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Versions: 00                                                            
Network Working Group                                         H. Lachman
INTERNET-DRAFT                             Netscape Communications Corp.
Expires: 30 September 1997                                    March 1997
Intended Category: Informational

                  LDAP-based Routing of SMTP Messages:
                       Approach Used by Netscape

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts 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.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   ftp.isi.edu (US West Coast).


   Directory services based on the Lightweight Directory Access Protocol
   (LDAP) [1] and X.500 [2] provide a general-purpose means to store
   information about users and other network entities.  One of the many
   possible uses of a directory service is to store information about
   users' email accounts, such as their email addresses, and how
   messages addressed to them should be routed.  In the interest of
   interoperability, it is desirable to have a common schema for such
   email-related information.

   This document discusses some of the fundamental questions to be
   considered in the development of a common schema for LDAP-based
   routing of SMTP [3] messages, presents an approach that has been
   implemented and deployed, and discusses possible extensions to that
   approach that may serve to make it more complete and general.  The
   intent is to provide information about an existing implementation,
   and to stimulate discussion about whether and how to standardize the
   relevant aspects of LDAP-based messaging management.

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1.  Background and Motivation

   LDAP-based directory services are currently being used in many
   organizations as a repository of information about users and other
   "network entities" (such as groups of users, network resources,
   etc.).  Some information is stored in the directory for the
   consumption of persons browsing for information (e.g., telephone
   numbers, postal addresses, secretary's name), while other information
   (e.g., login name, password, disk quota) is stored for use by one or
   more network applications or services.  It is the latter kind of
   information that is of interest in this discussion.  In general, it
   is advantageous for different network applications and services to
   refer to the directory for user account information, rather than each
   service keeping its own collection of user account records, which
   requires the network administrator to separately create or destroy
   user entities, passwords, etc., in many different systems each time a
   user joins or leaves the organization.  The goals of centralized user
   management and sharing of information with other service types drove
   our decision in the design of Netscape Messaging Server (an SMTP-
   based mail server product) to use LDAP-based directory services as a
   common repository for user account information.

   Now, if a given mail server can refer to the directory for its own
   users' account information, it follows that that same information is
   visible to other LDAP-aware mail servers in the same organization,
   and therefore that information can aid those other mail servers in
   correctly routing messages to users of the mail server in question.
   This assumes that there is an agreed-upon set of per-user attributes
   to support message routing.  If this assumption is met, we can
   obviate other means currently employed to specify per-user message
   routing (such as the Unix "aliases" database).  The benefit of this
   is to further consolidate per-user system information.

   If each vendor's mail server product has its own schema for LDAP-
   based message routing, then the above benefits can be achieved for
   single-vendor customers, but customers who have multiple vendors'
   mail server products would not be well served.  They will likely
   expect interoperability, which will require a common schema to be
   supported by the various vendors' products.  Thus, it is worthwhile
   to consider how to develop a common schema.

   This document does not attempt to define a standard.  It does attempt
   to define what the relevant questions are, and goes on to describe
   one vendor's solution plus possible extentions to generalize it.  It
   is hoped that this discussion helps to characterize the issue, and
   encourages the development of a common solution.

   This document considers only intra-enterprise SMTP message routing

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   using LDAP-based directory services.  Solutions and issues involving
   inter-enterprise routing, non-SMTP message handling, non-LDAP
   directory services, and other messaging management topics not related
   to message routing, are outside the scope of this document (except
   that the concepts presented may also be applicable in the case of
   X.500 directory services).

2.  Terminology

   In the context of this document, a "mail server" is a Simple Mail
   Transfer Protocol (SMTP) message transfer agent (MTA).  It either
   includes, or has associated with it, a local message delivery agent
   (MDA) that performs delivery to accounts that are considered local to
   the particular mail server.  A mail server may offer related services
   as well, such as providing a means for mail user agents (MUAs) to
   pick up messages, but that is outside the scope of this discussion.

   The term "account" is used to refer to any entity that can receive
   mail.  This includes mail user accounts, as well as mail group
   accounts (mail distribution lists).  A "delivery" is said to have
   occured when an MTA passes a message to the local MDA, having first
   ascertained that the message is destined for a particular account
   that can be delivered to locally.  The MDA may then place the message
   in a local message store, and/or take other actions as specified by
   the account's attributes.

   "Routing" and "forwarding" are distinct actions.  "Routing" is said
   to have occured when an MTA passes a message to a "next-hop" MTA,
   having ascertained that the addressed entity is not a local account
   but may exist elsewhere.  "Forwarding" is said to have occured when a
   message has successfully arrived at a particular account and the MDA
   determines that the message must be resent to one or more new
   destinations as specified by the account's attributes.  "Forwarding"
   may be configurable by the user, while "routing" is normally
   configurable only by a network administrator.  With this definition,
   it might also be said that when a message arrives at a mail group
   account, and the MDA resends the message to all of the individual
   group members, this is also "forwarding".

3.  Questions to Consider

   When a message arrives at an MTA, the MTA must determine whether to
   deliver the message to a local account, route the message to another
   MTA, or, in the case of an unresolvable recipient address, take some
   remedial action such as "bouncing" the message.  In the course of
   making this determination, an MTA may reference information from a
   variety of sources, including its own local configuration, one or
   more directory services, and perhaps other sources.  This document

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   discusses only per-account routing and addressing information
   provided by an LDAP-based directory service, and what role that
   information might play in helping the MTA determine what to do with a

   The question of how an MTA might use such information can be broken
   down into three subquestions:

   Question (1):  For a given recipient address, which LDAP entry does
   it belong to?

   Question (2):  For a given LDAP entry, should a message addressed to
   it be delivered locally or routed?

   Question (3):  If a message addressed to a given LDAP entry needs to
   be routed, to where should the message be routed?

   In order for these questions to be answerable by the MTA, LDAP
   entries that represent mail accounts should include attributes that
   specify addressing and routing information.  These attributes should
   be advertised to (i.e., readable by) the MTAs that are expected to
   act on them, and there should be a definition of what attributes are
   involved and how they are to be interpreted.  With this definition,
   an MTA can be implemented or configured to correctly use such
   information to answer the above questions, and therefore, correctly
   handle messages addressed to accounts represented as LDAP entries.

4.  Description of Solution Implemented

   In the design of Netscape Messaging Server, we defined two new LDAP
   object classes, 'mailRecipient', which is used to represent a mail
   user account, and 'mailGroup', which is used to represent a mail
   group account (mail distribution list).  An LDAP entry of either
   class may have attributes that are of an "account configuration"
   nature and are used solely by the MDA handling mail for the account,
   while other attributes are used by the account's "home" MTA as well
   as other MTAs.  It is this latter set of attributes that are of
   interest in this discussion, since we are concerned with the behavior
   of MTAs.

   Our MTA implementation uses the following attributes:

           mail                    primary email address
           mailAlternateAddress    additional email addresses
           mailHost                server hosting mail account
           uid                     user id (login name)

   The 'mail' and 'mailAlternateAddress' attributes are used to specify

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   the email addresses [4] that are considered valid for an account.
   They must all be complete email addresses (e.g., "joe@example.com",
   as opposed to "joe" or "joe@mars").  'mailHost' is the fully-
   qualified domain name [5] of the mail server that considers the
   account to be locally deliverable (e.g., "mars.eng.example.com").
   'uid' is the user's login name.  A 'mailGroup' is not expected to
   have a 'uid' attribute, and may or may not have a 'mailHost'
   attribute, but both attributes should be present for a
   'mailRecipient'.  For a detailed description of the 'mailRecipient'
   and 'mailGroup' object classes and associated attributes, refer to
   Appendices A and B.

   Our MTA implementation looks for the above attributes, and uses them
   to answer the three fundamental questions considered above, as

 4.1.  Mapping an Address to an LDAP Entry

   To resolve Question (1), we take the recipient address from the SMTP
   "envelope", and see if there is exactly one LDAP entry that
   advertises that address as one of its valid addresses.  Specifically,
   we search for an LDAP entry that has a 'mail' or
   'mailAlternateAddress' attribute whose value is the address in
   question.  The search is performed across all LDAP entries in a given
   directory subtree, which is configured in the MTA as its "base
   subtree" of interest.

   If the above search fails, we may also perform a fallback search.
   Specifically, if the above search yields zero matches, we split the
   address in question at the "@" sign, yielding a "local part" and a
   "domain part".  If the MTA configuration specifies that it is the
   final authority on messages addressed to the domain part in question
   (i.e., it has the authority to bounce messages addressed to that
   domain), then we search for an LDAP entry whose 'uid' attribute
   equals the local part.  If we get exactly one match, then we regard
   this as a successful match.

   In theory, the fallback search may not be required, but since our MTA
   rewrites envelopes to 'uid'@'mailHost' (as discussed in Section 4.3),
   it is clearly advantageous for receiving MTAs in this environment to
   be able to unconditionally recognize an address thusly rewritten.

 4.2.  Determining Whether or not to Perform Local Delivery

   To resolve Question (2), we look up the LDAP entry's 'mailHost'
   attribute.  If the value of this attribute matches the fully-
   qualified domain name (FQDN) specified in the MTA configuration, then
   the message is passed to the local MDA.

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   If the value of the 'mailHost' attribute does not match the MTA's
   FQDN, then the message is routed.

   If the LDAP entry has no 'mailHost' attribute, this is considered
   invalid for a 'mailRecipient', but for a 'mailGroup', the MTA will
   pass the message to the local MDA to perform group list expansion and
   forwarding to the individual recipients.  In other words, for a
   'mailGroup', a missing 'mailHost' means any mail server may perform
   group handling for the message.

 4.3.  Determining How to Route the Message

   To resolve Question (3), we look up the LDAP entry's 'mailHost' and
   'uid' attributes.  The 'uid' attribute is normally present for a
   'mailRecipeint', and is not normally present for a 'mailGroup'.  If
   the 'uid' attribute is present, we rewrite the recipient address in
   the SMTP envelope to 'uid'@'mailHost', i.e., we combine the
   respective values of these two attributes and add an "@" sign to
   formulate a new recipient address.  If the 'uid' attribute is not
   present, we do not rewrite the recipient address.

   The message is routed to the destination indicated in the 'mailHost'
   attribute.  This may or may not mean that the MTA will open an SMTP
   connection to the host identified as the 'mailHost', since the MTA
   configuration may specify routing rules that prevent this (e.g., in
   sites that are configured so that all message traffic follows a fixed
   "star" topology).  Also, some sites may use DNS MX records [6] for
   internal message routing.  For example, an MTA "mail.example.com" may
   receive a message for "joe@example.com", find that the 'mailHost' for
   this account is "mars.eng.example.com", and then discover that mail
   for "*.eng.example.com" is MX'ed to "hub.eng.example.com", which will
   then be the "next hop".

5.  Possible Ways to Generalize the Solution Implemented

   The following are serveral ways our approach could be extended to
   make it more general.  None of these suggestions are reflected in our
   existing implementation as of this writing.  We have no specific
   plans to follow or not follow these suggestions in any subsequent
   implementation.  The intent is to provide ideas as to what a more
   general approach might look like.  Whether or not these ideas should
   be implemented, or should become the basis for a future standard, are
   left as open questions.

 5.1.  More Flexible Envelope Rewrites

   One might argue that it is not really necessary for MTAs to rewrite
   envelopes when performing intra-enterprise message routing.  The

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   argument is as follows.  Taking an example from above, suppose Joe's
   account is on "mars.eng.example.com", and Joe's account advertises
   "joe@example.com" as one of its valid email addresses.  One would
   expect that Joe's "home" MTA knows what Joe's valid email addresses
   are.  When mail arrives on "mail.example.com" for "joe@example.com",
   and it finds Joe's LDAP entry that advertises this address, it should
   be able to route the message without rewriting the envelope under the
   assumption that Joe's "home" MTA (and other MTAs such as
   "hub.eng.example.com" that are "closer" to Joe's "home" MTA than
   "mail.example.com") can also correctly identify the address as
   belonging to Joe.

   However, existing practice in sites that use SMTP-based messaging
   often includes the rewriting of addresses to be host-specific.  In
   order to avoid going against existing practice, our MTA
   implementation rewrites envelopes to 'uid'@'mailHost', as explained
   above.  This is a fixed behavior, and some sites may desire more

   One way to provide more flexibility is to add an attribute, say:

           mailRoutingAddress      address for internal mail routing

   This could be added to the 'mailRecipient' and 'mailGroup' object
   classes as a way to explicitly specify how to rewrite the envelope
   when routing a message.  Then, if the 'mailRoutingAddress' is
   present, the envelope is rewritten to the indicated address,
   otherwise, the address is not rewritten.  This provides flexibility
   for site-specific policy governing whether or not envelopes are
   rewritten, and if so, how they are to be rewritten.  It obviates the
   fixed 'uid'@'mailHost' behavior in our implementation (see Section
   4.3), because the same information can then be stored in the
   'mailRoutingAddress' attribute.

   It should be noted that if the 'mailRoutingAddress' attribute were
   used as described here, that attribute would need to be added to the
   search specified in Section 4.1.  That is, an MTA receiving a message
   should search the directory for any entry whose 'mail',
   'mailAlternateAddress', or 'mailRoutingAddress' is the address in
   question, since any of those addresses could appear in the envelope.

   Also, the 'uid'@'mailHost' search could be removed from the method
   specified in Section 4.1, but some sites may still regard this as a
   desirable fallback, although in this case the reasons to keep it are
   more along the lines of the reasoning in Section 5.2.

   One might observe that 'mailRoutingAddress' and 'mailHost' may be
   partially redundant, and, in general, it is desirable to avoid

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   redundancy of information in the directory.  Having both attributes
   would be useful, however, if for some reason a network administrator
   wanted to separately control "next-hop" determination and envelope
   rewrites.  So if both attributes were present, 'mailHost' would
   determine where to route the message, and 'mailRoutingAddress' would
   determine how to rewrite the envelope.  If only 'mailRoutingAddress'
   were present, then the right-hand side (the domain part) of the
   routing address would determine the next destination.  If only
   'mailHost' were present, then the envelope would not be rewritten.

 5.2.  Localpart-only Searches

   Our implementation performs searches on email addresses as complete
   addresses (e.g., "joe@example.com").  We do not split the address at
   the "@" sign and search on the "local part", except in the case
   characterized above as a "fallback" search.  This approach is
   probably sufficient for most customers since they can always add more
   addresses to an account as needed.  It also reduces the risk of
   "namespace crossovers" that could result if customers with multiple
   distinct domains (e.g., with "joe" being a different person in each
   domain) did localpart-only searches.

   Nevertheless, some sites may desire the flexibility to configure
   their MTAs to perform "localpart-only" searches, once the MTA has
   ascertained that the domain part is considered to be "local".  They
   may then want the search to attempt matches against arbitrary
   attributes, like 'uid', 'cn' (with spaces and other illegal
   characters matching underscores or dots in the address), or some
   attribute whose purpose is to contain localpart-only email addresses.
   Site-specific needs can vary considerably in this area, and the most
   appropriate solution may be to make this part of an MTA's
   functionality as configurable as possible.

 5.3.  Mail Address Mappings

   Some sites have a need to perform what might be called a "transit
   service" whereby email sent to a given address is resent to another
   address (say, for a person who wants their former Internet service
   provider to resend their mail to their new account elsewhere).  This
   is often called an "alias", but, in a strict sense, "alias" means "an
   alternate name for something" (which is the purpose of
   'mailAlternateAddress'), so this might more properly be called a
   "mail address mapping".

   This effect can be accomplished with our existing implementation in
   several ways.  One way is to maintain a 'mailRecipient' entry that
   includes a forwarding address ('mailForwardingAddress' attribute).
   Another way is to maintain a "group of one" entry, i.e., a

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   'mailGroup' with only one member.

   However, some network administrators may not wish to represent such
   "transit" users in their directory service as being actual users or
   groups.  Therefore, it may be desirable to define a new object class
   for this purpose, e.g.:

           Object class:  mailAddressMapping
               Required attribute:
               Allowed attributes:

   The MTA would treat mail addressed to such an entry the same as it
   would for a non-local user who has a 'mailRoutingAddress' specified
   and no 'mailHost'; i.e., a message addressed to a
   'mailAddressMapping' entry (as per its 'mail' or
   'mailAlternateAddress' attributes) is resent to the address specified
   as its 'mailTransitAddress'.  The reason not to use
   'mailRoutingAddress' for this purpose is that the transit address
   must not participate in the Question (1) search (since doing so would
   cause the search to yield duplicate matches in the case where the
   targeted recipient is within the same organization).

 5.4.  More Configurability

   In lieu of a standard, mail server vendors could also achieve
   interoperability by providing a high degree of configurability in
   their products.  For example, each mail server product could provide
   a means to configure or program its methods of resolving each of
   Questions (1), (2), and (3); if all of the mail servers in a given
   site were configured to use the same methods, then they would, in
   theory, interoperate in terms of LDAP-based SMTP message routing as
   described in this document.  However, this approach to
   interoperability simply shifts the burden of standardization to the
   customer, and then there might still be a demand for a "recommended
   configuration profile" (i.e., a standard) for customers who desire
   solutions that work "right out of the box".

   On the other hand, some level of configurability with regard to the
   functionality discussed here may be desirable.

6.  Security Considerations

   As in all cases where account information is stored in an LDAP-based

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   directory service, network administrators must be careful to ensure
   that their directory service controls users' access to the entries
   and attributes stored therein, according to site policy (e.g.,
   allowing users to modify, say, their 'mailForwardingAddress'
   attribute, but not their 'mailHost' attribute).  Mail server products
   and their associated user management tools should help administrators
   to ensure this, and should also help administrators avoid
   configurations that would result in misdelivered mail due to
   "namespace crossovers" and other reasons.

7.  References

   [1]  W. Yeong, T. Howes, S. Kille, "Lightweight Directory Access
   Protocol", RFC 1777, March 1995.

   [2]  "Information Processing Systems - Open Systems Interconnection -
   The Directory: Overview of Concepts, Models and Service", ISO/IEC JTC
   1/SC21, International Standard 9594-1, 1988.

   [3]  J. Postel, "Simple Mail Transfer Protocol", RFC 821, August

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

   [5]  P. Mockapetris, "Domain names - concepts and facilities", RFC
   1034, November 1987.

   [6]  C. Partridge, "Mail routing and the domain system", RFC 974,
   January 1986.

   [7]  M. Smith, "Definition of the inetOrgPerson Object Class",
   Internet-Draft (work in progress), November 1996.

   [8]  "Information Processing Systems - Open Systems Interconnection -
   The Directory: Selected Object Classes", Recommendation X.521,
   ISO/IEC JTC 1/SC21, International Standard 9594-7, 1993.

   [9]  T. Howes, M. Smith, "An LDAP URL Format", RFC 1959, June 1996.

8.  Author's Address

   Hans Lachman
   Netscape Communications Corp.
   501 East Middlefield Road
   Mountain View, CA 94043

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   Phone: (415) 254-1900
   EMail: lachman@netscape.com

Appendix A.  mailRecipient Object Class and Attributes

   The following is an informal description of the 'mailRecipient'
   object class and associated attributes.  It was designed to be used
   as a "mix-in" object in combination with a person's LDAP entry (in
   our implementation, an 'inetOrgPerson' entry [7]) to enable a person
   to be recognized and handled as a mail user.

   Object class:  mailRecipient
       Required attribute:
       Allowed attributes:
                   Common name (person's full name).
                   "Primary" email address.  This is the address that
                   would likely be displayed by "white-pages" lookup
                   applications.  Must be a complete email address
                   (e.g., "joe@example.com").
                   Domains and IP addresses from which user may do POP
                   or IMAP login.
                   Email addresses that are considered valid for this
                   user in addition to their 'mail' address.  Must be
                   complete email addresses.
                   Auto-reply mode, may be one of:  'vacation' (send
                   reply text to sender, but only once per vacation),
                   'reply' (send reply text unconditionally), or 'echo'
                   (like 'reply' but include original message in the
                   Reply text to use with 'mailAutoReplyMode'.
                   Mail delivery option, one or more of:  'mailbox'
                   (deliver to user's POP/IMAP mailbox), 'native'
                   (deliver with platform's native delivery method,
                   e.g., "/usr/bin/mail"), or 'program' (perform program
                   delivery).  There must be at least one
                   'mailDeliveryOption' and/or 'mailForwardingAddress',
                   otherwise, mail to this account is undeliverable.
                   User-specifiable mail forwarding address(es).

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                   Fully-qualified domain name of the MTA that is the
                   final SMTP destination for mail addressed to this
                   account.  Used for routing (see Section 4.3), and
                   also used to determine which LDAP entries represent
                   accounts that are to be considered local to a given
                   mail server (see Section 4.2).
                   Identifier for the message store containing this
                   user's POP/IMAP mailbox.  Contains absolute path of
                   the message store directory (may be some other
                   identifier in the future).
                   Command text for program delivery.
                   Quota in bytes for user's POP/IMAP mailbox.
                   User-specifiable personal description.
                   User's login name.
                   User's password.

Appendix B.  mailGroup Object Class and Attributes

   The following is an informal description of the 'mailGroup' object
   class and associated attributes.  It was designed to be used as a
   "mix-in" object in combination with an LDAP group entry (in
   particular, a 'groupOfUniqueNames' entry [8]) to enable a group to be
   recognized and handled as a mail group.

   Object class:  mailGroup
       Required attributes:
                   "Primary" email address (as above).
       Allowed attributes:
                   Common name (name of the group).
                   Additional email addresses (as above).
                   FQDN of the MTA (as above).  For a group, if no
                   'mailHost' is specified, this implies that any mail
                   server may perform group handling for messages
                   addressed to this group (i.e., perform group list
                   expansion, and forward the message to the individual

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                   RFC 1959-style [9] specification of users who may
                   send mail to the group (if such restriction is
                   Domains from which users may send mail to the group
                   (if such restriction is desired).
                   RFC 1959-style filter expression specifying a search
                   criteria to identify users who should receive copies
                   of all messages to the group (in addition to the
                   formal group members, i.e., those who are specified
                   as members of the 'groupOfUniqueNames' with a
                   'uniqueMember' attribute), e.g., to include all
                   persons in the "Sales" organizational unit.
                   RFC 1959-style specification of users whom to notify
                   of mail delivery problems.
                   RFC 1959-style specification of a user whom to send
                   rejected messages (rejected because they came from
                   other than an "allowed broadcaster" or from outside
                   of the "allowed domains"), but only if the
                   'mgrpMsgRejectAction' attribute is present with value
                   Size in bytes of largest message that may be sent to
                   the group.
                   Action to take if a message is rejected due to
                   violating "allowed broadcaster" and/or "allowed
                   domain" restrictions (if any).  May be one of:
                   'reply' (send rejection notice to sender), 'bounce'
                   (send rejection notice to sender, including the
                   original message), or 'toModerator' (forward to
                   Text of rejection notice to use with
                   Email addresses of users who should receive copies of
                   all messages to the group (in addition to the formal
                   group members).  These users and those specified via
                   'mgrpDeliverTo' are also called "CC recipients",
                   since they are not formal group members.
                   Distinguished name (DN) of the person responsible for
                   the group.

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