NETWORK WORKING GROUP                                         K. Raeburn
Internet-Draft                                                       MIT
Updates: 4120 (if approved)                                       L. Zhu
Expires: December 27, 2006                                 K. Jaganathan
                                                   Microsoft Corporation
                                                           June 25, 2006

           Generating KDC Referrals to Locate Kerberos Realms

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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Copyright Notice

   Copyright (C) The Internet Society (2006).


   The memo documents a method for a Kerberos Key Distribution Center
   (KDC) to respond to client requests for Kerberos tickets when the
   client does not have detailed configuration information on the realms
   of users or services.  The KDC will handle requests for principals in
   other realms by returning either a referral error or a cross-realm
   TGT to another realm on the referral path.  The clients will use this

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   referral information to reach the realm of the target principal and
   then receive the ticket.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  4
   3.  Requesting a Referral  . . . . . . . . . . . . . . . . . . . .  4
   4.  Realm Organization Model . . . . . . . . . . . . . . . . . . .  5
   5.  Client Name Canonicalization . . . . . . . . . . . . . . . . .  5
   6.  Client Referrals . . . . . . . . . . . . . . . . . . . . . . .  7
   7.  Server Referrals . . . . . . . . . . . . . . . . . . . . . . .  8
   8.  Server Name Canonicalization (Informative) . . . . . . . . . . 10
   9.  Cross Realm Routing  . . . . . . . . . . . . . . . . . . . . . 10
   10. Caching Information  . . . . . . . . . . . . . . . . . . . . . 11
   11. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 11
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     14.1.  Normative References  . . . . . . . . . . . . . . . . . . 12
     14.2.  Informative References  . . . . . . . . . . . . . . . . . 12
   Appendix A.  Compatibility with Earlier Implementations of
                Name Canonicalization . . . . . . . . . . . . . . . . 13
   Appendix B.  Document history  . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
   Intellectual Property and Copyright Statements . . . . . . . . . . 16

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1.  Introduction

   Current implementations of the Kerberos AS and TGS protocols, as
   defined in [RFC4120], use principal names constructed from a known
   user or service name and realm.  A service name is typically
   constructed from a name of the service and the DNS host name of the
   computer that is providing the service.  Many existing deployments of
   Kerberos use a single Kerberos realm where all users and services
   would be using the same realm.  However in an environment where there
   are multiple trusted Kerberos realms, the client needs to be able to
   determine what realm a particular user or service is in before making
   an AS or TGS request.  Traditionally this requires client
   configuration to make this possible.

   When having to deal with multiple trusted realms, users are forced to
   know what realm they are in before they can obtain a ticket granting
   ticket (TGT) with an AS request.  However, in many cases the user
   would like to use a more familiar name that is not directly related
   to the realm of their Kerberos principal name.  A good example of
   this is an RFC 822 style email name.  This document describes a
   mechanism that would allow a user to specify a user principal name
   that is an alias for the user's Kerberos principal name.  In practice
   this would be the name that the user specifies to obtain a TGT from a
   Kerberos KDC.  The user principal name no longer has a direct
   relationship with the Kerberos principal or realm.  Thus the
   administrator is able to move the user's principal to other realms
   without the user having to know that it happened.

   Once a user has a TGT, they would like to be able to access services
   in any trusted Kerberos realm.  To do this requires that the client
   be able to determine what realm the target service principal is in
   before making the TGS request.  Current implementations of Kerberos
   typically have a table that maps DNS host names to corresponding
   Kerberos realms.  In order for this to work on the client, each
   application canonicalizes the host name of the service, for example
   by doing a DNS lookup followed by a reverse lookup using the returned
   IP address.  The returned primary host name is then used in the
   construction of the principal name for the target service.  In order
   for the correct realm to be added for the target host, the mapping
   table [domain_to_realm] is consulted for the realm corresponding to
   the DNS host name.  The corresponding realm is then used to complete
   the target service principal name.

   This traditional mechanism requires that each client have very
   detailed configuration information about the hosts that are providing
   services and their corresponding realms.  Having client side
   configuration information can be very costly from an administration
   point of view - especially if there are many realms and computers in

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   the environment.

   There are also cases where specific DNS aliases (local names) have
   been setup in an organization to refer to a server in another
   organization (remote server).  The server has different DNS names in
   each organization and each organization has a Kerberos realm that is
   configured to service DNS names within that organization.  Ideally
   users are able to authenticate to the server in the other
   organization using the local server name.  This would mean that the
   local realm be able to produce a ticket to the remote server under
   its name.  You could give that remote server an identity in the local
   realm and then have that remote server maintain a separate secret for
   each alias it is known as.  Alternatively you could arrange to have
   the local realm issue a referral to the remote realm and notify the
   requesting client of the server's remote name that should be used in
   order to request a ticket.

   This memo proposes a solution for these problems and simplifies
   administration by minimizing the configuration information needed on
   each computer using Kerberos.  Specifically it describes a mechanism
   to allow the KDC to handle canonicalization of names, provide for
   principal aliases for users and services and provide a mechanism for
   the KDC to determine the trusted realm authentication path by being
   able to generate referrals to other realms in order to locate

   Two kinds of KDC referrals are introduced in this memo:

   1. Client referrals, in which the client doesn't know which realm
      contains a user account.
   2. Server referrals, in which the client doesn't know which realm
      contains a server account.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Requesting a Referral

   In order to request referrals defined in section 5, 6, and 7, the
   Kerberos client MUST explicitly request the canonicalize KDC option
   (bit 15) [RFC4120] for the AS-REQ or TGS-REQ.  This flag indicates to
   the KDC that the client is prepared to receive a reply that contains
   a principal name other than the one requested.

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          KDCOptions ::= KerberosFlags
                   -- canonicalize (15)
                   -- other KDCOptions values omitted

   The client should expect, when sending names with the "canonicalize"
   KDC option, that names in the KDC's reply MAY be different than the
   name in the request.  A referral TGT is a cross realm TGT that is
   returned with the server name of the ticket being different from the
   server name in the request [RFC4120].

4.  Realm Organization Model

   This memo assumes that the world of principals is arranged on
   multiple levels: the realm, the enterprise, and the world.  A KDC may
   issue tickets for any principal in its realm or cross-realm tickets
   for realms with which it has a direct trust relationship.  The KDC
   also has access to a trusted name service that can resolve any name
   from within its enterprise into a realm.  This trusted name service
   removes the need to use an un-trusted DNS lookup for name resolution.

   For example, consider the following configuration, where lines
   indicate trust relationships:

                      /        \
                     /          \

   In this configuration, all users in the MS.COM enterprise could have
   a principal name such as alice@MS.COM, with the same realm portion.
   In addition, servers at MS.COM should be able to have DNS host names
   from any DNS domain independent of what Kerberos realm their
   principals reside in.

5.  Client Name Canonicalization

   A client account may have multiple principal names.  More useful,
   though, is a globally unique name that allows unification of email
   and security principal names.  For example, all users at MS may have
   a client principal name of the form "joe@MS.COM" even though the
   principals are contained in multiple realms.  This global name is
   again an alias for the true client principal name, which indicates
   what realm contains the principal.  Thus, accounts "alice" in the
   realm NTDEV.MS.COM and "bob" in OFFICE.MS.COM may log on as "alice@
   MS.COM" and "bob@MS.COM".

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   This utilizes a new client principal name type, as the AS-REQ message
   only contains a single realm field, and the realm portion of this
   name doesn't correspond to any Kerberos realm.  Thus, the entire name
   "alice@MS.COM" is transmitted as a single component in the client
   name field of the AS-REQ message, with a name type of NT-ENTERPRISE
   [RFC4120] (and the local realm name).  The KDC will recognize this
   name type and then transform the requested name into the true
   principal name.  The true principal name can be using a name type
   different from the requested name type.  Typically the true principal
   name will be a NT-PRINCIPAL [RFC4120].

   If the "canonicalize" KDC option is set, then the KDC MAY change the
   client principal name and type in the AS response and ticket returned
   from the name type of the client name in the request, and include a
   mandatory PA-DATA object authenticating the client name mapping:

     names          [0] SEQUENCE {
       requested-name [0] PrincipalName,
       real-name      [1] PrincipalName
     canon-checksum [1] Checksum

   The canon-checksum field is computed over the DER encoding of the
   names sequences, using the returned session key and a key usage value
   of (TBD).

   If the client name is unchanged, the PA-CLIENT-CANONICALIZED data is
   not included.  If the client name is changed, and the PA-CLIENT-
   CANONICALIZED field does not exist, or the checksum cannot be
   verified, or the requested-name field doesn't match the originally-
   transmitted request, the client should discard the response.

   For example the AS request may specify a client name of "bob@MS.COM"
   as an NT-ENTERPRISE name with the "canonicalize" KDC option set and
   the KDC will return with a client name of "104567" as a NT-UID, and a
   principal as the requested-name and the NT-UID "104567" principal as
   the real-name.

   It is assumed that the client discovers whether the KDC supports the
   NT-ENTERPRISE name type via out of band mechanisms.

   In order to enable one party in a user-to-user exchange to confirm
   the identity of another when only the alias is known, the KDC MAY
   include the following authorization data element, wrapped in AD-IF-
   RELEVANT, in the initial credentials and copy it from a ticket-

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   granting ticket into additional credentials:

   AD-LOGIN-ALIAS ::= SEQUENCE { -- ad-type number TBD --
     login-alias  [0] PrincipalName,
     checksum     [1] Checksum

   (Q: Wrapped inside KDCIssued too?  Use SEQUENCE OF PrincipalName?)

   The checksum is computed over the DER encoding of the login-alias
   field using (WHICH KEY?  If recipient can forge it, the KDC can't
   trust it when returned, and would have to verify that the alias is
   valid before copying it to additional credentials) and a key usage
   number of (TBD).

   The recipient of this authenticator must check the AD-LOGIN-ALIAS
   name, if present, in addition to the normal client name field,
   against the identity of the party with which it wishes to
   authenticate; either should be allowed to match.  (Note that this is
   not backwards compatible with [RFC4120]; if the server side of the
   user-to-user exchange does not support this extension, and does not
   know the true principal name, authentication may fail if the alias is
   sought in the client name field.)

6.  Client Referrals

   The simplest form of ticket referral is for a user requesting a
   ticket using an AS-REQ.  In this case, the client machine will send
   the AS-REQ to a convenient trusted realm, for example the realm of
   the client machine.  In the case of the name alice@MS.COM, the client
   MAY optimistically choose to send the request to MS.COM.  The realm
   in the AS-REQ is always the name of the realm that the request is for
   as specified in [RFC4120].

   The KDC will try to lookup the name in its local account database.
   If the account is present in the realm of the request, it SHOULD
   return a KDC reply structure with the appropriate ticket.

   If the account is not present in the realm specified in the request
   and the "canonicalize" KDC option is set, the KDC will try to lookup
   the entire name, alice@MS.COM, using a name service.  If this lookup
   is unsuccessful, it MUST return the error KDC_ERR_C_PRINCIPAL_UNKNOWN
   [RFC4120].  If the lookup is successful, it MUST return an error
   KDC_ERR_WRONG_REALM [RFC4120] and in the error message the crealm
   field will contain either the true realm of the client or another
   realm that MAY have better information about the client's true realm.
   The client SHALL NOT use a cname returned from a referral until that

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   name is validated.

   If the client receives a KDC_ERR_WRONG_REALM error, it will issue a
   new AS request with the same client principal name used to generate
   the first referral to the realm specified by the realm field of the
   Kerberos error message from the first request.  (The client realm
   name will be updated in the new request to refer to this new realm.)
   The client SHOULD repeat these steps until it finds the true realm of
   the client.  To avoid infinite referral loops, an implementation
   should limit the number of referrals.  A suggested limit is 5
   referrals before giving up.

   Since the same client name is sent to the referring and referred-to
   realms, both realms must recognize the same client names.  In
   particular, the referring realm cannot (usefully) define principal
   name aliases that the referred-to realm will not know.

   The true principal name of the client, returned in AS-REQ, can be
   validated in a subsequent TGS message exchange where its value is
   communicated back to the KDC via the authenticator in the PA-TGS-REQ
   padata [RFC4120].

7.  Server Referrals

   The primary difference in server referrals is that the KDC MUST
   return a referral TGT rather than an error message as is done in the
   client referrals.  There needs to be a place to include in the reply
   information about what realm contains the server.  This is done by
   returning information about the server name in the pre-authentication
   data field of the KDC reply [RFC4120], as specified later in this

   If the KDC resolves the server principal name into a principal in the
   realm specified by the service realm name, it will return a normal

   If the "canonicalize" flag in the KDC options is not set, the KDC
   MUST only look up the name as a normal principal name in the
   specified server realm.  If the "canonicalize" flag in the KDC
   options is set and the KDC doesn't find the principal locally, the
   KDC MAY return a cross-realm ticket granting ticket to the next hop
   on the trust path towards a realm that may be able to resolve the
   principal name.  The true principal name of the server SHALL be
   returned in the padata of the reply if it is different from what is
   specified the request.

   When a referral TGT is returned, the KDC MUST return the target realm

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   for the referral TGT as an KDC supplied pre-authentication data
   element in the response.  This referral information in pre-
   authentication data MUST be encrypted using the session key from the
   reply ticket.  The key usage value for the encryption operation used

   The pre-authentication data returned by the KDC, which contains the
   referred realm and the true principal name of server, is encoded in
   DER as follows.

          PA-SERVER-REFERRAL      25

          PA-SERVER-REFERRAL-DATA ::= EncryptedData
                                -- ServerReferralData --

          ServerReferralData ::= SEQUENCE {
                 referred-realm           [0] Realm OPTIONAL,
                                -- target realm of the referral TGT
                 true-principal-name      [1] PrincipalName OPTIONAL,
                                -- true server principal name
                 requested-principal-name [2] PrincipalName OPTIONAL,
                                -- requested server name

   Clients SHALL NOT accept a reply ticket, whose the server principal
   name is different from that of the request, if the KDC response does
   not contain a PA-SERVER-REFERRAL padata entry.

   The requested-principal-name MUST be included by the KDC, and MUST be
   verified by the client, if the client sent an AS-REQ, as protection
   against a man-in-the-middle modification to the AS-REQ message.

   (Note that with the forthcoming rfc1510ter, the AS-REP may include an
   option checksum of the AS-REQ, in which case this check would no
   longer be necessary.)

   The referred-realm field is present if and only if the returned
   ticket is a referral TGT, not a service ticket for the requested
   server principal.

   When a referral TGT is returned and the true-principal-name field is
   present, the client MUST use that name in the subsequent requests to
   the server realm when following the referral.

   Client SHALL NOT accept a true server principal name for a service
   ticket if the true-principal-name field is not present in the PA-

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   The client will use this referral information to request a chain of
   cross-realm ticket granting tickets until it reaches the realm of the
   server, and can then expect to receive a valid service ticket.

   However an implementation should limit the number of referrals that
   it processes to avoid infinite referral loops.  A suggested limit is
   5 referrals before giving up.

   Here is an example of a client requesting a service ticket for a
   service in realm NTDEV.MS.COM where the client is in OFFICE.MS.COM.

          +NC = Canonicalize KDCOption set
          +PA-REFERRAL = returned PA-SERVER-REFERRAL
          C: TGS-REQ sname=http/ +NC to OFFICE.MS.COM
          S: TGS-REP sname=krbtgt/MS.COM@OFFICE.MS.COM +PA-REFERRAL
             containing MS.COM as the referred realm with no
          C: TGS-REQ sname=http/ +NC to MS.COM
          S: TGS-REP sname=krbtgt/NTDEV.MS.COM@MS.COM +PA-REFERRAL
             containing NTDEV.MS.COM as the referred realm with no
          C: TGS-REQ sname=http/ +NC to NTDEV.MS.COM
          S: TGS-REP sname=http/

   Note that any referral or alias processing of the server name in
   user-to-user authentication should use the same data as client name
   canonicalization or referral.  Otherwise, the name used by one user
   to log in may not be useable by another for user-to-user
   authentication to the first.

8.  Server Name Canonicalization (Informative)

   No attempt is being made in this document to provide a means for
   dealing with local-realm server principal name canonicalization or
   aliasing.  The most obvious use case for this would be a hostname-
   based service principal name ("host/"), with a DNS
   alias ("foo") for the server host which is used by the client.  There
   are other ways this can be handled, currently, though they may
   require additional configuration on the application server or KDC or

9.  Cross Realm Routing

   The current Kerberos protocol requires the client to explicitly
   request a cross-realm TGT for each pair of realms on a referral
   chain.  As a result, the client need to be aware of the trust

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   hierarchy and of any short-cut trusts (those that aren't parent-
   child trusts).

   Instead, using the server referral routing mechanism as defined in
   Section 7, The KDC will determine the best path for the client and
   return a cross-realm TGT as the referral TGT, and the target realm
   for this TGT in the PA-SERVER-REFERRAL of the KDC reply.

   If the "canonicalize" KDC option is not set, the KDC SHALL NOT return
   a referral TGT.  Clients SHALL NOT process referral TGTs if the KDC
   response does not contain the PA-SERVER-REFERRAL padata.

10.  Caching Information

   It is possible that the client may wish to get additional credentials
   for the same service principal, perhaps with different authorization-
   data restrictions or other changed attributes.  The return of a
   server referral from a KDC can be taken as an indication that the
   requested principal does not currently exist in the local realm.
   Clearly, it would reduce network traffic if the clientn could cache
   that information and use it when acquiring the second set of
   credentials for a service, rather than always having to re-check with
   the local KDC to see if the name has been created locally.

   Rather than introduce a new timeout field for this cached
   information, we can use the lifetime of the returned TGT in this
   case.  When the TGT expires, the previously returned referral from
   the local KDC should be considered invalid, and the local KDC must be
   asked again for information for the desired service principal name.
   If the client is still in contact with the service and needs to
   reauthenticate to the same service regardless of local service
   principal name assignments, it should use the referred-realm and
   true-principal-name values when requesting new credentials.

   Accordingly, KDC authors and maintainers should consider what factors
   (e.g., DNS alias lifetimes) they may or may not wish to incorporate
   into credential expiration times in cases of referrals.

11.  Open Issues

   When should client name aliases be included in credentials?

   Should all known client name aliases be included, or only the one
   used at initial ticket acquisition?

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12.  Security Considerations

   For the AS exchange case, it is important that the logon mechanism
   not trust a name that has not been used to authenticate the user.
   For example, the name that the user enters as part of a logon
   exchange may not be the name that the user authenticates as, given
   that the KDC_ERR_WRONG_REALM error may have been returned.  The
   relevant Kerberos naming information for logon (if any), is the
   client name and client realm in the service ticket targeted at the
   workstation that was obtained using the user's initial TGT.

   How the client name and client realm is mapped into a local account
   for logon is a local matter, but the client logon mechanism MUST use
   additional information such as the client realm and/or authorization
   attributes from the service ticket presented to the workstation by
   the user, when mapping the logon credentials to a local account on
   the workstation.

13.  Acknowledgments

   Sam Hartman and authors came up with the idea of using the ticket key
   to encrypt the referral data, which prevents cut and paste attack
   using the referral data and referral TGTs.

   John Brezak, Mike Swift, and Jonathan Trostle wrote the initial
   version of this document.

14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

14.2.  Informative References

              Tung, B. and L. Zhu, "Public Key Cryptography for Initial
              Authentication in Kerberos",
              draft-ietf-cat-kerberos-pk-init-34 (work in progress),
              February 2006.

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   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

   [XPR]      Trostle, J., Kosinovsky, I., and M. Swift, "Implementation
              of Crossrealm Referral Handling in the MIT Kerberos
              Client",  Network and Distributed System Security
              Symposium, February 2001.

Appendix A.  Compatibility with Earlier Implementations of Name

   The Microsoft Windows 2000 and Windows 2003 releases included an
   earlier form of name-canonicalization [XPR].  Here are the

   1) The TGS referral data is returned inside of the KDC message as
      "encrypted pre-authentication data".

          EncKDCRepPart   ::= SEQUENCE {
                 key                [0] EncryptionKey,
                 last-req           [1] LastReq,
                 nonce              [2] UInt32,
                 key-expiration     [3] KerberosTime OPTIONAL,
                 flags              [4] TicketFlags,
                 authtime           [5] KerberosTime,
                 starttime          [6] KerberosTime OPTIONAL,
                 endtime            [7] KerberosTime,
                 renew-till         [8] KerberosTime OPTIONAL,
                 srealm             [9] Realm,
                 sname             [10] PrincipalName,
                 caddr             [11] HostAddresses OPTIONAL,
                 encrypted-pa-data [12] SEQUENCE OF PA-DATA OPTIONAL

   2) The preauth data type definition in the encrypted preauth data is
      as follows:

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          PA-SVR-REFERRAL-INFO       20

                 referred-name   [1] PrincipalName OPTIONAL,
                 referred-realm  [0] Realm

   3) When [I-D.ietf-cat-kerberos-pk-init] is used, the NT-ENTERPRISE
      client name is encoded as a Subject Alternative Name (SAN)
      extension [RFC3280] in the client's X.509 certificate.  The type
      of the otherName field for this SAN extension is AnotherName
      [RFC3280].  The type-id field of the type AnotherName is id-ms-sc-
      logon-upn ( and the value field of the type
      AnotherName is a KerberosString [RFC4120].  The value of this
      KerberosString type is the single component in the name-string
      [RFC4120] sequence for the corresponding NT-ENTERPRISE name type.

   In Microsoft's current implementation through the use of global
   catalogs any domain in one forest is reachable from any other domain
   in the same forest or another trusted forest with 3 or less
   referrals.  A forest is a collection of realms with hierarchical
   trust relationships: there can be multiple trust trees in a forest;
   each child and parent realm pair and each root realm pair have
   bidirectional transitive direct rusts between them.

   While we might want to permit multiple aliases to exist and even be
   reported in AD-LOGIN-ALIAS, the Microsoft implementation permits only
   one alias to exist, so this question had not previously arisen.

Appendix B.  Document history

   08 Moved Microsoft implementation info to appendix.  Clarify lack of
      local server name canonicalization.  Added optional authz-data for
      login alias, to support user-to-user case.  Added requested-
      principal-name to ServerReferralData.  Added discussion of caching
      information, and referral TGT lifetime.
   07 Re-issued with new editor.  Fixed up some references.  Started

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Authors' Addresses

   Kenneth Raeburn
   Massachusetts Institute of Technology
   77 Massachusetts Avenue
   Cambridge, MA  02139


   Larry Zhu
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052


   Karthik Jaganathan
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052


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