HEMS variable definitions
RFC 1024
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(October 1987; No errata)
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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.
Partridge & Trewitt [Page 1]
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).
Partridge & Trewitt [Page 2]
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
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