rfc1024.txt

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Network Working Group                                        C.Partridge
Request For Comment: 1024                                       BBN/NNSC
                                                              G. Trewitt
                                                                Stanford
                                                            October 1987

                       HEMS VARIABLE DEFINITIONS

STATUS OF THIS MEMO

   This memo assigns instruction codes, defines object formats and
   object semantics for use with the High-Level Monitoring and Control
   Language, defined in RFC-1023.

   This memo is provisional and the definitions are subject to change.
   Readers should confirm that they have the most recent version of the
   memo.

   The authors assume a working knowledge of the ISO data encoding
   standard, ASN.1, and a general understanding of the IP protocol
   suite.

   Distribution of this memo is unlimited.

INTRODUCTION

   In other memos [RFC-1021, RFC-1022] the authors have described a
   general system for monitoring and controlling network entities; this
   system is called the High-Level Entity Management System (HEMS).
   This system permits applications to read and write values in remote
   entities which support a simple query processor.

   In this memo we standardize the language instruction codes, the
   objects which can be read or written, and the meanings of any
   constants stored in the objects.  There are three parts to this
   standardization: (1) the assignment of an ASN.1 tag to each value,
   (2) the definition of the external representation of the value (e.g.,
   INTEGER, OCTETSTRING, etc.), and (3) the definition of the meaning,
   or semantics of a value (e.g., what types of packets a particular
   packet counter actually tracks).

   This definition is provisional, and the authors hope that it will be
   expanded and improved as the community becomes more experienced with
   HEMS.  Readers with suggestions for additional object definitions, or
   improved definitions are encouraged to contact the authors.






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RFC 1024                    HEMS Definitions                October 1987


MESSAGE FORMATS

   All HEMS values are conveyed between applications and entities using
   the High-Level Entity Management Protocol (HEMP) specified in RFC-
   1022.  All values specified in this memo are passed in the data
   sections of HEMP messages.  For all message types, the data section
   is a SEQUENCE of objects.  For requests, these objects are operations
   and their operands.  Replies contain a sequence of objects retrieved
   by a request.  Events contain an initial event object followed by an
   optional number of objects related to the event.

   Messages conforming to this memo should set the link field in the
   HEMP CommonHeader to 1, to indicate version 1 of HEMS.  The
   resourceId field should be set to NULL.


CONTROL LANGUAGE INSTRUCTIONS

   The HEMS Monitoring and Control Language defines a suite of
   operations which the query processor must be able to perform.  These
   operations and their operands are ASN.1 objects which are passed to
   the query processor over a network connection.  The operations and
   operands are sent in postfix form (the operation follows the
   operands). Operands are pushed onto a stack and are processed when
   the operation is encountered.

   To ensure that operations are easily recognized in the input stream,
   they are all encoded in a single application-specific type.  This
   type is shown below.

               Operation ::= [APPLICATION 1] IMPLICIT INTEGER {
                       reserved(0), get(1) begin(2), end(3),
                       get-match(4), get-attributes(5),
                       get-attributes-match(6), get-range(7),
                       set(8), set-match(9)
                   }

   When the query processor encounters an Operation object it consults
   the value to determine which operation is to be done (e.g., GET).

GENERAL COMMENTS ON OBJECTS STORED IN ENTITIES

   The High-Level Monitoring and Control Language requires the object
   space to have a tree-shaped type space.  Locating a particular object
   requires identifying that section of the tree in which the object
   resides.  (A more detailed explanation of the scheme is given in
   RFC-1023).




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RFC 1024                    HEMS Definitions                October 1987


   This memo defines a universal type space.  A subset of this type
   space is expected to be an appropriate type space for any entity
   (e.g., a gateway or a multi-user host).  The type space is divided
   into required and optional portions.  Implementors should implement
   the required portion of the type space plus that part of the optional
   type space which is appropriate for their particular entity.

   One problem with defining a universal type space is that certain
   interesting objects are not universal, but are instead very machine
   specific (for example, status registers on specialized hardware).  To
   allow implementors to retrieve such implementation-specific objects
   using the HEMS system, a special APPLICATION type is reserved for
   non-standard values.

   Putting objects in ASN.1 form implies an ability to map to and from
   ASN.1 format.  One of the design goals of this system has been to
   minimize the amount of ASN.1 compilation required by the query
   processor to reduce the expense of processing queries at entities.
   (This implies a certain willingness to force the applications
   querying entities to be more powerful).  We expect that most of the
   complex mapping will be done when objects are read; most writable
   objects have a simple format (e.g., an INTEGER, or OCTETSTRING).  As
   a result, we have made a heavy use of the ASN.1 SET type, which
   allows values to be presented in any order.  Applications which
   require particular fields in an object may use the template structure
   to specify particular fields to be retrieved, but this still permits
   the query processor to return the fields in whatever order is
   convenient.

   In addition to ease the problems of ASN.1 compilation, query
   processors are not required to reduce an INTEGER to the minimum
   number of octets as specified in ASN.1.  Applications should be
   prepared to receive INTEGERs which have leading octets with all zeros
   or ones.

   More generally, a design goal of HEMS was to try to limit the data
   processing done at the entity, and to place the burden of data
   reduction on the querying application.  As a result, the objects
   presented here are typically counters, or values which the entity has
   to compute already.  Object definitions which require the entity to
   do data reduction are not supported, although consideration might be
   given to making them optionally available.

   Finally, HEMS is required to support access by multiple network
   management centers or applications.  This constraint has some
   important consequences.  First, the SET operation cannot be applied
   to any Counter, since changing the value of a Counter may impair data
   acquisition by other centers.  More generally, there are questions



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RFC 1024                    HEMS Definitions                October 1987


   about competing or clashing SET requests from management centers.
   Currently HEMS does not provide any facilities for protecting against
   such requests.  If such facilities become necessary, the authors
   envision the enhancement of the object definitions to incorporate the
   idea of "owned" objects.


READING THE OBJECT DEFINITIONS

   Most of the rest of this memo is devoted to ennumerating the objects
   managed by the query processor.  Many of these objects are
   dictionaries, objects which reference other objects.  Defining
   dictionaries requires that we specify the class of objects they
   reference.

   Most significant objects, such as packet counts, reside at the leaves
   of the object data tree.  They need to be carefully defined to ensure
   that their meaning is consistent across all HEMS implementations.
   These values are defined using the following format:


   OBJECT:  This is the name of the object.

   Type:  This is the ASN.1 type of the object.

   Definition:  The meaning of the data the object contains.
           Implementations should ensure that their instance of
           the object fulfills this definition since an important
           feature of HEMS is that objects have consistent meaning
           across all machines.  It is better not to implement
           an object than to abuse its definition.

   Notes:  An optional section of the definition which is used
           to discuss issues not covered in other sections of
           this specification.

   Object Status:  An optional section of the definition which
           is used to indicate whether the object is required of all
           HEMS implementations, encouraged of HEMS implementations
           or simply considered useful.  Currently, there are four
           levels of status:

               Required:  The object is felt to provide critical
               information and must be included in a fully
               conforming HEMS implementation.

               Required On Condition:  The object is felt to
               provide critical information about an optional



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RFC 1024                    HEMS Definitions                October 1987


               feature of an IP entity (for example, support of
               the Transmission Control Protocol).  The object
               is required if the feature is implemented in the
               entity.

               Encouraged:  The object is felt to provide very
               useful management information and implementors
               are encouraged to implement it.

               Defined:  The object may be useful and has been
               defined so that all implementations of the object
               are consistent.

           If the object status is not specified, the object should
           be considered required.  If the parent dictionary is optional,
           then the object should be considered required if the parent
           dictionary is supported.

   Operations on Object:  The definition of how each monitoring
           and control operation acts on the object.  Many operations
           have the same effect on almost all values, so some
           default definitions are presented here.  In the absence
           of an operation specification, implementors should use
           the default operations defined here.

           BEGIN:  The default is for BEGIN to be defined for
               dictionaries, and an error if performed on leaf
               objects in the tree.

           CREATE:  The default is that CREATE is undefined.

           DELETE:  The default is that DELETE is undefined.

           END:  END is a stack operation and is defined for all objects.
               Note that END may fail if there is no object on the
               stack.


           GET-ATTRIBUTES:  The default is that GET-ATTRIBUTES is
               defined on the contents of all dictionaries specified
               in this memo.  The text description attributes
               are optional for values defined in this memo, but
               are required for implementation-specific objects.
               Any descriptions of object listed in this memo should
               cite this memo.  GET-ATTRIBUTES must be supported on
               all entity-specific values.  GET-ATTRIBUTES
               returns a Attributes object, which is defined in
               the well-known types section below.



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RFC 1024                    HEMS Definitions                October 1987


           GET-ATTRIBUTES-MATCH:  The default is that
               GET-ATTRIBUTES-MATCH is optionally defined on any
               object which supports GET-MATCH, and is an error
               otherwise.  The rules for attributes returned by
               GET-ATTRIBUTES-MATCH are the same as those for
               GET-ATTRIBUTES.

           GET:  The default definition of GET is to emit the operand
               specified is a leaf object, and if the operand is a
               dictionary, to recursively GET the entire dictionary and
               its subdictionaries.

           GET-MATCH:  Unless otherwise specified, GET-MATCH is not
               supported on an object.

           GET-RANGE:  Unless otherwise specified, GET-RANGE is not
               supported on an object.

           SET:  Unless otherwise specified, SET is not supported on an
               object.

           SET-MATCH:  Unless otherwise specified, SET-MATCH is not
               supported on an object.


ATTRIBUTES

   HEMS requires that remote applications be able to discover the
   meaning of an object by querying the entity in which the object is
   stored.  This is done through use of the GET-ATTRIBUTES operator,
   which causes an Attributes object to be returned to the application.
   The Attributes object is described below.

           Attributes ::= [APPLICATION 2] IMPLICIT SEQUENCE {
               tagASN1       [0] IMPLICIT INTEGER,
               valueFormat   [1] IMPLICIT INTEGER,
               longDesc      [2] IMPLICIT IA5String OPTIONAL,
               shortDesc     [3] IMPLICIT IA5String OPTIONAL,
               unitsDesc     [4] IMPLICIT IA5String OPTIONAL,
               precision     [5] IMPLICIT INTEGER OPTIONAL,
               properties    [6] IMPLICIT BITSTRING OPTIONAL,
           }

   The meanings of the various attributes are given below.

           tagASN1:  The ASN.1 tag for this object.
               This attribute is required.




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RFC 1024                    HEMS Definitions                October 1987


           valueFormat:  The underlying ASN.1 type of the object
               (e.g., SEQUENCE, or OCTETSTRING).  This attribute
               is required.

           longDesc:  A potentially lengthy text description which
               fully defines the object.  This attribute is optional
               for objects defined in this memo and required for
               entity-specific objects.

           shortDesc:  A short mnemonic string of less than 15 octets
               which is suitable for labelling the value on a display.
               This attribute is optional.

           unitsDesc:  A short string used for integer values to
               indicate the units in which the value is measured
               (e.g. "ms", "sec", "packets", etc).  This attribute
               is optional.

           precision:  For Counter objects, the value at which the
               Counter will roll-over.  Required for all Counter
               objects.

           properties:  A bitstring of boolean properties of the
               object.  If the bit is on, it has the given property.
               This attribute is optional.  The bits currently
               defined are:

                   0 -- If true, the difference between two values
                       of this object is significant. For example,
                       the changes in a packet count is always
                       significant, it always conveys information.
                       In this case, the 0 bit would be set.  On the
                       other hand, the difference between two readings
                       of a queue length may be meaningless.


IMPLEMENTATION SPECIFIC TYPES

   Each vendor or implementation specific value must be contained within
   an VendorSpecific object.  The format of the VendorSpecific object is
   shown below.

   Type:  VendorSpecific

            VendorSpecific ::= [APPLICATION 3] IMPLICIT SET of ANY






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RFC 1024                    HEMS Definitions                October 1987


   For a detailed discussion of the need for this type, see RFC 1023.

WELL-KNOWN TYPES

   There are some generally useful types which are defined across the
   system and are considered well-known.  These types support abstract
   notions that are frequently used in other definitions.

   TYPE: Error

           Error ::= [APPLICATION 0] IMPLICIT SEQUENCE {
               errorCode INTEGER,
               errorOffset INTEGER