
      4.4. Access permissions                                       16
   5. Reference implementations                                     17
   6. Acknowledgements                                              17
   7. Security considerations                                       17
   8. Bibliography                                                  17
   9. Abbreviations                                                 19
   10. Author's Address                                             19


1. Introduction


   Mail Based Servers (MBSs) are defined in C-bombs [13] as follows:

   An MBS is a process that automatically generates one or more messages
   (the output messages) as a result of receiving a message (the input
   message).

   Two main types are identified: repliers and forwarders. This
   documents deals only with the basic behaviour of a subclass of
   repliers: echo servers and vacation servers (jointly referred to as
   'answering servers').


2. General approach


   The overall approach for all MBS header requirements based upon C-
   bombs [13] is as follows.

   If all MBSs would agree to implement a common set of behaviour rules,
   this set could be fairly small. In practice however, there are some
   reasons why such a 'minimum approach' will not work:


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     - The most obvious reason is that one cannot realistically expect
       all networks and software developers to implement one common
       strict set of rules. In different mail communities, different
       MBS conventions have already been used for a long time. Some of
       these conventions can be unacceptable for other communities to
       implement.

     - MBSs can be built upon different underlying protocols. For
       instance, it is almost impossible to have a small set of rules
       that will prevent problems between any combination of MBSs, e.g.
       between an RFC 822-like MBS running over NJE and a P1 based MBS.
       More problems can be expected because header fields are crucial
       for the properly functioning of MBSs, and protocol gateways will
       not always map header fields bijectively.

     - Not all MBSs are controlled by software developers or network
       operators. Any user can write a simple program that will have
       the functionality of an MBS.

   Because the 'minimum approach' is not feasible, the bombs series
   follows the 'unilateral safety approach'. This means that any MBS
   that implements the complete set of rules should be safe from harm,
   regardless of what other 'dumb' MBSs it is interacting with.

   This approach results in quite a large number of recommendations, of
   which not every single one is strictly necessary to prevent problems,
   but none of them will 'hurt' the functioning of an MBS.

   From the previous paragraphs it follows that MBSs do not operate in a
   vacuum; they interact with other types of MBSs. As a result, the
   requirements in this document may sometimes look like an overkill
   when not seen in the light of the behaviour of other types of MBSs.
   To get an idea of the requirements for other MBSs, please refer to
   the H-bombs document [12] (which is the predecessor of the bombs
   series).

   As for the programming overhead caused by the recommendations, there
   is at least one example of an echo server (Echoput) that implements
   all a-bombs rules in two pages of (perl) code.

   In addition to the rules that protect against loops and explosions,
   there are also some rules reflecting common sense. For instance, if a
   user sends a message flagged 'urgent' to an echo server, he would
   expect not only his request message, but also the reply message to be
   handled with extra priority.

   The rules for vacation servers are the same as for echo servers, but
   due to the lifetime attribute and a vacation server not normally
   having a separate administrator, these servers have some
   additional/exceptional rules.



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3. Implementation levels and protocols


   Answering servers are normally implemented at UA level. If one wants
   to test connectivity at a lower level, a message can be sent to a
   'nosuchuser' address, which will result in an MTA-generated non-
   delivery message or report.

   To a user, it is often not known beforehand in which protocol world
   (RFC 822, X.400, others) an MBS is located. Also an MBS doesn't
   normally 'know' in which world a user lives. In order to come to a
   consistent echo server behaviour regardless of used protocols, this
   document describes recommendations for both RFC mail and X.400 echo
   servers. Note that a one hundred percent transparency cannot be
   reached (yet), because there exists no one-to-one mapping between all
   RFC mail and X.400 service elements.

   For the reader's convenience, the rules for MBSs in different
   implementation levels and protocols are explicitly stated in the
   appropriate terminologies. The rules are labelled as follows:

   For Internet mail:

     #RFC#     Applies to RFC 822 on top of RFC 821 (SMTP) based MBSs
    #1327#     Some the RFC 822 rules deal with non-standard headers as
                described in RFC 1327

   For X.400:

     #400#     Applies to X.400 (both 84 and 88) based MBSs
     #84#      Applies to X.400(84) based MBSs
     #88#      Applies to X.400(88) based MBSs
     #P1#      Applies to P1 (MTS) based MBSs
     #P2#      Applies to P2 (UA) based MBSs
     #P3#      Applies to P3 (MTA) based MBSs


4. Rules


   Depending on implementation level and protocol, answering servers
   follow, as a minimum, the requirements defined in RFC 822, RFC 821,
   RFC 1123, X.411, X.420, X.435 etc. For those requirements, the MBS
   must behave as an automated user or UA, depending on whether it is
   implemented at UA- or MTS-level, respectively. This chapter describes
   additional rules for answering servers in terms of RFC 821, RFC 822,
   P1, P3, and P2.






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4.1. Input message restrictions


4.1.1. Don't reply to automatically forwarded messages

   DISCUSSION: There is no need for a user to automatically forward his
   incoming messages to an echo server or a vacation server. Note that
   non-auto-forwarded messages can only be unambiguously identified in
   P2, Internet mail has no standard headers for this purpose. RFC 1327
   gateways map this attribute to a new RFC 822 header "Auto-
   Forwarded:". In the presence of this header, RFC based MBSs can
   safely assume that the message was indeed auto-forwarded.

   RULE: An auto-forwarded message is not valid as an input message. The
   result is the generation of an exception output message.

   ORIGIN OF RULE: This document.


4.1.2. Don't reply to threads

   DISCUSSION: It is very unlikely that a user will send a reply to
   another message as an input message to an answering server. Such a
   reply or follow-up should either have gone to the MBS administrator
   (due to the rules in this document) or to any other address that is
   not an answering server.

   RULE: An exception output message is generated if the input message
   contains either of the following headers or attributes:

     #RFC#          In-Reply-To:
                                References:

     #P2#           In-Reply-To
                                crossReferences

   ORIGIN OF RULE: This document.

   APPLICABILITY: should.


4.1.3. Valid input message types

   DISCUSSION #RFC#: An answering server is not to send automatic
   replies to (automatically generated) non-delivery messages, to avoid
   loops. In RFC mail, non-delivery messages can be recognised by the
   empty MAIL FROM: line.







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   RULE: Only the following types of input messages are valid as input
   messages. Any other type of input message (report, receipt
   notification) leads to the generation of an exception message.

     #RFC# Any message that does not have an empty MAIL FROM: line.

     #84#P1# UserMPDU
     #84#P2# IM-UAPDU

     #88#P1# Message
     #88#P2# IPM

   #400# P1.Probes are expected to be handled by the MTS and are thus
   not interpreted by the MBS.


4.2. Output messages


4.2.1. Relation to the input message


4.2.1.1. User can specify alternate output message recipient


   DISCUSSION: The user may decide that the output message should be
   sent to another address than his own. This is especially useful when
   the user is an automated process, e.g. a connectivity checker, with a
   complex distributed configuration.

   RULE: If the input message contains the following header or
   attribute, the output message is sent to that address. If this field
   contains more than one address, an output message is sent to at least
   the first address of this field. (Sending to the others is not
   recommended.)

     #RFC#          Reply-To:

     #84#P2#        replyToUsers

     #88#P2#        reply-recipients

   ORIGIN OF RULE: Common practice, RFC 821, RFC 822, RFC 1123, X.400.

   APPLICABILITY: must.


4.2.1.2. Make output messages traceable


   DISCUSSION: This rule allows the user to find know exactly to which
   message this output message belongs.


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   ORIGIN OF RULE: RFC 822, X.400, common practice, this document.


4.2.1.2.1. In reply to


   RULE: The following header or attribute of the output message has the
   value:

     #RFC#          In-Reply-To: : Message-ID of input message

     #84#P2#        inReplyTo : IPMessageID of input message

     #88#P2#        replied-to-IPM : this-IPM of input message

   APPLICABILITY: must.


4.2.1.2.2. Subject


   The following header or attribute of the output message has as value
   the string 'Re: ', concatenated with the subject of the input
   message.

     #RFC#          Subject:

     #P2#           subject

   APPLICABILITY: should.


4.2.1.2.3. References


   RULE: If the following header or attribute is used in the output
   message, it has the value:

     #RFC#          References: : Message-ID of input message

     #84#P2#        crossReferences : IPMessageID of input message

     #88#P2#        related-IPMs : this-IPM of input message

   APPLICABILITY: may.








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4.2.1.2.4. Alternate recipient can trace originator of the input message


   DISCUSSION: A user who receives mail from an MBS, without having
   ordered this information himself, has the right to know who was
   responsible for having messages sent to his mailbox. The semantics of
   both RFC 822 and X.400 header fields allow to specify that a message
   was sent from a certain address, but was authorised by someone else.
   This matches the semantics needed here. Another reason for using
   header fields for carrying this information is that the addresses
   will still be readable for the end-user after the message has crossed
   a protocol gateway.

   RULE:

   #RFC# If the output message is not sent to the originator of the
   input message, its From: field contains the addresses of the From:
   and the Sender: fields of the input message. In this case the Sender:
   field of the output message contains the address of the MBS
   administrator.

   #P2# If the output message is not sent to the P2.originator of the
   input message, its P2.authorizingUsers field contains the addresses
   of the P2.originator and the P2.authorizingUsers of the input
   message.

   ORIGIN OF RULE: This document, RFC 822, RFC 1327, X.400.

   APPLICABILITY: shall.


4.2.1.4. Body contents


   DISCUSSION: In order for the user to see what happened to his
   original input message on its way to the answering server (format,
   timing etc), the input message is reflected back to the user. Further
   info- and advertainment about the server can be included as well. See
   also 4.2.2.

   ORIGIN OF RULE: Common practice, this document.

   RULE: The input message (all headers and an optionally truncated part
   of the body) is included in the output message in an end user
   readable format, preferably as a MIME message body-part, an
   IPMS.ForwardedIPMessage bodypart, or in plain ASCII text.

   APPLICABILITY: must.






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4.2.1.5. Conservation


   DISCUSSION: There are a number of headers or attributes, set by the
   originator of the input message, that are to be set to the same value
   in the output message. For instance, a user will expect a high
   priority request to be handled with high priority. The output message
   will in this case have the same priority. Note that an MBS can, as a
   local decision, choose to spool all requests in order to spread the
   MBS load. As long as the local processing of high priority request
   can be guaranteed to be no slower than that of normal requests, and
   the following rules for the output messages are followed, these local
   processing delays will be transparent for the MBS users.


4.2.1.5.1. Retain privacy requests


   DISCUSSION: The server is to respect the originator's request for
   privacy.

   RULE:  The following headers or attributes have the same value in the
   output message as in the input message:

     #1327#         Sensitivity:
     #1327#         Importance:
     #1327#         Priority:

     #P2#           P2.sensitivity
     #P2#           Importance

     #P1#P3#        Priority

   ORIGIN OF RULE: this document.

   APPLICABILITY: must.


4.2.1.5.2. Answer in same type of content


   DISCUSSION: To minimise the chance of UAs not being able to handle a
   certain message content type, the content type of the output message
   is the same as that of the input message.

   RULE: The following headers or attributes have the same value in the
   output message as in the input message

     #RFC#          MIME-Version:
     #RFC#          Content-Transfer-Encoding:

     #84#P1#P3#     ContentType


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   ORIGIN OF RULE: this document.

   APPLICABILITY: should.


4.2.2. Restrictions


4.2.2.1. Don't ask for replies


   DISCUSSION: If an MBS would request some form of reply or report for
   an output message, other MBSs might as a result automatically send a
   message, report or (non)delivery message back to the MBS, which is to
   be avoided at all cost, or to the MBS administrator, which is highly
   undesirable.

   ORIGIN OF RULE: This document.


4.2.2.1.1. Don't give incentive to reply to output message


   DISCUSSION: Replies to the output message should be avoided,
   especially because they might be generated automatically.

   RULE: The following headers or attributes are not used in the output
   message:

     #RFC#1327#     Reply-By:
     #RFC#1327#     Expiry-Date:

     #P2#           Recipient.replyRequest
                                (defaults to FALSE)

     #84#P2#        replyBy
     #84#P2#        expiryDate

     #88#P2#        reply-time
     #88#P2#        Expiry Time
     #88#P1#P3#     Proof-of-delivery-request
                                (defaults to proof-of-delivery-not-
       requested)

   APPLICABILITY: shall








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4.2.2.1.2. Use of Reply-To functionality


   DISCUSSION: It is redundant to explicitly attract replies to the
   output message to the MBS administrator, as the other rules in this
   document will ensure such behaviour. If an MBS decides to explicitly
   attract replies to the output message to a certain address, that
   address is not to be the server's address, but preferably the
   administrators. Since this rule contains three different
   applicability levels, it is subdivided into 3 rules.


A. Don't use Reply-To functionality


   DISCUSSION: Other rules in this document will ensure that replies to
   the output message will automatically be sent to the right address
   (the administrator's).

   RULE: The following headers or attributes are not used in the output
   message:

     #RFC#          Reply-To:

     #84#P2#        replyToUsers

     #88#P2#        reply-recipients

   ORIGIN OF RULE: This document.

   APPLICABILITY: shall


B. Don't attract replies towards the server itself


   DISCUSSION: If the MBS decides, despite rule A, to attract replies to
   a certain address, that address is not this (or any other) answering
   server's.

   RULE: If the following field is used in the output message, it does
   not contain the address of the answering server.

     #RFC#          Reply-To:

     #84#P2#        replyToUsers

     #88#P2#        reply-recipients

   ORIGIN OF RULE: This document.

   APPLICABILITY: must.


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C. Attract replies towards the administrator


   DISCUSSION: If the MBS decides, despite rule A, to attract replies to
   a certain address, that address is the MBS administrator's.

   RULE: If the following field is used in the output message, it
   contains the address of the answering server's administrator.

     #RFC#          Reply-To:

     #84#P2#        replyToUsers

     #88#P2#        reply-recipients

   ORIGIN OF RULE: This document.

   APPLICABILITY: should.


4.2.2.2. Avoid non deliverable output messages to cause loops


   DISCUSSION: If the output message has an MTS-level originator with
   the address of the answering server itself, a loop can occur if the
   output message is undeliverable. Note that for X.400 answering
   servers, this rule affects a P1 attribute, but only when the output
   message is P2. For instance, consider a P1 distribution list that
   distributes another content type than P2, say Pc. Since Pc can be
   completely unstructured, changing the P1.originator would make it
   impossible to reply to the originator of the input message. Changing
   the P1.originator will also make sense for content types that have P2
   like header fields, e.g. for P35 messages.

   RULE: The following line or attribute of the output message has the
   value:

     #RFC#          MAIL FROM: : address of the MBS administrator

     #P2#           P1.originator : address of the MBS administrator

   ORIGIN OF RULE: Common practice, this document.

   APPLICABILITY: must.








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4.2.2.3. Avoid replies to the output message to go back to the server


   RULE: The following line or attribute of the output message has the
   value:

     #RFC#          From: : address of the MBS administrator

     #P2#           P2.originator : address of the MBS administrator

   ORIGIN OF RULE: This document.

   APPLICABILITY: must.


4.2.2.4. Avoid reports about the output message


   DISCUSSION: We don't want anything automatically generated in reply
   to the output message, to avoid loops.


A. PerRececipientFlags


   RULE: #84#P1#P3#  Every PerRececipientFlag in the output message has
   the following bits set:

     Report Request:            01
     User Report Request:       00

   (I.e. the Non-delivery Notification service will be prevented)

   ORIGIN OF RULE: this document.

   APPLICABILITY: must.


B. Don't request Reports or Notifications to the output message


   RULE: The following attribute is empty in the output message:

     #84#P2#        Recipient.reportRequest

     #88#P2#        NotificationRequests

   ORIGIN OF RULE: this document.

   APPLICABILITY: must.




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4.2.2.5. Extension Identifiers


   DISCUSSION: There is at least one case where not all
   P1.ExtensionIdentifiers being different has caused a mailing loop.
   Although this was due to a software bug, there is no good reason for
   not using different P1.ExtensionIdentifiers.

   RULE #P1#: All P1.ExtensionIdentifiers in the output message are
   distinct.

   ORIGIN OF RULE: Common practice, common sense, this document.

   APPLICABILITY: shall.


4.2.2.6. Body contents


   DISCUSSION: In order for the user to see what happened to his
   original input message on its way to the answering server (format,
   timing etc), the input message is reflected back to the user. Further
   info- and advertainment about the server can be included as well. See
   also under 4.2.1.

   ORIGIN OF RULE: Common practice, this document.

   RULE: Additional information is included in separate bodyparts of the
   output message.

   APPLICABILITY: may.


4.3. Logging


4.3.1. Logging for the administrator

   DISCUSSION: This rule allows the MBS administrator to track down
   malicious behaviour.

   RULE: The MBS logs the originator of the input message and all
   recipient(s) of the output/exception message(s).

   ORIGIN OF RULE: this document.

   APPLICABILITY: shall.







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4.3.2. Log output message IDs

   DISCUSSION: This will prevent all routing and MTS-redirection loops
   amongst MBSs. UA level MBSs, which create a new output message for
   each input message, will at least be safeguarded against mail storms
   from other MTS based MBSs.

   RULE: The MBS logs the message ID of every input message and every
   output message. It generates an exception message if the same message
   ID is encountered in the input message more than once.

   ORIGIN OF RULE: This document. Similar techniques are already being
   used in Netnews.

   APPLICABILITY: should.


4.3.3. Vacation logging

   DISCUSSION: Users of vacation servers don't normally want to use a
   server, but to reach another person. One output message stating that
   this person is on vacation will be enough.

   RULE: Vacation servers at least log the originator of the input
   message. During the lifetime of an vacation server, only one output
   message per input message originator is generated.

   ORIGIN OF RULE: This document. Similar techniques are already being
   used in Netnews.

   APPLICABILITY: must.


4.3.4. Black list

   DISCUSSION: Repliers are not to send output messages to addresses
   which are likely to be repliers themselves, to avoid loops.

   RULE: Repliers keep a list of loop-suspicious addresses, containing
   at least the following values for the local address designator
   (localpart, Surname, CommonName):

     autoanswer
     echo
     listserv
     mailerdaemon
     mirror
     netserv
     server

   In this respect, also echo servers can be thought to have a limited
   lifetime, during which a normal output message (with an extra


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   bodypart containing a warning) will be sent to loop-suspicious
   addresses only once. This can be implemented by automatically adding
   the exact suspicious address to a negative access control list.
   Whenever this list is cleared, the replier can be thought to start a
   new lifetime.

   The loop suspicious addresses are matched in any combination of upper
   and lower case.

   ORIGIN OF RULE: Tradition, this document.

   APPLICABILITY: shall.


4.4. Access permissions

   DISCUSSION: The user is is to be informed whether and why he has not
   been granted access to the server.

   ORIGIN OF RULE: Tradition, this document.

   DISCUSSION #RFC#: Note that in this case the not granted access is to
   be reported from MTS level, i.e. by the MBS administrator, owner or
   operator - and not by the MBS itself.

   RULE: #RFC# In case of an Access Permission violation an exception
   message is generated with the following text in the message body:

     "Originator not allowed to send to this address"

   APPLICABILITY: shall.

   DISCUSSION #84#: Note that also here the not granted access is to be
   reported from MTS level, i.e. by the MBS administrator, owner or
   operator - and not by the MBS itself. This holds for both options:

   RULE option 1: #84#: In case of an Access Permission violation a
   P1.DeliveryReportMPDU is generated with the following values:

     ReasonCode:    unableToTransfer(1)
     DiagnosticCode:uaUnavailable(4)
     SupplementaryInformation:
                                "Originator not allowed to send to this
       address"

   APPLICABILITY: shall.

   DISCUSSION: This option is preferred, but a P2 server may choose to
   respond more in line with RFC servers as follows:





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   RULE option 2: #84#P2# In case of an Access Permission violation an
   exception message is generated with the following text in the message
   body:

     "Originator not allowed to send to this address"

   APPLICABILITY: may.


5. Reference implementations


   There are a number of MBS implementations that follow most of the
   recommendations listed in this document. They include the following,
   all operating at UA level:

     Name           Protocols             Contact
     ----------------------------------------------------------------
     Concord        RFC, X.400(84)        klarenberg@netconsult.ch
     EAN echo serverX.400                 martinez@fundesco.es
     Echoput        RFC, MIME             klarenberg@netconsult.ch
     PP echo server X.400(84 and 88)      onions@xtel.co.uk


6. Acknowledgements


   Thanks for ideas, comments, flames and corrections: Harald Alvestrand
   (SINTEF), Allan Cargille (XNREN), Urs Eppenberger (SWITCH), Paul
   Klarenberg (NetConsult AG), Ignacio Martinez (Fundesco), Juan
   Pizzorno (DFN), Eric Thomas (SUNET), Johan Vromans (Multihouse), Jan
   van der Weele (Du Pont).


7. Security considerations


   Security issues are not discussed in this memo.


8. Bibliography


      [1]         Jonathan B. Postel, "Simple Mail Transfer Protocol",
                  RFC 821, University of Southern California, August
                  1982

      [2]         Crocker, D., "Standard of the Format of ARPA Internet
                  Text Messages", RFC 822, UDEL, August 1982.





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      [3]         R. Braden, Editor, "Requirements for Internet Hosts -
                  - Application and Support", RFC 1123, USC/Information
                  Sciences Institute, October 1989.

      [4]         Kille, S., "Mapping between X.400(1988) / ISO 10021
                  and RFC 822", RFC 1327, UCL, May 1992.

      [5]         Kille, S., "X.400 1988 to 1984 downgrading", RFC
                  1328, UCL, May 1992

      [6]         N. Borenstein, N. Freed, MIME (Multipurpose Internet
                  Mail Extensions), RFC 1341, June 1992

      [7]         J. Houttuin, "Concord functional specification",
                  COSINE MHS server, Mail: cosine-mhs-
                  server@nic.switch.ch, FTP: nic.switch.ch, Username:
                  cosine , File: tools/operational/concord/xxxxxxxx

      [8]         J. Houttuin, Allan Cargille, "Requirements for
                  concord echo servers and distribution lists", COSINE
                  MHS server, Mail: cosine-mhs-server@nic.switch.ch,
                  FTP: nic.switch.ch, Username: cosine, File:
                  procedures/echo-server-reqs

      [9]         "list of surnames/usernames not to automatically
                  reply to", RARE server, Mail: server@rare.nl, FTP:
                  ftp.rare.nl, File:
                  working-groups/wg-msg/div/dontreply

      [10]        CCITT Recommendations X.400 - X.430. Data
                  Communication Networks: Message Handling Systems.
                  CCITT Red Book, Vol. VIII - Fasc. VIII.7, Malaga-
                  Torremolinos 1984

      [11]        CCITT Recommendations X.400 - X.420. Data
                  Communication Networks: Message Handling Systems.
                  CCITT Blue Book, Vol. VIII - Fasc. VIII.7, Melbourne
                  1988

      [12]        Houttuin, J., "H-bombs: Header Behaviour of MBSs",
                  work in progress, November 1993.

      [13]        Houttuin, J., "C-bombs: Classification of Breeds of
                  MBSs", work in progress, April 1994.










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9. Abbreviations

      ASCII    American Standard Code for Information Exchange
      CCITT    Comite Consultatif International de Telegraphique et
               Telephonique
      COSINE   Co-operation for OSI networking in Europe
      EAN      MHS software (not an abbreviation)
      IESG     Internet Engineering Steering Group
      IETF     Internet Engineering Task Force
      IPM      Inter-Personal Message
      IPN      Inter-Personal Notification
      MHS      Message Handling System
      MBS      Mail based server
      MTA      Message Transfer Agent
      MTL      Message Transfer Layer
      MTS      Message Transfer System
      NJE      Network Job Entry
      PP       MHS software (not an abbreviation)
      RARE     Reseaux Associes pour la Recherche Europeenne
      SMTP     simple mail transfer protocol
      UA       User Agent


10. Author's Address


   Jeroen Houttuin

   RARE Secretariat
   Singel 466-468
   NL-1017 AW  Amsterdam
   Europe

   Tel +31 20 6391131
   Fax +31 20 6393289

   RFC 822     houttuin@rare.nl
   X.400 /S=houttuin/O=rare/PRMD=surf/ADMD=400net/C=nl/
















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