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Network Working Group C. Partridge Request For Comment: 1021 BBN/NNSC

                                                           G. Trewitt
                                                             Stanford
                                                         October 1987

           THE HIGH-LEVEL ENTITY MANAGEMENT SYSTEM (HEMS)

STATUS OF THIS MEMO

 An overview of the RFCs which comprise the High-Level Entity
 Management System is provided.  This system is experimental, and is
 currently being tested in portions of the Internet.  It is hoped that
 this work will help lead to a standard for IP internetwork
 management.  Distribution of this memo is unlimited.

INTRODUCTION

 Until recently, a majority of critical components in IP networks,
 such as gateways, have come from a very small set of vendors.  While
 each vendor had their own set of management protocols and mechanisms,
 the collection was small, and a knowledgeable system administrator
 could be expected to learn them all.

 Now, however, the number of vendors has grown quite large, and the
 lack of an accepted standard for management of network components is
 causing severe management problems.  Compounding this problem is the
 explosive growth of the connected IP networks known as the Internet.
 The combination of increased size and heterogeneity is making
 internetwork management extremely difficult.  This memo discusses an
 effort to devise a standard protocol for all devices, which should
 help alleviate the management problem.

 The RFCs that currently define the High-Level Entity Management
 System are this memo along with RFC-1022, 1024, and 1023.  This list
 is expected to change and grow over time, and readers are strongly
 encouraged to check the RFC Index to find the most current versions.

MONITORING AND CONTROL

 Historically, the IP community has divided network management into
 two distinct types of activities: monitoring and control.  Monitoring
 is the activity of extracting or collecting data from the network or
 a part of the network to observe its behavior.  Control is the
 activity of taking actions to effect changes in the behavior of the
 network or a part of the network in real-time, typically in an
 attempt to improve the network's performance.

Partridge & Trewitt [Page 1]  RFC 1021 HEMS Overview October 1987

 Note that the ability to control presupposes the ability to monitor.
 Changing the behavior of the network without being able to observe
 the effects of the changes is not useful.  On the other hand,
 monitoring without control makes some sense.  Simply understanding
 what is causing a network to misbehave can be useful.

 Control is also a more difficult functionality to define.  Control
 operations other than the most generic, are usually device-specific.
 The problem is not just a matter of providing a mechanism for
 control, but also defining a set of control operations which are
 generally applicable across a diverse set of devices.  Permitting
 remote applications to exercise control over an entity also implies
 the need for a suite of safeguards to ensure that unauthorized
 applications cannot harm the network.

 Because monitoring is the key first step, in this initial design of
 the system, the authors have concentrated more heavily on the
 problems of effective monitoring.  Although the basic control
 mechanisms are defined, many components need for control, such as
 strong access control mechanisms, have not been fully defined.

OVERVIEW OF THE HEMS

 The HEMS is made up of three parts: a query processor which can
 reside on any addressable entity, an event generator which also
 resides on entities, and applications which know how to send requests
 to the query processor and interpret the replies.  The query
 processor and applications communicate using a message protocol which
 runs over a standard transport protocol.

The Query Processor

 The query processor is the key to the management system.  It
 interprets all monitoring and control requests.  For optimal network
 management, we would like to see query processors on most network
 entities.

 To encourage the implementations of query processors, one of the
 primary goals in designing the query processor was to make it as
 small and simple as possible, consistent with management
 requirements.

 Defining the management requirements was no small task, since the
 networking community has not yet reached a consensus about what kinds
 of monitoring information should be available from network entities,
 nor what control functions are required to properly manage those
 entities.  The standards for HEMS were developed through discussions
 with several interest groups, and represent the authors' best effort

Partridge & Trewitt [Page 2]  RFC 1021 HEMS Overview October 1987

 to distill the varying sets of needs.

 The authors settled on a system which was extensible, robust and
 host-architecture independent, and as simple as possible, consistent
 with the other goals.  Extensibility was essential because it is
 clear that management needs will continue to evolve, and a closed
 system which could not be changed would be obsolete almost as soon as
 it was defined.  Unfortunately, extensibility is also the requirement
 least consistent with simplicity since the need to make the system
 extensible led the authors to use self-describing data formats and an
 interpreted query language.

 A robust system is required if the system is to be useful for
 diagnosing network failures.  If the monitoring system cannot survive
 at least moderate network failures, it is not useful.

 The query processor is designed to be highly extensible.  An
 application sends the query processor instructions about objects to
 be examined or changed.  The query processor locates the objects in
 its host entity, and performs the requested operations.  The objects
 are self-describing, using the binary-encoding scheme defined in ISO
 Standard ASN.1.  Care has been taken to use a limited set of the
 ASN.1 coding set, so that query processor's handling of data can be
 optimized.

 It is a key feature of HEMS that messages to the query processor
 contain multiple instructions.  The authors felt that this would give
 much higher performance than a remote procedure system which limited
 an application to one operation per message.

 The set of maintained objects is standardized across all entities.
 Every entity is required to manage a small set of objects.  In
 addition, entities of a particular type (e.g., a gateway) may be
 required to manage a larger set of objects, which are optional on
 other entities.  Entities are also permitted to make additional,
 entity-specific objects available to applications.  A method for
 discovering the existence of additional objects is defined.

 The combination of self-describing data, the ability to add to the
 standard data set, and a query language which can be easily enhanced
 appeared to offer the necessary extensibility.

Event Generator

 On many network entities, particularly critical network components
 such as gateways, it is necessary to have a way for the devices to
 send unsolicited status messages to network management centers.  In
 the IP community, these messages have historically been referred to

Partridge & Trewitt [Page 3]  RFC 1021 HEMS Overview October 1987

 as "traps", but for compatibility with the ISO nomenclature, in the
 HEMS system they are called "events".

 In the HEMS system, events are handled as slightly specialized
 replies to queries, and are sent to one or more management centers.
 Like all other HEMS messages, events are formatted in ASN.1 format.

 Each event is given a well-known code, which is standardized across
 all entities.  Provision is also made for entity specific event
 codes.

Applications

 The HEMS expects that applications will be more intelligent than the
 query processor.  Among other functions, the applications will have
 to be able to identify and parse entity-specific values which may be
 returned.

 The details of applications are largely not discussed in the HEMS
 specifications because there is very little that needs to be
 standardized.  Applications must send requests using the protocols
 discussed in the next section, but the interfaces the applications
 provide for displaying monitoring or control information are entirely
 application dependent.

Protocols

 Query processors and applications communicate using an application-
 specific monitoring protocol, the High-Level Entity Management
 Protocol (HEMP).  This protocol provides the formatting rules for the
 queries and their replies.

 HEMP runs over a standard transport protocol.  There was a certain
 amount of debate in the community about what type of transport
 protocol was best suited for monitoring.  The key issue was how
 reliable monitoring interactions needed to be.

 The authors expect that three types of management activities will
 predominate: status monitoring, firefighting, and event reporting.

 Status monitoring is envisioned as occasional retrieval of monitoring
 information, possibly in response to the receipt of event messages.
 In these situations, the network is expected to be in good working
 condition, and monitoring exchanges could probably comfortably work
 with an unreliable transport protocol.  The chance of data loss is
 small, and probably not a serious problem since the data is unlikely
 to be so important that it must be reliably delivered.  (However, it
 should be noted that some applications may prefer reliable delivery

Partridge & Trewitt [Page 4]  RFC 1021 HEMS Overview October 1987

 because it is more convenient.)

 Firefighting is a completely different situation.  In this scenario,
 one or more sites are using management applications to try to locate
 and fix a network problem.  Here we must assume that while the
 network functions (i.e., data can get through), it is not very
 healthy.  We should assume that packets are being lost, that network
 routes will be non-optimal and that it is essential that the
 monitoring data (which is presumably diagnostic) get back to the
 application and that control requests are reliably delivered to the
 entity.  In such circumstances, a reliable protocol is essential.

 Events provide yet another bit of complexity.  Events contain useful
 status information, but experience suggests that this information
 does not have to be delivered reliably.  If the problem is serious
 enough, it will re-occur and the event will be sent again.
 Furthermore, events will often be sent to more than one management
 center, which would appear to preclude the use of connection-
 oriented, reliable protocols such as TCP for events.

 The current decision has been to establish two possible transport
 options for HEMS.  More experimental systems may use the Versatile
 Message Transaction Protocol (VMTP), an experimental IP transaction
 protocol.  Near term production systems can use a combination of the
 Transmission Control Protocol (TCP) and the User Datagram Protocol
 (UDP), as described in RFC-1022.

Compatibility with Common Management Information Protocol (CMIP)

 Several groups have expressed interest in being able to develop
 applications which can use both HEMS and the emerging ISO-defined
 Common Management Information Protocol (CMIP).  It turns out that
 such a co-existence is feasible, and the authors have made an effort
 to accomodate it.

 At the highest level, both CMIP and HEMS perform operations on
 objects stored in remote entities, and both systems use ASN.1
 formatting to represent those objects.  This makes it possible to
 develop a standard set of interface routines which can be used to
 access either system, even though underlying mechanics of the systems
 are quite different.

Partridge & Trewitt [Page 5]