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Versions: 00 01 RFC 4407

   Internet Draft                                               J. Lyon
   Category: Experimental                                Microsoft Corp
   Document: draft-lyon-senderid-pra-01.txt                    May 2005


             Purported Responsible Address in E-Mail Messages


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Abstract

   This document defines an algorithm by which, given an e-mail message,
   one can extract the identity of the party that appears to have most
   proximately caused that message to be delivered.  This identity is
   called the "Purported Responsible Address" (PRA).















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Table of Contents

   1. Introduction...................................................2
   2. Determining the Purported Responsible Address..................3
   3. Security Considerations........................................4
   4. IANA Considerations............................................5
   5. Acknowledgements...............................................5
   6. References.....................................................5
      6.1 Normative References.......................................5
      6.2 Informative References.....................................5
   7. Author's Address...............................................5


Conventions used in this document

   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 [RFC2119].


1. Introduction

   Most e-Mail flows relatively directly from a sender to a recipient,
   with a small number of Mail Transfer Agents (MTAs) in between.  Some
   messages, however, are resent by forwarding agents, mailing list
   servers, and other such software.  These messages effectively result
   in two or more mail transactions: one from the sender to the
   forwarding agent, and another from the agent to the destination.

   In some cases, messages travel through more than one of these agents.
   This can occur, for example, when one mailing list is subscribed to
   another, or when the address subscribed to a mailing list is a
   forwarding service.

   Further complicating the situation, in some cases the party that
   introduces a message is not the author of the message.  For example,
   many news web sites have a "Mail this article" function that the
   public can use to e-mail a copy of the article to a friend.  In this
   case, the mail is "from" the person who pressed the button, but is
   physically sent by the operator of the web site.

   This document defines a new identity associated with an e-mail
   message, called the Purported Responsible Address (PRA), which is
   determined by inspecting the header of the message.  The PRA is
   designed to be the entity that (according to the header) most
   recently caused the message to be delivered.

   Note that the results of this algorithm are only as truthful as the
   headers contained in the message; if a message contains fraudulent or


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   incorrect headers, this algorithm will yield an incorrect result.
   For this reason, the result of the algorithm is called the "Purported
   Responsible Address" -- "purported" because it tells you what a
   message claims about where it came from, but not necessarily where it
   actually came from.

   This document does not prescribe any particular uses for the
   Purported Responsible Address.  However, [SenderID] describes a
   method of determining whether a particular MTA is authorized to send
   mail on behalf of the domain contained in the PRA.


2. Determining the Purported Responsible Address

   The purported responsible address (PRA) of a message is determined by
   the following algorithm:

   1. Select the first non-empty Resent-Sender header in the message. If
     no such header is found, continue with step 2.  If it is preceded
     by a non-empty Resent-From header and one or more Received or
     Return-Path headers occur after said Resent-From header and before
     the Resent-Sender header, continue with step 2.  Otherwise,
     proceed to step 5.

   2. Select the first non-empty Resent-From header in the message.  If
     a Resent-From header is found, proceed to step 5. Otherwise,
     continue with step 3.

   3. Select all the non-empty Sender headers in the message.  If there
     are no such headers, continue with step 4.  If there is exactly
     one such header, proceed to step 5.  If there is more than one
     such header, proceed to step 6.

   4. Select all the non-empty From headers in the message.  If there is
     exactly one such header, continue with step 5.  Otherwise, proceed
     to step 6.

   5. A previous step has selected a single header from the message.  If
     that header is malformed (e.g. it appears to contain multiple
     mailboxes, or the single mailbox is hopelessly malformed, or the
     single mailbox does not contain a domain name), continue with step
     6.  Otherwise, return that single mailbox as the Purported
     Responsible Address.

   6. The message is ill-formed, and it is impossible to determine a
     Purported Responsible Address.

   For the purposes of this algorithm, a header field is "non-empty" if
   and only if it contains any non-whitespace characters.  Header fields


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   that are otherwise relevant but contain only whitespace are ignored
   and treated as if they were not present.

   Note that steps 1 and 2 above extract the Resent-Sender or Resent-
   From header from the first resent block (as defined by section 3.6.6
   of [RFC2822]) if any.  Steps 3 and 4 above extract the Sender or From
   header if there are no resent blocks.

   Note that what constitutes a hopelessly malformed header or a
   hopelessly malformed mailbox in step 5 above is a matter for local
   policy.  Such local policy will never cause two implementations to
   return different PRAs.  However it may cause one implementation to
   return a PRA where another implementation does not.  This will only
   occur when dealing with a message containing headers of questionable
   legality.

   Although the algorithm specifies how messages that are not in strict
   conformance with the provisions of RFC2822 should be treated for the
   purposes of determining the PRA, this should not be taken as
   requiring or recommending that any systems accept such messages when
   they otherwise would not have done so.  However, if a liberal
   implementation accepts such messages and desires to know their PRA,
   it MUST use the algorithm specified here.

   Where messages conform to RFC822 rather than RFC2822, it is possible
   for the algorithm to give unexpected results.  An RFC822 message
   should not normally contain more than one set of resent headers;
   however the placement of those headers is not specified, nor are they
   required to be contiguous.  It is hence possible that the Resent-From
   header will be selected even though a Resent-Sender header is
   present.  Such cases are expected to be rare or non-existent in
   practice.


3. Security Considerations

   The PRA, as described by this document, is extracted from message
   headers that have historically not been verified.  Thus, anyone using
   the PRA for any purpose MUST be aware that the headers from which it
   is derived might be fraudulent, malicious, malformed and/or
   incorrect.  [SenderID] describes one mechanism for validating the
   PRA.

   A message's PRA will often be extracted from a header field that is
   not normally displayed by existing mail user agent software.  If the
   PRA is used as part of a mechanism to authenticate the message's
   origin, the message SHOULD NOT be displayed with an indication of its
   authenticity (positive or negative) without the PRA header field also
   being displayed.


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4. IANA Considerations

   This document contains no actions for IANA.


5. Acknowledgements

   The PRA concept was first published in [CallerID].  It as been
   refined using valuable suggestions from members of the MARID working
   group.


6. References

6.1 Normative References

   [RFC2119]   S. Bradner, "Key words for use in RFCs to Indicate
               Requirement Levels", RFC 2119.

   [RFC2822]   P. Resnick (editor), "Internet Message Format", RFC 2822.

6.2 Informative References

   [CallerID]  Microsoft Corporation, Caller ID for E-Mail Technical
               Specification, http://www.microsoft.com/mscorp/twc/
               privacy/spam_callerid.mspx.

   [SenderID]  J. Lyon and M. Wong, "Sender ID:  Authenticating E-Mail",
               draft-lyon-senderid-core-01.  Work in progress.


7. Author's Address

   Jim Lyon
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052
   USA
   jimlyon@microsoft.com











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