Subject: matched values 05

From: David Chadwick <[email-masked]>
Date: Tue, 12 Dec 2000 23:07:07 +0000

Could you please issue the attached ID, which is now ready for last call
in the LDAPEXT group

David
-- 
*****************************************************************

David Chadwick, BSc PhD
Post: IS Institute, University of Salford, Salford M5 4WT
Tel: +44 161 295 5351  Fax +44 161 745 8169
Mobile: +44 790 167 0359
Email: [email-masked]
Home Page:  http://www.salford.ac.uk/its024/chadwick.htm
Research Projects: http://sec.isi.salford.ac.uk
Understanding X.500:  http://www.salford.ac.uk/its024/X500.htm
X.500/LDAP Seminars: http://www.salford.ac.uk/its024/seminars.htm
Entrust key validation string: MLJ9-DU5T-HV8J

*****************************************************************

Internet-Draft                                      David Chadwick
LDAPExt WG                       		   University of Salford      
Intended Category: Standards Track                     Sean Mullan
								  Sun Microsystems
Expires: 11 June 2001                             11 December 2000

Returning Matched Values with LDAPv3
<draft-ldapext-matchedval-05.txt>

STATUS OF THIS MEMO

This document is an Internet-Draft and is in full conformance with 
all the provisions of Section 10 of RFC2026 [1].

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
time. It is inappropriate to use Internet-Drafts as reference 
material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

This Internet-Draft expires on 11 June 2001. 

Comments and suggestions on this document are encouraged. Comments on 
this document should be sent to the LDAPEXT working group discussion 
list:
                [email-masked]

or directly to the authors.

ABSTRACT

This document describes a control for the Lightweight Directory 
Access Protocol v3 that is used to return a subset of attribute 
values from an entry, specifically, only those values that match a 
"values return" filter. Without support for this control, a client 
must retrieve all of an attribute's values and search for specific 
values locally.

1. Introduction

When reading an attribute from an entry using LDAPv3 [2], it is 
normally only possible to read either the attribute type, or the 
attribute type and all its values. It is not possible to selectively 
read just a few of the attribute values. If an attribute holds many 
values, for example, the userCertificate attribute, or the subschema 
publishing operational attributes objectClasses and attributeTypes 
[3], then it may be desirable for the user to be able to selectively 
retrieve a subset of the values, specifically, those attribute values 
that match some user defined selection criteria. Without the control 
specified in this [ID/standard/document] a client must read all of 
the attribute's values and filter out the unwanted values, 
necessitating the client to implement the matching rules. It also 
requires the client to potentially read and process many irrelevant 
values, which can be inefficient if the values are large or complex, 
or there are many values stored per attribute.

This [ID/Standard/document] specifies an LDAPv3 control to enable a 
user to return only those values that matched (i.e. returned TRUE to) 
one or more elements of a newly defined "values return" filter. This 
control can be especially useful when used in conjunction with 
extensible matching rules that match on one or more components of 
complex binary attribute values.

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 RFC 2119 [5].

2. The valuesReturnFilter Control

The valuesReturnFilter control MAY be critical or non-critical as 
determined by the user. It only has meaning for the Search operation, 
and SHOULD only be added to the Search operation by the client. If 
the server supports the control and it is present on a Search 
operation, the server MUST obey the control regardless of the value 
of the criticality flag. 

If the control is marked as critical, and either the server does not  
support the control or the control is applied to an operation other 
than Search, then the server MUST return an 
unavailableCriticalExtension error.  If the control is not marked as 
critical, and either the server does not support the control or the 
control is applied to an operation other than Search, then the server
MUST ignore the control.

The object identifier for this control is 1.2.826.0.1.3344810.2.3

The controlValue is an OCTET STRING, whose value is the BER encoding 
of a value of the type ValuesReturnFilter.

        ValuesReturnFilter ::= SEQUENCE OF SimpleFilterItem

        SimpleFilterItem ::= CHOICE {
                equalityMatch   [3] AttributeValueAssertion,
                substrings      [4] SubstringFilter,
                greaterOrEqual  [5] AttributeValueAssertion,
                lessOrEqual     [6] AttributeValueAssertion,
                present         [7] AttributeDescription,
                approxMatch     [8] AttributeValueAssertion,
                extensibleMatch [9] SimpleMatchingAssertion }

         SimpleMatchingAssertion ::= SEQUENCE {
                matchingRule    [1] MatchingRuleId OPTIONAL,
                type            [2] AttributeDescription OPTIONAL,
--- at least one of the above must be present
                matchValue      [3] AssertionValue}

All the above data types have their standard meanings as defined in 
[2].

If the server supports this control, the server MUST make use of the 
control as follows:

(1) The Search Filter is first executed in order to determine 
which entries satisfy the Search criteria (these are the 
filtered entries). The control has no impact on this step.

(2) If the typesOnly parameter of the Search Request is TRUE, 
the control has no effect and the Search Request SHOULD be 
processed as if the control had not been specified.

(3) If the attributes parameter of the Search Request consists 
of a list containing only the attribute with OID "1.1" 
(specifying that no attributes are to be returned), the control 
has no effect and the Search Request SHOULD be processed as if 
the control had not been specified.

(4) For each attribute listed in the attributes parameter of the 
Search Request, the server MUST apply the control as follows to 
each entry in the set of filtered entries:

i) Every attribute value that evaluates TRUE against one or 
more elements of the ValuesReturnFilter is placed in the 
corresponding SearchResultEntry.
ii) Every attribute value that evaluates FALSE or undefined 
against all elements of the ValuesReturnFilter is not 
placed in the corresponding SearchResultEntry. An 
attribute that has no values selected is returned with an 
empty set of vals.

Note. If the AttributeDescriptionList is empty or comprises "*" 
then the control MUST be applied against every attribute.

3. Relationship to X.500

The control is a superset of the matchedValuesOnly (MVO) boolean of 
the X.500 DAP [4] Search argument, as amended in the latest version 
[6]. Close examination of the matchedValuesOnly boolean by the 
LDAPEXT group revealed ambiguities and complexities in the MVO 
boolean that could not easily be resolved. For example, it was not 
clear if the MVO boolean governed only those attribute values that 
contributed to the overall truth of the filter, or all of the 
attribute values even if the filter item containing the attribute 
evaluated to false. For this reason the LDAPEXT group decided to 
replace the MVO boolean with a simple filter that removes any 
uncertainty as to whether an attribute value has been selected or 
not. 

4. Relationship to other LDAP Controls

The purpose of this control is to select zero, one or more attribute 
values from each requested attribute in a filtered entry, and to 
discard the remainder. Once the attribute values have been discarded 
by this control they MUST NOT be re-instated into the Search results 
by other controls. 

This control acts independently of other LDAP controls such as server 
side sorting [10] and duplicate entries [7]. However, there might be 
interactions between this control and other controls so that a 
different set of Search Result Entries are returned, or the entries 
are returned in a different order, depending upon the sequencing of 
this control and other controls in the LDAP request. For example, 
with server side sorting, if sorting is done first, and value return 
filtering second, the set of Search Results may appear to be in the 
wrong order since the value filtering may remove the attribute values 
upon which the ordering was done. (The sorting document specifies 
that entries without any sort key attribute values should be treated 
as coming after all other attribute values.) Similarly with duplicate 
entries, if duplication is performed before value filtering, the set 
of Search Result Entries may contain identical duplicate entries, 
each with an empty set of attribute values, because the value 
filtering removed the attribute values that were used to duplicate 
the results.

For these reasons it is recommended that the ValuesReturnFilter 
control in a SearchRequest SHOULD precede other controls that affect 
the number and ordering of SearchResultEntrys.

5. Examples

All entries are provided in LDIF format [8].

The string representation of the valuesReturnFilter in the examples 
below uses the following ABNF notation:

 valuesReturnFilter = "(" 1*simpleFilterItem ")"
 simpleFilterItem = "(" item ")"

where item is as defined by RFC2254 [11].  

(1) The first example shows how the control can be set to return all 
attribute values from one attribute type (e.g. telephoneNumber) and a 
subset of values from another attribute type (e.g. mail).

The entries below represent organizationalPerson object classes 
located somewhere beneath the distinguished name dc=ac, dc=uk.

dn: cn=Sean Mullan, ou=people, dc=sun, dc=ac, dc=uk
cn: Sean Mullan
sn: Mullan
objectClass: organizationalPerson
objectClass: person
objectClass: inetOrgPerson
mail: [email-masked]
mail: [email-masked]
telephoneNumber: + 781 442 0926
telephoneNumber: 555-9999

dn: cn=David Chadwick, ou=isi, o=salford, dc=ac, dc=uk 
cn: David Chadwick
sn: Chadwick
objectClass: organizationalPerson
objectClass: person
objectClass: inetOrgPerson
mail: [email-masked]

An LDAP search operation is specified with a baseObject set to the
DN of the search base (i.e. dc=ac, dc=uk), a subtree scope, a filter 
set to (sn=mullan), and the list of attributes to be returned set to 
"mail, telephoneNumber". In addition, a ValuesReturnFilter control is 
set to ((mail=*hotmail.com)(telephoneNumber=*))

The search results returned by the server would consist of the 
following entry:

dn: cn=Sean Mullan, ou=people, dc=sun, dc=ac, dc=uk
mail: [email-masked]
telephoneNumber: + 781 442 0926
telephoneNumber: 555-9999

Note that the control has no effect on the values returned for the 
"telephoneNumber" attribute (all of the values are returned), since 
the control specified that all values should be returned.

(2) The second example shows how one might retrieve a single 
attribute type subschema definition for the "gunk" attribute with OID 
1.2.3.4.5 from the subschema subentry 

Assume the subschema subentry is held below the root entry with DN 
cn=subschema subentry, o=myorg and this holds an attributeTypes 
operational attribute holding the descriptions of the 35 attributes 
known to this server (each description is held as a single attribute 
value of the attributeTypes attribute). 

dn: cn=subschema subentry, o=myorg
cn: subschema subentry
objectClass: subschema
attributeTypes: ( 2.5.4.3 NAME 'cn' SUP name )
attributeTypes: ( 2.5.4.6 NAME 'c' SUP name SINGLE-VALUE )
attributeTypes: ( 2.5.4.0 NAME 'objectClass' EQUALITY
 objectIdentifierMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.38 )
attributeTypes: ( 2.5.18.2 NAME 'modifyTimestamp' EQUALITY
 generalizedTimeMatch ORDERING generalizedTimeOrderingMatch
 SYNTAX 1.3.6.1.4.1.1466.115.121.1.24 SINGLE-VALUE NO-USER-
 MODIFICATION USAGE directoryOperation )
attributeTypes: ( 2.5.21.6 NAME 'objectClasses' EQUALITY
 objectIdentifierFirstComponentMatch SYNTAX
 1.3.6.1.4.1.1466.115.121.1.37 USAGE directoryOperation )
attributeTypes: ( 1.2.3.4.5 NAME 'gunk' EQUALITY caseIgnoreMatch
 SUBSTR caseIgnoreSubstringsMatch SYNTAX
 1.3.6.1.4.1.1466.115.121.1.44{64} )
attributeTypes: ( 2.5.21.5 NAME 'attributeTypes' EQUALITY
 objectIdentifierFirstComponentMatch SYNTAX
 1.3.6.1.4.1.1466.115.121.1.3 USAGE directoryOperation )

plus another 28 - you get the idea.

The user creates an LDAP search operation with a baseObject set to 
cn=subschema subentry, o=myorg, a scope of base, a filter set to 
(objectClass=subschema), the list of attributes to be returned set to 
"attributeTypes", and the ValuesReturnFilter set to 
((attributeTypes=1.2.3.4.5))

The search result returned by the server would consist of the 
following entry:

dn: cn=subschema subentry, o=myorg
attributeTypes: ( 1.2.3.4.5 NAME 'gunk' EQUALITY caseIgnoreMatch
 SUBSTR caseIgnoreSubstringsMatch SYNTAX
 1.3.6.1.4.1.1466.115.121.1.44{64} )

(3) The final example shows how the control can be used to match on a 
userCertificate attribute value with a particular key usage bit set 
(in this case the key encipherment bit). Note that this example 
requires the LDAP server to support the certificateMatch matching 
rule defined in [9] and extensible matching.

The entry below represent a pkiUser object class stored in the 
directory.

dn: cn=David Chadwick + serialNumber=123456, ou=people, o=University
 of Salford, c=gb
cn: David Chadwick + serialNumber=123456
objectClass: person 
objectClass: organizationalPerson 
objectClass: pkiUser 
objectClass: inetOrgPerson
sn: Chadwick
mail: [email-masked]
userCertificate: {binary representation of certificate including key 
usage bit of digitalSignature (0)}
userCertificate: {binary representation of certificate including key 
usage bit of nonRepudiation (1)}
userCertificate: {binary representation of certificate including key 
usage bit of key encipherment (2)}
userCertificate: {binary representation of certificate including key 
usage bit of data encipherment (3)}

An LDAP search operation is specified with a baseObject set to 
o=University of Salford, c=gb, a subtree scope, a filter set to 
(sn=chadwick) and the list of attributes to be returned set to 
"userCertificate;binary". In addition, a ValuesReturnFilter control 
is set to ((userCertificate:2.5.13.35:=(USE'001'B)))

The search result returned by the server would consist of the 
following entry:

dn: cn=David Chadwick + serialNumber=123456, ou=people, o=University
 of Salford, c=gb
userCertificate;binary: {binary representation of certificate with 
key usage bit of key encipherment (2)}

6. Security Considerations

This [ID/standard/document] does not primarily discuss security 
issues. 

Note however that attribute values MUST only be returned if the 
access controls applied by the LDAP server allow them to be returned, 
and in this respect the effect of the ValuesReturnFilter control is 
of no consequence.

Note that the ValuesReturnFilter control may have a positive effect 
on the deployment of public key infrastructures. Certain PKI 
operations, like searching for specific certificates, become more 
practical when combined with X.509 certificate matching rules at the 
server, and more scalable, since the control avoids the downloading 
of potentially large numbers of irrelevant certificates which would 
have to be processed and filtered locally (which in some cases is 
very difficult to perform).

7. Acknowledgements

The authors would like to thank members of the LDAPExt list for their 
constructive comments on earlier versions of this 
[ID/standard/document], and in particular to Harald Alvestrand who 
first suggested having an attribute return filter and Bruce 
Greenblatt who first proposed a syntax for this control.

8. Copyright

Copyright (C) The Internet Society (date). All Rights Reserved.

This document and translations of it may be copied and furnished to 
others, and derivative works that comment on or otherwise explain it 
or assist in its implementation may be prepared, copied, published 
and distributed, in whole or in part, without restriction of any 
kind, provided that the above copyright notice and this paragraph are 
included on all such copies and derivative works.  However, this 
document itself may not be modified in any way, such as by removing 
the copyright notice or references to the Internet Society or other 
Internet organizations, except as needed for the purpose of 
developing Internet standards in which case the procedures for 
copyrights defined in the Internet Standards process must be 
followed, or as required to translate it into languages other than 
English.

The limited permissions granted above are perpetual and will not be 
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an 
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING 
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING 
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION 
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

9. References

[1] S. Bradner. "The Internet Standards Process -- Revision 3", RFC 
2026, October 1996.
[2] M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access  
Protocol (v3)", Dec. 1997, RFC 2251
[3] M. Wahl, A. Coulbeck, T. Howes, S. Kille, "Lightweight Directory 
Access Protocol (v3): Attribute Syntax Definitions", RFC 2252, Dec 
1997
[4] ITU-T Rec. X.511, "The Directory: Abstract Service Definition", 
1993.
[5] S.Bradner. "Key words for use in RFCs to Indicate Requirement 
Levels", RFC 2119, March 1997.
[6] Draft ISO/IEC 9594 / ITU-T Rec X.511 (2001) The Directory: 
Abstract Service Definition.
[7] J. Sermersheim. "LDAP Control for a Duplicate Entry 
Representation of Search Results", Internet Draft <draft-ietf-
ldapext-ldapv3-dupent-06.txt>, October 2000.
[8] G. Good. "The LDAP Data Interchange Format (LDIF) - Technical 
Specification". RFC 2849, June 2000.
[9] D. Chadwick, S.Legg. "Internet X.509 Public Key Infrastructure - 
Additional LDAP Schema for PKIs and PMIs", Internet Draft <draft-
pkix-ldap-schema-01.txt>, September 2000
[10] T. Howes, M. Wahl, A. Anantha, "LDAP Control Extension for 
Server Side Sorting of Search Results", RFC 2891, August 2000
[11] T. Howes. "The String Representation of LDAP Search Filters". 
RFC 2254, December 1997.

10. Authors Addresses

David Chadwick
IS Institute
University of Salford
Salford M5 4WT 
England

Email: [email-masked]
Tel: +44 161 295 5351

Sean Mullan			
Sun Microsystems
East Point Business Park
Dublin 3
Ireland
Tel: +353 1 853 0655
Email: [email-masked]

11. Changes since version 2

i) Revised the examples to be more appropriate
ii) Section on interactions with other LDAP controls added
iii) Removed Editor's note concerning present filter
iv) Tightened wording about its applicability to other operations 
and use of criticality field

Changes since version 3

i) Mandated that at least one of type and matchingRule in 
simpleMatchingAssertion be present
ii) Fixed LDIF mistakes in the examples
iii) Additional minor editorials only

Changes since version 4

i) corrected the ABNF for single items of valuesReturnFilter

---

From: Bruce Greenblatt <[email-masked]>
Date: Mon, 18 Dec 2000 08:37:07 -0800

While I appreciate all of the hard work that has gone into the development 
of the control, I don't feel that it is a good idea.  So, I guess the time 
has come (again) for me to state my opinion about the Matched Values Only 
(MVO) control.  IMHO the MVO control as described in this draft is a 
negative.  Throughout its progression, it has grown increasingly more 
complex.  As I recall, it started out life as a simple boolean.

The problem that is solved is mostly due to poor schema and DIT structure 
design.  There is no reason to have so many attribute values in the 
attribute of an entry.  As an example of how to solve this problem, I wrote 
a draft that described an alternative way of storing certificates (alas, it 
has expired).  With more thought given to the schema and DIT structure 
design, existing protocol and simple matching rules can be used to solve 
the problem that this complex MVO control solves.

I don't think that the Section 4 adequately address the problems of the 
interaction with other controls.  I am not convinced that the MVO control 
acts independently of server side sorting.  It seems to me that attribute 
values that are used in the sorting process can be subsequently affected by 
the MVO control.

My recommendation would be to not progress this draft at all.  If it is to 
be progressed at all, it should go only as Experimental since the impact on 
existing controls has not been explored, and since there are simpler 
solutions to the problem it claims to solve.

Thanks,

Bruce Greenblatt

==============================================
Bruce Greenblatt, Ph. D.
Directory Tools and Application Services, Inc.
http://www.directory-applications.com
See my new Book on Internet Directories: 
http://www.phptr.com/ptrbooks/ptr_0139744525.html

---

From: "Kurt D. Zeilenga" <[email-masked]>
Date: Mon, 18 Dec 2000 12:57:03 -0800

The data model supports multiple values.  Often only a
subset of the values are of interest.  I believe that such
functionality is generally useful in the presence of
attributes with (even a small number of) multiple values.

This I-D provides a well defined mechanism to request only
the return of values of interest.  This mechanism is, IMO,
technically sound and is quite suitable for publication on
standard track.

Kurt

---

From: David Chadwick <[email-masked]>
Date: Mon, 18 Dec 2000 22:58:35 -0800

> complex.  As I recall, it started out life as a simple boolean.

Correct, this was so it could conform to the X.50O extension. But after
much discussion on the list we found that the X.500 solution was
deficient. Hence the use of a simple filter (which servers will already
have implemented for the Search operation). I propose to align the
future X.50O with this control under the X.500 alignment work item that
it now active.

> attribute of an entry.  

I believe this is a fundamentally wrong statement. Sure, you can ensure
that no entry ever has more than single attribute values in it. But this
is not good DIT design, this is poor DIT design, as you are being forced
to bend the DIT to fit the LDAP protocol that is incapable of supporting
a more natural DIT design e.g. for a PKI I have to have several entries,
each containing one certificate, because LDAP cannot support a single
entry with multiple certificates. If you are interested in more details
about this and other LDAP limitations, I have a written a paper that is
currently under review for publication entitled "Deficiencies in LDAP
when used to support a PKI". Once the paper is accepted I will let you
have a copy if you wish.

>As an example of how to solve this problem, I wrote

Sure they can. Also, you can build a car with a hammer and nails if you
dont possess nuts, bolts and screws, but I would hardly call it good
design. Rather it is poor design forced upon you because of a lack of
proper tools.

> values that are used in the sorting process can be subsequently affected by

Ok. Please tell me where the ID is deficient in this area and we will
fix it. I have attempted to cover the overlaps in this section, but if I
have missed something please provide a concrete example and we will fix
it.

David

David Chadwick, BSc PhD

---

From: Bruce Greenblatt <[email-masked]>
Date: Tue, 19 Dec 2000 15:33:36 -0800

>have missed something please provide a concrete example and we will fix

If you process sort then MVO:  You sort entries based on certain attribute 
values which may be subsequently deleted by the MVO control processing.

If you process MVO the sort: The sorting is always done on the remaining 
entries.

It appears to me that there is the possibility that the search results will 
come back in a different order depending on how the MVO and sort controls 
are processed.  Section 4 states: "This control acts independently of other 
LDAP controls such as server side sorting".  To me if the controls were 
independent, I would get exactly the same data back in exactly the same 
order.  So, I don't think that MVO and the sort control are "independent".

Note that RFC 2891 requires the following processing method for 
multi-valued attributes: when an entry happens to have multiple values for 
that attribute and no other controls are present that affect the sorting 
order, then the server SHOULD use the least value (according to the 
ORDERING rule for that attribute).

So, I would require the MVO to be processed BEFORE the sort 
control.  Otherwise, the sorted entries may turn out to be no longer sorted.

---

From: "Kurt D. Zeilenga" <[email-masked]>
Date: Tue, 19 Dec 2000 18:37:43 -0800

At 03:33 PM 12/19/00 -0800, Bruce Greenblatt wrote:
>If you process sort then MVO:  You sort entries based on certain attribute values which may be subsequently deleted by the MVO control processing.

Yes.

>If you process MVO the sort: The sorting is always done on the remaining entries.

Yes.

>It appears to me that there is the possibility that the search results will come back in a different order depending on how the MVO and sort controls are processed.

Yes.

>Section 4 states: "This control acts independently of other LDAP controls such as server side sorting".  To me if the controls were independent, I would get exactly the same data back in exactly the same order.  So, I don't think that MVO and the sort control are "independent".

I would say that each control acts independently upon a set of inputs
provided to it but that the input to the control is dependent on
what factors proceed it in the processing of the operation. 

>Note that RFC 2891 requires the following processing method for multi-valued attributes: when an entry happens to have multiple values for that attribute and no other controls are present that affect the sorting order, then the server SHOULD use the least value (according to the ORDERING rule for that attribute).

This issue can be resolved by simply stating that the MVO control
may affect sort ordering in the presence of sort controls as it
may alter results after the sort has been completed.

>So, I would require the MVO to be processed BEFORE the sort control.  Otherwise, the sorted entries may turn out to be no longer sorted.

I disagree.  The sort control, like any other control, is subject
to other factors including extension.  It is perfectly reasonable
for the MVO control to extend search operations extended by a sort
control and to depend the semantics upon sequencing of controls
(it is a sequence after all).

Kurt