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RFC 2244

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Proposed STD
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ACAP -- Application Configuration Access Protocol

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Network Working Group                                          C. Newman
Request for Comments: 2244                                      Innosoft
Category: Standards Track                                    J. G. Myers
                                                                Netscape
                                                           November 1997


           ACAP -- Application Configuration Access Protocol


Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society 1997.  All Rights Reserved.

Abstract

   The Application Configuration Access Protocol (ACAP) is designed to
   support remote storage and access of program option, configuration
   and preference information.  The data store model is designed to
   allow a client relatively simple access to interesting data, to allow
   new information to be easily added without server re-configuration,
   and to promote the use of both standardized data and custom or
   proprietary data.  Key features include "inheritance" which can be
   used to manage default values for configuration settings and access
   control lists which allow interesting personal information to be
   shared and group information to be restricted.

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Table of Contents



 Status of this Memo ...............................................    i
 Copyright Notice ..................................................    i
 Abstract ..........................................................    i
 ACAP Protocol Specification .......................................    1
 1.       Introduction .............................................    1
 1.1.     Conventions Used in this Document ........................    1
 1.2.     ACAP Data Model ..........................................    1
 1.3.     ACAP Design Goals ........................................    1
 1.4.     Validation ...............................................    2
 1.5.     Definitions ..............................................    2
 1.6.     ACAP Command Overview ....................................    4
 2.       Protocol Framework .......................................    4
 2.1.     Link Level ...............................................    4
 2.2.     Commands and Responses ...................................    4
 2.2.1.   Client Protocol Sender and Server Protocol Receiver ......    4
 2.2.2.   Server Protocol Sender and Client Protocol Receiver ......    5
 2.3.     Server States ............................................    6
 2.3.1.   Non-Authenticated State ..................................    6
 2.3.2.   Authenticated State ......................................    6
 2.3.3.   Logout State .............................................    6
 2.4.     Operational Considerations ...............................    7
 2.4.1.   Untagged Status Updates ..................................    7
 2.4.2.   Response when No Command in Progress .....................    7
 2.4.3.   Auto-logout Timer ........................................    7
 2.4.4.   Multiple Commands in Progress ............................    8
 2.5.     Server Command Continuation Request ......................    8
 2.6.     Data Formats .............................................    8
 2.6.1.   Atom .....................................................    9
 2.6.2.   Number ...................................................    9
 2.6.3.   String ...................................................    9
 2.6.3.1. 8-bit and Binary Strings .................................   10
 2.6.4.   Parenthesized List .......................................   10
 2.6.5.   NIL ......................................................   10
 3.       Protocol Elements ........................................   10
 3.1.     Entries and Attributes ...................................   10
 3.1.1.   Predefined Attributes ....................................   11
 3.1.2.   Attribute Metadata .......................................   12
 3.2.     ACAP URL scheme ..........................................   13
 3.2.1.   ACAP URL User Name and Authentication Mechanism ..........   13
 3.2.2.   Relative ACAP URLs .......................................   14
 3.3.     Contexts .................................................   14

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 3.4.     Comparators ..............................................   15
 3.5.     Access Control Lists (ACLs) ..............................   17
 3.6.     Server Response Codes ....................................   18
 4.       Namespace Conventions ....................................   21
 4.1.     Dataset Namespace ........................................   21
 4.2.     Attribute Namespace ......................................   21
 4.3.     Formal Syntax for Dataset and Attribute Namespace ........   22
 5.       Dataset Management .......................................   23
 5.1.     Dataset Inheritance ......................................   23
 5.2.     Dataset Attributes .......................................   24
 5.3.     Dataset Creation .........................................   25
 5.4.     Dataset Class Capabilities ...............................   25
 5.5.     Dataset Quotas ...........................................   26
 6.       Command and Response Specifications ......................   26
 6.1.     Initial Connection .......................................   26
 6.1.1.   ACAP Untagged Response ...................................   26
 6.2.     Any State ................................................   27
 6.2.1.   NOOP Command .............................................   27
 6.2.2.   LANG Command .............................................   28
 6.2.3.   LANG Intermediate Response ...............................   28
 6.2.4.   LOGOUT Command ...........................................   29
 6.2.5.   OK Response ..............................................   29
 6.2.6.   NO Response ..............................................   29
 6.2.7.   BAD Response .............................................   30
 6.2.8.   BYE Untagged Response ....................................   30
 6.2.9.   ALERT Untagged Response ..................................   31
 6.3.     Non-Authenticated State ..................................   31
 6.3.1.   AUTHENTICATE Command .....................................   31
 6.4.     Searching ................................................   33
 6.4.1.   SEARCH Command ...........................................   33
 6.4.2.   ENTRY Intermediate Response ..............................   37
 6.4.3.   MODTIME Intermediate Response ............................   38
 6.4.4.   REFER Intermediate Response ..............................   38
 6.4.5.   Search Examples ..........................................   38
 6.5.     Contexts .................................................   39
 6.5.1.   FREECONTEXT Command ......................................   39
 6.5.2.   UPDATECONTEXT Command ....................................   40
 6.5.3.   ADDTO Untagged Response ..................................   40
 6.5.4.   REMOVEFROM Untagged Response .............................   41
 6.5.5.   CHANGE Untagged Response .................................   41
 6.5.6.   MODTIME Untagged Response ................................   42
 6.6.     Dataset modification .....................................   42
 6.6.1.   STORE Command ............................................   42
 6.6.2.   DELETEDSINCE Command .....................................   45
 6.6.3.   DELETED Intermediate Response ............................   45
 6.7.     Access Control List Commands .............................   45
 6.7.1.   SETACL Command ...........................................   46
 6.7.2.   DELETEACL Command ........................................   46

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 6.7.3.   MYRIGHTS Command .........................................   47
 6.7.4.   MYRIGHTS Intermediate Response ...........................   47
 6.7.5.   LISTRIGHTS Command .......................................   47
 6.7.6.   LISTRIGHTS Intermediate Response .........................   48
 6.8.     Quotas ...................................................   48
 6.8.1.   GETQUOTA Command .........................................   48
 6.8.3.   QUOTA Untagged Response ..................................   49
 6.9.     Extensions ...............................................   49
 7.       Registration Procedures ..................................   49
 7.1.     ACAP Capabilities ........................................   50
 7.2.     ACAP Response Codes ......................................   50
 7.3.     Dataset Classes ..........................................   51
 7.4.     Vendor Subtree ...........................................   51
 8.       Formal Syntax ............................................   52
 9.       Multi-lingual Considerations .............................   61
 10.      Security Considerations ..................................   62
 11.      Acknowledgments ..........................................   63
 12.      Authors' Addresses .......................................   63
 Appendices ........................................................   64
 A.       References ...............................................   64
 B.       ACAP Keyword Index .......................................   66
 C.       Full Copyright Statement

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ACAP Protocol Specification

1.       Introduction

1.1.     Conventions Used in this Document

   In examples, "C:" and "S:" indicate lines sent by the client and
   server respectively.  If such lines are wrapped without a new "C:" or
   "S:" label, then the wrapping is for editorial clarity and is not
   part of the command.

   The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT",
   and "MAY" in this document are to be interpreted as described in "Key
   words for use in RFCs to Indicate Requirement Levels" [KEYWORDS].

1.2.     ACAP Data Model

   An ACAP server exports a hierarchical tree of entries.  Each level of
   the tree is called a dataset, and each dataset is made up of a list
   of entries.  Each entry has a unique name and may contain any number
   of named attributes.  Each attribute within an entry may be single
   valued or multi-valued and may have associated metadata to assist
   access and interpretation of the value.

   The rules with which a client interprets the data within a portion of
   ACAP's tree of entries are called a dataset class.

1.3.     ACAP Design Goals

   ACAP's primary purpose is to allow users access to their
   configuration data from multiple network-connected computers.  Users
   can then sit down in front of any network-connected computer, run any
   ACAP-enabled application and have access to their own configuration
   data.  Because it is hoped that many applications will become ACAP-
   enabled, client simplicity was preferred to server or protocol
   simplicity whenever reasonable.

   ACAP is designed to be easily manageable.  For this reason, it
   includes "inheritance" which allows one dataset to inherit default
   attributes from another dataset.  In addition, access control lists
   are included to permit delegation of management and quotas are
   included to control storage.  Finally, an ACAP server which is
   conformant to this base specification should be able to support most
   dataset classes defined in the future without requiring a server
   reconfiguration or upgrade.

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   ACAP is designed to operate well with a client that only has
   intermittent access to an ACAP server.  For this reason, each entry
   has a server maintained modification time so that the client may
   detect changes.  In addition, the client may ask the server for a
   list of entries which have been removed since it last accessed the
   server.

   ACAP presumes that a dataset may be potentially large and/or the
   client's network connection may be slow, and thus offers server
   sorting, selective fetching and change notification for entries
   within a dataset.

   As required for most Internet protocols, security, scalability and
   internationalization were important design goals.

   Given these design goals, an attempt was made to keep ACAP as simple
   as possible.  It is a traditional Internet text based protocol which
   massively simplifies protocol debugging.  It was designed based on
   the successful IMAP [IMAP4] protocol framework, with a few
   refinements.

1.4.     Validation

   By default, any value may be stored in any attribute for which the
   user has appropriate permission and quota.  This rule is necessary to
   allow the addition of new simple dataset classes without
   reconfiguring or upgrading the server.

   In some cases, such as when the value has special meaning to the
   server, it is useful to have the server enforce validation by
   returning the INVALID response code to a STORE command. These cases
   MUST be explicitly identified in the dataset class specification
   which SHOULD include specific fixed rules for validation.  Since a
   given ACAP server may be unaware of any particular dataset class
   specification, clients MUST NOT depend on the presence of enforced
   validation on the server.

1.5.     Definitions


   access control list (ACL)
        A set of identifier, rights pairs associated with an object.  An
        ACL is used to determine which operations a user is permitted to
        perform on that object.  See section 3.5.

   attribute
        A named value within an entry.  See section 3.1.

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   comparator
        A named function which can be used to perform one or more of
        three comparison operations: ordering, equality and substring
        matching.  See section 3.4.

   context
        An ordered subset of entries in a dataset, created by a SEARCH
        command with a MAKECONTEXT modifier.  See section 3.3.

   dataset
        One level of hierarchy in ACAP's tree of entries.

   dataset class specification
        The rules which allow a client to interpret the data within a
        portion of ACAP's tree of entries.

   entry
        A set of attributes with a unique entry name.  See section 3.1.

   metadata
        Information describing an attribute, its value and any access
        controls associated with that attribute.  See section 3.1.2.

   NIL  This represents the non-existence of a particular data item.

   NUL  A control character encoded as 0 in US-ASCII [US-ASCII].

   octet
        An 8-bit value.  On most modern computer systems, an octet is
        one byte.

   SASL Simple Authentication and Security Layer [SASL].

   UTC  Universal Coordinated Time as maintained by the Bureau
        International des Poids et Mesures (BIPM).

   UTF-8
        An 8-bit transformation format of the Universal Character Set
        [UTF8].  Note that an incompatible change was made to the coded
        character set referenced by [UTF8], so for the purpose of this
        document, UTF-8 refers to the UTF-8 encoding as defined by
        version 2.0 of Unicode [UNICODE-2], or ISO 10646 [ISO-10646]
        including amendments one through seven.

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1.6.     ACAP Command Overview

   The AUTHENTICATE, NOOP, LANG and LOGOUT commands provide basic
   protocol services.  The SEARCH command is used to select, sort, fetch
   and monitor changes to attribute values and metadata.  The
   UPDATECONTEXT and FREECONTEXT commands are also used to assist in
   monitoring changes in attribute values and metadata.  The STORE
   command is used to add, modify and delete entries and attributes.
   The DELETEDSINCE command is used to assist a client in
   re-synchronizing a cache with the server.  The GETQUOTA, SETACL,
   DELETEACL, LISTRIGHTS and MYRIGHTS commands are used to examine
   storage quotas and examine or modify access permissions.

2.       Protocol Framework

2.1.     Link Level

   The ACAP protocol assumes a reliable data stream such as provided by
   TCP.  When TCP is used, an ACAP server listens on port 674.

2.2.     Commands and Responses

   An ACAP session consists of the establishment of a client/server
   connection, an initial greeting from the server, and client/server
   interactions.  These client/server interactions consist of a client
   command, server data, and a server completion result.

   ACAP is a text-based line-oriented protocol.  In general,
   interactions transmitted by clients and servers are in the form of
   lines; that is, sequences of characters that end with a CRLF.  The
   protocol receiver of an ACAP client or server is either reading a
   line, or is reading a sequence of octets with a known count (a
   literal) followed by a line.  Both clients and servers must be
   capable of handling lines of arbitrary length.

2.2.1.   Client Protocol Sender and Server Protocol Receiver

   The client command begins an operation.  Each client command is
   prefixed with a identifier (an alphanumeric string of no more than 32
   characters, e.g., A0001, A0002, etc.) called a "tag".  A different
   tag SHOULD be generated by the client for each command.

   There are two cases in which a line from the client does not
   represent a complete command.  In one case, a command argument is
   quoted with an octet count (see the description of literal in section
   2.6.3); in the other case, the command arguments require server

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   feedback (see the AUTHENTICATE command).  In some of these cases, the
   server sends a command continuation request if it is ready for the
   next part of the command.  This response is prefixed with the token
   "+".

        Note: If, instead, the server detected an error in a
        command, it sends a BAD completion response with tag
        matching the command (as described below) to reject the
        command and prevent the client from sending any more of the
        command.

        It is also possible for the server to send a completion or
        intermediate response for some other command (if multiple
        commands are in progress), or untagged data.  In either
        case, the command continuation request is still pending;
        the client takes the appropriate action for the response,
        and reads another response from the server.

   The ACAP server reads a command line from the client, parses the
   command and its arguments, and transmits server data and a server
   command completion result.

2.2.2.   Server Protocol Sender and Client Protocol Receiver

   Data transmitted by the server to the client come in four forms:
   command continuation requests, command completion results,
   intermediate responses, and untagged responses.

   A command continuation request is prefixed with the token "+".

   A command completion result indicates the success or failure of the
   operation.  It is tagged with the same tag as the client command
   which began the operation.  Thus, if more than one command is in
   progress, the tag in a server completion response identifies the
   command to which the response applies.  There are three possible
   server completion responses: OK (indicating success), NO (indicating
   failure), or BAD (indicating protocol error such as unrecognized
   command or command syntax error).

   An intermediate response returns data which can only be interpreted
   within the context of a command in progress.  It is tagged with the
   same tag as the client command which began the operation.  Thus, if
   more than one command is in progress, the tag in an intermediate
   response identifies the command to which the response applies.  A
   tagged response other than "OK", "NO", or "BAD" is an intermediate
   response.

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   An untagged response returns data or status messages which may be
   interpreted outside the context of a command in progress.  It is
   prefixed with the token "*".  Untagged data may be sent as a result
   of a client command, or may be sent unilaterally by the server.
   There is no syntactic difference between untagged data that resulted
   from a specific command and untagged data that were sent
   unilaterally.

   The protocol receiver of an ACAP client reads a response line from
   the server.  It then takes action on the response based upon the
   first token of the response, which may be a tag, a "*", or a "+" as
   described above.

   A client MUST be prepared to accept any server response at all times.
   This includes untagged data that it may not have requested.

   This topic is discussed in greater detail in the Server Responses
   section.

2.3.     Server States

   An ACAP server is in one of three states.  Most commands are valid in
   only certain states.  It is a protocol error for the client to
   attempt a command while the server is in an inappropriate state for
   that command.  In this case, a server will respond with a BAD command
   completion result.

2.3.1.   Non-Authenticated State

   In non-authenticated state, the user must supply authentication
   credentials before most commands will be permitted.  This state is
   entered when a connection starts.

2.3.2.   Authenticated State

   In authenticated state, the user is authenticated and most commands
   will be permitted.  This state is entered when acceptable
   authentication credentials have been provided.

2.3.3.   Logout State

   In logout state, the session is being terminated, and the server will
   close the connection.  This state can be entered as a result of a
   client request or by unilateral server decision.

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            +--------------------------------------+
            |initial connection and server greeting|
            +--------------------------------------+
                      || (1)                  || (2)
                      VV                      ||
            +-----------------+               ||
            |non-authenticated|               ||
            +-----------------+               ||
             || (4)      || (3)               ||
             ||          VV                   ||
             ||          +----------------+   ||
             ||          | authenticated  |   ||
             ||          +----------------+   ||
             ||            || (4)             ||
             VV            VV                 VV
            +--------------------------------------+
            |     logout and close connection      |
            +--------------------------------------+

         (1) connection (ACAP greeting)
         (2) rejected connection (BYE greeting)
         (3) successful AUTHENTICATE command
         (4) LOGOUT command, server shutdown, or connection closed

2.4.     Operational Considerations

2.4.1.   Untagged Status Updates

   At any time, a server can send data that the client did not request.

2.4.2.   Response when No Command in Progress

   Server implementations are permitted to send an untagged response
   while there is no command in progress.  Server implementations that
   send such responses MUST deal with flow control considerations.
   Specifically, they must either (1) verify that the size of the data
   does not exceed the underlying transport's available window size, or
   (2) use non-blocking writes.

2.4.3.   Auto-logout Timer

   If a server has an inactivity auto-logout timer, that timer MUST be
   of at least 30 minutes duration.  The receipt of ANY command from the
   client during that interval MUST suffice to reset the auto-logout
   timer.

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2.4.4.   Multiple Commands in Progress

   The client is not required to wait for the completion result of a
   command before sending another command, subject to flow control
   constraints on the underlying data stream.  Similarly, a server is
   not required to process a command to completion before beginning
   processing of the next command, unless an ambiguity would result
   because of a command that would affect the results of other commands.
   If there is such an ambiguity, the server executes commands to
   completion in the order given by the client.

2.5.     Server Command Continuation Request

   The command continuation request is indicated by a "+" token instead
   of a tag.  This indicates that the server is ready to accept the
   continuation of a command from the client.

   This response is used in the AUTHENTICATE command to transmit server
   data to the client, and request additional client data.  This
   response is also used if an argument to any command is a
   synchronizing literal (see section 2.6.3).

   The client is not permitted to send the octets of a synchronizing
   literal unless the server indicates that it expects it.  This permits
   the server to process commands and reject errors on a line-by-line
   basis, assuming it checks for non-synchronizing literals at the end
   of each line.  The remainder of the command, including the CRLF that
   terminates a command, follows the octets of the literal.  If there
   are any additional command arguments the literal octets are followed
   by a space and those arguments.

   Example:    C: A099 FREECONTEXT {10}
               S: + "Ready for additional command text"
               C: FRED
               C: FOOB
               S: A099 OK "FREECONTEXT completed"
               C: A044 BLURDYBLOOP {102856}
               S: A044 BAD "No such command as 'BLURDYBLOOP'"


2.6.     Data Formats

   ACAP uses textual commands and responses.  Data in ACAP can be in one
   of five forms: atom, number, string, parenthesized list or NIL.

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2.6.1.   Atom

   An atom consists of one to 1024 non-special characters.  It must
   begin with a letter.  Atoms are used for protocol keywords.

2.6.2.   Number

   A number consists of one or more digit characters, and represents a
   numeric value.  Numbers are restricted to the range of an unsigned
   32-bit integer: 0 < number < 4,294,967,296.

2.6.3.   String

   A string is in one of two forms: literal and quoted string.  The
   literal form is the general form of string.  The quoted string form
   is an alternative that avoids the overhead of processing a literal at
   the cost of restrictions of what may be in a quoted string.

   A literal is a sequence of zero or more octets (including CR and LF),
   prefix-quoted with an octet count in the form of an open brace ("{"),
   the number of octets, close brace ("}"), and CRLF.  In the case of
   literals transmitted from server to client, the CRLF is immediately
   followed by the octet data.

   There are two forms of literals transmitted from client to server.
   The form where the open brace ("{") and number of octets is
   immediately followed by a close brace ("}") and CRLF is called a
   synchronizing literal.  When sending a synchronizing literal, the
   client must wait to receive a command continuation request before
   sending the octet data (and the remainder of the command).  The other
   form of literal, the non-synchronizing literal, is used to transmit a
   string from client to server without waiting for a command
   continuation request.  The non-synchronizing literal differs from the
   synchronizing literal by having a plus ("+") between the number of
   octets and the close brace ("}") and by having the octet data
   immediately following the CRLF.

   A quoted string is a sequence of zero to 1024 octets excluding NUL,
   CR and LF, with double quote (<">) characters at each end.

   The empty string is represented as "" (a quoted string with zero
   characters between double quotes), as {0} followed by CRLF (a
   synchronizing literal with an octet count of 0), or as {0+} followed
   by a CRLF (a non-synchronizing literal with an octet count of 0).

        Note: Even if the octet count is 0, a client transmitting a
        synchronizing literal must wait to receive a command
        continuation request.

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2.6.3.1. 8-bit and Binary Strings

   Most strings in ACAP are restricted to UTF-8 characters and may not
   contain NUL octets.  Attribute values MAY contain any octets
   including NUL.

2.6.4.   Parenthesized List

   Data structures are represented as a "parenthesized list"; a sequence
   of data items, delimited by space, and bounded at each end by
   parentheses.  A parenthesized list can contain other parenthesized
   lists, using multiple levels of parentheses to indicate nesting.

   The empty list is represented as () -- a parenthesized list with no
   members.

2.6.5.   NIL

   The special atom "NIL" represents the non-existence of a particular
   data item that is represented as a string or parenthesized list, as
   distinct from the empty string "" or the empty parenthesized list ().

3.       Protocol Elements

   This section defines data formats and other protocol elements used
   throughout the ACAP protocol.

3.1.     Entries and Attributes

   Within a dataset, each entry name is made up of zero or more UTF-8
   characters other than slash ("/").  A slash separated list of
   entries, one at each level of the hierarchy, forms the full path to
   an entry.

   Each entry is made up of a set of attributes.  Each attribute has a
   hierarchical name in UTF-8, with each component of the name separated
   by a period (".").

   The value of an attribute is either single or multi-valued.  A single
   value is NIL (has no value), or a string of zero or more octets.  A
   multi-value is a list of zero or more strings, each of zero or more
   octets.

   Attribute names are not permitted to contain asterisk ("*") or
   percent ("%") and MUST be valid UTF-8 strings which do not contain
   NUL.  Invalid attribute names result in a BAD response.  Entry names

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   are not permitted to begin with "." or contain slash ("/") and MUST
   be valid UTF-8 strings which do not contain NUL.  Invalid entry names
   in the entry field of a command result in a BAD response.

   Use of non-visible UTF-8 characters in attribute and entry names is
   discouraged.

3.1.1.   Predefined Attributes

   Attribute names which do not contain a dot (".") are reserved for
   standardized attributes which have meaning in any dataset.  The
   following attributes are defined by the ACAP protocol.

   entry
        Contains the name of the entry.  MUST be single valued.
        Attempts to use illegal or multi-valued values for the entry
        attribute are protocol errors and MUST result in a BAD
        completion response.  This is a special case.

   modtime
        Contains the date and time any read-write metadata in the entry
        was last modified.  This value MUST be in UTC, MUST be
        automatically updated by the server.

        The value consists of 14 or more US-ASCII digits.  The first
        four indicate the year, the next two indicate the month, the
        next two indicate the day of month, the next two indicate the
        hour (0 - 23), the next two indicate the minute, and the next
        two indicate the second.  Any further digits indicate fractions
        of a second.

        The time, particularly fractions of a second, need not be
        accurate.  It is REQUIRED, however, that any two entries in a
        dataset changed by successive modifications have strictly
        ascending modtime values.  In addition, each STORE command
        within a dataset (including simultaneous stores from different
        connections) MUST use different modtime values.

        This attribute has enforced validation, so any attempt to STORE
        a value in this attribute MAY result in a NO response with an
        INVALID response code.

   subdataset
        If this attribute is set, it indicates the existence of a sub-
        dataset of this entry.

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        The value consists of a list of relative ACAP URLs (see section
        3.2) which may be used to locate the sub-dataset.  The base URL
        is the full path to the entry followed by a slash ("/").  The
        value "." indicates a subdataset is located directly under this
        one.  Multiple values indicate replicated copies of the
        subdataset.

        For example, if the dataset "/folder/site/" has an entry
        "public-folder" with a subdataset attribute of ".", then there
        exists a dataset "/folder/site/public-folder/".  If the value of
        the subdataset attribute was instead
        "//other.acap.domain//folder/site/public-folder/", that would
        indicate the dataset is actually located on a different ACAP
        server.

        A dataset can be created by storing a "subdataset" attribute
        including ".", and a sub-hierarchy of datasets is deleted by
        storing a NIL value to the "subdataset" attribute on the entry
        in the parent dataset.

        This attribute has enforced syntax validation.  Specifically, if
        an attempt is made to STORE a non-list value (other than NIL),
        an empty list, or one of the values does not follow the URL
        syntax rules [BASIC-URL, REL-URL], then this will result in a NO
        response with an INVALID response code.

3.1.2.   Attribute Metadata

   Each attribute is made up of metadata items which describe that
   attribute, its value and any associated access controls.  Metadata
   items may be either read-only, in which case the client is never
   permitted to modify the item, or read-write, in which case the client
   may modify the item if the access control list (ACL) permits.

   The following metadata items are defined in this specification:

   acl    The access control list for the attribute, if one exists.  If
          the attribute does not have an ACL, NIL is returned.
          Read-write.  See section 3.5 for the contents of an ACL.

   attribute
          The attribute name.  Read-only.

   myrights
          The set of rights that the client has to the attribute.
          Read-only.  See section 3.5 for the possible rights.

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   size   This is the length of the value.  In the case of a
          multi-value, this is a list of lengths for each of the values.
          Read-only.

   value  The value.  For a multi-value, this is a list of single
          values.  Read-write.

   Additional items of metadata may be defined in extensions to this
   protocol.  Servers MUST respond to unrecognized metadata by returning
   a BAD command completion result.

3.2.     ACAP URL scheme

   ACAP URLs are used within the ACAP protocol for the "subdataset"
   attribute, referrals and inheritance.  They provide a convenient
   syntax for referring to other ACAP datasets.  The ACAP URL follows
   the common Internet scheme syntax as defined in [BASIC-URL] except
   that plaintext passwords are not permitted.  If :<port> is omitted,
   the port defaults to 674.

   An ACAP URL has the following general form:

   url-acap  = "acap://" url-server "/" url-enc-entry [url-filter]
               [url-extension]

   The <url-server> element includes the hostname, and optional user
   name, authentication mechanism and port number.  The <url-enc-entry>
   element contains the name of an entry path encoded according to the
   rules in [BASIC-URL].

   The <url-filter> element is an optional list of interesting attribute
   names.  If omitted, the URL refers to all attributes of the named
   entry.  The <url-extension> element is reserved for extensions to
   this URL scheme.

   Note that unsafe or reserved characters such as " " or "?" MUST be
   hex encoded as described in the URL specification [BASIC-URL].  Hex
   encoded octets are interpreted according to UTF-8 [UTF8].

3.2.1.   ACAP URL User Name and Authentication Mechanism

   A user name and/or authentication mechanism may be supplied.  They
   are used in the "AUTHENTICATE" command after making the connection to
   the ACAP server.  If no user name or authentication mechanism is
   supplied, then the SASL ANONYMOUS [SASL-ANON] mechanism is used by
   default.  If an authentication mechanism is supplied without a user

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   name, then one SHOULD be obtained from the specified mechanism or
   requested from the user as appropriate.  If a user name is supplied
   without an authentication mechanism then ";AUTH=*" is assumed.

   The ";AUTH=" authentication parameter is interpreted as described in
   the IMAP URL Scheme [IMAP-URL].

   Note that if unsafe or reserved characters such as " " or ";" are
   present in the user name or authentication mechanism, they MUST be
   encoded as described in the URL specification [BASIC-URL].

3.2.2.   Relative ACAP URLs

   Because ACAP uses "/" as the hierarchy separator for dataset paths,
   it works well with the relative URL rules defined in the relative URL
   specification [REL-URL].

   The <aauth> grammar element is considered part of the user name for
   purposes of resolving relative ACAP URLs.

   The base URL for a relative URL stored in an attribute's value is
   formed by taking the path to the dataset containing that attribute,
   appending a "/" followed by the entry name of the entry containing
   that attribute followed by "/".

3.3.     Contexts

   A context is subset of entries in a dataset or datasets, created by a
   SEARCH command with a MAKECONTEXT modifier.  Context names are
   client-generated strings and must not start with the slash ('/')
   character.

   When a client creates a context, it may request automatic
   notification of changes.  A client may also request enumeration of
   entries within a context.  Enumeration simplifies the implementation
   of a "virtual scrollbar" by the client.

   A context exists only within the ACAP session in which it was
   created.  When the connection is closed, all contexts associated with
   that connection are automatically discarded.  A server is required to
   support at least 100 active contexts within a session.  If the server
   supports a larger limit it must advertise it in a CONTEXTLIMIT
   capability.

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3.4.     Comparators

   A comparator is a named function which takes two input values and can
   be used to perform one or more of four comparison operations:
   ordering, equality, prefix and substring matching.

   The ordering operation is used both for the SORT search modifier and
   the COMPARE and COMPARESTRICT search keys.  Ordering comparators can
   determine the ordinal precedence of any two values.  When used for
   ordering, a comparator's name can be prefixed with "+" or "-" to
   indicate that the ordering should be normal order or reversed order
   respectively.  If no prefix is included, "+" is assumed.

   For the purpose of ordering, a comparator may designate certain
   values as having an undefined ordinal precedence.  Such values always
   collate with equal value after all other values regardless of whether
   normal or reversed ordering is used.  Unless the comparator
   definition specifies otherwise, multi-values and NIL values have an
   undefined ordinal precedence.

   The equality operation is used for the EQUAL search modifier, and
   simply determines if the two values are considered equal under the
   comparator function.  When comparing a single value to a multi-value,
   the two are considered equal if any one of the multiple values is
   equal to the single value.

   The prefix match operation is used for the PREFIX search modifier,
   and simply determines if the search value is a prefix of the item
   being searched.  In the case of prefix search on a multi-value, the
   match is successful if the value is a prefix of any one of the
   multiple values.

   The substring match operation is used for the SUBSTRING search
   modifier, and simply determines if search value is a substring of the
   item being searched.  In the case of substring search on a multi-
   value, the match is successful if the value is a substring of any one
   of the multiple values.

   Rules for naming and registering comparators will be defined in a
   future specification.  Servers MUST respond to unknown or improperly
   used comparators with a BAD command completion result.

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   The following comparators are defined by this standard and MUST be
   implemented:

      i;octet
           Operations: Ordering, Equality, Prefix match, Substring match

           For collation, the i;octet comparator interprets the value of
           an attribute as a series of unsigned octets with ordinal
           values from 0 to 255.  When ordering two strings, each octet
           pair is compared in sequence until the octets are unequal or
           the end of the string is reached.  When collating two strings
           where the shorter is a prefix of the longer, the shorter
           string is interpreted as having a smaller ordinal value.  The
           "i;octet" or "+i;octet" forms collate smaller ordinal values
           earlier, and the "-i;octet" form collates larger ordinal
           values earlier.

           For the equality function, two strings are equal if they are
           the same length and contain the same octets in the same
           order.  NIL is equal only to itself.

           For non-binary, non-nil single values, i;octet ordering is
           equivalent to the ANSI C [ISO-C] strcmp() function applied to
           C string representations of the values.  For non-binary,
           non-nil single values, i;octet substring match is equivalent
           to the ANSI C strstr() function applied to the C string
           representations of the values.

      i;ascii-casemap
           Operations: Ordering, Equality, Prefix match, Substring match

           The i;ascii-casemap comparator first applies a mapping to the
           attribute values which translates all US-ASCII letters to
           uppercase (octet values 0x61 to 0x7A are translated to octet
           values 0x41 to 0x5A respectively), then applies the i;octet
           comparator as described above.  With this function the values
           "hello" and "HELLO" have the same ordinal value and are
           considered equal.

      i;ascii-numeric
           Operations: Ordering, Equality

           The i;ascii-numeric comparator interprets strings as decimal
           positive integers represented as US-ASCII digits.  All values
           which do not begin with a US-ASCII digit are considered equal
           with an ordinal value higher than all non-NIL single-valued

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           attributes.  Otherwise, all US-ASCII digits (octet values
           0x30 to 0x39) are interpreted starting from the beginning of
           the string to the first non-digit or the end of the string.


3.5.     Access Control Lists (ACLs)

   An access control list is a set of identifier, rights pairs used to
   restrict access to a given dataset, attribute or attribute within an
   entry.  An ACL is represented by a multi-value with each value
   containing an identifier followed by a tab character followed by the
   rights.  The syntax is defined by the "acl" rule in the formal syntax
   in section 8.

   Identifier is a UTF-8 string.  The identifier "anyone" is reserved to
   refer to the universal identity (all authentications, including
   anonymous).  All user name strings accepted by the AUTHENTICATE
   command to authenticate to the ACAP server are reserved as
   identifiers for the corresponding user.  Identifiers starting with a
   slash ("/") character are reserved for authorization groups which
   will be defined in a future specification.  Identifiers MAY be
   prefixed with a dash ("-") to indicate a revocation of rights.  All
   other identifiers have implementation-defined meanings.

   Rights is a string listing a (possibly empty) set of alphanumeric
   characters, each character listing a set of operations which is being
   controlled.  Letters are reserved for "standard" rights, listed
   below.  The set of standard rights may only be extended by a
   standards-track or IESG approved experimental RFC.  Digits are
   reserved for implementation or site defined rights.  The currently
   defined standard rights are:

   x - search (use EQUAL search key with i;octet comparator)
   r - read (access with SEARCH command)
   w - write (modify with STORE command)
   i - insert (perform STORE on a previously NIL value)
   a - administer (perform SETACL or STORE on ACL attribute/metadata)

   An implementation may force rights to always or never be granted.  In
   particular, implementations are expected to grant implicit read and
   administer rights to a user's personal dataset storage in order to
   avoid denial of service problems.  Rights are never tied, unlike the
   IMAP ACL extension [IMAP-ACL].

   It is possible for multiple identifiers in an access control list to
   apply to a given user (or other authentication identity).  For
   example, an ACL may include rights to be granted to the identifier
   matching the user, one or more implementation-defined identifiers

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   matching groups which include the user, and/or the identifier
   "anyone".  These rights are combined by taking the union of all
   positive rights which apply to a given user and subtracting the union
   of all negative rights which apply to that user.  A client MAY avoid
   this calculation by using the MYRIGHTS command and metadata items.

   Each attribute of each entry of a dataset may potentially have an
   ACL.  If an attribute in an entry does not have an ACL, then access
   is controlled by a default ACL for that attribute in the dataset, if
   it exists.  If there is no default ACL for that attribute in the
   dataset, access is controlled by a default ACL for that dataset.  The
   default ACL for a dataset must exist.

   In order to perform any access or manipulation on an entry in a
   dataset, the client must have 'r' rights on the "entry" attribute of
   the entry.  Implementations should take care not to reveal via error
   messages the existence of an entry for which the client does not have
   'r' rights.  A client does not need access to the "subdataset"
   attribute of the parent dataset in order to access the contents of a
   dataset.

   Many of the ACL commands and responses include an "acl object"
   parameter, for specifying what the ACL applies to.  This is a
   parenthesized list.  The list contains just the dataset name when
   referring to the default ACL for a dataset.  The list contains a
   dataset name and an attribute name when referring to the default ACL
   for an attribute in a dataset.  The list contains a dataset name, an
   attribute name, and an entry name when referring to the ACL for an
   attribute of an entry of a dataset.


3.6.     Server Response Codes

   An OK, NO, BAD, ALERT or BYE response from the server MAY contain a
   response code to describe the event in a more detailed machine
   parsable fashion.  A response code consists of data inside
   parentheses in the form of an atom, possibly followed by a space and
   arguments.  Response codes are defined when there is a specific
   action that a client can take based upon the additional information.
   In order to support future extension, the response code is
   represented as a slash-separated hierarchy with each level of
   hierarchy representing increasing detail about the error.  Clients
   MUST tolerate additional hierarchical response code detail which they
   don't understand.

   The currently defined response codes are:

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      AUTH-TOO-WEAK
           This response code is returned on a tagged NO result from an
           AUTHENTICATE command.  It indicates that site security policy
           forbids the use of the requested mechanism for the specified
           authentication identity.

      ENCRYPT-NEEDED
           This response code is returned on a tagged NO result from an
           AUTHENTICATE command.  It indicates that site security policy
           requires the use of a strong encryption mechanism for the
           specified authentication identity and mechanism.

      INVALID
           This response code indicates that a STORE command included
           data which the server implementation does not permit.  It
           MUST NOT be used unless the dataset class specification for
           the attribute in question explicitly permits enforced server
           validation.  The argument is the attribute which was invalid.

      MODIFIED
           This response code indicates that a conditional store failed
           because the modtime on the entry is later than the modtime
           specified with the STORE command UNCHANGEDSINCE modifier.
           The argument is the entry which had been modified.

      NOEXIST
           This response code indicates that a search or NOCREATE store
           failed because a specified dataset did not exist.  The
           argument is the dataset which does not exist.

      PERMISSION
           A command failed due to insufficient permission based on the
           access control list or implicit rights.  The argument is the
           acl-object which caused the permission failure.

      QUOTA
           A STORE or SETACL command which would have increased the size
           of the dataset failed due to insufficient quota.

      REFER
           This response code may be returned in a tagged NO response to
           any command that takes a dataset name as a parameter.  It has
           one or more arguments with the syntax of relative URLs.  It
           is a referral, indicating that the command should be retried
           using one of the relative URLs.

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      SASL This response code can occur in the tagged OK response to a
           successful AUTHENTICATE command and includes the optional
           final server response data from the server as specified by
           SASL [SASL].

      TOOMANY
           This response code may be returned in a tagged OK response to
           a SEARCH command which includes the LIMIT modifier.  The
           argument returns the total number of matching entries.

      TOOOLD
           The modtime specified in the DELETEDSINCE command is too old,
           so deletedsince information is no longer available.

      TRANSITION-NEEDED
           This response code occurs on a NO response to an AUTHENTICATE
           command.  It indicates that the user name is valid, but the
           entry in the authentication database needs to be updated in
           order to permit authentication with the specified mechanism.
           This can happen if a user has an entry in a system
           authentication database such as Unix /etc/passwd, but does
           not have credentials suitable for use by the specified
           mechanism.

      TRYLATER
           A command failed due to a temporary server failure.  The
           client MAY continue using local information and try the
           command later.

      TRYFREECONTEXT
           This response code may be returned in a tagged NO response to
           a SEARCH command which includes the MAKECONTEXT modifier.  It
           indicates that a new context may not be created due to the
           server's limit on the number of existing contexts.

      WAYTOOMANY
           This response code may be returned in a tagged NO response to
           a SEARCH command which includes a HARDLIMIT search modifier.
           It indicates that the SEARCH would have returned more entries
           than the HARDLIMIT permitted.

      Additional response codes MUST be registered with IANA according
      to the proceedures in section 7.2.  Client implementations MUST
      tolerate response codes that they do not recognize.


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